Bruce Andersop 1 with Michael Riordan .

Bruce  Andersop 1 with  Michael  Riordan .
.
‘,/
Bruce Andersop
with Michael Riordan
i’.
/ ‘.
.
1
Copyright 0 1987. 1976 by R.A.K. Publishing Co.
All rights reserved. Printed in the United States of America.
Acknowledgements
The first edition of this book. titled The SO/~J~HCJIW Book. was based on
Bruce Anderson’s master’s thesis. “Solar Energy and Shelter Design”.
for
the School of Architecture
at M.I.T. His manuscript was revised for book
publication
by Michael Riordan. This edition was produced by the staff of
Cheshire Books under the direction of Linda Goodman. Illustrations
were by
Edward A. Wong.
Revisions to bring the book up to date for the second edition were done by
Jennifer Adams, a designer with The Write Design and former engineering
illustraeditor of Solur Age magazine (now Progrrssiw Builder). Additional
tions were prepared by ANCO of Boston.
Publication of both editions
Katzenberg.
Library
has been tinanced
of Congress [email protected]
Anderson, Bruce,
The new solar
through the efforts
of Richard
Data
1947home book.
Rev. ed. of: The solar home borjk. clg76.
I'
Includes index.
1. Solar houses.
2. Solar energy.
I. Riordan,
Michael.
II. Anderson, Bruce, 1947. Solar home
book.
III.
Title.
TH7413.A53
1987
86-23214
697' -78
ISBN 0-931790-70-0
(pbk.)
For generations, Americans have viewed cheap
and plentiful energy as their birthright. Coal,
oil or gas have always been abundantly available to heat our homes, power our automobiles,
and fuel our industries. But just as the supply
of these fossil fuels begins to dwindle and we
look to the atom for salvation, we are beginning
to perceive the environmental havoc being
wrought by our indiscriminate use of energy.
Our urban and suburban skies are choked with
smog; our rivers and shores are streaked with
oil; even the food we eat and the water we drink
are suspect. And while promising us temporary
relief from energy starvation, nuclear power
threatens a new round of pollution whose severity is still a matter of speculation.
The residential use of solar energy is one step
toward reversing this trend. By using the sun
to heat and cool our homes. we can begin to
halt our growing dependence on energy sources
that are polluting the environment and rising in
cost. The twin crises of energy shortage and
environmental degradation occur because we
have relied on concentrated forms of energy
imported from afar. We had little say in the
method of energy production and accepted its
by-products just as we grasped for its benefits.
But solar energy can be collected right in the
home, and we can be far wiser in its distribution
and use.
Unlike nuclear power, solar energy produces
no lethal radiation or radioactive wastes. Its
generation is not centralized and hence not open
to sabotage or blackmail. Unlike oil. the sun
doesn’t blacken our beaches or darken our skies.
Nor does it lend itself to foreign boycott or
corporate intrigue. Unlike coal. the use of solar
energy doesn’t ravage our rural landscapes with
strip mining or our urban atmospheres with soot
and sulphurous fumes.
Universal solar heating and cooling could ease
fuel shortages and environmental pollution substantially. Almost I5 percent of the energy consumed in the United Statesgoes for home heating,
cooling, and water heating. If the sun could
provide two thirds of these needs, it would reduce
the national consumption of non-renewable fuels
by IO percent and world consumption by more
than 3 percent. National and global pollution
would drop by stmilar amounts.
But solar energy has the drawback of being
diffuse. Rather than being mined or drilled at
a few scattered places, it falls thinly and fairly
evenly across the globe. The sun respects no
human boundaries and is available to all. Governments and industries accustomed to concentrated energy supplies am ill-equipped, by reason
of economic constraints or philosophical prejudices. to harness this gentle source of energy.
These institutions are far more interestedin forms
...
111
Foreword
of energy that lend themselves to centralization
and control. Hence the United States govemment spends billions for nuclear power while
solar energy is just a subject for study-a future
possibility, maybe. but not right now.
This book speaks to the men and women who
cannot wait for a hesitant government to “announce” a new solar age. We can begin to fight
energy shortages and environmental pollution
in our own homes and surroundings. Solar heating and cooling are feasible t&q-not
at some
nebulous future date. The solar energy falling
on the walls and roof of a home during winter
is several times the amount of energy needed
to heat it. All it takes to harness this abundant
supply is the combination of ingenuity. economy and husbandry that has been the American
ideal since the days of Franklin and Thoreau.
Bruce Anderson
Harrisville. New Hampshire
Michael Riordan
Menlo Park. California
iv
Solar and Heat Basics 2
Measurement of Heat and Solar Energy
Solar Heating Methods 4
Other Solar Applications 5
3
Solar Position 9
Insolation I I
Diffuse and Reflected Radiation I3
Limitations of Insolation Data I5
eat
Conduction Heat Loss 17
Convection Heat Loss 21
Radiation Heat Flow 23
Heat Load Calculations 24
Seasonal and Design Heat Loads
26
3
Orientation and Shape 29
Color 31
Absorptance. Reflectance, and Emittance
32
Contents
Air Quality 36
Wind Control 36
Air and Vapor Barriers 37
Windows 38
High-performance Glazing 40
Insulation 4 1
5
ain Systems
Glazing 47
Shading 48
Sizing Overhangs 49
Sun Path Diagrams 50
Use of Sun Path Diagrams
6
e
52
ouse as a
emperature Swings
Heat Storage Capacities 56
Building with Thermal Mass 56
Storing Heat in a Concrete Slab 57
Sizing Mass 58
7
irect Gain Syste
Thermosiphoning Air Panels 60
TAP Variations 61
Mass Walls 62
Mass Wall Variations 63
Wail. Window, and Roof Collectors
Sunspaces 65
Passive Versus Active Systems 68
64
Batch Heaters 71
Thermosiphoning Water Heaters 73
Phase-change Systems 74
Freeze Protection 75
9
Active Solar
Recirculation 76
Draindown 79
Drainback 79
Antifreeze 82
PV-Powered 82
One-Tank vs. Two-Tank Systems 84
Installation Checklist 84
vi
Contents
art
ive
Cooling
Heat Transfer Fluids 89
Air System Designs 92
Liquid System Designs 93
Swimming Pool Heating 94
Controls 95
Performance and Cost 96
Solar Cooling 96
Absorption Cooling Principles
89
97
late Collectors
Tube Sizing and Flow Patterns 99
Tips on Corrosion Prevention 100
Absorber Plates 100
Absorber Coatings and Cover Plates IO1
Insulation IO3
Other Factors I04
late Collectors
1
Absorbers IO5
Air Flow and Heat Transfer I06
Absorber Coatings and Cover Plates I07
Other Design Factors 107
er Collector
es 109
Parabolic Collectors I
Compound Parabolic Concentrator
Evacuated-tube Collectors I IO
109
ante and Size
11
Collector Heat Losses I 14
Energy Flows in a Collector I I5
lnsoiation i I6
Collector Orientation and Tilt I I7
Sizing the Collector I I9
Estimating Collector Performance I22
Comparing Collectors 123
Estimating Collector Size 124
Storage an
Tanks of Water I29
Rock Beds I30
Phase-change Materials I32
Insulation 133
Storage Size I33
Estimating Storage Size 134
Heat Distribution I35
vii
Contents
Auxiliary Heating 136
Heat Pump Principles 137
Coefficient of Performance I37
16
otovoltaics: Electricity fro
the Sun
uniight to Electricity I38
Power Requirements 139
An Average Home 139
Estimating Array Size I40
Supplemental Power 143
Power inverters 143
Residential Installations I43
Financial Constraints I45
Life-cycle Costing I46
System Reliability 147
Solar Energy and the Construction Industry
Government Incentives I48
Solar Angles 149
Clear Day Insolation Data I54
Solar Radiation Maps I61
Calculating Solar Radiation I70
Degree Days and Design Temperatures
insulating Values of Materials I76
Heat Conduction Cost Chart I85
Air Infiltration Cost Chart I87
Emittances and Absorptances
of Materials I89
Specific Heats and Heat Capacities
of Materials I92
Metric/English Equivalents and
Conversion Factors 194
...
VIII
172
I47
13
NON* in II~LL~.~ with (1 south crsptw. the SIOI’S
my pcnetrcue into thr porticwrs in winter, hut
in .wt~itm~r th ptrth of the siiti is right over 0141
liiwls cirid uhow tlw Ao/l so tlicrt thw is shde.
!fl tlwn. this is th hcst ~wm,qtmw,
iii’ shtld
hilt1 t/w sorrtli sick IvjGr- to grt the hkter siui
trncl th tiortlr sitlt* Irmw to X;c~pout the ~wltl
brid.s.
Socrates. as quoted by
Xenophon in Mmonrhilict
‘fhc &sign of human shelter has oftrn retlcctcd
an undcrst;inding of the sun’s power. Primitive
shcltcrs in tropical arcas have broad thatched
roofs that provide shade from tire scorching
midday sun and ktxp out frequent rains. The
open wails of these structures allow cooling
hrrczcs to carry away accumulated heat and
moisture. in the American southwest. Pueblo
Indians built thick adobe wails and roofs that
kept the interiors cool during the day by absorbing the sun’s rays. By the time the cold
desert night rolled around. the absorbed heat
hxl penetrated the living quurtcrs to warm the
inhabitants. Communal buildings faced south or
southeast to absorb as much of the winter sun
as possible.
Even the shelters of more advanced civiiizations have been designed to take advantage
of the sun. The entire Meso-American city of
Teotihuacan. the size of ancient Rome, was laid
out on a grid facing I5 degrees west of south.
Early New England houses had masonry filled
wails and compact layouts to minimize heat loss
during frigid winter months. The kitchen. with
its constantly burning wood stove. was located
on the north side of the house to permit the
other rooms to occupy the prime southern exposure. Only in the present century, with abundant supplies of cheap fossil fuels available. has
the sun been ignored in building design.
Serious technical investigations into the use
of the sun to heat homes began in 1939. when
the Massachusetts institute of Technology built
its tirst solar house. For the first time, solur
d1ec~tor.s placed on the roof gathered sunlight
for interior heating. By 1960. more than a dozen
structures had been built to use modem methods
of harnessing the sun’s energy.
During the 1970s. following the Arab oil embargo, thousands of solar homes were built.
Hundreds of manufacturers produced solar coiIcctors. and the sun’s energy was used to heat
domestic water as well. But the steep rise in
crude oil prices also triggered conservation on
a scale that dramatically cut worldwide oil consumption, forcing crude oil prices back down.
Widespread popular interest in energy subsided
momentarily. but did leave behind a legacy of
real progress in the uses of renewable energy.
1
ULfRAVIOLET
NEAR
“S’OLE INF~RJCD
I
---L-v-MAJOR PORTION OFTHE
SPECTRUM OF ENERqIES
SHORT
WAVELWtj-P4
FROM THE SUN THAT RZACYI
THE utR7H’s
SURFACE.
MD10
WAVES
I
I
DEGRADED
ClEAT WEUtjY
FLD)urlN(.j [email protected]
J
llM+iz+H
THE EARTW
EMCK INTO spx.&
The electromagnetic
spectrum. (Miller,
SOLAR AND HEAT BASICS
Most of the solar energy reaching us comes in
the form of visible light and ir$urd rays. These
two forms of radiation differ only in their waveIcngths. When they strike an object. part of the
radiation is absorbed and transformed into an
equivalent amount of heat cncrgy. Heat is simply rhc motion of atoms and moiccuies in an
object. it is stored in the material itself or COUhcwl to surrounding matcriais. vVrarmingthem
in turn. Heat can also be carried off by air and
water Ilowing past these warm materials. in what
is called cornwtion heat flow.
That a material can be heated by the sun is
obvious to anyone who has walked barefoot
over a sun-baked pavement. What may not be
so obvious is that the puvemcnt also rdiutes
some of the heat energy away in the form of
infrared rays. You can feci this t/wrnru/ rculicrtion by ;JUtting
your hand near an iron poker
after it has been heated in a tireplace. it is this
radiation of energy back into space that keeps
the earth from overheating and frying us to a
crisp.
2
Living in the Environment.
Wadsworth.)
The amount of solar energy reaching the earth’s
surface is enormous. it frequently exceeds 2OO
Btu per hour on a square foot of surface, enough
to power a 60-watt light bulb if ail the solar
energy could be converted to electricity. But
the technology of solar electricity is in its infancy; we are fortunate if we can convert even
I5 percent. On the other hand. efficiencies of
60 percent are not unreasonable for the conversion of solar energy into heat for a house.
The energy failing on a house during the winter
is generdiiy several times what is needed inside.
so the sun can provide a substantial fraction of
its annual needs.
Glass is the “miracle” substance that makes
solar heating possible. Glass transmits visible
light but not thermal radiation. You can prove
this to yourself by sitting in front of a blazing
tire. Your face becomes unbearably hot if you
sit too close. But what happens if you place a
pane of glass in front of your face’? You can
still SPCthe tire but your face is not nearly as
hot as hefore. The iongwave infrared rays carrying most of the tire’s radiant energy are absorbed by the glass, while the shortwave visible
Introduction
rays penetrate to your eyes. In the same way,
once sunlight passes through a window and is
transformed into heat energy inside, this energy
cannot be radiated directly back outside. This
phenomenon, known as the greenhouse effect.
is responsible for the hot, stuffy air in the car
you left in the sun after the doors locked and
the windows rolled up. Other transparent materials. particularly plastics, also absorb this
thermal radiation. but none quite so well as
glass.
The basic principles of solar collection for
home heating and cooling are embodied in the
greenhouse. The sun’s rays pass through the
glass or transparent plastic glazing and are ab-
sorbed by a dark surface. The heat produced
cannot escape readily because thermal radiation
and warm air currents are trapped by the glazing. The accumulated solar heat i; then transported to the living quarters or stored.
There i> often an overabucdance of solar energy wheu it is not needed, and none at all when
it is most in demand. Some means is required
to store the collected solar heat for use at night
or during extended periods of cloudiness. Any
material absorbs heat as its temperature rises
and releases heat as its temperature falls. The
objects inside a house-the walls, ceilings,
floors, and even furniture-can serve as heat
storage devices.
Measurement of Meat and Solar Energy
There ure two btrsic tyes of Irtl~(l.stIrt’ItIl’Ilt used
to describe hrtrt ~~~~rr~~~-ttr,,lpercItlcr’e and
ylrtrntit~. Temperuttcre is (I meii.s~ux~of the (II*cv-qye ~~ibrtrtionul energy of molec~rles. For exiimple. the m0lri~44le.sin rcvrter at 40°c’ (degrees
C~ivitigrade) ure \*ihrating more rupid~~ than
molcv~~rlesin \~*uter at 10°C’. Hetrt cltctrntity is
d~vermint~d both b! lio~r~rirpid~~ mc)leclr~rs are
\*ibratiri!: irnd b,vhow munv m0L~crrle.vthere arc.
For cJ.\-ample.it takes . mLi Iarjyr qiuintity o/‘
heat to rtrise a .swimmin!: pool to 40°C’ than to
raise ti kettle of water to 40°C’. e1’en thoirgh the
tc’mperatrrre is the sume in both.
In the Engli.sh sytem of I?te~i.siiri’t~i~‘~lt.the
lrnit of’ heat ylurntitv is the British Thermal Unit,
or Btrr. the tmirnint of heat needed to raise one
powtd of I\vlter OIUJ&qree Ftrhrenheit (OF). In
the metric. .sF.stem,the irnit of hetrt cliiuntit~ is
the (ulorie. or id, the crmount of heat reqitired
to rtrise one gram of‘ water one degri*e C’rntigrade. One Btir is eqrri~~trlcnt to ul;oirt 2.52 cul.
It take.s the same ytwntit~ r!f heat. 100 Btlr or
,75.200 1~11.to heat 100 p~nd.s of lcyrter 1°F us
it dr~e.sto heut IO pounds oj’ ,tv&r 10°F.
Herrt is one form of energv und sunlight is
~urt,tlter-rudiiult
energy. An important churtrcteri.stic of‘ energ! is that it is ne1yr lost-
energy mu! change ,fi-om one jbrm to another,
blct it ne\ler d;.~trppeurs. Thus rc*ecun describe
the amolrnt oj’ .solur energv striking u surj&e
in terms of un equi~vdrnt umount of heat. We
meusiire the sokur energy striking a siuji~ce in
u given time period in trnits of Bttr!ft’lhr or cull
cm’lmin. Olitside the eurth’s atmosphere. jbr
lwrnlplt~, solur energy strikes at the u~~eruge
rute c:f 429 Btlc!f?lhr or I .Y4 ctrllcm~lmin.
The radiunt energy reuching us from the sun
hus u distribution of wavelengths (or colors).
We describe these wu\*elengths in units of microns. or millionths of u meter. The wavelength
distribution of solur energy striking the earth’s
atmosphere and reaching the grotrnd is shown
in the accompanying chart.
Abolct half of the solar radiation reaching
the grattnd fulls in the visible range, 0.4 to 0.7
microns. Most of the radiution in the ultrarGlet
runge, rcpithwur*riengths below 0.4 microns. is
ubsorbed in the lcpper atmosphere. A substantiul portion of the infrared radiation. with wavelengths greater than 0.7 microns, reaches the
eurth’s surface. A warm body emits e\*en longer
wave infrared rudiution. Since glass transmits
very little radiation at these longer wavelengths. it traps this thermal radiation.
3
The New Solar Home Book
HOT AIR
m HOUSE
COOL AIf2
F&m
tfiX5E
A typical active system for solar heating.
SOLAR HEATING METHODS
The great variety of methods used to trap solar
radiation for home heating can be grouped into
two broad categories-passive and active. In
pussive systems, the sun’s energy is collected,
stored, and transmitted without the use of electrical or mechanical energy. Passive systems
can be further subdivided into direct gain and
indirect gain systems. Direct gain systems are
the simplest way to solar heat. They require at
most a rearrangement of standard construction
practices. Almost all solar homes employ some
direct gain, unless poor orientation or unsightly
wiews prohibit it.
Indirect gain systems collect the sun’s energy
before it enters the home. Then they either di-
4
rect the heat into the building to be stored there.
or use ingenious adaptations of the natural thermal properties of materials to store and distribute the heat. The energy flows to rooms without
the help of complex ducts, piping, or pumps.
Such systems are often an integral part of the
home itself. Although they may call for nonstandard building practices, they can be simple
and effective.
Active systems for solar heating generally use
rooftop solar collectors and separate heat storage devices, although if small enough. they too
can use the mass of the house itself for storage.
Heat moves from the collectors to storage or to
interior spaces through pipes or ducts. Pumps
Introduction
or fans circulate a fluid through the collector
and back to the house or to an insulated heat
storage container. In the second case. if the
house needsheat. the tluid from the central heating system is warmed by the stored heat and
circulated through the rooms. Such heating systems are called utviw because they rely on mechanical and electrical power to move the heat.
Most active solar heating systems use an array of Jtlt-pltrttj twl1t~t~tor.sto gather solar energy. These collectors hake one or more glass
or plastic cover plates with a black absorber
beneath them. The cover plates reduce the loss
of energy through the front. and insulation behind the absorber reduces the heat loss through
the back. Heat from the absorber is conducted
to a transfer fluid, either a gas (usually air) or
a liquid (water or antifreeze). which tlows in
contact with it and carries off the heat.
In t~o~ii~eritrtttin~~iwl1t~t~tor.s. reflective surfaces concentrate the sun’s rays onto a very
small area-often an evacuated tube. This solar
energy is then absorbed by a black surface and
converted to heat that is carried off by a Huid.
Concentrating collectors can produce very high
temperatures. and some require mechanical devices to track the sun across the sky. They are
most often seen in large scale applications. such
as industrial heating or generation of electricity.
Depending on the climate, the house. and the
solar heating system design. SO to 90 percent
of a house’s heating needs can be readily supplied by the sun. However. solar heating systems almost always require a backup. or auxiliary
heating system. l?areiy is it economical to build
a heat storage unit with the capacity to carry a
house through long periods of cold and cloudy
weather.
OTHER SOLAR APPLICATIONS
Two other uses of sunlight have a strong place
in the market: systems for heating domestic hot
water and attached greenhouse solariums called
sunspaces. A third application. photovoltaics.
is still struggling to achieve a cost-benefit ratlo
that will attract major attentmn. but it has longterm promise.
Solar heating of domestic hot water (DHW)
is a smaller scale application of the same concepts and techniques used for home heating. It
can have a lower tirst cost and can tit in easily
with existing conventional water heating systems.
Sunspaces are a modern version of traditional
sunporches or attached greenhouses. designed
to serve many purposes. Depending on the particular design combination, sunspaces can be
attractive living spaces. economical sources of
auxiliary heat, a place for growing plants. or a
combination of ail three.
In photovoitaics. a way of getting electricity
directly from the sun. solar ceils use the srmiconducting properties of materials such as siiicon to convert sunlight to electricity.
Photovoitaics has enormous potential. At present. however. only in remote areas can solar
ceils compete on overall cost with other methods of generating electricity.
Using sunlight for heat and energy goes back a
long way in human history. But the last forty
years have seen the most dramatic progress in
developing solar technology. The purpose ol
this book is to present the principles of solar
design, so that you can understand how and why
these principles can be applied to using the free
and abundant energy of the sun.
Is it rwl by the \ibrtrtiorrs gitvert lo ir by 1114srtti
hit light trppecrrs to II.~: wid muy it not be thut
euq otw of’ the it$nitcl~ .stnirll ~dwtrtint~s. strikitl,q uvwnot~ tmtter bcith CIcw-tuitl jiww. ettttw
its .wh.stum*i~. is hrld thaw 1J.vtrttriri*tion utrd
crirgtmvitd by .wcw.s.siw \dmition.s, till the
nrtrttiv has t-rcvi\vd us twcA tls their jimxj cut1
driw into it:’
is it riot t/ur.stkcrt tlie sr&cc 0f this globe is
IicWed /JJ sdi ri~ptwi~cl dw~rti0ti.s itt the clqv.
1rtrdcwoled b! t/w e.scu~?e
of IA4 hrt WhPnthose
rdmrtims we cliscwt~titiued itI the ni,qht?
Benjamin Franklin.
Loose Thmghts OHtl Utnkrscrl Fluid
Before you design and build a solar home. you
need to become familiar with your surroundings. You need to know the position of the sun
in order to orient a house or collector to receive
its warm rays. To gauge the solar heat flows
into a house you must calculate the solar radiation hitting the walls. windows. roofs and collector surfaces. You also need to calculate the
heat escaping from a house in order to select
the best methods to slow it down. Only when
you have grasped the fundamentals can you take
advantage of these natural energy flows.
First you need to understand some of the
language others use to describe and measure
energy. Become familiar with climatic data and
the properties of common building materials.
The aim of this is to aquaint you with these and
other essentials that will help you use the abundance of solar energy falling all around you.
Some of this may seem tedious. but it is all very
important to good solar home design.
7
After centuries of observation. ancient astronomeis could accurately predict the sun’s motion
across the sky. Stonehenge was probably a gigantic “computer” that recorded the movements of the sun and moon in stone. From their
earthbound viewpoint. early peoples reckoned
that the sun gave them night and day by moving
in a path around the earth. But today, thanks to
the work of the sixteenth-century Polish astronomer Copernicus, we know that the earth travels
in an orbit around the sun and that the rotation
of the earth, not the motion of the sun, gives
us the cycles of night and day.
The earth actually follows an elliptical teggshaped) path around the sun. As it travels this
orbit. its distance from the sun changes
slightly-it is closest in winter and most distant
in summer. The amount of solar radiation striking the earth’s atmosphere is consequently most
intense in winter. Then why are winters so
dreadfully cold’?
This seeming paKIdOX is readily explained.
The earth’s axis is tilted relative to the plane of
its orbit, as shown in the first diagram. The
north pole is tilted torwrd the sun in summer
and a~u~fiont the sun in winter. This angle is
called the der’linutiotl angle. From our viewpoint here on earth. this tilt means that the sun
is higher in the sky in summer, and lower in
winter. Consequently. the sun’s rays have a
greater distance to travel through the atmosphere in winter, and they strike the earth’s surface at a more glancing angle. The amount of
solar radiation eventually striking a horizontal
surface is less during the winter, and the weather
is colder.
This tilt of the earth’s axis results in the seasons of the year. If the axis were perpendicular
to the orbital plane, there would be no noticeable change of seasons. Each day the sun would
follow the same path across the sky. and the
weather would be uniformly dull. Likewise, if
the earth did not rotate on its axis, the sun would
creep slowly across the sky. and a single day
would last a whole year. The diurnal (daily) and
seasonal cycles that we take for granted are a
direct result of this rotation of the earth about
a tilted axis.
SOLAR POSITION
Most people have probably noticed that the sun
is higher in the sky in summer than in winter.
Some also realize that it rises south of due east
in winter and north of due east in summer. Each
day the sun travels in a circular path across the
sky. reaching its highest point at noon. As winter proceeds into spring and summer. this circular path moves higher in the sky. The sun
rises earlier in the day and sets later.
TIME
OF DAY
6
SEW 23
The earth’s elliptical path around the sun. The tilt of the
earth’s axis results in the seasons of the year. The declination angles on June 22 and Dec. 22 are +23.5 and
-23.5, respectively. The declination angles on Mar. 21
and Sept. 23 are both 0.
The actual position of the sun in the sky
depends upon the latitude of the observer. At
noon on March 3 I and September 23. the vernal
and autumnal ecpitmxrs. the sun is directly
overhead at the equator. At 4O”N latitude, however. its angle above the horizon is SO” (90 40”). By noon on June 22. the smttwr solstiw
in the Northern Hemisphere. the sun is directly
overhead at the Tropic of Cancer, 23.S”N latitude. Its angle above the horizon at 40”N is
73.5” (90” + 23.5” - 40”). the highest it gets
at this latitude. At noon on December 22. the
sun is directly overhead at the Tropic of Capricorn. and its angle above the horizon at 40”N
latitude is only 26.5” (90” - 23.5” - 40”).
A more exact description of the sun’s position is needed for most solar applications. In
the language of trigonometry. this position is
expressed by the values of two angles-the solar altitude and the solar azimuth. The solar
trltitu& (represented by the Greek letter theta
0) is measured up from the horizon to the sun,
;r,hile the solar uzitnuth (the Greek letter phi +)
is the angular deviation from true south.
These angles need not be excessively
mysterious-you can make a rough measurement of them with your own body. Stand facing
the sun with one hand pointing toward it and
the other pointing due south. Now drop the first
hand so that it points to the horizon directly
below the sun. The angle that your arm drops
IO
--Q
6
ER
SOLSTICE
SUMMER
SOLSTICE
EOUINOX
E
The sun’s daily path across the sky. The
sun is higher in the sky in summer than
in winter due to the tilt of the earth’s axis.
Measuring the sun’s position. The solar altitude (theta
8) is the angle between the sun and the horizon, and
the azimuth (phi 4) is measured from true south.
Solar Phenomena
SOLAR
AM
PM
Notes:
Jan 21
Feb 21
Mar 21
POSITIONS
Apr21
FOR 40ON LATITUDE
May 21 Jun 21
Jul21
Aug21
1.9
114.7
4.2
117.3
2.3
115.2
7.4
98.9
12.7
105.6
14.8
108.4
13.1
106.1
7.9
99.5
Sep21
0ct21
Nov21
Dec21
4.3
72.1
11.4
80.2
18.9
89.5
24.0
96.6
26.0
99.7
24.3
97.2
19.3
90.0
11.4
80.2
4.5
72.3
x.1
55.3
14.8
61.6
‘-I 5
u-._
69.6
30.3
79.3
35.4
87.2
37.4
90.7
35.8
87.8
30.7
79.9
22.5
69.6
15.0
61.9
8.2
55.4
5.5
53.0
16.X
44.0
24.3
49.7
32.x
57.3
41.3
h7.2
46.8
76.0
48.8
80.2
47.2
76.7
41.8
67.9
32.8
57.3
24.5
49.8
17.0
44.1
14.0
41.9
23.x
30.‘)
37.1
35.4
41.6
41.9
51.3
51.4
57.5
60.9
59.8
h5.X
57.9
61.7
51.7
52. I
41.6
41.9
-3’-. 4
35.6
24.0
31.0
20.7
29.4
2x.4
16.0
37.3
I X.6
47.7
‘2.h
58.7
29.2
66.2
37.1
69.2
41.9
66.7
37.9
59.3
‘9.7
47.7
22.6
37.6
18.7
28.6
16.1
25.0
IS.2
30.0
0.0
39.2
0.0
so.0
0.0
hl.6
0.0
70.0
0.0
73.5
0.0
70.6
0.0
62.3
0.0
SO.0
0.0
39.s
0.0
30.2
0.0
26.6
0.0
Top number in each group is altitude angle. measured from the horizon. Second number is azimuth
measured from true south. Angles given in degrees. and solar times used.
altitude (8) and the angle between
your arms in the final position is the solar azimuth (4). Much better accuracy can be obtained
with better instruments. but the measurement
process is essentially the same.
The solar altitude and azimuth can be calculated for any day. time, and latitude. For 40”N
latitude (Philadelphia, for example) the values
of 0 and d, are given at each hour for the 21~1
day of each month in the accompanying table.
Note that 4 is always zero at solar noon and
the 8 varies from 26.6” at noon on December
21 to 73.5” at noon on June 21. You can find
similar data for latitudes 24”N. 32”N. 48”N.
56”N. and 64”N in the table titled “Clear Day
Insolation Data” in the appendix. This appendix also shows you how to calculate these angles
directly for any day. time. and latitude.
is the solar
angle.
Why do you need to know these solar positions? A knowledge of the sun’s position helps
you detemline the orientation of a house and
placement of windows to collect the most winter
sunlight. This knowledge is also helpful in positioning shading devices and vegetation to block
the summer sun. Often the available solar radiation data only applies to horizontal or southfacing surfaces, and exact solar positions are
needed to convert these data into values that are
valid for other surfaces.
INSOLATION
Arriving at a quantitative description of the solar
radiation
striking
a surface,
or the
ittsolutiot~
(not to be confused with insulation). is a difficult task. Most of this difficulty arises from
II
The New Solar Home Book
m
WEAN DAILY
N PtRCENTAGt
SOL
OF POSSIBLE SCNSHINL.
the many variables that affect the amount of
solar radiation striking a particular spot. Length
of day. cloudiness, humidity, elevation above
sea level. and surrounding obstacles all affect
the insolation. Compounding this difficulty is
the fact that the total solar radia!ion striking a
surface is the sum of three contributions: the
dircc? radiation from the sun, the [email protected] rudinrim from the entire sky, and the rejected
rudiurion from surrounding terrain, buildings,
and vegetation. Fortunately, however, we do
not need exact insolation data for most lowtemperature applications of solar energy.
12
4NNUU
Although insolation data has been recorded
at about 80 weather stations across the country,
much of it is inaccurate and incomplete. The
information is usually provided in units of lung1eF.sstriking a horizontal surface over a period
of time, usually a day. A langley is one calorie
of radiant energy per square centimeter, and one
langley is equivalent to 3.69 Btu per square
foot, the more familiar English measure. An
example of the information available is the map
of “Mean Daily Solar Radiation, Annual” presented here. You can find monthly maps of the
mean daily solar radiation in the appendix. These
Solar Phenomena
Diffuse and Reflected Radiation
The total solar radiation srriking a surface is
rhe sum of three components: the direct solar
rudiurion (It)),).rhe diffuse sky radiation (Id), and
the rudiurion rejlecred from surroundings (I,).
The direct component consists of rays coming
srruighr from the sun-casting srrong shadows
on a cleur duy. if ail our days were ciear, we
could simply use the Clear Duy lnsolarion Dura,
add u small percentage for ground reflection.
and huve a very good esritnare of rhe total insolution on our wails. roofs, and collec’rors. But
ail of us cun’t live in Phoenix or Albuquerque,
so we musf learn to deal with cloudy weather.
As ir pusses through rhe urmosphere, suniighr
is stuttered b! uir molecules. dust. clouds. ozone,
and water vupor. Coming uniformly from rhe
entire sky. this scutrered rudiurion makes rhe
sky blur on clear du!s und grey on hazy days.
Although this diffuse rudiution umounrs to beI,r*eett 10 uttd 100 percenr of rite rudiurion
reaching the earth’s surfuce, iirde is known about
irs strength and variubiiie.
The Cieur Day Insoiurion Catu aren’f much
help on a cloudy du!. But frequently we otti>
need to know rhe uveruge daily insoiarion o\ler
(I period of u monrh. In such a case we can use
rhr ttuttiihl! maps of the percent of possible
.~utt.shitii~lo help us eslittiate rhis a\‘erage. If P
is the percenragr of possible sunshine for the
ttiotiih anti iocatioti in question. then we cotnputt a firctor F uccording to
F = 0.30 + 0.65( P/100)
The ttwttber.s 0.30 and 0.6s are coejicirnt.s rhar
acWai!\. \vrt? wifh clittiate. locafioti. and surfit(*e orientation. But their \~ariation is not loo
severe. und we cut1 use these uwruge w1ue.s
for r.stittitrriti,~ average daily insolation. If I,, is
the Clear Da! Itt.soiatiott (whole day roral) on
u piutii~ .surfitce. then \\qr cottipirre rhe ii\*erage
daii! it~soiatioti (I,,) according to
I,, = Ffl,,)
These f~trtttu1u.s estitnate rhe [email protected];ilr.seradiation
that still strikes !!tc surj~~ceon cioucl~ and part!\
cloudy days. Even in a complerely cloudy month
(P = 0). we would still be receiving 30 percent
(F = 0.30) of the clear day insolarion, according to these equations. This is perhaps a
bit high, but the coefJicients have been selected
to produce accurate results under normal conditions, not blackours.
For example, calculate rhe average daily insolation striking a horizonral roof in Philadelphia during rhe tnonths of June and Junuuty.
Using thejrst equation and P = 65 (June) and
49 (January) from before, we ger for June:
F = 0.30 + 0.65(65/100)
= 0.72
For January:
F = 0.30 + 0.65(49/100)
= 0.62
Therefore, rhe average daily insolarion is. for
June:
I,, = 0.72(2618)
= 1907 Btulft’
For Januury:
I, = 0.62(948)
= 588 Brulfrz
These nutnbers may be cotnpured wirh rhe 1721
Brulft’ and 464 Brulf? cuicuiared earlier. If we
include [email protected] radiation during cloudy weuarher.
our results are IO to 20 percent higher than
before.
The di’use and rejected radiation striking u
surjace uiso depend upon the orientation of the
sut-jke. Under rhe sutne sky condirions, a horizonral roof (which “sees” the entire shy) receives abour twice the diffiise radiation hirring
a vertical wail (which “sees” only one half the
sk?). Tilted surfaces receive some average of
these two. Ground rejlection depends a ior upon
rhe shape and te.rxtut-eof the surroundings and
rhe altitude of rhe sun. Snow rejects much tnore
sunlighr than green grass, and more reflection
occurs when the sun is lower in rhe sky. During
the winter, us much us 30 percenr of rhe hori:otttui clear duy insolation mu! be reflected up
onto the surfttce of a south facing wall. But a
roof recei\*es no rejected radiarion in any seuson, becuuse it fucrs the .sh~, not rhe ground.
13
The New Solar Home Book
CLEAR
DAY
INSOLATION
TOTAL
FOR 40°N LATITUDE
INSOLATION.
Btu/ft*
South facing
21sl
Day
January
February
March
April
May
June
July
August
September
OClObN
November
December
Normal
Surface
2182
2640
2916
3092
3160
3180
3062
2916
2708
2454
2128
1978
Horizontal
Surface
948
1414
1852
2274
2552
2648
2534
2244
17KX
I348
942
782
300
400
SO0
60°
900
I660
2060
2308
2412
2442
2434
2409
2354
2210
I962
I636
I480
1810
2162
2330
2320
2264
2224
2230
2258
7378
--2060
1778
I634
I906
2202
2284
2168
21140
1974
2006
2104
2lR2
20’38
I8’;‘O
17.10
1944
2176
2174
I956
I760
I670
1738
I894
2074
2074
I908
1796
I726
I730
I484
1022
724
610
702
978
1416
I654
1686
1646
data apply only to horizontal surfaces, and can
be misleading. Complicated trigonometric conversions, which involve assumptions about the
ratio of direct to diffuse radiation, are necessary
to apply these data to vertical or tilted surfaces.
The trigonometric conversions are also discussed the the appendix.
The weather bureau also provides information about the percentage of possible sunshine,
defined as the percentage ot time the sun “casts
a shadow .” An example of these data is the
map shown here titled “Mean Percentage of
Possible Sunshine, Annual.” In the appendix
you will tind monthly maps that are more useful
for calculations of insolation. By themselves,
these maps tell us little about the amount of
solar radiation falling on a surface, but when
coupled with the “Clear Day Insolation Data,”
they make a powerful design tool.
Clear Day Insolation tables, prepared by the
American Society of Heating, Refrigerating. and
Air-Conditioning Engineers (ASHRAE). provide hourly and daily insolation (and solar positions) for a variety of latitudes. Tables for
24”N. 32”N. 40”N. 48’N, and 36”N latitude are
14
surface tilt angle
reprinted in the appendix. The values of the
daily insolation from the 40”N latitude table are
included here as an example. These tables list
the average clmr duy insolurion on horizontal
and normal (perpendicular to the sun) surfaces,
and on five south-facing surfaces tilted at different angles (including vertical). The insolation figures quoted include a diffuse contribution
for an “average” clear sky, but do not include
any contl-ibution for reflections from the surrounding terrain.
Hourly and daily insolation data are given in
the appendix for the 2lst day of each month.
You can readily interpolate between these numbers to get values of the insolation for other
days, times, latitudes, and south-facing orientations. Trigonometric conversions of these data
to other surface orientations are explained there.
When multiplied by the appropriate “percentage of possible sunshine,” these data provide an estimate of the hourly and daily insolation
on a variety of surface orientations. You will
note, for example, that the total clear day insolation on a vertical south-facing wall in Philadelphia (40”N) is 610 Btulft’ on June 21 and
Solar Phenomena
1726 Btu/ft’ on January 2 I -almost three times
greater! Multiplied by the percentage of possible sunshine for this locale (about 65% in June
and 49% in January). the total insolation becomes 396 Btu/ft’ in June and 846 Btu/ft’ in
January, or still a factor of two greater. On the
other hand. the clear day insolation on a horizontal roof is 2648 Btu/ft’ in June and only 948
Btulft’ in January. or almost a factor of four
smaller. Clearly, the roof is taking the heat in
summer and the south walls are getting it in
winter.
LIMITATIONS
OF INSOLATION
DATA
You must be careful to note the limitations of
the Clear Day Insolation table. These data are
based upon “average” clear day conditions. but
“average” can vary with locale. Many locations are IO percent clearer, such as deserts and
mountains. and others. such as industrial and
humid areas, are not as clear as the “average.”
Reflected sunlight from vegetation and ground
cover is not included in the values given in the
tables. Another IS to 30 percent more sunlight
may he retlected onto a surface than the alnount
listed. In the winter. even more radiation will
be reflected onto south-facing walls because the
sun is lower in the sky and snow may be covering the ground.
Other difficulties arise from the subjective
evaluations ot “percentage of possible sunshine.” In the method of calculating average
insolation described above. an assumption was
made that the sun is shining full blast during
the “sunshine” period and not at all during
other times. In reality, up to 20 percent of the
clear day insolation may still be hitting the surface during periods of total cloudiness. During
hazy periods when the sun still casts a shadow,
only 50 percent of the clear day insolation may
be striking the surface. More accurate calculations, in which the diffuse and direct components of solar radiation are treated separately.
are provided in the appendix.
Another problem is the variability of weather
conditions with location and time of day. The
weather maps provide only area-wide averages
of the percent of possible sunshine. The actual
value in your exact building location could be
very different from your county average. On
the other hand, the cloudiness in some areas,
particularly coastal areas. can occur at specific
times of the day, rather than being distributed
at random over the entire day. There may be a
morning fog when the sun is low on the horizon.
and a clear sky from mid-morning on, but this
would be recorded as 75 percent of possible
sunshine. while 90 percent of the total clear day
insolation was actually recorded that day.
You may need more detailed information than
is available from national weather maps. Occasionally. friendlier-than-usual personnel will
assist you at the local weather station. but you
will almost always be referred to the National
Weather Records Center in Asheville, North
Carolina. This center collects, stores, and distributes weather data from around the country.
and makes it available in many forms. You should
first obtain their “Selective Guide to Climate
Data Sources,” to give you an overview of the
types of data available. You may obtain a copy
from the Superintendent of Documents there.
15
Heat energy is simply the motion of the atoms
and molecules in a substance-their twirling,
vibrating. and banging against each other. It is
this motion that brings different atoms and molecules together in our bodily Huids. allowing
the chemical reactions that sustain us. This is
why our bodies need warmth. Seventeenth-century natural philosphers thought heat was a
fluid--“phlogiston”
they called it-that was
released by tire and flowed from hot bodies to
cold. They were correct about this last observation, for heat always flows from warm areas
to colder ones.
The rate of heat flow is proportional to the
temperature difference between the source of
the heat and the object or space to which it is
flowing. Heat flows out of a house at a faster
rate on a cold day than on a mild one. It there
is no internal source of heat, such as a furnace
or wood stove, the temperature inside the house
approaches that of the outdoor air. Heat always
Hows in a direction that will equalize temperatures.
While the rate of heat How is proportional to
the temperature difference, the quantity of heat
actually flowing depends on how much resistance there is to the flow. Since we can do little
about the temperature difference between inside
and outside, most of our effort goes into increasing a building’s resistance to heat Row.
16
The actual mechanisms of heat flow are numerous, and so are the methods of resisting
them. Therefore. we will review briefly the three
basic methods of heat flow-conduction,
convection and radiation.
As children, we all learned about heat conduction intuitively by touching the handle of a
hot skillet. When an iron skillet sits on a hot
stove for a while, heat from the burner flows
through the metal of the skillet to the handle.
But the rate of flow to the handle of an iron
skillet is much slower than if the skillet were
made of copper. The heat flow through copper
is quicker because it has a greater conductance
(less resistance to heat flow) than cast iron. It
also takes less heat to warm copper than iron.
and therefore less time to heat the metal between
the burner and the handle. These principles are
basic to the concept of conduction heat flow.
Convection is heat flow through the movement of fluids-liquids or gases. In a kettle of
water on a stove. the heated water at the bottom
rises and mixes with the cooler water above.
spreading the heat and warming the entire volume of water far more quickly than could have
been done by heat conduction alone. A house
with a warm air furnace is heated in much the
same way. Air is heated in the firebox and rises
up to the living spaces. Since the house air is
cooler than the hot furnace air, the heat is trans-
Heat Flow Calculations
ferred from the hot furnace air to the cooler
room air and then to the surfaces in the rooms.
Heated fluids can move by natural convection
or forced convection. As a fluid is warmed. it
expands and becomes less dense. making it
buoyant in the surrounding cooler fluid. it risei
and the cooler fluid that flows in to replace it
is heated in turn. The warmed fluid moves to a
cooler place where its heat is absorbed. Thus
the fluid cools down. becomes heavier and sinks.
This movement is known as ttaturtrl cotnwtim
or thermosiphoni,I!:. When we want more control over the heat flow. we use a pump or a
blower to move the heated Huid. This is called
jbrca!
conwctiot~
Note that convection works hand-in-hand with
conduction. Heat from a warm surface is conducted to the adjacent fluid before it is carried
away by convection, and heat is also conducted
from a warm Ruid to a cool surface nearby. The
greater the temperature difference between the
warm and cool surfaces. the greater the heat
how between them.
Thermal radiation is the flow of heat energy
through an open space by electromagnetic waves.
This Row occurs even in the absence of any
material in that space-just as sunlight can leap
across interplanetary voids. Objects that stop
the flow of light also stop thermal radiation.
which is primarily invisible longwave radiation.
Warmer objects constantly radiate their thermal
energy to cooler objects (as long as they can
“see” each other) at a rate proportional to their
temperature difference.
We experience radiative heat flow to our bodies when we stand in front of a fireplace or hot
stove. The same transfer mechanism. although
more subtle and difficult to perceive. is what
makes us feel cold while sitting next to a window on a winter night. Our warm bodies are
radiating energy to the cold window surface.
and we are chilled.
Of the three basic kinds of heat loss. radiation
is the most difficult to calculate at the scale of
a house. Calculation of convection heat loss
through open doors or cracks and around window frames is educated guesswork. Conduction
heat loss through the exterior skin of the house
(roofs. walls. and floors) is perhaps the easiest
to estimate. Fortunately. this is the thief that
can pilfer the most heat from our homes.
CONDUCTION
HEAT LOSS
The ability of a material to oermit the How of
heat is called its thermal conductivity or conductance. The cmduc’tcurw (C) of a slab of material is the quantity of heat that will pass through
one square foot of that slab per hour with a 1°F
temperature difference maintained between its
two surfaces. Conductance is measured in units
of Btu per hour per square foot per degree Fahrenheit. or Btu/thr ft’ “F).
The total conductance of a slab of material
decreases as its thickness increases. While IO
Btu per hour may flow through a l-inch slab of
polystyrene. only S Btu per hour will flow through
a Z-inch slab under the same conditions.
The thicker a slab, the less heat it conducts.
17
The New Solar Home Book
The opposite of conductance is resistance.
the tendency of a material to retard the flow of
heat. All materials have some resistance to heat
Llow-those with high resistance we call insulation. The rvsistoncc (R) of a slab of material
is the inverse of its conductance, R = (l/C).
The higher the R-value of a material, the better
its insulating properties. R-values are expressed
in (hr ft’ “FkBtu. In the table you can find Rvalues for a few common building materials.
More detailed lists are provided in the appendix
under “Insulating Value of Materials.”
A related quantity. the overall catffic~ient of‘
hrtrf mrn.smi.s.sion( U). is a measure of how well
a wall, roof. or Hoor conducts heat. The lower
the U-value of a wall, the higher its insulating
ability. Numerically. U is the rate of heat loss
in Btu per hour through a square foot of surface
with a I degree (“F) temperature difference between the inside and outside air. Similar to conductance. U is expressed in units of Btu/(hr ft’
“F). To tind the conduction heat loss (AH,,,,).
through an entire wall. we multiply its U value
by the number of hours (h). the wall area (A).
and the temperature differer 1 (AT). between
the inside and ol*tside air:
RESISTANCES
OF COMMON
MATERIALS
BUILDING
Thickness
Material
(inches)
I .o
Hardwood (oak)
I .o
Softwood (pine)
0.5
Gypsum board
lapped
Wood shingles
Wood bevel siding
lapped
4.0
Brick. common
Concrete (sand and gravel)
8.0
Concrete blocks (filled cores)
X.0
Gypsum fiber concrete
8.0
Minera; fiber (ban)
3.5
6.0
Mineral fiber (baItI
Molded polystyrene beads
I .O
I .o
Fiberglass board
I .o
Extruded polystyrene
I .o
Cellular polyurethane
I .o
Polyisocyanurare
I .o
Phenolic foam
Loose fill insulation:
I .o
Cellulose fiber
I .o
Mineral wool
I .o
Sawdust
0. I’S
Flar glass
Insulating glass (0.2.5” space)
R-Value
($
OF hr)/Btu
0.91
I.3
0.45
(I.87
0.81
0.80
0.64
I .93
4.80
I I .oo
19.00
3.85
4.35
5 .oo
6.25
7.04
8.33
3.13-3.70
2.93
3 31
-.-0.9 I
I .h9
AH,,,, = (U)(h)(A)(AT)
SOURCE: ASHRAE
SAMPLE
CALCULATIONS
Wall Construction
Component3
OF II-VALUES
___-
llninsulalcd
R-values
Insulaled
Oul’ride air film. 15 mph bvind
0.75” beveled wood \iding. IapPed
0.50” plywood \heathinp
3.5” air space
3.5” mineral fiber halt
(Vi” gypsum board
Inside air film
0. I7
0.8 I
0.h’
I.01
0.4s
O.hX
0. I7
0.X I
0.6’
I I .oo
0.4s
O.hX
TOTALS
(R,,
3.74
13.73
ll-Vulucs
(11 = I/R,,
0.27
0.07
-__
18
Hundhoook.
19X5 Fundamenruls.
To tind the heat loss through a SO sq ft wall
with a U-value of 0.12 over an X-hour time
span, when the inside temperature is 65°F and
the outside temperature is 30°F. multiply:
AH,,,,, = (0.1’)(8)(50)(65 - 40) = I200 Btu
If the inside temperature is 70°F instead of 65°F.
then the heat loss is 1440 Btu over the same
time span.
The U-value includes the thermal effects of
all the materials in a wall. roof, or floor-including air gaps inside. and air tilms on the inner
and outer surfaces. It can be computed from the
conductances or resistancesof all these separate:
components. The total resistance R, is the sum
of the individual resistances of these components. As U is the conductance
of the entire
building section, it is the inverse of R,. or
Heat Flow Calculations
U=(I/R,)=
l/(R, + Rz + Rj + . . .+ R,)
Thus, computation of U involves adding up all
the R-values, including R-values of inside and
outside air films, any air gap greater than three
quarters of an inch, and all building materials.
As an example. the U-values of two typical
walls, one insulated and the other uninsulated,
are calculated here. Note that the uninsulated
wall conducts heat almost four times more rapidly than the insulated wall.
This is a simplified version of the heat flows.
Heat will pass more quickly through the framing
of the wall than through the insulation. If the
total R-value through the framing section of the
wall is 7. I. and the framing takes up 20 percent
of the wall cavity. then the weighted R-value
of the insulated wall is:
R, = 0.20(7.1) + 0.80( 13.73) = 12.4
The weighted R-value of the uninsulated wall
is:
R,. = 0.20(7.1) + 0.X0(3.74) = 4.4
Notice that the weighted R-value of the insulated wall is now less than three times better
than the uninsulated wall.
Once you have calculated the U-values of all
exterior surfaces (windows, walls. roofs. and
floors) in a house. you can begin calculating
the total conduction heat loss. One important
quantity is the hourly heat loss of the house at
outside temperatures close to the lowest expected. These extreme temperatures are called
design renl~rr~rturc’s. A list of the recommended
design temperatures for a number of U.S. cities
is provided here; those for many other locations
in the United Statesare provided in the appendix
under “Degree Days and Design Temperatures.”
The following approach is used to tind the
Btu per hour your heating system will have to
supply in order to keep your house warm under
all but the most extreme conditions. Subtract
the design temperature from the normal inside
temperature to tind the temperature difference
(AT). Next, determine the total area (A) of each
type of exterior building surface and multiply
it by the temperature difference and the appropriate U-value (U,). to get the total conduction
heat loss (AH,) of that surface per hour:
AH, = UA,)(AT)
The total conduction heat loss of the house is
merely the sum of the conduction heat losses
through all these building surfaces. For example. the conduction heat loss of the 50-square
foot insulated wall with a U-value of 0.07 under
design temperature conditions ( - 2°F) in Denver, Colorado, is
AH, = 0.07(50)[70 -- ( - 2)] = 252 Btu/hr.
To compute the total conduction heat loss for
a single heating season, you must first grasp the
concept of degree days. They are somewhat
analogous to man-days of work. If a man works
one day, the amount of work he does is often
called a man-day. Similarly, if the outdoor temperature is one degree below the indoor temperature of a building for one day. we say one
degree duv (D) has accumulated.
Standard practice uses an indoor temperature
of 65°F as the base from which to calculate
degree days. because most buildings do not require heat until the outdoor air temperature falls
between 60°F and 65°F. If the outdoor temperature is 40°F for one day. then 65 - 40 = 25
degree days result. If the outdoor temperature
is 60°F for live days, then 365 - 60) = 25
degree days again result. (When we refer to
degree days here. we mean degrees Farenheit
(OF). unless otherwise noted.)
The Weather Service publishes degree day
infommation in special maps and tables. Maps
showing the monthly and yearly total degree
days are available in the Climatic Atltrs. Tables
of degree days, both annual and monthly. are
provided for many cities in the appendix under
“Degree Days and Design Temperatures.” Your
local 011 dealer or propane distributor should
also know the number of degree days for your
town.
To compute the total conduction heat loss
during the heating season. you first multiply the
total degree days for your locality by 24 (hours
19
The New Solar Home Book
DEGREE
State
City
DAYS AND DESIGN TEMPERATURES
(HEATING
SEASON)
Design
Temperature
Degree
Days
(OF)
(OF day:.)
-
Alabama
Alaska
Arizona
Arkansas
California
Birmingham
Anchordge
Phoenix
Little Rorh
Los Angeles
I9
-25
31
I9
41
2.600
10.900
1,800
3.200
2.100
California
Colorado
Connecticut
Florida
Georgia
San Francisco
Denver
Hartford
Tampa
Atlanta
42
--3
I
36
IX
3,000
6.300
6.200
700
3,000
Idaho
Illinois
Indiana
Iowa
Kansas
Boise
Chicago
Indianapolis
Des Moines
Wichita
4
-4
0
-7
5
Kentucky
Louisiana
Maryland
Massachusetts
Michigan
Louisville
New Orleans
Baltimore
Boston
Detroit
Minnesota
Misstssippi
Missouri
Montana
Nebraska
Minneapolis
Jackson
St. Louis
Helena
Lincoln
Design
Temperature
State
City
Nevada
New
New
New
New
Reno
Hampshire Concord
Albuquerque
Mexico
Buffalo
York
New York
York
(OFI (OFdays)
2
-II
I4
3
II
6.300
7.400
4.300
7,100
4.900
I6
3.400
-24
2
8.900
5.700
3.900
4.600
Ohio
Oklahoma
Oregon
Raleigh
Bismarck
Columbus
Tulsa
Portland
6.200
6,600
5.700
6.600
4,600
Pennsylvania
Pennsylvania
Rhode Island
South Carolina
South Dakota
Philadelphia
Pittsburgh
Providence
Charleston
Sioux Falls
II
S
6
26
-14
6800
x
32
I2
6
4
4.700
I.400
4,700
5,600
6.200
Tennessee
Texas
Texas
Utah
Vermont
Chattanooga
Dallas
San Antonio
Salt Lake City
Burlington
IS
I9
25
5
-12
3.300
2.400
I .sOO
6. IO0
8.300
-I4
21
4
-17
-4
8.400
2,200
4.900
x.700
5 .wo
Virginia
Washington
West Virginia
Wisconsin
Richmond
Seattle
Charleston
Madison
Wyoming
Cheyenne
I4
2x
9
-9
-6
3 i\(#l
4:400
4.500
7.900
7.400
per day) to get the total dqrc~e how.s during
that time span. Now your calculation proceeds
as in !he earlier example: you multiply the area
of each section (A,) by its U-value (II,) and the
number of degree hours (24D) to get the seasonal heat loss through that section:
Seasonal 1H, = A, (U,)(ZJ)(D)
The seasonal conduction heat loss from the entire house is the sum of seasonal heat losses
through all the building surfaces. A short cut is
North Carolina
Degree
Days
North Dakota
I2
‘I
5.100
6.000
I.800
7.800
to multiply the U-value of each section times
the area of each section to get the “UA” for
that section. Add together all the UA’s and then
multiply by 24D to get the total seasonal conductive heat loss:
Seasonal AH = (UA, + UA, + UA3 . . .
+ U&)(X)(D)
But to get the total seasonal heat loss, you must
include the convection heat losses described in
the next section.
Rest Flow Calculslrtions
CONVECTION
HEAT LOSS
There are three modes of convection which influence the heat loss from a building. The first
two have already been included in the calculation of conduction heat losses through the
building skin. They are the convection heat flow
across air gaps in the wall and heat flow to or
from the walls through the surrounding air. These
two effects have been included in the calculation
of U-values by assigning insulating values to
air gaps or air films. The third mode of convection heat flow is air injilrrution through
openings in walls (such as doors and windows)
and through cracks around doors and windows.
In a typical house. heat loss by air intiltration
is often comparable to heat loss by conduction.
The first mode of convection heat loss occurs
within the walls and between the layers of glass
in the skin of the building. Wherever there is
an air gap. and whenever there is a temperature
difference between the opposing surfaces of that
gap. natural air convection results in a heat flow
across that gap. This process is not very efficient. so air gaps are considered to have some
insulating value. For the insulating value to be
significant. the width of the air gap must be
greater than 314 inch. However. a quick glance
at the insulating values of air gaps in the appendix reveals that further increases in the width
don’t produce significant increases in insulation. Wider air gaps allow freer circulation of
the air in the space. offsetting the potentially
greater insulating value of the thicker air blankct.
Most common forms of insulation do their
job simply by trapping air in tiny spaces to
prevent air circuhnion in the space they occupy.
Fiberglass blanket insulation. rigid board insulation. cotton. feathers. crumpled newspaper.
and even popcorn make good insulators because
they create tiny air pockets to slow down the
convection flow of heat.
Conduction heat tlow through the exterior
skin of a house works together with air movements within the rooms and winds across the
exterior surface to siphon off even more heat.
Interior surfaces of uninsulated perimeter wails
are cooler than room air. They cool the air film
right next to the wall. This cooled air sinks
down and runs across the floor, while warmer
air at the top of the room flows in to take its
place. accelerating the cooling of the entire room.
The inside surface of a well-insulated wall will
have about the same temperature as the room
air. But the inside surface of a window will be
much colder. and the air movement and cooling
effects are severe. Heating units or warm air
registers have traditionally been placed beneath
windows in an effort to eliminate the cold draft
coming down from the glass surfaces. While
this practice improves the comfort of the living
areas, it substantially increases the heat losses
to the outdoors. With the advent of new. higher
R-value glazing materials, better insulated walls.
and lower infiltration rates, this location isn’t
as important in energy-conserving home.
Though not very large. the insulating value
of the air tilms on either side of a wall or roof
do make a contribution to the overall U-value.
The air tilms on horizontal surfacesprovide more
insulation than those on vertical surfaces. (Convection air How. which reduces the effective
thickness of the still air insulating him. is greater
down a vertical wall than across a horizontal
surface.) Similarly. the air film on the outside
surface is reduced by wind blowing across the
surface. The higher the wind speed, the lower
the R-value. The heat that leaks through the
wall is quickly transmitted to the moving air
and carried away. The outer surface is cooled.
drawing more heat through the wall. These heat
losses can be reduced by wind screens or plantings that prevent fast-moving air from hitting
the building skin.
Air infiltration heat losses through openings
in buildings and through cracks around doors
and windows are not easy to calculate because
they vary greatly with tightness of building construction and the weatherstripping of windows.
doors. and other openings. Small openings such
as holes around outside electrical outlets or hose
faucets can channel large amounts of cold air
into heated rooms. Every intersection of one
21
The New Solar Home Book
building material with another can be a potential
crack if care isn’t taken during construction.
This is why. in home construction today, air/
vapor barriers of 6-mil polyethylene sheets are
commonly (and carefully) installed around the
warm side of the building frame. They slow the
passage of warm air (and moisture vapor) from
inside to outside. Air barriers. sheets of polyethylene fibers that allow vapor, but not air, to
pass through, are also installed around the outside of many buildings before the siding is installed. They keep cold air from passing through
cracks between materials-cold air that forces
warm air out the leeward side of the building.
In both cases, special care is also taken around
doors and windows. between floors, and around
electrical and plumbing penetrations, to seal
against the infiltration of cold air. This cold air
has to be heated to room temperature. In the
following calculations, we assume that the general wall construction is air-tight, and that only
the infiltration through windows and doors needs
to be considered.
The magnitude of air infiltration through cracks
around doors and windows is somewhat predictable. It depends upon wind speeds and upon
the linear footage of cracks around each window
or door, usually the perimeter of the opening.
If the seal between a window frame and the
wall is not airtight, you must also consider the
length of this crack. From the table “Air Infiltration Through Windows,” you can approximate the volume of air leakage (Q) per foot of
crack. With the temperature difference (AT) be-
41R INFILTRATION
THROUGH
WINDOWS
Air leakage (0)’ at
Wind velocity (mph)
Window
Type
Double-hung
wood sash
Double-hung
metal sash
Rolled-section
steel sash
Remarks
5
10
I5
20
2s
Average fitted’
non-weatherstripped
7
‘I
3’)
55,
x0
Average fitted2
weatherstripped
4
I.3
24
36
49
Poorly fitted3
non-weatherstripped
27
hY
IS4
IYY
Poorly fitted3
weatherstripped
6
IO
34
51
71
Non-weatherstripped
20 47
74
lo4
137
Ill
Weatherstripped
6
IY
32
46
60
Industrial
s2
108
I76
244
304
14
32
52
76
IO0
pivoted2
Residential
casement4
I. Air leakage. Q. is measured in cu ft of air per ft of crack per hr.
3. Crack = 3/X inch.
4. Crack = l/32 inch.
2. Crack = I/lb inch.
SOURCE: ASHRAE.
22
Handbook
of Fundamenrals.
eat Flow Calculations
tween inside and outside, you can determine the
amount of heat required to warm this air to room
temperature (AHi”r):
AH,,r = (c)(Q)(LMh)(AT)
where c = 0.018 Btu/(ft”“F) is the heat capacity
of air, L is the total crack length in feet, and h
is the time span in hours.
With 10 mph winds beating aginst an average
double-hung. non-weatherstripped, wood-sash
window, the air leakage is 2 1 cubic feet per
hour for each foot of crack. Assuming the total
crack length is I6 feet and the temperature is
65°F inside and 40°F outside, the total infiltration heat loss during an eight-hour time span is:
AH,,, = 0.018(21)(‘6)(8)(65
- 40)
= 1210 Btu
If the same window is weatherstripped (Q =
13 instead of 21). then the infiltration heat loss
is 749 Btu over the same time span. You can
make a multitude of other comparisons using
the Q-values given in the table.
Apply the above formula to the total crack
length for each different type of crack leakage.
The total crack length varies with room layout:
for rooms with one exposure, use the entire
measured crack length; for rooms with two or
more exposures, use the length of crack in the
wall having most of the cracks; but in no case
use less than one-half of the total crack length.
You can also use this formula to calculate
the heat loss through infiltration under the worst.
or “design” conditions your house will undergo. For these conditions. use the outdoor
design temperatures and average wind speed for
your area. Fortunately, the design temperature
does not usually accompany the maximum wind
speed. Average winter wind velocities are given
for a number of localities in the Clkwtic Atius
of the United Stat&s.
The total seasonal heat loss through air infiltration is calculated by replacing h x AT with
the total number of degree hours. or 24 times
the number of degree days:
Seasonal AH,,l- = c(Q)(LM24NDI
Infrared photographs showing thermal radiation from
a conventional house. Note that more heat escapes from
an uninsiriated attic (top) than from an insulated one
(bottom). SOURCE: Pacific Gas and Electric Co.
Radiation works together with conduction to
accelerate heat flow through walls. windows,
and roofs. If surrounding terrain and vegetation
are colder than the outside surfacesof your house.
there will be a net flow of thermal radiation to
these surroundings. Your roof will also radiate
substantial amounts of energy to the cold night
sky. If the relative humidity is low. as much as
30 Btu per hour can be radiated to the sky per
23
The New Solar Hame Book
square foot of roof. This radiation can rapidly
cool your roof surface to temperatures lower
than the outside air temperature, thereby increasing the temperature difference across the
roof section and the overall heat flow through
the roof.
In summer, this radiative heat flow provides
desirable nocturnal cooling. particularly in arid
2reas. In the winter, however, this nocturnal
cooling is an undesirable effect. Well-insulated
roofs are necessary to prevent excessive losses
of heat.
If the interior surfaces of walls and windows
are colder than the objects (and people!) inside
a room, there will be a net flow of thermal
radiation to these surfaces. A substantial flow
of heat radiates to the inside SUrfaceS of windows, which are much colder during winter than
adjacent walls. This flow warms the inside surface of the glass, and more heat is pumped to
the outside air because of the greater temperature difference across the glass. Extra glazing,
special glazing, or window insulation can reduce this flow drastically.
In both examples above, radiation heat flow
enhances the transfer of heat from warmer to
cooler regions. Its effects are included in the
calculation of conduction heat loss through surfaces of the house. But don’t ignore radiation
heat flow when taking preventive measures.
Heat Load Calculations
So fur. you have learned to cukulate the heat
losses through the individual sutj&es and cracks
of u house. To calculate the over& heat loss
(or heat load) of u house, ~-ou merely sum the
losses through all surfaces and trucks. The heat
loud of u house depends on its construction and
insulation and varies with the outside tempcruture und wind ve!o<ity .
To indicute just how bad things cun get, let’s
use u draj-8, uninsulated. wood-frame house us
un e..rample. Assume it’s 40 feet long and 30
ft>et wide. It has uninsulated stud walls and a
hurdwoodJoor above a ventilated crawl space.
The low-sloped ceiling has acoustical tile but
is otherwise uninsulated. under a roof of plywood and asphalt shingles. The house has eight
single-pane. double-hung. wood-sash windows
(each 4 feet high by 2.5 feet wide) and two solid
oak doors (each 7 feet by 3 feet).
First we need the U-values oj’euch surface.
From the “Sample Calculations of U-values”
given earlier in this chapter, we know that an
uninsulated stud wall has a U-value of 0.27.
From the appentlir, we get U = I. 13for singlepune windows, and R = 0.91 fdr one inch of
oak. Adding the resistance of the inside and
outside air films. we get:
24
R, = 0.68 + 0.91 + 0.17 = 1.76 or
U = 111.76 = 0.57
for the doors.
The culculution of the U-values of the floor
and ceiling is a bit more invol\yd. The hurdwoodJIoor has three layers-interior
hurdwood
finish (R = 0.68). felt (R = 0.06). und wood
subfroor (R = 0.98)-and
essentiull~ still air
films above and belo#l (R = 0.61 euch). The
resistances of all jive layers ut-e udded to give
R, = 2.94, or U = 112.94 = 0.34.
About half the floor area is covered by carpets
(an additional R = I .23 including the rubber
pad), and this half has a U-value of 0.24. The
total resistance of the ceiling and roof is the
sum of the resistances of eight different lavers,
including the acoustical tile (R = 1.19). gypsum board (R = 0.45). rafter air space (R =
0.80), plywood (R = 0.62), building paper (R
= 0.12), asphalt shingles (R = 0.44), and the
inside and outside air films (R = 0.62 and
0.17). These add to R, = 4.41, and the U-value
of the ceiling is U = 114.41 = 0.23.
For a 1°F temperature difference between
indoor and outdoor air, the conduction heat loss
Heat Flow Calculations
HEAT
LOAD
CALCULATIONS
Conduction
Surface
Walls
Windows
Doors
Bare floor
Carpeted floor
Ceiling
Pm
(ft2)
Btu/(hr
998
80
42
600
600
I200
Total Conduction
l0F temp diff
U-value
ft2 OF)
0.27
I.13
0.57
0.34
0.24
0.23
Btu/(hr
OF)
269
90
24
3S°F outside
Btu/hr
I44
276
8,084
2,712
718
6,120
4.320
8.280
I.007
30.234
204
Heat Losses
heat losses
Infiltration
Heat Losses
35OF outside
Length
Q-value
1°F temp diff
around:
m
(ft2 hr ft)
Btu/(hr OF)
Window sash
Door
Window &
Door frames
62
20
111
220
124
79
3.716
2,376
82
I1
16
487
219
6,579
Crack
Total Infiltration
Heat Losses
All calculations
assume 15 mph wind.
through euch surface is the product
qf the u.rea
of the surface times the U-value of the surface.
If the design temperature is .?YF. .for e-rumple,
we multiply by (65 - 35) to get the design heat
loss through that surface. The conduction heat
losses through all surfaces are summarized in
the table.
Infiltration heat losses are culculated using
Q-values from the table “Air lnjiltration Through
Windows. ’ ’ Poorly fitted double-hung wood-sash
windows have u Q-value of I I I in a I5 mph
wind. Assume that around poorl! fitted doors,
the injiltration rate is twice that: 220 fr’lhr for
each crack foot. Also assume that there is still
some injiltration through cracks around windott
and doorframes as well. with a Q-v&e of II _
These Q-values ure then multiplied by the
heat capa+ of (I cubic foot of air /0.018 Btul
(jii’ OF)/ and the total length of each type of
BN/hr
crack to get the infiltration heat loss. Onlv windows and doors on two sides of the house (that
is, four windows and one door) are used to get
total crack lengths. The injltration heat losses
through all cracks are also summarized in the
table.
In a 15 mph wind, the conduction heat loss
of this house is 1007 Btulhr for a 1°F temperature difference between indoor and outdoor
air. Under the same conditions, the infiltration
loss is 219 Btulhr, or a total heat load of of
1226 B!ul(hr OF). Over an entire day, the house
loses 24 (hours) times 1226 (Btu per house) for
each 1°F temperature difference, or 29,424 Btu
per degree day. Under design conditions of 35°F
and a 15 mph wind, the heat load of this house
is 34,813 Btulhr (30. 234 -I- 6,579). The fltrnace has to crank out almost 37.000 Btulhr to
keep this house comfy during such times.
25
The New Solar Home Book
SEASONAL AND DESIGN HEAT LOADS
The total heat escaping from a house is the sum
of the conduction heat loss and the convection
heat loss through air infiltration, because the
effects of radiative heat flow have already been
included in these two contributions. The total
conduction heat loss is itself the sum of conduction losses through all the exterior surfaces,
including walls. windows, floors, roofs, skylights, and doors. The total conduction heat loss
is generally one to four times the total convection heat loss through air infiltration, which includes all convection heat losses through cracks
in walls and around windows and doors.
The ratio of the two losses depends heavily
on the quality of construction. For example, the
total conduction heat loss from a typical poorly
insulated 1250 square feet house may be 1000
Btu/(hr “F) temperature difference between the
inside and outside air. while the convection heat
loss is only 250 Btu/(hr “F). If the temperature
drops to 45°F on a typical winter night, the
house loses a total of
l250(65 - 45) = 25,000 Btu/hr
assuming the indoor temperature is 65°F.
The design temperatures introduced earlier
allow us to estimate the maximum expected heat
loss from a house. The design temperature for
a locality is the lowest outdoor temperature likely
to occur during winter. Houses are often rated
in their thermal performance by the number of
26
Btu per house that the heating system must produce to keep the building warm during these
conditions. The design temperature for Oakland, California, is 35’F, so that
1250(65 -35)
= 37,500 Btu/hr
is the design heat load that the heating system
must be able to produce in the above house.
The same house would have design heat loads
of 62,500 Btu/hr in Chattanooga, Tennessee,
where the design temperatureis 15”F, and 98,750
Btu/hr in Sioux Falls, South Dakota, where the
design temperature is - 14°F. The cost to heat
the house in Sioux Falls might persuade the
owner to add some insulation!
Degree day information allows us to calculate
the amount of heat a house loses in a single
heating season. The greater the number of degree days for a particular location, the greater
the total heat lost from a house. Typical homes
lose 15,000 to 40,000 Btu per degree day, but
energy conservation measures can cut these by
more than half. Our example house loses
(24)(1250) = 30,000 Btu per degree day, for
example. If there are 2870 degree days, as in
Oakland, California, the total heat loss over an
entire heating seasonis 86.1 million Btu [(30,000)
(287O)j or about 1230 therms (I therm = 100,000
Btu) of gas burned at 70 percent efficiency 186.I/
(100,000~(0.7>~. In most other regions of the
country, where seasonal heat loads are much
greater and energy costs higher. energy codes
are more stringent.
As the position of the heavens with regard to a
given tract on the earth leads naturally to different characteristics, owing to the inclination
of the circle of the zodiac and the course of the
sun, it is obvious that designs for homes ought
similarly to conform to the nature of the country
and the diversities of climate.
Vitruvius,
Ten Books on Architecture
Energy conservation is the first step in good
shelter design. Only the house that loses heat
begrudgingly can use sunlight to make up most
of the loss. Some people might think it rather
dull to let sunlight in through the windows and
keep it there, but others delight in its simplicity.
In fact, conserving the sun’s energy can often
be more challenging than inventing elaborate
systems to capture it.
Nature uses simple designs to compensate for
changesin solar radiation and temperature. Many
flowers open and close with the rising and setting sun. Many animals find shelters to shield
themselves from intense summer heat, and bury
themselves in the earth to stay warm during the
winter.
Primitive peoples took a hint or two from
natxe in order to design shelters and clothing.
But as we learned to protect ourselves from the
elements, we lost much of this intuitive understanding and appreciation of natural phenomena. We lely more on technology than nature
and the two are often in direct conflict.
27
The New Solar Home Book
The earth’s heat storage capacity and atmospheric greenhouse effect help to moderate temperatures on the surface. These temperatures
fluctuate somewhat, but the earth’s large heat
storage capacity prevents it from cooling off too
much at night and heating up too much during
the day. The atmosphere slows thermal radition
from the earth’s surface, reducing the cooling
process. Because of these phenomena, afternoon temperatures are warmer than morning,
and summer temperatures reach their peak in
July and August.
A shelter design should reflect similar principles. Weather variations from one hour to the
next or from cold night hours to warm daytime
hours should not affect a shelter’s internal ciimate. Ideally. not even the wide extremes of
summer and winter would affect it. There are
countless examples of indigenous architecture
based on these criteria. Perhaps the most familiar of these is the heavy adobe-walled homes
of the Pueblo Mians. The thick wails of hardened clay absorb the sun’s heat during the day
and prevent it from penetrating the interior of
the home. At night. the stored heat continues
its migration into the interior. warming it as the
temperatures in the desert plummet. The cooiness of the night air is then stored in the wails
and keeps the home cool during the hot day. in
many climates houses made of stone. concrete,
or similar heavy materials perform in a like
fashion.
A shelter should moderate extremes of temperature that occur both daily and seasonally.
Caves, for example. have relatively constant
temperatures and humidities year round. Like-
wise, you can protect a house from seasonal
temperature variations by berming earth against
the outside wails or molding the structure of the
house to the side of a hill.
On sunny winter days, you should be able to
open a house up to the sun’s heat. At night, you
should be able to close out the cold and keep
this heat in. In the summer, you should be able
to do just the opposite: during the day close it
off to the sun, but at night open it up to release
heat into the cool night air.
The best way to use the sun for heating is to
have the house collect the sun’s energy itself,
without adding a solar collector. To achieve
this, a house must be designed as a total solar
heating system and meet three basic requirements:
The house must be a heat trap. It must be
well insulated against heat loss and cold air
infiitration. There’s no point in making the house
a solar collector if the house isn’t energy-conserving. This is done with insulation, weatherstripping, shutters, and storm windows, or
special glazings.
The house must be a solar collec.tor. it must
use direct-gain systems to let the sunlight in
when it needsheat and keep it out when it doesn’t;
it must also let coolness in as needed. These
feats may be accomplished by orienting and
designing the house to let the sun penetrate the
living space during the winter and by using
shading to keep it out during the summer.
The house must be a heat storehouse. it must
store the heat for times when the sun isn’t shining. Houses built with heavy materials such as
stone and concrete do this best.
The best way of using the sun’s energy to heat
a liouse is to let it penetrate directly through the
roof. walls. and windows. You should attempt
to maximize your heat gain from insolation during cold periods, and minimize it during hot
weather. You can do this with the color of your
house, its orientation and shape, the placement
of windows, and the use of shading.
Traditionally, solar heat gains have not entered into the computation of seasonal heating
supply or demand. Unfortunately, most of the
reseachdone on solar gain applied to hot weather
conditions and to reducing the energy required
for cooling. But all that changed in the early
1980s. Still. the data that apply to heating are
difficult to understand and difficult to use in
building design. This chapter is an attempt to
translate these data into useful design tools.
ORIENTATION
AND SHAPE
Since solar radiation strikes surfaces oriented in
different directions. with varying intensity, a
house will benefit if its walls and roofs are otiented to receive this heat in the winter and block
it in the summer. After much detailed study of
this matter, a number of researchershave reached
the same conclusion that primitive peoples have
always known: the principal facade of a house
should face within 30 degrees of due south (between south-southeastand south-southwest), with
due south being preferred. With this orientation,
the south-facing walls can absorb the most radiation from the low winter sun. while the roofs,
which can reject excess heat most easily, catch
the brunt of the intense summer sun.
In his book Design With Clirnare, however,
Victor Olgyay cautions against generalizing to
all building locations. He promotes the use of
“sol-air temperatures” to determine the optimal
orientation. These temperatures recognize that
solar radiation and outdoor air temperatures act
together to influence the overall heat gain through
the surfaces of a building. Because the outdoor
air temperatures are lower in the morning and
peak in the mid-afternoon, he suggests that a
house be oriented somewhat east of due south
to take advantage of the early morning sun when
heat is needed most. In the summer, the principal heat gain comes in the afternoon, from
the west and southwest, so the house should
face arvu~ from this direction to minimize the
solar heat gain in that season. Depending upon
the relative needs for heating and cooling, as
well as upon other factors (such as winds), the
optimum orientation will vary for different regions and building sites. The accompanying
diagram gives the best orientations for four typical
U.S. climate zones, as determined by Olgyay’s
sol-air approach.
29
The New Solar Home Book
N
Optimum house orientations for
four different U.S. climates.
A house also benefits in solar heat gain because of different ratios of length to width to
height. The ideal shape loses the minimum
amount of heat and gains the maximum amount
of insolation in the winter, and does just the
reverse in the summer. Olgyay has noted that:
0 In the upper latitudes (greater than 40”N).
south sides of houses receive nearly twice as
much solar radiation in winter as in summer.
30
East and west sides receive 2.5 times more in
summer than they do in winter.
* At lower latitudes (less than 35”N) houses
gain even more on their south sides in the winter
than in the summer. East and west walls can
gain two to three times more heat in summer
than the south walls.
0 The square house is not the optimum form in
any location.
* All shapes elongated on the north-south axis
work with less efficiency than the square house
in both winter and summer. The optimum shape
in every case is a form elongated along the eastwest direction.
Of course, other factors influence the shape
of a house, including local climate conditions
(e.g., early morning fog), the demands of the
site, and the needs of the inhabitants. But energy conservation can often be successfully integrated with these factors.
The relative insolation for houses with various shapes, sizes, and orientations can be a
very useful aid at the design stage, particularly
for placement of the windows. The first chart
shown here lists the relative insolation for different combinations of house shape, orientation, and floor and wall area. Values in this
chart are for January 2 1, and are based on the
next chart. “Solar Heat Gain Factors for 40”N
Latitude.” The ASHRAE Handbook of Fundumenruls provides similar information for many
other latitudes. These factors represent the clear
day solar heat gain through a single layer of
clear, double-strength glass. But they can be
used to estimate the insolation on the walls of
a house.
From the relative solar insolation data, you
may note that a house with its long axis oriented
east-west has the greatest potential for total solar
heat gain, significantly greater than that for a
house oriented north-south. The poorest shape
is the square oriented NNE-SSW or ENE-WSW.
In doubling the ground floor area, the optimal
east-west gain increases by about 40 percent
because the perimeter increases by 40 percent.
If you doubled the floor area of a house by
INSOLATION
FACADE
b
a
1
‘ob
dnb
‘Fib
:
DOUBLE
DOUBLE
c
lo 0
0
B
C
118
84
168
II8
236
123
87
174
123
246
127
C
90
DOUBLE
BUILDING
A
(Btu/day)
SIZES
Varlarton
RELATIVE
B 0,
WALL
c
C
AND
I80
127
254
265
I88
376
265
530
.OOR
d
C
508
722
361
1016
_ 508
828
II80
590
1656
828
1174
1670
835
2348
1174
1490
2120
1630
II60
2320
1630
-3260
1490
dQb Do”BLE;
DOUBLE
Vmatmn
ON WALL
ORIENTATIONS
1060
2120
1490
2980
II74
835
1670
II74
2348
- 828
590
1060
II80
2980
1490
828
1656
Total
2764
508
2668
722
3210
361
3780
1016
. 508 . 4512
2706
265
2703
376
3072
I88
3799
530
4319
265
2602
127
2775
I80
2775
90
3903
254
3903
127
2706
123
3072
174
2703
87
4319
246
123 _ 3799
AREAS
Varlrlmn
double
B
o, double
C
Relative insolation on houses of different shape and orientation on January 21 at 40”N
latitude. Listed values represent the insolation on a hypothetical house with w = 1 foot.
To get the daily insolation on a house of similar shape with w = 100 feet, multiply these
numbers by 100.
1
adding a second floor, the wall area and the
total solar insolation would double.
This study does not account for the color of
the walls, the solar impact on the roof. the variations in window location and sizes, or the
effects of heat loss. A detailed analysis would
also include the actual weather conditions.
However. this study does produce relative values to help you make preliminary choices.
COLOR
The color of the roofs and walls strongly affects
the amount of heat which penetrates the house,
since dark colors absorb much more sunlight
than light colors do. Color is particularly im-
portant when little or no insulation is used, but
it has less effect as the insulation is increased.
Ideally, you should paint your house with a
substance that turns black in winter and white
in summer. In warm and hot climates, the exterior surfaces on which the sun shines during
the summer should be light in color. In cool
and cold climates, use dark surfaces facing the
sun to increase the solar heat gain.
Two properties of surface materials. their ubsorptmce (represented by the Greek letter alpha, a). and emirrunce (representedby the Greek
letter epsilon, E). can help you estimate their
radiative heat transfer qualities. The ubsorprunce of a surface is a measure of its tendency
to absorb sunlight. Emitfurrcr gauges its ability
31
SOLAR
Jan
N
NNE
NE
ENE
E
ESE
SE
SSE
s
ssw
SW
wsw
W
WNW
NW
NNW
HOR
II8
I23
I77
265
508
828
I174
1490
1630
1490
II74
xx
sax
265
I27
I23
706
HEAT
GAIN
FACTORS
FOR 40° N LATITUDE.
WHOLE
Btu(ft*
day): Values for 21st of each month
DAY TOTALS
Feb
Mar
Apr
May
Jun
Jul
Aus
Sep
Ott
Nov
Dee
I62
200
225
439
715
IO1 I
12X5
224
300
422
691
961
11x2
1318
306
400
654
91 I
1115
1218
II99
1081
978
1081
II99
406
550
813
1043
II73
1191
1068
848
712
848
IO68
484
700
894
422
550
821
IO41
II63
1175
1047
831
I22
123
I32
260
504
815
II51
1462
98
loo
103
205
430
748
II04
1430
I.566
1596
1482
1191
I:15
911
bSX
400
I924
II73
1043
x13
550
2lbb
232
300
416
666
920
II31
1266
1326
1344
I326
12b6
II31
920
666
416
300
1476
166
200
226
431
694
971
1234
1454
1218
322
400
656
903
IO90
l/x8
1163
1049
942
1049
II63
118x
I090
903
1454
1234
971
694
431
226
200
IO70
1462
II51
815
504
260
I32
123
706
1430
II04
748
430
205
IO3
100
564
1509
1376
Ih3
1X-M
1509
1370
12x5
IO1 I
71s
439
22.5
‘00
IO92
1318
llX2
9bl
691
322
300
1538
1108
1200
II79
1007
761
b22
761
1007
1179
1200
1108
894
700
2242
b94
831
1047
117s
llb3
1041
831
550
‘148
b56
400
1890
Figures in bold type: Month of highest gain for given orientations.
Figures in itdic,: Orientations
of highest gairl in given month.
Figure3 in buld italic: Both month and orientation
of highest gains.
SOURCE: ASHRAE,
Handbook
of
Fwtdamcnrals.
Absorptance, Reflectance, and Emittance
Surdight srriking u sutj~c~e is either uhsorbed
or rcjlec*ted. The uhsorptunce ((.u)of’lhe srrrjtice
is rhr rutio oj’ the sniur energy uhsorbed lo the
wlur crqqsrriking that surf~~ce: 01 = i,,il,
\c*her-r I,, is ubsorbed soiur twr,qy md I is inc.idem soiur energy. A hypothericui “blackbody” bus un ubsorptmc*e oj’ I -ir ubsorbs ail
rhe rudiation hirting it. und liquid be tomi!\
Muck lo our eyes.
But cdl reui mb.smx*e.s reflect some portion
of’ rhe sitniighr hirririR them. e\*en if only ii j&r
percent. The rejiwmnce ( p) of u .su~-j&~eis rhe
rufio of soiur energ! rcjiecred fo rhur striking
ir: p = Idl. dirt-e I, is ri~jiei~trti .solur enet-gj
tuld I is inc*ide,l1 soiur energy. A I~~porhericui
biuckbod~ bus u rejiccwnce ~$0. The sun1 of’ (Y
und p j+ oprrqiu~ .surj&.s is uiwu~.s 1.
Ail wmn 1~odie.sernir thennui rudiufion, .some
betret- rhun orhers. The ernimnce (e) of u muteriui is the rutio of rhennai energy being rudiurcd by dwr mureriui lo rhe ~hermul energy
rudiuwd by u blwkbody crf Ihut sume temnper32
urure: E = RJR,,, where R is rudiurion from rhe
muteriul und R,, is rudiarion from the biackbody. Therefore. a biuckbo<lF has an ernirrunc*e
of 1.
The possible vuiues of 0~. p and E lie in a
rungt~ from 0 to I. Vail4es for a few common
surfuce mureriuis ure listed in die acconipunying table. More extensive iistirlgs can be found
in the appendi.x under “Absorptances artd Emittuncvs oj’ Materials. ’ ’
The values listed in rhis table (and those i,r
the apperldix) will help you compare the response oj \wrious materials and surjbces to soiur und thermal radiation. For e.wrnpie, Jrat
biuck puinr (with OL = 0.96) will absorb 96
perwrrr cf the incornirlg sunlight. Bur green paint
(with 01 = 0.50) will absCrb only 50 percent.
Both parrs (with emittances c.f 0.88 and 0.90)
emit thermal radiation at ubout the same me
$rhey ure ut the sume templ,eratrtre. Thus, black
paint (\bith u higher \wiue of OLIE)is a better
ubsorber of sunlight and will bccorne horter brhen
esposrd to the sun.
The House as a Solar Heating System
to emit thermal radiation. These properties are
explained further in the sidebar given here, and
sample values of (Yand E are listed in the table.
Also listed in the table are the materials’ reflectance values (represented by the Greek letter
rho. p).
Substances with large values of CYare good
absorbers of sunlight; those with large values
of E are good emitters of thermal radiation. Sub-
ABSORPTANCE.
Material
White plaster
Fresh snow
While paint
White enamel
Gwen paint
Red brick
Concrete
Grey paint
Red paint
Dry sand
Green roll roofing
Water
Black tar paper
FI,II Hack paint
Granite
Griiphile
Aluminum
toil
<idvani,<ed steel
REFLECTANCE,
stances with a small value of OL,particularly
those with a small value of o/E, like white paint,
are good for surfaces that will be exposed to
the hot summer sun (your roof and east and
west walls, for example). Those that have a
large value of 01, particularly those with large
a/r, like black paint, are good for south-facing
surfaces, which you want to absorb as much
winter sunlight as possible.
AND
EMI-ITANCE
Absorptance
Reflectance
0.07
0. I3
0.30
0.35
030
0.55
O.hO
0.75
0.74
0.82
0.M
0.Y-l
0.93
0.96
0.55
0.78
0. I5
O.bS
O.Y3
0.87
0.x0
0.65
0.50
0.45
0.40
0.3
0.3b
0. I x
0. I?
O.Ob
0.07
0.04
0.4s
0 .-7’
0.x.s
0.3s
OF MATERIALS
Emittance
0.9 I
0.82
0.9 I
O.YO
0.00
0.Y7
0.88
O.YS
0.90
O.YO
O.Y4
O.Yb
0.93
0.88
0.44
0.4 I
0.05
0. I3
Absorptancel
Emittance
0.08
O.Ib
0.22
0.39
0.56
Oh0
0.68
0.79
0.H’
O.YI
0.M
0.98
I .oo
I .OY
I.25
I .YO
3 .oo
5.00
33
If you design a house to collect and store solar
heat. you should design the house to hold that
heat. The escape of heat from ;1 house during
winter is usually called its “heat loss.” In addition. houses also absorb heat through their
walls and windows during summer-their “solar heat gain.” Retarding this movement of heat
both into and out of a house is the essence of
energy conservation in housing design. Fortunately. most efforts to reduce winter heat loss
also help reduce summer heat gain.
There are three primary ways that heat escapes from a house: ( I ) by conduction through
walls. roofs, and tloors. (2) by conduction
through windows and doors. and (3) by convection of air through openings in the exterior
surface. Conduction works together with radiation and convection-within the walls and floors
and at the inner and outer wall surfaces-to
produce the overall heat flow. The third mode
of heat loss includes air infiltration through open
windows, doors, or vents, through penetrations
in the building “envelope.” and through cracks
in the skin of the house or around windows and
doors.
Depending upon the insulation of the house,
the number and placement of windows, and the
movement of air. the ratios of the three modes
to the total can vary widely. If the total heat
loss is divided evenly among these modes. and
34
any one mode is reduced by half. the total heat
loss is reduced by only one sixth. Clearly, you
should attack all r/tree tttod~~.sof heat loss with
the same vigor if you want to achieve the best
results.
AIR INFILTRATION
People require some outdoor air for ventilation
and a feeling of freshness. and the penetration
of air through the cracks in the surface of a
house usually satisfies this need-particularly
if cigarette smoking is avoided. You should make
every effort. however, to reduce such uncontrolled air infiltration. As you reduce other heat
loss factors, the penetration of outdoor air becomes a greater part of the remaining heat loss.
Air infiltration can account for 20 to 55 percent of the total heat loss in existing homes. In
those with some insulation, the heat loss from
air infltratation exceeds conduction losses
through the walls, ceiling, and floor by up to
25 percent. Insulating older homes often requires a major overhaul, and tackling infiltration
problems is the logical place to begin.
Measures for reducing infiltration include
general “tightening up” of the structure and
foundation, caulking and weatherstripping the
doors and windows, redesigning fireplace air
Conservation First: The House as a
flow. creating foyer or vestibule entrances, installing a vapor barrier in or on the walls. and
creating windbreaks for the entrances and the
entire house.
One of the main reasons historically for installing building paper between the plywood
sheathing and the exterior siding of houses was
to reduce air infiltration through cracks in the
walls. However. the fragile material did little
to reduce infiltration after being penetrated by
siding nails. Today’s energy-conserving homes
have a polyethylene-!iber air barrier wrapped
around the sheathing-a continuous sheet that
blocks the infiltration of cold air. but not the
exfiltration of moisture vapor. Good trim derails
on a house exterior also reduce air penetration.
Mortar joints in brick and concrete block facades should be tight and complete.
To tighten up your house. start with the obvious defects. Close up cracks and holes in the
foundation. and replace missing or broken shingles and siding. Hardened. cracked caulking on
the outside of the house should be removed and
replaced with fresh caulking. Be sure to retit
obviously ill-lilting doors and windows. And
plug up interior air leaks around moulding.
baseboards. and holes in the ceiling or Hoor.
More important than cracks in the wall surfaces,
however. are those around the windows and
doors. Different types of windows vary greatly
in their relative air intiltration heat losses. but
weatherstripping improves the performance of
any window, particularly in high u inds. because it checks infiltration where the edges of
doors and windows meet ior don’t quite meet!)
their frames.
Weatherstripping is readily rlvailable at the
local hardware store. Different types are required for different applications. For example,
gasket compression-type weatherstripping (with
a fabric face. peel-off back. and adhesive coating) is best :;uiied for hinged door and casement
and awning type windows. For sliding windows. the spring bronze or felt-hair weatherstripping is more appropriate. Forget about the
cheap. spongy stuff. Its effectiveness deteriorates rapidly and so does its appearance.
Fixed windows save the most energy. You
need operable windows for ventilation, but how
many do you really need? In an existing house
some windows can usually be rope-caulked shut
for the winter. Double-hung windows are more
of a problem. Your best bet is to rely on storm
windows or insulating shutters to reduce infiltration and conduction losses. Be sure to caulk
around the perimeter of storm window frames.
Before you try to stem infiltration between
windows or doors and their frames. make sure
no air is leaking around the ourside edges of
the frames. Caulking will remedy any such
problems and prevent water seepage during
driving rainstorms. The caulking compounds with
superior adhesive qualities are generally called
sealants. It is worthwhile to obtain high quality
sealants because caulking is a lot of work, and
inferior compounds can decompose after one
winter!
It you plan to install new windows, you should
be careful in selecting them. Operable windows
should be chosen for their tight fit when
closed-not only when first installed, but also
after being used hundreds of times over a period
of decades. Pivoted and sliding windows are
the loosest. and casement and awning windows
are among the closest fitting.
One obvious energy conservation step is to
close a fireplace when not in use. If the tireplace
is old and doesn’t have a damper. install one.
You can get more heat from a fireplace by using
a C-shaped tubular grate to cradle the burning
wood. Cold air is drawn into the tubes at the
bottom. warmed and delivered back into the
room from the top by thermosiphoning. Fireplace insert packages are available with builtin vents. and glass doors that block room air
from rising up the chimney. Another way to
extract more heat is to install vents in the Rue
that reclaim heat from hot air rising up the chimney. You should also provide a fresh intake air
inlet for the fireplace or woodstove to draw air
directly from the basement or outdoors. Otherwise, the fire will continue to draw warm air
from the rooms. effectively cooling the same
living space you want to heat by causing cold
air to be pulled in from the outside.
35
The New Solar Home Book
AIR QUALITY
Better energy-conserving building techniques
and infiltration control can mean less fresh air
passing through the house. Houses with less
than 0.5 air changes per hour can have excessive
levels of carbon dioxide and moisture from occupants and cooking; formaldehyde from plywood, furniture, carpets, and tobacco smoke;
radon from soils and groundwater; and combustion gases from kerosene heater, woodstoves. gas appliances. and furnaces. Controlling
indoor air pollution at each source is the best
remedy. Choose building materials and fumiture without urea-formaldehyde glues or foams.
Vent combustion appliances and provide separate outside combustion air. Install fans (on
timers) in kitchens and bathrooms. Seal cracks
and openings around penetrations to the basement. These will take care of the problem in
the average home.
In superinsulated houses with air-infiltration
rates less than 0.5 air changes per hour. an airto-air heat exchanger may be necessary. Heat
exchangers remove the heat from stale exhaust
air and transfer it to fresh intake air. Air-to-air
heat exchangers can be centrally located or wallmounted like an air-conditioner. Central heat
exchangerscan provide fresh air to all the rooms.
In average construction, an exhaust-only fan
may suffice. As it exhausts air. the negative
pressure it createsinside the building draws clean
air in through tiny cracks around windows and
between floors. but does nothing to recover heat
from the exhausted air. In houses where indoor
air pollution is controlled at each source, occasional opening of a window may be all that’s
necessary.
WIND CONTROL
Wind is the arch-culprit in the moment-to-moment variation of the amount of air that penetrates a house. Olgyay reports in Design With
Chafe that a 20 mph wind doubles the heat
loss of a house normally exposed to 5 mph
36
Proper orientation and vegetation shields protect a
house from the wind.
winds. He also notes that the effectiveness of
a belt of sheltering trees increases at higher wind
velocities. With good wind protection on three
sides, fuel savings can be as great as 30 percent.
Buildings should be oriented away from prevailing winter winds or screened by natural vegetation to block heat-pilfering air flows around
windows and doors. Vegetation should be dense
and eventually reach as high as the house. The
distance from the house to the wind-break,
measured from the home’s leeward side, should
not exceed five times the building height. Local
agricultural extension services can suggest the
trees and shrubs best suited to your climate and
the appropriate planting distance from the house.
Man-made windscreens, such as baffles, can
also be very effective.
Winter winds blow, whistle, and wail from
ihe north and west in most locales. Entrances
should not be located on these sides, and the
Conservation First: The House as a
number of windows (the smaller the better!)
should be kept to a minimum. Wind directions
do vary, however, with locality and season.
Monthly maps of the “Surface Wind Roses,”
in the Climatic Atlas of the United States. can
be most helpful in the layout of windows and
doors. These maps give the average wind velocity at many weather stations and show the
percentage of each month that the wind blows
in various directions. But remember that wind
direction and speed depend very much on local
terrain.
Plenty of cold outside air flows into a house
every time you open a door, particularly if the
door is on the windward side. However, a foyer
or vestibule entrance can reduce this problem
by creating an “air lock” effect. If a door opens
into a hallway. another door can be positioned
about four feet into that hallway to make a foyer.
If your entrance has no hallway. you can build
two walls out from either side of the door, add
a roof. and install a second door. This addition
can be simple and inexpensive-you needn’t
insulate the vestibule walls, just weatherstrip
both doors. And be sure that the new door opens
outwards for rapid exit in case of fire!
It may come as a surprise that air can infiltrate
the walls themselves. Wind pressure forces air
through the tiny cracks in the wall materials. A
good vapor barrier will keep that cold air from
reaching the living spaces while fulfilling its
primary purpose, that of maintaining comfortable indoor humidity. For older homes with no
such vapor barrier, installing one is only practical if the inside surface of the walls is being
removed for extensive rehabilitation or remodeling. If your home needs a vapor barrier but
you have no intention of ripping your walls
apart. certain paints with low permeabilities are
sold as vapor barriers.
AIR AND VAPOR BARRIERS
The daily activites of a family of four can produce two to three gallons of water vapor per
day. Water vapor also flows into the buildin
from basements and crawlspaces. Just as heat
flows from areas of greater to lower temperatures (hot to cold). vapor migrates (or diffuses)
from areas of greater to lower vapor pressures.
It also travels in the air that infiltrates through
cracks in walls, ceilings, and floors. Almost all
of the moisture is carried by infiltration. Less
than two percent moves by diffusion in a home
with a typical vapor barrier and an infiltration
rate of one air change per hour.
Moisture in the warm air will condense in
the wall and ceiling cavities if it meets a cold
surface. If enough of the moisture vapor condenses, it can saturate the insulation. reducing
its R-value and eventually causing rot and decay. Vapor flow due to diffusion can be prevented with the use of a vapcr barrier, such as
polyethylene film or aluminum foil. Infiltration
and exfiltration. which force moisture vapor
through the building cracks and accounts for as
much as 50 percent of the heat loss in a wellinsulated home. can be slowed with a reasonably tight uir barrier.
The following guidelines will help control
condensation in homes:
0 Avoid trapping moisture within a cavity. Use
materials in the outer skin that are at least live
times as permeable as the inner skin. Seal all
cracks and joints. The air barrier should be as
tight as possible.
. There should be at least twice as much insulation outside the vapor barrier as inside. In
high-moisture areas, the vapor barrier should be
on the warm side of all the insulation.
g Avoid any gaps in the insulation that could
cause cold spots and result in condensation.
Condensation on double-glazing may indicate that you have problems in walls and ceilings that have inadequate vapor barriers. To
prevent moisture infiltration from crawlspaces.
vent them in all seasons except the dead of
winter. Place a continuous 6-mil polyethylene
moisture barrier across the floor of the crawlspace. and provide a tight vapor barrier above
the floor insulation. If winter humidity levels
37
The New Solar Home Book
Relative heat losses through various types of windows acd
wails. These values represent only conduction heat loss.
inside the living space are kept below 40 percent. the potential for damage in wall and ceiling cavities will be limited.
Besides chimneys and flues. sources of intiltration that carry cold air in and moisture out
are walls and basement (60 percent), windows
and doors (20 percent). and ceilings (30 percent). Air enters through joints in materials in
the building envelope and holes in the vapor
barrier. To limit infiltration, use high-quality
caulking to seal gaps between surfaces that do
not move, such as where windows and framing,
trim and siding. or sill and foundation meet.
Weatherstrip all doors and windows. Air
barriers-high density polyethylene fiber films
stretched around the outside of the building
frame-are also used to reduce infiltration. They
allow moisture vapor to pass. but testing shows
they can reduce infiltration from 35 to 47 percent in the average home.
As the infiltration rate is reduced below 0.5
air changes per hour. the relative humidity increases rapidly. The tighter the infiltration con38
trols. the more important the vapor barrier
becomes. Vupor barriers-large thin sheets of
transparent polyethylene around the inside of
the building envelope-limit moisture migration. If you seal it very carefully at every seam
and at window. door, plumbing, mechanical,
and electrical penetrations, it will also serve as
an air barrier. Installing two separate barriers,
one air and one vapor, may be only slightly
more expensive than installing one very tight
vapor barrier, and they offer a greater defense
and are easier to install. An alternative to the
very tight polyethylene vapor barrier is caulking
between the frame and the subfloor, and between the gypsum wallboard and the framing,
and then painting the interior walls with a vapor
barrier paint.
WINDOWS
The conduction heat losses through the surfaces
of a house also increase with wind velocity.
The lower the R-value of a surface. the more
Conservation First: The
ouse as a Heat Trap
b
CONDUCT/ON
INFILTRATIOW
Wfli
SfNGLE GLASS
STORM W/NDsw
Relative heat conduction and air infiltration losses fom various
windows. An added storm window cuts both kinds of heat loss.
you need to protect it from wind. A single-pane
window or skylight needs much more wind protection than a well-insulated wall, because the
air film clinging to its exterior surface contributes more to its overall thermal resistance. As
the air film thickness decreaseswith the increase
in the air velocity striking it. the effective insulating value of the Mm decreases. The decrease is large for doors and windows but almost
negligible for well-insulated walls.
Various types of window and wall constructions differ widely in the amount of heat they
transmit. Under the same indoor and outdoor
air conditions, a single pane of glass will conduct I I5 Btu. double glass will conduct 60 Btu.
and a well-insulated wall will conduct only 4
Btu. You will lose the same quantity of heat
through a well-insulated wall 30 feet long and
8 feet high as through a single glass window 2
feet wide and 4 feet high! A single-pane window
The New Solar Home Book
loses heat about 20 times as quickly as a wellinsulated wall of the same total area. There are
a number of ways you can cut these losses, with
high-performance glazings. storm windows. and
window insulation.
In existing buildings, a storm window almost
halves conduction heat loss for single-pane windows and also reduces air infiltration. A twowindow sash (the standard single-pane window
in combination with a storm window) can be
superior to a single-sash insulating glass (because of the larger insulating air space) as long
as it is sealed well around the perimeter. A
standard window of double glazing in conjunction with a storm window is even better.
Iru;ulating curtains are made of tightly woven
material lined with loose stuffing, a blankettype insulating materiai. or o!her heavy material. They are fitted at the top and bottom and
travel in tracks at the sides to create a tight seal
yet permit opening during times of winter sun.
These curtains create a dead air space. With a
rehecting layer on the outer surface, such insulating curtains can also be used to reduce solar
heat gains in summer.
Another option is to insulate the windows
with insulating shutters. Depending upon its
thermal resistance. an insulating shutter can reduce conduction heat loss through a window by
a factor ranging from two to ten. Shutters also
reduce radiative heat transfer from warm bodies
to the cold window glass and, depending upon
construction. can practically eliminate air leakage. But window insulation can be very expensive. and it is not effective unless it is properly
used more than 75 percent of the time. If its
use cannot be guaranteed when operated manually, it should either have automatic controls
that respond to light levels. or it should be passed
up in favor of a special glazing. which works
24 hours a day without help.
HIGH-PERFORMANCE
GLAZING
High-performance glazings are making their mark
in new home construction, and may soon take
40
the place of three layers of glass or two layers
of glass with night insulation. These special
windows are made to reduce heat loss. Their
main advantage is that they cut heating bills
over a 24-hour period.
There are only two approaches to improving
the performance of a window: design it to transmit more light (i.e., heat) into the house, or
manufacture it to lose less heat out. For years,
low-iron glass was the solution to the first. Multiple glazings and window insulation solved the
second.
In the 1970s. a vacuum-coated polyester film
called Heat [email protected] was introduced. Suspended between two panes of glass, the film
allows less heat to escape by creating two insulating air spaces. But in addition, the coating
itself is very selective in the wavelengths of
radiation it transmits. Visible and near-infrared
light pass through easily. But once changed to
heat, the energy has a hard time passing back
out because Heat [email protected]’ reflects long-wave
radiation (heat) back in. When placed between
two layers of glass, it makes a lighter window
than one made with three panes of glass (also
cheaper to install). and has an R-value greater
than 4.0.
Glass manufacturers took this vacuum-deposited metal oxide “sputtering” process a step
further in the early 1980s and developed a process to “soft” coat glass. The coating is placed
on an inner surface of an insulated glass unit.
By being inside the sealed air space between
the two layers of glass, the low-emissivity coating is protected from moisture, which can destroy it. These soft-coated windows have Rvalues greater than 2.0. (See table, “Glazing
Properties.“) That is slightly less than tripleglazed units, but the small difference is made
up in lower costs and lighter weight.
In the mid-1980s. glass manufacturers developed a way to make a tougher coating that
needs no special handling or protection. As the
glass comes off the float line, the metal-oxide
coating is sprayed onto the hot glass and becomes an integral “hard” coat as the glass
cools. This pyrolitic process produces windows
Conservation First: The House as a Heat Trap
GLAZIMG
Glazing
I /8”
Single
Double
Triple
Double.
with low-e
soft coat on outer
surface of inner pane
Double,
with low-e
hard coat on outer
surface of inner pane
Triple,
with Heat Mirror
between two
glass panes
Tripane,
with anti-reflective
film between two
glass panes
Quadpane.
with two anti-reflective
films between two
glass panes
Air
Space
PROPERTIES
Transmittance
(Solar)
-
Winter
U-Value
1.16
0.55
0.39
l/4”
l/4”
0.85
0.74
0.61
1.oo
0.90
0.85
l/4”
0.52
0.74
l/4”
0.51
0.83
318”
0.46
0.62 /
0.25
318”
0.66
0.85
0.36
318”
0.63
/
0.82
0.26
slightly
lower R-values than the soft-coats,
but a longer life makes them more attractive.
Another high-performance film increases
window efficiency by transmitting more light.
Anti-reflective glazings such as 3M [email protected]
have R-values of 3.85 when’ two layers are suspended in a double-glazed window called [email protected] These windows have a solar transmittance
of 0.63-better than the quadrupled glass transmittance of 0.50. The units are also available
with one layer of tilm called Tripanem. Added
benefits of both low-emissivity and anti-reflective glazings are warmer interior glass surfaces
for greater comfort and less condensation, lighter
weight for easier installation, and less interior
fabric fading becausethey block more ultraviolet
light.
with
Shading
Coefficient
/
/
’
INSULATION
The only way further to reduce heat loss through
air-tight walls, floors, and roofs is to add more
resistance to this heat flow. Insulation retards
the flow of heat, keeping the interior surfaces
warmer in winter and cooler in summer. Because of radiation heat transfer from your body
to the walls (which can be 8°F to 14°F colder
than the room air during winter if the walls are
poorly insulated), you can feel cold and uncomfortable even when the room air is 70°F. Eliminate this “cold-wall effect” by adding insulation
and you will feel comfortable at lower thermostat settings.
Lowering the thermostat is the easiest way
to reduce winter heating cost (but perhaps the
41
The New Solar Home Book
most difficult for many of us to accept). The
heat loss through walls and windows is proportional to the difference between indoor and
outdoor temperatures. Reducing this difference
can definitely reduce your heat loss. You can
do this without undue discomfort by wearing an
extra sweater, or by using more blankets while
sleeping. The accompanying table shows that
lowering the thermostat at night does save energy. A nightly 10°F setback reduces energy
consumption by at least 10 percent in every city
listed.
Well-insulated buildings also foster more
uniform distribution of air temperatures. The
air adjacent to cold. uninsulated walls cools,
becomes more dense, and falls to the floor, displacing the warm air. These “ghost” drafts are
considerably reduced in well-insulated houses.
You can reduce the U-value of an exterior surface, and consequently its heat loss. by adding
more insulation. However, your investment for
insulation will quickly reach the point of diminishing return. For example, by adding two
inches of polystyrene board insulation to the
exterior of a concrete wall, you can reduce its
U-value from 0.66 to 0.11, an 83 percent decrease in heat loss. Adding another two inches
of polystyrene lowers the U-value to 0.06, an
additional savings of only 7.5 percent. Your
money may be better spent on extra window
glazing and weatherstripping, depending on your
climate.
R-VALUES
OF COMMON
INSULATORS
R-Values
Insulation
Material
Verrmculite
Mineral wool
Fiberglass
Polystyrene
Cellulose fiber
Urethane
Polyisocyanurate
Phenolic foam
For one
inch
2.5
3.0
3.5
4.0
4.5
6.5
7.0
x.3
Inside 2 x 4 Inside 2 x 6
stud wall*
stud wall*
Il.9
13.7
IS.5
17.2
18.9
x.9
27.7
_3’-.- ’
16.9
19.7
22.4
‘5 -J
-.27.9
38.9
41.7
48.8
* Includes insulating value of siding, sheathing, and
air films. but not the effects of direct conduction
through framing versus insulation.
SOURCE: E. Eccli, LOW-Cosr Enerp-/Z&Cm
Shclrcr.
PERCENT FUEL SAVINGS
WITH NIGHT
THERMOSTAT
SETBACK
FROM 75O F
(g-hour setback: IO pm to 6 am)
Setback
City
5OF
7.5OF
Atlanta
Boston
Buffalo
Chicago
Cincinnati
Cleveland
Dallas
Denver
Des Moines
Detroit
Kansas City
Los Angeles
Louisville
Milwaukee
Minneapolis
New York City
Omaha
Philadelphia
Pittsburg
Portland
Salt Lake City
San Francisco
St. Louis
Seattle
Washington.
DC
II
7
6
7
8
8
II
7
7
7
8
12
9
6
8
8
7
8
7
9
7
IO
8
8
9
13
9
8
9
IO
IO
13
9
9
9
IO
14
II
8
IO
IO
9
IO
9
I1
9
12
IO
IO
11
SOURCE: Minneapolis-Honeywell
lOoF
IS
II
IO
11
12
I2
15
II
II
II
12
16
13
10
12
12
II
12
II
13
II
14
12
I2
13
Data.
The placemenr of insulation is also important. First, you should insulate roofs and the
upper portions of walls. Warm air collects at
the ceilings of rooms, producing a greater temperature difference there between indoor and
outdoor air.
Six inches of fiberglass batt insulation (R19) for roofs and 3.5 inches (R-l 1) for walls
were once the standards in cold climates. These
standards are being quickly accepted in mild
climates and greatly upgraded to R-38 and R19 in cold ones. In extreme cold. builders are
installing R-60 roofs and R-25 to R-40 walls.
These are the “superinsulated” buildings. Rigid
board insulation installed outside the framing
/
FOIL-FAc6D
FlBERtpII
BATTS
AIR
/
SPMQ
RlqlD
VAPOR
/
BARRIER
BOARD
Adding rigid board insulation to exterior stud walls-plan
of conventional stud walls gives even better Rvalues because it slows the heat loss through
the uninsulated wood frame, which can represent 15 to 25 percent of the total wall area.
Construction details for adding such insulation
are shown in the diagrams.
Attic insulation is the most crucial because
substantial amounts of heat are lost in winter
and gained in summer. An R-valut af 20 to 30
can be obtained by applying thick blanket. batt,
loose-till. or poured insulation on the ceiling
framing or directly on top of existing insulation.
If the attic roof is too low. you can have a
contractor install blown insulation. If neither is
possible, wait until re-roofing time and add rigid
board insulation.
The insulation of an existing stud wall is
limited by the wall thickness. The only insulating materials that approach or exceed the desired resistance of I9 in a standard 2 x 4 stud
wall are cellulose tiber or urethane. Mineral
wool and fiberglass insulation won’t do it (see
table). Trying to compress these insulators to
increase their resistance will have the opposite
effect after a point-compression reduces the
air spaces needed to slow the Row of heat. If
you are fortunate enough to have 2 x 4 studs,
you have many choices.
view.
Cellulose fiber or polystyrene beads can be
blown into wood frame walls. although holes
will have to be bored in the interior wall between studs and above and below the timbreak.
Later somone will have to patch the holes, providing you with an opportunity to use a vapor
barrier paint. The other alternative-installing
blanket or batt insulation-requires ripping out
the interior walls. This makes sense only if the
walls need replacing.
For masonry walls, one method is to blow
loose-fill or foam insulation into the existing air
spaces. This approach is possible if the plate
for the ceiling rafters doesn’t cover the concrete
block cores or the cavity wall construction. Rigid
board insulation can also be placed on the outside of a masonry wall and replastered or COVered with siding.
Insulate floors and foundations last. Tacking
foil-backed insulation supported by wire mesh
to the underside of the floor (leaving a half inch
air space) can provide a high resistance. If there
isn’t enough room to get under the house, seal
the foundation but leave a few ventilation openings. For basements being used as living space,
insulate the foundation walls all the way to the
floor (interior) or footing (exterior).
43
Once the house is insulated well to retain solar
or auxilary heat, it can be designed to act as a
solar collector. Although the color, orientation,
and shape of the house are important, the most
significant factors in capturing the sun’s energy
are size and placement of windows. Openings
in shelters are the origin of present-day windows: they were used for the passageof people
and possessions. and for natural ventilation and
lighting. These openings aiso allowed people to
escape from indoor drudgeries by gazing off
into sylvan surroundings. But the openings also
had their discomforts and inconveniences. Animals and insects had free access, the inside
temperature was difficult to regulate. and humidity and air cleanliness could not be controlled.
Although glass has been dated as early as
2300 BC. its use in windows did not occur until
about the time of Christ. And only in the present
century have the production and use of glass
panes larger than eight or twelve inches on a
side become possible. As the technology and
economics improve, glass is replacing the traaitional masonry or wood exterior wall. But the
design problems accompanying this substitution
have often been ignored or underrated.
Besides reducing the amount of electricity
needed for lighting. glass exposed to sunlight
captures heat through the greenhouse effect ex44
plained earlier. Glass readily transmits the shortwave visible radiation, but does not transmit the
long-wave thermal radiation emitted after the
light changes to heat when it hits an interior
surface. Almost all this thermal radiation is absorbed in the glass and a substantial part of it
is returned through radiation and convection to
the interior space.
Experimental houses were built in the 1930s
and 1940s with the major parts of south-facing
walls made entirely of glass. The most extensive
work with these “solar houses” was done by
F. W. Hutchinson at Purdue University. In 1945,
under a grant from Libbey-Owens-Ford Glass
Company, he built two nearly identical houses.
They were thermally and structurally the same,
except that one house had a larger south-facing
window area. Based on the performance of these
two houses, Hutchinson reported that “the
available solar gain for double windows in south
walls in most cities in the, U.S. is more than
sufficient to offset the excess transmission loss
through the glass. ”
Hutchinson also conclnded that more than
twice as much solar energy is transmitted through
south-facing windows in winter than in summer. If the windows are shaded in summer. the
difference is even greater. For a fixed latitude,
the solar intensity does not vary strongly with
the outside air temperature, but heat loss does.
Direct Gain Systems
SOLAR
AVeEIge
Percent
Possible
Sunshine
City
Heating
Season
Temperature
BENEFIT
Net Heat Gain
Btu/(hr ft?)
Single Double
glass
glass
VALUES
City
Percent
Possible
Sunshine
Average
Heating
Season
Temperature
Net Heat Gain
Btu/(hr ft2)
Single Double
glass
glass
Albany, NY
Albuquerque, NM
Atlanta. GA
Baltimore, MD
46
77
52
55
35.2
47.0
3 I .s
43.x
-12.x
IX.0
9.0
2.0
5.6
30.2
18.8
IS.9
Jacksonville,
FL
Joliet, IL
!,incoln. NB
Little Rock. AR
40
53
61
51
62.0
40.8
37.0
5 I .6
13.9
2.9
-2.2
8.5
18.1
12.8
IS.3
18.3
Birmingham.
AL
Bismarck. ND
Boise. ID
Boston. MA
51
55
5-l
s4
53.8
24.6
49.7
3x. I
10.9
-20. I
‘7
--. 9
5.2
:9.5
4.0
16.0
I I.7
Louisville.
KY
Madison, WI
Minneapolis,
MN
Newark, NJ
51
SO
53
55
45.3
37.8
29.4
43.4
I.5
-7.6
-15.7
1.4
14.6
9.5
5.X
IS.5
Burlington.
VT
Chattanooga. TN
C’hcycnne. WY
Cleveland. OH
42
so
67
41
31.5
49.x
41.3
37.2
-19.5
5.0
5.7
-13.7
.9
16.7
‘0.9
3.7
New Orleans, LA
Phoenix, A%
Portland. ME
Providence. RI
37
so
52
54
61.6
so.5
33.x
37.2
I I.7
‘1.9
-7.2
-6. I
16.1
27.5
12.0
I I.3
Columbia.
SC
Concord. NH
Dallas. TX
Davenport. IA
51
52
47
s-1
54.0
33.3
.?’L._5
40.0
I I.2
-12.0
7.1
-3. I
19.6
7.4
16.4
12.x
Raleigh. NC
Reno. NV
Richmond. VA
St. Louis. MO
s7
t-d
5’)
.57
50.0
45.4
47.0
43.6
- IO.0
X.6
x.0
7.h
20.6
21.7
20.2
! 6.6
Denver. CO
Detroit. Ml
Eupenc. OR
Harrishurp. PA
70
43
44
so
3x.9
3.5.x
so.2
43.b
.57.I-t.1
2.7
-1.5
21.7
44.0
13.2
12.5
Salt Lake City. UT
San Francisco, CA
Seattle, WA
Topeka. KS
59
62
34
61
40.0
54.2
46.3
42.3
0.0
17.3
-7.3
3.x
IS.9
75.7
5.2
I x.4
Hartford. C’T
Hclcna. MT
Huron. SD
Indi~~n;tpolis. IN
43
s2
5x
51
42.X
40.7
2X.2
40.3
-..3
-3.3
-14.1
-4.h
14.1
I’ -.- ’
x.0
I I.’
Tuls,~. OK
Vicksburg.
MS
Wheeling. WV
Wilmington,
DE
Sd
45
41
Sh
4x.2
56.X
46. I
4.5.0
7.4
- 10.7
3.7
3.7
IO.0
17.7
9.0
16.9
Consequently.
tential
the use of glass
for reducing
winter
has greater
heating
demand
poin
than in cold climates.
The table of “Solar Benefit Values” gives
us plenty of evidence for this potential. Many
of the cities :<tudied showed net energy gains
through single glass (a negative number represents a net loss). and all 48 cities studied
showed net gains through double glass. The
losses through single glass in some cities should
be compared to the heat loss through a typical
wall that the glass replaces.
mild
climates
There are a number of reasons that the quantity of solar energy that gets through a south
window on a sunny day in winter is more than
that received through that same window on a
sunny day in summer.
I. There are more hours when the sun shines
directly on a south window in winter than in
summer. At 40”N latitude, for example. there
are I4 hours of possible sunshine on July 2 I.
but the sun remains
north of east until
X:00 a.m.
and goes to north of west at 4:00 p.m.. so that
direct sunshine occurs for only eight hours on
4s
The New Solar Home Book
r,,
WINTER
SUUMER
f W’F
T,” = 7PF
row -25°F
r,, = 1O.F
247
>
lRANSMISSIt?N
AND REFLECTX%V
/67
107
89
17
213
2Y7
2Y7
58%
c/UN
-RAW”
-
Solar heat gains through clear, heat-absorbing, and
reflecting single glass. Listed values are in Btu per hour.
the south wall. But on January 2 I, the sun is
shining on the south wall for the full ten hours
that it is above the horizon.
2. The intensity of sunlight hitting a surface
perpendicular to the sun’s rays is about the same
in summer and winter. The extra distance that
the rays must travel through the atmosphere in
the winter is offset by the sun’s closer proximity
to the earth in that season.
46
3. Since the sun is closer to the southern
horizon during the winter. the rays strike the
windows closer to perpendicular than they do
in the summer when the sun is higher in the
sky. This means less is reflected and more is
transmitted. At 40”N latitude, 200 Btu strike a
square foot of vertical window surface during
an average hour on a sunny winter day, whereas
100 Btu is typical for an average summer hour.
Direct Gain Systems
--fiLM4
. SMAD.WG
NEEDEO
1
S
Different glass types are recommended
for limiting summer heat gain for various
window orientations.
In addition to these effects. the diffuse radiation
from the winter sky is double that from the
summer sky.
GLAZING
The type of glazing you use can have a significant effect on energy gains and losses. Single
sheets of clear, heat-absorbing. and reflecting
glass all lose about the same amount of heat by
conduction. But there is a great difference in
the amount of solar heat transmitted through
different types of glass. as shown in the tirst
table. The percentage summer and winter heat
gains for single-glazed units of clear, heat-absorbing. and reflecting glass are summarized in
the second table. The accompanying diagrams
will give you an idea of the net heat gains for
various combinations of single and double glass.
The percentage of solar heat gain includes a
contribution from heat conduction through the
glass. The heat gains are approximate for the
sunny day conditions shown, and no attempt
has been made to account for the differing solar
angles in summer and winter.
To reduce summer heat gain. yol: might use
reflecting glass on the outside and clear glass
on the inside of two-pane windows facing into
the sun. Unfortunately, this combination drastically reduces the winter heat gain, and is not
recommended for south-facing glass. Two clear
panes of glass, low-emissivity double-glazed
units (with the special coating on the outer surface of the inner pane), or anti-reflective tripleor quadruple-glazed units, are generally recommended for windows used for solar heat gain
in winter. In either case, you must still use
shading, natural and artificial, to keep out the
hot summer sun.
In many climates, keeping the sunshine out
during warm weather is very important to human comfort. In such areas, the use of special
glazings is one alternative, especially for the
east and west sides. The important factors to
consider in the use of specialized glass bear
repeating:
I. Such glass does reduce solar heat gain,
which can be more of a disadvantage in the
winter than an advantage in the summer.
2. Except for their higher insulating values,
special glazings are almost always unnecessary
on north, north-northeast, and north-northwest
orientations. Reflecting and heat-absorbing glass
only helps to control glare.
3. In latitudes south of 40%. heat absorbing
and reflecting glass should not be considered
for south-facing windows.
4. The use of vegetation or movable shading
devices is a more sensible solution than the use
of heat-absorbing or reflecting glass for south,
southeast. and southwest orientations.
SOLAR TRANSMITTANCE
Glazing
Type
Single. clear
Double,clear
Triple, clear
Triple, low-e film
0.85
0.74
0.61
0.46
Quad. clear
Double. low-e coating
0.50
0.52
Triple, anti-reflectivefilm
Quad.anti-reflectivefilm
0.66
0.63
47
The New Solar Home Book
PERCENTAGE
HEAT GAIN THROUGH
HEAT-ABSORBING
AND REFLECTIVE
Summer
Glass Type
Single Glazing
Clear
CLEAR.
GLASS
Winter
97
68
Heat-absorbing1
86
41
Reflective?
58
19
83
6R
74
5’
so
42
42
28
31
I7
Double Glazing
Clear outside & inside
Clear outside/
hear-absorbing inside
Clear outside/
reflective inside
Heat-absorbing outside/
clear inside
Reflective outside/
heat-absorbing inside
I. Shading
2. Shading
coefficient
coefficient
the sun rather than the climatic seasons. The
middle of the summer for the sun is June 21,
but the hottest times occur from the end of July
:o the middle of August. A fixed overhang designed for optin-;a1shading on August 10 causes
the same shadow on May I. The overhang designed for optimal shanding on September 2 I.
when the weather is still somewhat warm and
solar heat gain is unwelcome, causes the same
shading situation on March 2 1, when the weather
is cooler and solar heat gain is most welcome.
= 0.5.
= 0.35.
The four (or more) sides of a building need
not, and in fact should not. be identical in appearance. Substantial savings in heating and
cooling costs will result from the use of wellinsulated walls on the north, east and west. The
few windows needed on these sides of the house
for lighting and outdoor views should use the
glazing methods advocated here. In most areas
of the United States. double-glazed clear glass
windows or high-performance glazings on the
south sides provide the optimum winter heat
gain.
SHADING
Through the intelligent use of shading. you can
minimize the summer heat gain through your
windows. Perhaps the simplest and most effective methods of shading use devices that are
exterior to the house, such as overhangs or awnings. One difficulty with fixed overhangs is that
the amount of shading follows the seasons of
48
L&X&M&3?
21
Shading a south window with a fixed
overhang (at solar noon).
Direct Gain Systems
Sizing Overhanps
Overhangs can be effective shades fur large
south-facing vertical window areus. How much
shade you want and when you want it depends
on the home’s heating and cooling load. You
can size an overhang by choosing what months
you want shade and how much of the window
you want shoded (e.g., all or half the window).
The depth of the overhang (0) und how high it
is sepuruted from the window (S) ure found with
simple trigonomet~:
The ruble lists the declination angles for the
2 1st day of mch month. Irt this cusp:
0 = Hl(tun A - ton B)
In rhis cuse, the o\*erhung would need to be
ulmosf m*o feet deep md irs lower edge would
he owr hrtJ.ftW
trbow
the window. If yr could
uccepr jdl shcrdc m Jwle 2lst. !xt no shade
on December 2 Isr (u?ld I~PWCsome shtrdiq on
September 2 1st). the o\~erhun,q could be shullower and t~loser- w the rap of he window:
S = D fun B
where H is the height of the shudow (meusured
dobrn fkom rhe bottom of rile or*erhunSg). A is
the summer noon projle ungle, und B is the
winier noon projile uncle.
The profile ungle is d#iculr to envision. The
jigure .show.s thur it is the ungle benveen rhe
horkon und the sun’s ruy. in u ,*erktrl plune
perpendiicxlur lo Ihe \raindo,c*. The noon profile
ungle is equul Irt (90 - L + D). where L is
the lutirude of rile site und D is the declinurion
of rhr .SlOl.
Ler’s .wy .wu liwd ut JWN Iurirrrdt~, und FOII
HWlttYl
fltll .duldt~ oil u fourlfimt
high ,~*indo,c~
on Jurrd 21.~1und no shude on September ,‘lSY.
MONTHLY
SOLAR
Month (Day 1 I )
Decemkr
January/Navemkr
FcbruaryKktoher
March/September
April/August
May/July
June
A = 90 - L + D = 90 - 40 + 23 = 73
B = 90 - 40 + 0 = 50
0 = Hl(tan A - tan B)
= 4litun 73 - fun 50) = I .92
S = D fun B = 1.92 run 50 = 2.29
A = 90 - 40 + 23 = 7-Z
B = 90 - 40 - 23 ‘= 27
0
s
73 - mn 27)
= I .45 fl (deep)
=
Jl(tm1
27
= 0.74 .fi (8 irr) ubo,*e the window.
=
I .JS
DECLINATIONS
Declinarion
-23
-20
-IO
0
+I I.h
+20
+23
SOLAR
ALTITUDE
A
fill1
The New Solar Home Book
Vegetation, which follows the climatic seasons quite closely. can provide better shading
year round. On March 21, for example, there
are no leaves on most plants, and sunlight will
pass readily (except through oak trees, which
do not lose their leaves until late fall). On September 21. however. the leaves are still full,
providing the necessary shading. Placement of
deciduous trees directly in front of south-facing
windows can provide shade from the intense
midday summer sun. But watch out for trees
with dense. thick branches that still shade even
without their leaves. Even better is an overhanging trellis with a climbing vine that sheds
its leaves in winter. Unfortunately, stalks remain and produce considerable shading in the
winter as well. so the vines must be cut back
in the fall.
Movable shading devices are even more amenable to human comfort needs than fixed overhangs or vegetation. but they have their own
problems. Movable shading devices on the outsides of buildings are difficult to maintain and
can deteriorate rapidly. Awnings are perhaps
the simplest and most reliable movable shading
devices. but their aesthetic appeal is limited.
The requirement for frequent human intervention is often seen as a drawback. Operable shading placed between two layer% of glass is not
ah effective as an exterior device, but it is still
more effective than an interior shading device.
UV-TRANSMITTANCE
<iking
Mini-blinds between glass panes can be expensive. Interior shading devices, such as roller
shades and draperies, give the least effective
shading but offer versatile operation by the people inside. And they do keep direct sunlight
from bleaching the colors of walls, furniture,
and floors. (The high-performance glazings are
also effective in reducing this bleaching effect
because they block more ultraviolet light than
ordinary glass. The table lists the ultraviolet
transmittance of different glazings. )
East- and west-facing glass is extremely difficult to shade because the sun is low in the sky
both early morning and late afternoon. Overhangs do not prevent the penetration of the sun
during the summer much more than they do
during the winter. Vertical louvers or extensions
are probably the best means of shading such
glass, but you might consider reflecting and heatabsorbing glass or high-performance glazings.
For this purpose, you should be familiar with
the values of the shading coefJicient of the various glasses. A single layer of clear, doublestrength glass has a shading coefficient of I.
The shading coefficient for any other glazing
system, in combination with shading devices,
is the ratio of the solar heat gain through that
system to the solar heat gain through the doublestrength glass. Solar heat gain through a glazing
system is the product of its shading coefficient
times the solar heat gain factors. The solar heat
factors for 40”N latitude were listed earlier. The
ASHRAE Handbook of Fundanlenrals has a
complete list for other latitudes.
Type
Single, char
Double. clex
Triple, char
Triple. low-c film
@wt. ckar
Double. low-e coating
Triple. ah-retlcctivc
I’ilm
Quad. mli-retlectivc
film
0.7x
Oh4
03 I
0.43
O.-l I
0.39
O.Oh
0.0’
SUN PATH DIAGRAMS
It is usually necessary to describe the position
of the sun in order to determine the size of a
window shading device. Earlier, we described
the sun’s path in terms of the solar altitude angle
(0) and the azimuth angle (4). These can be
determined for the 2 1st day of any month by
Direct Gain Systems
COEFFICIENTS FOR VARIOUS SHADING CONDITIONS
Coefficient
Condition
Clear double-strength
glass. l/8”. unshaded
Clear plate glass, l/4”. unshaded
Clear insulating glass. two panes l/8” plate. unshaded
Clear insulating glass. three panes l/g” plate, unshaded
Double-glazed
unit, l/8” glass with low-e hard coat on surface 3, unshaded
Clear insulating glass. two panes l/4” plate. unshaded
Clear insulating glass. threi: panes l/4” plate. unshaded
Double-glazed
unit, I/X” glass with low-e soft coat on surface 3. unshaded
Clear glass with dark interior draperies
Heat-absorbing
I/4” plate glass. unshaded
Triple-glazed
unit. I/X” glass with low-e soft coat on surface 5. unshaded
Triple-glazed
unit. l/X” glass with low-e suspended film. unshaded
Blue reflective l/4” glass. unshaded
Clear glass with light interior Venetian blinds
Heavy-duty grey heat-absorbing
l/2” glass, unshaded
Heavy-duty grey heat-absorbing
l/2” glass with dark interior drapes
(or medium Venetian blinds)
Silver reflective l/4” glass, unshaded
Silver reflective l/4” glass with interior drapes or Venetian blinds
Clear glass with exterior shading device
tables, or can be calculated directly from
formulas. Another method for determining solar
altitude and azimuth for the 2lst day of each
month is the use of sutl path dia,qratns. A different diagram is required for each latitude. although interpolation between graphs is reasonably
using
accurate.
52”N
Diagrams
are provided
for latitudes
in the appendix.
from
24”N
to
The 40”N
diagram is reprinted here as an example.
You can also use sun path diagrams to determme the effects of shading devices. There
are two basic categories of shading-a horizontal overhang above the window or vertical
fins to the sides. As shown in the diagram, the
shading angles a and b of these two basic obstructions are the two important variables available to the designer. The broader the overhang
or fin. the larger the corresponding angle.
Each basic shading device determines a specific shading musk. A horizontal overhang determines a “segmental” shading mask while
I .OO
0.95
0.90
0.85
0.84
0.83
0.78
0.72
0.69
0.68
0.67
0.62
0.58
0.55
0.50
0.42
0.23
0.19
0.12
vertical fins determine a “radial” one. These
shading masks are constructed with the help of
the shading mask protractor proGded here. These
masks can then be superimposed upon the appropriate sun path diagram for your latitude to
determine the amount of shading on a window.
Those parts of the diagram that are covered by
the shading mask indicate the months of the
year (and the times of day) when the window
will be in shade.
Sun path diagrams and the shading mask protractor can also be used to design shading devices. If you specify the times of year that shading
is needed and plot these on the appropriate sun
path diagram, you have determined the shading
mask for your desired condition. The shading
angles a and b can be read from this mask using
the shading mask protractor. From these angles
you can then figure the dimensions of the appropriate shading devices.
51
The New Solar Home Book
Use of Sun Path Diagrams
A sun parh diagram is a projection of rhe s&
vault, just as a world map is a projection of the
globe. The paths of the sun across the sky are
recorded as lines superimposed on a grid that
represents rhe solar angles. Sun path diagrams
can be used to determine these angles for an?
dure and time. Diflerenr sun path diagrams are
needed for different latiludes.
As an esample. find rhe solar alrirude and
azimurh angles at 4.00 p.m. on April 2i in
Philadelphia (JO’%). First locale the April
line--the durk line running left to right and
numbered *‘IV” for the fourth month-and [he
-1:OOp.m. line--&e dark line running verlical!\
und numbered ’ ‘4. ” The interseclion of these
lines indicates the solar posirion ul thut time
and day. Solur alrinrde is read from the concenrric circles-in (his case it’s 30 degrees. The
solar azimuth is read from the radial lines-in
this case ir’s 80 degrees west of true south. If
you wust your judgement, you can also use these
diagrams TOgive you the solar positions on days
other than (he 2 1st of each month.
The shading mask protractor provided here
will help you TOconstrucl masksfor any shading
situation. First derermine the shading angle of
rhe horizontal overhang or verficaljns, as shown
in the figure. For a horizontal overhang, find
the arc corresponding to angle ’ ‘a” in the lower
half of the.shading mask protractor. All the area
above rhar arc is rhe segmental shading mask
for that overhang. For vertical jins. find the
radial lines corresponding to rhe shading angle
“b” in the upper half of the shar!ing mask protractor. All The area below these lines is rhe
radial shading mask for (hose jins.
,I,,. r-4
.
.
_.--.
.
.
v(. . -* . 2:,- ~.. _. . , ;o;L:
,:..
‘. ,.
.‘/,. (1,. sI. $0.
. ;<I*
40-r-4LATITUDE
eHAOlN0
MABK
bO~:KCE. Ramsey and Sleeper. Archirr~rr~rrul Gwphic
52
PROTRACTOR
S~undurds.
irect Gain Systems
SEGMENTAL
MASK
RADIAL
MASK
Determining masks for horizontal and vertical shading obstructions. Use the shading mask protractor to convert a particular
shading angle into the corresponding mask.
53
A vital question in a solar-heated house is where
to store the heat. When the house is used as the
solar collector. it needs a method of “soaking
up” or storing heat so it doesn’t become too
hot when the sun is shining, and retains some
of this heat to use when it isn’t. Probably the
most efficient heat storage container is the material of the house itself-the walls, floors, and
roofs. All materials absorb and store heat as
they are warmed. For example, water or stone
will absorb more heat for a fixed temperature
rise than straw or wood. Heavy materials can
store large quantities of heat without becoming
too hot. When temperatures around them drop,
the stored heat is released and the materials
themselves cool down.
This heat storage cupuci~ of various materials can be used to store the sun’s heat for later
use. Solar energy penetratesthrough walls, roofs,
and windows to the interior of a house. This
solar heat is absorbed in the air and surrounding
materials. The air in the house is likely to heat
up first. It then distributes this heat to the rest
of the materials via convection. If they have
already reached the temperature of the air or
cannot absorb the heat quickly, the air continues
to warm and overheats. The greater the heat
storage capacity of the materials in the house,
the longer it will take for the air to reach uncomfortable temperatures and the more heat can
be stored inside the house.
54
If it is cold outside when thfe sun sets, the
house begins losing heat through its exterior
skin, even if it is well insulate,d. To maintain
comfortable temperatures, this heat must be replaced. In houses which have not stored much
solar heat during the day, auxiliary heating devices must provide this heat.
If the interiors are massiveenough, however.
and the solar energy has been allowed to penetrate and warm them during the day, the house
can be heated by the sun, even at night. As the
inside air cools, the warmed rmaterials replace
this lost heat. keeping the rooms warm and cozy.
Depending upon the heat storage capacities of
the inside materials. the amount of solar energy
penetrating into the house, and the heat loss of
the house, temperatures can remal n comfortable
for many hours. Really massive houses can stay
warm for a few days without needing auxiliary
heat from fires or furnaces.
During the summer, a massive house can also
store coolness during the night for use during
the hot day. At night, when outside air is cooler
than it is during the day, ventilation of that air
into the house will cool the air and all of the
materials inside. Since they will be cool at the
beginning of the next day, they can absorb and
store more heat before they themselves become
warm-cooling
the indoor air as they absorb
heat from it. Thus, if the materials are cool in
the morning. it will be a long time before they
The House as a Heat Storehouse
have warmed to the point that additional cooling
is needed to remove the excess heat.
TEMPERATURE
70
,I, DROP IN OUTDOOR TEMPERATURE
SWINGS
The effects of varying outdoor temperaturesupon
the indoor temperatures can be very different
for different types of houses. The first graph
shows the effects of a sharp drop in outdoor
temperature on the indoor temperatures of three
types. Of the three, a lightweight wood-frame
house cools off the fastest. It has little heat
stored in its materials to replace the heat lost to
the outside. A massive structure built of concrete, brick, or stone maintains its temperature
over a longer period of time if it is insulated on
the outside of the walls. The heavy materials
which store most of the heat are poor insulators,
and they must be located within the confines of
the insulation.
A massive house set into the side of a hill or
covered with earth has an even slower response
to a drop in the outdoor air temperature. Ideally,
the interior concrete or stone walls in this house
are insulated from the earth by rigid board insulation. One or two walls can he exposed to
the outside air and still the temperature will drop
very slowly to a temperature close to that of the
earth.
The second graph shows the effects of a sharp
rise in outdoor temperature on the same three
houses. Again, the lightweight house responds
the fastest to the change in outdoor temperature;
in spite of being well-insulated. its temperature
rises quickly. The heavy houses. however. absorb the heat and delay the indoor temperature
rise. The house set into a hill or covered with
earth has the longest time delay in its response
to the outdoor air change; if properly designed,
it may never become too warm.
The effects of alternately rising and falling
outdoor air temperatures on indoor air temperatures are illustrated in the third graph. Without
any sources of internal heat the inside air temperature of the lightweight house fluctuates
widely. while that of the earth-embedded house
remains almost constant near the temperature
10
10
IIll RISE IN OUTDOOR
100 -
7”
70
00
EO
,,I,, DAILY
IllI
FLUCTUATIONS
TEMPERATURE
IN OUTDOOR
TEMPERATURE
1
ELAPSED
_---- - -.-.-.-.-.-.-.
-----a
-
ouldoor
amr
hak, mrdrld
-.--
-
-
-
MllWC
ma,*,“*
burr
howe
rrood
TIME
ham
hovrc
l”l”llmd
on o”,rlde
embedded
I” earth
Effects of changes in outdoor air temperature on
the indoor air temperatures of various houses.
of the earth. We say that massive houses, whose
indoor temperatures do not respond quickly to
fluctuations of outdoor temperature, have a large
thertnd tnuss. or thertnal itrertict .
If a house responds slowly to outdoor temperature fluctuation. you don’t need heavy duty
auxiliary equipment to keep the place comfortable. Although the furnace in a lightweight,
uninsulated wood-frame house might not be used
much on a cold, sunny day, it might have to
55
The New Solar Home Book
labor at full throttle to keep the house warm at
night. The massive earth-embedded house, on
the other hand. averages the outdoor temperature fluctuations over a span of several days or
even weeks. A bantamweight heating system
(such as a wood stove, for example) could operate constantly to assure an even comfort level
throughout the house.
HEAT STORAGE CAPACITIES
All materials vary in their ability to store heat.
One measure of this ability is the specijk heat
of a material, which is the number of Btu required to raise one pound of the material 1°F.
For example, water has a specific heat of 1.0,
which means that 1 Btu is required to raise the
temperature of 1 pound of water 1°F. Since one
gallon of water weighs 8.4 pounds, it requires
8.4 Btu to raise it 1°F.
Different materials absorb different amounts
of heat while undergoing the same temperature
rise. While it takes 100 Btu to heat 100 pounds
of water 1°F. it takes only 22.5 Btu to heat 100
pounds of aluminum 1°F. (The specific heat of
aluminum is 0.225.) The specific heats of various building materials and other common materials found inside buildings are listed in the
accompanying table.
The heat wpacity. or the amount of heat
needed to raise one cubic foot of the material
I “F. is also listed along with the density of each
material. Although the specific heat of concrete,
for example. is only one-fourth that of water.
its heat capacity is more than half that of water.
The density of concrete compensates somewhat
for its low specific heat. and concrete stores
relatively large amounts of heat per unit volume. As heat storage devices. concrete or stone
walls insulated on the outside are superior to
wood-framed walls having a plywood exterior
and a gypsum wallboard interior with fiberglass
insulation stuffed between them.
56
BUILDING
WITH THERMAL
MASS
Thermal mass is one of the most underrated
aspects of current building practice. Unfortunately, heavy buildings are hardly the favorite
children of architects and building contractors,
because the visual weight of buildings is an
important aesthetic consideration. Well-insulated homes with reasonable amounts of south
glazing (no more than six percent of the floor
area) usually have enough thermal mass in the
standard building materials without adding more.
Extra thermal mass is now looked at more as
an “option” than a “necessity.”
Massive fireplaces. interior partitions of brick
or adobe, and even several inches of concrete
or brick on the hoor can greatly increase the
SPECIFIC
HEATS AND HEAT CAPACITIES
OF COMMON
MATERIALS
Material
Water (40°F)
Steel
Cast iron
Copper
Aluminum
Basalt
Marble
Concrete
Asphalt
Ice (32°F)
Glass
White oak
Brick
Limestone
Gypsum
Sand
White pine
White fir
Clay
Asbestos wool
Glass wool
Air (75°F)
Specific
Btu/(lb
I .OO
0. I2
0. I 2
0.002
0.314
0.20
0.2 I
0.33
0 .-a
3’
0.387
0. I x
0.57
0.10
0.’ I7
0.26
0.191
0.67
0.6.5
0.23
0.20
0. IS7
0.34
Heat
OF)
Density Heat Capacity
lb/ft3
62.5
489
450
556
I71
I80
16’
144
131
57.5
IS4
47
123
I03
78
94.6
27
37
6.7
36
‘5
_3 .-_
0.07s
Btu/(ft3
“F)
62.5
sx.7
s4.0
51.2
36.6
36.0
34.0
31.7
29.0
18.0
27.7
26.8
24.6
x.4
‘0.3
x. I
X.1
7.6
3.9
7.2
0.5 I
0.0 I 8
The House as a Heat Storehouse
u
/
RIGIP BOWD
N~ULATfmJ
mm&CTIVE
/SURFiNS
Insulation on the exterior of a house must be protected from weather and vermin to at least one foot
below grade.
thermal mass of a house. Placing containers of
water within the building confines, especially
in front of a window, is a simple solution.
Putting insulation on the outside of a house
is not standard construction practice and involves some new problems. Insulation has customarily been placed between the inner and outer
surfaces of a wall. Insulation on the outside of
a concrete or masonry wall requires protection
from the weather and contact with people or
animals.
In the example shown, three inches of rigid
board insulation covers the outside surface of a
poured concrete foundation. Above the surface
of the ground. this insulation must be protected
from rain, physical abuse, and solar radiation
-particularly
ultraviolet rays. Below ground
level, it must be protected from the unmerciful
attacks of moisture and vermin. The insulation
could be placed inside the formwork before the
concrete is poured, and the bond between the
two materials would be extremely strong. But
the insulation must still be protected above ground
Storing Heat in a Concrete Slab
Consider u 20 X 40 .fimt hoiise ,vith u billinsulutrd concrete sluh 9 intks thick. By 5 pm.
on Junutrrv 2 I. the sluh bus ~rwmed ~rptn 75°F
fi-om sunlight flooding in the .south ,tirrdmr*.s.
From that rim until rurly the ne.vt niorning,
the o:rtdoor tenrperuturc~ ur*eruge.s 25°F. ,\hile
the indoor uir uverugr.s 65°F. !f the house is
well-insuluted and 1osr.s heat ut u rule of 300
Btul(hr “F). and there is no source r,f‘umiliu~
hrut, what is the temnperutirw of the sluh ut 9
o’clock the ne.rt tnorning~~
The total heat lost from the house during that
period is thr product cf the rute of heut loss
(UA). times the number cd’hours (h). times the
average temperature diff~rcnw between the indoor und outdoor uir (AT). or
AH = (UA) (h)(AT)
= 300(16)165 - 25)
= 192.000 Btu.
With a tutu1 ~vhrme of 600 cubic -feet
(2ONJONO.7.5) und u heat captrci~ ($32 Btul
(fi” “F). the concrete slab .store.s19.200 Btlt.fiv
u I “F rise in its temptwrtwc. For- u I “F drop
in its tempertrture, the slab reltwses the sumtl
19.200 Btu. [f’ the slub drops 10°F. *from 75°F
to 65°F. it will reltwe just enough heat to repluc~e thut lost b! the hrse during the night.
So the .slub drops to u temperatuw c$65”F b!
9:00 thr nert morning.
In reali&, things are a bit more cwmplicuteci.
But this twrcise kelps to give a rough i&w of
how mwh heat you cm store in u concrete slab.
!f the house has 200 square feet of soirtli windows. and u solur heut gain of 1000 Btulft’ is
typical ,fiw a .sunny Junuu~ day. the slab cm
.storr the 200.000 Btu of solur energ! with a
temperuture rise of about 10°F. The stored solar
heut is the11releused at night to keep the house
warm as the inhabitants sleep.
57
The New Solar Home Book
level. The most popular alternative is to plaster
the insulation with a “cementitious” material
such as fiberglass-reinforced mortar.
dWUlOTL
MASS
The three kinds of thermal mass, based
on location.
strikes so that you can get by with as little mass
as possible. This saves on costs.
The table shows the surface area of the three
types of mass needed for each square foot of
south glazing, for different materials of various
thicknesses. The mass must be directly exposed
to the room. Mass in the floor doesn’t count if
it’s covered with carpet. Concrete or brick walls
don’t count as mass if they are hidden behind
a frame wall.
In a well-insulated, iight-frame house (R- 19
walls, R-38 ceiling, triple-glazing), the building
materials themselves are enough thermal mass
to allow six to seven square feet of south glazing
for every 100 square feet of heated floor area.
In the average house (R-l I walls R-19 ceiling,
double-glazed), the building material’s thermal
mass allows 11 to 14 square feet of south glazing for every 100 square feet of heated floor
area. To have more glazing than that, thermal
mass must be added to the building to avoid
overheating.
SIZING MASS
Thermal mass in a building stores heat and releases it later to the space when the air temperature around it begins to drop. When sized
properly. the mass can prevent overheating on
a sunny afternoon. and can keep the auxiliary
heat from turning on until later in the evening.
There are three ways mass can be “charged.”
that is. heated up. If it is “direct” mass, it is
directly hit by the sun for at least six hours a
day. If it is “indirect” mass. the sunlight hits
another surface tirst and is reflected onto the
mass. If it is “remote” mass. it is charged by
warm air that tlows by its surface through natural or forced convection. Direct. indirect. and
remote mass al! charge and discharge on one
side only, as shown in the figure.
The effectiveness of the mass is directly related to its location. Direct mass stores more
heat than the same surface area of indirect mass.
and much more than remote mass. When you
design. try to place the mass where the sun
MASS SIZING: SQUARE FEET OF MASS
NEEDED FOR EACH SQUARE FOOT OF SOUTH
GLAZING
Miltt+ll
Thickness
Direct
Mass
Indirect
Mass
Remote
Mass
Concrete
4”
6”
X”
4
3
3
7
5
s
14
14
I5
Brick
7”
i,.
x
5
s
IS
9
10
30
IX
Is)
76
38
I14
57
X”
Gypsum
tKMKi
0.5”
I”
II4
57
Hardwood
I”
17
2X
3’
S0ftw00d
I”
21
36
39
A grasp of the principles of thermosiphoningwhere the natural bouyancy of heated air or
water is used to circulate heat-is crucial to an
understanding of the passive uses of solar energy. When heated, air expands and becomes
lighter than the surrounding air. The heated air
drifts upward and cooler air moves in to replace
it.
You have observed the process of thermosiphoning, also called natural convection, at work
in a fireplace. Because the hot air just above
the fire is much lighter than the surrounding air,
it rises rapidly up the chimney. Cooler, heavier
room air replaces it, bringing more oxygen to
maintain the flames. Most of the fire’s heat is
delivered to the outdoors by this “chimney effect.” Thermosiphoning is also a strong force
in passive solar heating systems.
COOL AIR
t%dCK
lq.4s
A thermosiphoning air collector-two
OR
AESCRE
PLASTI
4
views.
59
Added fan provides heat control.
Damper prevents reverse thermosiphoning.
‘I’HERIW~SIPHONING AIR PANELS
The simplest fommof a thermosiphoning air panel.
or TAP. is illustrated in the diagrams. The air
in the space between the glass and the blackened
absorber wall is heated. II expands and becomes
lighter. rises through the collector. and flows
into the room from a vent ;LI the top. Cool room
air is drawn through another vent at the base ot
the wall. heated in turn. and returned IO the
room at the top. This process continues as long
as thcrc is enough sunlight to push the temperature of the absorber wall above the room air
lcmprrature.
Undesirable reverse thermosiphoning
1 cools the room.
Chimney effect induces natural ventilation in
summer.
To provide greater control of air Ilows. you
can add a fan to the supply duct of the solar
collector. Faster movement of air across the
absorber surface boosts the c,)llector efficiency
and allows the use of a smaller air gap between
absorber and glass. These hybrid collectors are
called forced air panels, or FAPs. They differ
from active air collectors only in their smaller
scale and their dependence on the thermal mass
of the building itself IO prevent the space from
overheating. A fan can also deliver warm air to
other parts of the house. such as north rooms.
or heat storage bins (in which case. they are
Indirect Gain Systems
active systems). Using fans with a proper combination of windows and wall collectors, you
can simultaneously heat the rooms exposed to
the sun and those in the shade.
Dampers help to control the air flow and prevent the cooling effect of ~et’~~e thermosiphoning. When the sun isn’t shining, the air in
the collector loses heat by conduction through
the glass and radiation to the outside. As this
air cools, it travels down the absorber face and
flows out into the room. Warm room air is drawn
in at the top to the collector and cooled in turn.
Although this reverse thermosiphoning could be
a benefit in summer, it is most undesirable in
winter. It can be prevented by shutting dampers
at night.
Dampers can operate manually or automatically. Natural air currents or fan pressure can
open or close them. You can also use dampers
in summer to prevent overheating by inducing
natural ventilation through houses. Cool air can
be drawn into the house from the north side and
warm air expelled by the “chimney” exhaust
system shown here. As with all air-type solar
heating devices, dampers should be simple in
design and operation. They should close tightly
and there should be as few of them as possible.
TAP VARIATIONS
A number of variations on the basic design of
TAPs can improve their performance. These
variations include insulation, improved absorber surfaces, and dampers and fans to regulate the How of air.
During a sunny winter day, no insulation is
needed between the back of the absorber and
Detail of tin cans
Low-cost thermosiphoning
air collector built onto an exterior wall.
61
The New Solar Home Book
Cross-section of CNRS wall collector.
the room. To reduce room heat losses on cloudy
days or at night. however, the wall should be
adequately insulated behind the absorber.
A metal absorber plate isn’t an absolute necessity for a TAP. Since the temperature of the
collector wall does not get extremely high.
blackened masonry or wood surfaces are also
possible. and costs need not be excessive. Altcmatives that increase the total absorber surfact can he particularly effective. if they do not
hinder the natural convection air flow. Rough
surfacc~make better absorbersthan smooth ones.
Pebbles cast in t blackened concrete wall are a
good example of such an absorber surface. Special absorber sheets, made of a selective surface
with an adhesive back. can greatly improve TAP
performance.
Another option (shown in the ligure) has tin
cans cut into quarters and mounted on the standard plywood sheathing of conventionally-framed
houses,
Some of the most significant work in thermosiphoning air collectors was done at the C’ettrre
Nu~ionul tk lu Reck-&
Sciet+tue
(CNRS)
in Odeillo. France. Under the direction of Professor Felix Trombe. this laboratory developed
62
several low technology approaches to solar
heating. The main building remains an excellent
example of the passive use of solar energy. Its
south. east, and west walls are a composite of
windows and thermosiphoning air panels, which
supply about half of the building’s winter heat.
The TAPS are installed below the windows, between floors so that the view to the exterior isn’t
blocked.
A cross-sectional view of these collectors is
shown in the accompanying diagram. Blackened corrugated metal sheets are located behind
a single pane of glass. Solar radiation passes
through the glass and is absorbed by the metal.
which is contained entirely within the volume
defined by the glass and duct. As the metal
heats, so does the air between the absorber plate
and the glass. The heated air flows upward
through vents into the rooms. Simultaneously.
cooler room air falls through a lower vent and
sinks down between the back of the absorber
and the duct wall. This air returns to the face
of the absorber where it. too, is heated and
expelled into the rooms.
No provision has been made to store the solar
heat, other than the thermal massof the building
itself-particularly the reinforced concrete slab
tloors. Consequently, the system is most effective when the sun is shining-almost POpercent
of the daytime hours in Odeillo. The air temperature in the offices and laboratories remains
relatively constant during the day. Even during
February, auxiliary heat is required only at night
and on overcast days. Outdoor temperatures are
relatively cool in summer, allowing the use of
east and west facing collectors. which would
overheat most buildings in hot climates.
MASS WALLS
Heat storage capacity can be added directly to
TAP vertical wall collectors. The overall simplicity of this synthesis of collector and heat
storage is compelling. Large cost reductions are
possible by avoiding heat transport systems of
‘ucts. pipes, fans, and pumps. Operation and
Indirect Gain Systems
PfDtWAL
VE NT5
A concrete mass wall collector. Solar collection, heat storage,
and heat distribution are combined in one unit.
maintenance are far simpler, and comfort and
efticiency generally greater. than collectors with
remote storage.
The schematic diagram shows a concrete wall
used as a solar collector and heat storage device.
When sunlight strikes the rough blackened surface. the concrete becomes warm and heats the
air in the space between wall and glass. Some
of the solar heat is carried off by the air. which
rises and enters the room, but a large portion
of this heat migrates slowly through the concrete. The wall continues to radiate heat into
the house well into the night. after the thermosiphoning action has ceased. In energy-conserving buildings with proper insulation levels
and infiltration control. mass walls can be sized
to maintain comfort for two or three days of
sunless weather.
Mass walls are usually constructed without
the vents for spaces used primarily at night. All
day tong. the sun’s heat is driven through the
thick wall. until it reaches the inside surface at
the end of the day. The thicker the wall. the
longer it will take for the heat can be conducted
through the wall. Once it reaches the inside
surface, the heat is delivered to the living space
through radiation and convection currents.
MASS WALL VARIATIONS
A number of variations in the design of concrete
walls is possible. The wall can be constructed
from poured concrete, or hollow masonry blocks
tilled with sand or concrete. Empty voids can
be used as air ducts for thermosiphoning. Brick
or adobe can also be used instead of concrete,
and need not be painted black if dark enough.
There are advantages in making the space
between the concrete wall and the glass covers
wide enough for human use. The space can be
used as a porch or vestibule, or even as a greenhouse. But the thermosiphoning heat flow to the
interior does not work very well for such large
spacesbecause the air does not get quite as hot.
Fortunately, there will still be large heat flows
by conduction through the wall and radiation to
the rooms.
Mass walls can also be constructed of other
materials, using water or phase-change materials to store the heat. Special containers for
water walls and whole manufactured units for
phase-change materials are available.
The pioneering work in mass wa!ls was done
at Odeillo under the direction of ProfessorTrombe
and architect Jacques Michel. Thus mass walls
63
The New Solar Home Book
made of concrete are often referred to as Trombe
(pronounced Trohm) walls. The first buildings
were two four-room houses, each with a floor
area of 818 square feet and a collector area of
5 I6 square feet. The collectors operate in a fashion similar to the one in the previous diagram,
except that to prevent reverse thermosiphoning.
the lower ducts are located above the bottom of
the collector. Cool air settling to the bottom at
night is trapped there.
Because they are not very well insulated, the
houses lose about 22.000 Btu per degree day.
Nevertheless. the concrete wall collectors supply 60 to 70 percent of the heat needed during
an average Odeillo winter, where temperatures
frequently plummet to 0°F. From November to
February. the collectors harvest more than 30
percent of the sunlight falling upon them. Over
a typical heating season, this passive system
supplies about 200.000 BIU (or the usable heat
equivalent of 2 gallons of oil) per square foot
of collector.
December and January, the coldest months, and
the least occurs in June and July. The midwinter clear day insolation on vertical south walls
is only about 10 percent less than that on tilted
roofs facing south. With an additional IO-50
percent more sunlight reflected onto vertical
surfaces from fallen snow. they can actually
receive more solar heat gain than tilted surfaces.
Other types of reflectors, such as swimming
pools. white gravel, and concrete walks, work
well with vertical collectors. South walls can
be shaded easily in summer, preventing the collector surface from reaching high temperatures.
At first glance, it seems foolish to remove a
window which admits light and heat directly,
only to replace it with an opaque wall solar
collector. But don’t forget the advantages of a
mix of windows and collectors: direct gain, indirect gain, view, ventilation, and egress. Interior wall surface is lost if the entire south
facade is glass, and excessive sunlight can dam-
30-
WALL, WINDOW, AND ROOF
COLLECTORS
25 -
Ease of construction is perhaps the most important reason for the emphasis on vertical wall
collectors rather than sloping roof collectors.
Gla/.ing is much easier to install. weatherproof.
and maintain in a verticai orientation. The cost
difference between windows and skylights is
testimony to this t&t. Builders estimate you can
install three windows for every skylight at about
the same cost. It is much easier I \ keep weather
out of ver!ical surfaces than tilted or horizontal
ones. There are fewer structural complications
with wa.lls than with roofs. and you needn’t
worry much about hail or snow build-up. Another important architectural constraint of large,
steeply-pitched roofs is that interior space under
such roofs is difficult to use.
The total amount of clear day solar heat gain
on south walls follows the seasonal need very
closely. In most of the United States. the greatest heat gain on vertical south walls occurs in
6-a
IO .’
v+
15 -
10 4
5-
1
OIAN
1
FEB
1
M4R
I
APR
I
MAY
I
JUN
I
JUL
I
AUG
a
SEP
tihjh?
Clear day insolation on horizontal surfaces,
and on south-facing vertical and tilted surfaces. Reflected radiation not included.
Indirect Gain Systems
age furniture. floors, and fabrics. A section of
wall provides an interior space where you may
place delicate objects that could not take direct
sunlight. People also can be very uncomfortable
when the sun shines directly on them. Overheating is often a problem with an all-glass wall,
even with massive floors and partitions. But
with solar collectors and heat storage in the
south walls. the excess heat can be transported
to cooler parts of the house or trapped and stored
for later use.
SUNSPACES
Sunspaces are the modem solar equivalent of
attached greenhouses.Originated in Roman times
to meet the demands of Tiberius Caesar for fresh
cucumbers out of season. greenhouseswere used
in I Wh-century Europe as both a supply of yearround food and a source of winter heat from
the sun. Called “conservatories” in England.
they were built in sizes ranging from small window units to room-sized structures.
Today. site-built or prefabricated sunspaces
have become a very popular mode of capturing
the benefits of passive solar heat and making
winter living more cheerful. As a heat source.
a sunsptlce with 100 square feet of glazing can.
in one winter. offset up to 30 gallons of heating
oil. 6560 kilowatts of electricity. or 3000 cubic
feet of natural gas. As a greenhouse rather than
primarily a heat source. a sunspace can work
wonders on houseplants. and supply fresh vegetables all winter long. Or a sunspace can he
used mainly ;LSadditional year-round living space.
(In this case it is a direct-gain system and not
truly a sunspace.) How the space is to be used
will affect choices of glazing. heat storage materials. and heat transfer methods.
glass, with solar transmissivity greater than ordinary glass, will improve sunspace performance. Polycarbonate sheets, fiberglass-reinforced
polyesters, and polyethylene glazings are other
choices in sunspace glazing. They can be less
expensive, lighter in weight, and less likely to
break than glass, but they age more quicklv.
losing their strength and appearance.
If the sunspace will only be used as a solar
collector, and closed off from the house at night,
single glazing will do. Single glazing may also
be suitable if you use insulated shutters or shades
that store out of the way in the daytime and
cover the glazing at night. Otherwise, double
glazing shoud be used. (Triple-glazed glass is
only worth the extra cost in severe climates.)
New high-performance glazings are available
for site-built sunspaces with a layer of high
transmittance film sandwiched between two glass
layers, or a low-emissivity coating on the inner
pane of double glazing. They offer a high degree of protection from 24-hour heat loss.
A sunspace should face due south, but up to
IS degrees either east or west will have little
effect on perfomlance. Facing it slightly east
offers desirable sunlight and warmth in the
morning. The space should not be shaded during
the day. Direct sun should be allowed in at least
from IO:00 a.m. to 2:OO p.m.. and more is
better.
The best glazing angle for collecting winter
sun is between 50 and 60 degrees. But sloped
glazing is difficult to seal against leaks yearround. and is harder than vertical glazing to tit
with window insulation in winter and sh:ldes in
summer. Sunspaces with sloped glazing are
colder at night in the winter and hotter all summer. Vertical glazing solves these problems.
with only a IO to 30 percent loss in efficiency.
Ground reflectance from snow. gravel. or sidewalks can cut this loss in half.
Glazings and Orientation
Sunspaces collect solar heat through south-facing glass or plastic glazing. Glass can last a
long time-up to 50 years if no one throws a
rock through it in the meantime. Using low-iron
Storing and Moving the Heat
If a sunspace is to be more than a solar collector.
it will need thermal mass to retain the heat when
the sun goes down. A brick or concrete floor
65
The New Solar Home Book
works well. So do water-filled containers painted
a dark color. Phase-change salts have four to
five times the heat storage capacity of water,
but are expensive and have often proved unreliable. Whatever is used should be positioned
to capture direct and reflected solar radiation,
and have as great a surface area as possible.
At least three square feet of concrete or brick
tloor (4 to 6 inches thick) or three gallons of
water are required for every square foot of south
glazing. Heat can also be stored in a rockbed
under the sunspace and/or living room floor by
blowing warm air from the sunspace through it.
There are several choices of how to connect
the sunspace and the house. The two spacescan
be open to one another so that the sun’s heat
penetrates directly. If you choose this design,
use high-performance glazings or very good insulating shades over double-glazing at night,
and insulate well around the rest of the structure. or heat loss will be high. (The sunspace
would really be a direct-gain space in this case.)
Double-glazed sliding glass doors are a second option. Direct solar radiation can still penetrate the living space, and the view through
the sunspace to the outside can be maintained.
If the doors are left open during the day. warm
air can freely pass to the living area. When the
doors are closed at night, the sunspace acts as
a thermal buffer, reducing heat losses in adjacent rooms.
A standard wood-frame wall with R-IO to RI5 insulation prevents solar radiation from passing through, but warm air can pass through open
windows and doors. A solid masonry wall offers
a combination of storage and distribution. If it
is meant to warm only the sunspace, it should
be six to eight inches thick and insulated on the
house side. If the wall’s stored heat is to be
shared by the sunspaceand living space, it should
be I2 to I6 inches thick. and uninsulated.
A wall made of water containers 6 to 12
inches thick also works well. Water can hold a
great deal of heat and deliver it easily to both
rooms through convection currents in the water
itself. Vents or windows and doors will help
transfer warm air to the living space earlier in
the day.
66
Wall openings that allow warm air to pass
through should be included no matter what type
of common wall is used. Warm air can flow
from sunspace to living space Lhrough the top
of a doorway or window and cool air can return
through the bottom. This can also be done with
pairs of high and low vents. There should be at
least 8 square fee: of openings for each 100
square feet of south glazing. When they are
closed, the sunspace will act as a buffer against
living area heat loss.
Any outside surface not used for collection
or storage should be well insulated. The side
walls of a sunspace should only be glazed if it
is to be used for serious plant growing. Eastwest exposure supplies a negligible amount of
solar heat. If the layout of the house permits,
it can “wrap around” the sides of the sunspace.
thus partly enclosing it. This design reduces
heat loss, offers a place for more thermal mass
than the add-on sunspace. and transfers heat to
a larger area of the house itself.
As in any energy-efficient construction, care
must be taken in putting together a sunspace to
limit air infiltration. Tight co;lstruction. weatherstripping, and careful caulking to seal cracks
are all important, especially around windows
and vents that open and close.
Shadesand vents are absolutely necessary for
summer comfort. A properly-sized fixed overhang on vertical south glazing can be sufficient
to keep the sunspace tolerably cool. If the glazing is tilted. a fixed overhang is impossible. and
exterior shading with vegetation and interior
shading with movable insulation or window
shades are the only options.
Warm air in the winter carries desirable heat
through doors and windows in the common wall
to living rooms inside the house. But in the
summer. that warm air must be vented .utside.
This is usually done with vent openings placed
high and low on exterior walls of the sunspace.
By convection, hot air rises out the top and
draws cooler air in through the bottom.
Fans and therinostatic controllers may be
necessary to move air. especially in the summer
if sloped glazing is used. They can also be used
to raise and lower movable insulation, to keep
ndirect Cain Systems
Sunspaces are attractive additions to a home, for living space, winter greenery, extra
heat, or a combination of all three. (Photo courtesy of Sunplace, Inc.)
67
The New Solar Home Book
temperatures more nearly constant automatically.
Sunspace Uses
The combination of design factors in a particular
sunspace depends on its purpose. Is it to be a
greenhouse? Is it purely a solar collector? Will
it be used as a daytime and/or nighttime living
space’?Or will it be a combination of all three’?
Each type has its own design requirements.
A sunspace for serious year-round plant
growing needslight as much as heat, so it should
be glazed all around. Extreme temperatures are
hard on plants, so thermal mass should be included to moderate them. Movable insulation
or high-performance glazing are needed for night
protection. Yentilation (both summer and winter) is also necessary to promote growth and
control humidity.
If you opt for a solar living room. night heat
losses are high. so vertical glazing insulated
with movable insulation or special glazings is
called for. To save heat. the east and west walls
should be well insulated or enclosed by the house
itself.
A sunspace used purely as a solar collector
should have this same end-wall protection. because east-west glass gains little. Close it off at
night and you can eliminate the cost of movable
insulation and thermal mass.
The combination sunspace is the most
popular-even though it requires some sacrifice
in the efficiency of any one purpose. Most people want the space to serve many purposes. It
should have as many of the features described
above as possible. but compromises will be necessary to balance the various requirements of
comfort during the day and night, winter and
summer.
PASSiVE VERSUS ACTIVE SYSTEMS
The real beauty of passive solar design is its
ability to function without external power sources.
But as the three functions of solar heating68
collection, storage, and distribution-become
more distinct. external mechanical power is
needed to transport the heat. Natural air flow
on a large scale requires very large ducts that
are too expensive, so a pump or fan is necessary. But how do you know which system will
perform best in your particular situation’?
The advent of hand-held calculators and microcomputers has made three major solar calculations generally usable. For passive solar
calculations. Los Alamos National Laboratory’s
Load Collector Ratio (LCR) and Solar Load
Ratio (SLR) are the most popular. Each predicts
the performance of direct gain systems, mass
walls. and sunspaces. The LCR method helps
you determine cznnual performance in the early
stages of designing. The LCR is the building’s
heat loss per degree day divided by the area of
south glazing.
The SLR method determines performance on
a monrhlv basis. Its more detailed outputs are
useful later in the design process. The SLR is
the ratio of solar gain to heat load. and is used
to find the Solar Savings Fraction (SSF)-the
ratio of the energy savings from solar to the net
heating load of the same buiding without solar
heat. Outputs include monthly and annual SSF,
annual auxiliary energy use. and life-cycle solar
savings.
F-Chart, from the University of Wisconsin.
estimates the performance of passive and active
solar energy systems for space heating. swimming pool heating, and domestic hot water. With
this interactive program, you can analyze air or
liquid systems, passive direct gain and mass
walls. and swimming pool heating systems.
Typical outputs include the solar gain, load. and
fraction of the load met by solar.
Most of the microcomputer programs for determining system performance include life-cycle
costing that help determine the most cost-effective design for a particular application. They
also usually include weather data for several
hundred locations. Other kinds of interactive
solar software are coming onto the market, including programs for evaluating photovoltaic
systems, daylighting. shading, and even wind
turbines.
I believe the grolcnd rules can be transformed
so that technology simplifies life instead of cmtinually complicating it.
Steve Baer
The use of solar energy to heat household water
supplies has been technically feasible since the
1930s. when solar water heaters were commonly used in California and Florida. Solar domestic water heating made a comeback in the
1970s and continued to sell even when oil prices
dropped in the 1980s and the sale of active solar
space conditioning systems plummeted. Their
smaller scale and lower cost put them within
closer reach of the homeowner’s pocketbook.
Furthermore. they integrate more easily with
existing water heating systems.
Hot water needs are fairly constant throughout the year. The collector and other parts of
the system operate year round, and initial costs
can be recouped more quickly than with space
conditioning systems. A solar space heater is
fully operational only during the coldest months
of the year, and the payback period is longer.
A solar water heater can also be sized more
closely to the average demand. A water heater
has roughly the same load day in and day out
and doesn’t have to accommodate wide fluctuations in demand.
A problem common to crll types of solar heating is the fluctuation of available sunlight. But
variable weather conditions are less problematic
for household solar water heating because hot
water requirements are more flexible. If the supply of hot water runs out during extended periods of cloudy weather, the consequences are
69
The New Solar Home Book
less severe than if the house were to lose its
heat. It’s the difference between letting the laundry wait a bit longer or having the pipes freeze
and burst. If you can tolerate occasional shortages of hot water, your solar water heater can
have a very straightforward design-free of :he
complications that provide for sunless periods.
When a more constant hot water supply is
needed the existing, conventional water heater
can make up the difference. Controls are simple
enough because this auxiliary heater can boost
incoming water temperatures to the desired supply temperature. If the solar heater is providing
full-temperature water. the auxiliary remains off.
If not, the auxiliary comes on just long enough
to raise the solar-heated water to the required
temperature.
In addition to all these advantages,solar water
heaters are a lot smaller than solar spaceheaters.
The initial cost of a solar water heating system
is lower. and it can be installed and operating
within a very short time.
How large a system you need depends on
how much hot water you use daiiy. and whether
you will draw it directly from the solar heater.
or use the solar heater to preheat the water for
a !inal boost by a conventional water heater.
On average. a solar water heater will supply 50
to 75 percent of your annual hot water needs;
25 to 35 percent in the winter. and SO to 100
percent in the summer. depending on how much
water you use and what system you choose.
Where should you locate your collectors? The
tirst thing to consider is that the collector should
face south or as nearly south as possible. Because domestic hot water is required year round
in about the same daily amounts. the collector
should be tilted for about the same solar gain
in all seasons. Relatively constant daily insolation strikes a south-facing collector tilted at
an angle equal to the local latitude. Steeper tilts
(up to latitude + IO“) may be needed in areas
with limited winter sunshine.
If the roof can support the collector. it is
much less expensive to put it there than to build
70
a separate structure. (Before installing the collectors on the roof, be sure the roof is soundif it is an older house, it may need reinforcemerit.) True south orientation and latitude angle
tilt may not be feasible on many houses. Fortunately, the loss in efficiency is relatively small
( 10 to 15 percent from idea!) if the roof faces
within 25” of true south and its tilt is within 15”
of the latitude angle. This efficiency reduction
can be easily recouped by making a proportionately larger collector.
How much storage capacity should you have?
Two days is generally the optimum. A larger
tar will carry a household through longer cloudy
periods (assuming enough collector area), but
will require more time to reach the desired hot
water temperature. Every attempt should be made
to stratify the hot water in the storage tank above
the cold. The hot water then goes directly to
the load, and the cold water to the collectors,
increasing efticiency and allowing the use of a
larger tank. The larger tank costs more but a
smaller one requires more frequent use of auxiliary heat. In genera!, the collector should be
large enough to provide a single day’s hot water
needs under average conditions. Beyond this
point. a larger collector experiences the law of
diminishing returns.
After the system is installed. check regularly
on the ouside racks, fasteners, mounts, and insulated piping. They should be repaired immediately if damaged by weather. Follow the
manufacturer’s maintenance instructions and
schedules faithfully.
Just as in space heating, there are passive and
active solar DHW systems. The passive group
includes batch or integral collector storage (KS).
thermosiphoning. and phase-change systems.
The active group-those powered by an electrically-driven pump-include antifreeze. drainback, draindown. and recirculation.
A third group, photovoltaic-powered systems. can be totally passive because the sun
provides the electricity to power the pump. But
the system uses a!! the same components as an
active system with the addition of the photovoltaics panel.
Passive solar domestic hot water heaters come
in a wide range of shapes, sizes, efficiencies,
and costs. Many can be totally passive by using
the sun’s power in the collection mode and city
water pressure in the storage and distribution
modes. Many are termed “passive” but are actually hybrid systems. because although their
collection modes may be passive, their storage
and distribution loops require auxiliary power
for pumping.
BATCH HEATERS
In the batch heater. the solar storage tank itself
is the collector and all the water is heated together in one “batch.” One or more pressure
tanks are painted black and placed inside a glazed.
insulated box. House water pressure draws the
hot water from the tank and takes it directly to
the user. or to the conventional water heater.
The batch design is the easiest to do yourself.
The inside tank of a conventional water heater.
minus the metal cover and insulation, do nicely.
You can even hang one or more tanks in a
sunspace or greenhouse.
Batch systems are the least efficient solar
DHW systems, but they are also the least expensive. If it is going to be used year round in
a cool climate. all the exposed pipes require
heavy insulation, and double or triple glazing
is recommended for the box. If installed in a
freezing climate. the batch heater is usually
drained before the first freeze and lies dormant
for the winter.
The “bread box” is a batch water heater that
retains the virtues of low cost and simplicity of
design. Variations of this water heater were first
used in the 1930s.
The bread box minimizes heat loss from the
stored hot water by means of an insulated box
that encloses the tank at night and during cloudy
weather. During the day. a top panel is raised
and the front panel on the south face is lowered
to expose the glass and tank to sunlight. The
inside surfaces of the panels and the box itself
are covered with a material such as alun,inum
foil to increase collection by reflection of sunlight around to the sides and back of the tank.
When the panels are closed these surfaces reflect
thermal radiation back to the tank.
The tank can be filled with water from either
a pressurized or non-pressurized source. Once
in the tank. the water is heated slowly but uniformly. Convection currents and conduction
through the tank metal distribute the heat
throughout the water, and little heat stratilication occurs. In unpressurized systems, the hot
water is nearly used up before the tank is refilled. In pressurized systems. cold replacement
71
The New Solar Home Book
The Bread Box: a simple and effective storage-type water heater.
water is drawn into the tank as hot water is
dmwn off and sonre mixing occurs. lfdual tanks
arc used. the bread box has less difticulty with
mixing. Hot water is drawn from one tank while
cold wuter flows into the other.
Frec/.ing in cold weather i> not much of a
problem becuuusr of the large volume of warm
water in the tank. However. the pipes must be
protected with insulation and electric helit tape
(it‘ the system isn’t drained for the winter).
Integral collector stordgc ( ICS) systems
the modern. m~nufacturctl vcrsicrn of the bread
box. The units are better insulated, and many’
manutastureres claim that if they ;rrc plunrhed
with polyhutylene pipe. they can withstand nwltiplc freezes. However. many Ioc;~l plumbing
codes still Jo not allow the use of pcAybutylene
pipe in potable water lines.
;irC
72
Batch and ICS heaters are really DHW “preheaters.” since their lower cfliciencies rarely
let them achieve the temperatures needed by the
average family. Their efliciency is hurt drdstitally by the high heat loss from the tank to the
ambient air at night-a loss other DHW tanks
located inside the house don’t experience. But
if the demand for hot water is concentrated into
the early evening when the water is its hottest
and before outside temperatures drop, their
maximum efticiencies can be reached.
ICS manufacturers are striving to make their
units more attractive by lowering the protile of
the collectors to look more like their “flat-plate”
collector counterparts. One manufacturer does
this by having many smaller stainless steel tanks
plumbed in series in one collector instead ot
one or two larger diameter tank>. The smaller-
Passive Solar DHW Systems
if the collector is near the south wall. That way.
the heat lost will flow to the rooms. If the tank
has to be outside. it should be shielded from
the winds and lavishly insulated.
Manufactured thermosiphoning systems are
installed as one unit, with collector and tank
together. They are available in the open-loop
systems described above, where water is used
for collection and storage. or in closed-loop systems.
In a closed-loop system. the heat transfer
tluid follows the same pattern. but passesthrough
a heat exchanger in the storage tank. The heat
transfer Huid is usually a mixture of glycol and
water or other non-freezing mixture. This protects the collector from freezing. and a wellinsulated tank protects the water within it from
freezing. But the piping from the cold water
supply and to the auxiliary heater in the house
must still be protected.
THERMOSIPHONING
WATER
To eliminate the large heat loss from the tank
HEATERS
above the collector. many manuf’acturers have
rcplrrd
the tank with a heat exchanger. The
The least complicuted type ot’ Ilat-plate Jar
collector is one that thermosiphons. It has no
heat exchanger is connected to the storage tank,
or between the supply line and the storage tank.
pumps. controllers. or other moving parts in the
If the collector heat exchanger (or tank. as in
collection loop. All that moves is the water. It
,he above case) is plumbed between a pressuroperates on the principle of natural convection:
the hot water rises from the collector to a tank
ilcd supply line and the storage tank, it is rotally
hjcatcd uhovc the top of the collector.
passive. If the heat exchanger (or tank) is conThe older thcrmosiphoning water heater denected only to the storage tank, then the storage
signs and site-built systems have a complctcl~
loop must hc pumped, and the system is considcred hybrid. When the heat exchanger above
scparute collector and storage tank. Insulated
pipe5 connect a tilted Hat-plntc c~~llectorwith a
the collector is connected to another heat exwell-insulated tank. In an open-loop system. the
changer in the remote storage tank, and an antiwater in the collector is heated by the sun, rises.
freeze solution is used. the system is completely
cntcrs a pipe. and tlows into the top of (1,~‘ protected against freezing.
In addition to lowering the heat loss from the
storage tank. Simultaneously the cooler water
at the bottom of the storage tank Ilows through
tank. the use of a heat exchanger instead of a
bulky ::mk above the collectors. lowers the proanother pipe leading down to the bottom of the
collector. As long as the sun ahincs. the water
tile of the collectors. It also eliminates the dead
cirCUlatc5 and becomes warmer.
load >,f mtjre than 40 gallons of water on the
roof.
Collector backs. pipes. and tank should all
he insulated, with the insulation around the tank
For thermosiphoning solar water heaters. the
collector location must allow placement of the
ax thick as your pockcthH)k permits. Four inches
storage tank at a higher level. For roof-mounted
01’ tihergla.ss isn’t 4xccssivc. If possible. pIact
collectors. the storage tank may be placed under
the tank indoors-in the attic if the collector i>
the roof ridge or even in a false chimney. The
on the root’, or in the room behind the collector
profiled collecton can be roof mounted like other
collectors, but the concentrated dead load of the
30 to 40 gallons of water on the roof, and the
nighttime heat loss. still remain.
Another manufacturer solves all three problems by using a phase-change ma!erial in the
collector instead of water. Copper pipe in tin
tubes runs through long rectangles of wax. The
wax melts during the day. storing latent and
sensible heat (see the discussion of phase-change
collectors below). It transfers the heat to the
water running through the pipes when there is
a demand. As it cools. the wax solidilies from
the outside in. insulating itself against high evening heat losses. The collector only holds 5
gallons of water at one time, and is only 5.5
inches high.
73
The New Solar Home Book
Closed-loop solar water heater with heat exchanger inside the tank.
roof structure must be strong enough to support
the weight of a full tank. One alternative is to
build a collector support structure on the ground.
detached from the house or leaning against it.
The collector then feeds an elevated tank located
beside it or inside the house. Another possible
location is on the roof of a lean-to greenhouse
built onto the south-facing side of a house.
PHASE-CHANGE
SYSTEMS
Materials can store IWOdifferent kinds of heat,
larent and sensible. LUIPI~~heat is stored when
a material changes phcrsc~from a solid to a liquid
and released when the material changes back to
74
a solid. Other phase-change materials change
from a liquid to a gas and hack. Latent heat is
stored at one particular temperature. For example. when one pound of ice at 32°F melts.
it absorbs I44 Btu. but the cold waler’s temperalure is also 3,7°F. When it freezes. it gives
up I44 Btu. but the temperature of the ice is
still 32°F. Stvrsihle heat is stored when a material is heated and rises in temperature. A single BIU is stored when you raise one pound of
water 1°F. Phase-change materials. such as
Freon. eutectic salts. or wax. change phase in
a tempcrarure range better suited to solar energy
systems than water’s 33°F freezing point and
212°F boilingc point. For example. some salts
melt at 84°F and store 75 Btu! I b-perfect for
Passive Solar DHW Systems
passive systems. Others melt at 97’F. and store
I14 Btu-perfect for low-temperature active
systems. (The pros and cons of phase-change
materials are discussed in Chapter 15.)
Traditional phase-change collectors use a refrigerant that changes from a liquid to a gas
when heated by the sun. The gas bubbles rise
in the collector, pass over the heat exchanger.
and condense to a liquid again as they give up
their latent heat. The condensed fluid flows by
gravity to the bottom of the collector to begin
the process again. Since the Huid is able to
change phase and store latent heat, more energy
is collected over the same temperature rise than
by thermosiphoning collectors, which store only
sensible heat. Manufacturers claim increased
eflicicncies of 30 to 30 percent.
Although the collectors cannot freeze. the
potable water side of the heat exchanger can.
Anti most phase-change systems still need to
!IXV~ their heat exchanger installed above the
top of the collector. Another drawback to phasrchungc refrigerant systems is the need for silversolder connections. which increases cost.
A new phase-change system has been developed that is t(~tally passive and yet still allows
the !+rayt’ tank to he located one story below.
An cvacuatcd closed-loop system is filled with
a w;itcr-alcohol heat transfer fluid that changes
phase thoilh) at a low temperuturc. Its change
in phase drives the Iluid through the loop, and
doesn’t transfer IlCilt hctwccn collectors and
\toragc (as in truditional phase-change collectors). The lluid boils. carrying a mixture of gas
and liquid to a riser across the top of the collectors. This causes a pressure difference in the
closed Il~~p. which the system naturally tries
to overcome. The liquid portion of the mixture
fl~~wsdown to the storage tank. gives up its heat
in the heat exchanger. and is forced up again
lo the collectors by the difference in pressure.
Mcanwhilc. the gas in the header travels to the
Vapor condenser. whet-c it condcnscs against the
cooler liquid returning from the heat exchanger.
l’hc comhincd liquid falls through a pipe to the
hotrom 01’the collectors.
The wax ICS unit dcscribcd in the section on
integral collector storage systems is another type
of phase-changesystem. But this time, the phasechange material is the collector’s built-in storage and not the heat transfer fluid. The collector
is plumbed between the cold water supply line
and the auxiliary DHW tank. As hot water is
drawn from the water heater, it is replaced by
warm water from the collectors. The collector’s
absorber is made of wax, encased in long extruded-aluminum canisters coated with a selective surface. Copper-fin tubes, which help
conduct the heat from the wax to the passing
water in the tubes, are embedded in the wax.
The wax holds onto its heat longer into the
evening than traditional flat-plate collectors can.
And it has a lower heat loss than other ICS units
because the wax insulates itself as it cools and
hardens from the outside in.
FREEZE PROTECTION
When air temperatures drop below 32°F. freezing water can burst the pipes or collector channels of a solar water heater. Less obvious but
more dangerous is freezing caused by radiation
to the night sky. Copper pipes in collectors have
frozen and burst on clear, windless nights whet1
air temperatures never dipped to freezing. The
heat lost by thermal radiation was greater than
that gained from the surrounding air.
To protect a passive solar DHW sy::tem that
uses potable water in locations where freezing
temperatures occur only now and then, heat tape
fastened to the back of the absorber is a simple
and inexpensive safeguard. Heat tape. commonly used to prevent ice dams on eaves. looks
like an extension cord and has a small resistance
to electricity. A thermostat turns it on when the
outside trmper:lture falls to 35°F. and the current flowing through it heats the absorber. in
more scverc climates where I( would be called
on frequently. heat tape would be too hard on
the electric bill. and the batch heater, ICS. OI
open-loop thermosiphoning system would have
to be drained for the winter.
7s
The USC 01’ pumps cun remove many of the ar~hitcctural constraints of thcmmosiphoningwater
heaters. A purnpd
system is commonly used
when piping runs would bc loo long or an clcvittcd tank is impossible. The penalty paid for
choosing iin Mivt! systt’111 is the additional lirst
costs of the pump and controls. and the electricity needed to run them. On the positive side.
;;ou have more freedom in the systcn~ layout.
A pumped systcnl can have 3 collccror on the
roof and the storage tank located qwhcre you
iikc. The hc~t advuntilge ix the additional us~t’ul
cncrgy produced per squxc foot of collector
xxx.
Active hol;rr domestic hot watt’r systc~lls prevent t’rccx-ups
in one ot’ three ways: by using
ibn antifrcc/,c solution; by draining the ct~llcc.
tot-s; or by circulatmg warm water through the
colltxtors. Biisically . syste111s are divided into
open- and closed-loops. Open-lcxjps circulate
polablc wuttx through the collectors. The colIcctor array is pumped directly zo the tank. Hot
Hater is drawn from the top of the timk. and
fold water from the supply line r~pli~c~s it iIt
the bottom. Coc~lwater from the bottom of the
tank is p~nlpd to the collecttws and solar-heated
water is returned through ;I dip tuhc to the middl~ 01‘ the tank. ;IS shokvn in the ligurc.
‘l‘wo lllgii~r problems cim plague open-loop
5ystl’nls: c~~rrosionillld trcc/.in~. I!‘ water qUillit)
76
is poor. or freezing is 3 coninion ivintor occurrence. you’d he better r~l’l’with iI closed-loop
system.
CIOS~Z~-IOO~
SySlcfllS
have sSepiIri\tccollector
and storrrgt‘ IOO~S that pass through iI hcut exchanger. l%h IOO~ mily have its O\VII punlp.
The heat transfer lluid in the collector loop c;ln
be distilled Wilkr. trtxtcd water (to rcducc carrosion). or an anlifrtxLc solution.
Active solar DHW sy~knls xc further classifietl by their specilic rncrhod ol’ from protcclion. Open-loops inc*ludc “rccirculrttion” and
“drilindiwn”
systems. Clos13I-l~~opsinclude
“drainback” and “i:ntifrtx~e.”
RECIRCULATION
l&circulation systems arc only rccc~mn~endt2cl
for ;trCiIswhere frtxzing tcmptxuures occur less
than 70 days ;Lyear. When iI t’rcc/.c snap switch
on the collector header scnscs the tcmpcraturc
hils dropped to 30°F. it “snaps” shut and sends
3 signal through the controller to the pump. ‘The
pump circulates warm water from the storage
tank through the collectors until the tcmpcrrtturc
of the snap switch rises over 50°F. when it opens
and signals the pump to turn off.
l’hcrc arc two mil.jor disadvimttigcs to this
type of system. First. it’s the cncrgy collcctcd
Active Solar
/
D CITY
WATER
W Systems
IA
In a pumped system the collector can be located above the storage tank.
HOT WATER
TO LOAD
WATER FROM COLLECTOR5
TANK
COLD WATER SUPPLY
WATER
TO IXKLECTO RS
The Oprn-Loop System: Polable water from the tank also serves as the
heat transfer fluid in the collectors.
I
77
FROM COLLEtTORS
COLD WATER
COLLECTORS
The Closed-Loop System: Treated water or antifreeze solution is circulated in the collection loop, which is separated from the storage loop
by a heat exchanger.
j 91 R VENT/
PRESSURE.
RELIEF
FREEZE-SNAP
SWITCH
>
WARM WATER
FROM
COLLECTORS
I
DI FFE RENTIAL
CONT~OLLE
R
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b
R
:..,:j:;1.‘;,‘;
;:lj,:,,‘:.1:;
HOT
STORAGE
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[email protected]
COOL
WATER
Tb COLLECTORS
WATER
l
t-4J
cmnhb
:.,z.z
WA I tn
rnt n
Y”.
b
SUWL,
E
Recirculation System: When the freeze-snap switch senses the approach
of freezing temperatures, it signals the controller to turn on the pump.
Warm water from the storage tank circulates through the collectors
until the snap switch temperature rises over 50”F, when it signals the
coatroller to shut off tbe pump.
78
Active Solar DHW Systems
during the day that keeps the collectors and
pipes from freezing at night. You don’t want a
recirculation system if that is going to happen
often. Second, the freeze snap switch is electrically powered. If the freeze is accompanied
by power outage, the snap switch won’t be able
to activate the controller and freeze damagecould
occur. Its one good point is that it uses water
-the heat transfer fluid with the greatest heat
storage capacity.
Like most active systems. the recirculation
system uses a differential controller, which senses the temperature difference between the collectors and storage. The controller typically turns
the pump on when the collectors are IO to 15°F
warmer than the storage tank. and off when they
drop to only 2 to 3°F warmer than the tank.
(You can buy a very accurate-but more
expensive-differential controller with a 5”Fon and IoF-off control strategy. This would allow the system to collect more energy.) The
collector sensor is mounted on the absorber plate
or collector header. and the storage sensor is
mounted on the side of the tank under the insulation or on the supply lint to the collectors
where it exits the tank.
A recirculation design for pressurized systems relics on thermally-activated valves and
city or well-pump water pressure to protect it
from frccling. The freeze-protection valves.
sometimes called dribble valves or bleed valves.
USCFreon or wax 10change phase and open the
valve at just above -lOoF. The valve is placed
on the return pipe leaving the last collector.
Warm water from the pressurized tank tlows
through the collectors and spills out the valve
onto the roof. Once the valve’s temperature rises
lo above 50°F. the Freon or wax changes phase
ac;rin and closes the valve.
‘fhc advantage is that the freeze protection
doesn’t rely at all on electricity. But the system
still dumps heat from the storage tank IO warm
the collectors. Two to three gallons of warm
water can pass through the valve bcforc it closes. And if the valve sticks--open or closrdit could mean ;Llot of water lost down the roof,
or daniagcd collectc~s.
DRAINDOWN
The first type of draindown system is basically
the recirculation system with the addition of an
electrically powered draindown valve. The valve
is normally closed, except when the electricity
to it is cut off. When the temperatures drop to
just above 40°F, the snap switch signals the
controller to cut the power to the valve. The
valve automatically opens, and water drains out
of the collector and supply/return piping. I! is
very important in systems that drain for freeze
protection to pitch the collectors and pipes to
If they don’t, trapped water
drain c-omnpletely.
could freeze and burst a collector riser or a pipe.
In pressurized draindown systems, another
valve that is normally open is installed between
the tank and collectors. When the snap switch
closes. it signals the controller to also cut the
power to the second valve so that it closes and
prevents city water from refilling the collectors.
When the snap switch opens, so does this second valve, 2nd the collectors refill.
In either case, you must completely refill the
system. and the pump must be large enough to
overcome the pressure in the loop. An air vent/
vacuum breaker. located at the highest point of
the collector array, opens to purge or draw in
air when the system tills or drains. Unfortunately, the major problem with draindown systems is that the air vent can freezeshut. preventing
the collector loop from draining.
An extra advantage of the draindown system
is that since there is no tluid in the collectors,
the system doesn’t have to wait as long in the
morning for the collectors to warm up before
the controller can signal the pump to start.
DRAINBACK
Drainback systems have two separate loops for
collection and storage with a heat exchanger
between them. The collector loop is tilled with
a small amount of water. which is either distilled or treated to prevent corrosion. The sytem
depends on a differential controller to activate
the pump and till the collector from a small tank
79
The New Solar Home Book
Al
AIR VWT/VACUUM
BREAKER
SAlAP
EZE
swiT-cH
WARM WATER FROM COLLECTORS
HOT WATER
TO LOAD
SENSOR
I
4
I
I
COOL
WATER
TO
COLLECfORS
I
4
A
FROM
COLD
WATER
SUPPLY
Draindown System: When temperatures approach 46”F, the freeze-snap
switch signals the controller to open the draindown valve.
(8 to 12 gallons) that holds the heat transfer
fluid. When the c:ontroiier shuts off the pump
at the end of the collection day. or when the
c‘oiieclor temperature approaches 4PF. the lluid
80
drains by gravity back into the tank. Just as in
the draindown sytem. you have to be sure the
collectors and piping are pitched properly to
drr?in.
Active Solar D
DRAINBACK TANK WITH
BUILT-IN HEAT EXCHANGER
AND COLLECTION PUMP
STORAGE
PUMP ( c
l
STORAGE TANK
FROM COLD
WATER SUPPLY
Drainback System: When the collector sensor signals the controller that
temperatures are approaching freezing, it activates the drainback module to open its valves to drain the collector and return/supply piping.
Several manufacturers make drainbuck moduies that include the insulated tank. heat t‘xchanger. pump. and differential controller. One
module has an air-vent/pressure-relief valve built
into it that allows air to enter and escape when
the system drains and tills. The tank completely
tills with water when the system drains. When
the pump tills the system. it creates ;L siphon
81
The New Solar Home Book
that helps pull the water through the loop, so
you can use a smaller pump. Unfortunately. the
entry of outside air can mean increased corrosion in the collectors, piping, or tank. Another
manufacturer’s module has a tank with enough
room for both air and water so that the system
can be completely closed to outside air. This
reduces corrosion. but it means you’ll need more
pump horsepower to overcome the pressure in
the loop.
If the water in the tank is distilled or treated
with a potable non-toxic corrosion inhibitor, a
single-walled heat exchanger can be used. But
if a toxic solution is used, the heat exchanger
must be double-wailed to prevent leakage into
the potable water supply. Double-wailed heat
exchangers are less efficient than single-wailed,
reducing the total system efticiency. They’re
also more expensive.
Drainhack systems require two pumps-one
t’or collection and one for storage-unless the
heat exchanger is part of the main storage tank.
Tanks are available with hr,eat exchangers inside
them or wrapped around them. which transfer
heat through natural convection currents in the
water inside the tank. If the heat exchanger is
separate from the tank. a circulator is needed
to pump water to it tram the tank.
The differential controller requires at least
IWOsensors. with additiona! stnsors and freeze
snap switches recommended for extra freer.e
protection. Drainback systems are second only
to anlifre3.e systems in popularity and freeze
pr:Gcction.
ANTlFRElME
AntifrccLe systemscircuialc a non-freezing heat
transtcr fluid through a cii)scd collector ioc~p.
The collectors transfer their heat to storage
through ;I heat exchanger. The primary advantage of these systems is that. dcpcnding on the
hcut transfer lluid. they can withstand freezing
0931 in the most severe climates. in addition.
a smaller circulator ta low horhepowcr pump)
can be used. reducing tirst costs and annual
82
operating costs. The disadvantages are slightly
lower efficiencies because of the less-effective
heat transfer fluids and the heat exchangers they
use.
There are many different types of heat transfer fluids for active systems. The most popular
is a propylene glycol and water mixture. Propylene giycol is a non-toxic antifreeze, so a
single-walled heat exchanger can be used. A
double-walled heat exchanger must be used with
ethylene glycoi, its toxic cousin.
Other fluids include silicone. aromatic oils,
paraftinic oils, and synthetic hydrocarbons. They
each have their drawbacks, ranging from less
desirable viscosities and lower flash points, to
corrosion and toxicity.
Another non-freezing heat transfer fluid is
air. It’s non-corrosive, non-freezing or boiling,
free. and it doesn’t cause damage when it leaks.
Unfortunately, its specific heat and density are
much lower than the others, and coupled with
an air-to-liquid heat exchanger, air solar DHW
systems are much less efticient. The higher material and installation costs for ductwork over
copper piping make them even less popular if
you’re only heating domestic hot water.
Just like drainback systems, antifreeze systems can have one pump or two. depending on
the location of the heat exchanger.
PV-POWERED
Solar domestic hot water systems that include
a photovoltaic (PV) panel arc included in this
section because they share many of the same
components as active systems. The major difference is that the PV panel replaces the differential controller. power from the electric
company. or both.
PV panels produce direct current (dc) eiectrinity irom the sun (see Chapter 16). The current can be used to signal the pump that there
is enough solar insolation to begin collecting,
or that there isn’t enough to continue. In this
case. the PV panel is only used to control. and
you :;tiii purchase the electricity to run an al-
Active Solar D
DIFFERENTIAL
CONTROLLER
HOT WATER FROM
HEAT EXCHANGER
EXCH4NGER
1I
HOT
WATER
PUMP
\\
COOL WATER
TO HEAT
EXCHANCZjER
Antifreeze System: The collection loop circulates an antifreeze mixture
through the collectors and the heat exchanger. The storage loop circulates potable water from the tank through the other side of the heat
exchanger.
troller must be high enough to take into account
temating current (ac) pump. If the electricity
the temperature of the absorber (which depends
from the panel is also used to power a dc pump.
heavily on the ambient temperature) and the
then the system can be considered passive in
storage temperature (which depends on prenature.
vious day’s collection and water use). There
Systems that rely on PV panels for control
could be little energy to collect if the absorber
have their design problems. The insolation level
panel were slow to warm up because of subat which the panrt sends a signal to the con-
83
The New Solar Home Book
freezing outdoor temperatures. Its shut-off insolation level must be set low enough to make
sure you can still collect the energy left in the
collector’s materials themselves at the end of a
warm day. It also shouldn’t allow the pump to
cycle on and off every time a cloud passes in
front of the sun. You have to carefully match
the PV panel to the pump to be sure it’s big
enough to produce the extra burst of energy the
pump needs to start.
The more insolation available the warmer the
absorber plate, and the more energy being produced by the PV panel to power the pump. The
pump begins circulating slowly, increasing the
temperature of the water returned to the tank.
As the insolation level increases, the collector
absorber temperature increases and the PV panel
produces more electricity. The extra power makes
the pump circulate t.tster, increasing collector
efficiency by keeping the fluid temperatures
lower. Unfortunately. unless care is taken to
make sure tank water stays stratified in hot and
cold layers. the faster pump rate could stir up
the water in the tank and send warmer water to
the collectors-negating the increase in efticiency .
PV-powered controls and pumps are usually
designed for closed-loop. pressurized systems.
PV panels arc expensive. and you need more
panels to produce enough power to overcome
the pressure in an open loop. A PV-pumping
system for an open hop can cost more than
three times as much as one for a closed loop.
And since the panels control the systems based
on changes in insolation and not tcnipcraturc.
a separate f’rccle-protection mechanism muht hc
added.
ufactured with the burner at the top. The cold
water supply line and ths collector supply line
are connected to the bottom of the tank. The
collector return line and the line to the hot water
demand are connected at the top. as long as
water entering the tank moves slowly, the hot
water will stratify above the coid, so that only
cool water goes to the collectors. You can buy
special diffusers to slow the entry of water into
the tank. One-tank systems save you money
since you aren’t buying an extra tank.
In two-tank systems, a tank for sr-!ar storage
is plumbed between the backup tank and the
collectors. the solar tank has no heating elemcnts in it. but only stores solar energy. The
cold water supply line enters the bottom of the
\:)lar storage tank. The top of the solar tank is
p!umbed to the bottom of the backup heater,
which has a heating element or burner at the
top. As hot water is drawn from the top of the
batLup tank. it is replaced from the top of the
solar-heated tank.
A two-tank system increases collector efhciency by returning cooler water to the collectors. Unfortunately, its standby losses are also
greater because of the increased storage volume. If the fJn:ily is small, you may find the
single-tank system adequate, as long as you take
precautions to stratify the hot water above the
cold.
ONE:-TANK VS. TWO-TANK
system:
CHECKLIS’I
It is very important that you follow the manufacturer’s installation. operations. and maintcII;IIICC instruction. But it will also be helpful to
ask yourself the i‘ollowing questions when dcsigning and installing a solar domestic hot water
SYSTEMS
~y~lclns USCthe existing domestic water
hcatcr tier fhc storage tank an(J;I backup hcatcr.
It‘ the tank uses electricity to heat the water.
rcmovc the lower healing element so that hcatcd
wutcr isn’t being pumped to the collectors. It
it’s gas-lircd. the tank ~nu~t bc specially nIan-
01wtanh
INSTAI,l~ATION
Arc the collectors oricntcd properly? 110they
have an unobstructed vicu bctwccn 9 a.m. and
3 p.m.‘. Have you tilted them within acccptablc
liniils?
Have you arranged the system components to
bc easily accessible for scrvicc and repair’!
l
l
Active Solar DHW Systems
HOT WATER
TO LOAD
HEATING
ELEMENT
COLD
WATER
SUPPLY
A one-tank solar DHW system.
HOT WATER
TO LOAD
HEATING, ELEMENT
SOLAR
BACKUP
I
A two-tad
COLD
TANK
WATERSJPPLY
solar DHW system.
85
The New Solar Home Book
. If you’re planning on mounting the collectors
on the ground, are they arranged so that they
don’t block drifting snow, leaves. and debris?
If you’re mounting the collectors on the roof,
will the roof be able to support the additional
load?
0 Is the collector frame designed to support collectors under the most extreme local weather
conditions’! Can the frame material resist corrosion? Are the roof penetrations caulked or
tlashed to prevent water leakage? Have you installed the collectors so that water Howing off
warm collector surfaces can’t freeze in cold
weather and damage the roof or wall’? In areas
that have snow loads over 20 pounds per square
foot or greater, have you made sure that snow
or ice sliding off won’t endanger persons or
property?
* Have you designed the system to follow the
local and national codes that apply? Have you
obtained the required building. plumbing. and
electrical permits’!
* Are all the pipes properly insulated to maintain system efticiency? Have you protected all
exposed insulation from the weather and ultraviolet rays? Do you have enough pipe hangers.
supports, and expansion devices to compensate
for thermal expansion and contraction? If you’re
installing a draindown system. are the collectors
86
and pipes properly pitched to drain all the fluid
in areas where fluid might freeze?
. Have you designed in isolation valves so that
major components of the system (pumps. heat
exchangers, storage tank) can be serviced without system draindowns? Have suitable connections been supplied for filling. flushing, and
draining’? Do you have temperature and/or
pressure relief valves to prevent system pressures from rising above working pressure and
temperatures?
Is the storage tank insulated well’? Are the
piping connections to the tank located to promote thermal stratification? Is the storage tank
properly connected to the backup water heater’!
. Are all system, subsystems, and components
clearly labeled with appropriate flow direction.
till weight. pressure, temperature. and other information useful for servicing or routine maintenance’? Have all outlets and faucets on
nonpotable water lines been marked with a
warning label’?
l
. Are you sure you know how the system operates. including the proper start-up and shutdown procedures. operation of emergency
shutdown devices. and the importance of routine maintenance’!Does the owner’s manual have
all instructions in simple. clear language?
Who tloc~.~ not rrtt~rtthrr
wlwtr
wtirtig
lw Iookecl
tlw ititerest
h-itlt brhic4
tit shel~~itig rock~.
~rppro~t~~li tit it (ul’c :’ It 11w.vthe trtrturtrl
or try
ycwrtiitig
1!/‘ tliirt portiott
01 oiu ttiost pritttiti\v
trtrwstot
~~hicli still .wr\~irxd
iti its. f‘rltttt the cii\*i~ w
ltm3i~ trtl~*m~~id
to ror$v
trttcl ltcrrrs~lis, cff‘ liticw
,grm.~ mcl strcm~. if
c~f’ltdtti
\~w*cti
ttocrrtls
stotw.~ lrttrl ti1c.s. At Itrst.
is to li\*c itr tlw
clottw.vtic. in ttiort’
letrr~~s. c!f‘ hit-k
md strcwliivl.
trtttl sliitrgl~~s.
NY ho,\.
trot \\hcrt
oprti irir. litid oi4r li\*cs
wttws
tlitrti u-c think.
Henry David
(!I’
(ff‘
it
urc
Thoreau.
L~‘tlllli’ll
Active wl;lr hating
and cooling sy5lcuis use
laryc cupanscs of tilted. glass-cot’mxi
surtaccs
to collccl solar energy and cmvt’rt it to heat. A
tluid---cithcr
ilir or iI liquid-carrics
this heat
through pipes or ducts to the living ;wxs or lo
\tt)riigc unils. As c~ppc~~d 10 the methods and
~~~tt‘lll~
discussccl in preious ChilptCl3.
ilCliVC
sykms
involve mm ccwplw uncl inkrdependent coniponcnts. I‘hcir clahocw cdlcctors. tluid
trilllSporl
s~stc‘nls. and hear sl<)riigt‘ cmtaincr~
rquirc
nctwcvk of conlrols. vulvcs. pumps.
t’ans, and hat cschangers. Frotii ;I cost standpoint. solar SpilCC heating and cooling sysknis
IlLI)
hC tnorc applI~priate
tar ilpiltIlllCllt
huiltlinss. schools. and oflicc buildings
than for sin?“IC t‘illllil~ d~bcllings. Bllr coordiniirion hctwccn
the owners. iirchitccr. cnginecr and ctmtractor
cm product an aclivc solar cncrgy systcni tipprOpriilk lo rhC rt3i&mtiid
SCillC. ‘IIlC IllilJOr role
in deigning
rcsidwtial
solar sprrcc heating and
Carolina svstcins is to “liwp
it sitiiplc.”
il
87
Simplicity
is the watchword in the overall dcsign of 3 solar energy systtm. It’s tempting to
design mart’ and more ccmiplrx system-always trying to squce/x one niorc ounce ot‘ pcrt’ormancc or a little mart’ comfort out ot’ them.
But this added compkuity
usually means higher
initial costs illld grcatt’r opcriiling
ilnd niaintenanct’ txpcnscs. It’s better to design il siniplc
system that may require the inhahitants to toss
iI lo? or two in a fire every now and then.
It’ you’re installing the systtm in ;I new house.
design the house to incorporate passive solar
systems to collect and store solar heat in the
walls and Floors. On ;1sunny winter day. enough
solar energy streams through ;t hundred square
teet of south-fncing
windows or &ylights
to
keep ;L well-insulated
house toasty wiu-m long
into the evening. And it’ the house has ;I concrctc
tloor slab or masonry walls insulated on the
outside. any excess heat can he stored tar use
lutcr at night. The solx heat gathered in the
iictive collectors can then he stored away until
it is nwll~ needed rather than squandered hrating the house during ;L cold. sunny day.
HEATTRANSFERFLUIDS
When designing an active solar system. you
must choose it Huid for transporting the heat.
There are usually two primary heat transport
loops: one links the solar collector to the heat
storage container: the other delivers the heat
t’r~m storilgt to the house. Liquids or gases rnu~
be used as the heat transfer Iluid in either loop.
Liquids includin, 0 IVilttT. ethylcnc flycOl. illIll
propylene glycol have predominuttxl.
Air is the
cmly gas that has been used. The following
critcria intlucncc the selection ()I‘ a heat transport
fluid:
Personal nerds and conitcxt.
* Compatihilit~
with the hackup
l
heating
sys-
tern.
Compatibility
with other mechanical
Climate t notrrhly I’rec1ing).
* RCliltiVicosi i initial.
oprilling.
nance 1.
Kclativc complexity.
. Long term reliabilitv.
l
devictx.
l
mainte-
l
When personal comfort requires only space
heating. forced-air systems are favored hecause
of their relative simplicity
and long lifetimes.
When domestic hot water must also be provided, cold inlet water can be prcdwotcd before
reaching the hot water heater where it is then
raised to its final temperature. This preheating
can he accomplished by passing the cold water
supply through a heat exchanger in contact with
89
The New Solar Home Book
STOUAGE
I
VALVE,
“‘m
--I
t/&AT
RJMP,2Q
A
\I
L ---
FAN
1
HEATEV
d---+
EXCUPA&~
Basic components of an active solar heating system. There are two primary heat
transport loops from collector to storage and from storage to the room.
the solar heated fluid in the return air duct to
the stor;qc bin or tank in ;tn air system. or in
the t;tnk itwll in ;1 liquid hysttrm.
II’ c~~cllin~ is ncedcd in addition to heating. ;I
Iicfutd \! \tcnl 14 ;I niorc likeI> choice. Although
uww rcsc;rrch ha hwn done v ith ;tir. most
~~~liir-p~~~~crcil cooling systcrns USCliquids. I‘hc
WIIK th~rlllc,~lvnariii~
and physical propcrtics
that labor licluid~ in convcnticwal
ct,oling units
;ll\cr I;rvcw thcln in \~diir ccloliiig systcnis. t\ir
4\ k ‘III\.
hobcwr.
ciiii hc used wcccssl’l~lIv
1’~~
4cjlllC I\ pC\ cll’CcN~ling. In arid pitI?\ ~J!‘tllc countr>. Ior ~uan~plc. coc~l ntght air can hc hllwn
through ii r(bcC,hctl and the co~,liic\\ 5tiwcd f’or
1la> t1111c ll\C
I‘hc nlcthc~il of di~trihutin~
heat or ~~~~lnc~s
to the l-o~wt4 Itl;i! help ~cw dctcmiinc
Iihich
11uId\ to u\c. f-clr~xxl-air circui;ltlcN~ i\ ii1041 c‘~binptll~le
\+ Ill1 ;iir \>ktcii!s. fG)rccd-air cx~llcctors
ktorc their heat in hin4 lillcd \iith roc.h4. C\‘hcn
the hour calls for hciit. rooiii air I\ hlrw 11through
the njchhin 10 ilcli\cr the heat to the I!\ inp S~;ICCS.
f3ut Iorcctl-air dcli\,cry \> 4tctils ciiii ;1140Iw tidal
Li Ith Iicfuid scalar cx~llcc.t~w\. ~‘;iirII or C.CN)I ~;ltcl
1rcwl the \t(jr;t!:C t;lnh IS Pascal thrcwsh f’;u-coil
units or hc;tt cuchanycr4. \i her-c the air hlou n
;tc’ro\\ thcnt i\ hcatctl or coc~lcd ;tld deli\ crcd
to tflc hc~llsc.
k~~iiilsc
01. the hot or cold ~fr;if’ts
that occur. I~wccLair hc;iting and co~\liiig \b\90
terns 1tm htz uncotnt‘ort~hlr
to the people using
than and they rnlrst be designed caret’ully. But
of grt’ater simplicthey do have the adva~ltagt’
ity.
M,)st radiant hciiting systenls
USC kvater to
transport the heat. but some USC hot air circu!rrtcd through wall. wiling. or floor panels. Hot
water radiant S~S~CWS. such iis hawhourd radiators, work well with licfuid systcrns.
H\)t water
l‘rom hish-tcmpcrature
collectors can circulatt’
directly through ;L hascfward hc;rting system or
hc sent to the htxt storage tank. The main dis~~dvtintit~t’ is the high ( 130°F to Ic)O”F) water
tcnlperaturcs. The higher the \vatcr tcmpcratute
used, the lower the overall cfticiency
of’ the
s\ll;lr hcatins systt’m. Stcitnl hcatq
syst~n~+ arc’
$CllCKl!lY
incc~~npatihle with solar collccticw he,
c;iusc of the poor cjpcrating cfficicncics
ol‘ colIcctrws ;It those high tcnipcraturt3
(with the
cuccpticjn of‘ the niort’ txpcnsivc concentrating
;Illtl c\~;lCUatcd tlthc collect~ws). Bllt IllilIIy
desi~iicrs arc illstilllillg
liquid systems IhiIt circulittc their fluid through polyhutylcnc
tubes in
concrctc floors. I‘hc concrt’tc stores the hcilt and
radiates it to the living space.
active~h;ir~~:passiv~-ilischitrF~
systems xc htxxwiinp
wry popular since they dlm’t require the higher
tcmpcr;tturcs thitt lxtscho;trd hwting systems dc-
m3~
Illand.
COMPONENT
OPTIONS FOR ACl’TVE HEATING
SYSTEMS
Collector Fluid
Heat Storage
Heat Distribution
Air
Building itself
Natural convection
Rocks or gravel
Tbermosiphoning
Small containers
Forced convection
of water
Small containers of
phase-change materials
Air feri radiant panels
or concrete slabs
Building itself
Natural convection
Large tanks of water
or other liquids
Baseboard radiators
or fan-coil units
Water-anrifreeze
solutions
Polybutylene tubing
in concrete slabs
Water-fed radiant panels
or concrete slabs
Oil and other liquids or
phasechange materials
Large tanks of water
or othnr liquids
Forced convection past
water-to-air heat
exchangers
or heat pumps
Water
The amount of space allotted to heat storage
is often a citicaf factor in the choice of fluids.
lintil phase-change materials are cheap and reliable, the mam choices for heat storage are
water and rock. Water tanks occupy from one
third to one half the volume of rock beds for
the same amount of heat storage. This fact alone
may dictate the choice of a liquid system. The
options available for collection.
storage. and
distribution
of heat are summarized in the accompanying chart.
A choice of heat transfer fluids is available
for residences-but
not for larger buildings. The
larger the solar-heated building. the greater the
amount of heat that must travel long distances.
If the fluid temperature is kept low to increase
collector efficiency. either a heat pump is needed
!a raise the delivery fluid temperature. or more
fluid must be circulated to provide enough heat
to the building.
Liquid heat delivery is better
suited to large buildings because piping occupies less valuable space than ductwork. For air
to do a comparable job. large ducts or rapid air
velocities are necessary. Both alternatives are
usually expensive. and the latter can be very
uncomfortable.
Climate rnay dictate the choice of tluid. In
cold climates. where a house may require only
heating, air systems could he the most likely
choice. When a liquid system is subject to freezing conditions. an antifreeze and water solution
may be necessary. An alternative is to drain the
water from the collector when the temperatures
approach freezing.
First costs for materials and installation
are
also a factor. Storage and heat exchangers (or
the lack of them) can cost less for air systems.
Local labor economics often favor the installation of air ducts over water pipes. But don’t
underestimate the cost of fans and automatic
dampers.
Air systems can be cheaper to maintain because air leaks are nowhere near as destructive
as water leaks. Antifreeze
solutions in liquid
systems deteriorate and must be changed every
two years. True enough. the cost of changing
91
The New Solar
antifreeze in cars and trucks is minimal. but ;L
residential liquid-type
solar heating system requires up IO SO rinws as much antifreeze as a
car. Air systems. on the other hand. can be more
costly to operate than liquid systems because
more electrical power is required to move heat
with air than with water.
In all Huid transport systems. the network ot
ducts and piping should be kept simple. Pipes
or ducts should k wdl insulated and as sijort
;I\ pohsiblc.
AlK SYS’lXM DESIGNS
I’hc very 4mpks1 active solar heating system
has collector\ that functicrr only nhcn the sun
is shining and the house needs hritt. Air is ducted
from the house to the collector. heated by the
sun. and fan-forced
into the room. The only
heat storage container is the fabric of the house
itself-and
the heavier it is. the better. The fan
operates when the collector temperature is warmer
than that inside the house. It shuts off when the
collector cools in the late afternoon or when
room temperatures become unbearably hot. The
more massive the house. the more heat it can
store before temperatures get out of hand. and
the longer it can go without backup heating.
Thi\ type of sys!em eliminates the controls,
ductwork, and storage unit of the more expensivc systems. and i5 becoming very popular.
Another simple active system delivers solar
heated air to ;L shallow rod storage bin jud
kneath the house Hoor. Heat can tlou up to
NO HEPr NEMD
IN k%DMS
COuECTOQ
When the sun shines, the collectur heats the storage. Storage is hypassed if the rooms call for he&
WEQT NtXDED
1
I
When the sun isn’t shining:. stored heat is delivered to the rooms as needed. If there is
no heat in storage, the furnace comes on.
92
Space Heating and Cooling
the rooms by natural convection through grilles.
or if the rockbin is below a concrete slab, the
heat is conducted through the tloor and radiates
to the space.
More storage for longer periods without sun
is possible with another fan added to blow solar
heat from a storage bin to the rooms. The fan
Jra\\ s cool room air through the storage bin and
blows warm air back to the rooms. The backup
heater (which can be ;L wood stove. electric
hratcr. or an oil or gas furnace) an be in line
with the \olar storage. or be completely indepcndtznt of the solar heating system. Ideally hcrtt
tram storqt’ isn’t need4 when the sun is shining hccrruse the solar heat gain through windows
kps
the house warm. The heat from the colkctors can be stord for later USC. But if the
house cdls for heat. the collectors bypass the
storage loop and supply it directly to the house.
Whrn the \un isn’t shining and the house needs
heat. \olar hcut is drawn from the siorugt’ bin
--~-il. availahlc. If not. the backup heater is put
10 USC. There art‘ four possible modes of opcration for this systt’m and they arc‘ deti~iltd in
the di;lgr;lms.
In larger houses. it’s expensive to have scpXiltC dclivtq
4ystcms for solar and ilUXiliilr>
heat. Intcrgrirting the two into ;L 5inple delivtq
\y3tcni rcquircs extra dampers and controls but
can bc chcilpcr in the Ions run since no nc\\
duct\vork
I\ uddcd. In ilny air systt’m. estra
tlucting and dampers should btz kept IO I minimum.
R&WANT HEATIN
(IRNAWVE.
FOQCED
CON&FCTUW)
Piping system design for a simple liquid system.
IJQUII~ S&STEM DESIGNS
llsuidly
;L liquid solar energy system is not ils
~~onomicd ils ;tn air system for heating 3 singletanlily Jwdlinp.
But with larger dwellings and
increasing nods for domestic w;Itcr heating imd
ilhs~Wptiol3
tiding.
il liquid system bcComes
mot-c feasible. It doesn’t hilvc to be elaborate.
A very basic liquid system for non-frce!ing
climato is illustrated in the first of the two ticcl~mpan~ing Ji:tgrams. Water from the storage
Ftzv?cw
CcvvvECTlQN
(UTFLZPIIATNI:
~IPNT
UEATNC)
A liquid system designed for forced-convection
heating and preheating of domestic hot water.
tank is heated by the auxiliary.
if necessary.
hefore delivery to the baseboard radiators or
radiant heating panels. Only two pumps are
93
The New Solar Home Book
needed to circulate the water through the two
heat transport loops.
A somewhat more complex system is illustrated in the second diagram. Because of the
threat of freezing. a water-glycol
solution circulates through the collector and surrenders its
solar heat to a heat exchanger immersed in the
storage tank. Heat is distributed to the rooms
by a warm air heating system that uses a fan to
blow cool room air past a water-to-air heat exchanger. Cold inlet water from a city main or
well pump passes through yet another heat exchanger immersed in the storage tank. This water
is preheated before it travels to the conventional
hot water heater. This type of system can be
si/.ed only slightly larger than a solar DHW
system for each fan-coil unit connected to it.
An active solar energy system can be much
more involved than these simplified diagrams
indicate. Additional pipes. controls. and valves
arc required for the various modes of operation.
Each heat exchanger degrades the overall performancc of the system. The use of a heat exchanger substantially
increases the collector
operating trmperrrture and lowers its cfliciency.
The greater the number of heat exchangers in
a system. the lower the collector etliciency. Even
more pumps. t’ans. and heat exchangers are
necdcd than shown in the diagrams if solar ahsorption cooling is desired.
SWIMMING
POOL HEATING
Solar collccton can be used to heat swimming
pool wutcr too. For outdoor swimming pools.
inexpensive unglalcd collectors can extend the
swimming
season carlicr into the spring and
later in the f4l. How much longer you’ll be
able to swim depends on how cold ir is in your
arca. Glazed collectors can be used in freezing
climates to heat water year-round for indoor
hwiniming
pools.
e)ngla/.ed plastic or metal collectors perform
well because they operate at low temperatures,
usually in ranges that are only IO to 20°F above
the \ummcr outdoor temperature.
Since the)
don’t need glass or plastic covers, they are less
expensive than collectors that produce higher
temperatures for space heating, cooling. and
domestic hot water.
Because the pool water can be very corrosive
when proper pH and chlorine levels aren’t maintained, polybutylene
or PVC plastic collectors
are recommended more often for open-loop systems than copper collectors. They are more resistant to corrosion, but less resistant to ultraviolet
radiation. Closed-loop systems-where
treated
water or antifreeze is spearated from the pool
water by a heat exchanger-are
recommended
for metal collectors to avoid corrosion. But the
closed-loop system can be less efficient and more
expensive since it requires an extra pump and
heai exchanger.
Most solar swimming pool heating systems
have :lpen loops and use the swimming pool’s
existing filtration pump. When the collectors are
hot enough. the differential
control signals a
diverter valve to s(:nd the pool water through
the co,~tors
before returning to the pool. The
diverter valve i\ located after the tilter so th:lt
only clean water passcq through the collectcr:s.
Closed-loop systems have an extra pump to circulatc heat transfer fluid. and use the filtration
pump lo circulate pool water only.
PV-controlled
systems are available for pool
heating. At a preset time in the morning. pool
water begins circulating through the filter. When
the intensity of the sunlight reaches a preset
level. the PV panel signals the controller
to
divert water through the collectors. When pool
water reaches a preset temperature. the diverter
valve bypasses the collectors and sends the water
straight back to the pool. If pool water drops
below the desired temperatrqre. the diverter valve
sends the u*dter bsck through the collectors. At
the end of every day. when the sunlight drops
to a preset level. the valve diverts the pool wa!er
back to the pool filtration loop again. Finall; at
a preset time. the circulation pump turns off.
In hot-arid climates. the cycle can be reversed IO cool the pool during the summer. Pool
water is circulated through unglazed collectors
at night to radiate the heat to the night sky. A
eating and Cooling
COLLECTORS
DIVERTER
,
30LAR
LOOP
LTER
n
PUMP
VALVE/
Solar swimming pool heating.
timer turns the pump on al night and off in the
morning for a more refreshing water temperaturc.
Another pool heating system has polybutylene pipes buried in a poured concrete slab around
the pool. Pool water is circulated through the
tuhes. cooling the solar-heated patio as it warms
the pool water.
Ho\v large a collector area is needed will
depend on the kind of coiiector choosen. Unglazed plastic collectors usually have an area
one-half to three-quarters the pool area. Glazed
collector systems require much less: 40 to SO
percent of the pool area. Patio systems must be
bigger because they are less efticient: about I30
percent of the pool area.
CONTROLS
One set of controls governs the delivery of
heat (or coolness) to a house from the collector.
heat storage. or backup heating Ior cooling) system. Its operation is determined by the needs
of the household and the limits of the entire
system. In general. the thermostat governing the
energy flow from storage can operate at a different temperature level than the thermostat on
the backup heater. Often a two-stage thermostat
is installed. The tirst setting might be at 70°F
and the second at 68°F. If the heat storage cannot maintain 70°F room temperatures. the backup
system springs into action when the temperature
falls below 68°F.
Controls to govern collector operation are
relatively simple and are readily available. Most
of these controls determine collector operation
by comparing the collector temperature and the
storage temperature. One temperature sensor is
placed directly on the absorber. The other sits
in the storage tank or near the return pipe to the
collectors. Customarily the collector pump starts
working when the collector is 10 to 15°F warmer
than the storage. For air systems, a temperature
difference of as much as 20°F may be needed
before the circulation
fan is triggered. A time
delay of about 5 minutes is necessary to prevent
the system from turning on and off during intermittent sunshine. Some liquid systems may
need controls that prevent liquid temperatures
from rising to the point where pressures can
cause piping to burst or degrade the heat transfer
fluid.
95
The New Solar Home Book
Photovoltaic
panels, that convert solar energy to electricity, are also being used to control
and pump solar systems. The photovoltaic panel
turns the pump on when solar insolation reaches
a certain level and turns it off when it falls below
another level.
PERFORMANCE
AND COST
The tradeoff between performance and cost is
crucial to the design of any solar energy system.
The performance of a system is measured by
the amount of energy it can save a household
per year. The dollar value of the energy saved
is then compared with the initial (and operating)
costs of the system. The initial costs must not
get so high that they can never be recouped over
the life of the system. One doesn’t have to be
quite as careful in the design of conventional
heating systems because the fuel costs are far
and away the major heating expense. But the
initial costs of an active solar heating system
are usually so high that more than IO years of
trouble-free operation are needed before the energy savings make it a good investment.
SOLAR COOLING
Active solar energy systems can also cool a
house during the summer. And the sun is usually shining the brightest when cooling is needed
most. The hottest months and times of day occur
at times of nearly peak solar radiation. Systems
that provide both heating ~lnd cooling can operate the year round-with
additional fuel savings and a shorter payback period.
Solar cooling seems paradoxical.
How is it
that a heat source can be used to cwol a house?
One answer is that solar energy is also a source
of power that can move room air in ways that
enhance comfort.
Substantial cooling can be obtained by using
nocturnal radiation to cool the storage container
at night. Warm objects radiate their heat to the
cooler night sky-particularly
in arid climates.
96
Warm air or water from storage is cooled as it
circulates past a surface exposed to the night
sky. The cooled fluid returns to the storage container, which is cooled in the process. The next
day the storage is used to absorb heat from the
house.
Solar collectors can provide the heat needed
soby an absorption cooling device-making
lar-powered air conditioners a distinct possibility. An absorption cooling unit uses two working
absorbent such as water, and a refluids-an
frigerant such as ammonia. Solar heat from the
collector boils the refrigerant out of the less
volatile absorbent. The refrigerant
condenses
and moves through a cooling coil inside the
room. Here it vaporizes again, absorbing heat
from the room air. The refrigerant vapor is then
reabsorbed in the absorbent, releasing heat into
cool water or the atmosphere.
Unfortunately,
most absorption cooling devices work best with fluid temperatures between
250°F and 300°F. The lowest possible working
fluid temperature
that can be used is about
l8O”F-where
flat-plate collectors have sharply
reduced efficiencies.
And the collectors have to
operate at temperatures about 15°F to 20°F above
this lower limit.
If 210°F water is supplied by a collector. the
working fluids will receive solar heat at 180°F
and the water will return to the collector at 200°F.
On a hot summer day. a square foot of collector
might deliver 900 Btu-or
about 40 percent of
the solar radiation hitting it. About 450 Btu will
be removed from the interior air, so that a 600square-foot collector can provide a daytime heat
removal capacity of about 270,000 Btu or 30.000
Btu per hour.
Solar collectors designed for absorption cooling systems are more expensive than those used
only for winter heating. But substantial fuel savings are possible if the same collector can be
used for both purposes. Concentrating
collectors and evacuated-tube
collectors are particularly well suited to absorption cooling because
they can supply high temperatures at relatively
high efficiency.
Almost all absorption cooling
equipment requires liquid collectors.
eating and Cooling
Complete packages are available that combine solar space heatmg and domestic hot water,
and evaporative
and desiccant cooling. The
cooling cycles are not solar, but first costs are
lower for the whole system since it comes as a
manufactured unit.
Absorption Cooling Principles
Just like window uir conditioners and heat pumps,
un uhsorption cooling device uses the ewporution of u fluid refrigerant to remo\v heurfrom
the uir or wuter being cooled. But window uir
cw1ditiorter.s und heut pumps use /urge yuuntiries of ektricity to compress this iwporuti~d
jiltid so thur ir cvmd~v~st~.s
rrnd releuws this hear
. ’ ’ The condensed jhki then wlo rhe ’ ‘out.sid~~
twn.s to th4 rlwpwxting cwi1.sfor trnothi>rcyie.
In un ui~.sorption cooling cyie. (hi> PINTOruted wj-iglwmt is ubsorbed in a ,SPCO~I~ fluid
*‘ub.sorbwt. ” The rcwiting solution
l*~liir~d
the
is pirmprd to rhr ’ ‘ril-SrnPrcrtor’ ’ by (I !ow-pohw
i?ttitll). Here. u .sourct~of’ heut- \r*hich cut1 bt’
&J.s.sii,fiwi or .soiur twergy-di.stii1.s thr w/i-igcrunt ji-om the soirrtirvi.
The less ~~oiatiieabsorbent remuins a liquid
and returns to the absorber. The refrigerant
liquid returns to the evaporating coils-where
it evaporates and cools the room uir. completing rhe cycle.
Absorption cooling devices cun use hot fluid
*from u soiur collector to boil the refrigerant
from the absorbent. U~~fortunatel~. most ab.sorption cooling devices work best \rith fluid
temprrutures behveen 250°F and 300°F. Fiatpiutc cwlitwors ure inejficient at such high temperutwes, but wnctwtruting and elwc.lctrttJd-tlrbp
m’iemws can prodme thestj ~en~peraturesPNSily . [f’rheir c0.st.sund cwmpit~si~ cun be brought
down. tht>Fmu! somrdtry jind un upplicvrtio,l in
soiur ubsorption cooling.
Absorption Cooling Cycle.
The primary component of an active system is
the solar collector. It converts the sun’s radiant
energy into useful heat energy that is carried
into the house by a fluid. The distinguishing
feature of a Hat-plate collector is that the sun’s
energy is absorbed on a flat surface. Flat-plate
or
collectors fall into two catagories--licluid
(Gr-according
to the type of nuid which circulates through them to carry off the solar heat.
A new circulatory
fiuid-phase
change-falls
into the liquid catagory, since it also circulates
through tubes.
The basic components of a liquid flat plate
collector are shown in the diagram. The absorber stops the sunlight. converts it to heat,
and transfers this heat to the passing liquid.
Usually the absorber surface is black to improve
efficiency.
To minimize heat loss out the front
of the collector, one or two transparent cover
plates are placed above the absorber. Heat loss
out the back is reduced by insulation.
All of
these ccbmponents are enclosed in a metal box
for protection from wind and moisture. Most
contractors will buy a manufactured collector.
but they should look closely at what goes into
them before they buy. The materials and design
of a collector are crucial in determining
its efficiency and how long it will last.
98
There are two types of absorber designs-each
characterized by the method used to bring liquids in contact with the absorber plate. The tirst
category includes open-faced sheets with the
liquid
flowing
over the front surface. The
Thomason absorber. with water Howing in the
valleys of corrugated sheet metal, is a good
collectors. The
example of these “trickle-type”
second more papular category uses tubes connected to a metal absorber plate. A variation on
the tube-in-plate is the extruded plastic collector
used in swimming pool heating.
I pJSVLRTl6N
A liquid flat-plate collector.
Liquid Flat Plate Collectors
soldered, welded. wired, or clamped to them.
Thousands of experimenters
all over the world
have struggled to develop cheap, effective
methods of bonding tubes to plates. Good thermal bonds are of paramount importance. Most
commercially
available collectors have copper
tubes soldered to copper plates.
TUBE SIZING AND FLOW PATTERNS
z3mFTMETPL
Thomason’s trickle-down
absorber.
A typical sandwich-type absorber.
‘l‘hc open-t’acc Thomson
absorhcr shown in
the diagram has the advantage of simplicity.
(‘001 water from storage is pumped to a header
pipe at the toll of the collector and Ilows out
into the corrugation5 through holes on top ol
each valley. A gutter at the base of the collector
sathers the warm water and returns it to the
storasc tank. Its clearest advanta?e is that it is
self-dmining
and needs no protection against
c~,rro+n or freezing. One disadvantage is that
condensation can form on the underside of the
covc’r plate. Another is that the trickling water
may eventually erode the black paint.
In most of the early experimental
work with
the absorber plates conIlat-plate collectors.
sisted of flat metal sheets with copper tubes
The choice of tube size for an absorber involves
tradeoffs between fluid flow rate. pressure drop,
a:id cost. If cost were the only factor. the tube
diameter would be as small as possible. But the
smaller a tube. the faster a liquid must travel
through it to carry off the same amount of heat.
Corrosion increases with tluid velocity. And the
faster the fluid flows. the higher the pumping
costs.
Typically,
the ri.srr.s (the tubes soldered directly onto the ahsorber plates) are I/2 inch in
diameter. hut this ultimately depends on the size
of the system and the liquid being used. I’he
Iwcrrkvx (those tubes running along the top and
hottom of the plate) are 34 to I inch in diameter.
The pattern of the tubes in the absorber plate
is also important to the overall performance of
the collector. Strive to attain uniform fluid Row.
low pressure drops. ease of fabrication. and low
cost. Uniform tluid flow is the most important
of these. “Hot spots” on the absorber plate will
lose more heat than the other areas--lowering
overall efficiency.
Since most applications
call for more than
one collector. you will have to connect a numher of independent collector panels together.
Series or parallel networks are the simplest.
Again. the important criteria are uniform fluid
Ilow. low pressure drop. and the ability to fully
drain the liquid in drainback systems. A network of collectors piped in series has uniform
How but a high pressure drclp. while a parallel
hookup has just the opposite. For a large num-
99
The New Solar Home Book
Tips on Corrosion Prevention
Because o-x-ygencan be veryvcorrosive under
certain conditions, air should be preventerl.from
entering the heat transfer liquid. This can be
veT dij%ult in self-draining sytems.
The pH qf the transfer liquid (a measure oj
its acidity) is the most critical determinant of
corro.sion. Liquid.s coming in cvmtac’twith aluminum must be neutral-with a pH around 6
or 7. Any deviation. whether hurler (more acidic)
or higher (more basic) .se\‘ert~!\’increaxs the
rate oj‘ corrosion. The pH must be measured
frequrntly to prelvnt de~~iatinn.s.fr~~ni
the norm,
Antifreeze should be replaced at 12-mcnth intervak.
Swtems in which the tran.$er liquid$ows in
contact with a number of different metals are
susceptible to galwnic corrosion. If possible,
you should a\*oid using several diflerent metals.
In particular. aluminum should be isolatedfrom
cwmpo~lent.smade from other metals.
PARALLEL
Reverse return piping systems help balance the flow through
the collectors. The first collector plumbed to the supply
line is the last plumbed to the return line.
ber of independent collector panels. a seriesparallel network is your best bet. In any network. the exterior piping should he at least I
inch in diameter and well-insulated.
Many collectors are available with integral top and bottom headers. Connections
are made directly
between collectors, reducing pipe costs and heat
loss.
The pluml>ing configuration
most often used
is the reverse-return method. that follows the
lirst-in. last-out rule. The first collector to receive liquid from storage is the last connected
to the return to storage.
ABSORBER PLATES
Absorber plates are usually made of copper or
aluminum. But plastics are taking over the lowtemperature applications. such as swimming pool
heating systems.
A metal need not be used for the absorber
plate if the liquid comes in direct contact with
100
every surface struck by sunlight. With almost
all liquid systems now in use. however. the
liquid is channelled through or past the plate.
Heat must be conducted to these channels from
those parts of the absorber that are not touching
the fluid. If the conductivity
of the plate is not
high enough. the temperatures of those parts
will rise, and more heat will escape from the
collector-lowering
its efficiency.
To reduce
this heat loss. the absorber plate will have to
be thicker or the channels more closely spaced.
With a-metal of high conductivity
such as copper. the plate can be thinner and the channels
spaced further apart. To obtain similar performance. an aluminum plate would have to be twice
as thick and a steel sheet nine times as thick as
a copper sheet.
The accompanying
graph illustrates the variation in absorber efficiency
(the “efficiency
factor”
gauges the deviation from optimum)
with tube spacing for various types and thicknesses of metals. Cost rises jkster than efficiency for increasing
thickness of copper.
Liquid Flat Plate Collectors
100
ABSORBER PLATE
Thicknezs
Type
Copper
90
0.040”
Aluminum
0.040”
Copper
0.020”
Aluminum
0.020”
Steel
0.040”
Steel
0.020”
60
TUBE DIAMETER
5c
I
1
1
I
= %”
1
I
1
6
4
5
3
2
TUBE SPACING, INCHES (center to center)
I
The variation in collector efficiency with tube spacing and absorber type.
Optimum cost and efficiency is achieved with
a 0.0 IO-inch-thick copper sheet with tubes spaced
at intervals of 4 IO h inches. Copper has become
the most popular absorber choice in manufactured collectors.
ABSORBER COATINGS
AND COVER PLATES
The primary function of the absorber surface or
coating is to maximize the percentage of sunlight retained by the absorber plate. Any surface
reflects ~rd absorbs different amounts of the
sunlight striking it. The percentage it absorbs
is called its crhsorptcrrtw (a). Enritttrncx~(EJ is
the tendency of a surface to emit longwave thermal radiation. An ideal absorb,:r coating would
have CL = I and E = 0. so tha: it could absorb
all sunlight striking it and emit no thermal ra-.
d&ion.
But there is no such substance. and we
usually settle for Hat black paints. with both OL
and E close to I.
There are a few substances called .sdrctir*c
s~rrfir~s which ha\ e a high absorptance (greater
than .OS) and low emittance (less than .7). Selective surfaces absorb most of the incident sunlight but emit much less thermal racliqtion than
ordinary black surfaces at the same temperature.
Collectors with selective absorber surfaces attain higher collection efticiencies at higher temperatures than normal collectors. But they are
necessary for systems which operate at temperatures below I W’F.
The absorber coating should be chosen IOgether with the collector cover plate.They have
similar functions-keeping
the solar heat in-and complement each other in a well-designed
collector. For example. a selective surface with
The New Solar Home Book
PROPERTIES
OF SELECTIVE SURFACES FOR SOLAR ENERGY
APPLICATIONS
- -- _._
Surface
Absorptance
for Solar Energy
“Nickel Black” on polished Nickel
“Nickel B!ack” on galvanized Iron*
CuO on Nickel
CojOj on Silver
CuO on Aluminum
Ebanol C on Copper*
CuO on anodized Aluminum
PbS crystals an Aluminum
0.92
0.89
0.x I
0.90
0.93
0.90
0.85
0.x9
Emittance for
Long Wave Radiation
0.1 I
0.12
0.17
0.27
0.1 I
0.16
0.1 I
0.20
*Commercial processes. (Source: Duftie and Be&man. Soiur Ener,cp Thermal Processes.)
a single cover plate is usually more efficient
than flat black paint with two cover plates. The
accompanying graph compares the performance
of Ilat black and selective surfaces for one and
two cover plates. For collector temperatures below 150°F. a second cover plate may be supertluous. blrt for temperatures above I 80°F (for
process heat or absorption cooling) a second
cover plate or a selective surface may be necessary. For temperatures below IOO”F. a sclcctivc surfxc
performs no better than flat black
paint.
C’o\w /~/tr~c~sarc transparent sheets that sit
about an inch above the abscjrber. Shortwave
sunlight penetrates the cover plates and is convcrted to heat when it strikes the absorber. The
cover plates retard the escape of heat. Thq
absorb thermal r&iation from the hot absorber.
returning some of it to the collector. and create
a dead air space to prt3ent convecticjn currents
from stealing heat. Commonly used transparent
i:latcriuls
include glass. libcrcelass-reinti,rc~d
polyester. and thiu plastics. They vary in their
ability tn transmit sunlight ancl trap thermal radiation. They also vary in wtaight, east of himdling. clurability. and ctjst.
Glass is clearly the favorite. It has very pooch
solar tnlnsmittance and is fairly opaque to thermal radiation. Depending OK. the iron content
of the glass. between XS and 9h percent of the
sunlight striking the sur:‘;Lce of l/X-inch sheet
102
of glass (at vertical incidence) is transmitted. It
is stable at high temperatures and relatively
scratch-and weather-resistant.
Glass is readily
available and installation
techniques are familiar to most contractors.
High transmittance solar glass with a low iron
content is used almost exclusively today in commercial solar collectors.
Viewed on edge. the
greener the glass. the higher the iron content
and the lower the transmittance.
Alternatives
to glass include plastic and fiberglass-reinforced
polyester. Plastics. many of
them lighter and stronger than glass. have a
slightly higher solar ttxnsmittance because many
are thin films. Unfortunately,
plastics transmit
some of the longwave radiation from the absorber plate. Longwave transmittance as high
as X0 percent has been measured for some very
thin films. The increased solar transmittance
mitigates this effect somewhat-as
does the use
of a selective surface. But good thermal traps
become very important at higher collector temperatures. and many plastics can’t pass muster
under these conditions.
Almost all plastics deteriorate after continued
exposure to the ultraviolet rays of the sun. Thin
films are particularly
vulnerable to both sun and
wind fatigue. Most are unsuitable for the outer
cover but could be used for the inner glazing.
with glass as the outer glazing. Some of the
thicker plastics yellow and decline in solar
Liquid Flat Plate Collectors
necessary in New England and only one in Florida. The majority of the collectors on the market
have one layer of glass as the cover plate, and
a selective surface.
:
40-
:
u
;
k
30-
INSULATION
;
w
2
2 70-
I”-
0
6
0
I3
!
10
1‘J
511l.11 ry,~,~~~
TIME
7
4
1
6
Flat-plate collector performance of
selective and flat black absorber
coatings.
transmittance.
even though they remain structurally sound. Other plastics like Plexigl:iss’““’
and acrylics soften at high temperatures and
remain permanently deformed. In dirty or dusty
regions, the low scratch resistance of many plastics make them a poor choice. Hard. scratchresistant coatings are available at an increased
cost. Newer plastics are being introduced with
special coatings to protect against ultraviolet
degradation.
with limited warranties up to IO
years against yellowing.
scratching.
and hail
hlllilg~.
Additional cover plates provide extra barriers
to retard the outward few of heat and insure
higher collector
temperatures.
Double-glared
commercially
available collectors
most commonly use two layers of glass. The more cover
plates. the greater the fraction of sunlight absorbed and rellected by them-and
the smaller
the percentage of solar energy reaching the absorber surface.
In general. the lower the temperature requir :d t’rom the collector. the fewer the cover
plates. For example. solar collectors that heat
swimming pools usually, don’t require iI cover
plate. For co&r climates. additional cover plates
may be needed. To obtain the same collector
performance.
for exanlplc. IUO covers may be
Insulation
is used behind the absorber to cut
hehi losses out the back. If the collector is integrated into the wall or roof. heat lost out the
back is transferred directly into the house. This
can be an advantage during winter but not in
the summer. Except in areas with cool summer
temperatures, the back of the absorber should
be insulated to minimize this heat loss and raise
collector el’ticiency. Six inches of high temperature liberglass insulation
or its equivalent
is
adequate for roof collectors, and as little as 4
inches is sufficient for vertical wall collectors
it‘ they are attached to a living space. Where
the collector sits on its own support structure
separate from the house an K- I7 back and K-X
sides should be the minimum.
Choose an insulation made without a binder.
Tine binder will vaporize at high temperatures
and condense on the underside of the glaling
when it ~~~1s. cutting transmittance.
The insulation should be separated from the absorber
plate by at least a j/d-inch air gap and a layer
ol’ rellective foil. This foil renects thermal radiation back to the absorber-thereby
lowering
the temperaturt: ot’ the insulation and increasing
collector et’licicncy.
Most collectors use a foilt’aced cellular plastic insulation at the back of
the absorber. separated from it with a layer of
binderless tiberglass and a layer of foil. Both
insulations are made spc~(fic*trll~ for high temperatures because the collector could stagnate
above 300°F.
The perimeter of the absorber must aIs0 be
insulated to reduce heat losses at the edges.
Temperatures
alon, o the perimeter of the absorber are gcncrally lower than those at the middle. So less insulation can be used. but it too
should be made for high temperatures in case
of
[email protected]
103
The New Solar Home Book
OTHER FACTORS
Smaller issues should also be addressed when
choosing a collector. Glazing supports and mullions can throw shade on the absorber so look
for collectors with the standard low-profile aluminum extrusions. Gaskets and sealants should
be able to resist ultraviolet
radiation and high
temperatures. The glazing details should provide for drainage and keep out snow, ice, water,
and wind.
A tilled collector weighs between I and 6 pounds
per square foot. This is well below the roof
design load of most houses. Wind loads on wall
collectors or integral roof collectors are no problem either, since these surfaces must withstand
wind conditions anyway. But, wind loads are
important in the design of raised support structures for separated collectors.
Snow loads have not been a problem. The
steep collector tilt angles needed at higher latitudes (where most of the snow falls) are usually
adequate to maintain natural snow run-off. Even
when snow remains on the collector, enough
sunshine can pass through to warm the collector
and eventually cause the snow to slide off. As
a last resort. warm water from storage can be
circulated through the collector in the morning.
Solar heating systems that use air as the heat
transport medium should be considered for all
space heating applications-particularly
when
absorption cooling and domestic water heating
are not important. Air systems don’t have the
complications
and the plumbing costs inherent
in liquid systems. Nor are they plagued by
freezing or corrosion problems.
The relative simplicity
of air solar heating
systems makes them very attractive to people
wishing to build their own. But precise design
of an air system is difficult.
All but the simplest
systems should be designed by someone skilled
in mechanics and heat transfer calculations. Once
built. however, air systems are easy to maintain
or repair. Fans, damper motors. and controls
may tail occasionally.
but the collectors. heat
storage. and ducting should last indelinitely.
The construction of illl air collector is simple
compared to the difficulty
of plumbing a liquid
collector and lindinp an absorber plate compatible with the heat transfer liquid. Except for
Thomason’s collector. the channels in a liquid
collector absorber must be leakproof and pressure-tight and be faultlessly
connected into a
larger plumbing system at the building site. But
the absorber plate for an dir collector is usually
a sheet of metsl or othor material with a rough
surface. Air collectors must be built with an eye
on air leakage and thermal expansion and contraction.
ABSORBERS
The absorber in an air collector doesn’t even
have to be metal. In most collector designs. the
circulating air flows over virtually every surface
heated by the sun. The solar heat doesn’t have
to be conducted from one part of the absorber
to the flow channels-as
in liquid collectors.
Almost any surface heated by the sun will surrender its heat to the air blown over it.
This straightforward
heat transfer mechanism
opens up a wide variety of possible absorber
sulfates: layers of black screening. sheets of
glass painted black, metal lath, or blackened
aluminum plates. Many of these can be obtained
very cheaply-as
recycled or reused materials.
The entire absorber surface must be black, must
be heated directly by the sun. and must come
in contact with the air flowing through the collector .
A sheet metal absorber plate. tile old standby
for liquid collectors, is probably the lkst choice.
Metal is preferable for collectors in which the
105
The New Solar Home Book
sun cannot reach every last surface in contact
with the movirig air. Because of its high conductivity. metal can also alleviaie the “hot spots”
caused by an uneven air flow. Excess heat is
conducted to other areas where the air is making
better contact.
AIR FLOW AND HEAT TRANSFER
Just where to put the air passage relative to a
blackened metal absorber is a question that merits some atttention. Three basic contigurations
are shown in the diagram. In Type I. air flows
between a transparent cover and the absorber:
in Type II. another air passage is located behind
the absorber: and in Type III. only the passage
behind the absorber is ussd. The Type II COIlector has the highest effciency
when the collector air temperature is only slightly abo1.e that
outdoors. But as the collector temperature increases. or the ambient air temperature decreases. Type 111ts dramatically
better because
of the insulating dead air space between the
cover and the absorber.
The rate of heat flow from the absorber to
the passing air stream is also crucial. The /MYI/
trmsjk
coefticient
h is one measure of this
flow. It is similar to the rJ-value. which is a
measure of the heat flow through a wall or roof.
The higher the value ofi’ 11. the better the heat
transfer to the air stream and the better the collector performance. Good values of Ir fall in the
range ofb to IZ Btu/(hr ft’ “Ft. At a temperature
3°F above that of the air stream, one square
foot of good absorber surface will transfer I SO
to 300 Btu per hour to passing air-almost
as
much solar radiation as is hitting it. The value
of r’t can be increased by increasing the rate of
air flow. by increasing the effective surface area
of the absorber. or by making the air how more
turbulent.
As long as costs to run the fan or
noise levels do not get out of hand. higher values of 19 are definitely preferred.
Whether the absorber surfaces are metal or
not. turhuhrl
flow of the air stream is very
important. Poor heat transfer occurs if the air
Hows over the absorber surface in smooth. un106
The three types of warm air solar collectors.
disturbed layers. The air next to the surface is
almost stilt and becomes quite hot, while layers
of air flowing above it do not touch the absorber
surface. Two levels of turbulent tlow will help
improve this situation. Turbulence on the macroscopic level can be observed with the naked
eye when smoke blown through the air tumbles
over itself. Turbulence on the microscopic level
involves this tumbling right next to the absorber
surface.
Air Flat-Plate Collectors
To create turbulent flow on either level. the
absorber
surface should
be irregular-not
smooth. Finned plate and “vee”
corrugations
create macroscopic turbulence by breaking up
the air flow-forcing
the air to move in and
out. back and forth. up and down. To create
microscopic
turbulence. the surface should be
rough or coarse. with as many tine, sharp edges
as possible. Meshed surfaces and pierced metal
plates do the trick.
But increased air turbulence means a greater
pressure drop across the collector. 1‘00 many
surfaces and too much restriction of air flow
will require that a larger fan be used to push
the air. The added electrical energy required to
drive the fan may cancel out the extra solar heat
gains.
ABSORBCCR COATINGS
PLATES
AND COVER
While considerutions for absorber coatings. seIcctivt. surfLlces. and cover plates are similar for
air and liquid collectors.
there are a few differcnceh. One of the primarq drawbacks of a
non-metalic
absorber. such as in a plastic thin
him collector. is the extreme difficulty
of applying a selective surl&e to it. Until this technology improves. metal absorbers are preferred
in applications where a selective surface is desirable. Low-cost. eflicient air collectors will
be readily available if selective surt’aces can
ever bc applied to non-metal absorbers with ease.
As with liquid collectors, the use of a selective surface is about equivalent to the addition
of a second cover plate. For Type I and II collectors. in which air tlows between the absorber
and the glazing. the addition of a second cover
plate may be preferred because it creates a dead
air space in front of the absorber.
The use of a “vee” corrugated absorber plate
is somewhat analogous to the use of a selective
surfiice. The vees create more surface area in
the same square footage of collector area. It
also increases the overall solar absorption (and
hence the “effective”
absorptance) because direct radiation striking the vees is retlected several times, with a little more absorption occuning
at each bounce. oriented properly. its absorptance is higher than that of a flat metal sheet
coated with the same substance. But the increase in the emitted thermal radiation is small
by comparison.
OTHER DESIGN FACTORS
Air leakage. though not as damaging as water
leakage in a liquid collector. should be kept to
a minimum. Because the solar heated air is under some pressure, it will escape through the
tiniest crack. Prevention of air leakage helps to
raise the collector efficiency. Take special care
to prevent leakage through the glazing frames.
By using large sheets instead of many small
panes you can reduce the number of glazing
joints and cut the possibility of leakage. And
just as storm windows cut the air infiltration
into your home, second and third cover plates
reduce air leakage from a collector. Air leakage
is the biggest factor in decaying efficiency and
occurs throughout the system: collectors, ducts,
and storage.
For Type I and II collectors, the turbulent
flow through the air space in front of the absorber results in somewhat larger convection
heat loss to the glass than is the norm with liquid
collectors. Thermal radiation losses from the
absorber are therefore a smaller part of the overall heat loss. The absorber in a Type I collector
becomes relatively
hot and loses a lot of hea1
out the back. so more insulation is required.
But in Type I! and III collectors. a turbulent air
tlow cools the back side somewhat and less
insulation may be required.
One drawback of air as a heat transfer fluid
is its low heat capacity. The specific heat of air
is 0.24 and its density is about 0.075 pounds
per cubic foot under normal conditions.
By
comparison, water has a specific heat of I .O and
a density of 62.5 pounds per cubic foot. For
the same temperature rise, a cubic foot of water
can store almost 3500 times more heat than a
cubic foot of air. It takes 260 pounds. or about
3500 cubic feet of air, to transport the same
amount of heat as a cubic foot of water.
107
The New Solar Home Book
Because of this low heat capacity, large spaces
through which the air can move are neededeven in the collector itself. Air passageways in
collectors range frcm l/2 to 6 inches thick. The
larger the air space. the lower the pressure drop,
but the poorer the heat transfer from absorber
to air stream. And larger passages mean higher
.
108
costs for materials. For flat. sheet-metal absorber surfaces. the passageway usually is l/2
to I inch. Passageways ranging from t-112 to
2-l/2 inches are standard for large collectors
using natural convection
or having unust,ally
long (more than IS feet) path lengths-the
distance from the supply duct to the return duct.
Corr~vatr~rtin!:~ and focwsing
collecmrs
may
someday emerge as favorites. These collectors
use one or more retlecting surfaces to concetrate
sunlight onto a small absorber area. Collector
performance is enhanced by the added sunlight
hitting the absorber.
Depending upon their total area and orientation, Hat reftectors can direct SO to IO0 percent
more sunlight at the absorber. Focusing collectors only retlect direct sunlight onto the absorber. Concentrating collectors direct and diffuse
radiation. so they also work well in cloudy or
hazy weather-when
diffuse sunlight is coming
from the entire sky.
PARABOLIC
COLLECTORS
Parabolic collectors have ;r rellecting
surface
XYU-IYYI to direct incoming sunlight onto I very
small area. A deep parabolic surface (a Hy ball
hit to tile outlield traces out a parabolic path)
can focus sunlight on an area as small as a
hlackcned pipe with tluid running through it.
Such a focusing collector
will perform extremely well in direct sunlight but will not work
at all under cloudy or hazy skies because only
a few of the rays coming from the entire bowl
of the sky can be caught and reflected onto the
blackened pipe. And even in sunny weather,
the retlecting SUifaCC must pivot to follow the
sun so that the absorber remains at the focus.
The mechanical devices needed to accomplish
this tracking can be expensive and failure-prone.
But the higher the temperatures and efficiencies
possible with a focusing collector are sometimes
worth this added cost and complexity for hightemperature applications.
COMPOUND PARABOLIC
CONCENTRATOR
The compound parabolic concentrator was developed at the Argonne National Laboratory by
physicist Dr. Roland Winston. His collector uses
an array of parallel reflecting troughs to concentrate both direct and diffuse solar radiation
onto a very small absorber-usually
blackened
copper tubes running along the base of each
trough. The two sides of each trough are sections of parabolic cylinder-hence
the name
“compound
parabolic concentrator”
or CPC.
Depending upon the sky condtion and collector
orientation.
a three- to eight-fold concentration
of solar energy is possible. The collector performs at SO percent efficiency while generating
temperatures 150°F above that of the outside
air.
The real beauty of the CPC collector is its
ability to collect diffuse sunlight on cloudy or
hazy days. Virtually all the rays entering a trough
109
The New Solar
PARABOLIC
REFLECTOR
~ARABOLI C
DISH
-COLD
FLUID
IN
COLD FLUID IN AhfC
ffOT FLUID OUT
@OUC
PARABOLIC
R&t=ECTOR
TROUcjH
Typical concentrating collector with parabolic reflectors. Direct rays from the sun
are focused on the black pipe, absorbed and converted to heat.
are funneled to the absorber at the bottom. With
the troughs oriented east-tn-west. the collector
need not track the sun. You merely adjust its
tilt angle every month or so. After publishing
his initial designs, Wins!on discovered that *he
same optical principles have been used by horseshoe crabs for thousands of centuries. These
antediluvian
creatures have a similar structure
in their eyes to concentrate the dim light that
strikes them as they “scuttle across the floors
of silent seas.”
EVACUATED-TUBE
COLLECTORS
One of the biggest problems with flat-plate collectors is their large surface area for losing heat.
Since the best insulator is a vacuum. a more-
110
lit collector invented
Other Collector Types
SELECTIVE
SURFACE
SELECTIVE
COLD
SURFACE
FLUID
HOT FLUID
Early evacuated tube design.
efficient Hat-plate collector would have a vacuum between its absorber plate and the cover
sheet. The vacuum would eliminate the convective currents that steal heat from the absorber
and pass it to the cover plate. which conducts
it through to the outside. Better still would be
an absorber with a vacuum on all six sides.
But vacuums cannot be created easily in a
rectangular box without atmospheric pressure
pushing in the cover. Researchers years ago
applied
the fluorescent
tube manufacturing
process to make solar collectors. One glass tube
is piaced within another and the space between
them is evacuated-like
a Thermos”?: bottle.
The inner tube has a selective coating on its
outer surface. and an open-ended copper tube
inside it, as shown in the figure. Air or water
enters the copper tube from the header, and is
forced out the open end of the copper tube.
gathering heat absorbed in the inner glass tube
before it returns to the header.
Most of the recent evacuated tube designs
feature a U-shaped copper tube with a small
selective-surface
copper absorber. The copper
plate absorbs the sun’s energy and passes it to
the heat transfer Huid flowing through the Ushaped tube.
IN
OUT
VACUUM
Another design, called the heat-pipe evacuated tube. has a closed metal tube and plate
inside the evacuated glass tube. The end of the
metal tube, which extends just beyond the glass
tube, protrudes into the header across the top
of the collector.
The refrigerant heat transfer
fluid in the metal tube vaporizes when warmed
by the sun, rises to the top of the tube. and
condenses after conducting its heat to the water
passing through the header. The condensed liquid falls down the side of the tube, to boil again.
The 3- to 4-inch diameter evacuated tubes
are arranged side-by-side connected at one end
to a header or heat exchanger (depending on
the design) and supported at the other end.
Evacuated-tubes are available for liquid, air, or
phase-change systems. When pitched properly,
they can even be used in drainback systems.
No matter what the fluid or design, the collectors drastically cut heat loss from the absorber,
and can have higher annual efficiencies
than
Hat-plate collectors in cold, cloudy climates or
in higher temperature applications.
Because the “cover plate” on an evacuated
tube is a cylinder. less sunlight is reflected over
the whole day than from a flat sheet of glass.
And since many of the collector designs feature
111
The New Solar
@ASS
I
COLD
WATER
IN
*
VACUUM
COPPER
U-TUBE
TO METAL SEAL
\
HOT
WATER
OUT
SURFACE
COPPER
FIN
COPPER U -TUB
Subsequent evacuated tube design.
HLAT
PIPE
ELLCTI VE
EVACUATED
SURFACE
TUBE
Heat pipe evacuated tube.
flat or CPC-like reflectors underneath the tubes.
they can collect diffuse as well as direct sunlight. This means they can collect energy on
days when flat-plate collectors may be lying
dormant.
Whether or not you need an evacuated-tube
collector depends on your local climate and the
temperature needed. In side-by-side tests, evacuated-tubes with CPC reflectors outperformed
flat-plates during the winter, and performed about
the same during the warm months, when their
112
lower heat losses aren’t as important. This reveals the secret of concent&ing
collectors: it
isn’t their ability to cone ntrate energy that’s
important. but the fact t’rat they have such small
/
heat losses.
/
The two major Drawbacks
to evacuated-tube
d
collectors are thprr high cost and tube breakage.
Harder glass ‘and better manufacturing
processes and d& signs have reduced the second, but
production volumes are achieved,
be more expensive than flat-plate col-
Other Collector Types
*..
CPC evacuated-tube collector.
123
The performance of flat-plate collectors has been
studied extensively. Most researchers try to predict the collector efficieny-the
percentage of
solar radiation hitting the collector that can be
extracted as useful heat energy. A knowledge
of the efficiency is very important in sizing a
collector. If you know the available solar energy
at your site, the average collector efficiency,
und your heating needs, you’re well on your
way to determining the size of your collector.
The collector efficiency depends upon a number of variables-the
temperature of the collector and outside air, the incoming temperature
of the heat transfer fluid, the rates of insolation
and Huid Row through the collector, and the
collector construction and orientation.
By manipulating the variables, a designer can improve
overall collector perfomrance.
Unfortunately,
few gains in efficiency are made without paying
some penalty in extra cost. Beyond the obvious
requirements of good collector location and orientation. many improvments
in efficiency just
aren’t worth the added expense. Keep a wary
eye turned toward the expenses involved in any
schemes you devise to improve the efficiency.
COLLECTOR
HEAT LOSSES
A portion of the sunlight striking the collector
glazing never makes it to the absorber. Even
I14
when sunlight strikes a single sheet of glass at
right angles, about IO percent is reflected or
absorbed. The maximum possible efficiency of
a flat-plate glazed collector is therefore about
90 percent. Even more sunlight is reflected and
absorbed when it strikes at sharper angles-and
the collector efticiency is further reduced. Over
a full day, less than 80 percent of the sunlight
will actually reach the absorber and be converted to heat.
Further decreases in efficiency can be traced
to heat escaping from the collector. The heat
transfer from absorber to outside air is very
complex-involving
radiation, convection, and
conduction heat flows. While we cannot hope
to analyze all these processes independently,
we
C’UII describe some important factors, including:
*
*
*
.
average absorber temperature
wind speed
number of cover plates
amount of insulation.
Perhaps you’ve already noticed that very similar
factors determine the rate of heat escape from
a house!
More heat escapes frJ,m collectors having hot
absorbers than from those with relatively cool
ones. Similarly,
more heat escapes when the
outdoor air is cold than when it is warm. The
diJ%wwce. in temperature between the absorber
Collector Performance and Size
Energy Flows in a Collector
Because energy never disuppeurs. the total sulur energ.v received by the ubsorber eyrruls the
sum of the heat energy escaping the collecto,
and the useful heat energy extructed from it. If
H, represents the rate of solar heat gain (espressed in Btul(fi’ hr)) by the absorber, and H,.
is the rate of heat escupe. the’1 the rute ofusefttl
hea; c.dli;ciitjn (H,.) is Rivet; b!:
H,. = H,, - H,
Usuall! H,. and H,, are the easiest yuuntities
to cwkulate. und H, is e.rpressed as the difference between them. The rate of solar heut collection is easily determined by measuring the
@cl Jaw rute (R. in lbl(fr’ hr)) und the inlet
und outlet temperatures t T,,, and T,,,,,. in “F).
The solar heut e.vtructed. in Btu per square foot
of collector per hour, is then:
H, = WC,,NL,,,
- -I-,,,)
where C,, is the specijc heat of the ,jlrtid--I .O
Btull b.fi>r wuter and 0.24 Btull b for uir. Knolc*ing H,. und the rute of insolution (I), yol; can
immediuteiy culc14lute
the collector [email protected]
(E, in percent):
E = IOO(H,.lI)
Of the total insolation the amount actually
con\.erred to heat in the absorber (H,) is reduced by the transmittance (represented by the
Greek letter tau. or T) of the cover plates and
by the absorptance (represented by the Greek
letter alpha, or a) qf the absorber. The value
of H, is further reduced (by 3 to 5 percent) b>
dirt on the cover plates and by shading from
the glazing supports. Therefore, the rate of solar heat gain in the absorber is about
H,, = (O.%)fcx)(~)(I)
Both OLand 7 depend upon the angle at which
the sunlight is striking the collector. Glass and
plastic transmit more than 90 percent of the
sunlight striking perpe,ldic.lairrl~. But d:crir:g a
single day, the uverage transmittunce can be as
low as 80 percent for single glass, and lower
for double glas.s. The absorptances +muterials
commonly used for collector coatings ure usu~11~Setter thun 90 percent. Jf rio rudiation is
converted to heut absorbed in the collector juid.
tht-n
Hc = H,, = 0.9tqaj(~j(1j
The instuntuneous qfficienc! cun be culcrclured by taking this rutto ut uny selected moment. Or an uveruge tlfjcienq muy be determined
by dividing the totul heut collected o\*er u certain time period (suy un hour) by the totul in.solution during thut period.
dq) izvuld still be less Ihun 80 percent. Unfortunately. there are large heat losses from u
flat-plate collector, and efJiciencies rarely get
above 70 percent.
and the outdoor air, AT = Tahb - T ,,,,,, is what
drives the overall heat flow in that direction.
The heat loss from a collector is roughly proportional to this difference.
As the absorber gets hotter. a point is eventually reached where the heat loss from a collector equals its solar heat gain. At this
equilibrium
temperature,
the collector
efficiency is zero- no useful heat is being collected. Fluids are usually circulated through a
collector to prevent this occurence. They carry
away the accumulated
heat and keep the absorber relatively cool. The higher the fluid flow
rate, the lower the absorber temperature and the
higher the collector efficiency.
Some fluids cool an absorber better than others. Although it has the disadvantages of freezing and corrosion, water is unmatched as a heat
transfer fluid. It has the advantages of low viscosity and an extremely high heat capacity. So-
und the uveruge collector elfJicrenc:v
(&jr a whole
I15
The New Solar Home Book
lutions of propylene glycol in water solve the
freezing problems but they have a lower heat
capacity. For the same flow rates, a 25 percent
solution of glycol in water will result in a 5
percent drop in collector efficiency.
As a heat transport fluid, air rates a poor
third. While optimum water flow rates are 4 to
IO pounds per hour for each square foot of collector, I5 to 40 pounds of air are usually needed.
And the rate of heat transfer from absorber to
fluid must also be considered. Rough corrugated
surfaces work best in air collectors. Good thermal bonds and highly conductive metal absorbers are needed with liquid collector.The
faster
the heat transfer to the passing fluid, the cooler
the absorber and the higher the collector efficiency.
As with houses, the collector heat losses can
be lowered by adding insulation or extra glass.
But extra glass also cuts down the sunlight
reaching the absorber. The relation of these two
factors to the collector efficiency
is illustrated
in the next two graphs. In the first, the equivalent of 2 inches of fiberglass insulation is placed
behind the absorber. The back of the second
ab:;orber is very heavily insulated-so
that virtually no heat escapes. In all cases, the db;lly
average collector efficiency falls with increasing differences between the absorber and outside air temperatures. For absorber temperatures
less than 40°F. the extra cover plates and insulation are obv5usly
helpful. These remarks
apply to a specific collector, but are generally
true for most others. For very cold climates or
very high tluid temperatures. the evacuated-t&e
collectors have the lowest heat losses.
The collector cfliciency also depends upon the
amount of sunlight hitting it. Under cloudy
morning conditions. for example. the absorber
will be much loo cool to have tluid ci:c-:l: lling
through it, and no useful heat can be extracted.
But at noon on ;I sunny day. the collector will
hc opcrsting al full tilt. tlelivcring
60 percent
II6
4
w
G
k
w
5
Lw
J
-1
8
>;
80
60
40
20
2
TEMPERATURE
DIFFERENCE
BETWEEN
ABSORBER PLATE AND OUTDOOR AIR. ‘F
Performance of a moderately-insulated
collector.
::
w
60
;
40
I
-:
e
2
7
I
II
I
II
I
II
I
II
I
iI
I
II
I
II
I
I
I
I
II
II
II I
I 1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
20
2
0
0
20
40
50
80
1
100
TEMPERATURE
DIFFERENCE
BETWEEN
ABSORBER PLATc AND OUTDOOR AIR. OF
Performance of a well-insulated collector.
of the solar energy to storage. If the fuid flow
can be increased to keep a constant absorber
temperature.
the collector efliciency
will inCI’P~LSCas the insolation rate increases.
The actual va!ue of the insolation at a particular spot is very difficult to predict. Weather
conditions vary by the hour. day, month. and
year. and a collector designed for average cond-
Collector Performance and Size
tions may perform quite differently at other times.
For example. the collector described above may
have an average daily efficiency of 40 percent.
but its efficiency at any one moment can be
anywhere in the range form 0 to 60 percent.
Usually, we have to resign ourselves to using
the clv~-trge collector efficiency
in our caIculations. But that’s not as bad as it may sound.
The average daily insolation multiplied by the
average collector efficiency
givles us the solar
heat collected per square foot on a typical day.
With sufficient heat storage to tide us over times
of shortage. why worry?
The Clear Day Insolation Data and the Percentage of Possible Sunshine Maps in the appendix provide a suitable method for calculating
the insolation at most sites. They are limited to
south-t’acing surfaces. Unless you do a little trigonometry. They are capable of providing good
estimates of the average insolation for any day
and time. And fortunately a precise knowledge
of the insolation isn’t critical. Variations of IO
percent in the insolation will change collector
efticiency
by only 3-4 percent out of a total
efficiency of 40 percent.
COLLECI‘GR
TILT
ORlENTATlON
TTMPEHATVRE
DIFFERENCE
BETWEEN
OUTER
GLASS
AND
ABSORBER
( FI
The effect of different insolation rates on collector efficiency. The outdoor air temperature is
assumed constant.
rool
AND
Two other fact,>rs that determine a collector’s
performance are its orientation
and tilt angle.
A collector facing directly into the sLn will receive the most insolation. But Hat-PI: te collectors are usually mounted in a fixed pol;ition and
cannot pivot to follow the sun as it sweeps across
the sky each day or moves north and south with
the seasons. So the question naturally arises.
“What is the best orientation and tilt angle for
my collector‘!”
In addition. designers need to
know how much they can deviate from optimum.
Although true south is the most frequent choice
for the collector orientation.
slightly west of
south may be a better choice. Because of early
morning haze, which reduces the insolation. and
higher outdoor air temperatures in the after-
s
EarW
SE dr SW
WALL
AZIMUTH
ldegrees dewatmn
from
SouthI
The percentage of insolation on vertical walls
for orientations away from true south.
noon. such an orientation can give slightly higher
collector efficiencies.
On the other hand. afternoon cloudiness in some localities may dictate
an orientaion slightly east of south. Fortunately.
deviations of up to 15°F from true south cause
relatively
small reductions in collector efhciences. The designer has a fair amount of Aexibility in his choice of collector orientation.
A useful diagram shows the approximate decrease in the insolation on a vertical wall col-
117
The New Solar Home Book
BOSTON,
CHARLESTON,
MASSACHUSETTS
% INCREASE IN
COLLECTOR
AREA
SOUTH
CAROLINA
% INCREASE IN
COLLECTOR
AREA
5-30%
lo-60%
30-90%
The change in area of a vertical wall collector with orientations away from true south.
The collector (shaded areas) has been sized to provide 50 percent of the winter heating
needs of a well-insulated home in Boston and Charleston.
Icctor facing ilway from true south. The graph
is valid for latitudes between WN and JS’Nalmost the whole United States. if we forget
Ala&a and Hawaii. And it applies to the coldest
part ot’ the yea-from
Novemhcr 21 to January
21. YOU can uw this Lgraph together with the
Clear Day Insolation Data to get ;I rough estimate of the clear day insolation on surfaces that
do not face true south. Simply multiply the data
by the percentage appropriate to the orientation
you have selected.
The effect orientation has upon the required
size of 9 collector is illustrated by two examples
in the next diagram. The vertical wrll collectors
in all cases are sized to provide SO percent of
the heating needs of a IOW-square-foot
house
in either Boston or Charleston. South Carolina.
Note that southwest (or, for that matter. southeast). orientations require an extra collector area
of only IO percent in Boston and 30 percent in
Charleston.
Tilted surfaces are affected even
less by such variations.
Collector Performance and Size
The collector tilt angle depends upon its intended use. A steeper tilt is needed for winter
heating than for summer cooling. If a collector
will be used the year round, the angle chosen
will be a compromise.
If heating and cooling
needs do not hr<e equal weight, your selection
of a tilt angle should be biased toward the more
important need.
The general consensus holds that a tilt angle
of IS’ greater than the local latitude is the optimum for winter heating. For year-round uses,
the collector tilt angle should equal the local
latitude. And for maximum summer collection,
the best choice is 15” less than the latitude. But
your house won’t freeze up or boil over if you
don’t have exactly the right tilt angle. The collector can be tilted 15-20” away from the optimum and still get more than 90 percent of the
maximum possible insolation.
For areas with severe winter cloudiness.
steeper tilts may be required. Sometimes vertical wall collectors are a good idea. Such a
collector receives its peak insolation in the winter months. but gets very little in the summer.
so don’t expect to heat a swimming pool with
it. When reflections from the ground are added
to the sunlight hitting a vertical collector directly. its performance can surpass that of a
collector tilted at the “optimum”
winter heating
angle (latitude + 15”). Clean. fresh snow has
the highest reflectance-87
percent-of
any
common surface. It can add another IS to 30
percent to the solar heat output of a vertical
collector. Other surfaces such as asphalt. gravel.
concrete. and grass have reflectances ranging
from 10 to 33 percent.
The accompanying diagram shows the relationship between tilt angle and collector size.
The collectors in all four cases are sized to provide 50 percent of the January heating load for
a typical Minneapolis home oriented true south.
Relatively
small changes in collector size can
compensate for the reductions in solar collection
resulting from changes to the January “optimum” (50”) tilt angle. With additional sunlight
reflected from fallen snow. a vertical wall collector can be 14 percent smrller than the rooftop
collector tilted at SO”.
MINNEAPOLIS,
MINNESOTA
% CHANGE IN
COLLECTOR
AREA
TILT
ANGLE
90”
NOTES:
+6%
60”
0%
50”
+18%
40”
1. Base tilt angle = 50’.
2. Operating temperature
3. Due South orlentatlon
.9O”F
Small changes in collector size are required
when the tilt angle differs from the optimum.
SIZING THE COLLECTOR
Accurate performance
predictions are needed
for si7ing a collector.
It is important to know
whether 35 or 40 percent collector efticiencies
(on the average) can be expected in any given
month. With this knowledge and some predictions of the average daily insolation for that
month, you’ll have a good idea of how much
solar heat you can expect from each square foot
of collector. The size then follows from the
average monthly heat demands.
119
The New Solar
ome Book
colleclor
Lze’
111:
I
OLAR
S.-p
OCl
Now
Dvc
HEAT
AVAILABLE
Jdll
Frb
2ow
Mar
API
MW
The solar heat delivered per square foot of collector decreases as the collector size increases. The shaded area
represents the portion of the heating demand supplied by
a 500-square-foot collector. Doubling the collector size may
double the solar heat supplied in January, but it doesn’t
help much in October or April.
But the performance of a collector is even
more difficult to predict when it is tied into an
entire heating system. Although some rules of
thumb have emerged. the subject is still clouded
in mystery as far as the average homeowner or
builder is concerned. Site conditions. the heating demands of the house. and specilic design
choices (such as the collector tilt. the operating
temperature. and the heat siorage capacity) all
affect the average collector performance.
A well-designed
collector might be able to
collect I200 Btu per square foot on a sunny
winter day. But not all of this heat will reach
the rooms. There will be heat losses from the
heat storage container. which may already be
too hot to accept additional heat. There will also
be heat lost from the ducts or pipes between the
collector and the storage tank and the rooms.
Solar energy will be rejected during extended
periods of sunny weather-even
if the outdoor
temperatures are quite cold. Only when the proper
sequence of sunny and cloudy days occurs can
all the available solar energy be used.
Consider a 1000-square-foot
house with a
heating demand of 12.000 Btu per degree day.
or 84-million
Btu in a 7000-degree-day climate.
This demand is distributed over the heating sea-
120
son (late September to early May) as shown in
the diagram. Little heat is needed in October or
April and the bulk of the heat is needed from
December through February--just
when sunlight is at a premium.
Such a house needs about 600.000 Btu on a
day when the outside temperature averages 15°F.
On a sunny day. this amount of solar heat can
be supplied by 500 square feet of a good collector. And with a 3S”F temperature rise (from
85°F to 120°F. for example). about ZOO0 gallons
of water will absorb all of the solar heat. But
under crwrqy operating conditions.
a square
foot of this collector gains only 350 Btu of usable solar heat per day-or
84.000 Btu for the
entire seven-month
heating season. The 500
square feet of collector will supply only half of
the seasonal heating load of 84 million Btu.
Contrary to what you might expect, doubling
the collector size from 500 to 1000 square feet
does rror provide 100 percent of the heating
load. Instead. it provides about 75 percent because the larger collector does not work to full
capacity as often as the smaller one. In this
particular system. the usable heat per square
foot of collector drops from X4.000 Btu to h9.000
Btu because the house just cannot use the added
Collector Performance and Size
SIZE AND SOLAR HEAT DELIVERED
-~
COLLECTOR
Total Solar Heat
Supplied per Season
of Demand
MMBtu+
Solar Heat Used
per Square Foot
of Collector
Collector
Size*
(f&
‘ow
I SW
IO00
S(H)
4xw
x4
sh.ow
69.0(N)
7s
6.3
x4.000
42
*S1orage site remains fixed. +I MMBtu
100%
X9%
75%
50%
= I million Bw.
heat in the fdll and spring. And the system must
reject more heat during four or five successive
days of January sunshine. Even if the storage
silt were doubled. the system would have to
reject cxccss heat durin! the autumn and spring.
As collector size increases for a fixed heat demand. the amount of solar energy provided by
each square fool drops because of the decreased
/ocrd,/irc~or- on each additional square foot. This
is the law of’ tlirv~ir~i.d~ir~c~
rumrx~.
A simplified method for collector sizing. lies
somcwherc between educated guess-work and
detailed analytical calculations.
It is accurate to
within 20 percent and is biased toward conscrvative
results. Architects.
designers. and
owner-huildcn
often need such a “tirst-cut”
c&mate of collector si/.c to proceed with their
designs.
First the monthly output of a Hat-plate collector is calculated as the product of the usejitl
sunshine hours in the month times the average
hourly solar heat output of the collector during
the month. The number of useful sunshine hours
is less than the total number of sunshine hours
because the insolation rate is not high enough
in early morning to justify collector operation.
The two most influential
factors that determine
the hourly solar output-the
hourly insolation
rate and the temperature difference between the
absorber and the outside air-change
rapidly
during a single day. Therefore, we only try to
determine an UIWU,+J hourly solar output. The
method outlined in “Estimating
Collector Performance”
is an attempt to determine the reasonable mean monthly output from average
insolation
rates and temperature differences.
More detailed analyses are usually made using
the F-chart computer simulations described earlier.
A sample calculation of mean monthly solar
heat output is provided for iiiustrstion.
The hypothetical collector is sited in Boston and oriented south at a tilt angle 0: 60”. At an average
operating temperature of 120°F. this col!ector
has an efliciency of 38 percent and gathers 9880
Btu per square foot during the month of January .
.
0
0
25
50
75
100
125
150
175
TEMPERATURE
DlFFERtNCE
BETWEEN
ABSORBER f’LATE AND OUTDOOR AIR. “F
0
25
50
75
100
125
150
175
TEMPERATURE
DIFFERENCE
BETWEEN
ABSORBER PLATE AND OUTDOOR AIR. “F
Performance curves for single- and double-glazed collector. Use these graphs to estimate
collector efficiency from the insolation rate and temperatures of the absorber and outdoor
air.
SOCRCli
Re~rc C’oppcr and Bras\ (‘11
121
The New Solar Home Book
Estimating Collector Performance
The.following method will help you estimate the
average eSJiciencyand monthI! solar heat output of a well-built solar collector. This method
is accurate to about 20 percent and results ;n
conservative estimates of per$ormance. A running example is included for illustration.
I.
Find the total number of hours of sunshine
for the month jLom the “Mean Number of
Hours of Sun,hine ’ table in the appendix:
for e.\-crmple,148 hours for Boston in Jan-
7
h.
Find the averuge day length,fin- the month
from almanucs or the weather bureau; 10
hours for Boston in Junuury .
From the Clear Day Insolation tables in
the appendix. calculate the number of
’ ‘collection hours ’ per duy . For the selected collector tilt angle, this is the number of hours that the insolution is greater
than 150 Btulft’. Count II2 hour for insolution rutes between 100 and 1.50 Btrrl
.ft’; 7 hours for 4O”N lutitude und 60” tilt.
Determine the totul collection hours per
month by multiplying the sunshine hours
per month (#I) by the collection hours per
duy (#3) and dividing by the average da!
length (#2); 148 hours (7 hours) + 10
hours = IO4 hours.
Determine the totul daily usefitl insolation,
dejined us t!le total insorirtiotr during cullection hours, b!: adding the hourly insolation rutes (from the Cleur Dq Insolution
tables) for those collection hours described
above; 187 + 254 + 293 + 306 + 293
+ 254 + I87 = 1774 Btul(ft’ dir!).
Determine the uveruge hourly insolution
rate (I) during the collection period by dividing the total daily useful insolation (#.5)
b! the number of collection hours per da>
1iCl~.
3.
4.
5.
6.
122
(#3); 1774 Btul(f+ day) + 7 hours/day =
253 Btul( f+hr).
7. Determine the average outdoor temperature (T,) during the collection period from
II2 the sum of the normal daily mcruimum
temperature and the normal daily average
temperaturefor the month and locale. These
are uvuilable from the local weather bureau and from the Climatic Atlus of the
UnitedStates . T(,,,, = 1/2(38”F) + (30°F)
= 34°F.
8. Select the averuge operating temperature
iTc,I,.r)of the collector absorber and$nd the
difference (AT = Tcrhs- T,,,,,). In general,
you should e..ramine a runge elf possible
vdue.s for Tcrh.,.AT = 120°F - 34°F =
86°F.
9. Refer to a performance curve *for the collector to determine the averuge collector
eficienq from a knowledge of AT (#8) and
I (#6). The sumple curves provided apply
to u tube-in-plate liquid-type collector. but
they should be.fairly accurate for most,flatplate collectors of moderate to good construction; average colle~*tor crfJicienc*y=
38 percent for a double-glazed collector.
IO. Determine the averuge hourly collector
output by multiplying the a\Berage hour!\
insolation rate (I from #6) by the average
collector eficiencv (#9): 0.38(253 Btul(ft’
hr)) = 96 Btul(f;? hr).
1I. The useful solar heat collected during the
month is then the uverage hourly collector
output (#IO) multiplied by the number of
collection hours (#4) for that month: 96
Btul(jiihr)(l04 hclurslmonth) = 9984 Btul
ft’ for Japuan, in Boston.
This procedure should be repeated for a number
of other collector operating temperatures and
tilt angles.
Collector Performance and Size
ESTIMATES
OF MONTHLY
COLLECTOR
OUTPUT (in Boston)
Average Solar Heat Collected (Btu/ft2)
Collector
Sep
Ott
21.700
14.615
18.600
12.700
16,275
IO.lhO
19.630
19.781
15.855
16,006
I 2,835
12,986
OF
Tilt
90
90
I20
120
140
140
60°
900*
60°
900*
60°
900*
*With
Zp ywrcent ground reflection.
Nov
Dee
Jan
13.780 13.080 12,480
14.310 14.170 13.000
II.130
9.810 9.984
l I.660 l I.455 10.400
9.010
8.175 7.800
9.540 9.265 8.320
Monthly solar output for the rest of the heating season has been calculated with the same
method and listed in the accompanying
table.
The output of a vertical collector (including 20
percent ground reflectance) is included in the
table. as are the monthly outputs when 90°F and
140°F operating temperatures are allowed. The
seasonal output is the sum of all these monthly
tigures. In your design work. it’s extremely useful to consider a number of alternative collector
tilts and operating temperatures-instead
of
proceeding
single-mindedly
with a preconccived design. Almost every collector operates
over a range of temperatures and its efficiency
varies in a corresponding
fashion. It’s irrstructive to determine the solar heat collection for a
few of these conditions.
In general, the larger the percentage of house
heating you want your collector to supply, the
more diffcult it is to estimate its size using these
simplified methods. The actual sequence of sunny
and cloudy days becomes more important as the
percentage of solar heating increases. If a full
week of cold, cloudy days happens to occur in
January, your collector (or storage) would have
to be enormous to insure 30 percent solar heating. But good approximations
of collector size
can be made for systems that are designed to
supply 60 percent or less of the seasonal heating
needs.
Feb
Mar
14.640
15.250
Il.590
12.200
9.150
9.750
8.000
0.925
4.250
8,625
I I.250
5,750
A pr
May
TOTALS
14.720
5,040
I 2. I60
3,240
10.240
2.160
5,225
2.520
2,325
I.800
9,425
I.080
143,255
109.61 I
Il5.704
88,076
94. I60
69.02 I
A simplified
method of calculating the collector size from monthly output and heating demand figures is outlined in “Estimating Collector
Size.” The monthly output tigures are those of
our hypothetical collector, tilted at 60” and operating at an absorber temperature of 120°F. The
heating demand figures are for a Boston home
of 1000 square feet. that loses 9500 Btu per
degree day. In this particular example. we strive
to provide 50 percent of the seasonal heat demands of the house. If the initial guess at the
appropriate collector size does not provide the
desired percentage, it can be revised up or down
and the calculations
repeated until the desired
results are achieved.
The final size of the collector should reflect
other factors besides heating demand-for
example, the size of the heat storage container,
the solar heat gain through the windows. available roof or wall area, and cost.
COMPARING
COLLECTORS
Some states require that manufactured solar cotlectors be tested and rated on how much thermal
energy they produce. The Solar Rating and Certification Corporation
(SRCC) is a non-profit
organization
incorporated
in 1980 to develop
123
1 he New Solar Home Book
Estimating Collector Size
The following procedure helps you to estimate
the collector size needed to supply a desired
percentage of the yearlv_ heating demand. To
use it, you need the monthly output per square
foot of collector, as calculated in “Estimating
Collector Performcnce.” The Boston example
the colis continued here for illustration-with
lector tilted at 60” and operating at 120°F.
I. For the tilt angle and operating temperature
selected, enter the month/y output per square
foot of collector in column A. Add them to
get the heating season output for one syuare
foot.
2. Enter the monthly degree days of the location (jIrom the “Degree Days and Design
Temperatures” table in the appendk) in column B.
3. Enter the monthly heat loss of the house in
column C. This is the product of the monthly
degree days times the heat loss per degree
day-or 9500 Btu per degree day for our
Boston home.
4. Add the entries in column C to determine
the seasonal heat loss. Divide this total
by the total of column A (step I) and take
60 percent of the result as a first guess at
the collector area needed to supply 50 percent of the seasonalheat demand: 0.6Oc.53.46
Month
September
October
November
December
January
February
March
April
May
Heating
Season
Totals
A
Collector
Output
(Btu/ft’)
I X.600
lS.HSS
I I.130
9.x10
9,884
I I.590
14.250
12.160
12,325
I 15,704
B
C
Degree
Heat
Days
Loss
(OF days) (MMBtu*)
98
316
603
983
1.088
972
846
513
208
5,627
MMBtu) (100,OOOBtu) f 115,704 Btulft’ =
277.2 f?.
5. Multiply this collector area by the entries in
column A and enter the resulting solar heat
collected in column D.
6. Subtract entries in column D from those in
column C to obtain the heat demand NOT
met by solar energy durirq the month. If a
negative result occurs, solar energy is supplying more than can be used, and a zero
should be recorded in column E.
7. Subtract entries in column E from those in
column .C to get the total solar heat used by
the house in the month. Enter these resuits
in column F.
8. Divide entries in column F by those in column C and multiply by 100 to get the percentage of monthly heat losses provided by
solar (column G).
9. Divide the seasonal total of column F by
that of column C to get the percentage of
the seasonal heat loss provided by solar, or
47 percent in the Boston home. If this result
is too low (or high) the collector area can
be revised in step 4 and steps 5 to 9 repeated
until satisfaction is achieved.
The total of column F is the “useful”solar energy output of the collector. it can be used to
predict the economic return on rhe initial e-rpenses of the system.
D
Solar Heat
Collected
(MMBtu*)
0.93
3.00
5.73
9.34
10.34
9.23
8.04
4.87
I .98
5.16
4.40
3.09
2.72
2.74
3.21
3.95
3.37
3.42
53.46
32.06
E
Auxiliary
Heat
(MMBtu*)
0
0
F
Solar Heat
USed
(MMBtu*)
ci
Percent
Solar
Heated
2.64
6.62
7.60
6.01
4.09
I.50
0
0.93
3.00
3.09
2.72
2.74
3.21
3.95
3.37
I .98
100
100
54
29
26
35
51
69
lo0
28.46
24.99
47
*Millions of Btu. House Heat Loss: 9500 Btuldeg day. Collector Area: 277.2ft’.
Collector Performance and Size
SGSS-Single-glazed
selective surface
DGFB-Double-glazed
flat black
SGFB-Single-glezed
flat black
UGP-Unglazed
plastic
0
.l
.2
.3
.4
.5
.6
.7
.8
.9
1.0
it, -t,j
Four sample thermal efficiency curves. (Solar Age)
and implement certification
programs and national rating standards for solar equipment. The
SRCC’s collector
certification
program provides a means to evaluate the maintainability,
structural integrity, and thermal performance of
solar collectors under strict laboratory conditions. The tests. paid for by each n,;inufacturer,
are conducted by independent laboratories accredited by the SRCC.
Collectors or whole systems are randomly
selected and inspected upon receipt to check the
original condition after shipping. The collector
then undergoes a pressure test to see if it leaks,
and is exposed to the weather for 30 days. After
exposure, the collector is checked for signs of
degradation. A series of tests, from thermal shock
to thermal performance, is conducted before the
collector is taken apart and inspected one last
time.
The thermal performance test determines the
instantaneous efficiency of the collector. With
the outside air temperature and incident solar
radiation level held constant. the inlet temperature is varied four times to see how well the
collector operates in four different temperature
ranges. The data collected is plotted on the collector’s thennul [email protected] cww. The curve helps
you compare the instantaneous efficiencies of
different collectors, so that with the cost of each
collector, you can decide which collector is right
for your location and application.
The figure shows the thermal efficiency curves
for four collectors: an unglazed collector with
a plastic absorber, a single-glazed collector with
a flat-black painted absorber. a double-glazed
collector with a flat-black absorber. and a single-glazed collector with a selective-surface
absorber.
Following
our last example, if the average
insolation rate (I) in January is 253 Btu/(ft’hr)
in Boston, the average daytime temperature (T,)
is 34°F. and the collector inlet temperature (T,)
is 120°F. you can use the thermal efficiency
curves to find each collector’s average instantaneous efficiency in this application. The fluid
parameter, plotted along the x-axis, is equal to
the inlet temperature (Ti) minus the daytime
temperature (T,). all divided by the insolation
rate (I) :
Fluid parameter
= (Ti - T,) / I
In this case, the fluid parameter equals ( I20 - 34)
/253. or 0.34. Starting at that point on the xaxis, mark the intersections with the efficiency
curves, and read the efficiency from the y-axis.
The efficiency
for the single-glazed
selective
125
The New Solar Home Book
SGSS-Single-glazed
selective surface
DGFB-Double-glazed
flat black
SGFB-Single-glazed
flat black
UGP-Unglazed
plastic
Pool collector
.81
range
.8
/
Domes?ic
hot water
collector
range
.6
6
q
.4
.2
0
.l
.2
.3
.4
.5
.6
7
.Q
.9
.l
(to -5’
.8 t
.8
.6
.6
.2
.3
.5
.4
.6
Industrial
rl
q
.4
.7
.8
.9
processes or
.4
.2
.2
0
0
1
2
.3
.4
.5
.6
.7
.8
.9
It, -1,)
.1
.2
.3
.5
.4
.6
.7
.8
It, -‘J
It
General fluid parameter boundaries for different applications. (Sob Age)
surt‘dce collector if 0.45. for the double-glazed
flat-black collector is 0.36. and for the singleglazed Ilat-black collector is 0.28. The unglazed
plastic collector cannot compete in this range
--it only performs well in low-temperature
applications. such as pool heating or domestic hot
water in very warm climates.
The thermal efhciency curve can tell you a
lot about the collector. The point where it intersects the left side of the graph represents the
maximum efliciency the collector can achieve
tat that point its losses equal zero). If the manufacturer tells you his collector can deliver half
the energy it receives. and its collector efticiency curve intersects the y-axis at 0.50. he’s
exaggerating. The curve doesn’t account for the
rest of the system losses!
126
The steeper the slope of the curve, the less
efficient the collector is at higher temperatures.
As the Huid parameter increases. collector efticiency decreases. Swimming pool coiiectors
have the steepest slopes because their losses are
high. They are best suited in the fhud parameter
range below 0. IO (see figure). Collectors
for
solar domestic hot water have less steep s!opes
so that they can collect energy better in the 0.10
to 0.30 fluid parameter range. Space heating
collectors are made for the 0.30 to 0.50 range.
and industrial
process heating or absorption
cooling collectors need to perform well in the
0.3 to 0.8 fluid parameter range.
Single-glazed flat-black collectors have steeper
slopes than double-glazed
collectors since their
losses ate higher.
But selective surfaces on
.9
Collector Performance and Size
single-glazed,
double-glazed, or evacuated-tube
collectors outperform the rest since their radiation losses are cut significantly.
The instantaneous efficiencies
only help tc
compare collectors and shouldn’t
be used to
determine annual performance,
since it is only
under optimum
the instarmu~eous efficiency
condittons. It doesn’t account for the difference
in collector efficiency at the beginning or end
of the day versus that at noon, when the sun’s
rays are more perpendicular to the absorber and
insolation rates are higher. It doesn’t say what
happens under hazy skies. In both cases, collectors such as evacuated-tubes can perform better than flat-plates. Another drawback to the
collector test is that it only tests the collector
efticiency.
and doesn’t subtract the tank or distribution losses or pumping power required.
The SRCC tests for complete systems do take
into account losses from the tank and how much
pump power is used. Tank losses at night are
important if you’re comparing an aclive system
(with the tank in a “heated”
basement) to an
integral storage system (with its tank exposed
to the cold night air). Taking pump power into
account is important if you’re comparing passive and active systems. But remember again
that the results you see are only those gathered
under strict laboratory conditions, and not what
you’ll get for the same systems in your location
and your operating conditions.
Comparing efficiency curves is like comparing EPA automobile gas milage ratings. There
are many other things that should be looked at
before you decide which collector to buy: cost,
appearance, and anticipated lifetime.
127
Some capacity for storing solar heat is almost
always necessary because the need for heat continues when the sun doesn’t shine. And more
heat than a house can use is generally available
when the sun is shining. By storing this excess,
an active system can provide
energy as
needed-not
according to the whims of the
weather.
If costs were not a factor, you would probably design a heat storage unit large enough to
carry a house through the longest periods of
sunless weather. A huge. well-insulated
storage
tank (say IS-20.000 gallons) in the basement
could store heat from the Summer for use in
winter! But most of us do not have the money
to spend on an enormous storage tank, and our
designs are limited by what we can afford. Some
of the major factors influencing
heat storage
costs are:
0 choice of storage medium
= amount or size of the storage medium
l
type and size of a container
0 location of the heat storage
. use of heat exchangers, if required
l
choice of pumps or fans to move the heat
transfer fluid
In designing solar heat storage, you must
weigh these cost considerations against the performance of the system. All of the above factors
128
influence performance
to some extent. Other
factors include the average operating temperature of the entire system, the pressure drop of
the heat transfer fluid as it passes through or by
the storage medium, and the overall heat loss
from the container to its surroundings.
In general, the heat storage capacity of common storage materials varies accordjng to their
specific heat-the
number of Btu required to
raise the temperature of I pound of a material
1°F. The specific heats of a few common heat
storage materials are listed in the table along
with their densities and heat capacities-the
amount of heat you can store in a cubic foot of
the material for a 1°F tempe, sture rise. Heat
energy stored with an accompanying rise in temperature is called sensibk heat. and it is reclaimed as the temperature of the storage medium
falls. It takes high temperatures or large volumes of material to store enough sensible heat
(say SOO,OOOBtu) for a few cold, sunless days.
Rocks and water are by far the most common
storage media because they are inexpensive and
plentiful,
Some materials absorb a lot of heat as they
melt, and surrender it as they solidify. A pound
of Glaubers salt absorbs IO4 Btu and a pound
of paraffin 65 Btu when they melt at temperatures not far above normal room temperatures.
The heat absorbed by the change in phase from
liquid to solid and solid to liquid is called latent
Storage and Distribution
PROPERTIES OF HEAT STORAGE
MA’i’ERIALS
Heat Capacity [ Btu/(ft3 OF))
Materizl
WZtel
Scrap Iron
Scrap Aluminum
Concrete
Stone
Brick
Spc:cific Heat
lr,tu/(lb OF)]
Density
[lb/ft3]
I .oo
0.12
0.2!
0.22
0.2 I
0.20
heat, and is stored or released without a change
in temperature. For example, when one pound
of ice melts, it absorbes I44 Btu, but stays at
32°F. This is latent heat storage. To raise its
temperature to 33’F. it takes I Btu-sensible
heat storage.
The storage of heat in phase-changing materials
can reduce heat storage volumes drastically. 3ut
continuing problems of cost. containment.
and
imperfect re-solidification have limited their use.
A solar collector and heat storage medium
should be chosen together. Liquid collectors almost always require a liquid storage medium.
Most air c:ollectors require a storage medium
consisting of small rocks. or small containers
of water or phase-change materials. These allow
the solar heated air to travel around and
between-transferring
its heat to the medium.
Within these basic categories of heat storage,
there are many possible variations.
TANKS OF WATER
Water is cheap and has a high heat capacity.
Relatively small containers of water will store
large amounts of heat at low temperatures. From
I to 2 gallons are needed per square foot of
solar domestic hot water collectors, and I to IO
gallons are needed per square foe* of space heating collectors-or
SO0 to SO00 gallons for a
SOO-square-foot space heating collector.
An-
62
490
171
144
170
140
No voids
62
59
36
32
36
78
30% voids
43
41
26
22
25
20
other advantage of water heat storage is its compatibility with solar cooling. But there are several
problems with water storage, such as the high
cost of tanks and the threat of corrosion and
possible leakage.
Water containment has been simplified in recent years by the emergence of good waterproofing
products and large plastic sheets.
kPreviously, the only avaiiable containers were
leak-prone galvanized steel tanks. Their basement or underground
locations made replacement very difficult and expensive. Glass linings
and fiberglass tanks helped alleviate corrosion
problems but increased initial costs. Until recently, the use of poured concrete tanks has
been hampered by the difficulty of keeping them
water tight-concrete
is permeable and develops cracks. But large plastic sheets or bags now
make impermeable liners having long lifetimes.
And with lightweight wood or metal frames supporting the plastic, the need for concrete can be
eliminated.
The most straightforward
heat storage system
(see diagram) is a water-filled container in direct
contact with both the collector and the house
heating system. The container shown is made
of concrete or cinder blocks with a waterproof
liner, but it might well be a galvanized or glasslined tank. The coolest water from the bottom
of the tank is circulated to the collector for solar
heating and then returned to the top of the tank.
Depending upon the time of day, the temperature difference between the bottom and top of
129
The New Solar Home Book
COi’CRETE Oip
CIlVLsR l3Loc8s -
Heat storage tank is tied directlJ to both
the collector and the house heating system
in an open-loop system.
CLWCRETE of=!
C/NDER BLOWS
Use of a heat exchanger to extract solar heat
from a storage tank, in a closed-loop system.
a j-to 4-foot high tank can be I5 to 24°F. Sending the coolest water to the collectors i.mproves
collector efficiency. In an open-loop system, the
warmest water from the top of the tank is circulated directly through baseboard radiators. a
water-to-air heat exchanger, or radiant heating
panels inside the rooms.
If the system is a closed loop. it might have
a heat exchanger-a
copper coil or finned
tube--immersed
in the tank of solar-heated
water. Water or another liquid circulates through
the heat exchanger. picks up heat, and carries
it to the house. Warm water in the tank can also
be pumped through heat transfer coils located
in an air duct. Cool room air is blown past the
coils and heated.
Heat exchangers are necessary when the water
in the tank cannot be used for purposes other
than heat storage. For example, an antifreeze
solution used in a solar collector is often routed
through a heat exchanger to prevent mixing with
130
water in the tank. And heating engineers often
insist that water in the tank not be used in the
room radiators--particularly
when the tank wzter
is circulated through the collector-because
of
corrosion. Because of their large size, some of
these heat exchangers can be expensive. For a
typical metal heat exchanger submerged in the
water tank, the total metal surface can be as
much 2s Ii3 the surface area of the solar collector.
For the designer who wishes to include heat
storage 2s an integral part of 2 total design, the
p’lacement of a large unwieldly
tank can be a
problem. Self-draining
systems require a tank
located below the bottom of the collector. and
thermosiphoning
systems need it above the collector top. If the storage tank is linked to other
equipment such 2s a furnace, pumps. or the
domestic water heater, it will probably have to
be located near them.
One-gallon or smaller containers of water can
and hzve been used 2s the heat storage medium
in air systems. They are arranged in racks, on
shelves. or in any fashion that allows an unobstructed air flow around them. Possible containers include plastic, glass or aluminum jars,
bottles. or cans.
ROCK BEDS
Rocks are the best known and most widely used
heat storage medium for air systems. Depending
upon the dimensions of the storage bin. rock
diameters of I to 4 inches will be required. But
through much of New England, for example.
the only available rock is I- to I l/2 -inch gravel.
Even if the proper size is available, a supplier
may be unable or unwilling
to deliver it. Collecting rocks by hand sounds romantic to the
uninitiated but becomes drudgery after the hrst
thousand pounds.
And many thousands of
pounds-from
100 to 400 pounds per square
foot of collector-are
required because of the
low specific heat of rock.
A large storage bin must be built to contain
the huge quantities of rock needed. With 30
percent void space between the rocks, the bin
Storage and Distribution
requires abcut 2 l/2 times as much volume as
a tank of water to store the same amount of heat
over the same temperature rise. And the large
surface area of rock storage bin leads to greater
heat losses.
Rock storage bins can be used in systems
v;hich combine cooling and domestic water
heating with space heating. Cool night air is
blown over the rocks. and the coolness stored
for daytime use. To preheat domestic hot water,
cold water from city mains can pass through a
heat exchanger located in the air duct returning
from the collector to the storage bin.
The location of a rock storage bin must take
into account its great volume and weight. It can
be located in a crawl space under the house or
under a poured-concrete
slab at a small additional cost. Putting it inside the basement or
other living space is usually more difficult and
expensive.
To distribute heat from a rock storage system, air is either blown past the hot rocks or
allowed to circulate through them hy gravity
convection. From there. the air carries solar heat
to the rooms. In gcncral. a fan or a blower is
needed to augment the natural circulation
and
give the inhabitants better control of the indoor
temperature.
A basic method of transferring
heat to and
from a heat storage bed is shown schematically
in the diagram. Solar heated air from the collector is delivered to the to/> of the bin. It is
drawn down through the rocks and returns to
WARM
AIR OUT
OR
.ocKs
This approach uses both water and stone as
storage media.
BLOW
Schematic diagram of an air system with rock
heat storage.
the collector from the bottom of the bin. To
heat the house, cool air is drawn in at the bottom
and is heated as it rises through the warm rocks.
The warmest rocks at the top transfer their heat
to the air-just before it ib sent to the rooms. The
furnace heating cycle (also shown) draws even
slightly pre-heated air from the top of the rock
storage. boosts it to the necessary tempemture.
and delivers it through the same ductwork to
the house. The furnace is placed in line c!fic~r
the rock bed.
Solar heated air is brought in at the top of 2
rock storage bed in order to encourage temperature strati tication. House air can then be heated
to the highest possible temperature by the warmest rocks at the top. But if solar heated air comes
in at the bottom, the heat percolates upward and
distributes itself evenly through the ntire bed
-resulting
in lower temperatures throughout.
Bringing cool room air in at the warm top also
promotes this unwanted even heat distribution.
The shape of 2 rock storage bin is closely
related to rock size. The farther the air must
travel through the rocks, the larger the rock
131
The New Solar Home Book
WATER
SALT
SALT
WATER
SALT
WATER
Btu Stored
1250
5000
9375
Temperature
Range (“F)
80”-100”
50”-130”
50”-200”
20”
80”
150”
Temperature
Difference (OFI
TVe volume of Glaubers salt needed to store the same amount of heat as
a cubic foot of water. The salt volume indicated includes 50 percent voids
between the containers of salt.
diameter required to keep the pressure drop and
fan size small. If the path length through the
rock bed is more than 8 feet, the rocks should
be at least Z inches in diameter-and
larger for
longer paths. For shorter path-lengths I- to 2inch gravel can be used.
The optimum rock diameter depends a lot on
th- velocity of the air moving through the rocks.
The slower the air speed. the smaller the rock
diameter or the deeper the bed of rocks can be.
And the smaller the rock diameter. the greater
!hc rock surface area exposed to the passing hot
air. A cubic foot of l-inch rock has about 30
square feet of surface area while the same volmile of 3-inch rock has about I13 as much. In
general. the rocks, stones, gravel, or pebbles
should he large enough to maintain a low pressure drop but small enough to insure good heat
tranfer.
ANGE MATERIALS
Phase-change materials, such as cutectic salts,
are the only real alternative to rocks ard containers of water 2s the heat storage for an air
system. A eutectic salt absorbs a large amount
of heat as it melts at a low temperature and
132
releases that heat as it solidifes.
A pound of
Glaubers salt. the most widely studied and used,
absorbs 104 Btu 2s it melts at 90°F and about
Z I Btu as its temperature rises another 30°F. To
store the same I25 Btu in the same temperature
range requires about 4 pounds of water or 20
pounds of rocks.
Much smaller storage volumes are possible
with eutectic salts. Consequently. they offer unusual versattlity
in storage location. Closets.
thin partitions. structural voids. and other small
spaces within 2 house become potential heat
storage bins.
But this advantage is less pronounced when
you increase the temperature range over which
the salt cycles. The diagram illustrates the volume of Glaubers salt needed to store the same
amount of heat as 2 cubic foot of water over
three different temperature ranges. With 50 percent voids between the containers of salt, twice
as much total volume is needed. Clearly. the
advantages of phase-change materials decline
as the storage temperature range increases.
But the costs of these salts can often demolish
the bes: ‘laid plans of enthusiastic designers. Off
the shelf. Glauhers salt costs little more than 2
cents a pound. But preparing and putting it in
a container can run the costs up. It is unlikely
Storage and Distribution
that Glaubers salt will ever be installed in an
active solar heat storage system for less than 20
cents a pound. The other salts can cost significantly more.
INSULATION
Every storage system-whether
of water. rock.
or phase-change material-requires
2 massive
amount of insulation.
The higher its average
temperature and the colder the surroundings. the
mote insulation required. For low temperature
(below 120°F) storage units inside the house.
at least h inches of liberglass insulation (or its
equivalent)
is the noml. The same unit in the
basement needs 8 inches of tiberglass or more.
And if located outside. it must be shielded from
the wind and insulated even more heavily. The
ground can provide insulation if the water table
is low. But be careful-even
a small amount
of moisture movement through the soil will ruin
its insulating value.
All ducts or pipes should be just as well insulated as the storage unit. Heat loss from the
ducts or pipes can be further reduced by putting
the collector close to the storage. The shorter
the ducting or piping, the lower the total heat
loss. And you’ll save on construction and operating costs too.
STORAGE SIZE
The higher the temperature a storage medium
can attain. the smaller the storage bin or tank
needs to be. For example. 1000 pounds of water
(about I20 gallons or I6 cubic feet) can store
20.000 Btu as its temperature increases from
80°F to 100°F. and 40.000 Btu from 80°F to
IN’F.
It takes almost 5000 pounds of rock (c)r
40 cubic feet. assuming 30 percent voids) to
store the same amounts of heat over the same
temperature rises.
Offhand. you might be tempted to design for
the highest storage temperatures possible in order to keep the storage size down. But the storage temperature is linked to those of the collector
and the distribution system. If the average stor-
age temperature is 120°F. for instance. the heat
transfer fluid will not begin to circulate until
the collector reaches 135°F. And collector efficiency plummets as the temperature of its absorber rises. A collector operating at 90°F may
collect hvice as much heat per square foot as
one operating at 140°F. On the other hand, the
storage must be hot enough to feed your baseboard radiators, fan coil units, or radiant panels.
For example. a fan coil unit that delivers 120°F
air cannot use storage tank temperatures of 100°F
without an auxiliary
boost in temperature.
In
general, the upper limit on the storage temperature is determined by the collector performance
and the lower limit by the method of heat distribution.
You can increase the possible range
of storage temperatures and keep the storage
size at a minimum by using collectors that are
efficient at high tempertures and heat distribution systems that operate at low temperatures.
It’s 2 good idea to allow some flexibiltity
in
yoclr initial designs so that you can alter the heat
storage capacity after some experience under
real operating conditions. For example, an oversized concrete water tank can be tilled to various
levels until the best overall system performance
is attained. If you’re not too sure of your calculations, the storage should be oversized rather
than undersized-to
keep its average temperature low.
The capacity of a heat storage unit is often
described as the number of sunless days it can
keep the house warm. But this approach can be
misleading. A system that provides heat for two
sunless days in April is much smaller than one
that can do so in January. It’s better to describe
the heat storage capacity as the number of degree days of heating demand that a system can
provide in the absence of sulight. For example,
a 1200-square-foot
house in Minneapolis
loses
about 10.000 Btu per degree day. In the basement, a tank with 15,000 pounds of water (about
2000 gallons) stores 600.000 Btu as its temperature rises from 80°F to 120°F. Assuming
the heating system can use water at 80°F. this
is enough heat to carry the house through 60
degree days (or through one full sunless day
when the average outdoor temperature is 5°F).
133
The New Solar Home Book
Estimating Storage Size
Thefollowing procedure helps you cakulafe the
volume of water or rocks needed to store all
the solar heat coming from a collector on an
average sunny day. It assumes that the collector performance and size have ulready been
detertnined according to procedwes described
earlier.
First yrr need to deizrtttitte the ttt~~\ittwttt
.storuge tempemtwe to be e.rpected. This is 5°F
less ihuti the ttia~ittiittti collector opertiring
trtttpmrturl~--c.clrr sidered curlier in ’ ‘E.siimuiittx Collecwr Pet-fitrtnuttw. ** The temperutwe
ruttge oj’ the .stortrge tnedium I 9 the tnuvitttwtt
dot-uge temperutiire tttittir.s the lowe.sr icwtperuttrrc thut the htwt di.stribttiotr .systerttcutt use.
For e.wmple. if the collector cut1 operute (11
IJO”F, the tttu~itnttttt storuge tetnpcruturc is
135°F: urrd if the heutittg .s~.stetttcutt use 85°F.
the remperuture ruttge is ( I35 - X5) = 50°F.
Next. detertnine the umouttt of heui you ctm
.store in tt cubic foot of the .stot-u,qetnediwtt o\w
this tettiperulure rutige. This uttwimt is the specijic heut of the tttediuttr titnes the density of. the
ttredium titnes the tetnperuture rtrtt,qe. For exutttple. tt cubic foot of wuter can store
1.0 Bircl(lb”F)(62.4 lblfi%5O”F) = 3120 Birr(ft’
over u 50°F tetnperuture runge. or 417 BIU per
gullott. If the collector guthers IO00 Bitdfi’ on
un uwruge .stttrtt~ duy in winter, you need
; IWO + 4 I 7) = 2.4 gullotis of wtiltv twui stortt,qe per syttut-ejitot c>fcollector. For Ihe collector on the Botiott home. with un ureu of
2 77.2 fi’. thui’.s 665 ~0lloti.s of wter ut the \a0
ttiitiitnum.
134
We recall from the Boston e.wtnplr that there
are 1088 degree days in Januuty. or 35 per
day. The house loses 9500 Btu per dearee day,
or (35)(95W) = 332.500 Bia Ottun average Januaty day. But the 666 gallons of water can store
only 278.055 Btu over a 50°F tetnperuture rise.
Therefore, the storuge volume must be ittc-reusedto 796 gallons. or 2.9 ,qallonsper squure
fctot of collector. to sutisfi the storuge t1eed.sof
u single Junuuty dqy.
If rock were the storage tnedium. even tnore
wlutne would be ttecessury. A cubic foot of
solid rock weighs about I70 po1ord.sund hus a
specific*heut of 0.2 I B~ul(lb”F). so it U:N store
0.2 I Btrcl(lb”F)(l7(1 lb/j? (50°F)
= I 785 Brdji-’
over the sttme 5il”F temperuture ruttge. To Store
the 1000 Btu from u .sin,qlesqutrre ,fitot of collecior, you need ( 1000 + 1785) = 0.56 cvbicj>ei
or 95 porrt1d.sof rock. To store the 332,500 Btlr
required for un averuge Juttuaqv due. you need
(332.500 + 1785) = 186 cubic ji~et (!f solid
rock.
The totul ~wlrrttte ocwrpied tly the heut storii,qe cotiiuitrt~r tnii.st itrr*!itde wit! .spuce.sin the
.storu,~ettledi!!:;i to let the uir puss. !f there we
30 !vrc*ettt voids berwt~twthe rocks jbr eurtttple.
this Boston hotne wortld need u 266cubic:fiwt
storuge bitt for the rocks. Or (f’ cottiuitteri~ed
wuter with 50 percent voids were used. the 796
gu1lon.sor I Oti cubic.feet of wuter would occttp~
2 t 2 cubic*ji>et of’ house wluttte.
Storage and Distribution
Generally. t!te storage should be large enough
to supply a home with enough heat for at least
one average January day. In Minneapolis,
there
are about 1600 degree days each January, so
the storage unit in our Minneapolis
example
should be designed to supply at least 52 degree
days of heating demand-or
520.000 Btu. Depending upon available funds, the storage can
be even larger. Or you can sink your money
into better collectors that are efficient at higher
temperatures. Solar heat can then be stored at
high temperatures-increasing
the effective
storage capacity of a tank or bin.
At the very least, the storage should be large
enough to absorb all the solar heat coming from
the collectors in a single day. If you can be
satisfied with 60-percent solar heating or less.
the simplified method described in “Estimating
Collector Size” will be useful. First you size
the collector according to the method provided
earlier under “Estimating
the Collector Size.”
Then determine the volume of storage medium
required per square foot of collector. Multiplying this volume by the total collector area gives
a reasonable “first-cut”
estimate of stomge size.
This estimate should be close enough for preliminary
design work. If this storage volume
fails to meet the heat demand for an average
January day. revise your estimate upward until
it does.
To get more than 60-percent solar heating,
it helps to know the normal sequence of sunny
and cloudy days in your area. If sunny days
followed cloudy days one after the other, you
would only have to size the collector and storage
for one sunny day and the following cloudy day.
Almost 100 percent of the heating demand could
then be provided if the system were designed
for the coldest two-day period. If the normal
sequence were one sunny day followed by two
cloudy days, both collector and storage size would
have to be doubled to achieve the same percentage. At the Blue Hills weather staiion near
Boston, for example. about 80 percent of the
sunless periods are two days long or less. A
collector and storage system that could carry a
house in Blue Hills through two cloudy days of
the coldest weather will supply more than 80
percent of the home’s heating needs. But the
wide variaticn in weather patterns at a single
location makes such a practice little more than
educated guesswork. And this kind of weather
data is hard to obtain.
HEAT DISTRIBUTION
An active solar heating system usually requires
another sub-system to distribute the heat to the
rooms. With integrated solar heating methods
such as mass walls and direct-gain windows,
the solar heat is absorbed directly in the fabric
of the house and heat distribution
comes naturally. But an active system usually needs more
heat exchangers. pipings ducts. pumps. fans.
and blowers to get the heat inside the house.
And there must still be some provision for backup
heating in the event of bad weather.
The heat distribution
system should be designed to use temperatures as low as 75°F to
80°F. If low temperatures can be used. more
solar energy can be stored and the collector
efficiency
increases dramatically.
In general.
warm air heating systems use temperatures from
80°F to 130°F. while hot water radiant heating
systems require temperatures from 90°F to 160°F.
Steam heating is rarely combined with a solar
array since it needs temperatures over 212°F.
so incorporating
solar heating into an existing
house equipped with steam heating will require
a completely separate heat distribution
system.
Many designers of new homes opt for forced
w;rm air systems or radiant heating systems.
By using larger volumes of air or oversized
panels (such as concrete floors). these solar
heating systems can operate at lower temperatures. Radiant slabs take time before the room
is comfortable and usually require slightly higher
storage temperatures. But because their radiant
heat warms occupants dircct!y. the air temperature can be kept 2 to 3°F lower. reducing the
home’s heat loss.
135
The New Solar Home Book
AUXILIARY
HEATING
Even a system with a very large storage capacity
will encounter times when the heat is used up.
So the house must have an auxiliary heat source.
This is a major reason why solar heating has
not yet met with widespread acclaim-you
still
have to buy the conventional
heating system.
The severe consequences of a single sustained period of very cold, cloudy weather are
enough to justify a full-sized conventional heating system as backup. Small homes in rural
areas can probably get by with wood stoves. If
the climate is never too severe, as in Florida
and most of California.
a few small electric
heaters may do the trick. But most houses will
require a full-sized gas, oil. or electric heating
system. Solar energy is a means of decreasing
our consumption of fossil fuels-not
a complete
substitute. Energy conservation in building construction. the first step to a well-designed
solar
home, lowers the building’s
peak heat load,
which means you can buy a smaller. less-expensive auxiliary heating system.
If the auxiliary system won’t be needed very
often, you might well consider electric heating.
But remember that 10.000 to 13.000 Btu are
burned at the power plant to produce I kilowatthour of electricity-the
equivalent of only 3400
Btu in your house. At an efficiency of 65 percent. an oil furnace bums only S-400 Btu to
achieve the same result. And electric heating
can be very expensive, although the tirst cost
of electric heaters is cheaper than a gas or oil
furnace.
The auxiliary
heater should not be used to
heat the storage tank or bin because the collector
will operate at a higher femperture and lower
efficiency.
And there will be costly heat losses
from the storage container if an auxiliary sytem
provides continuously
higher storage temperatures. The heat lost from the storage container
is already 5 to 20 percent of the solar heat collected.
The heat pump has served as a combination
backup and booster in a number of solar energy
136
systems. It is basically a refrigeration
device
working in reverse. The heat pump takes heat
from one location (the heat source) and delivers
it to another (the heat sink). The heat source is
cooled in the process and the heat sink is warmed.
A heat pump can deliver about three times
the energy required for its operation. For every
2 Btu which a heat pump takes from a source,
it needs the equivalent of I Btu of electricity
for its operation. It delivers all 3 Btu to the heat
sink. Thus, its Coeficient of Performance (COP),
or the ratio of the heat energy delivered to the
energy required for operation, is 3. Typically,
this coefficient ranges from 2.5 to 3 for good
heat pumps. By contrast, electric resistance
heating has a COP equal to I, because it delivers
I Btu of heat for every I Btu of electricity
expended.
When heat pumps are used1 in conjunction
with solar heating sytems, the stored heat is
useful over a wider temperature range. Without
heat pumps, a forced warm air system would
use storage tempertures from 80°F to 130°F and
a hot water radiant system would use 90°F !o
160°F. But with a heat pump, both systems can
use 40°F storage temperatures! The heat pump
takes low grade heat from storage and delivers
it at a higher temperature to the heat distribution
system. This increased temperature range results in an increased heat storage capacity and
markedly enhanced system efficiency. The extra Btu that a cool collector can gather each year
often justify the added cost of installing a heat
pumpBut heat pumps require electricity for operation. About 10.000 to 13.000 Btu are burned
at the power plant when a heat pump uses I
kilowatt-hour
of electricity (or 3400 Btu) to deliver a total of 10.200 Btu. So, including losses
at the power plant and in electrical transmission.
the real Coefficient of Performance is closer to
I than 3. And electricity
is expensive. High
electricity bills have been a major shortcoming
l\f past solar heating systems that relied on heat
pumps.
Storage and Distribution
Heat Pump Principles
A heat pump is a me~hani~ul device that trunsfers heat from one medium to unother, thereb!
cooling the first and wurming the second. It cun
be used to heat or cool a body of air or CItank
of water, or even the earth. The cooled medium
is called the “heat source” and the wurmed
medium is the “heut sink.” A household refrigerator is N heat p”mp that tukes heat from
the food compurtments (the heut source) und
dumps it in the kitchen uir (the heut sink).
The heut pump trunsfers heut against the
gruin-from c,~1 ureas to wurm. This sleightof-hand is uccomplished by circulating u heut
trun.TferJuid or ’ ‘refii,yerunt’ ’ (such us the Freon
cornmonl~ used in household refrigrrutors) between the sour(‘e und sink und inducing thi.sJuid
to evuporute und condense. Heat is ubsorbed
frotn the source when the heut trunsfiv- jhfld
evuporutes there. The vupor is then compressed
und pumped through a heut e..rc*hun~erin the
sink, where it condenses-releu.siyg its I’atent
heut. The condensed liquid retxrns to the heut
eschunger in the .sourcethroc;gh und expunsion
vulue. which muintuins the pres.sure [email protected]
creuted b! the compressor.
The put~kuged,se!f-c.ontrrinedheat pump used
in resldentiul upplicutions generull~ reserses
the direction of the rejkigerunt jiow to chunge
.from heming to cooling or \ise-\vrsu. A jburnwy ~*uluereverses the direci ion of.jlo,c*through
the compressor so that high pressure vapor condenses inside the conditioned space when heating is needed and low pressure liquid evaporates
inside when cooling is desired. Heut pumps are
classified according to the heat source and sink,
thejuid used in each. and the operating cycle.
The heat pump shown here is a water-to-water
pump with reversible refrigerantjow. A household refrigerutor is an uir-to-air hent pump with
u fixed refrigerant jlow. Ground-coupled heat
pumps ure usually water-to-air. but are occasionally wuter-to-wcrter.
HEAT
HEAT
SOURCE
I
I
-l
HEXT W’.‘!sFER
EVApDRATE5
FLUID
I I
CLLII
1
III
HEAT
l
TWWSFEU
CONDENSES
COMPRESSOR
Coefficient of Performance
A heat pump uses electrical energ! fo munipulute heut trunsferfrom .source to sink. The heat
deposited in the sink is N combinutiov of the
heat ge~leruted by compressing the refrigerunt
(which requires electricul power) cind the lutent
heat rei’pased by the condensing vapor. The heat
removedfrom the heat source is the lvent heat
of evuporution. The eflectiveness of a heartpump
is indicuted bx its Coeffiunt of Pet-ftn-munce,
or COP, which equuls heut energ! deposited
SINK
(or removed) divided by electrical energy consumed.
The electrical energy reqitired to run the
compressor (in hlvh) can be converted to Btu
b? multiplying by 3413 Btulkwh. Because the
heat oj’compression is part of the heat deposited
in the sink, the COP of a heat pump used.f~~r
heating is usuctll~ greclter than the COP qf the
.sume hsut pump used for cooling.
137
II
FL’UID
Of all the benefits the sun can give us, potentially the most far reaching is direct generation
of electrical
power. Photovoltaic
(PV) cells
can generate electricity
from sunlight.
Like
sunlight.
electricity
is an essentially
benevolent form of energy: silent, invisible,
quick,
nonpolluting,
far reaching.
PV-powered
vehicles and aircraft are now where Henry Ford
and the Wright brothers were with their fossilfueled inventions
less than a century ago.
Imagine a future where pollution-free
cars and
airplanes whisk us silently from one place to
another: where trucks, buses, machines, and
factories no longer intrude upon the natural
landscape and atmosphere.
The use of solar cells to generate electricity
is a very recent achievement. Although the photovoltaic phenomena was tirst discovered in the
19th centrury. it was not until the 1950s that
scientists built the tirst working solar cells. Because of the high costs involved,
solar cells
were used initially in military and research projects and where the cost of ol,taining conventional power was too expensive. During the t 460s
and 1970s. solar cells were used to provide power
in remote installations, such as electronic relay
stations. irrigation pump facilites. and navigational buoys. as well as for homes and other
installations
that were not tied to conventional
power lines. Other applicatons were developed
by using solar ceils to charge storage batteries.
138
providing a steady source of power for devices
such as marine radios, lights, and recorders.
High cost is still a serious disadvantage of
PV systems. Although considerably cheaper than
the $5,000 per watt cost of 25 years ago, PV
power is still more expensive than conventional
utility power.
SUNLIGHT
TO ELECTRICITY
Photovoltaic
or solar cells generate electricity
when exposed to sunlight. Although the amount
of electricity produced by a single solar cell is
small, a group or array of cells can generate a
considerable amount-almost
l/2 kilowatt per
square foot of cell surface. A sufticiently
large
army-555
square feet for a small residential
installation-can
generate enough electricity to
meet a substantial percent of the needs of a
single-family
home.
In practice, electricity
generated by PV arrays can be used as direct current (dc). stored
in batteries for subsequent use, or changed to
alternating current (ac) for immediate use. With
a good-quality
power inverter (a transformerlike device that converts dc to ac). surplus electricity can be fed into the local power grid to
be credited against later power drawn from that
system when the photovoltaic array is not gen-
Photovoltaics: Electricity from the Sun
erating electricity.
Local power companies are
required by federal law to accept and pay for
such customer generated power. This eliminates
the need for costly battery storage.
When bundles of solar energy, called photons, penetrate two-layered solar cells, *hey knock
loose electrons. transfening
energy to them.
These loose electrons move to one side of the
cell. creating a negative charge. On the other
side, a deticit of electrons creates a positive
charge. As they move about, these loose electrons are quickly caught up in an electrical held,
forming a weak electric current at the junction
of the two layers.
To generate electric current in this manner,
solar cells use a semiconductor,
typically two
layers of silicon. The negative layer on the top
facing the sun is treated or “doped”
with phosphorous to create an excess of electrons; the
positive layer on the bottom of the cell is treated
with boron to create vacancies or “holes”
for
new eiectrons to till. As photon energy is absorbed by the negative layer. millions of excess
electrons are captured by the electrical held at
thejunction
between the two layers. The voltage
difference between them pushes the electrons
through a wire grid on the front of the cell.
which is connected in turn to the wire grid on
the back of the next cell. As current tlows through
a seiies of cells. its voltage continues to build.
The ~~11sare sandwiched together between a
substrti*e and a superstrate to form a rwdule.
The aluminum-framed
modules are connected
together to form p~w1.s that are installed in an
crrrtr~. The ultimate current and voltage produced by the array depends on how the modules
and panels arc wired together.
Most PV semiconductors
arc made of crystalline silicon in an expensive manufacturing
process. Among alternative semiconductor materials less expensive to produce, amorphous
silicon offer:; much of the same capability as
crystalline
silicon. Amorphous silicon actually
absorbs visible light better than the crystalline
form, but it is less efticientabout 3 percent
compared with I2 to I6 percent for crystalline
in converting light to electricity.
Nevertheless.
the lower material and fabrication
cost make
amorphous silicon a pnme candidate to replace
crystalline
silicon as a low-cost source of PV
power in the future.
POWER REQUIREMENTS
A typical residential PV installation consists of
an array of solar modules, an inverter to change
solar-cell direct current to alternating current
and, where local utility power is unavailable,
a
bank of batteries to store excess electricity.
In designing a system, you must take into
account factors such as geographical locations.
availability
of local utility power and how much
electricity you need.
GeogrEtphic location affects the amount of
potential power available from a system, because the percentage of sunshine available varies greatly in different sections of the country.
A home in the southwestern U.S. can count on
a much higher percentage of daily sunshine than
one in the northeast or coastal northwest.
A
residential PV system in Arizona for example,
will generate almost twice as much electricity
as a similar system in New Hampshire. To detemtine the amount of sunshine available in your
location you can refer to the Clinrutic Atlas oj
the Ullited States which lis s percentages of possible sunshine by geographic area. The less sunlight available. the larger your PV array must
be to meet given power during needs.
If you live in an area where local utility power
is unavailable. you must design your system to
store power for use when there is not enough
sunlight for PV operation. Storage requires a
bank of batteries with sufficient capacity to provide power at night and during cloudy weather.
AN AVERAGE
HOME
The table that follows lists the example requirements for an average home. But how much of
this load could be met by a PV system’!
Let’s say that the house with the electrical
demand we just calculated is located in Phila-
139
The New Solar Home Book
Estimating Array Size
A prime consideration in your design is the
amount of power your home needs and the pattern of daily usage. Thesefactors determine the
size of the system and its components. Chances
are that the power needs of appliances and
lighting in your home will exceed the capability
oJL‘most moderate-size residential installations.
The solution is energy corservation-reducing
and planning your needs.
First determine your need for power by making a list of your appliances and other electrical
devices. Record their power requirements in
bvatts.how many hours each is used on a weekly
basis, and the percentage of time the appliance
is normally running. With this information, calculate your initial average overall requirements
per day. For example, to calculate how much
power your refrigerator will require on an average day, figure the weekly average and divide
by 7. Its rated power requirement is 400 watts.
Since the refrigerator is used 24 hours a day
(168 hours in 7 days:)and it runs about one half
(50%) of the time, multiply these figures together to get its energy requirement:
delphia (40” north latitude). There is room on
its SOO-square-foot roof for the PV array. The
roof is pitched at a 40” slope. First, we must
calculate how much solar radiation is available
every month. For example, in January, the average daily insolation (from the “Clear Day
Insolation”
tables in the appendix) on a 40”
slope is 1810 Btu/(ft’ day). From the “Mean
Percentage of Possible Sunshine”
map in the
appendix, we see that Philadelphia receives only
50 percent of the possible sunshine in January.
Multiplying
0.50 by 18 IO Btu/(ft’ day) and 500
square feet, we calculate that the roof would
receive 452.500 Btu/day of sunshine. Since there
are 3412 Btu in a kwh. that is 133 kwh/day.
But not all that energy can be converted into
electricity.
If the solar cells only have an efficiency of 0.10. then the array only produces
(133)(0.10) or 13.3 kwh/day. The inverter and
other balance of system parts lose another 15
percent in conversion of the dc power to ac. so
the system output
is further
reduced by
(13.3NO.85) to 11.3 kwh/day.
If our daily average demand is 19.4 kwh. the
PV array could provide ( 1 I .278/ 19.4)( 100) or
58 percent of tb.e electricity used. Since not all
the electricity
is used during daylight hours,
some of the demand would have to be met by
the power company, but some of the power
produced by the PV array would be stored in
batteries for later use, or flow back into the grid
to be credited against the power bought.
The second table lists the average radiation
that strikes the roof each month, the percent
possible sunshine for that month, the total daily
insolation on the roof, how many kwh a day
the system produces after subtracting for cell
efficiency
(0.10) and balance of system efhciency (0.85). The last column is the percent
of the daily demand supplied by the array.
Remember that this only gives you an estimate of what the array may produce, and not a
guarantee of how much energy you’ll collect.
That depends on your system components and
local climate.
If you have unlimited roof area and would
like to size the array based on the demand, you
can find a range of areas that will help you
decide. With the monthly insolation values and
percent possible sunshine, you can find the maximum and minimum array you need.
In our example, December gets the least sun
with only 815 Btu/(ft’ day) from (1634 Btu/(ft2
day))(0.5).
In August, the roof gets the most
sun with 1400 Btu/(ft’ day) from (2258 Btu/(ft’
day))(0.62).
Converting the solar gains to kwh,
140
400 watts( 168 hoQrs)( .50) = 33.6 kwh per week1
7daysper week = 4.8 kwhlday
Photovoltaics: Electricity from the Sun
EXAMPLE
POWER REQUIREMENT
Power
Required (Kw)
Appliance
Refrigerator
Dish washer
Clothes washer
Clothes dryer
Stove
Water heater
Oil furnace pump
Toaster
Record!tape player
Television set
Radio
Electric saw
400
loo0
600
4500
3500
3ootl
250
loo0
100
200
40
450
Hours/
Week
168
5
2
2
7
168
I68
O.G7
7
7
14
2
Percent
Running Time
50
loo
10
IO0
100
10
20
100
100
100
100
100
Average
Kwh/Week
33.6
5.0
1.2
9.0
24.5
50.4
8.4
0.07
0.70
I .40
0.56
0.90
Total Kwh per week 135.73
Average Kwh per day 19.39
EXAMPLE
Month
January
February
March
April
May
June
July
August
September
October
November
December
GAIN FROM 500 SQ F-f ROOF ARRAY
(I)
(2)
Surface Daily
Total Insolation
Btu / (ft’ Gay)
Percent
Possible
Sunshine
:x10
2162
7330
7370
2264
7374
--“30
mm_
“SX
339x
--2060
I778
1634
0.50
0.60
0.5s
0.55
0.60
0.62
0.60
0.63
0.60
0.60
0.50
0.50
(3)
(4)
Total Dairy
System-Produced
Insolation (Kwh,‘day) Energy (Kwh/&y)
(I) x (2) x 500 / 3312 (3) x 0. I x 0.85
I33
I 90
18X
IX7
I99
202
I96
20s
I96
I81
I30
I20
II.3
16.2
16.(r
IS.9
16.9
17.3
16.7
17.4
16.6
IS.4
I I.1
10.2
Average
Percent
Supplied
(4) / 19.4 x loo
58%
83
82
82
87
8’)
86
YO
86
79
57
52
78%
The New Solar Home Book
SIZING THE ARRAY TO DEMAND
System Size (ft*)
Based on Monthly
Insolation
Month
January
February
March
April
May
June
July
August
September
October
November
December
Energy Produced
with 953 ft* Array
Kwh/Day
860
600
608
610
573
555
582
556
583
630
876
953
minimum
area =
=
area =
=
% Deficit
+I I%
59
57
56
66
69
64
71
64
51
9
0
12.6
18.0
17.8
17.7
18.8
19.1
18.6
! 9.4
18.5
17.2
12.3
II.3
-35%
Average
+48%
Average
-13%
!9.4/!0.24(0.10)(0.85))
953 ft’
!9.4/[0.41(0.
!0)(0.85:]
557 ftl
Depending on how much money you want to
spend on the system, and how much you want
to invest in battery storage (for the excess) or
how much power you want to buy from the
electric company (from the deficit). your array
should be between 557 and 950 square feet.
The next table shows how much excess or deficit energy those two PV array sizes will produce each month. The first column shows how
big the array should be based on the average
energy produced per day that month. The second shows the energy produced per month if
the system were sized at the maximum and the
142
Kwh/Day
21.5
30.8
30.4
30.3
32.3
32.7
31.8
33.2
31.7
29.3
21.1
19.4
the maximum gain is 0.41 kwh/(ft’
day) and
the minimum is 0.24 Btu/(ft’ day).
The areas needed to supply the demand can
be found by: area (ft’) = daily demand/!(so!ar
gain) (cell efficiency) (balance of system efficiency)].
In our example:
maximum
% Excess
Energy Produced
with 557 ft* Array
8
0
3
I
4
0
4
I2
36
42
third how much energy would be produced if
the system were sized to the miminum.
Unless
your power company is paying top dollar for
the electricity it buys from you, you’d be better
off sizing the collector toward the minimum side
to save on the high first cost of the system.
Your initial overall requirement for power is
likely to be substantial. we!! beyond the capacity of a residential PV system. To reduce the
amount of power needed to a more practical
level. conserve energy first. Reduce the number
of electrical appliances. Use them less. When
the appliance requires a large amount of power,
e.g., a refrigerator, hot water heater. or clothes
dryer, install more efficient units or replace them
with non-electrical devices, such as a solar water
heater, or a clothes line for drying. Your goal
is to reduce the gap between the amount of
power needed and the amount your PV system
will generate.
Daily residential
power use usually shows
peak consumption at meal times and in the evening with an additional low level of steady usage
24 hours a day. Your system must be able to
provide adequate power at these times plus when
Photovoltaics: Electricity from the Sun
it is dark or cloudy. It is important to note that
electric motors, such as those used in refrigerators and washing machines, require a large
surge of electricity when they start. Your system
has to meet these extroardinary peak needs also.
SUPPLEMENTAL
POWER
To meet a!! of your power needs, you will probably have to supplement your PV power with
either local utility power or bstttery storage.
Local utility power enters a residential PV
system through an inverter. which also converts
dc power from the colar cells to ac. As ac power
from the utility is used, it is metered in the usual
way to determine the number of kilowatt hours
used. When power from the PV system reaches
a sufticient level, the inverter cuts off utility
Power. The inverter also directs excess PV-systern-generated power into the local utility lines,
in effect running the meter backward. Because
power fed into a utility power line must meet
certain standards of electrical quality, inverters
must be properly matched to the utility system.
Battery storage can be used to meet both 24hour and peak-load needs. (Batteries are essential where no local utility power is available.)
Charged by electricity from solar cells during
hours of sunlight, batteries store power, making
it available for use by appliances and lights when
needed. Where appliances and lights can be operated on dc. no inverter is necessary. However,
most appliances use ac. and so need an inverter
to convert dc to ac current.
The number and overall voltage and current
output of the batteries depend on power needs
as we!! as on the amount of power provided by
the PV system. High voltages (32 to 38 volts)
are much more efficient than low voltages and
are necessary to meet most modern residential
power requirements. Depending on geographic
location. a PV system may need to rely on battery power for two or three weeks of cloudy
weather at a time. A gasoline generator can be
used to charge batteries during sunless periods,
reducing the number of batteries required.
POWER INVERTERS
Power inverters are essential in ac systems to
convert solar-cell generated dc to residentialsystem ac. They are also essential as a go-between with the utility line.
The inverter is the control center of the PV
system. It turns on the system when sufficient
power is available from solar cells. It turns it
off when power drops below a set level. The
inverter also determines the quality of the a!temating current waveform, which powers resDevices such as stereo
idential appliances.
turntables require a high-quality waveform. When
a PV system is tied to local utility lines, the
inverter also must provide an ac waveform compatible with the utility line. Only a high-quality
inverter--so!id
state or synchronous-can
meet
these requirements.
RESIDENTIAL
INSTALLATIONS
A typical residential PV system has SO- to 860square-feet of roof-mounted,
interconnected PV
modules. Such an array can produce 5 to 8
kilowatts at maximum voltages of I60 to 200
volts. At efficiencies
of 85 to 90 percent, an
inverter will yield a peak rate between 6400 and
9000 kwh of power in regions with moderate
sunshine (for example. the Northeast) and an
additional 80 percentI I.500 kwh to 16,200
-in
the Southwest.
While such a system may not provide 100
percent of a family’s
electrical needs, it can
provide 50 to 90 percent, depending on location
and amount of sunshine.
Residential PV panels can be mounted in severa! ways; stand-off, direct, rack, or integrai.
A stand-off mount places PV panels several
inches above the roof, which allows air to circulate behind the cells to coo! them and increase
their efficiency.
These panels are attached to
mounting rails which, in turn. are attached to
the roof rafters.
Direct-mount
arrays are those attached directly to roof sheathing, replacing the rooting
material. Like shingles. these special PV mod-
143
The New Solar Home Book
ules overlap each other, forming an effective
sea! against water and wind. However, operating temperatures will be higher and efficiency
lower because of the lack of air circulation around
the back of the cells to coo! them. For this
reason, direct-mount arrays are few and far between.
Rack-mounted
PV panels are used on flat
roofs to position PV cells at the proper angle to
the sun. Although more complicated
to build
than a stand-off mount, this arrangement provides excellent air circulation,
increasing system efficiency.
Integral-mounted
PV systems replace ordinary roof sheathing and . !. :.ngles. PV pane!s are
attached directly to rafters, the space between
them sealed with gaskets. Other edges are sealed
with silicone sealant or held down by aluminum
battens. Attic ventilation
keeps backside temperatures at efficiently coo! levels.
Integral mounts can produce hot air for space
heating or solar domestic hot water by directing
144
the air used to cool the back of the cells to the
load or to a storage container.
In designing a mounting system, there are
several factors to consider. Panels must be installed so that they are easily accessible. For
example, debris must be brushed away from
time to time and accumulated dirt washed off
cell surfaces. Stand-off and rack. panels must be
easy to remove from their mountings for repair
or module replacement.
Panels installed intergrally must be water-tight and panel backs accessible for repair and cooling.
Whenever
possible t’V panels should be mounted to allow
air to circulate behind them to cool the underside of the cells.
The appearance of the PV array is also
impottantespecially roof-mounted arrays that
are clearly visible. Rectangular and square ce!!s
and dark anodized metal frames blend in better
with most roofs than modules of round cells and
polished aluminum frames.
S&r energy, in rhe lusr unuly.si.s. bus ulrcw.~.s ing upon size and complexity, a solar heating
system could add 2-10 percent to the building
been rhe hu.si.snor only oj’ i~ir~ilixrion. hlrr of
cost of a new house. A system fitted to an exI(fv: .from the primecul sun-5uskin,q plunkron lo
isting house costs more. Financing such an exmodern tnun harvesring his fields utd brrrnin,q
penditure is particularly
difficult during periods
cwal und oi! betwuth his boilers. .solur energ>
when costs are burdensome, interest rates high,
bus provided rhe ul~itnute movin,q.fbrc*r. But irs
direr-r itrilixticm iiI u higher Ie~el of‘rechnolo~~~ and mortgage money difficult to obtain. Financing is one of the principal reasons people
is u tie\i’ ph~~tiottietr!ttt.und rich wirh Neil* podecide against using solar energy.
tenMitie.s ut rhis stuge of human uffirirs.
Peter van Dresser,
Lund.sc*upe.Spring I956
There are sti!! many obstacles blocking
the
widespread use of the sun’s energy for heating
and cooling. Most of these problems are nonrechnicul in nature, having to do with solar energy’s impact upon and acceptance by society
as a whole. Whenever a new building method
bursts upon the scene. financial institutions and
the building trades are understandably conservative until that method has proved itself. But
with the long-range depletion of cheap fossil
fuels and the rising energy needs of our developing world, the rapid development o!.‘!his heretofore neglected power source is inevitable.
FINANCIAL
CONSTRAINTS
The greatest barrier to the immediate home use
of solar energy is the high initial cost. Depend-
Part of the problem. of course. is that solar
energy is sti!! not a major established alternative
to conventional
heating systems. Banks are reluctant to fund an expensive addition that they
consider unlikely to pay back. As more solar
heating systems come into genera! use, however, and bear out the claims of lowered heating
costs, loans for these systems are becoming more
readily available.
Compounding
the financing difficulties
is the
fact that an auxiliary
heating system must be
provided, even in solar-heated homes. People
prefer complete heating systems rather than systems that provide only 50-90 percent of their
heating needs. But IOO-percent solar heating
systems are usually far too large to be practical.
so the additional expense of a conventional heating system must also be borne.
If long-range predictions are true, and sources
of conventional
fuels dry up over time. arguments against a big cash outlay will lose their
145
The New Solar Home Book
Ufe-Cycle Costing
L$e-cycle costing is an estimating method that
includes the ftlrure costs of energy consumption,
maintenance and repair in the economic comparison of severul alternatives. ‘hese furure coss
can make an initially cheaper system costlier
over the life cycle of rhe system. Life-cycle cosfing methods make such costs visible af the OUIset, und rhey include the economic impact of
interest rates and injarion. They ure iderllly
suited for comparing the costs oj’ solar heating
with those of conventional methods.
1n order to obtain consistent cost cotnpurisons among several alrernarives, ull rhe co>fs
of each sysretn (over a selected “life cycle” )
ure reduced to total costs over a unit of titne.
usually rhe jirsr year. Furure suvings such as
lon*er fwl costs ure discounred to ’ ‘presentvulue” doll~!rs, ,vhich is rhc atnount of money
rhut, jf it:\tesred roduy. would grow lo the value
of rhe suvings in the inrervening years. And if
the unnuul operating and maintenunce expenses
can be predicred to grow at some steady injiarion t-ale. the presenr-value total of those expendintres over rhe lije c*ycleoj* the system (P,)
can be calculared using the jbllorcing equation..
P, = A(R)(R” - !)/(R - I)
where R = (I + g)/(I + i) and i and g are
the .frucriottal rates of interest und injiation. In
this equation, [he current annual e.~pen.se(A)
is tnulriplied b! a fuctor rlhich accout1t.sfor rhe
number of yeurs in the lift> cycle 01) uttd fhe
rate ut which rhe annual expense (A) is expected
to increase
. .
Example: A.s.sumerhar 1111’
refail cosf of‘hetrring oil is $1 .OOper gullon and that it will increase 5 percenf per Jear. What is lhe presenr
\vrlue of rhe e.\-penditure.sj%r one gallon of oil
euch year for the next 30 yeurs?
146
Solution: Assuming an annual interest rate
of 10 percent, the ratio R equals I .OSlI .12, or
G.9375. Applying the equation, wefind the present value of 30 gallons of oil expended over the
next 30 years is $12.84:
.P, = $I .00(0.9375)(0.9375 - !)/(0.9375 - 1)
= ($!.00)(12.84) = $12.84
To ger the life-cycle costs of a system, the
purchase and installation prices are added to
the present value of the total operating and
maintenance cosCs.
For example, an owner-builder might want
lo compare the life-cycle costs of insulated 2x4
stud walls lo the cost of insulated 2x6 stud walls.
He estimates that the 2x4 walls will cost $7140
CObuild but will lose 48.4 million Bru per year:
the 2x6 walls will cost $7860 and loge 34.8
million Bru. Assutning that a gallon of hearing
oil produces 100,000 Btu of useful hear. the
house will require 484 or 348 gallons per year,
depending upon rhe wall construclion. Over a
30-year l[fe cycle, rhe operuring costs will be
($12 .X4)(484) = $62 1.5for the 2x4 walls and
($12.X4)(348) = $4468 ji)r the 2x6 walls, in
present-value dollars. Maintenance expensesare
equal for the two alternatives and hence are
ignored. Adding operating und insrallution costs,
the owner-builder jinds rhat the 30-year lift>cycle COSISof rhe two alrernutir’es are:
2x4 walls = $7140 + $6215 = $13.355
2x6 wulls = $7860 + $4468 = $12,328
Lij&cycle costing suggests that the initially
more e.xpensive2x6 wall is actually more thun
$1000 cheaper over titne. Sitnilar cosring can
be used to cotnpare the COSC.S
of solar heating
and oj’ conventional systems.
Epilogue
clout and the avuilubili~
of fuel will become
the real issue. People who find themselves without fuel will decide that the shortage is reason
enough for using solar energy. that the initial
costs of the system are less tmportant.
But home-financing plans can encourage such
an investment even now because lower heating
bills over rhe lifprimc of the system make it a
sound buy. Ail too frequently.
financial institutions disregard the ever-increasing
operating
costs of a conventionally
heated home and focus
upon the large initial costs of a solar heating
system. Some lending institutions, however, are
using life-cycle costing methods, which compare the higher initial costs of solar to lower
operating and maintenance costs. These meth-,
ods emphasize the lowest total monthly homenwning costs (mortgage payments plus utilities). Lenders should allow higher monthly
mortgage payments if monthly energy costs are
lower. Most progressive lending institutions are
doing just that.
SYSTEM RELIABILITY
A major difticuity
in the custom design and
manufacture of active boiar heating systems for
particular sites is the necessary combination of
low cost. good performance. and durability. The
building designer must have a thorough understanding of the principles of solar energy and
of the pitfalls discovered in the past. Even then.
many things can go wrong with such complex
systems. and many architectural and engineering firms hesitate to invest extra time and money
in custom designs. Most active systems today,
however. are designed by the supplier of the
equipment.
leaving the building designer with
the responsibiity
only to seiect the best system
from the most reliable supplier.
One of the most appealing aspects of solar
heating has been that custom design and on-site
construction
often seems a cheaper alternative
than buying manufactured collectors. However.
this turns out usually not to be the case as construction costs rise and prices of manufactured
systems drop. (The exception would be systems
built by the do-it-yourseifer.
or the owner-builder.
whose time and labor are not usually counted
:n the cost.) There are now hundreds of excellent solar products, and the competition is fierce.
Resulting price reductions are inevitable in the
long run, but they will come slowly.
Passive solar systems are often cheaper, more
efficient. and more reliable than active systems.
and are usually more appropriate for custom
design and on-site construction of new houses.
Here, too, there are now many products to choose
from.
SOLAR ENERGY AND THE
CONSTRUCTION 1NDUSTRY
The housing industry and the laws that regulate
it have a record of slow adaptaticn to change.
The industry is very fragmented, with thousands
of builders, and 90 percent of all work is done
by companies who build fewer than lo0 units
per year. The profit margin is small. and innovation is a risk that few builders will take.
But the fragmented nature of the construction
industry is essential to the localized industries
that have sprung up around solar energy. Even
if a few large manufacturers
achieve low-cost
solar collectors.
interstate transportation
costs
will remain relatively high. adding one or two
dollars per square foot to collector costs. Onsite construrtion
or local fabrication of components will be a viable alternative for years to
come.
Some contractors and developers install solar
equipment in order to evoke interest in their
recent housing developments.
And, despite
lessening public concern for energy. most buiiders and contractors are building energy efficient
homes, and more and more are including solar
water heating and passive solar room heating
systems.
147
The New Solar Home Book
GOVERNMENT
INCENTIVES
Some local governments still acknowledge the
benefits of solar energy in their tax laws. The
extra employment
stimulated by solar energy,
which requires local labor to build and install
components. can be a boon to local economies.
Annual cash outllows for gas and oil from energy-poor areas like New England amount to
billions of dollars that can be saved through
energy conservation and the use of solar energy.
Also. reductions in pollution levels result from
lowered consumption
of burning fuels. Local
taxes can discourage solar systems. if instaiiation costs mean higher property taxses for the
owner. Lowering taxes to encourage the use of
solar energy is a desirable goal. and many communities now do not add the value of a solar
energy system to the assessed value of a home.
Government
incentives
and development
programs are important to the further development of solar energy. Solar energy must overcome many obhtacies. apart from competition
with established.
government-subsidized
energy suppliers such as the nuclear and oil industries. The importance of solar energy to global
and national welfare is more than adequate justification for equal promotion of it.
But more than technological
innovation and
government incentive will be needed to make
solar systems a universal reality. In a larger
sense. a nation’s energy future rests in the personal choices of its people. The consumption
practices learned in an age of plentiful and cheap
fossil fuels cannot be supported by a solar economy. We can enjoy clean. inexhaustible
solar
energy much sooner if we insist that our energyconsuming
possessions (houses, cars. appiiantes. etc.) be energy efficient. Also. we should
take a cue from the more intimate relationship
between humans and their natural world that
prevailed in the centuries prior to the availability
of cheap fossil energy. People had simple needs
that could be supplied by the energy and materials around them. They interacted with their
climates to take full advantage of natural heating
and cooling. These attitudes of efficiency
and
harmony with the environment must once again
become standard. A new solar age will dawn
when we can forego our high-energy ways of
life and return to our place in the sun.
uORTH
-T-T7-Tl
JAN
FEE
MAR
API3
MAY
JUN
JUL
AUG
SEP
OCT
NOV
OEC
Solar declination
The sun’s position in the sky is described by
two angular measurements. the solar altitude
(represented by the Greek letter theta or 8) and
the solar azimuth (represented by the Greek ietter phi or 4). As explained earlier in the book.
the solar altitude is the angle of the sun above
the horizon. The azimuth is its angular deviation
from the true south.
The exact calculation of theta or phi depends
upon three variables: the latitude (L). the declination (represented by the Greek letter delta
or 6). and the hour angle (H). Latitude is the
angular distance of the observer north or south
of the equator; it can be read from any good
map. Solar declination
is a measure of how far
north or south of the equator the sun has moved.
149
The New Solar Home Book
15
DEC
JAN
FEE
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
Equation of time
At the summer solstice. 6 = + 23.5’. while at
the winter solstice 6 = - 23.5” in the northern
hemisphere; at both equinoxes. 8 = 0”. This
quantity varies from month to month and can
be read directly from the first graph shown here.
The hour angle (H) depends on Local Solar
Time, which is the ume that would be read from
a sundial oriented south. Solar time is measured
from solar noon. the moment when the sun is
highest in the sky. At different times of the year,
the lengths of solar days (measured from solar
noon to solar noon) are slightly different from
days measured by a clock running at a uniform
rate. Local solar time is calculated taking this
difference into account. There is also a correction if the observer is not on the standard time
meridian for his time zone.
To correct local standard time (read from! an
accurate clock) to local solar time, three steps
are necessary:
I ) if daylight savings time is in effect, subtract
one hour.
2) Determine the longitude of the locality and
the longitude of the standard time meridian
(75” for Eastern. 90” for Central. 105” for
Mountain,
120” for Pacific, 135” for Yukon,
150” for Alaska-Hawaii).
Multiply
the difference in longitudes by 4 minutes/degree.
150
If the locality is east of the standard meridian, add the correction minutes; if it is west,
subtract them.
3) Add the equation of time (from the second
graph shot\ n here) for the date in question.
The result is Local Solar Time.
Once you know the Local Solar Time,
obtain the hour angle (H) from:
H = 0.25(number
you can
of minutes from solar noon)
From the latitude (L), declination (8). and hour
angle (H), the solar altitude (0) and azimuth (4)
follow after a little trigonometry:
sin 8 = cos L cos 6 cos H + sin L sin 6
sin $I = cos 8 sin H/cos 8
As an example, determine the altitude and
azimuth of the sun in Abilene, Texas, on December I. when it is I:30 p.m. (CST). First
you need IO calculate the Local Solar Time. It
is not daylight savings time, so no correction
for that is needed. Looking at a map you see
that Abiiene is on the iOO”W meridian. or IO”
west of the standard meridian, 9O”W. Subl.ract
the 4( IO) = 40 minutes from local time; I:30
- 0:40 = i2:50 p.m. From the equation of
Solar Angles
time for December I, you must add about I I
minutes.
12:50 + 0: 11 = I:01 Local Solar
Time, or 61 minutes past solar noon. Consequently, the hour angle is H = 0.25(61) or about
15”. The latitude of Abilene is read from the
for
same map: L = 32”, and the declination
December I is 6 = - 22”.
You have come this far with maps, graphs,
and the back of an old envelope. but now you
need a scientific calculator or a table of trigonometric functions:
sin 0 = cos(32”)cos( - 22”)cos( 15”)
+ sin( 32”)sin( - 22”)
= 0X5(0.93)(0.97)
+ 0.53( -0.37)
= 0.76 - 0.20 = 0.56
Then 8 = arcsin(0.56)
horizon. Similarly:
=
34.12”
above the
Sun Path Diagrams
In applications
where strict accuracy is superfluous, solar angles can be quickly determined
with sun path diagrams. In these diagrams, the
sun’s path across the sky vault is represented
by a curve projected onto a horizontal
plane
(see diagram). The horizon appears as a circle
with the observation point at its center. Equaiiyspaced concentric circles represent the altitude
angles (0) at IO” intervals, and equally spaced
radiai lines
represent the azimuth angles (4) at
the same intervals.
The elliptical curves running horizontally
are
the projection of the sun’s path on the 2ist day
of each month; they are designated by two Roman numerals for the two months when the sun
follows approximately
this same path. A grid
of vertical curves indicate the hours of the day
in Arabic numerals.
sin 8 = cos( - 22”)sin( 15”)/cos(34.i2”)
= (0.93)(0.26)/0.83
= 0.29
Then + = arcsin(0.29)
= 16.85” west of true
south. At I:30 p.m. on December I in Abiiene,
Texas. the solar altitude is 34.12” and the azimuth is 16.85” west.
.
‘
.
0
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LATITUDE
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Solar Angles
The shading mask protractor shown here can
be used to construct shading masks characteristic of various shading devices. The bottom
half of the protractor is used for constructing
the segmental shading masks characteristic
of
horizontal devices (such as overhangs), as explained earlier. The upper half, turned around
so the the 0” arrow points down (south) is used
to construct the radial shading masks characteristic of vertical devices. These masks can be
superimposed on the appropriate sun path diagram to determine the times when a surface will
be shaded by these shading devices. (Source:
Ramsey and Sleeper, Architecturul Grtrphic
Stundurds, Wiley . )
BI-IADINO
MABK
PROTRACTOR
153
ASHRAE
has developed tables that give the
clear day insolation on tilted and south facing
surfaces, such as those commonly used for solar
collectors. For north latitudes (L) equal to 24”.
32”. 40”, 48”. and 56”. insolation
values are
given for south facing surfaces with tilt angles
equal to L- 10’. L, L+ 10” L+ 20’. and 90”
(vertical).
Values are also given for the direct
normal (perpendicular
to the sun’s rays) radiation and the insolation on a horizontal surface.
The values listed in these tables are the sum of
the direct solar and diffuse sky radiation hitting
each surface on an average cloudless day. Data
are given for the 21st day of each month; both
hourly and daily total insolation are provided.
A brief examination of the 24” N table reveals
that the insolation of south-facing
surfaces is
symmetrical about solar noon. The values given
for 8 a.m. are the same as those for 4 p.m.,
and they are listed concurrently.
Moving from
left to right on any fixed time line, you encounter values of: the solar altitude and azimuth
in degrees: the direct normal radiation and the
insolation on a horizontal surface in Btu/(hr ft’);
and the insolation of the five south facing surfaces discussed above in Btu/(hr ft*). Below
these hourly data are values of the daily total
insolation for each of these surfaces (in Btu/ft’).
An example will help to illustrate the use of
these tables.
154
Example:Determine
the optimum tilt angle
for a flat plate collector
located in Atlanta,
Georgia (32” N latitude). Select the tilt angle to
maximize the surface insolation for the following three periods: a) heating season, b) cooling
season, and ::\ the full year.
1) The lrc;,;rirg season in Atlanta lasts from
October through April: the cooling season
from May to September.
2) Using the 32” N table, we sum the surface
daily totals for the 22” tilt for the months
October through April, and get 14,469 Btu/
ft*. We do the same for the 32”, 42”, 52”,
and 90” tilts and get totals of 15,142; 15,382;
15.172; and 10,588.
3) Comparing these totals, we conclude that
the 42” tilt, or latitude + lo”, is the best
orientation
for solar collection
during the
heating season.
4) A similar set of totals is generated for the
cooling season, using the data for the months
May through September. These are 11,987
Btu/ft* for 22”; 11,372 for 32”; 10,492 for
42”; 9,320 for 52”; and 3,260 for 90” tilt.
5) Comparing these totals, we conclude that the
22” tilt, or latitude
- lo”, is the best for
summer cooling.
6) Using the data for the whole year, we get
totals of: 24,456 Btu/ft’ for 22”; 26,514 for
32”; 25,874 for 42”; 24,492 for 52”; and
13,848 for 90” tilt.
ay Insolation Data
7) Comparing these totals, we choose the 32”
tilt, or latitude, as the best for year-round
collection.
These conclusions are useful for the designer
as they stand, but a little closer scrutiny is instructive. For example, the 42” tilt is best for
heating, but the heating season totals for 32”
and 52’ are within 2 percent of the 42” total.
Thus, other design considerations
(such as
building layout, structural framing, height restrictions) can enter the decision process without seriously
affecting
the final collector
efficiency.
The Clear Day Insolation
Data are an extremely valuable design tool, but their limitations should be kept in mind. For instance, there
is no ground reflection included in the listed
values. This can lead one to underestimate the
clear day insolation on a vertical surface. In the
example above. the heating season total for a
90’ surface is about 30 percent below the 42”
maximum. In reality, the insolation on a vertical
surface is only 10 to 20 percent lower than this
maximum during the heating season because of
the contribution of these data is their assumption
of an “average”
clear day. Many locations are
clearer than this (high altitudes and deserts), and
many are less clear (industrial and dusty areas).
To correct for this assumption, the numbers in
these tables should be multiplied by the areawide clearness factors listed in the ASHRAE
Handbook of Fundamentals. Finally, the Clear
Day Insolation Data do not account for cloudy
weather conditions,
which become quite important for long term predictions. (Source: Morrison and Farber, “Development
and Use of
Solar Insolation Data in Northern Latitudes for
South Facing Surfaces,”
symposium paper in
Solar Energy Applications, ASHRAE. Used by
permission. )
155
I
I I I I
I I
I I
I I
I 1
I I
I1
I 14 t
I I
I I
I I
I I
I I
I I
I I
I I
I
‘:t
“7--,,
!
f I
Ttp-.
. .
-I-L------++
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~.
lf
+j-mm
*+
.--
-----t
C, F.
J
I.>
- m,
-T
.+-. . ._.-
----
- .---
-__-j
;
iI
The quantity of solar radiation actually available
for use in heating is difficult
to calculate exactly. Most of this difficulty
is due to the many
factors that influence the radiation available at
a collector location. But most of these ktors
can be treated by statistical methods using longterm averages of recorded weather data.
The least modified and therefore most usable
solar radiation data is available from the U.S.
Weather Bureau. Some of these data. averaged
over a period of many years. have been published in the Clittwtic Ath
o$ the Utlitrd States
in the form of tabks or maps. A selection of
these al:‘:rage data is reprinted here for convenience. They are taken to be a good indicator of
future weather trends. More recent and complete information may be obtained from the National Weather Records Center in Asheville.
North Carolina.
As one example. daily insolation has been
recorded at more than X0 weather stations across
the United States. The available data have been
a*;erqed over a period of more than 30 years;
these averages are summarized in the tirst I2
tone for each month) contour maps: “Mean Ciaily
Solar Radiation.”
Values are given in langleys.
or calories per square centimeter.
Multiply
by
3.69 to ,:nvert to Btu per square foot. These
figures represent the monthly average of the
daily total of direct. diffuse. and reflected ra-
diation on a horizontal
surface. Trigonometric
conversions
must be applied to these data to
convert them to the insolation on vertical or
tilted surfaces.
Other useful information includes the Weather
Bureau records of the amount of sunshine, which
is listed as the “hours of sunshine” or the “peri’entage of possible sunshine.”
A device records
the cumulative total hours from sunrise to sunset
to get the percentage of possible sunshine.
Monthly
averages tif this percentage are provided in the next 1Z contour maps here. “Mean
Percentage of Possible Sunshine.”
These values can be taken as the average portion of the
daytime hours each month when the sun is not
obscured by clouds.
Also included in each of these I2 maps is a
table of the average number of hours between
sunrise and sunset for that month. You can multiply this number by the mean percentage of
possible sunshine to obtain the mean number of
hours of sunshine for a particular month and
location. A tab!e at the end of this section lists
the mean number of hours of sunshine for selected locations across the United States.
These national maps are useful tar getting an
overview or approximation
of the available solar radiation at a particular spot. For many locations. they may be the only way of finding a
particular \ ue. As a rule. however. they should
161
The New Solar Home Book
be used only when other more local data are
unavailable.
Many local factors can have significant effect, so care and judgement are important when using interpolated data from these
x7 SOLiR
-DAILY-
SOLAk
4.
162
national weather maps. (Source: Environmental
Climatic AtScience Services Administration.
las of the United States, U.S. Department of
Commerce. )
R&IATiO~nt&vs)
RADIATION
.=pyp
(&jl,&)
* A
Solar Radiation Maps
lIATiON
I! ” ..._. -,+-i--
&m&Yd
-+
‘-
165
Solar Radiation Maps
t
t
c
169
The total solar radiation is the sum of direct,
diffuse. and reflected radiation. At present, a
statistical approach is the only reliable method
of separating out the diffuse component of horizontal insolation. (The full detail of this method
is contained in the article by Liu and Jordan
cited below; their results are only summarized
here. )
First ascertain the ratio of the daily insolation
on a horizontal surface (measured at a particular
weather station) to the extraterrestrial
radiation
on another horizontal surface (outside the atmosphere). This ratio (usually called the percent of ertruterrc~.striulruditrtim or percent ETR)
can be determined from the National Weather
Records Center: it is also given in the article
by Liu and Jordan. With a knowledge of the
percent ETR. you can use the accompanying
graph to determine the percentage of diffuse
radiation on a horizontal surface. For example,
SO percent ETR corresponds to 38 percent diffuse radiation and 62 percent direct radiation.
You are now prepared to convert the direct
and diffuse components of the horizontal insolation into the daily total insolation on southfacing tilted or vertical surfaces. The conversion
factor for the direct component (Ft,). depends
on the latitude (L). the tilt angle of the surface
(represented by the Greek letter Beta, or PI.
[email protected]
and the sunset hour angles (represented by the
Greek letter Omega, or w). of the horizontal
and tiled surfaces:
horizontal
surface: cos w
= -tan L tan 6
tilted surface: cos w*
= - tan (L - B)tan 6
where the declination 6 is found from
in Appendix I and B = 90” applies
surfaces. Depending on the value of
angles w and w’, the calculation of F.
different. If w is less the w’. then:
cos(L
b
=
-
cos L
p, x sinwsin w -
the graph
to vertical
these two
is slightly
wcosw’
w cos w
If w’ is smaller than w. then:
h, =
cos (L cos L
B) x sin w’ -
w’ cos w’
sin w - w cos w
The direct component of the radiation on a tilted
or vertical surface is I’t, = Ft, (It,). where It,
is the direct horizontal insolation
The treatment of diffuse and reflected radiation is a bit different. The diffuse radiation is
Calculating Solar Radiation
assumed to come uniformly from all comers of
the sky, so one need only determine the fraction
of the sky exposed to a tilted surface and reduce
the horizontal diffuse rddiation accordingly. The
diffuse radiatior: on a surface tilted at an angle
P is:
I’d = I + cos B/2(Id)
where IJ is the daily horizontal
40
PERCENT
I’d = p( I -
on a tilted surface is:
cos (3/2)(I,
+ Id)
where p (the Greek letter Rho) is the reflectance
of the horizontal surface. (Source: Liu. B.Y.H.
and R.C. Jordan “Availability
of Solar Energy
for Flat-Plate Solar Heat Collectors.”
in Lou
7’emperature Etqineering Applications of Solar
Energy, edited by Richard C. Jordan, New York:
diffuse radiation.
30
The reflected radiation
OF
ASHRAE.
50
EXTRATERRESTRIAL
1967.)
60
70
60
RADIATION
171
The hourly, monthly. and yearly heat losses
from a house depend on the temperature difference between the indoor and outdoor air, as
explained earlier. To aid in the calculation of
these heat losses, ASHRAE publishes the expected winter design temperatures
and the
monthly and yearly total degree days for many
cities and towns in the United States.
The maximum heat loss rate occurs when the
temperature is lowest, and you need some idea
of the lowest likely temperature in your locale
in order to size a conventional heating unit, The
ASHRAE HtrttdhooX-c~f’Ftrrr~l~rttt~~ttttr1.s
provides
three choices-the
“median
of annual cxtremes” and the “YY% ” and “07 I/Z4 *’ dc<ign
temperatures. The median of annual extremes
is the average of the lowest winter temperatures
recorded in each of the past 30 to 40 years. The
“YY% ” and”Y7 I/2% ” design temperatures arc
the temperatures which arc normally exceedcd during those percentages of the time in
December. January and February. We list the
“Y7 l/2’% ” temperatures here together with the
average winter temperatures. For example, the
temperature will tall below IY”F for 2 I/2 per-
172
cent of the time (about 2 days) during a typical
Birmingham
winter.
Consuit the ASHRAE
Hmdbaok oj’ Funduttretmls for more detailed
listings.
Degree days gauge heating requirements over
the long run. One degree day occurs for every
day the average outdoor temperature is 1°F below 65°F. which is the base for degree day
calculatiuns because most houses don’t require
any heating until outdoor temperatures fall below this level. For example. if the outdoor air
temperature remained constant at 30°F for the
entire month of January. then 3l(hS - 30) =
IO85 degree days would result. Both monthly
and yearly total degree days are listed in these
tables, but only the months from September to
May are included here because very little heating is needed in summer. The yearly total degree days are the sum over all I2 months. More
complete listings of monthly and yearly degree
days can be found in the ASHRAE Gui& utuf
Dater Book or Hatuihoctk of Ftttt~krtttc~tttols.
(Sources: ASHRAE. G’uidt~utrd Duta Book. 1070;
ASHRAE. Hatulbaok rtj Ftut~crtttetttcrls.19X1,
reprinted by permission. )
Degree Days and Design Temperatures
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175
The conduction heat flow through a wall. window. door. roof. ceiling. or hoor decreases as
more rrsistonce is placed in the path of the how.
All materials have some resistance to conduction heat Row. Those that have high resistance
are called insulators; those with low resistance
are called c*ort~~~~tors.
Insuiators are compared to one another according to their R-values, which are a measure
of their resistance. The R-value of a material
increases with its thickness-a
Z-inch thick sheet
of polystyrene has twice the resistance of a Iinch sheet. And two similar building materials
that differ in density will also differ in R-value.
Generally. though not always, the lighter material will have a higher R-value because it has
more pockets of air trapped in it. Finally. the
average temperature of a material also affects
its R-va!ue. The colder it gets. the htter most
materials retard the flow of heat.
Knowledge of the R-values of insulators and
other components permits us IO calculate the
heat trrmsmission through a wall or other building surface. Toward this end, we list the Rvalues of many common buliding materials in
the tirst table. R-values are given per inch of
thickness and for standard thicknesses. If you
I listed in the table. use
have some odd size
the R-value per inch thickness and multiply by
its thickness. Unless otherwise noted, the R-
176
values are quoted for a temperature of 75°F.
Further tables list R-values for surface air
films and for air spaces, both of which have
insulating values. These R-values vary markedly with the reflectance of the surfaces facing
the air film or space. Radiation heat flow is very
slow across an air space with aluminum toil on
one side. for example, and the R-value of such
an air space is correspondingly
high. This is
why tiberglass batt insulation is often coated
with an aluminized
surface. In the tables we
have used three categories of surface: nonreflective (such as painted wood or metal). fairlyrefective (such as aluminum-coated
paper). and
highly-reflective
(such as metallic foil). The Rvalue of an air film or air space also depends
on the orientation of the surface and the direction of heat How that we are trying to retard.
These differences are reflected in the tables.
The total resistance R, of a wall or other
building surface is just the sum of the R-values
of a!! its components-including
air films and
spaces. The coefficient of heat transmission. or
U-value. is the inverse of the total resistance
(U = I/R,). To get the rate of heat loss through
a wall. for example. you multiply its U-value
by the total surface area of the wall and by the
temperature difference between the indoor and
outdoor air. The next table in this section lists
U values for windows and skylights. Here, again.
Insulating Values of Materials
U ,,,,,,,
NOTES
Fanal U ~.,l,w nbtamw,
bv dddmg
ar, dnrunm,
of ,hcvm~I
6.u
rrr~rt~ncc.
the U-value depends upon the surface orientation. the direction of heat flow, and the season
of the year. The U-values in these tables apply
only to 1he glazing surfaces; to include the effects of a wood sash, multiply these U-values
by about 80 to 90 percent. depending upon the
area of the wood.
For the avid reader \eeking more detailed
information about the inhulating values of building materials. we recommend the ASHRAE
Hutulbook of Fundumentuls.
from which most
of the present data were taken. Tables there list
resistances and conductances of many more materials than are given here. Sample calculations
of the U-values of typical frame and masonry
walls. roofs, and floors are also provided there.
Recently hard coat, low-e glass. a new kind
of low-emissivity
(low-e) coated glazing. became available. It stands up better than soft coat
low-e glass and shares many of soft coat’s energy conserving properties. U-values are shown
F 11: hr
,=i ,o a,> P~,,-~NuI rh,n havurq
a co,-lf,r,vn~
ul hrd,
,rmrm,rr~~~~.
U,,,,I,
,,
below for hard coat. soft coat and suspended
film. low-e glass
Cilazing
Double
soft coat
hard coat
Triple
soft coat
suspended
film
Air Space
U-Values
Su:,,mer
Winter
I/4”
1/Y
0.44
0.32
0.48
0.32
l/4”
l/2”
0.52
0.40
o.s4
0.44
l/4”
l/3”
0.22
0.22
0.37
0.37
-3:8”
I/4”
l/2”
0.3
0.3 1
0.23
0.28
0.35
0.25
(Data from the Sealed Insulated Glass Manufactu&
Association.)
177
The New Solar Home Book
In all of this discussion, no mention has been
made of the relative costs of all the various
building alternatives. To a large extent, these
depend upon the local building materials suppliers. But charts in the next two sections of
this appendix will help you to assess the savings
in fuel costs that can be expected from adding
insulation.
By adding insulating materials to a wall or
other building surface, you can lower its Uvalue, or heat transmission coefficient.
But it
takes a much greater amount of insulation to
lower a small U-value than it does to lower a
large U-value. For example, adding 2 inches of
polyurethane insulation (R, = 12) to a solid 8inch concrete wall reduces the U-value from
0.66 to 0.07. or almost a factor of IO. Adding
the same insulation to a good exterior stud wall
reduces the U-value from 0.069 to 0.038, or
less than a factor of 2. Mathematically,
if U, is
the inital I-J-value of a building surface, and R
is the resistance of the added insulation, the final
U-value (Ur) is:
Ur = Ui/l
+ RUi
If you don’t have a pocket calculator handy,
the following
chart will help you to tell at a
glance the effects of adding insulation to a wall
or other building surface. The example shows
you how to use this chart.
Example If 3 l/2 inches of fiberglass insulation (R = 11) is added to an uninsulated stud
wall having a U-value of 0.23, what is the final
U-value? When adding the insulation,
you remove the insulating value of the air space (R
= I .Ol ) inside the wall, so the net increase in
resistance is R = 10. To use the chart, begin
at Ui = 0.23 on the left-hand scale. Move horizontally to intersect the curve numbered R =
IO. Drop down from this point to the bottom
scale to find the final U-value, Ur = 0.069.
With this information,
you can now use the Heat
Conduction Cost Chart in the next section to
find the fuel savings resulting from the added
insulation.
Thermsrl Properties of Typical Building rod Insulating hh¶tt!riSdS-fkSiRIl Values’
Description
Density
lb/[email protected]
Resistance c(R)
Per inch
thickness
(I/A)
h= Hz =
F/Btu
BUILDING
BOARD
Boards. Pnaels. Subflooring. Sheathing
Woodboard Panel Products
Asbestoscement board. ........................
Asbestos-cement board .................
Asbestos-cement board ..................
Gypsum or plaster board. ...............
Gypsum or plaster board. .................
Gypsum or plaster board. ...............
Plywood (Douglas Fir)O ........................
Plywood (Douglas Fir) ..................
Plywood(DouglasFir)
.................
Plywood (Douglas Fir) ...................
Plywood (Douglas Fir) .................
Plywoodorwoodpanels
.................
Vegetable Fiber Board
Sheathing, regular density ...............
.0.125
.0.25
.0.375
.0.5
.0.625
BUILDING
in.
in.
in.
in.
in.
.0.25 in.
.0.375in.
.0.5 in.
.0.625 in.
.0.75in.
.0.5
.0.78125
Sheathing intermediate dens& ................ ... .0.5
Nail-base sheathing ....................
.0.5
Shingle backer. .....................
.0.375
Shingle backer .....................
.0.3 I25
Sound deadening board. ................
.0.5
Tile and lay-in panels, plain or
acoustic .................................
..............................
0.5
.0.75
Laminated pa~perboard’ .......... : ........ : .. : : : : ....
Homogeneous board from
repulped paper. ...........................
Hardboard
Medium density. ............................
High density. service temp. service
underlay ................................
High density, std. tempered. ...................
Particleboard
Lcw density. ...............................
Mecub ..- density. ............................
Htgh density ...............................
Underlayment ......................
.0.625
Wood suLfloor ........................
.0.75
in.
in.
in.
in.
in.
in.
in.
::
50
::
in.
in.
0.25
-
I,25
-
:t
34
34
I8
I8
t:
I8
lg
15
2.50
-
0;3
0.06
0.32
0.45
0.56
0%
0.47
0.62
0.77
0.93
1.32
2.06
1.22
1.14
ii%
1135
1.25
1.89
-
1:
30
2%
30
2.oc
50
1.37
55
63
1.22
1.00
37
1.85
1.06
0.85
-
0.82
0.94
-
-
0.06
-
-
0.12
Negl.
-
-
2.98
1.23
0.28
-
-
0.68
-
IP
:$
::5
40’
-
MEMBRANE
FLOORING
MATERIALS
Carpet and fibrous pad. ........................
Carpet and rubber pad .........................
Corktile..
..........................
.O.IZSin.
Terrazzo ................................
I in.
Tile-asphalt,
linoleum, vinyl, rubber. .............
vinyl asbestns ..............................
ceramic ...................................
Wood, hardwood tinish .................
.0.75 in.
INSULATING
120
I20
I20
18
in.
in.
..
Vapor-permeable
felt .........................
Vapor-seal,
2 layers of mopped
15lb felt ..................................
Vapor-seal,
plastic film. .......................
FINISH
For thicknew &ted
U/$2
hmft l
F/Btu
PE
MATERIALS
Blanket mad Battd
Mineral Fiber, fibrous form processed
from rock, slag. or glass
approx.e 3-4 in ............................
approx.e 3.5 in ............................
approx.c 5.5-6.5 in .........................
approx.c 6-7.5 in. .........................
approx.c 9-loin ...........................
approx.r 12-13 in ..........................
0.3-2.0
0.3-2.0
0.3-2.0
0.3-2.0
0.3-2.0
0.3-2.0
-
22*
3od
38*
Thermal Properties of Typical Building and Insulating Materials-Design
Dewtptioa
Board and Slabs
CelIular glass . . . . . . . . . . . . . . . . . ,
Glass fiber, organic bonded . . . . . . .
Expanded perlite. organic bonded. . .
Expanded rubber (rigid) . . . . . . . . . .
Expanded polystyrene extruded
Cut ceil surface . . . . . . . . . . . . . . .
Smooth skin surface
.........
Expanded polystyrene. molded beads
Density
lb/[email protected]
Values’
Resistmcc L(RI
Per inch
thickness
(l/A)
b- It2 l
For tbickmss listed
F/Btu
F;Bt:
I!‘#+)
..
..
_.
..
.....
.....
.....
.....
8.5
4-9
I.0
4.5
2.86
4.00
2.78
4.55
-
..
..
..
*. . . .
.....
.....
I.8
1.8-3.5
1.0
I .25
I.5
1.75
2.0
I.5
4.00
-
;::
4.00
4.17
4.17
4.35
6.25
-
15.0
7.20
3.45
3.6
7.2
14.4
-
16-17
18.0
21.0
2.94
2.86
2.70
-
23.0
2.38
Cellular polyurethanef (R-l I exp.)(unfaced)........
Cellular
polyisocyanurate”(R-I
I exp.) (foil
faced,
glass fiber-reinforced
core) ....................
Nominal0.4in
...
..........................
Nominal I .O in. .............................
Nominal 2.0 in. .............................
Mineral fiber with resin binder ...................
Mineral fiberboard, wet felted
Core or roof insulation .......................
Acoustical tile. .............................
Acoustical tile. .............................
Mineral fiberboard, wet molded
Acoustical tileh .............................
Wood or cane fiberboard
Acoustical tiles .......................
.0.5 in,
Acoustical tiles ......................
.0.75 in.
Interior finish (plank, tile). ......................
Cement fiber slabs (shredded wood
with Por:land cement binder ..............
....
Cement fiber slabs (shredded wood
- with magnesia oxysulfide binder). ...............
2.0
-
-
IS.0
2.86
25-27.0
2.0-1.89
1.25
1.89
-
22.0
1.75
-
..
... .
. ..
..
2.3-3.2
8.0-15.0
2.0-3.5
2.0-4. I
4.1-7.4
7.4-l I.0
3.70-3,13
2.22
3.33
3.7-3.3
3.3-2.8
2.8-2.4
-
.
0.6-2.0
0.6-2.0
0.6-2.0
0.6-2-O
LOOSE FILL
Cellulosic insulation (milled paper or
woodpub)
..................
Sawdust or shavings .............
Wood fiber, softwoods. ..........
Perlite, expanded ...............
Mineral fiber (rock. slag or glass)
approx.c 3.75-5 in.. . . . . .
. . . . .
approx.c 6.5~R.75in.
.
.
.
approx.c 7.5-loin..
.. .
.
.
approx.c 10.25-13.75 in.. .
.
Mineral fiber (rock, slag or glass)
approx.c3.5 in. (closed sidewall application)
Vermiculite, exfoliated . .
FIELD APPLIED
Polyurethane foam.
.
Urcaformaldehyde
foam.
Spray cellulosic fiber base
PLASTERING
...
.
..
.
2.0-3.5
7.0-8.2
4.0-6.0
11.0
19.0
22.0
30.0
273
2.27
12.0-14.0
-
. .
.
. .
1.5-2.5
0.7-I .6
2.0-6.0
6.25-5.26
3.57-4.55
3.33-4.17
1
-
ii
II6
-
0.20
-
0.0s
0.15
-
0.32
0.39
0.67
0.47
-
MATERIALS
Cement plaster, sand aggregate.
Sand aggregate.
.
.
Sand aggregate
Gypsum plasfer:
Lightweight aggregate.
.
1 ighiweight aggregate
...
Lightweight agg. on metal lath.
Perlite aggregate . .
.
...
iii;;
.0.75 in.
0.5 in.
b:625 in.
.0.75 in.
. .
.
45
45
.s
Thermal Properties of Typical Building and lnsulatiag Materials--Design
Density
ksctiptton
Values’
ResistanceC(R)
Ib/ft’
Per inch
thickness
(l/U
b.f?’
PLASTERING
MATERIALS
Sandaggregate.............................
Sandaggregate
. . . , . . . . . . . . . . . . . . . . . . . .O.Sin.
Sand aggregate. . . . . . . . . . . . . . . . . . . . . . .0.625 in.
Sand aggregate on metal lath . . . . . . . . . . . .0.75 in.
Vermiculite aggregate . , . . . . . . . . . . . . . . . . . . . . .
MASONRY
F/Btu
F/Btv
s
0.18
0.59
0.09
0.11
0. I3
-
II6
0.20
-
51
120
100
80
60
40
:8
0.60
0.19
0.28
0.40
0.59
0.86
1.11
1.43
1.08
1.41
2.00
-
140
0.11
-
I40
II6
Z:E
-
120
130
0.20
0.11
-
I05
I05
105
MATERIALS
Concretes
Cement mortar. . . . . . . . . . . . . . . . . . . . . . . . . .
Gypsum-fiber concrete 87.5% gypsum,
12.5% woodchips..
. ...... ...............
Lightweight aggregates including expanded shale, clay or slate; expanded
slags; cinders; pumice; vermiculite;
also cellular concretes
Perlite. expanded
. ... ......
. . .
.... ..
Sand and gravel or stone aggregate
(ovendried)..
.............................
Sand and gravel or stone aggregate
(notdried).................................
stucco......................................
-~
MASONRY
::
40
UNITS
Brick, common1 ..............................
Brick, face’. .................................
Clay tile, hollow:
lcelldeep..
...........................
I celldeep .............................
2cellsdeep ............................
Zcellsdeep ...........................
2 cells deep ............................
3 cells deep ............................
Concrete blocks, three oval core:
Sand and gravel aggregate ................
.................
.............
Cinderaggregate
.....
.::::.
............
........................
........................
........................
Lightweight aggregate ....................
(expanded shale, clay, slate. .............
or slag; pumice): ......................
.3in.
.4in.
.6in.
..Ein.
IO in.
I2 in.
-
-
0.80
1.11
1.52
1.85
2.22
2.50
.4 in.
8 in.
I2 in.
.:i;.
-
-
0.71
1.11
1.28
0.86
1.11
1.72
1.89
1.27
1.50
2.00
2.27
-
-
-
-
1.6s
2.99
2.18
5.03
2.48
0%
5.82
-
-
-
1.26
1.35
1.67
8 in:
I2 in.
3 in.
.4 in.
.8 in.
I2 in.
Concrete blocks, r~k&&l.&
core:r:; ..........
Sand and gravel aggregate
Zcore.Bin.36Ib.
.........................
Same with filled cores’ ......................
Lightweight aggregate (expanded shale,
clay, slate or slag, pumice):
3core.6in.
l9lb. .........................
Same with filled cores’ ......................
2core.Sin.24Ib.
.........................
Same with filled cores’ ......................
3core. 12in.38lb ..........................
Same with filled cores’ ......................
Stone, lime or sand. ...........................
Gypsum partition tile:
30 120 30in.solid..
.......................
3 0 I2 30in.4~cell .........................
.
4 I2 30 in. 3-cell . . . . . . . .
l
l
For tbickness listed
l
...
. .
-
-
-
-
--
1.04
1.93
Thermal Properties of Typical Building and Insulating Materials-Design
Description
Demtty
Vaiues’
Resistance c(R)
lb/[email protected]
Per inch
thickness
u/u
b*ft2’
F/Btu
For thickness listed
120
-
:x
70
-
-
0.21
0.15
0.44
0.33
0.05
0.94
120
-
-
0.21
0.87
1.19
1.40
0.67
0.21
0. is
1.46
i-o
-
-
0.79
0.81
1.05
0.59
-
-
0.61
-
-
1.82
-
-
2.96
0.10
41.2-46.8
42.6-45.4
39.8-44.0
38.4-41.9
0.89-0.80
0.87-0.82
0.94-0.88
0.94-0.88
-
35.6-41.2
33.5-36.3
31.4-32.1
24.5-31.4
21.7-31.4
24.5-28.0
1.00-0.89
1.06-0.99
1.11-1.09
1.35-1.11
1.48-1.11
1.35-1.22
-
l!‘!tF
Fy Bt:
ROOFING”
Asbestos-cement shingles .......................
Asphalt roll roofing ...........................
Asphalt shingles ..............................
Built-up roofing ......................
.0.375 in.
Slate .................................
.0.5 in.
Wood shingles. plain and plastic film faced. .........
SIDING MATERIALS
(on flat surface)
Shingles
Asbestos-cement ............................
Wood, I6 in., 7.5 exposure ....................
Wood, double, Idin., 12.in. exposure. ...........
Wood, plus insul. backer board, 0.3 I25 in. ........
Siding
Asbestos-cement, 0.25 in., lapped ...............
Asphalt roll siding. ..........................
Asphalt insulating siding (0.5 in. bed.). ...........
Hardboard siding, 0.4375 in ....................
Wood, drop, I l 8 in. ........................
Wood.bevel.0.5
l
Bin..lapped..
..............
Wood, bevel.0.75 l IOin.,lapped ..............
Wood, plywood, 0.375 in., lapped. ..............
Aluminum or Steelm, over sheathing
Hollow-backed
.............................
Insulating-board
backed nominal
0.375in. ................................
Insulating-board
backed nominal
0.375 in., foil backed. ......................
Architectural glass ............................
WOODS (12% Moisture
-
-
ContentP-P
Hardwoods
Oak.. ....................................
Birch .....................................
Maple ....................................
Ash ......................................
Softwoods
Southern Pine. .............................
Douglas Fir-Larch. ..........................
Southern Cypress ...........................
Hem-Fir, Spruce-Pine-Fir.
....................
West Coast Woods, Cedars ....................
California Redwood .........................
PExccpt where otherwise noted. all values are for a mean temperalure of 75 F. Rcprcscmative values for dry materials. selected by ASHRAE TC 4.4, are intended
as design (not specificalion) values for materials in normal use. lnsulatlon malerials in actual service may huvc thermal values that vary from design values depending
on rhelr in-situ propertics (e.g.. density and moisture conlcm). For properties of a particular product. use Ihe value supplied by the manufacturer or by unbiased tests.
bTo obtain thermal conductivirie: in But/h*ftZ*F. divide the Avalue by I2 in./ft.
c Resistance values are the reciprocals of C before rounding off C 10 IWOdecimal places.
*Does not include paper backing and facing, if any. Where insulation forms a boundary (reflective or otherwise) of an air space, see Tables 2A and 28 for the
insulating value of an air spacewith Ihe appropriate effective emirrance and lempcrature conditions of the space.
CConductivity varies with fiber diameter. (See Chapter 20. Thermal Conductivity section.) Insulation is produced in different densities, therefore. rherc is a wide
variation in thickness for the same R-value among manufacturers. No effort should be made 10 relate any specific R-value to any specific density or thickness.
‘Values are for aged, unfaced. board stock. For change in conducrivity with age of expanded urethane. see Chapter 20, Factors Affecting Thermal Conductivity.
alnsuladng values of acoustical rile vary, depending on density of the board and on type. size and depth of perforadons.
hASTM C 855-77 recognizes the specification of roof insulation on the basis of the C-values shown. Roof insulation is made in thickness to vcct these values.
‘Face brick and common brick do noI always have these specific densities. When densily differs from that shown, there will be a change in thermal conductiviry.
‘At 45 F mean tempcralurc. Data on rectangular core concrete blocks differ from the above data on oval core blccks. due IO core configuration, different mean
rcmperatura. and possibly differences in uni1 weights. Weight data on the oval core blocks tested arc not available.
h Weights of units approximarcly 7.625 in. high and 15.75 in. long. These weights are given as a means of describing the blocks ~cstcd. but conducrance values are
all for I h2 of area.
‘Vermiculite, pcrlilc. or mineral wool insulation. Where insulation is used, vapor barriers or other precautions must be considered 10 keep insulation dry.
mValues for metal siding applied over flat surfaces vary widely. depending on amount of ventilation nf air space beneath the siding; whether air space is reflective
or nonreflective; and on thickness. type. and applicarion of insulating backing-board used. Values given are averages for USCas design guides, and were obtained
from several guarded hotbox tests (ASTM C236) or calibrated hotbox (ASTM C 976) on hollow-backed types and types made using backing-boards of wood fiber,
foamed plastic. and glass fiber. Departures of*50% or more from the values given may occur.
“Time-aged values for board slack with gas-barrier quality (0.001 in. thickness or greater) aluminum foil lacers on tow major surfaces.
=‘SecRef. 5.
PScc Ref. 6. 7. 8 and 9. The conductivity values listed are for hear transfer across rhe grain. The thermal conductivi!y of wood varies linearly with the density and
the density ranges listed arc those normally found for rhe wood species given. If the dcnsify of Ihe wood species is not known. use the mean conductivity value.
Insulating Values of Materials
R-VALUES
Direction
of
Heat Flow
Orientation
& Thickness
of Air Space
Horizontal
J/”
4”
‘A”
4”
OF AIR SPACES
I
UP*
UP
t
%”
1%”
4”
down*
J/4
1%”
downt
4”
4s” slope
Nonreflective
surface
Fairly
reflective
surface
Highly
reflective
surface
0.87
0.94
0.76
0.80
1.71
1.99
1.63
1.87
2.23
2.73
2.26
2.75
1.02
1.14
1.23
0.84
0.93
0.99
2.39
3.21
4.02
2.08
2.76
3.38
3.55
5.74
8.94
3.25
5.24
8.03
2.02
2.13
1.90
1.98
2.40
2.75
2.09
2.50
2.78
3.00
2.81
3.00
2.36
2.34
2.10
2.16
3.48
3.45
3.28
3.44
J/k”
4”
UP*
0.94
0.96
‘A”
4”
J/$.4”
UPt
0.8 1
down”
1.02
1.08
0.84
0.90
4”
1/”
4”
Vertical
R-value for Air Space Facing: *
0.82
down t
across*
J/4’
4”
:lClWSS+
‘A’
4”
1.01
1 .Ol
0.84
0.9 1
*0ne sick of the air \pacc is a non-reflective
iWinter conditions.
Summer conditions.
ASIlKAI<,
SO~JH<:I<:
Hadhook
R-VALUES
Type and
Orientation
of Air Film
Xrection
of
Heat Flow
3.57
4.41
3.34
4.36
1
surface
(J/‘I:ltndurrtenru/s.
1972. Reprinted
hy permission.
OF AIR FILMS
R-value for Air Film On:
1
Nonreflective
surface
Fairly
reflective
surface
Highly
reflective
surface
Still air:
klorizonral
tlorizontal
45O slope
45O slope
Vertical
UP
down
“P
down
across
0.61
0.92
0.62
0.76
0.68
1.10
2.70
1.14
1.67
1.35
1.32
4.55
1.37
2.32
1.70
Moving air:
15 mph wind
7% mph wind
anv*
an;,+
0.17
0.25
-
-
*Winter conditions.
t Summer conditions.
ASIIKAI:.
SOUKCE:
Ilumfhook
o/‘~undunrc~r~k?~s. 1972. Reprinted
by permission
183
The New Solar Home Book
U-VALUES
OF WINDOWS
AND SKYLIGHTS
Description
U-values’
Winter
Vertical panels:
Single pane flat glass
lnsularing glass-double2
3/l 6” air space
l/4” air spa0:
l/2” air space
Insulating glass-trlple2
l/4” air spares
l/2” air spaces
Storm windows
air space
l-4”
Glass blocks”
6 X 6 X 4” thick
8 X 8 X 4” thick
same, with cavity divider
Single plastic sheet
I lorizontal panels:4
Single pane flat glass
insulating glass--double’
3/l 6” air space
l/-C” air space
l/2” air space
Glass blocks11 X 11 X 3” thick,
with cuvity divider
12 X 12 X 4” thick,
with cavity divider
Pl;lstIc bu hblJ
single-walled
dout,le-walled
Summer
1.13
1.06
0.69
0.65
0.5 8
0.64
0.61
0.56
0.47
0.36
0.45
0.35
0.56
0.54
0.60
0.56
0.48
1.09
0.57
0.54
0.46
1.00
1.22
0.83
0.75
0.70
0.66
0.49
0.46
0.44
0.5 3
0.35
0.5 1
0.34
1.15
0.70
0.80
0.46
’ in unit\ of Ktu/hr/fr’PI;
-I
&ulde and triple rrfcr to thr number of lights of glass
.l
nc~m~nsl Jimcn\icln\
4~~-valucs for horirontal panels arc for heat flow U/I in
writer and Ilorcvt in summer.
Sl~awd cm arca of opening. not surface.
SOl’Kt:IC.
ASI IKAbl. tlutrllbrJok O] Furrllul?llvrruls. 1972.
I<cprintcJ by pernwsion.
The Heat Conduction Cost Chart provided here
simplifies the calculation of total seasonal heat
loss through wall, roof, or floor. It also facilitates the study of alternative constructions and
possible savings due to added insulation.
An
example of the use of the chart is included:
* For a building surface with a U-value of 0.58.
start at point ( I ).
* Follow up the oblique line to the horizontal
line representing the total heating degree days
for the location, in this case 7.000 degree days
(2).
* Move vertically from this point to find a heat
loss of 95.000 Btu/ft’ per season for the surface (3).
9 Continue vertically to the oblique line representing the total area of the surface, 100
square feet (4).
Moving horizontally
from this point, the total
heat loss through the entire surface for the
season is 9.SOO.000 Btu (5).
* Continue horizontally
to the oblique line representing the cost per million Btu of heat energy, in this case $9 per million Btu (6).
* Moving vertically down from this point. the
total cost for the season of the heat through
that surface is $86 (7).
l
The lower right graph converts the apparent
cost to a “real cost of energy” through the use
of a multiplication
factor. This factor might reHect:
! I ) Estimated future cost of energy-design
decisions based on present energy costs make
little sense as costs soar.
(2) Real environmental
cost of using fossil
fuels-this
particularly
includes pollution
and the depletion of natural resources, both
directly as fuels bum and indirectly as they
are brought to the consumer from the source.
(3) Initial investment cost--use of the proper
multiplication
factor would give the quantity of increased investment made possible
by resultant yearly fuel savings.
For example, heat costs may increase by a factor
of IO. Continue down from the last point until
you intersect the oblique line representing the
multiplication
factor IO (8). Then move horizontally left to arrive at the adjusted seasonal
heating cost through the building surface of $862
(9).
The numerical values of the chart can be
changed by a factor of ten. For example, to
determine the heat transfer through a really good
exterior wall, U = 0.05. use U = 0.5 on the
chart and divide the final answer by ten. Each
of the graphs can be used independently of one
another. For example, knowing a quantity of
energy and its price, the upper right graph gives
the total cost of that energy.
The Heat Conduction
Cost Chart can help
you compare the energy costs of two different
methods of insulation.
For example, an insulated stud wall has a U-value of 0.07 and an
uninsulated wall has a U-value of 0.23. The
d#hxv~ce 0.23 - 0.07 = 0.16 can be run through
the chart in the same way as done for a single
U-value. Assuming 5000 degree days. 100 square
feet of wall. and $9 per million Btu. the savings
in heating costs for one year is about $21. or
rnore than the cost of insulation.
185
r---m--r-
J
P
/
1/’
l/i+7
I
The use of this chart to calculate the costs of
heat loss through air intiltration is similar to the
USC‘of the Heat Conduction Cost Chart. An example of it5 use is included:
l
l
l
l
l
Start at point t I) for an air infiltrution
rate ot
45 ft’ithr ft).
Follow up the oblique line to the horizontal
line representing the total heating degrer days
I’or the location. in this case 7O(liI degree days
(2).
Move vertically
from this point to tind that
l4-1.tHW) Btu are consumed each heating season per crack foot (3).
Continue vertically
to the oblique lint carrcxponding to the total crack length, in this
case 30 feet (3).
Move horizontally
from this point to the total
seasonal heat loss through the window crack4.4oo.ooo
Htu (5).
(‘ontinue hori/.ontally tu the oblique line rtprcscntiq
the cost per million Btu of heat energy. in this case Ir;h per million Btu (h).
. Move vcrticully down from this point to the
total cost for heat lost through the crack during
an entire heating season. or !$%.75.
l
As with the Heat Conduction
Cost Chart. the
bottom right graph permits a conversion of this
apparent cost to a “real cost of energy” through
the use of a multiplication
factor. In this example. a factor of IO is used. Contintie down
from the last point until you intersect the oblique
line representing a multiplication
factor IO (8).
Then move horizontally
left to arrive at an adjusted heating cost of about $270 per heating
season (9).
As with the previous chart. you can use this
Air intiltration
Cost Chart to make quick evaluations of the savings resulting from changes
in the rate of air infiltratrion.
For example. if a
wood-sash. double-hung
window is weatherstripped. the air intiltration
ratr will drop from
39 to 2-I cubic feet per hour per crack foot. By
moving through the chart from a starting point
of IS ft ‘/(hr ft). you arrive ultimately at the
savings resulting from weatherstripping.
Assuming 5000 degree days. IS feet of crack. and
$9 per million Btu. we get an immediate savings
of about $6 in the tirst heating season. Since
weatherstripping
costs a ftw cents per foot, it
can pay for itself in fuel savings within a few
,veeks.
187
il;:
p
,
!.
6
.i
,
,.
:
.:
c
i
p1.
ef:
..,
(.!.
3
;_
(
:,
!
f
.
,
*
*
+
-.
$8
.
.
.
.
.
.
.
3
I 3
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
t.+
K.
.
*
.
.
r++
.
.
.
.
Radiation is an important method of heat transfer between two surfaces. As explained earlier,
any warm body emits energy in the form of
electromagnetic
radiation. We might say every
warm object has an “aura” that is invisible to
our eyes.
Sunlight is one form of electromagnetic
radiation and thermal radiation is another. They
differ only in wavelength.
Sunlight comes in
wavelengths ranging from 0.3 to 3.0 microns.
The wavelengths of thermal radiation from warm
bodies (say 1OC”F) range from 3 to 50 microns
When radiation strikes a surface, of any material, it is either absorbed, refiected. or transmitted. Each material absorbs, reflects, and
transmits radiation differently-according
to its
physical and chemical characteristics
and the
wavelength of the incoming radiation. For example, glass transmits most of the sunlight hitting it but absorbs almost all thermal radiation.
We can assign numerical ratings that gauge
the percentage of radiation absorbed, reflected,
or transmitted by a material. These numbers
depend upon the temperature of the material and
the wavelength of the radiation. We usually define the absorptance (represented by the Greek
letter Alpha, or a) of a material as the ratio of
solar energy (in the wavelength range 0.3 to 3.0
microns) absorbed to the total solar energy incident:
a = I,/1 = absorbed solar energy!
incident
solar energy
The reflectarrce (represented by the Greek letter
Rho, or p) and transtnittmm (represented by
the Greek letter Tau. or 7) are similarly defined
ratios:
p = 141 = re fleeted solar energy/
incident
solar energy
T = 1,/I = transmitted
incident
solar energy/
solar energy
Because all the sunlight is either absorbed, reflected, or transmitted, a t p + T = I. For
opaque solids, no energy is transmit!ed, so that
a + p = I or p = I - a. If we know the
absorptance of an opaque material. we also know
its reflectance.
Once absorbed, this radiant energy is transformed into heat energy-the
motion of molecules. The body becomes warmer and emits
more radiation of i!.; own. The emittance (represented by the Greek letter Epsilon, or E of a
material is a numerical indicator of that material’s propensity to radiate away its energy. The
emittance is defined as the ratio of the thermal
radiation emitted from a material to the thermal
189
The New Solar Home Rook
radiation emitted iy a hypothetical
“blackbody” with the same shape and temperature:
E = R,/R,
= radiation
radiation
from material/
from blackbody
With t = 1. the blackbody is a theoretically
“perfect”
emitter of thermal radiation.
A knowledge of the absorptances and emitlances of materials helps us to evaluate their
relative thermal performance. For example, brick.
masonry and concrete have emittances around
O.%--so they are better heat radiators than galvanized iron, which has an emittance between
0. I3 and 0.28. With an absorptance greater than
0.9. asphalt paving absorbs much more of the
sunlight than sand (a = 0.60 to 0.75). as any-
one who has walked barefoot from parking lot
to beach can testify
The ratio a/e of the absorptance (of shortwave solar radiation) to the emittance (of longwave thermal radiation) has special importance
in the design of solar collectors. In general, you
want materials with high values of Q/E for the
absorber coating. Then a large percentage of
solar radiation is absorbed, but only a small
amount lost by re-radiation. Materials with high
values of both a and a/r are called “selective
surfaces. ”
The ensuing tables list absorptances and
emittances of many common and some uncommon materials. They are grouped into two categories according to whether a/c is less than or
greater than I .O.
Different matcrlais abosorb different amounts
of heat while undergoing the same temperature
rise. F<,r example. ten pounds of water will
abso~l) 100 Btu during a 10°F temperature rise,
but IO pounds of cast iron will absorb only I2
Btu over the same range. There are two common measures of the ability of material to absorb and store heat-its
specific heat and its
heat capacity.
The specific heat of a material is the number
of Btu absorbed by a pound of that material as
its temperturc rises 1°F. .All specific heats vary
with temperature and a distinction must be made
between the true and the mean specilic heat.
The true specilic heat is the number of Btu abI
Matcrhl
r
.Alr art t ~~rrmephrrr)
AlUnllnUnl (JllliV I loo)
Xhcst,B\ fllK-r
:\SlwsI~v4 Inwl.lr,~~r~
;\dlc\. \%OO‘l
;\sph;lll
IlJhCllIC
ISrIck. hu~ld~ng
Hrda. rcll (H5”,, (:u. 15". %111
Ilr.iss. ycll~~w (65"t8 (III. 35". %n)
Itrtlnfe
~:c4lult~w
(imirnr IPorrl.ind cllnkcr)
c:llJlli
l:harc~d ( WIMI~)
192
sorbed per pound per “F temperature rise at a
tixed temperature.
Over a wider temperature
range. the mean specific heat Is the average
number of Btu’s absorbed per pound per “F
temperature rise. In the following table only true
specitic heats are given-for
room temperature
unless otherwise noted.
The heat capacity of a material is the amount
of heat absorbed by one cubic foot of that material during a 1°F temperature rise. The heat
capacity is just the product of the density of the
material (in Ib/ft ‘1 times its specitic heat (Btu!
(Ib”F)). Specitic heats. heat capacities,
and
densities of common building materials and other
substances are given in the following
table.
spc .iic tklr
(Hr”/lb/“t~)
02-l [,‘I
0.2 I-t
0.15
0.20
0.20
0.22
0.35
II.2
O.OY
0.09
0.104
0.32
0.16
0.215
I).-,0
Dcnsiry
(Iblft’ t
ttcrt Capacity
(Hru/ft’l”Ft
0.075
I71
151)
36
40
132
81
I?3
548
5 I9
530
3.-t
I20
l-13
15
0.01x
36.6
37.5
7.2
H.0
29.0
28.-l
24.6
4Y.3
46.7
55.1
I.1
19.2
30.8
3.0
Spc- ific Heats and Heat Capacities of Materials
Material
-Ilay
:oal
bicrete
(stone)
:oppr- (elcctrulytic)
.:ork cgrdnuiated)
‘:otion (f&m)
lithyl dct:hol
Flreclav brlcii
‘;iars, crown tsoda-hmc)
Xlass, flmt (lead)
3lass. pyrcx
:;Ll\s. ‘WCNII”
C;vvutn
I’
I I~~P:~ ! fl!wr1
ICC
Irlln. i.w
! .CJII
I.llllrsrllne
%la~gnc\wm
%larldc
N,<kCl
.lctdnc
?apcr
Pdrafiln
P~wceldui
IZ,)Cl \JIl
i;.llt w.Ircr
L(.IIl,l
SIIIC.1
Silver
Srccl
(Illllll)
Sronc tquxrwl)
Specific
I
tiedt
(Btu/lb/‘F)
0.22
0.3
0.22
0.092
0.485
0.319
0.68
0.198 (2121
0.18
0.117
0.20
0.157
0.25Y
0.323
0.487 1321
it I’l2l2l
0.03 1
0.2 17
0.241
0.2 1
0.105
0.5 I
0.32
0.69
0.1 H
0.2 I9
0.75
0. I4 I
1; 316
0.056
0. I2
0.2
‘rlfl
().I156
I’unprrn
Wdtrr
\V,,cd. whnc c~li
\VOI~. whnc fir
W~Nld. \vhltc pnr
Zinc
0.032
I.0 1391
0.5 70
0.65
0 67
0.092
‘~alurr
.trc for room rcmperarurc unlcu orhcrww
Density
(Iblft’ )
65
90
144
556
5.4
95
49.3
112
I54
267
139
3.25
78
93
57.5
450
707
103
1on
162
555
43.9
58
56
I62
136
72
Y4.6
I40
654
-MY
95
455
12lrb
02.4
47
27
27
445
Heat Capacity
(Btu/ft310F)
13.9
27.0
31 7
51.2
2.6
30.3
33.5
22.2
27.7
31.2
27.8
0.5
20.2
30.0
28.0
54.0
21.8
22.4
26.0
34.0
58.3
22.4
18.6
38.6
29.2
29.8
54.0
18.1
44.2
36.6
58.7
19.0
25.5
3H.7
62.4
26.8
17.6
18. I
40.9
nerd 111hrrdwt\
193
METRX
English Measure
Inch
illOf
\,.ird
llllk
(statute)
yu.lrc
squ.!rc
square
square
Inch
foot
!,ard
mile
~NlIlc‘1’
pt )u nd I 111.1s~)
shcu-t fan
flud
~IUIlL‘C
pint
quart
g.illon
cubic fl)clt
cuh~c vard
Iitu
pound (I‘orcr)
Metric
2.51
30.50
0.Y i
1 .60
6.45
Y2Y.00
0.84
2.60
J ENGLISH
EQUIVALENTS
Equivalent
Metric Measurr
centimeters
centimeters
meter
kilometers
millimc-cr
ctnrimeter
meter
meter
kilometer
0.04
0.39
3.28
1.09
0.62
square cc*-?ri-1f:rf-r
sqllx
mctcr
square kilometer
O. 16 square inch
I. 19 square yards
0.38 square mile
gram
kilogram
ton ( 1,000 kg)
,035 ounce’s
2.20 pounds
1.10 short tons
milliliters
liter
liter
suhic meter
cubic meter
0.03
1.06
0.26
35.3
t .3
square
square
square
squarr
centimeters
centimeters
meter
kil~mcxers
28.30 grams
0.45 kllogram
907.Otl k Ilograms
29.60
O.-i7
0.95
3.78
0.03
0.76
inilliliters
Ilter
liter
liters
cuhc meter
cubic meter
25 I .98 rah~ries
English Equivalent
0.00-l Btu
newton
0.2?5 pound (force)
TEMPERATURE
I
water
freezes
I
8o
fluid ounce
quarts
gallon
cubic feet
cubic yards
calorie
4.45 nrwttjns
20
inch
inch
feet
yards
miles
3,7
I
I
::A:
80
I
temperature
80
I
160
100
212
water
boils
..
.-
VI
5
2 3
D2
= x
g
absorbent--the
balance of system--the components
working
device.
than the photovohaic
photovoltaic system.
less volatile of the two
fluids used in an absorption cooling
absorber--the
blackened surface in a collector that absorbs the solar radiation and converts it to heat energy.
absorptance--the ratio of solar energy absorbed by a surface to the solar energy striking it.
active system--a solar heating or cooling
system that requires external mechanical power
to move the collected heat.
air-type collector-a collector with air as
the heat transfer Huid.
altitude--the angular oistance from the horizon to the sun.
ambient temperature--the temperature of
surrounding outside air.
aperture-solar collection area.
RAE--abbreviation
for the American
and
Society of Heating. Air-Conditioning
ltefrigerating
Engineers.
auxiliary heat--the extra heat provided by
a conventional
heating system for periods of
cloudiness or intense cold, when a solar heating system cannot provide enough.
azimuth--the angular distance between true
south and the point on the horizon directly
below the sun.
196
other
cells that make up a
British thermal unit, or Btu--the quantity
of heat needed to raise the temperature of I
pound of water 1°F.
calorie--the quantity of heat needed to raise
the temperature of I gram of water 1°C.
closed-loop-any loop in the system separated from other loops by a heat exchanger;
often closed to the atmosphere as well.
coefficient of heat transmission, or U-value
--the rate of heat loss in Btu per hour through
a square foot of a wall or to the building
surface when the difference between indoor
and outdoor air temperatures is 1°F. measured in Btu/‘hr ft’“F).
collector-any of a wide variety of devices
used to collect solar energy and covert it to
heat.
collector efficiency--the ratio of heat energy
extracted from a collector to the solar energy
striking the cover, expressed in percent.
collector tilt--the angle between the horizontal plane and the collector plane.
concentrating collector-a device which uses
reflective surfaLes to concentrate the sun’s
rays onto a smaller area, where they are absorbed and coverted to hear energy.
Glossary
conductance-a
property of a slab of material equal to the quantity of heat in Btu per
hour that flows through one square foot of
the slab when a I “F temperature difference
is maintained between the two sides.
conduction--the
transfer of heat energy
through a material by the motion of adjacent
atoms and molecules.
conductivity-a
measure of the ability of a
material to permit conduction heat flow through
it.
convection-the
transfer of heat energy from
one location to another by the motion of a
tluid (air of liquid) which carries the heat.
cover plate-a sheet of glass or transparent
plastic that sits above the absorber in a tlatplate collector.
declination-angle
between the sun and the
earth’s tilt.
degree-day-a
uni! that represents a I “F deviation from some tixed reference point (usually WF)
in the mean daily
outdoor
rcmperatuir.
design heat load---the total heat loss from a
house under the mosl sevl:re winter
tions likely to occur.
design temperature-a
temperature
the Iowcst expected for a location.
determine the design heal load.
condiclose to
used 10
differential
controller
or differential
thermostat--a
device that receives signals
from tempcralure sensors. measures difttrcnccs. and sends commands !o pumps, fans.
valves. or dampers.
diffuse radiation-sunlight
that is scattered
from air ni0le~ul.c~. dust. and water vapclr
and comes frr!sn ihe entire sky vaul[.
direct gaizr-iechniques
of solar healing in
which .sunlight enlers a house through the
win&j&s and is ahsorbctd inside.
direct radiation--solar
radiation that come\
straight from the sun. castmg clear shadows
on a clear day.
domestic hot water--poUk
washing.
eater used for
coc)king. and cleaning.
doped-treating
a substance with phosphorous (to create a negative charge) or boron
(to create a positive charge).
double-glazed-covered
by two panes of glass
or other transparent material.
emittance-a measure of the propensity of
a material to emit thermal radiation.
eutectic salts -a group of phase-change materials that melt at low temperatures, absorbing large quantities of heat.
evacuated-tube collector-a
collector made
of small absorbers in evacuated glass cylinders.
f-chart-computer
simulation that estimates
the performance of active and passive solar
energy systems.
flat-plate collector-a
,iolar collection device in which sunlight is converted to heat
on a plane surface, withuut the aid of reflecting surfaces to concentrate the rays.
focusing collector-a
colleclor
that uses
mirrors to reflect sunlight onto a small receiver.
forced convection---the transfer of heat by
the tlow of warm tluids. driven by fans. blowers. or pumps.
Glaubers salt- sodium sulfale (Na2S0,
10H20). a eutectic salt that melts at 90°F and
absorbs about I04 Btu per pound as it does
SO.
gravity
convection--the
natural movement
of heat through a body of tluid that occurs
when a warm fuid rises and cool Huid sinks
under the influence of gravity.
header--the pipe that runs across the top (or
bottom) of an absorber plate. gathering (or
distributing)
the heat transfer Ruid from (or
‘CO)rhe grid :,f pipes that run across tht absorber surface.
beat capacity-a
propeny of a material. detine4 as the quantity of heat needed to raiz
one cubic foot of the material 1°F.
heat exchanger-a
device, such as a coiled
copper lube immersed in a ranl, of water. that
is used to transfer heat from one fluid to anorher through an intervening metal surface.
197
The New Solar Home Book
heating season--the period from about
October1 to about May I, during which additional heat is needed to keep a house warm.
heat pump-a mechanical device that transfers heat from one medium (called the heat
source) to another (the heat sink), thereby
cooling the first and warming the second.
heat sink-a medium or container to which
heat flows (see heat pump).
heat source-a medium or container from
which heat flows (see heat pump).
heat storage-a device or medium that absorbs collected solar heat and stores it for
periods of inclement or cold weather.
heat storage capacity-the ability of a material to store heat as its temperature increases.
hybrid system-a
system that uses both active and passive methods to co!lect. distribute. and store heat.
indirect system-an active solar heating or
cooling system in which the solar heat is collected exterior to the building and transferred
inside using ducts or piping and, usually, fans
or pumps.
infiltration--the
movement of outdoor air
into the interior of a building through cracks
around windows and doors or in walls, roofs,
and floors.
infrared radiation-electromagnetic
radiation. whether from the sun or a warm body,
that has wavelengths longer than visible light.
insolation--the
total amount of solar radiation. direct. diffuse. and reflected, striking a
surface exposed to the sky.
bsulation--s
or R-value
material with high resistance
that is used to retard heat flow.
Integrated system-a passive solar heating
or cooling system in which the solar heat is
absorbed in the walls or rcof of a dwelling
and flov:s to the rooms without the aid of
complex piprng. ducts, fans, or pumps.
kilowatt-a
measure of power equal to one
thousand watts, approximately
I l/3 horsepower, usually applied to electricity.
198
kilowatt-hour--the
amount of energy equivalent to one kilowatt of power used for one
hour-3,413
Btu.
langley-a measure of solar radiation, equal
to one calorie per square centimeter.
latent heat-the amount of heat, in Btu,
needed for a material to change phase from
a liquid to a gas, or liquid to a solid, and
back again.
life-cycle costing-an estimating method in
which the long-term costs such as energy consumption, maintenance, and repair can be included in the comparison of several system
alternatives.
iiquid collector-a collector with a liquid as
the heat transfer fluid.
load collector ratio (LCR)-a building’s hea!
loss per degree day divided by the area of
south glazing.
natural convectian-see
gravity convection.
nocturnal cooling--the
cooling of a building or heat storage device by the radiation of
excess heat into the night sky.
open loop--any loop in the system where
potable water is used for collection and .%torage; open to the atmosphere as well.
parabolic collector-a concentrator-y collector with a parabolic-shaped
reflector.
passive system-a
s&u heating or cooling
system that uses no external mechanical power
to move the collected solar heat.
percentange of possible sunshine--the percentage of daytime hours durmg which there
is enough direct solar radiation to cast a
shadow.
phase-change material-a
substance that
stores and releases latent heat when it changes
from a liquid to a solid or gas.
photons-particles of light energy that transfer energy to electrons in photovoltaic
cells.
photosynthesis--the conversion of solar energy to chemical energy by tire action of chlorophyll in plants and algae.
photovoltaic cells-semi-conductor
devices
that convert solar energy into electricity.
Glossary
present valtie--the
value in today’s dollars
of something to be received at a later date.
profile angle--the
angle between the horizon and the sun’s rays, in a vertical plane
perpendicular to a window; used to size fixed
overhangs for shading.
radiant panels-panels
with integral passages for the flow of warm fluids. either air
or liquids. Heat from the fluid is conducted
through the metal and transferred to the rooms
by thermal radiation.
radiation--the flow of energy across open
space via electromagnetic waves, such as visible light.
reflected radiation-sunlight
that is reflected from surrounding
trees, terrain. or
buildings onto another surface.
refrigerant-a
liquid such as Freon that is
used in phase-change collectors or cooling
devices to absorb heat from surrounding air
or liquids as it evaporates.
tendency of a
resistance ar K-value--the
material to retard the tlow of heat, measured
in (hr ft’“F)/Btu.
retrofitting--the application of a solar heating or cooling system lo an existing building.
r&t-s---the flow channels or pipes that distribute the heat transfer liquid across the face
of an absorber.
R-value-see resistance.
seasonal efficiency--the
ralio of solar energy collected and used to that striking the
collector. over an entire heating season.
selective surface--an absorber coating lhat
absorbs most of rhe sunlight hitting it but
emits very liule thermal radiation.
shading coefficient---the
ratio of the solai
heal gain through a specific glazing system
to the total solar heat gain through a single
layer of clear, double-strength
glass.
shading mask--a section of a circle that is
characteristic of a particular shading device:
Ihis mask is superimposed on a circular sun
path diagram Lo determine the time of da)
and the months of the year when a window
will be shaded by Ihe device.
solar declination-see declination.
solar house-a dwelling that obtains a large
part. though not neccessarily
from the sun.
solar load ratio (SLR)-ratio
to heat load.
all, of its heat
of solar gain
solar radiation-electromagnetic
radiation
emitted by the sun.
solar savings fraction (SSF)--ratio
of solar
savings to net reference load.
specific heat-the amount of heat, in Btu.
needed to raise the temperature of 1 pound
of a material 1°F.
sun path diagram-a
circular projection of
the sky vault. similar to a map, that can be
used to determine solar positions and to calculate shading.
sunspace-a glazed room on the south side
of a building that collects solar energy to heat
that building.
thermal capacity--the quantity of heat
needed to warm a collector up to its operating
temperature.
thermal
mass, or thermal inertia--the
tendency of z building with large quanfities
of heavy materials to remain at the same ternperature or to fluctuate only very slowly; also.
the overall heat storage capacity of a building.
thermal radiatian-electromagnetic
radiation emitted by a walm body.
thermosiphoning-we
gravity convection.
tilt angle--the angle that a flat collector surface forms with the horizc?ntal.
trickle-type collector-a collector in which
the heat transfer liquid Hews down channels
in the front face of the absorber
tube-in-plate absorber-copper sheet metal
absorber plate in which the heat transfer fluid
flows through tubes in the plate.
ultraviolet
radiation-electromagnetic
ra-
diation. usually from the sun. with wavelengths shorter than visible light.
unglazed collector-a
collector
with no
transparent cover plate.
U-value-see coefficient of heat transmission.
I99
Ahsorbcnt.
Yl
Absorber coatings. IOI-10.3. 107
Absorhcr &sip.
OX. YY
Absorber cflicicncy.
IOtt
Absorber platch. X-l. IW 101. IO5
Absorbers.
5:
ahsorptancc.
rcllsctancc
and cniittancc.
32. 32: tlatplate collector.
OX; Thoma>on. YY: tube silt. YY- I()():
air blat-plate collector. 105; hcnt loss. I I-I. I 15. S&J t~/.~,~
Absorber
coatings. Ahsorbcr
&sign:
Ahsorbcr cfhciency: Ahsorbcr pla~rs: Absorbers, ;ltr typ
Ahsorbcrs.
au type. 105. I07
Ahsorptancc.
3 I-33
Absorption
coohng. Yh. 07
Act&
heating \y\tcm. YO-0’
Active dnr DHW \y\trms.
7h
Actlvc \olar systsmh. X7
ACtIvc systems. 4
Xlhrhc. I. 2x. 5h
Air harrier. ?(,
Air Ilat-plate collector\.
105. IOh
Air Ilow. ItK. It17
Air mhltmtion.
21.23. 34. 35
Air leakapc. 107
Air quality. 36
AII svstcms. Y2
Air-to-air
heat cxchangcrs. 36
Atr-to-lquid
heat crchangcr\.
X2
Air-type ccdlectors. IO7
Altitude. dar,
IO. I I. 50
Antlfrccre
\y\tcms. 82
Antgfrcczc. Y I. OZ. 04
Argonne National I ,ahrr;ltom
ASHRAE.
14. IH. 22. 30. ;i ‘t)”
ASHRAE Hatullmd
o/’ ~rtrrckrnrc,rrrc!Is. 50
Attic insulation. 43
Autumnal cquinclx. IO
Auxi1i.q
heating. 5. 136. IJS
Arimuth angle. 50
Arimuth.
solar. IO, I I
Backup hcatcr. X4
Batch hcatcrs. 7 l-73
Bimfy
\toragc. 112
Bread box hatch hcatcr. 71-72
Brick. Sh. hh. hX
British Thcrmd Vnit. 3
Calorie. 3. I2
c’cllulosc lihcr. -13
Ccntrc Nation&
dc Ia Rcchcrchc Scicntiliquc
(CNRS).
67
Chimney cffcct. 5Y
Clear day msolation. rd. IJO
Clear day insolation data. 1% IS. I I7
Climatic Atlas of the llnitcd States. 19. 23. 37. 139
Closed-loop
DHW syhtcms. 74. 76. Srr dsn Domestic
hot war
(DHW)
Cloud-loop
sy\tcni. 7X
C’NRS wall collector.
h2
Cocflicicnt
of heat transmission.
IH. I9
Cocfliclcnt
of Performance
(COP). 136. I37
(‘old-wall
cffcct 4 I
Collector cfticicncy.
I 14-l 15;
two-tank system, K-I: effect of heat exchanger. 94; absorber cflictency.
I(W)- IO1 : effect of temperature.
I33
Collector orientation.
X3. I I7- I I9
201
Index
Collector performance.
114-I 19: estimating performance,
121, 122
Collector.
size. 114. 119. 120-124
Colcr of roofs and walls 31
Compound parabolic concentrator (WC).
109-l IO
Concentrating
collectors.
5. 96. 109. I IO
Concrete collector,
63
Concrete slab, 57. 95
Concrete wall collectors. 62
Concrete walls. 63
Concrete. 66. 6X. X9
Conductance.
I7- I9
Conduction.
I6 I Y
Conduction
heat How. 21
Conduction
heat loss. 24
Conductivity.
17
Conservatory.
6X
C~rnstruction industry. I-17
Controls, YS
Convection.
2. 16; heat loss . 2 I-23; thcmmsiphoning.
SY
Cooling. solar. I2Y
Corrosion.
X6. YJ. YY
Corrosion prcvcntion.
I(X)
Cover plates. 5:
iiqutd hat-plutc collectors.
IOI- 103: air hat-plate colIcctors. 107; evacuated tube collector.
III
CPC cva;ualrd
tube collector.
I I3
Crack Icnpth. 23
Danipcrs. 61. Y I
1)cclination
angle. Y
Dcgrcc day. IY. 20
Dcgrcc d;iy4 and dcstgn tcmpcraturcs.
20
Dcgrcc hrjurs. 20
Dcstpn heat load. 24. 26
I)rsign tcmpcraturcs.
I 9. 2-l
Diffcrcntial
cturtrollcr.
76. 7Y-X7
Ihll’uw
rad(at(on. I?. I?
Direct pain systems. -1. 4-t. 6X
Direct mass. 5X
Direct radiation.
I. I3
I43
Dtrccl-mount
PV may.
IIomcstrc hot watsr IIIHWI.
S. 6Y. 70;
~WAIVC
solar. 7 I. XIIVC
solar. 76-77; wtth qcc
Ill&!.
XY
I)rainback
systems. 70-X2. YY
Draindown
systems. X0
t+ctromapnctic
qh2ctrur11.
2
fmittancc
(c ). 3 I-33
Fncrgy conservation.
27;
an mliltrutnnr.
35; auxihary hcatmg.
I<\.
I-80
Energy-cflicient
conWuchon.
66
Estimating array LIIIC. I40
202
hcat-
136; photovolta-
Estimating collector performance.
See Collector
ance
Estimating collector size, I24
Estimating storage size, I34
Eutectic salts, 74. 126. 132
Evacuated closed-loop
system, 75
Evacuated-tube
collectors,
96, I IO- I I2
Evacuated-tube
&sign,
I I I. I I2
perform-
F-Chart. 6X
Fans, 60. 66, 91
Flat-plate collector. 5. 72. 73, 9X
Focusing collectors,
107
Forced-air panels (FAP), 60
Forced-air systems, 89. 90
Forced conveciion.
I7
Freeze protection,
75. 79
Freeze snap switch, 76. 79
Glass. 7;
heat loss through windows.
3Y; in high-performance
glazing. 40. 41; history, 44; solar benefit values, 45;
specialized types, 47: shading, SO; cover plates. 102
Glaubers salt, 128. 132. 133
Glazing properties. -I I
Glazing: greenhouse effect. 3:
solar heat gain and loss, 47. 4X: sunspaccs. 65. 66:
cover plates, I03
Government
incentives,
14X
Gravel heat storage. 130. I32
Greenhouse effect. 3. 28
Grcenhou\es.
6X. 71. 74
Hard-coat glass. 40
Heat capacities of common materials. 56
Heat capacity, 56
Heat distribution.
135 I36
HcJt cxchangcrs:
thermosiphoning
water heaters, 73; drainback systems.
70-X2; space heating. XY, 94: with a water tank. I30
Heat how. I6IX;
radiation,
23-24: calculations.
24
Heat Illad calculations.
2-t-25
Heat loss. 34;
conduction,
17. IX; convection , 21: radiation.
23-24;
windows.
3X-40: insulation to reduce, 41
Heat Mirror’*.
40
Heat pipe cvacuatcd tube. I I2
Heat pump. 136. lY6
Heat storapc. 3. 2X. 12X-135;
nicasurcmcnt of, 4: capacities of materials. S4
Heat storage bm. 60
Hcaf storage capacity. S-1, 62. 17X
Hsat storage materials,
I 29
Heat transfer. IOS- I07
Index
Heat transfer coefficient.
I06
Heat transfer fluids. 89-92:
thermosiphoning
systems, 73; closed-loop
systems. 76:
antifreeze systems. X2; air systems. lO7- I I6
Heat trap. 28, 34
Heating season. 24
High-performance
tilm. 41
High-performance
glazing. 40. 66. 68
House orientation.
30. 31
House shape. 29. 31
Hutchinson,
F W.. 44
Indirect gain systems. 5. 59
Indirect mass. 5X
Indirect systems. 149-207
Inliltration
heat loss, 23. 25
Infrared radiation. 2. 23
Insolation and house orientation.
3I
Insolation and house shape. 3 I
Insolation,
I I-IS. 31, II6
Insulating curtains. 40
Insulating shutters. 40
Insulation.
41-43, lO3-10-I;
R-values.
IX; thermal mass and temperature
55, 57: storage systems. I33
lntcgral collector storage (ICS). 72
Integral-mount
PV array. 14-l
Natural
convection.
Odcillo.
Olgyay.
One-tank
Open-loop
Water
Operating
of houses. I, 29-30. 68
48-49. 51. 53. 66
Parabolic collectors,
107. I IO
Passive heating systems. 4. 27, 13.5
Passive solar design, 68
Passive solar DHW systems, 71-75
Percentage of possible sunshine. 14-15
Performance and cost, 96
Phase-change collectors,
75
Phase-change materials, 73. 129. 132
Photons. I39
Photovoltaic
(PV) array. I38
Photovoltaic
(PV) panels. 82-84. ‘96, 13% I39
Photovoltaics
( PV). 5. I 3% I44
Plexiglasas.
103
Polyethylene
moisture barrier, 22. 37-38. 42
Polyethylene-tiber
air barrier, 32
Polystyrene,
42
Polystyrene beads, 43
Power invertcrs.
13% 139. 143
Protile angle, 49,
Pumps. 76. 7Y. X2-X4. 93
swings.
Q-values, 25
Quadpane**.
4I
R-values of common insulators. 42
R-values, 19. 21. 41-42
Rack-mount
PV arrays. I44
Radiant heating, 90
Radiant panels, 135 I36
Radiation, 2-4. I6- I7
Radiation heat Aow. 23-74
Recirculation
systems, 76-79
Rcllcctance,
32-33
Rcllccted radiation.
12. IS. 30
Refrigerant.
96-07. I36- I37
Remote mass. 58
Residential PV system, l43- I44
Resistance. IX
Reverse return piping system. 102
Reverse thermosiphoning,
6I
Rigid board insulation.
43
Rock bed, 90. 91
Rock heat storage. 92. I30- I32
Roof collectors.
64
Langlcy.
I2
Latent heat. 74, IZX
Life-ryclr
costing. I46
Liquid collectors.
9X. I29
Liquid flat-plate collectors. YX
I.iquid heating slstcms. Y3
Liquid system dcsipns. 03
Load Collector
Ratio (LCR). 6X
Los Alamos National Laborator) . 68
Masonry walls. XY
Mass walls, 62. 63
Mean Daily Solar Radiation.
I2
Mean percentage of possible sunshine,
Michael, Jacques. 63
MIT solar houses. I
Movable insulation. 6X
Movable shading devices. SO
Orientation
Overhangs,
I4
17. 2 I
62-64
Victor. 29-30. 36
system. X4, XS
DHW systems. 76-77. SPP CI/SODomestic
(DHW)
temperatures,
I23
Hot
Seasonal heat loads. 76
Seasonal heat loss, 20. 23
Seasonal heating needs. I23
Selective surface. 101. 107
Semiconductor.
I39
Sensible heat, 74, I28
Shading, 4X-50
Shading coefficients.
SO-S I
203
Index
Shading devices, 50
Shading mask. 5 l-53
Shading mask protractor. 52
Shelter design. 27-28
Silicon,
I39
Sizing mass, 58
Sizing overhangs. 49
Skyltght. 39
Soft-coat glass. 40
Solar altitude.
II
Solar azimuth, I I
Solar benefit values. 4S
Solar cells. 5. 13X
Solar collectors.
2X
Solar cooling. 96
Solar domestic hot water (DHW). See Domestic hot water
(DHW)
Solar heat gain, 29. 30. 44. 46-47
Solar Load Ratio t SLR). 6X
Solar position. Y-l I . 5 I
Scalar Rating and Certification
Corporation
(SRCC), I23
Solar transmiltancc,
47
Solar water heaters. SPY Domestic hot water (DHW)
Space heating and cooling. X9
Sprcilic heat. 56. 12X. I30
Spccilic heats of common materials, 56
Spring equinox.
IO
Storage size. 134. 13.5
Storage tank:
thcrmosiphoning
water hcatcrs. 73: active systems. 77;
one- and two-tank systems. X4; installation
checklist.
X6
Storage tcmpcraturc.
I ?3- I34
Storapc-type
water heaters. 7 I
Storm window. 40
Summer solticc. IO
Sun path diagrams. SO-52
Sun paths. IO
Sun’s daily path. IO
Sungain’q’. II
Sunspaccs. 5. M-6X
Supcrinsulatcd
bulldings. 36. 42
Swimming
pool heating. Y&Y5
Temperature
fluctuations,
55
Temperature
swing of a house, 55-56
Thermal efficiency
curves. 125, I26
Thermal mass. 34, 56-58
Thermal radiation, 2, 17. 24;
effect of atmosphere. 2X; emittance, 33; absorber
loss. 107
Thermosiphoning
air panel (TAP), 60-62.
Thermosiphoning,
17. 2 I. 59-63
Thermostat setback, 42
Thomason absorber, 9X-99
Thomason’s collector,
I31
Tilt angle of a collector,
I 17-l I9
Trickle-type
collectors.
9X
Tripanew,
4I
Trombe walls, 64
Trombe. Felix. 62-63
Tube-type absorbers. 9X
Turbulent flow, IO6
Two-tank system. X4. 85
U-value.
1X-20;
convection heat loss, 24; cffcct
Ultraviolet
(UV) light, 41
Uitraviolet
(UV) rddiakxk.
?
Utility power. I43
IIV transmittance.
SO
Vapor barrier. 37-3X
Ventilation.
34. 6X
Vernal equinox. IO
Wall collectors.
64
Water containers. 66
Water tanks. Y I. I20
Water vapor. 37
Wcathrrstripping.
3.5
Wind control. 36
Window collectors.
64
Windows.
3.5. 3X-40. 17. SO
Winston, Dr. Roland. IOY-I IO
Winter so!sticc. It)
of insulation
41
heat
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