Solar System Components, Configurations and Operating Principles

Solar System Components, Configurations and Operating Principles
3 Solar system components, configurations and operating principles
CHAPTER 3
Solar System Components, Configurations
and Operating Principles
Table of contents
3.1
Principles of water heating .............................................................. 52
3.2
Collector types and operating principles.......................................... 59
3.3
Storage tanks .................................................................................. 70
3.4
Close coupled solar water heaters .................................................. 79
3.5
Pump-circulated (or split) systems................................................... 88
3.6
Thermosiphon remote storage systems .......................................... 99
3.7
Heat pump water heating systems ................................................ 104
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3 Solar system components, configurations and operating principles
3. SOLAR SYSTEM COMPONENTS,
CONFIGURATIONS AND OPERATING PRINCIPLES
3.1
Collector types and operating principles
What this section is about
To install and maintain water heating systems for efficient operation, it is
essential to understand the key physical principles underpinning their
operation. This section is a summary of the key principles that apply to solar
water heaters. These principles are covered in more detail elsewhere such as
in the TAFE resource book, Solar Water Heating Systems (see Bibliography
for details).
This section provides an understanding of:
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•
•
•
the concepts of conduction, convection and radiation as ways in which
heat moves between hot and cold bodies
the stratification principle in hot water storage tanks
the changes in temperature, volume and pressure as water is heated
the concept of thermosiphon flow for non-pumped water circulation in
solar water heating systems.
Conduction, convection and radiation
Heat is another name for thermal energy, or energy stored in a body due to its
temperature. We use temperature as a way to measure this thermal energy.
The three processes by which heat is transferred in water heaters are
conduction, convection and radiation. These processes determine:
•
•
the rate of heat absorption and transfer by the solar collector to the
water
the rate of heat loss from the solar collector and storage tank back to
the surrounding air.
These two processes control the overall effectiveness (or efficiency) of
conversion of solar radiation into hot water. This in turn affects the size of the
solar collectors and the water storage tank. For example, if a collector is poor
at conducting heat in the absorber to the water in the riser tubes or manifold,
then more heat will be lost back to the surrounding air. Alternatively, if too little
insulation is wrapped around the storage tank or pipe work, then more heat is
again lost to the outside air. Both these factors would reduce the performance
of the solar water heater. To compensate, either a bigger system would need
to be installed or more boost energy would need to be used. Let us now look
at each process and how it affects solar water heater performance.
Conduction is the transfer of heat via atomic particles vibrating within a
material and bumping into one another, giving some of their vibrational energy
to neighbouring particles. The hotter the material, the faster the particles
vibrate. It is the main way heat moves through solids and, to some extent, in
liquids. Heat does not easily move through gases by conduction as the
molecules are spread far apart compared with solids or liquids.
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We know that some materials are good conductors of heat; e.g. copper,
aluminium and steel. Other materials, such as plastics, are poor conductors.
This is due to their differing atomic structures.
How does this heat transfer mechanism influence the performance of solar
water heating systems? Firstly, performance is improved if the heat absorbed
by the absorber plate can be quickly conducted to the water in the riser tubes
or their equivalent. For this to happen we need materials that conduct well.
Metals are far better conductors than plastics. Copper is roughly two times
better than aluminium at conducting heat, which is two times better than steel.
Hence, if using copper we can use one-quarter of the thickness of a steel
plate; or if using aluminium we can use half the thickness of the steel to
conduct heat over the same distance. So if we use steel instead of copper for
our absorber plate, we need to either use a thicker plate or place the water
passages closer together to get similar or better performance. This is why
steel absorber plates use the flood-plate design compared with the fin and
tube design of copper absorber plates.
Secondly, we need to use materials that are poor conductors around the sides
and back of the collector to insulate the hot absorber plate from the
surrounding area. This slows the rate of heat loss to the surrounding air, but
does not prevent it completely.
Convection is the transmission of heat within a liquid or gas due to the bulk
motion of the fluid. The rising of hot water from the bottom to the top of a
saucepan as it is heated is one example. This occurs because the particles of
water bump against one another more vigorously as they are heated and push
themselves further apart, making the water less dense and hence lighter. This
is called natural convection and is the main process of heat transfer in
liquids and, to a lesser extent, in gases. We can also create convection by
mechanical means, such as fans to blow cooling air across surfaces. The
cooling fans in computers that help keep the electronic components operating
at safe temperatures are one example.
In a solar water heater, convection heat transfer occurs in two main places.
The first, in a flat plate collector, is under the glass cover between the hot
absorber plate and the glass. The air immediately above the absorber plate
heats up, becomes less dense and rises up to the glass cover above, where it
heats the glass. The air is then cooled and sinks back to the absorber plate.
This process is also happening on all outside surfaces of the collector box and
glass cover. As a result, heat is lost to the surrounding air.
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Ta
RADIATION
CONVECTION/
RADIATION
CONVECTION
RADIATION
Tc
CONVECTION
CONDUCTION
THROUGH BACK
CONDUCTION
THROUGH SIDES
CONVECTION/
RADIATION
Figure 3.1.1 – Heat transfer via conduction, convection and radiation in a collector
Figure 3.1.1 shows all three heat transfer modes operating inside and from
the outside surfaces in a solar collector with a glass cover on top, black
absorber plate inside a box with insulated edges and back (note: Ta = air
temperature, and Tc = collector absorber temperature).
Radiation is the transfer of heat via direct rays of electromagnetic energy.
This is the main way heat is transferred through the air or even across
vacuums. We feel this process when we stand in the sun or close to an
electric radiator. In the sun, we feel all the electromagnetic parts of the sun’s
energy being absorbed into our skin and heating it. This includes the
ultraviolet (UV) rays, which will eventually burn us, and short infra-red (IR)
rays, both of which we can’t see. And of course solar radiation includes the
light rays that we can see. When we stand next to an electric radiator, we feel
much longer infra-red rays.
In a solar collector, solar energy is transmitted as short-wave electromagnetic
radiation to the absorber plate. Heat is also re-radiated to the surrounding
space from all surfaces of the collector that are at a higher temperature than
their surroundings. Mostly, the heat is re-radiated from the absorber plate to
the glass above it, and then from the glass to the sky above, as these are the
hottest surfaces in the collector.
In all the processes above, two important things occur. Heat always moves
eventually from hotter to colder areas or objects, and the total amount of
energy in the whole system remains the same. One way to think of this is as
follows. If one litre of hot water at 60ºC is mixed into one litre of cold water at
20ºC, we will end up with two litres of hot water at 40ºC.
This means that heat collected by our solar heater and stored in our storage
tank will always try to return and heat the surrounding air, which is at a lower
temperature. By good design, we are able to capture some of the solar energy
(typically about 40%) and transfer it to the taps as hot water. Even the energy
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in this hot water will eventually go back to heating our surroundings, after it
has been used by us.
Pressure, temperature and expansion
Water is the main heat transfer fluid used in open loop water heating systems.
It has some important properties that affect the design of water heaters.
Firstly, when water is heated, several changes can occur (in closed loop
systems the heat transfer fluid is a glycol-type fluid, which has many similar
characteristics to water):
•
•
•
an increase in temperature
an increase in volume (hot materials expand)
an increase in pressure if the volume is kept constant.
For example, one litre of water at 20°C will expand to 1.006 litres at 40°C. At
100°C that same litre of water would occupy 1.042 litres just before it boils. If
this water is contained in an unvented cylinder (such as our hot water storage
tank) and heated, then the volume of the tank cannot increase as the tank is
not a very flexible material, and so the pressure inside will increase. The
cylinder of our storage tank must be able to handle a certain amount of this
pressure without making the tank too heavy and too costly. If the pressure
becomes excessive, then there must be a system to relieve that pressure.
Failure of that system could result in the explosive bursting of the cylinder.
To cope with this increase in pressure, mains pressure tanks must be
designed to operate up to a maximum pressure that is specified in
engineering Standards such as AS/NZS3500. As well, a safety valve, such as
a pressure and temperature relief valve, may be fitted that will relieve
pressure in the tank at a preset temperature and pressure by draining some
water out of the tank to ground. Gravity feed tanks operate at atmospheric
pressure. These require an expansion tank above the storage tank. This small
tank also contains a float valve to top up the tank as hot water is drawn off.
Details of these systems are shown in subsequent chapters.
It should also be remembered that water boils at 100°C at atmospheric
pressure (101 kPa), but at 200 kPa the boiling point is 121°C. If water at this
temperature is released to the atmosphere it will immediately turn to steam –
explosively! These high temperatures and resulting pressures could injure
people or damage equipment.
Secondly, when water cools, it initially contracts in volume (under constant
pressure) until about 4ºC when it expands. This has practical impacts upon
the design of solar water heaters for freezing conditions, as this expansion of
water can crack pipes and tanks. System design features and precautions to
cope with freezing are explained in subsequent chapters.
Principles of thermosiphon flow
In a thermosiphon solar water heating system, there is no pump to circulate
the water from the collectors to the storage tank. Yet the water heated in the
collectors flows to the storage tank and the cold water from the bottom of the
storage tank flows to the bottom of the collectors. What causes this
circulation?
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The circulation occurs because the water expands and becomes less dense
as it is heated. This makes the water lighter than the cold water, so it floats on
top of the colder water. Due to the location of the storage tank above the
collectors and the continuous upward rise of the collector riser and header
tubes, and the return pipe from the collector to the tank, the less dense hot
water can rise to the top of the collector, up through the return pipe to the hot
water storage tank. Its place is taken by the colder, denser water from the
bottom of the tank that will sink slowly down the collector supply pipe.
Thermosiphon flow is the name given to this flow and it is an example of
natural convection. The driving force is due to the change in density of the
water as it is heated. Figure 3.1.2 below shows this circulation in a typical
close coupled thermosiphon system.
Image: Courtesy Conergy
Figure 3.1.2 – Thermosiphon flow in a close coupled system
A thermosiphon system has a cost advantage over a pump-circulated system.
A pumped system requires a pump and a pump controller as well as a power
supply to run the pump. Pumps and controllers can add some extra
complexity to a solar water heater system.
Stratification in hot water storage tanks
Stratification is the tendency for stored water to remain in layers of different
temperature, with hot water at the top layer and cold at the bottom. Heated
water expands, becomes less dense and can float on colder denser water.
Since water is a poor conductor of heat, the water can remain in different
temperature layers until it is agitated:
Relative density of water at 20°C = 0.998kg/m3
Relative density of water at 60°C = 0.983kg/m3
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The stratification in a tank of hot water allows the hottest water to be drawn
from the top of the tank. Incoming cold water is fed through a baffle or
spreader pipe so that it will create the least disturbance to this stratification.
Figure 3.1.3 below shows a cut-away through a typical close coupled,
thermosiphon solar system tank.
Image: Courtesy Solahart
Figure 3.1.3 – Cross-section through a close coupled system storage tank
This diagram shows the stratification of the water in the tank with the hotter
water at the top and the colder water at the bottom. To prevent or minimise
mixing of hot and cold water, cold water is brought into the bottom of the tank
through a specially designed spreader pipe that slows the water velocity and
spreads it along the bottom of the tank.
Conversely, at the top of the tank the hot water outlet pipe is scooped
upwards inside the tank to draw water from the hottest level. Note also that
good insulation is wrapped around the tank, and the tank is located further to
the bottom of the insulation so that the insulation is thicker at the top where
the hottest water is located.
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Key points
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Heat is thermal energy due to the vibratory motion of atomic particles
within a solid material, liquid or gas. We measure heat energy by
measuring temperature.
Heat always flows from hotter to colder areas or materials and the total
amount of energy in the whole system remains the same.
Heat is transferred in all solar energy systems via conduction,
convection and radiation.
As they heat up, solar collectors and storage tanks try to lose heat back
to the surrounding air via conduction, convection and radiation. Good
system design tries to limit this.
The choice of materials in the construction of solar collectors affects
their performance. For example, absorbers need to be good conductors
of heat to efficiently transfer solar energy to the heat transfer fluid (often
water). The absorber plate and the storage tank need to be insulated to
prevent heat loss to the air.
The properties of the heat transfer liquid affect the construction
materials of the collector and storage tank. Water is the most common
heat transfer liquid used and designers must account for its properties,
or for the properties of other heat transfer fluids if they are used.
Water expands as heated, and in closed mains pressure systems exerts
a pressure on the inside of the storage tank and collectors. Both must
be strong enough to cope and have some form of safety protection to
prevent rupture if the pressure is too high. This is usually a
pressure/temperature relief valve that drains some water from the tank
to ground if the pressure or temperature exceeds set limits.
Water boils at about 100ºC at atmospheric pressure and at higher
temperatures as the pressure increases. If steam forms inside the
system, it could cause tanks to rupture and cause injury to people.
Water contracts in volume at constant pressure until about 4ºC when it
expands again. This can cause cracking of pipes if the design does not
take this into account.
Thermosiphon flow is used in many systems to transfer the heated
water in the collector to a storage tank. It uses the natural convection
principle that a heated fluid will expand and become lighter, and so rise
to the top of the container that encloses it. This means that the storage
tank must be higher than the solar collector and the pipe work must all
rise continuously upwards from the collector to the tank.
Stratification of hot water occurs when hot water is stored in a stationary
condition in a tank. The hottest water rises to the top and the coolest
falls to the bottom. A small amount of conduction will occur between
layers. This stratification assists the operation of the hot water system
by allowing the hottest water to be drawn off from the top of the tank.
To prevent mixing of hot and cold water, specially designed pipes are
used to feed and spread the cold water into the bottom of the tank at
low speed. At the top of the tank, the draw-off pipe is scooped upwards
to take water from the hottest level.
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3.2
Collector types and operating principles
What this section is about
Different solar collectors are designed to operate efficiently over different
temperature ranges and using different heat transfer fluids. For example, pool
heating collectors look different from residential hot water collectors because
they only have to heat water to a maximum of about 30ºC. Residential hot
water collectors, however, need to be able to heat water to 60ºC or higher. In
this section, we examine the features and construction of different collectors
for domestic hot water supply, including:
•
•
•
•
the construction and components of flat plate collectors and evacuated
tube collectors
the effect that each of the structural components has on the operation of
the collector as a whole, in relation to maximising absorption of solar
radiation and minimising heat losses by conduction, convection and reradiation
selective absorber surface coatings and their effect on the performance
of a solar collector
the effect that winter and summer conditions have on the solar collector.
Flat plate collector systems
Flat plate collectors have been used since 1950 in Australia. Their principle of
operation is relatively simple, but there have been advances in the
construction technology, materials and absorber surface that are used by
different manufacturers. All have a box, usually made of steel or aluminium
sheet. The absorber plate, with tubes attached for a heat transfer fluid to pass
through, is in the middle of the box. Beneath the absorber plate is insulation to
prevent the loss of heat out through the bottom of the box. Above the plate is
a transparent cover, usually glass, which is designed to trap solar radiation
and convert it to heat in the absorber plate. It also prevents cold air from
blowing over the plate and taking this heat away. Figures 3.2.1 and 3.2.2
show the various components.
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Transparent
cover
Header with
connection to:
Strainer
Non-return
valve
Absorber
plate
Collector
box
Pressure relief
valve
Vacuum break
valve
Insulation
riser tubes
header tube
Figure 3.2.1 – Construction details of a flat plate collector
Sealing strips
Transparent cover
Absorber plate
80mm–100mm
25mm–50mm
Bottom insulation
Side insulation
Figure 3.2.2 – Cross-section of a flat plate collector
Principle of operation
Two processes happen at the same time inside a solar collector:
•
When the sun shines through the glass cover, the solar radiation, both
the direct and diffuse parts, is absorbed by and heats the absorber
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•
plate. The absorber plate conducts heat to the water in the riser tubes
connected to it.
Heat is lost from the absorber plate back to the surrounding air via the
transparent cover. This is because heat always tries to flow from hotter
to cooler areas. As well, the transparent cover is not a good insulator so
it allows heat to escape from the top surface.
Heat is, in fact, lost from all external surfaces of the collector via convection,
conduction and re-radiation. These energy losses can be reduced by:
•
•
•
for conduction – bulk insulation such as rock wool or fibreglass around
the sides and back of the collector
for convection – a transparent cover to protect the absorber plate from
winds
for re-radiation – a transparent cover to trap energy re-radiated from the
absorber plate, and selective surfaces on the absorber plate to reduce
the amount of re-radiated energy from the absorber surface.
The function and design of the components of flat plate collectors are
discussed in more detail below.
Absorber plate
For best performance, the absorber plate should have the following features:
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•
highly absorbing surface for the incoming solar radiation (typically a
dark colour); that is, a high absorptance factor (such as 0.95; i.e. 95%)
re-radiation of very little heat (infra-red) radiation from the absorber
surface as it gets hotter; that is, a low emittance factor (such as 0.05;
i.e. 5%)
high conduction of heat to the heat transfer fluid (typically water or
water/glycol)
suitable mechanical strength and corrosion resistance.
How the heat transfer fluid (water or water/glycol) is brought into contact with
the plate also affects the efficiency of heat transfer from the plate to the fluid.
It is typically done in one of three ways:
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Header pipes at the top and bottom of the collector are joined by risers.
The risers are in thermal contact with the absorber plate. These are
called fin and tube collectors.
The absorber plate is made of two sheets with welded seams joining the
two together. Waterways are formed between the welded seams and
the fluid passing through is heated directly by the sun. These are called
flooded-plate collectors.
There are also available plastic, rotationally moulded systems that
integrate collectors with a storage tank. These require less protection
from over-heating and ‘hard’ water quality (see Chapter 5 for details of
water hardness).
Serpentine collectors have a pipe coiling backwards and forwards
across the absorber plate. This pipe is thermally bonded to the absorber
plate and as the water passes through the pipe it absorbs heat from the
plate.
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•
The ‘heat sheet’ collector is a lightweight, flat-plate heat pipe,
consisting of two sheets of steel seam-welded together at the edges
and carrying a pattern of indentations. The indentations create a vapour
space within the heat-sheet that is evacuated and into which a working
fluid is introduced, providing a two-phase thermosiphon process. The
working fluid collects heat from the collector, transfers it to the water via
a copper tube heat exchanger and the condensate returns via gravity
from the condenser end to the evaporator end.1
Photo: Courtesy Solahart
Figure 3.2.3 – Types of absorber plates
Figure 3.2.3 shows a cut-away of typical absorber plates including a fin and
tube collector (lower) and a flooded plate collector (upper). The fin is wrapped
around the tube to better conduct heat to the tube. A thermal paste is used
between the fin and tube to prevent corrosion if different metals are used.
Typically, either the fin is aluminium and the tube is copper, or both are
copper. See Figure 3.2.4 (below) for more examples.
The flooded plate collector uses two pressed metal sheets that are then spot
welded together to form passageways for the heat transfer fluid. Because the
fluid is in contact with almost the whole absorber inner surfaces, the heat
transfer to the fluid is improved, and cheaper, lower conductivity absorber
materials can be used such as steel or even plastic.
Serpentine collectors are less frequently used due to their lower efficiency of
transferring the absorbed heat to the heat transfer fluid. But they are a little
cheaper to make.
1
Information courtesy Thermocell Ltd New Zealand (www.thermocell.co.nz).
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Photos: Andrew Blair
Figure 3.2.4 – Fin and tube collectors – a stack of riser/headers (left) and absorber
plates (right) ready for assembly
Absorber plate surface coatings
Simple coatings, such as black paint, work satisfactorily to absorb solar
radiation but they are also high emitters of heat from the absorber surface as
it gets hotter. This heat leaves the absorber surface as infra-red radiation and
is absorbed by the glass above. This glass gets hot and loses heat to the
surrounding air. This makes the collector less efficient at heating water.
To improve the performance of the absorber surface, special surface coatings
have been developed called ‘selective surfaces’. Common selective surfaces
include chromium, copper, nickel or titanium oxides that are electro-chemically
or chemically applied to the absorber surface. They provide close to the ideal
characteristics required for the absorber plate; i.e. high absorptance of solar
radiation and low emittance of infra-red re-radiated energy from the hot
absorber surface.
Selective surfaces improve the efficiency of solar radiation collection and
reduce heat losses from the collector’s transparent cover to the surrounding
cooler air. They are therefore most beneficial in colder climates. They are also
used in commercial applications where higher water temperatures may be
required.
Transparent collector cover
The essential purpose of the transparent cover is to:
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•
•
•
transmit the maximum amount of solar radiation to the absorber plate to
heat it
trap the re-radiated infra-red (heat) radiation that is emitted from the
absorber plate towards the transparent cover
prevent wind from blowing directly over the absorber plate and
removing heat
be strong and long lasting
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•
be cheap and readily available.
Some glasses and plastics contain these characteristics. The most common
material for the transparent cover is toughened, low-iron glass. Compared
with normal window glass and many plastics, low-iron glass has the following
improved qualities:
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•
•
•
It has very high transmittance and hence reflects and absorbs less solar
energy as the sun’s rays pass through it. More energy gets to the
absorber plate.
Less energy is re-radiated and lost from the cover to the surrounding air
because it stays cooler due to its lower absorptance.
It can be etched on one surface to further reduce reflection of solar
radiation.
It is very long lasting and strong, and will resist hail damage and many
other ‘neighbourhood’ projectiles.
Collector box
The outer box of the collector must be able to do the following:
•
•
•
•
•
protect the absorber plate from water ingress, hail, snow, dust and
corrosion
provide sufficient mechanical strength against both thermal stresses
and wind forces, and be UV-resistant
support and protect the insulation around the sides and back of the
absorber
be light enough to handle and easy to install
provide a long, low-maintenance life.
Aluminium is now a commonly used box material.
Evacuated tube collectors
In flat plate collectors, significant heat is lost mostly by convection and reradiation through the top surface of the collector. This heat loss increases as
the water temperature in the collector gets hotter during the day. So while the
collector is highly efficient at the beginning of the day (e.g. 70%), the
efficiency decreases as the water circulating through the collector gets hotter.
In evacuated tube systems, this heat loss is reduced by almost totally
eliminating conduction and convection heat losses. This is because the space
between the absorber and the glass outer tube is evacuated. With little air to
move and transfer heat by conduction and convection, heat loss is further
reduced. Radiation losses are reduced by incorporating a selective surface on
the absorber, similar to flat plate collectors. As a result, evacuated tube
collectors can operate at temperatures above 100ºC, compared with about
100ºC for flat plate collectors.
The principle of operation is similar to a flat plate collector in that solar
radiation (both direct and diffuse) enters through the glass tube and is
absorbed by the absorber plate, which transfers the heat into a heat transfer
fluid inside the collector tube. The heat transfer fluid (generally an anti-freeze
liquid) can be passed in one of three ways through the evacuated tube
collector:
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•
•
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in one direction only between top and bottom manifolds
into (down) and out (up) the collector through concentric tubes or a
U-shaped tube,
up (as vapour) and down (as liquid) in the same inner sealed tube, often
copper which is itself evacuated. This is known as a heat pipe.
Cross-sections of a heat pipe and concentric tube types are shown in Figure
3.2.5 (below).
Collector tube
Absorber plate with
selective coating
Glass outer cover
Selective surface
on glass tube
Vacuum
Transfer fluid
flows out in
this passage
50mm
80mm
Heat transfer Vacuum
fluid
Glass tube
Transfer fluid
flows in through
this inner tube
(a) Evacuated Tubular Collector with Absorber
Surface within Single Glass Tube
(b) Evacuated Tubular Collector using
Concentric Glass Tubes
Figure 3.2.5 – Cross-section of evacuated tube collector types
The U-tube type of construction is shown in Figure 3.2.6 (below). The
photographs show the copper U-tube with an aluminium fin mounted inside to
form a roughly cylindrical shape. This is then inserted in an evacuated glass
tube shown to the left in each photograph. Each tube is inserted into an
insulated manifold box at the top, which captures the heat and transfers it to a
storage tank.
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Photos: Andrew Blair
Figure 3.2.6 – Constructions details for a U-tube type evacuated tube collector
Figure 3.2.7 (below) shows the principle of the heat pipe in action. The heat
pipe is the sealed copper tube in the centre of the evacuated tube. An
absorber fin is attached. The tube contains a refrigerant (often water) at a
reduced pressure. The water in the tube will boil well below 100ºC. For
example, depending on the pressure, it may boil at 40ºC. When it boils it
forms water vapour that fills the tube and rises rapidly to the top. Heat is
removed at the top of the tube as a fluid passes over the heat exchanger at
the top of the heat pipe.
The vapour condenses to a liquid and flows back down the tube where it is
reheated. The heat pipe must be inclined to transfer heat up the tube. The
heat pipe is like an electronic diode or a plumbing non-return valve. The heat
is transferred up but not down the tube.
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c
d
(1) Insulated manifold
(2) Condenser
e
f
(3) Water flow
(4) Heat pipe
(5) Absorber plate
(6) Liquid coming down
i
g
(7) Liquid going up
h
(8) Glass tube
j
Image: Courtesy Endless Solar
Figure 3.2.7 – Heat pipe evacuated tube system
Winter and summer performance of collectors
What are the factors that affect winter and summer performance of collectors?
Firstly, as the collector heats up during the day, hot air rises from the absorber
to the glass cover of the collector. If the air above the glass is warm, as in
summer, then the heat stays beneath the cover as heat flows from hot to cold
areas. If the air above the glass is cool, then heat is lost to the atmosphere by
conduction of heat through the thin layer of glass, and then by convection and
re-radiation from the cover surface to the surrounding air. This means that in
winter when the day is cold, a lot of heat is lost to the atmosphere through the
glass, even on a bright sunny day.
Secondly, the weather patterns vary throughout Australasia. This includes
periods of rainfall, cloud and solar radiation, ambient temperatures, wind
speeds and dust. For example, in Melbourne, Adelaide and Perth the climate
tends to bring most rain in winter, day lengths are much shorter than in
summer and the sun is lower in the sky all day. These factors reduce solar
radiation levels. By comparison, in Brisbane, Rockhampton, and Alice Springs
the winters are often sunny with most rain falling in summer. Days are not
much shorter and the sun is higher in the sky than it is further south.
Overall, this means that in most parts of Australasia, solar collectors produce
more hot water in summer than in winter for the following reasons:
1. The days are longer and the sun shines for more hours, heating the
water for a longer period.
2. In southern coastal parts of Australia, there is less cloud during the
summer and more in winter due to winter rains. So again the water is
heated for a longer period of time.
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3. Because New Zealand is an island, cloud cover is persistent yearround. However, winter rains and low temperatures reduce solar gain in
winter.
4. The air surrounding the collectors is warmer in summer so there is less
heat lost through the glass, back to the atmosphere.
5. The cold water supply is usually warmer in summer than in winter.
Because it is warmer before heating starts, it is possible for it to reach a
higher final temperature.
Key points
•
Flat plate collectors consist of the following components:
o a transparent top cover (typically glass) to allow solar radiation to
pass through to an absorber plate
o an absorber plate to absorb solar radiation and convert it to heat
o a series of passageways through, or attached to, the absorber plate
through which a heat transfer fluid passes to collect the heat energy
o an insulated box to prevent weathering and reduce heat loss from
the absorber plate.
•
Evacuated tube collectors consist of the following components:
o a cylindrical glass tube to allow solar radiation to pass through to an
absorber plate
o an evacuated space between the glass tube and the absorber
surface
o a long, thin absorber plate either as a flat metal fin, cylindrical metal
fin or cylindrical glass tube within the outer clear tube
o a glass passageway, metal U-tube or one-way metal tube on the
inside of the absorber to pass a heat transfer fluid through to collect
the heat.
•
•
•
The principle of operation is that solar radiation passes through and is
trapped by a transparent top cover in a flat plate collector. This trapped
radiation is absorbed by the absorber plate, converted to heat and
conducted to a heat transfer fluid circulating through the collector. This
heat is then circulated and stored in an insulated storage tank for later
use.
The heating ability of a collector is determined by how effectively it
collects solar radiation and how little heat is lost from the hot collector
back to the surrounding air.
To maximise collection of incoming solar radiation from flat plate
collectors:
o use highly transparent top covers that allow maximum solar
radiation through but limit re-radiated energy from the absorber
plate
o use highly absorbing surfaces on the absorber plate.
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•
To minimise heat losses from the absorber plate of a flat plate collector:
o insulate the back and sides of the box
o use absorber surfaces that limit re-radiation from the absorber plate
o use one or more transparent covers to reduce heat loss by wind
from the top of the collector
o evacuate the space between the transparent cover and the
absorber plate.
Sections 3.1 & 3.2 questions
1.
Solar radiation passes through the transparent surface of solar flat
plate collectors and is absorbed on the absorber plate.
a. How does the heat energy reach the water in the collector?
b. Not all of the heat energy heats the water in the collector. What
happens to the heat that is not transferred into the water?
c. What purpose does the glass serve in flat plate solar hot water
collectors?
d. Modern flat plate collectors use low-iron glass rather than window
glass used in some older collectors. Low-iron glass is more expensive
than window glass so why is it used?
e. Solar swimming pool heating collection material does not have
glass covering it. Why is glass covering comparatively unimportant for
pool heating?
2.
Flooded collectors are able to be used for solar hot water systems
where the absorber material is plastic or steel. Why would plastic be
unsuitable for fin and tube flat plate collectors?
3.
Australian scientists invented the use of ‘selective surfaces’ for solar
hot water systems. Why is a selective surface so much better than just plain
black paint for the plates (fins) of solar collectors?
4.
Why are evacuated tube collectors cylindrical? Why aren’t flat plate
collectors made with a vacuum between the absorber plate and the glass
cover?
5.
What is a ‘heat pipe’?
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3.3
Storage tanks
What this section is about
Storage tanks should be robust, long lasting and well insulated to store the
heat energy from the solar collectors. This section examines some of the
features of storage tanks for solar water heating systems. It covers the
following topics:
•
•
•
•
•
•
•
•
tank materials and construction requirements
insulation
outer casing
stratification and preventing mixing
tank shape
draw-off
tank connections for solar collectors and/or solid fuel heaters
heat exchangers and tanks.
The storage tanks can be subjected to harsh water conditions and thermal
stressing. To be robust and provide good life, they must be constructed of
materials that are:
•
•
•
•
durable
safe to use and handle
cost-effective
low impact environmentally.
The most common materials used are:
•
low-pressure tanks
o copper
o plastics such as polyethylene and ABS
•
mains pressure tanks
o vitreous enamel-lined mild steel
o 316 stainless steel
Tank construction
Durability and safety
Domestic storage tanks must be light enough to be handled by two people.
They must be strong enough not to rupture and expose workers or users to
scalding hot water. They must be able to handle the stresses of expanding
water, cyclic thermal stresses of daily expansion and contraction, partial
vacuum conditions, corrosion, hail, snow and ultra violet (UV) radiation attack.
There are pros and cons of using different materials. For example, stainless
steel forms a thin oxide layer that is generally very corrosion resistant. But it
can be corroded around stresses in the welds, for example, under exposure to
chlorinated water. By comparison, mild steel does not have the corrosion
resistance of stainless steel and must be coated with a glass vitreous enamel
lining to stop direct contact with water. Generally two layers are used: one that
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bonds with the steel well and the other that resists cracking or crazing at
higher temperatures and cycling thermal stresses.
Sacrificial anodes that will corrode in place of the steel are also used in
vitreous enamel-lined tanks for further protection. This may increase
maintenance costs. Copper can also be corroded, particularly when in contact
with other metals or via stress corrosion cracking. Plastics, while highly
corrosion resistant, are subject to attack by UV radiation and may become
unstable at high temperatures.
All tanks in the Australian and New Zealand market have to meet stringent
engineering Standards such as AS/NZS 2712 – Solar and Heat Pump Water
Heaters: Design and Construction. As a result of these Standards, all systems
on the Australasian market provide long life when properly installed and
maintained. In all cases, maintenance of tanks and collectors increases and
system life decreases when conditions are extreme; e.g. very harsh water
supplies.
Insulation
Insulation reduces the heat loss to the air. This is wasted energy that is no
longer available at the hot water taps. The insulation requirements and
allowable heat loss rate from storage tanks are specified in AS/NZS 4692 –
Storage Water Heaters. In the past, this Standard has allowed excessive heat
losses from storage tanks, in excess of 30%. In recent years, these Standards
have been improved with the introduction of Minimum Energy Performance
Standards (MEPS). Solar water heater tanks have generally been better
insulated than normal electric or gas water heater tanks.
Typically, high density polyurethane foam insulation is injected between the
inner tank containing the drinking (potable) water and the outer
weatherproof shell. The inner tank is located within the outer shell to increase
the thickness of the insulation at the top of the tank. Figure 3.3.1 shows this in
a cut-away view of a horizontal storage tank.
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Image: Courtesy Solahart
Figure 3.3.1 – Cut-away view of a typical horizontal storage tank
Environmental impact
All water heaters have some environmental impacts, either as a result of their
construction materials or the energy used in service. Ideally, the materials
used for the construction of solar collectors and tanks should be:
•
•
made in production processes that minimise use of energy and toxic
materials
easily reusable or recyclable at the end of their service life.
These criteria are now being written into Australian and international
standards.
Outer casing
The outer casing must protect the insulation and inner tank from the weather.
The most common materials used are:
•
•
•
anodised aluminium
colour bond aluminium
UV-resistant plastic.
The outer casing must also be strong enough to handle and provide
protection during transport. It must also protect the electric boost element.
Tank use
Stratification and preventing mixing
Stratification is explained in an earlier section – ‘Principles of Water Heating’.
It assists with the delivery of hot water by allowing the hottest water to be
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drawn off from the top of the tank. To prevent or minimise mixing of hot and
cold water, cold water is brought into the bottom of the tank through a
specially designed spreader pipe that slows the water velocity and spreads it
along the bottom of the tank (see Figure 3.3.1).
Tall tanks have much better stratification, and maintain this stratification better
than long, low or squat tanks. This is because it is easier for heat to be
conducted between hot and cold layers and the tank walls in a long, low tank
since the surface area for conduction is much greater. Figure 3.3.2 (below)
shows two tanks of the same shape and volume. One is mounted horizontally
(tank A) and one vertically (tank B). The shaded area marks the size of the
surface area of conduction between hot and cold layers halfway up the tanks.
Tank A clearly has a much larger area across which heat is conducted. Hence
the lower layers become warmer and the hot upper layers cool a little.
TANK A
TANK B
Figure 3.3.2 – Surface area for conduction between layers in storage tanks
Tank shape and size
Tank shape affects the following:
•
•
•
the rate of heat loss from the tank
heat transfer between hot upper layers and cooler lower layers by
conduction
the mixing between hot and cold layers in the tank as water is circulated
through the tank and/or drawn off from the tank.
Long, thin tanks have a larger surface area for a specified volume and this
increases the rate of heat loss for the same thickness and type of insulation.
Squat tanks have a smaller surface area and less heat loss.
As water either circulates through the tank due to thermosiphon heating by the
collector, pumped circulation from the collector, or draw-off of hot water, then
some mixing of the hot layers with the cooler layers can occur. This can
reduce the temperature of the water at the hot outlet, making it appear as
though the tank has lost a lot of heat.
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Tank size is chosen to meet the requirements of the user. Most solar water
heater tanks are sized to roughly 75 litres volume of hot water for every 1m2 of
collector area – or about one-and-a-half to two days’ demand of hot water – to
cater to some extent for bad weather conditions. These figures have been
determined by testing and optimisation of solar water heating system
configuration and costs for various hot water loads. This is explained in more
detail in Chapter 6.
System performance, draw-off volume and recovery rate
The performance of solar water heating and heat pump systems can be
determined from Standards; in particular, AS/NZS 4234 – Water Heaters:
Domestic and Heat Pump – Calculation of Energy Consumption; and AS/NZS
2984 – Solar Water Heaters – Method of Test for Thermal Performance –
Outdoor Test.
The latter gives the volume of hot water (draw-off) and energy that a system
with a given collector area and storage tank size can deliver above 57ºC or
45ºC.
AS/NZS 4692 covers the hot water delivery rating (or draw-off volume) of a
given tank size. This is the volume of hot water that can be drawn off at a set
flow rate until the temperature falls to a specified temperature. For example, a
300 litre tank may be rated for a draw-off of 280 litres down to 60ºC. They are
tested at a 12 litre per minute flow rate and a temperature drop of 12ºC.
Manufacturers also specify the rate of recovery for their tanks. This is the
volume of water that the tank can continue to deliver per hour for a specified
temperature rise of the water; say 15ºC to 60ºC (i.e. a rise of 45ºC) for each
tank and booster heating element size. For example, the heat recovery rate of
the gas-boosted tank may be rated at 200 litres per hour for a 45ºC
temperature rise.
Tank connections for solar collectors and/or solid fuel heaters
Solar collectors and solid fuel heaters operating under thermosiphon flow
require connections to the storage tank that are larger than 13mm and are
typically 20mm to 25mm diameter. The connection also has to be located
appropriately. The supply pipe from the storage tank to the collector must be
located as close as practicable to the tank bottom to draw off the coolest
water. With vertical tanks used with either remote thermosiphon storage or
split (pumped) storage systems, the return pipe from the collectors to the tank
can be returned at about one-third to two-thirds of the tank height. This
arrangement helps to create a stratified pocket of hot water at the top of the
tank that is largely undisturbed by the thermosiphon flow. However, piping
arrangements vary from manufacturer to manufacturer.
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Diagram: Courtesy Rinnai
Figure 3.3.3 – Pipe connections between collector and tank – split system
Retrofitting of existing tanks requires the use of a five-way RMC connector
that fits into the cold inlet fitting at the bottom of the tank. This special
connector allows cold water to be drawn off to the collectors and the hot water
returning to the tank to be injected into the tank through the centre of this
connector. This is not an ideal arrangement unless the hot water inlet pipe is
bent up to allow the hot water to be above the cold, as it tends to causing
mixing of hot and cold water at the bottom of the tank, hence increasing the
temperature of the water circulated to the collectors. This in turn reduces the
efficiency of the collectors.
For close coupled thermosiphon systems, the collector supply and return
pipes exit from the bottom and top of the tank respectively to try to avoid the
mixing of hot and cold water inside the tank. Cold water spreader pipes are
essential in this design as hot–cold mixing can easily occur.
Heat exchange tanks
In some environments, the water quality is very poor. As well, freezing
conditions can cause the water in collectors and connecting pipes to freeze
and burst. To overcome these problems, heat exchangers are used to
separate the drinking (potable) water from the water circulating through the
solar collectors. This allows the use of water/glycol anti-freeze and corrosion
inhibiting fluids to be used in the separate collector circuit. Heat exchangers
come in many forms. The most common types used in solar water heating
system tanks form an integral part of the tank. This is achieved by having an
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outer tank around the outside of the main inner, drinking (potable) water,
horizontal cylindrical tank (see Figure 3.3.4). The section on frost protection
will cover heat exchangers in more detail.
Image: Courtesy Solahart
Figure 3.3.4 –Heat exchanger as a tank around the inner drinking (potable) water tank
For split systems, a copper coil (or calorifier) can be set inside the storage
tank through which the drinking (potable) water is passed (see Figure 3.3.5).
This is mostly done for larger commercial systems.
Figure 3.3.5 – Mains pressure copper coil heat exchanger inside the storage tank
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Key points
•
To be robust and provide good life, storage tanks must be constructed
of materials that are:
o durable
o safe to use and handle
o cost-effective
o low impact environmentally.
•
The most common materials used are:
o low-pressure tanks – copper, and plastics such as polyethylene and
ABS.
o Mains pressure tanks – vitreous enamel-lined mild steel and 316
stainless steel
•
•
•
•
•
•
Good quality, thick insulation will ensure low heat loss from the tank to
the surrounding air.
Consideration should be given to minimising the life cycle environmental
impact of storage tanks and solar collectors by reducing toxic material
and energy use during construction and using recyclable materials.
The outer casing of the tank should protect the system from the weather
and allow ease of handling.
Tall, vertically mounted tanks increase temperature stratification within
the tank. However, the larger surface area can increase heat losses if
not adequately insulated.
Long, horizontally mounted tanks are prone to mixing of hot and cold
layers and require spreader pipes to minimise this adverse effect.
In poor water quality or freezing environments, heat exchangers are
used to separate the drinking (potable) water from the water circulating
through the solar collectors. This allows the use of water/glycol antifreeze and corrosion inhibiting fluids to be used in the separate collector
circuit.
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Section 3.3 questions
1.
Hot water storage tanks are much the same whether they are for solar
hot water systems or electric or gas storage systems. They all consist of an
inner tank in which the hot water is stored and an outer case. Between the
inner and outer cases is insulation designed to reduce heat loss from the
stored hot water.
a. What materials are commonly used for the inner hot water storage
tank?
b. Suggest a reason why the insulation is thicker at the top of a
Solahart hot water storage tank than at the bottom, as shown in the
small inset in Figure 3.3.1.
c. The outer case must be able to withstand various environmental
factors. What are these factors that might otherwise damage the hot
water storage tank?
2.
What does the word ‘potable’ mean when used to describe water?
3.
Hot water storage tanks are nearly always cylindrical. Why?
4.
Most non-solar hot water storage tanks are cylinders, standing on end.
Close coupled solar storage tanks are cylinders that are mounted horizontally.
a. Why are the tanks mounted horizontally?
b. What problem does this present in terms of stratification?
c. Suggest why a person with a close coupled solar hot water system
may find that their hot water is a lot cooler in the morning than it was
when they went to bed.
5.
A standard 315 litre hot water storage tank is likely to contain 340 litres
of water. Why do we call the tank a 315 litre tank and not a 340 litre tank?
6.
What is a heat exchanger and why are they commonly used in solar
water heating systems?
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3.4
Close coupled solar water heater systems
What this section is about
The close coupled solar water heating system has become the most popular
domestic solar hot water system in Australia. This is because of the advantages
listed below, and the fact that the product has been vigorously marketed, particularly
by Solahart. After significant research, development and market research, Solahart
has concluded that a 300 litre unit with 4m2 of collector area is the most practical
option.
This section covers the following aspects of close coupled solar water heaters:
•
•
•
•
•
•
the layout of a typical close coupled system
principles of operation
the components of a close coupled system
heat exchanger type of system and the benefits
methods of boost heating, including electric and gas boosting
advantages and disadvantages.
Principle of operation
Close coupled systems are so called because the collectors and the storage tank are
close together. The storage tank is mounted directly above the collectors. The most
common installation has the collectors and the storage tank mounted on the roof.
The hot water from the collectors rises by thermosiphon flow through the very short
connecting pipes and into the storage tank. The storage tank is able to withstand
higher pressures than a gravity-feed, in-ceiling tank, providing hot water at ‘mains
pressure’.
Close coupled systems – features
The low-profile storage cylinder is a feature of all Australasian manufactured close
coupled solar water heating systems. This has the advantages of:
•
•
•
•
Not being too prominent on the roof,
Spreading the load across a number of roof rafters,
Being stable on the roof,
Being relatively easy to install.
Figure 3.4.1 (below) shows typical components of a close coupled system including:
•
•
•
•
•
•
storage tank (direct heating or integral heat exchanger types available)
collectors
auxiliary heating or booster element
sacrificial anode (vitreous enamel tanks only)
hot water pressure/temperature relief valve
cold water valves
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Image and photo: Courtesy Charters & Prior and Solahart.
Figure 3.4.1 – Typical close coupled system and components
Storage tank
Typical storage volume is about 300 litres. Tank sizes vary from 150 to 440 litres.
Tanks may use:
•
•
direct heating of the potable water
indirect heating of potable water via an integral heat exchanger shell tank that
surrounds the inner potable water tank
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•
indirect heating of potable water via a heat exchanger external to the tank and
collectors.
The tank may be supplied with cold water at:
•
•
•
full mains pressure
reduced pressure via a pressure reducing valve
low pressure via a float control valve (vented systems only).
Depending on the operating pressure, the tank may be made from:
•
•
•
vitreous enamel-lined mild steel (high-pressure systems)
stainless steel (high-pressure systems)
moulded plastic (low-pressure systems).
Tanks are reasonably well insulated with about 50mm thickness of high density
polyurethane foam insulation.
Collectors
For domestic applications, one, two or three collectors of approximately 2m2 area
each are used depending on storage tank size. A rule of thumb is 2m2 of collector per
150 litres of storage (for construction details, see Section 3.2 – Collector types and
operating principles).
Absorber
The absorber is enclosed in a glass-covered metal box made from zinc-alume or
aluminium sheeting, insulated at the back and sides. The absorber surface may be
painted matte black or finished with a selective surface (e.g. AMCRO or Black
Chrome). The glass cover may be toughened low iron, anti-reflective (higher
transmittance) or plain window glass (cheaper models).
Absorbers are made from:
•
•
•
copper pipe attached to copper or aluminium sheeting (fin and tube design)
all moulded plastic (low-pressure design)
mild steel (flooded plate design – heat exchange systems only).
The number of collectors and the storage tank size can be changed to meet the
demand for hot water.
Cold water entry
Cold water from the mains supply enters the bottom of the tank via a series of valves
specified in AS/NZS 3500 (see Figure 3.4.1). These are often in a cluster called a
combination set. Their purpose is as follows:
•
•
•
•
•
Isolating valve – allows isolation and maintenance of the system.
Line strainer – filters larger particles in the water.
Non-return valve – prevents back-flow of water into mains (not required for a
low-pressure system with float valve).
Pressure reducing valve – reduces mains pressure to below the maximum
rated pressure of tank and collector (not required in all situations or on lowpressure, vented systems).
Cold water expansion valve – releases cold water rather than hot water due
to pressure build-up as the water in the storage tank is heated and expands.
This prevents wastage of hot water and protects the tank from excessive
pressure. Pressure setting should be about 200kPa less than the
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•
pressure/temperature relief valve (typical cold water expansion valve pressure
setting is 500kPa).
Diffuser or spreader pipes – the cold water inlet pipe connects to a diffuser or
spreader pipe running part way or the full length of the tank at the bottom. It
reduces the water velocity and limits mixing of hot and cold layers in the tank.
This helps maintain stratification of water temperature and hence keep the
hottest water at the top of the tank.
Hot water exit
After water is heated in the collectors, the hot water passes into the tank through the
hot water inlet at the opposite side of the tank to the cold water inlet. It is usually
located about halfway up the tank.
The hot water entering the tank rises to the top of the tank, causing some mixing of
the water in the top half of the tank as it enters, though the hottest water will always
rise to the top of the tank. Hot water is drawn off from the very top of the tank through
either a top diffuser pipe or scoop.
The water exits via a pressure/temperature relief (PTR) valve. Its purpose is to
protect against excessive temperature (>99ºC) and pressure (>1MPa) (typical
pressure setting is 700kPa). If either of these conditions is exceeded, the valve
opens and dumps a large quantity of hot water through a drain to ground (or via the
roof).
Other system features
Heat exchanger systems
Some manufacturers provide heat exchangers to provide:
•
•
frost damage protection in frost prone areas
protection from hard water supplies, particularly bore water in outback regions.
Details of their construction and installation requirements can be found in Section 3.3
(Storage tanks) and Section 5.2 (Frost protection).
Sacrificial anode
Storage tanks made of mild steel and lined with vitreous enamel must be fitted with a
sacrificial anode to prevent corrosion of the tank. The anode runs horizontally along
the inside of the tank. It is made from either aluminium or magnesium alloys,
depending on water quality. It corrodes over time and must be replaced periodically.
Manufacturers specify replacement periods according to site water quality.
Stainless steel tanks and plastic lined tanks do not require a protective anode.
Over-temperature heat dissipater or dump
Some older systems used a heat dissipater device to reduce the likelihood of
overheating in summer if the system is not used for some weeks (e.g. a summer
holiday). This comprises a finned copper heat pipe, positioned at the back of the
storage tank and running into the middle of the tank as shown below. The heat pipe
is sealed and partially evacuated with a little water in it. This water boils at about
75ºC. The steam rises up to the radiator fins and condenses, giving up its heat to the
air via the fins. The cooled water trickles back down the pipe to be heated again.
Once the temperature falls below 75ºC, it ceases to boil, transfer heat and lose heat
from the storage tank. Figure 3.4.2 shows how this works. More detail is provided in
Section 5.1.2 (Over-temperature protection).
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Image: Courtesy of Solahart.
Figure 3.4.2 – Over-heating dissipater
Supplementary heater or booster types
A booster element can be either electric or gas. Electric boosters are usually located
in the middle of the tank so that they heat the water above. Elements fitted to tanks
for the colder regions have a curved or ‘sickle’ element fitted with the sickle pointing
down to boost heat more water when connected to off-peak electricity tariffs. A
thermostat switches the element off when the water reaches the desired
temperature, typically about 60ºC. They are generally rated at 2.4 or 3.6 kilowatts of
power.
Gas boosters can either be tank mounted to heat the tank water directly or a
separate in-line instantaneous gas booster after the storage tank. Gas boosters are
generally rated at between 13 (tank mounted) and 200 Megajoules per hour (in-line
instantaneous type), providing fast recovery time. Electronic ignition systems are
used to avoid wasting gas with a pilot light. Electricity is required to operate the
electronic control, which turns on the gas when the water temperature drops and a
spark ignites the gas. A big advantage of the electronic ignition is the ability to control
it from a remote location and disable the burner during daylight hours. Figure 3.4.3
shows details of a close coupled system with integral tank gas boosting and a heat
dump pipe at the rear of the storage tank. The inlet and outlet valves are also shown.
This system provides 97% of the hot water to the Brisbane home of the author and
his wife.
Details of booster operation and control are given in Chapter 4 (Boost Heating).
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Photo: Courtesy of Trevor Berrill.
Figure 3.4.3 – Close coupled system with integral gas boosting and heat dissipater
Other system types
Most solar water heaters use collectors with metal absorber plates that maximise
conduction of heat to the water. However, in regions of high solar radiation all year,
such as northern Australia and SE Asia, these systems can overheat in summer. As
well, in many regions, hard water supply can lead to shortened life of metal tanks and
collectors. One solution is a plastic, rotationally moulded, integrated collector and
tank system. This aims to be a low-cost, low-maintenance system in regions of high
annual radiation and hard water. As the absorber plate is less conductive, the
collector is less efficient than conventional metal absorber designs, so overheating is
less of a problem and performance is satisfactory. The systems have electric
boosting.
These all plastic systems are low-pressure systems. They can be used as preheaters in cooler regions. Figure 3.4.4 (below) shows a cut-away through such a
system.
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Diagram: Courtesy of Solco.
Figure 3.4.4 – Close coupled low-pressure, plastic system
Advantages of close coupled systems
•
•
•
•
•
•
•
•
•
•
•
•
•
The collectors and storage tank being close together means that there is less
heat loss in the pipe work connecting the two.
Installation is reasonably straightforward. It is more likely to be installed
correctly. There is no difficult roof flashing to be undertaken. The entry of the
cold water supply, and the hot water delivery pipes and the electric power
cable or pipe for gas boosting, is fairly simple.
There is no scrambling in restricted ceiling spaces. The installation is out in the
open.
The system can be installed on houses where there is no ceiling space at all;
for example, houses with cathedral ceilings.
As long as the roof has sufficient slope, there is no problem with reverse
thermosiphon flow at night.
Most systems operate on mains pressure. This ensures a good water flow to
each hot tap.
Usually all the components required for the installation are supplied.
Selling a close coupled unit is easier than a remote storage system. Most
people are familiar with close coupled systems.
Giving a quotation is reasonably straightforward.
A leak should not result in water running through the ceiling.
Replacement of the storage tank does not mean having to open up the roof.
No spill tray is required under the storage tank.
The systems still operate during power outages.
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Disadvantages of close coupled systems
•
•
•
•
•
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The wide, low storage tank has poor stratification and heat is conducted down
from the hot water to the cold water beneath it.
The roof has to be strong enough to support the weight of the water in the
storage tank. Sometimes it is not and additional strengthening is sometimes
not possible.
Some people find the tank on the roof aesthetically unattractive.
The steel storage tank, and particularly the double jacket heat exchange unit, is
heavy. Getting it onto a roof, especially a steep one, can be very difficult
without lifting equipment.
Safety equipment is now required by legislation, adding to installation cost.
If the roof on which the close coupled unit is to be mounted does not face the
sun (north in the southern hemisphere), a support frame is required. This can
often be aesthetically most unattractive and is usually expensive. The support
frame, in turn, requires significant support through the roof to the building
frame.
Connection to a combustion cooker or heater as a boost source is possible but
is not straightforward. Vitreous enamel-lined steel cylinders should not have
water in them in excess of about 75ºC as this can shorten their life. The higher
the average water temperature, the more adverse the effect.
Key points
•
The typical components of a close coupled system include:
o storage tank (direct heating or integral heat exchanger types)
o collectors
o supplementary heating or booster element
o sacrificial anode (vitreous enamel tanks only)
o hot water pressure/temperature relief valve
o cold water valves.
•
•
•
The standard size system uses about a 300 litre storage tank and 4m2 of
collector area.
Tanks and collectors come in a range of sizes to meet varying demands
although most collectors are about 2m2 in area.
A typical system has the following:
o inlet valves isolated to prevent back-flow, reduce pressure and relieve
pressure from the storage tank
o a pressure/temperature relief valve on the hot outlet
o a sacrificial anode to protect vitreous enamel-lined tanks – not used in
stainless steel or plastic tanks
o an electric or gas booster either in the storage tank or mounted in-line after
the storage tank
o heat exchanger tanks if the system is used in frost prone areas or those
with very poor water quality.
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Section 3.4 questions
In the 1980s the close coupled hot water system became the ‘standard’ solar water
heating system. It was easily recognised. It was easily marketed. It was reasonably
easily installed. All the customer had to decide was, ‘Do I buy one or not?’
1.
What are the features, dimensions, etc. of a standard close coupled solar hot
water system?
2.
What fittings are normally included?
3.
What are the positive features of a horizontal close coupled tank?
4.
In what situations can a close coupled solar hot water system be installed
when other types of systems can’t be installed?
5.
Close coupled solar hot water systems can use direct or indirect heating. What
is the difference?
6.
Starting at the isolating valve, list the fittings and valves that get the cold water
to the hot water storage tank.
7.
What is the purpose of the diffusers on the cold supply into a horizontal hot
water storage tank?
8.
Why is a heat dissipater unit included in some solar water heaters? What type
of storage tanks might have a heat dissipater?
9.
Why is there such an enormous difference between the 13MJ/hour gas rate
for the boosting unit in the close coupled system and the instantaneous gas heater,
which may have a gas rate of 200MJ/hour?
10.
Why is the all plastic Solco solar hot water system not available as a mains
pressure unit?
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3.5
Pump-circulated (or split) systems
What this section is about
Pump-circulated systems have become popular in recent years due to their cleaner
appearance with only collectors roof-mounted. They are known by various names,
including ‘pumped storage’ and ‘split’ systems. Both flat plate and evacuated tube
collectors are used for these systems.
This section aims to provide an understanding of:
•
•
•
•
•
the principles of a pumped circulated system
the important components of the system
the requirements for the pump
the operation of the pump controller
methods of connection to a storage cylinder.
Reasons for split system use
The ideal solar water heating system operates on the thermosiphon flow principle as
it is simpler, cheaper and often more efficient. However, this is not always possible
for the following reasons:
•
•
It may not be possible to get the hot water tank high enough above the
collectors because the roof pitch is low and the customer does not wish to use
a mounting frame.
The customer may not wish to have the tank on the external roof for aesthetic
reasons.
In these situations we need to consider a forced circulation system where a pump is
used to circulate the water from the tank at a lower position, up through the collector
panels and back down to the tank. This allows us to locate the tank at ground level
and possibly some distance from the collectors. It also allows us to use smaller pipe
sizes and to operate multi-collector installations.
Principles of operation
This type of system consists typically of a ground-mounted tank and roof collector
panels. A small circulation pump is used to pump water through the collectors. A
differential temperature controller with two or more temperature sensors is used to
control the pump operation. This is described in detail below.
Components
A typical system configuration and components are shown in Figures 3.5.1, 3.5.2 and
3.5.3. It consists of:
•
•
•
•
•
•
•
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storage tank (direct heating or heat exchanger types available)
collectors
auxiliary heating or booster element
sacrificial anode (vitreous enamel tanks only)
hot water pressure/temperature relief valve
cold water valves
circulation pump
pump controller.
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Figure 3.5.1 – Pump-circulated system
Storage tanks
Typical storage volume is about 300 litres. Tanks sizes vary from 160 to 420 litres. A
significant difference between close coupled thermosiphon system and split systems
is that split systems use vertical tanks that more easily maintain temperature
stratification.
Tanks may use:
•
•
•
direct heating of the potable (drinking) water (softer water supplies and mild
climates)
indirect heating of potable water via a internal coil heat exchanger or a jacketed
shell heat exchanger (i.e. a thin tank surrounding the large inner tank with
potable water)
indirect heating of potable water via a heat exchanger external to the tank and
collectors.
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Image: Courtesy of Conergy.
Figure 3.5.2 – Direct (open circuit) pump-circulated system
Heat exchange systems are used to overcome problems of corrosion and freezing in
hard water areas and cold climates respectively.
The tank may be supplied with cold water at:
•
•
full mains pressure
reduced pressure via a pressure reducing valve.
Depending on the operating pressure, the tank may be made from:
•
•
vitreous enamel-lined mild steel (high-pressure systems)
stainless steel (high-pressure systems).
Tanks are reasonably well insulated with about 50mm thickness of high density
polyurethane foam insulation. Government regulation via Minimum Energy
Performance Standards (MEPS) is gradually improving insulation levels to decrease
heat losses.
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Photo: Courtesy Rinnai.
Figure 3.5.3 – Forced circulation (pumped) system
Collectors
Both flat plate and evacuated tube collectors are used. For domestic applications,
one, two or three flat plate collectors of approximately 2m2 area each are used,
depending on storage tank size. A rule of thumb is 2m2 of flat plate collector per 150
litres of storage. For evacuated tube collectors, the number or tubes in a bank can be
varied. Typically, they come in lots of about 10 tubes (for collector construction
details, see Section 3.2 – Collector types and operating principles).
Rule of Thumb 3.1 – For flat plate collectors, use 2m2 of collector area per 150 litres of storage
Absorber
Flat plate absorbers are enclosed in a glass-covered metal box made from zincalume or aluminium sheeting, insulated at the back and sides. The absorber surface
may be painted matte black or finished with a selective surface (e.g. AMCRO or
Black Chrome). The glass cover may be toughened low-iron, anti-reflective (higher
transmittance) or plain window glass (cheaper models).
Flat plate absorbers are made from:
•
•
copper pipe attached to copper or aluminium sheeting (fin and tube design)
mild steel (flooded plate design – heat exchange systems only).
Evacuated tube absorbers are made as cylindrical glass tubes from:
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•
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Double-layered glass tube – a tube within a tube
Single-layered glass tube with a copper U-tube and attached cylindrical or flat
aluminium fin inside
Single-layered glass tube with a copper heat pipe and attached flat aluminium
fin inside.
With both types of collectors, the number of collectors (or tubes) and the storage tank
size can be changed to meet the demand for hot water.
Cold water entry and hot water exit valves
These are similar to those explained in Section 3.4 – Close coupled solar water
heater systems. The main difference is the use of:
•
•
•
a non-return valve after the pump to prevent reverse thermosiphon at night
a pressure/temperature relief (PTR) valve at the top of the collectors to protect
against damage from excessive pressure or temperature or both
an air eliminator valve at the highest point in the circuit to bleed air out of the
circuit to prevent air locks restricting the flow (see Chapter 5 for details).
Pumps
Time and experience have helped to determine the most suitable pumps for this
application. Most importantly, they must be able to operate for long periods with
temperatures of the pumped fluid reaching 100ºC. The most commonly used pumps
are the Grundfos UP 15-14B or 20-60B, which draw around 20 to 40 watts of
electrical power. This is a brass pump with a brass impellor and has three speed
settings. It can be fitted with small isolating valves on the connections, which enable
the pump to be removed from the circuit without emptying the lines. Other pump
brands/models include Wilo and Salmson NSB 04-15. Generally the annual energy
used by the pump is less than 5% of the total solar energy harvested and the cost of
running the pump is less than $15 per year.
Pump controllers
Two types are now available: simple controllers that switch the pump only on and off,
and smart controllers that control both the pump and booster switching.
Simple controllers for pump only
The pump must be controlled so that it does not run continuously and thereby cause
the water to be cooled down at night. Several methods have been used to ensure the
pump only runs when solar energy is available.
These are:
•
•
A 24-hour timer or a photoelectric cell–operated switch to turn the pump on and
off. The 24-hour timer can be set to operate the pump between say 9am and
4pm. It does not have an automatic sensing system to tell it to turn on when a
frost is imminent. Also, if the flow rate through the collector is too high or the
flow persists for too long under poor solar conditions, then the water may in
fact be cooled rather than heated. The alternative is a photoelectric cell
operated switch that senses the light level and turns the pump on during
daylight hours. Again, similar problems exist.
An appropriately sized photovoltaic (PV) module to provide power to a DC
pump. In this case, no differential controller is required as the PV output and
hence the pump flow rate will increase and decrease in proportion to the solar
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•
energy available. However, a maximum power point electronic controller will be
required between the PV module and the DC pump.
Differential temperature controllers. These units rely on temperature
information received from thermistors placed at various points on the hot water
circuit. Some controllers use two sensors while others use three or more. With
two sensors, one sensor is placed at the outlet from the collectors and one at
the bottom of the tank. They measure the temperatures at each location and
send that back to the temperature controller. When the controller sees a
difference of 7º to 10º between the sensors (it varies a little from controller to
controller), then it will turn the pump on. As the water is pumped through the
collectors this difference in temperature will be gradually lost until the controller
turns the pump off at about 2º difference. Under this situation the pump will be
turning on and off all day. Similar units are used by solar pool heating
manufacturers.
Other controller functions
The controller will also turn the pump on if the water temperature drops to 3ºC to
5ºC as an anti-freeze function. In some cases, there is a third sensor, mounted at
the bottom of the collectors, which is used to switch the pump on when freezing
conditions occur. Remember water starts to expand at 4ºC and continues expanding
until freezing is completed at 0ºC. This expansion will burst the tubes in the collectors
or pipe work. The pump switches off when the water in the bottom of the collector
reaches about 7ºC. This method is, of course, wasting heat energy stored in the tank
by passing hot water through the collectors and heating them a little.
Some manufacturers advise that the use of anti-freeze dump valves should be
combined with the pump circulation freeze protection, as a power failure will prevent
the pumps and controller from working.
The controller can also turn the pump off to prevent overheating of the water in
summer for safety reasons or to prevent damage to vitreous enamel linings in mild
steel tanks. Over heating might occur if the system is unused during holidays. Some
controllers would then switch the pump off when the bottom tank sensor reaches
65ºC.
The thermistors are best fitted in a sealed tube which protrudes into the water flow.
The sealing is important as water can greatly affect the accuracy and operation of the
thermistor. For this reason it is inadvisable to simply tape the sensors to the side of
the pipe. If that is the only option, the thermistors should be set in heat-conducting
paste and then covered with sealing tape.
Smarter controllers for pump and booster
More intelligent controllers are now available that aim to optimise the solar
contribution while minimising booster use and meeting user hot water demands in all
weather conditions. For example, the Solarit controller uses three sensors, the third
one being at the centre of the tank. The collector outlet and tank bottom sensors do
the usual pump control for solar heating, freeze and over-temperature control. This
third sensor allows monitoring of the amount of hot water in the tank and control of
the boosting. By following an adjustable two-hourly temperature profile over the day,
the third sensor will limit boosting to a preset (but user adjustable) temperature for
each two-hour period of the day. This allows the user to boost sufficiently to meet
their patterns of hot water demand, but avoid excessive heating during daylight hours
when you want the collectors to do most of the work (see Chapter 4 – Boost Heating
for more details).
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Calibration of controllers
It is important to check the calibration of the controller and sensors to ensure the
correct temperature settings are used to turn the pump on and off for the most
efficient operation.
Auxiliary heater or booster types
The booster element can be either electric or gas. Electric boosters are usually
located either at the bottom of the tank or towards the top of the tank. A thermostat
switches the element off when the water reaches the desired temperature – typically
about 60ºC, but it may be set higher. The electric elements are generally rated at 2.4
or 3.6 kilowatts of power, with 4.8kW elements also available.
Gas boosters can either be tank mounted on the side of the tank to heat the tank
water directly or a separate in-line instantaneous gas booster after the storage tank.
Gas boosters are generally rated at between 13 (tank mounted) and 200 Megajoules
per hour (in-line instantaneous type), providing fast recovery time. Electronic ignition
systems are used to avoid wasting gas with a pilot light. Electricity is required to
operate the electronic control, which turns on the gas when the water temperature
drops and a spark ignites the gas. A big advantage of the electronic ignition is the
ability to control it from a remote location and disable the burner during daylight
hours. Boosting is shown diagrammatically in Figure 3.5.4 (below).
Details of booster operation and control are given in Chapter 4 – Boost Heating.
Other system types
Figures 3.5.4 and 3.5.5 show schematic diagrams of evacuated tube collector,
pumped systems.
The system configuration is very similar to conventional, flat plate collector pumped
systems. The main difference with this particular system is that the collectors are a
bank of evacuated tubes connected to a manifold. Cold water from the tank is
pumped through the manifold at the top of the tubes. Each tube contains a heat pipe
inside the evacuated tube that conducts its heat via a heat exchange condenser to
the pump-circulated water in the manifold (see Section 3.2 – Collector types and
operating principles for more details). The pump is controlled in a similar way to that
outlined above.
Evacuated tube collectors are generally more efficient than flat plate collectors for
higher temperature applications or in colder climates due to their lower conduction
and convection losses. Therefore, a smaller collector area should be able to be used
for the same application. However, their efficiency for lower temperature applications
may be little different to selectively coated flat plate collectors. The only way to
assess this is to compare the collector efficiency data for Australian Standards
testing requirements.
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Figure 3.5.4 – Evacuated tube split system with electric boosting
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Diagrams: Courtesy of Endless Solar.
Figure 3.5.5 – Evacuated tube split system with in-line gas boosting
Finally, both flat plate and evacuated tube systems can be retrofitted to existing
vertical storage tanks via either:
•
•
additional fittings already in the storage tank to accept the collector circuit
A five-way connector fitted into the cold inlet at the tank bottom (see Section
3.3 – Storage tanks for details).
Alternatively, a complete split system could be added as a pre-heater to an existing
electric or gas storage tank or instantaneous heater.
Advantages and disadvantages of split systems
Compared with close coupled or remote thermosiphon systems, split systems have
the following advantages and disadvantages:
Advantages:
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•
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no ‘unsightly’ tank on the roof
can sometimes be retrofitted to use existing storage tanks
less roof work
tank or collectors can be located in more accessible or appropriate locations.
Disadvantages:
•
•
more expensive than thermosiphon systems due to the pump and controller
costs
they require power to run the pump and temperature differential controller –
this can be done with a photovoltaic module to drive the pump; the pump will
then run faster when it is sunnier so a controller is not required.
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Key points
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Pump-circulated or split systems have been around a long time but have
become more popular due primarily to aesthetic reasons; i.e. no ‘ugly’ tank on
the roof.
As the tank is located below the collectors, the water must be pumped up
through the collectors and back to the tank. This requires a very small pump
(20 to 40 watts power) that uses less than $15 of electricity per year.
A significant difference between close coupled thermosiphon system and split
systems is that split systems use a vertical tank that more easily maintains
temperature stratification. This helps keep the hottest water at the top of the
tank ready to be drawn off.
Collectors can be either flat plate or evacuated tube type. The latter should
perform better for higher temperature loads or in colder regions due to reduced
heat loss.
Split systems require a couple of extra valves. These include:
o a non-return value after the pump to prevent reverse flow
o an air eliminator at the highest point in the circuit to vent trapped air
o a PTR valve at the top of the collectors to release excess pressure.
•
The pump operation is controlled by a:
o timer, photo-cell-controlled switch
o PV module and DC pump
o differential electronic controller
o smart controller.
•
Differential electronic controllers use two or more sensors located at the top of
the collectors and the bottom of the tank to:
o switch the pump on and off so it operates only when there is adequate
solar energy to heat the water
o protect the system against the extremes of freezing and overheating.
•
•
Smart controllers are available that use more sensors and can control both the
pump operation and boost heating to optimise solar collector contribution.
Split systems can be retrofitted to existing tanks via a five-way valve at the cold
inlet to the tank or as a pre-heater to an existing storage or instantaneous
heater.
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Section 3.5 questions
1.
What types of collectors are suitable for use with pump circulation systems?
2.
Is there any difference between the collectors used in a pump circulation
system and a close coupled system or a remote (in-ceiling tank) system?
3.
Suppose a family of six people has a 400 litre hot water storage tank. How
many 2m2 collectors would you recommend be used in conjunction with that tank?
4.
Why is a circulating pump required in some solar systems?
5.
What does the circulating pump do? How is the pump controlled?
6.
The circulating pump is often called a ‘circulator’. Why is it not just called a
pump? Is there any difference to any other pump?
7.
An electric storage tank is used as the hot water store for many pump
circulation systems, whether the electric boost element is used or not. Some systems
adapt other tanks for use as the hot water storage tank. List the characteristics of the
following mains pressure storage tanks:
a. stainless steel
b. vitreous enamel-lined mild steel.
8.
The valves on the cold water supply to a pump circulation solar system are the
same as for a close coupled solar hot water system. There are, however, two other
valves not normally part of a close coupled system:
a. The air eliminator valve. Where is this located and what is its function?
b. The non return valve associated with the circulating pump. Why is it
installed?
9.
Other fittings that are required to go with the circulating pump controller are
the pockets (tubes sealed at one end) for the sensor probes. The sensor pockets are
often made on site from small diameter copper tube (often 10mm outside diameter).
Where are these positioned, and what is their function?
10.
The differential temperature sensors are not the only temperature sensors in
the system. What other temperature sensors are there?
11.
What advantages do pump circulation systems have to offer?
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3.6
Thermosiphon remote storage systems
What this section is about
Thermosiphon systems with remote storage are low-pressure, open-vented, solar
water heaters that have been used for many years in southern parts of Australia and
in homes not serviced by reticulated water. They are largely being replaced by mains
pressure feed, close coupled thermosiphon systems and split (pumped) storage
systems. This is unfortunate because lower water pressure usually means less water
is wasted. However, they still have a role to play as the lower pressure gives long life
and allows back-up heating from combustion heaters. In well-designed systems, they
provide more than adequate low-pressure hot water supply and can improve the
overall energy and water efficiency of any water supply system by reducing the need
for inefficient pressure pumps.
This section gives a summary of their features. It covers:
•
•
•
typical system configuration
factors that affect system performance
reverse thermosiphon.
Principles of operation
These systems operate by thermosiphon flow. The water is heated in the collectors
and rises naturally up to the storage tank. The heavier, cold water from the bottom of
the storage tank flows to the bottom of the collectors. Hence, there is no need for a
pump to circulate the water.
Vent to outside atmosphere
Hot pipe
run
300mm rise or 1:20 rise
of hot water outlet pipe
from collectors
Cold pipe
run
Feed tank
Roof
Cold water
mains
Storage
tank
Hot water
Figure 3.6.1 – Typical thermosiphon remote storage system configuration
Figure 3.6.1 shows a typical system configuration. The storage tank is usually
located in the roof space and is kept full with a feed tank and float valve control.
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This is similar to a toilet cistern valve in operation. This reduces the pressure at the
feed tank to one atmosphere. The feed tank is usually attached to the side of the
storage tank. The storage tank is vented, usually constructed of thin copper sheet
and hence is not designed for high pressure. A safety tray is required under the
tanks with a drain to the outside. The collectors are usually roof-mounted at least
300mm below the bottom of the tank as shown. The location of collectors and tanks
can be quite flexible depending on roof space and distance. For example, the
collectors could be ground mounted, with the tank in a roof space. Alternatively, the
whole system could be ground mounted.
A thermosiphon system has a cost advantage over a pump-circulated system. A
pump system requires a pump and a pump controller as well as a power supply to
run the pump. Pumps and controllers can fail – therefore a thermosiphon system is
more reliable.
Factors affecting system performance
Important factors that affect the performance of thermosiphon remote storage
systems are as follows:
•
•
•
•
•
The bottom of the tank should be at least 300mm above the top of the
collectors unless special piping arrangements, tanks or valves are used to
prevent reverse thermosiphon flow.
Thermosiphon flow is driven by a relatively weak force due to a small density
and pressure difference between the hot and cold water. Therefore, the pipes
between the collector and the storage tank should rise continuously upwards at
a minimum slope of about 1:20 to avoid air locks that would prevent or slow
thermosiphon flow. The steeper the slope, the easier it is for thermosiphon flow
to circulate.
The connecting pipes between the collector and tank should be adequately
sized to prevent friction slowing the thermosiphon flow. A 25mm diameter
copper pipe is a common size.
The distance between the collector and tank should be minimised to reduce the
need for larger pipes to, in turn, reduce friction that would restrict the
thermosiphon flow. This also helps to reduce pipe heat losses.
The connecting pipes should be well insulated because the heat loss increases
with surface area and pipe length. Bigger diameter pipes have a much larger
surface area; e.g. a 25mm diameter pipe has roughly four times the surface
area and so four times the heat loss rate per metre of pipe compared with a
13mm diameter pipe.
The installation requirements to account for these factors are explained in Chapter 6
– System Design and Installation.
Reverse thermosiphon flow
Reverse thermosiphon flow is a process whereby hot water cools when exposed to
cold air temperature conditions, becomes denser and sinks to the lowest part of a
plumbing circuit. It can occur at night, particularly when clear sky conditions exist
causing the air temperature to be generally lower. It can occur with:
•
split (or pumped) systems because the collectors are mounted above the
storage tank. In this case, cold water in the external pipes connecting the
collector and tank can cool and sink down to the storage tank, pushing hot
water into the outside pipes towards the collector. This is the case with most
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•
•
split systems. Non-return valves must be used to prevent this happening in split
systems.
thermosiphon systems where the collector is only just below the tank as with
close coupled systems. Reverse thermosiphon occurs when the water in the
connecting pipe supplying water to the bottom of the solar collector from a
storage tank becomes colder than the water inside the bottom of the collector.
A small pressure difference will then be created by the heavier, colder water.
The water in this supply pipe can potentially sink down to the bottom of the
collector. In doing so, it would displace some water from the collector back to
the tank, and push some warm water from the bottom of the storage tank into
the collector supply pipe. As a result, warm water from the tank would be
cooled as it comes into contact with the outside air temperature. This can
happen on cold clear nights when uninsulated external connecting pipes are
used between the collector and tank.
thermosiphon systems where the collector is at the same height as the tank.
The same process as in the previous case occurs, but to a greater extent.
Reverse thermosiphon flow is prevented by:
•
•
•
•
insulating external piping
mounting the tank base at least 300mm above the top of the collectors (see
Figure 3.6.1)
using a special non-return valve that can operate at very low flow rates and
pressure (see Section 6.2.4.2 – Solutions to the problem of collectors being too
high); a normal non-return valve of either the spring or flap type is not suitable
arranging the connecting pipes so that the supply pipe from the tank to the
collectors exits the storage tank at about the same height as the hot return pipe
from the collectors (see Figure 3.6.2 below).
The following diagram shows the external plumbing to prevent reverse thermosiphon
flow. The cold supply pipe exits the tank at the same height as the hot return pipe.
Inside the tank, the cold supply pipe is extended to the bottom of the tank to draw
water from this level. Note that this type of tank is called a squat tank and is
especially designed to fit in lower pitch roofs where roof space is limited. Figure
6.2.14 shows the internal pipe connections of this type of tank.
Diagram: Courtesy of Rinnai.
Figure 3.6.2 – Plumbing connections that prevent reverse thermosiphon flow
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Advantages and disadvantages
The advantages and disadvantages of thermosiphon remote storage systems are:
Advantages:
•
•
•
•
•
•
•
•
•
•
•
simple with no moving parts
flexibility to design to suit customer needs
usually cheaper than other system configurations
longer life than mains pressure systems due to all-copper tanks, and lower
pressure and stress on tank and collectors
no ‘unsightly’ tank on the roof
the tank is more upright and so promotes temperature stratification, improving
system efficiency
tank is more protected from the weather and therefore has lower heat losses
less weight on the roof and therefore no need to reinforce the roof
can be connected to another uncontrolled heat source such as a wood heater
or slow combustion stove
can provide hot water without external power
can be more energy efficient if header tanks are used to supply water pressure
instead of pressure pumps.
Disadvantages:
•
•
•
•
more complex installation compared with the close coupled type, especially if
the system has to be installed in an existing house
requires space in the ceiling
low water pressure and if pipes are undersized, flow to taps will be restricted
access to replace or service the tank is more difficult.
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Key points
•
•
•
•
Thermosiphon remote storage systems are low-pressure solar water heating
systems with longer life than mains pressure systems. They continue to be
used in non-reticulated water supply or remote areas with combustion stoves
or wood heaters as back-up. Their use on reticulated mains pressure water
supply systems is reducing due to their lower water pressure and more difficult
installation. This is unfortunate because lower water pressure usually means
less water is wasted.
Care needs to be taken during installation to prevent restriction on
thermosiphon flow through correct pipe sizes, minimising pipe lengths and
optimising pipe slope from collector to tank.
Reverse thermosiphon flow can occur in poorly installed systems. Take
appropriate measures such as installing the collectors below the tank,
insulating external connecting pipes or using purpose-made tanks and nonreturn valves.
They offer a range of advantages including lower cost, longer life and flexibility
in installation arrangements.
Section 3.6 questions
1.
Describe the features of an in-ceiling gravity feed hot water system, without
solar collectors attached.
2.
How would you convert such a hot water system to a solar system?
3.
Supposing the bottom of the storage tank was not 300mm or more above the
top of the collectors. How would it be possible to use the tank?
4.
If you were selecting a tank especially for the job it might be possible to have a
tank with both the cold and hot solar connections (flow and return) near the top of the
tank. Such a tank is made by Rinnai Beasley. Why is it so important to have the solar
collectors lower than the connections on the storage tank?
5.
It is most important that the pipe slopes uphill (however slightly) from the
collectors to the storage tank. Why?
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3.7
Heat pump water heating systems
What this section is about
A heat pump system is a form of solar water heating system. It may use the standard
refrigeration cycle to transfer heat from the ambient air outside the house into the
water in the storage tank or it may rely on solar radiation to directly heat a refrigerant
fluid in a collector.
This section outlines the types and operation of heat pump water heaters. It covers:
•
•
•
the refrigeration cycle used by heat pumps
the difference between split solar, split air and compact systems
the advantages and disadvantages of heat pumps compared to other solar
water heaters.
Principle of operation
The heat pump operates like a refrigerator or an air conditioner. It pumps heat from
one place to another. In a refrigerator the heat is pumped out of the food and into the
grille at the back of the refrigerator, where it is dissipated as waste heat to the air. Air
conditioners pump heat out of houses in summer (and into houses in winter if they
have the reverse-cycle capability). They all work on the refrigeration cycle.
In a typical refrigerator, there are four main components:•
•
•
•
Evaporator – a heat exchanger consisting of either a flat plate with tubes
attached or a set of fins attached to a network of tubes. In a refrigerator, it is
the plate inside the cabinet at the back that gets cold. It absorbs heat from the
food in the refrigerator.
Condenser – a heat exchanger consisting of either a flat plate with tubes
attached or ‘grille’ (like fins and tubes) at the back of the refrigerator that
becomes hot and dumps heat collected from the food to the air.
Compressor – this pumps the refrigerant around the system.
Expansion device – this controls the rate of refrigerant flow through the
evaporator.
For water heating, the condenser has to be on the inside, wrapped around or
immersed in the hot water storage tank. The evaporator has to be on the outside to
extract heat from the surrounding air. This is the reverse of a refrigerator. Figure
3.7.1 (below) shows the components of two types of heat pump water heater
available on the market today: split and compact types.
For water heating, the heat pumps work as follows:
•
•
When the system is turned on the compressor pressurises the refrigerant so
that its pressure and temperature are raised. The refrigerant is then pumped
through the condenser as a hot gas. The condenser is a long coil of tube or a
mantle-like array of channels in contact with the inner shell of the storage tank,
or a coiled arrangement inside the tank. The water in the tank is at a lower
temperature than the refrigerant in the condenser and as a result, heat flows to
the water and raises its temperature.
After going through the condenser, the refrigerant has cooled and is then
pumped through the expansion valve. This has a small orifice that lowers the
temperature and pressure of the refrigerant as it enters the evaporator. By now
this refrigerant is cooler than ambient and so it absorbs heat energy through
the evaporator from the surrounding air.
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•
The refrigerant then returns to the suction side of the compressor where the
cycle starts again. In essence, the heat pumped from the hail, rain or sun is
dumped into the water tank.
Image: Courtesy of Quantum.
Figure 3.7.1 – Components of a split (left) and compact (right) heat pump water heater
In the heat pump water heater, the evaporator is usually:
•
•
a fan-cooled fin and tube or flat plate heat exchanger in a self-standing fan-coil
box mounted beside the storage tank (split air system)
a fan-cooled fin and tube or flat plate heat exchanger mounted on the top of the
storage tank (compact system).
In all cases for heating, system performance will be improved if the whole system is
located in a sunny, warmer, wind protected location.
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Photos: Courtesy Quantum & Rheem.
Figure 3.7.2 – Split and compact heat pumps
The first commercial heat pump water heaters had the evaporator as a set of
unglazed bare black aluminium plates. These collectors (evaporators) did not suffer
from frost damage like normal water-filled solar collectors. These evaporators were
usually roof-mounted and absorbed heat from the sun, wind and rain, operating at
temperatures as low as -10ºC. However, the installation of the solar evaporator panel
and connection of the refrigeration system required people trained in refrigeration
techniques, which limited the number of people who could install the systems.
After development of the air-source heat pump, again working down to as low as 10ºC, split solar systems have become uncommon. More recently these compact airsource heat pumps have been available from most major hot water manufacturers
and all have a compact fin coil as the evaporator.
Atmospheric air provides the heat and the condenser is either a coil wrapped around
a water storage tank or a flat plate heat exchanger. The heat is transferred to the
water in the tank either through the wall of the tank or as it is pumped through the
heat exchanger. Installers for these compact systems do not have to be trained in
refrigeration.
The storage tank is insulated on the outside to reduce heat loss. Figure 3.7.2 shows
some examples from the real world and their location.
Advantages and disadvantages
Advantages:
•
A relatively small amount of energy is required to heat the water when
compared with a conventional system using an electrical element. The only
energy used is by the compressor to pump the refrigerant around the system.
The result is that for each kilowatt hour of energy used by the compressor, 2.0
to up to 5.0 kilowatt hours of useful hot water is produced.
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•
•
•
•
•
The compressors in heat pump systems use around 0.4 to 1.3 kilowatts of
power, whereas the equivalent standard electrical water heater uses 2.4kW or
3.6 kW to produce the same result. This means that some heat pumps can
operate from a standard 10 amp power outlet. Bigger units may require a 15
amp circuit with separate circuit breaker.
A thermostat controls the temperature to a maximum of 60ºC, just like the
refrigerator, so it turns the compressor on and off as required.
Systems can be expected to operate for three to 12 hours per day depending
upon water usage, air and water temperatures, and the size of the storage
tank.
While the system works best in sunny conditions, it also operates in cold,
cloudy and wet conditions and even at night. This makes it particularly
attractive for situations where shading from the sun is hard to avoid and a
conventional solar water heater with collectors is not suitable.
Siting is flexible and systems are generally ground mounted.
Disadvantages:
•
•
•
•
Systems are expensive and may use more energy to run than the boosting
energy for a conventional solar water heater.
Depending on the solar fraction for a solar water heater, a heat pump may
produce more greenhouse gas emissions and pollutants, unless it uses ‘green
power’. For high-efficiency units this may happen when the solar fraction is
above 75%, but it also depends on the environmental conditions such as air
and water temperatures.
It is not recommended to connect compact heat pumps to off-peak electricity
tariffs in cooler regions due to the limited time for the compressor to run and
hence the likelihood of running out of hot water. In addition, operating at night
when the ambient air is cooler reduces the overall efficiency of the heat pump
system. Intermediate tariffs that offer up to 18 hours per day of available power
may be suitable.
Current refrigerants can still damage the ozone layer, but to a much lesser
extent than previous refrigerants. Alternative, truly environmentally friendly
refrigerants are not yet available in Australia.
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Heat pump rate of heating
The rate of heating water is
determined by the ambient (air)
temperature. The table (right)
shows the hot water produced
per hour at different ambient
temperatures (water heated from
20ºC to 60ºC) by the Quantum
270L system. It also shows that
with high temperatures more hot
water can be generated per hour
or conversely it takes less time to
heat a given volume of water.
These figures came from the
Quantum Energy website but do
not indicate relative humidity,
which also has an influence on
how long it
takes to heat the water.
Table 3.1 - Litres of hot water per
hour by ambient temperature
Ambient
º
C
temperature Litres of hot water in
one hour
35
112
30
100
25
88
20
75
15
59
10
48
5
38
0
31
-5
26
-10
24
Co-efficient of performance
As the ambient temperature increases, the quantity of water heated increases. The
amount of electricity being used by the heat pump remains
more or less fixed, so with an increase in ambient temperature
the efficiency of the heat pump increases. For every kilowatt
hour of electricity used, more kilowatt hours of energy go into
the water. This ratio is called the co-efficient of performance
(COP).
This heat pump water heater draws air in from the grille on the
left-hand side and passes the air across the compressor and
out through the grille on the right. The heat from the air is
drawn into the evaporator before cold air is discharged out of
the unit.
Placing the unit in the hottest location possible will increase the
efficiency of the unit.
Photo: Andrew Blair
Figure 3.7.3 – Rheem heat pump
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One manufacturer, Rheem, has produced a heat pump using a compact heat
exchanger. The heat exchanger is not around the outside of the storage tank, but in a
unit at the top of the tank. The two parts (refrigeration unit and storage tank) of the
unit are assembled during installation. This can make transport and installation
easier.
Figure 3.7.4 – Separate sections of compact heat pump
•
•
•
•
Water is circulated
by the pump
through the heat
exchanger
Hot water is returned
to the top of the
storage cylinder.
Cold water is drawn
from the bottom of
the cylinder.
Figure 3.7.5 – Cutaway view of Rheem heat pump
Water is drawn from the hot water storage tank, passed through a heat exchanger
where it is heated to 60ºC and then discharged into the top of the storage tank. The
water is circulated at a variable speed to ensure that it is heated to 60ºC in a single
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pass, regardless of the atmospheric conditions. It then enters the top of the storage
tank at 60ºC.
An advantage of this arrangement is that hot water is available very soon after the
unit starts running. The whole tank of water does not need to be heated before hot
water is available.
6
1
7
5
4
2
Receiver drier
8
3
Diagram and information: Courtesy Rheem Australia.
Figure 3.7.6 – Rheem heat pump refrigerant flow diagram
1.
2.
3.
4.
5.
6.
7.
8.
Hot, high-pressure refrigerant vapour leaves the compressor and is passed to the heat
exchanger.
The refrigerant condenses as heat transfers from it to the water passing through the heat
exchanger.
High-pressure, liquid refrigerant leaves the heat exchanger and passes to the TX valve.
High-pressure liquid refrigerant expands through the TX valve to low-pressure liquid
refrigerant.
Low-pressure, low-temperature liquid refrigerant is passed to the evaporator.
The low-temperature liquid refrigerant evaporates as it gains energy from the warmer air
passing over the evaporator.
Warm, low-pressure refrigerant vapour is passed to the compressor.
Low-pressure refrigerant vapour is compressed to hot high-pressure refrigerant vapour by the
compressor.
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Key points
•
•
Heat pumps use the refrigeration cycle to transfer heat from the surrounding
environment into a storage tank.
The main components are:
o evaporator – absorbs heat from the air
o condenser – dumps heat collected from the air into the water
o compressor – pumps the refrigerant around the system
o expansion valve – controls the rate of refrigerant flow through the
evaporator.
•
There are two types of system:
o Split systems – where the evaporator is located remote from the storage
tank and compressor. This allows the evaporator to be located in a sunny
location to benefit from solar energy absorption.
o Compact systems – where the evaporator is fan cooled and located
integral to the top of the storage tank. This is a more flexible design that
can be more easily located.
•
•
•
•
•
•
Systems cost about the same as a solar water heater and are most suitable in
locations where shading excludes the use of conventional solar water heaters
due to lack of sunshine.
Warm locations will always improve the performance of heat pumps for water
heating.
The main outstanding feature of the heat pump system is the small amount of
energy required to heat the water when compared with a conventional system
using an electrical element. The only energy used is by the compressor to
pump the refrigerant around the system. The result is that for each kilowatt
hour of energy used by the compressor, two to four kilowatt hours of useful hot
water is produced.
The compressor in heat pump systems uses around 1100W, whereas the
equivalent standard electrical heater uses 3.6kW or 4.8 kW to produce the
same result. This means that most heat pumps can operate from a standard 10
amp power outlet. An electrician is required to install the power point, but once
in place the unit simply needs to be plugged in which can be done by anyone,
including the installing plumber.
Because the system can operate 24 hours a day, a smaller hot water storage
tank is required than for a normal solar hot water system. It is logical to install a
timer so that water heating occurs only during the day when the air
temperature is warmer than during the night.
The thermostat controls the temperature to 60ºC. Like a refrigerator, it turns
itself on and off as required and can be expected to be in operation for three to
eight hours per day depending upon water usage, and, very importantly, the
temperature of the air from which the heat is extracted. In frosty districts if hot
water is used in the late evening the unit can run all night attempting to reheat
the water.
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Section 3.7 questions
1.
A heat pump hot water system has been described as being like a normal
household refrigerator in reverse. What is meant by this?
2.
Where does the heat that heats the water in the storage tank come from?
3.
Is there any difference in the plumbing for a heat pump system than for an
electric storage hot water system?
4.
Is there any difference in the electrical connection between the heat pump
system unit and the electrical or peak storage system?
5.
Why would someone consider installation of a heat pump instead of:
a. a mains pressure electric storage hot water system?
b. a solar hot water system?
6. Why do manufacturers speak about the ‘co-efficient of performance’ in relation to
heat pump water heaters? What does it mean?
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