Solar Thermal Energy Systems, 2 Collecting Thermal - Lab-Volt

Solar Thermal Energy Systems, 2 Collecting Thermal - Lab-Volt
Information Job Sheet
2
Collecting Thermal Energy
A solar collector captures the Sun’s radiant energy (light) and converts it into
thermal energy (heat) by transferring (or exchanging) the heat to a fluid (liquid or
gas). Water and air are two of the most commonly used fluids in modern solar
thermal energy systems.
Basic operation
Figure 6 shows the anatomy of a basic solar collector. When sunlight shines on
the glass pane, some of the light (long wave radiation) is reflected or refracted,
but most of the light (short wave radiation) passes through the glass and onto
dark-colored collection material, which absorbs a large portion of the radiant
electromagnetic energy and converts it into thermal energy as heat. The heat is
thermally transmitted from the absorber material to the copper tubing via physical
contact. The copper is highly conductive (both thermally and electrically). The
collected heat conducts through the copper and into the fluid inside the pipe. In
this case, the liquid is water. In addition, a greenhouse effect is created by the
longwave and shortwave infrared (IR) radiation inside the collector enclosure.
This effect helps to heat up the gas (air in this case) that is trapped inside the
panel. The copper tubing conducts heat from the warmed air to the water inside.
As water moves through the tubing, the conducted heat is transmitted to other
parts of the hydronic (water-based) system.
Glazing
Riser tubes
Absorber plate
Manifold
Frame
Insulation
Figure 6. Basic solar collector anatomy.
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Job Sheet 2 – Collecting Thermal Energy
Basic construction
The clear glass pane in Figure 6 is called glazing. It is commonly manufactured
to minimize iron content that is normally present in plate window glass in order to
pass a larger spectral portion of infrared (IR) energy. Thermal insulation placed
below and on the sides of the absorber material helps to minimize thermal
losses, or the inadvertent transfer of heat to the ambient air that surrounds the
solar collector enclosure. Baffles and manifolds are used to increase surface
area and to control the flow of fluid through the solar collector in such a way as to
improve the thermal transfer efficiency.
Collector types
There are many different types of solar collectors in use today. Some of the most
common collector types are listed below. Each one includes an explanation of its
typical operation and construction.
Passive water collectors
x
Integrated collector storage (ICS), or solar batch heater - suitable for
moderate climates, this simple collector uses its internal water tank (or
multiple tanks) for both collection and storage of solar heat. Each tank is
coated to absorb solar energy, and the entire assembly is enclosed in a
thermally insulated and glazed box.
Figure 7. Integrated collector storage (ICS) solar collector.
x
x
22
Thermosiphoning water panel (TWP) - a flat-plate type of convection
heat storage (CHS) system that requires storage tank placement above
the collector.
Evacuated-tube - another convection heat storage (CHS) system that
also requires storage tank placement above the collector. The heat pipe
style uses long borosilicate glass tubes with pure copper tube absorbers
called heat pipes inside to indirectly heat an insulated copper manifold at
the top of the unit by way of evaporated heat pipe fluid. The purified
water with additives boils at only 86°F due to an internal vacuum. The
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Job Sheet 2 – Collecting Thermal Energy
cooled vapor forms back into liquid by way of condensation and the
heating process repeats continuously. A vacuum between a clear outer
glass and an aluminum nitride coated inner glass absorber tube virtually
eliminates heat loss by natural convection, so these units are highly
efficient. The vacuum is maintained by a barium coating at the bottom of
each tube. Normally silver in color, a cloudy white coloring at the tube
end indicates a loss of vacuum. Heat pipes require a minimum tilt angle
of 25° to operate properly. The direct flow style heats liquid directly inside
the inner glass tube, which contains a long copper feed tube that forces
the liquid up through the glass tube. This style is not normally passive in
operation. It typically requires an active circulator pump. Unlike heat pipe
designs, if the glass tube breaks (due to freezing conditions), the thermal
transfer fluid can escape and drain the system. These units, and similar
devices, called U-tube (or U-pipe) collectors can be positioned
between 0° and 90°. In a U-pipe design, a U-shaped copper tube (inside
the inner glass tube) contains the thermal transfer fluid. No liquid enters
the glass vacuum tube, so breakage is less common and not as critical.
Figure 8. Evacuated-tube (heat pipe) solar collector (photo courtesy of Ra Boe).
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Job Sheet 2 – Collecting Thermal Energy
Active water collectors
x
Flat-plate - this collector type is very common for residential and
commercial buildings, due to its high efficiency and low cost. It uses a
grid of copper tubing inside a thermally insulated enclosure. A glass
window on the top exposes dark-colored copper fins or aluminum
sheeting as an absorber plate that is thermally connected to the flow
tubes. Unenclosed formed-plastic styles are available also for less
demanding applications. Water flow is forced through the tubing by a
motorized electric pump.
a
The training system uses a flat-plate solar collector.
Figure 9. Flat-plate solar collector.
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Job Sheet 2 – Collecting Thermal Energy
x
Web and tube (pool) - made of formed black plastic to avoid chemical
corrosion issues, this collector integrates multiple tubes across a plastic
membrane. It is not contained in an enclosure and is not covered by a
glass pane. Glazing is not required for the low temperature rises typically
needed to warm swimming pools, hot tubs, and spas. The water is
commonly pumped between the pool and the collector via the water filter
system. The total collector area is typically sized to half of the pool
surface area.
Figure 10. Pool solar collector.
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Job Sheet 2 – Collecting Thermal Energy
x
Concentrating - known as concentrating solar power (CSP) devices,
there are several styles of these solar collectors that all work by
passively reflecting and focusing sunlight into a smaller absorbing area.
These high-temperature collectors are often used by solar power plants
to generate electricity. The water temperatures obtained with this
approach often exceed levels needed for domestic hot water (DHW)
applications. Automated tracking of the sun is typically required with
these collectors as well, and molten salts (sodium and potassium
nitrates) are commonly used as a thermal transfer fluid. Synthetic oil or
pressurized steam is also used to transmit thermal energy to the heat
engine. Here are a few different kinds of concentrating collectors:
x
Parabolic trough - by far the most commonly used type of
concentrating collector, it uses a cylindrical trough-shaped
reflector to concentrate sunlight on an insulated (Dewar) tube,
also called a heat tube. Transfer fluid inside the absorber tubing
is circulated by pump to distribute the heat to the power station,
often a steam-powered electric generator. Single- axis solar
tracking is required for the trough.
Figure 11. Parabolic trough solar collector.
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Job Sheet 2 – Collecting Thermal Energy
x
Parabolic dish - this collector is the most powerful type of solar
concentrator. Its bowl- shaped reflector directs sunlight to a
single focal point, where the heat absorber is positioned. As one
of the most energy-efficient solar collectors, very little energy is
lost when using this approach. Dual-axis solar tracking is
required for the dish. A Stirling engine (heat engine) is commonly
used for converting the thermal energy to mechanical energy
and finally into electrical energy by using an electric generator.
Figure 12. Parabolic dish solar collector.
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Job Sheet 2 – Collecting Thermal Energy
x
Power tower - this tall, large-scale collector has a central tower
surrounded by numerous rotating mirrors, called heliostats that
track the sun’s location and focus sunlight at the tower top,
where the heat can be absorbed and stored in purified graphite
or ceramic pellets. As needed, heat is drawn from the thermal
storage and pumped to the power station below, which is usually
a boiler and steam-driven turbine generator.
Figure 13. Power tower solar collector.
x
x
x
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Solar pyramid - this pyramid-shaped collector draws in and
heats air to move electric turbines at a power station.
Linear Fresnel reflector (or lens) - these collectors use either
long, narrow mirrors, or a flat, clear plastic lens with longitudinal
grooves on one side to concentrate sunlight into a focal line
along its heat tube absorber, also called a receiver.
Solar pond - this large-scale collector consists of a salty lake or
pond that uses evaporation to process brine solutions into a
concentrated solid. The heavy solid and dense salt water stays
at the bottom, absorbing heat, while the lighter fresh water at the
top provides enough thermal insulation for the bottom to retain or
store the thermal energy. An organic Rankine cycle (ORC)
engine is used to generate electricity. Although the process is
passive, several pumps actively circulate the fresh water and salt
water.
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Passive air collectors
x
Thermosiphoning air panel (TAP), or Trombe wall - a glass window
upon a vertically positioned and well insulated box with a darkly colored
interior allows the air inside to become heated by the sun. Natural
convection causes the air to flow from bottom to top.
Summer sun at 68°
Winter sun at 11°
Figure 14. Trombe wall solar collector.
x
x
x
Window box - a flat-plate type of collector, it is a smaller version of the
TAP or Trombe wall that fits inside a window exposed to the sun.
Matrix - similar to the flat-plate, this collector uses an absorber material
with a large amount of total surface area, such as expanded metal lath or
fiberglass fibers within an insulated enclosure. A glass window on the
box top exposes the dark-colored material inside to heat the internal air.
Designs that restrict air flow should only be used in active systems.
Solar chimney - a large structure that uses sunlight and natural
convection to heat and move the air inside it. A tall center chimney is
surrounded by a round greenhouse type of solar collector. The up-draft
created by natural convection spins turbine generators at the chimney
base to produce electricity.
Active air collectors
x
x
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Flat-plate - this collector uses baffles to direct air flow through a grid
within a thermally insulated enclosure. A glass window on the top
exposes the dark-colored interior of the box to heat the air inside. Air flow
is forced by an electric blower fan.
Rock box - a glass window upon an insulated and stone-filled box allows
the air flowing inside to become heated by the sun-warmed rocks. Air
flow is forced by an electric blower fan.
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Job Sheet 2 – Collecting Thermal Energy
x
Matrix - similar to the flat-plate, this collector uses an absorber material
with a large amount of total surface area, such as expanded metal lath or
fiberglass fibers within an insulated enclosure. A glass window on the
box top exposes the dark-colored material inside to heat the internal air.
Designs that permit plenty of air flow can also be used in passive
systems.
Collector efficiency
During radiant to thermal energy conversion, the amount of surface area
exposed to the sun is the largest single contributor to solar collector efficiency.
Also, high flow rates that keep low temperature differentials between the solar
fluid and ambient air is usually more efficient than low flow rates and high
temperature differentials.
Array configurations
Solar collectors can be connected in series or in parallel to increase volume or
flow rate, respectively (see Figure 15). Series configurations are not normally
used in residential applications. The high water temperatures from seriesconnected arrays are sometimes required for commercial installations.
Series
Parallel
Parallel
Figure 15. Series and parallel connected arrays.
Basic plumbing techniques are used to interconnect the solar collector panels.
The use of brass unions permits easy collector replacement, and can minimize
rooftop soldering.
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Job Sheet 2 – Collecting Thermal Energy
Mounting methods
There are several common techniques used to secure solar collectors on
residential and commercial buildings, in addition to alternative ground
installations. Roof mounting styles include vertical, horizontal, flush, and
sawtooth, as shown in Figure 16. The method used to attach the mounts is
based upon the roof material, such as clay tile, asphalt shingle, built-up gravel,
etc. Wind and snow expectations for the area must also be considered.
Ground mounting styles include metal or wooden racks or concrete pillars. This
method is most commonly used for closed-loop, antifreeze systems. Racks are
typically anchored with concrete footings.
Vertical roof mount
Saw tooth roof mount
Flat (flush) roof mount
Horizontal roof mount
Pillar ground mount
Awning mount
Rack ground mount
Leaning ground mount
Figure 16. Solar collector mounting.
In a typical installation, the solar collector panels are positioned pointing to the
southern sky. Arrays pointing east and west are also used when exposure to the
south is limited. Areas of roof penetration, such as where mounting bolts enter
the attic, often require flashing and weather sealant, or where the solar collector
inlet and outlet (feed and return) pipes enter the home often require a roof boot
and roofing cement. Horizontal pipes should be sloped to avoid air traps. Passive
ICS and CHS systems are more challenging to install on a building due to the
additional weight of the storage tank and its liquid contents. The roof construction
must be strong enough to support the heavy water tank and solar collector that is
being mounted.
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Job Sheet 2 – Collecting Thermal Energy
Collector sizing
Several key factors determine the required size or capacity of a solar collector
array for any given thermal system. These include the solar power required, the
available space, and the orientation of the collectors. The array must have large
enough surface area to capture the required solar power for a particular system
and geographical location. Also, unless a ground installation is being planned,
the entire array must be sized correctly to fit properly on the building’s roof. More
detailed information regarding solar collector sizing is provided in the manual
Solar Thermal Systems, p/n 30-87330-20.
Roof pitch and collector angle
The space required to mount the solar collector array on a roof also depends
upon the roof pitch versus the tilt angle required for each solar collector. The site
latitude essentially equals the optimal solar collector tilt angle for year-round
operation. You can subtract 10° to compensate for weather variations. In the
Northern Hemisphere, for winter operation only, add 15°, or for summer
operation only, subtract 15°. The dimensions of the roof (ratio of rise and run)
determine the roof pitch or slope, as shown in Figure 17.
Rule
Level
30 cm run
10 cm rise
Sloped roof
18° angle
Figure 17. Checking roof pitch.
Some common roof pitches are listed in Table 4.
Table 4. Roof pitches.
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Rise/Run Ratio
Roof Pitch (°)
20:12
60
12:12
45
7:12
30
4:12
18
0:12
0
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Job Sheet 2 – Collecting Thermal Energy
System performance
This job involves installing and operating a single closed-loop space (floor or air)
heating system, which has the following advantages and disadvantages.
System type
x
Active, direct, closed-loop, unvented, floor (surface) or radiator (air)
heating without storage
Advantages
x
x
Heats directly
Only used during heating season
Disadvantages
x
x
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Requires antifreeze, which can overheat
Not for hot climates
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Job Sheet
2
Collecting Thermal Energy
OBJECTIVE
In this job, you will install a single closed-loop space (floor or air) heating system
as part of a solar thermal energy system. You will also become familiar with
techniques that are commonly used to collect solar thermal energy.
PROCEDURE
Equipment required
Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required for this job.
Safety procedures
Before proceeding with this job, complete the following checklist.
‰ You are wearing safety glasses.
‰ You are wearing safety shoes.
‰ You are not wearing anything that might get caught such as a tie,
jewelry, or loose clothes.
‰ If your hair is long, tie it out of the way.
‰ The working area is clean and free of oil.
‰ The floor is not wet.
‰ Your sleeves are rolled up.
‰ The wheels of the system are locked in place.
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Job Sheet 2 – Collecting Thermal Energy
Installation
1. Figure 18 and Figure 19 show direct solar heating systems without thermal
storage. Choose one of those systems, then continue this procedure using
the selected system.
Warm surface
Shutoff
valve
Auto air vent
Expansion
tank
Pressurerelief valve
Shutoff
valve
Radiant
floor
Fill bowl (manual air
vent/fluid feeder)
Solar collector
Funnel
Pump
Ball valve
Ball valve
Figure 18. Closed-loop, floor-heating system schematic.
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Job Sheet 2 – Collecting Thermal Energy
Radiator
Fan
Cold air
inlet (AS)
Warm air outlet
Power input (ES)
Auto air vent
Shutoff
valve
Expansion
tank
Shutoff
valve
Pressurerelief valve
Fill bowl (manual air
vent/fluid feeder)
Solar collector
Funnel
Pump
Ball valve
Ball valve
Figure 19. Closed-loop, air-heating system schematic.
2. Ensure that the following parts are mounted on the mobile workstation as
shown in Figure 20 or Figure 21.
x
x
x
x
x
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Solar collector
Circulator pump
Thermostat controller (with 1 sensor)
Expansion tank
Radiator
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Job Sheet 2 – Collecting Thermal Energy
x
x
x
x
x
x
x
x
x
Radiant floor
Thermometers (2)
Rotameter
Pressure gauge
Pressure relief valve
Shutoff valves (2)
Fill bowl
Automatic air vent
Electrical panel
Primary loop
Hot flow
Cold return
Warm path
Secondary loop
Hot flow
Cold return
Warm path
Figure 20. Closed-loop, floor-heating system configuration.
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Job Sheet 2 – Collecting Thermal Energy
Primary loop
Hot flow
Cold return
Warm path
Secondary loop
Hot flow
Cold return
Warm path
Figure 21. Closed-loop, air-heating system configuration.
3. Ensure that the electrical wiring is complete by referring to the electrical
wiring instructions in Appendix F.
4. Adjust the solar collector tilt angle to 90° (vertical) to simplify illumination.
Be careful when rotating the solar collector. Improper use can lead to injuries.
5. Connect the necessary hoses as shown in Figure 18 and Figure 20 (or
Figure 19 and Figure 21).
a
Always use the minimum hose length required for each circuit connection.
6. Connect the training system to an ac power source.
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Job Sheet 2 – Collecting Thermal Energy
7. Fill the system with water (you can refer to the System filling procedure in
Appendix C).
Be careful to prevent water from being in contact with electrical components. Use
a bucket and mop if some leakage occurs.
8. Turn the power on by setting the circuit breaker to I.
9. Start the pump by setting the differential controller override switch to
Override.
Then, set the pump to III.
10. Evacuate the air present in the training system by performing the priming
procedure described in Appendix C.
This time, while performing the priming procedure, make sure you use the fill
bowl with its valve open and that it is at least half-filled with water.
Commissioning
11. Set the valves as shown in Table 5.
Table 5. Valve states.
Valves
States
Automatic air vent
closed
Fill bowl ball valve
closed
Solar collector ball valve
open
Shutoff valves (2)
open
12. If installed, turn on the radiator and set the fan to minimum speed (Switch
settings: S1 = 2, S2 = ĹĹ).
13. Turn the pump on by setting the override switch under the thermostat
controller to the “Controller” position. Then, adjust the set point temperature
between 3°C to 6°C above the ambient temperature.
Ambient temperature ൌ
Set-point temperature ൌ
40
°C
°C
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Job Sheet 2 – Collecting Thermal Energy
14. Measure (with the pump on) the different parameters listed in Table 6, then
record the values in the “Initial ” column of the table. After that, position and
power up the two 500 W work lights so that most of the radiant energy floods
the solar collector panel evenly.
a
The pressure gauge port must be vertically level with the rotameter pressure
port.
a
If the rotameter does not indicate any flow, it means that the controller has
turned the pump off.
a
Use the magnetic surface thermometer to measure ambient air temperature in
front of the radiator output.
The solar thermal training system should be operated under supervision at all
times. Never let the system operate unattended.
The surface of the work lights can become very hot. Whenever you manipulate
them, take great care to avoid direct contact with the skin.
Table 6. Test results.
Values for different measuring times
Device
Initial
At 15 min
At 30 min
Units
F1-1 (flow rate)
L/min
PI-1 (pressure)
kPa
F1-2 (flow rate)
L/min
PI-2 (pressure)
kPa
TI (collector input)
°C
TI (collector output)
°C
TE-S1 (COIL)
°C
TE-S2 (TST)
°C
TE-S4 (TRF)
°C
Floor temperature
°C
Air temperature
°C
15. After about 15 minutes, measure (with the pump on) the different parameters
listed in Table 6, then record the values in the “At 15 min” column of the
table.
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Job Sheet 2 – Collecting Thermal Energy
16. After about 30 minutes, measure (with the pump on) the different parameters
listed in Table 6, then record the values in the “At 30 min” column of the
table.
17. Use the thermostat override switch to turn the pump off.
18. Turn off the system by setting the circuit breaker to O.
Also turn off the work lights.
19. Disconnect the power cords and drain the system by performing the System
draining procedure in Appendix C.
20. Ask the instructor to check your work.
REVIEW QUESTIONS
1. Did the system work correctly?
‰ Yes
‰ No
2. Describe and briefly explain the system behavior.
3. Name two locations or devices in the system where thermal energy was
transferred through a containment wall.
4. Where might a similar system prove to be effective or useful?
5. Based upon the measured values, what was the overall temperature rise of
the water?
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Job Sheet 2 – Collecting Thermal Energy
6. Based upon the measured values, what was the overall temperature rise of
the floor or air?
Name: ______________________________
Instructor's approval:
© Festo Didactic 52668-20
Date: ____________________
______________________________________________
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