ME 252 Lab – Solar Collector Experiment
Objective: to measure the performance of an active, evacuated tube solar collector
system for domestic hot water. Specifically,
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determine the system efficiency at two different flow rates.
Background: Evacuated tube solar collectors are more efficient than conventional flat
plate collectors since heat losses from the absorber are suppressed by an evacuated
space between the transparent glazing (glass) and the absorber surface. A perfect
vacuum eliminates conduction and convection heat transfer; thus, the only thermal
losses are due to radiation exchange between the absorber surface and glazing. In
practice, a perfect vacuum is not achievable and conduction losses will also occur
through other parts of the system such as the collector framework.
The collector system to be tested was manufactured by the Sunmaster Corporation and
uses the Owen-Illinois “Sunpack” evacuated tubes. The design consists of concentric
glass tubes with the annulus evacuated. The inner glass tube is coated with a selective
surface – it has high solar absorptivity and low infrared emissivity. A small copper tube
is located at the center of the absorber tube. Water enters the collector at the bottom of
the annular space separating the absorber tube and the copper tube; it then flows
upward to the top of the collector and returns downward through the copper tube. The
original Sunmaster panels consisted of eight evacuated tubes connected in parallel
through a single header, or manifold. Refer to handouts.
The experiment was designed, built, and tested by a Chico State Mechanical
Engineering senior design team in Spring 1997. It was configured as an active, closedloop water heater by incorporating a small centrifugal pump and a five-gallon storage
tank. Measurement of water flow rate and water temperature are accomplished by a
glass rotameter and several thermocouples, respectively. Only six of the original eight
evacuated tube collectors are currently operational.
Theory: Solar collector efficiency is the fraction solar energy intercepted by the
collector aperture that is ultimately transferred to the circulating fluid (water). It is
computed from
c 

mc p,av Tf  T i
Ac  Gsun t

(1)
where m is total mass of water in the system, cp,av is the average specific heat of the
stored water, Tf is the average final temperature of the water, T i is the average initial
temperature of the water, Ac is the collector aperture area, Gsun is the instantaneous
solar irradiation, and t is the measurement time interval. The summation is performed
over the testing time period.
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Apparatus and Instrumentation:
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Sunmaster evacuated tube solar panel
Grundfos variable-speed centrifugal pump
Gate valve for controlling flow
Glass tube rotameter
Three K-type thermocouples
LI-COR LI-200SA pyranometer
Kipp & Zonen pyranometer
Fluke Hydra data logger
PC with Hydra data acquisition software
Rotameter Calibration:
2
Qind  1.9172Qact  0.0302Qact
where Qind = indicated volume flow rate in gpm
Qact = actual volume flow rate in gpm
Experimental Procedure:
Special Precautions
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Never operate the pump dry as this may cause impeller failure. Always make sure
that the storage tank has been filled before starting the pump.
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Always fill the system with water before moving it to a sunny location. Failure to do
so may result in thermal shock and glass tube breakage when the system is filled.
Pre-start
1. Inspect the system to ensure that all plumbing connections are secure. Note that the
water flows from the bottom of the storage tank to the pump, through the rotameter,
through the collector tubes, and then back to the top of the storage tank.
2. Measure the collector aperture dimensions of the six operational tubes. Be sure to
include the parabolic reflectors.
3. Fill the system with water using the calibrated water jug. First, open the gate valve
and fill the storage tank with five gallons of water. Turn on the pump (plug in power
cord) to fill the collector tubes. Then add four more gallons of water to the storage
tank. Secure lid to tank and unplug the pump.
4. Move the solar water heater unit and data acquisition cart to a sunny location. Use
the hydraulic jack to tilt the collector so that it is approximately perpendicular to the
sun’s rays. This alignment can be done by using the shadow cast by a frame
member.
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Data Collection
1. Turn on the pump and adjust the gate valve to yield a volume flow rate of 2.0 gpm
(refer to the rotameter calibration equation).
2. Insert the input module into the Fluke Hydra data logger. Turn on the data logger
and boot the PC. Start up the Hydra software and check the configuration. Make
sure you are using the correct setup file. Set the measurement scan to “slow” and
the time interval to 30 seconds. Enable data recording and create a file that data will
be written to. Make sure the time tag is set to “Elapsed time” and the data recording
is set to “Amend to file”.
3. Start data logging and set up a Quick Plot to monitor temperatures and solar
irradiation. Make sure the sensors are yielding reasonable values.
4. Collect data at this flow rate for approximately one hour, then stop data logging.
5. Adjust the gate valve to yield a flow rate of 5.0 gpm. Reorient the collector if
necessary.
6. Start data logging for another hour.
Shutdown
1. Stop data logging and download your data file to a floppy disk. Exit the program,
turn off the Hydra, and shut down the PC.
2. Unplug the pump and move unit into the shade. Empty the system by attaching a
garden hose to the fitting at the top of the storage tank. Run the garden hose to the
grass and start the pump. Open the gate valve completely and run the pump until
the tank is empty. Allow all water to drain from the system after unplugging the
pump.
3. Disconnect the input module from the Hydra and roll the equipment back into the
lab.
Results:
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Plot the storage tank temperature versus time for the 2 and 5 gpm tests on the
same graph. Connect data markers with a straight line (no smoothing).
Compute the (overall) collector efficiency at each flow rate for the entire testing
period.
Divide each test into 10 minute periods and compute the collection efficiency for
each period. Summarize these results in a neat table.
Questions:
1. Are the temperature versus time plots linear? Explain why or why not.
2. Compare the overall collection efficiencies for the two flow rates. Are they
significantly different? Explain why they may be different.
3. What trends, if any, do you observe in the collection efficiencies computed for the
10-minute periods? Explain these trends.
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