Measuring Output of A Prototype Solar Collector

The picture below shows a simple setup to measure the output of small test solar water collector.


The aim was to see how well CPVC pipe and extruded aluminum "heat spreaders" as used in the radiant floor business would do as an easy to build solar collector. 

I wanted to measure the thermal output of the collector, and to get an idea how efficient the heat transfer was from the alum extruded fins to the CPVC pipe.

In this setup, water is pumped by a small submersible pump in the bucket through the collector and back into the bucket.  The performance of the collector can be determined by measuring the flow rate, temperature in and temperature out.



In the picture:

The output of the collector is:


    Qout = (Tout - Tin) (Flow Rate ) (Specific Heat of Water)



In this case, Tout and Tin were measured with thermocouples that were placed near the pump inlet and in the outflow pipe.  Ordinary hardware store alcohol thermometers could also have been used.


The flow rate was measured by timing how long it took to fill up the 2 quart mixing bowl in the picture.  This is very accurate (better than most  flow meters).  The clamp on the hose was used to adjust the flow rate to get a reasonable temperature rise.


As an example:

Flow rate = 0.5 gpm

Tin   = 91F

Tout = 98F


Then   Qout = ( 98F - 91F)(0.5 gal/min)(8.2 lb/gal) (1.0 BTU/lb-F) = 28.7 BTU/min or 1720 BTU/hr


In addition to measuring flow rate, thermocouples were placed on the outer edge of the extruded fin, the middle of the fin, and on the CPVC pipe wall near the fin.  This gives an idea how much thermal resistance is in the alum fin vs how much is in the connection from the fin to the pipe wall.  The thermocouples were of the type that has an adhesive pad that can be stuck onto the surface whose temperature is to be measured.  These thermocouples are  in the neighborhood of $10 each -- not too expensive.


Here is one way to estimate the efficiency of the collector:


- Set up as shown above.

- Point the collector due south, and tilt it so that at noon it is approximately perpendicular to the sun.

- Do the test on a very clear day.

- Start measuring the output of the collector a few minutes before solar noon, and measure it repeatedly until a few minutes after solar noon.

- Average the measurements to get a good estimate of the output at solar noon.

- Look up the solar radiation on the collector in a handbook for your latitude and for the date of the test.  These tables are included in the back of many solar reference books.



The efficiency is:     Efic = (Measure heat output)/(Looked up Solar Input)


You can compare this to similar collectors documented at the NRCC site to see how well your collector is doing.


The total outlay for the instrument setup shown here is around $100.

If only the collector output was needed, just as good a job could have been done with a couple of $2 hardware store thermometers.  This very small outlay for instruments and simple testing can be a big help in determining if your collector is working well, and in evaluating the effect of design changes.


Building and testing a small working prototype like this can often save a lot of money and time by pointing the way to a better design.


The thermocouples that measure the fin temperatures being mounted on the back of the fins (under the blue tape)

Note that the PVC glazing and the polystyrene (pink) insulation I used for this little test are both very bad choices for real collectors -- they melt.