You can click on any picture here to see a full size enlargement.
I built the two collectors and mounted them in a test jig (described here) which can be oriented and tilted to face the sun. The collectors are connected to identical small insulated reservoirs which contain the same volume of water. During the test, the water is pumped from the reservoir through the collector and back to the reservoir. As the test progresses, the water in the reservoir heats up. My test is modeled after the technique described by Gary Reysa at BuildItSolar. As Gary says "The collector with the better performance heats the water in its reservoir to a higher temperature, and the difference in final temperatures in the two reservoirs is an indicator of how much better one collector performed than the other."
How to build a Tracking Parabolic Solar Collector. The sun's energy reflects from a mirror bent in the shape of a parabola and is concentrated onto a single copper collector pipe which is positioned at the focus. The water flowing in the pipe is heated by conduction with the copper.
For this test, I used a half length four foot long reflector. It is in all ways the same as the full size version except for the length. The shorter version was chosen to make the test jig more manageable. The test jig incorporates the motor drive and solar sensor as described in the book. The motor drive can control the positioning of both of the collectors in the jig although in this test, I fixed the flat plate collector stationary to match the way it is traditionally used, in a fixed position.
I purchased the preformed aluminum plates from Tom Sullivan of U.P. Solar Solutions. The base and the sides of the box are Baltic plywood. The copper pipe was joined with conventional solder fittings. The aluminum absorber plates got a bead of silicon caulking prior to being crimped onto the copper pipe using Tom's excellent modified vise grip pliers and then stapled in place onto the base. The entire surface of the aluminum then received three coats of flat black rust paint (chromate based).
I incorporated insulation (1" polystyrene) on the back and sides of the flat plate but did not include a cover for this test. A cover will be included in a later test of "insulated collectors".
One of my goals was to make the solar size (the aperture or the effective area that "sees" the sun) of the two collectors the same so that the results could be compared directly. I didn't get that quite right:
|Parabolic concentrator||19.25 x 48.0||924||0.596|
|Flat plate||20.625 x 46.0||948||0.612|
So my flat plate collector has an actual effective area about 2.6% greater than my concentrator. For future tests, I may mask 24 sq. inches of the flat plate to make the two truly equivalent. For this test, I have ignored the difference. Later I also did not make corrections, the test results are as recorded.
The solid line is the parabolic concentrating collector, the dashed line is the flat plate. The temperatures measured are the temperature of the water exiting each reservoir at the inlet to each collector. Throughout much of the chart, the concentrating collector temperature exceeds the flat plate by as much as 11 degrees.
This was my first test in a series that I plan to do. The weather looks overcast for most of the rest of the week. At the first opportunity, I will repeat this test, but with the flat plate ganged to the concentrator, so that they will both rotate to face the sun.
Subsequent tests are planned with insulation added to both collectors.
Thank you for your interest.