Tuesday, September 27, 2011

1 Comparing concentrator to flat plate solar collector

A test is described which compares a traditional flat plate solar water heater made from copper tubing and aluminum fins to a parabolic concentrating collector. Both types can be home built for significantly less than the cost of commercial units. This test compares how each design heats an identical volume of water side by side in the same sun.

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."

The parabolic concentrator design (parabolic trough) is my own and is described in my plan book 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.

The flat plate collector consists of a network of copper pipe and aluminum absorber plates which are thermally coupled to the copper. As the aluminum heats up in the sun, the sun's energy is transfered by conduction to the water flowing in the copper pipe and heats the water. The flat plate collector was constructed using the excellent information at BuildItSolar.com.

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:

Solar Aperture
Size (inches)Area
sq. in.
Parabolic concentrator19.25 x 48.09240.596
Flat plate20.625 x 46.09480.612

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.

Here is the solar test jig from the sun end. The apparatus to the right is the motor drive and the solar sensor for the parabolic concentrating collector. All the plastic pipe runs are the same length and sizes for the two collectors. The fine wires hanging from the collectors are the temperature sensors for the HOBO data recorder discussed here.

Viewed from the back, the two reservoirs (coolers) can be seen at the bottom. The reservoirs each contain 45 lbs (20.5 kg) of water and a small aquarium pump. The white control panel holds flow meters and flow adjusting valves to equalize the water flow in the two systems. The flow was measured on the flow meters to be 0.91 GPM (3.44 LPM). The pumps contribute a small amount of heat. I measured each of the pumps operating in the system and each was drawing 11 watts so the heat contribution of the pumps is small and the same for each collector.

The test was begun at 8am and discontinued about 19:30. The day began with an overcast sky but the sun broke through about 11:30 and the sky cleared until about 14:45 when unbroken clouds rolled in.

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.

The beginning of the day is interesting as the flat plate heats more than the concentrator. As the sun was not visible until about 11:30, I believe that the larger metallic surface of the flat plate was picking up heat from the air more readily than the much smaller collector tube of the concentrator. Once the tracker locked onto the rising sun, the concentrator heats quickly. In fact, both heat rapidly once the sun is visible.

Looking at the most active, mid-day portion of the chart, I have added straight lines which by eye show the heating (red) and cooling (blue) rates of several sections of the chart. It seems that the two systems heat at similar rates (the slopes of the red lines are parallel) but the flat plate cools more quickly that the concentrator (the blue lines for the flat plate slope downward more than for the concentrator. Or maybe it is just my eyes? Could it be that the larger metallic surface area of the flat plate provides better heat transfer to the air which causes it to lose heat more quickly?

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.

Index - Comparing concentrator to flat plate solar collector


JC-Solar said...

George what is the distance between the flow tubes of the collector?

george plhak said...

Hello John, the distance between the flow tubes in the flat plate collector is six inches.