Solar Collector Glazing Perromance Tests

This page provides some data from tests of glazing materials done by Alan Rushforth.

Alan is working on developing an inexpensive, long life, low maintenance solar water heating collector for commercial buildings (e.g. apartment buildings). He is looking for a glazing material that offers high collection performance at a low price, and for a a cost effective way to transfer heat from the collector absorber to the working fluid (water).

I think that he has developed a really simple way to compare the collection efficiency of glazing materials.

Glazing Performance by Stagnation Test

In this stagnation test, Alan made several identical solar collector boxes. Each box is made from 1 inch thick rigid foam board, and is painted flat black inside. The collectors boxes are about 8 by 10 inches, and about 1 inch deep. Each is glazed with a different material (glass, polycarbonate, Teflon, ...). The glazing material is sealed to the collector box in such a way that there is no air leakage. The collector boxes are mounted on a rack such that they all see the same sun conditions. The boxes are exposed to sunlight which is nearly normal to the glazing surface, and the stagnation temperature in each collector box is measured.

The idea is that the the stagnation temperature depends on 1) how much sun is transmitted by the glazing, and 2) how much heat is lost through the glazing. The stagnation temperature also depends on the losses from the sides and back of the collector box (but this is the same for all the boxes), and on the ambient temperature. So, the test determines the highest performing glazing for the ambient temperature at which the test was run -- the results will be different for other outside temperatures.

The glazing material that produces the highest stagnation temperature has the highest thermal performance at this ambient temperature.

It appears to me that this simple test quickly and easily determines the comparative performance of the glazing tested, including the combined effect of:

1) On 2/5/07 the collectors were in a location with some light twig shading plus there was an occasional light cloud. On 2/6/07 the collectors were in full sun with zero clouds.

2) Gray numbers were from non-oven type thermometers that have a 158 limit and were reading lower than the oven thermometers. Dark numbers are all from oven thermometers.

A Thin Water Film Collector

Alan has been working on finding a way to make water spread out between a substrate and an overlying layer of film. The idea being that if this could be accomplished, the thin film of water would efficiently collect heat from the absorber, and also be inexpensive.

The early tests with commonly available films resulted in the water not spreading out uniformly, but instead concentrating into rivulets. Alan then tried the same test on small samples of Teflon and Tefzel film from DuPont -- these are the initial results:

Water beads up like mercury on both the Teflon and Tefzel. It beads on painted metal, and it beads on the foil faced foam, BUT when you put the Teflon or Tefzel on top and trickle water in between, the water behaves totally differently. The water glues the two together and SPREADS. A small dribble of water poured in between at the top, quickly spreads to nearly full width by the bottom of the 8 1/2" x 11" sheet, at any tilt. Once the area is fully wetted, it says wet, stays glued, and is nearly invisible. You can hold the panel upside down and the film stays put. Also I found a piece of used white epdm roofing, and the films behave the same way on that. All this, by the way, is with NO wetting agents.

To test fragility of the films, I tried tearing both 1 mil and 2 mil at an edge, wondering if once a tear started, would it move across like a potato chip bag tear. It stretched a little, but did not want to tear. I took a nail and poked the films (near an edge) down against the work bench, and pulled. Even with a sharp point on it, first they stretched, THEN they tore. In other words they are tough. One thing is a little weird. Some of the samples are the type that are treated on one side. I cannot detect which side it is, so I am not sure what good that is. By the way, scotch tape (I used for labeling) sticks to this stuff.

I am thinking since water glues the 2 mill just as well as the 1 mil, that the 2 mill may be the best all purpose film to order. It costs a bit more, but it would have 1/2 of the vapor transmittance and would have added toughness.

Time	Ambient Temp.	Window glass	Tefzel 200 LZ (2 mil)	Teflon FEP500C (2 mil)	Corrugated polycarbon-ate	8 mm twinwall polycarb.
2/5/07 - 9:55AM just before test went outdoors	68F	67F	70F	70F	67F	67F
10:00 sun	24	114	95	85	114	114
10:05	24	128	103	91	131	141
10:10	25	110	102	96	148	155
10:16 light cloud	25	99	85	85	123	146
10:34	26	90	78	70	101	116
11:28 sun	28	145	122	106	163	194
12:48 cloud	28	81	78	71	103	118
1:14	28	95	100	82	111	109
1:20	29	105	107	92	127	134
1:40	30	114	113	98	132	144
2:04	30	60	64	54	67	96
3:00	29	105	113	90	105	125
3:25	29	95	99	89	106	136
4:30 (low sun behind trees)	28	34	37	34	37	41
2/6/07 -9:55 AM	16		105/83	72	112	112
10:00 full sun all day	17		105/80	67	111	122
	17		138/101	95	158	188
100:00w/ reflector added	17		221/hi	143	228	250
“	17		229/hi	146	237	263
“	17		238/hi	152	247	280
“	17		246/hi	hi	255	287
12:00 w/o reflector	17		207/hi	134	212	252
“	17		207/hi	134	212	252
“	17		208/hi	142	216	248
1:05 with reflector	17		239/hi	hi	246	281
1:15 with reflector	17		237/hi	hi	241	281