This is a simple test comparing the performance of twinwall polycarbonate glazing to Plexiglass glazing for a solar collector under cold weather conditions.
Bottom line is that the twinwall glazed collector added about 10% more heat to its water charge over the 3 hour test.
The Plexiglas is a clear acrylic glazing that was about 1/8 inch thick. The Twinwall is polycarbonate glazing 10 mm thick. The twinwall is a double glazing with the inner and outer layer separated from each other by extruded polycarbonate webs -- see picture. It is used a lot for greenhouse glazing.
The two collectors are identical very simple collectors made from insulation board with a glazed front face. Each collector has a full 2 liter plastic pop bottle filled with water and painted flat black. The collectors are detailed a bit more here...
The left collector is glazed with the twinwall polycarbonate, and the right with the single wall Plexiglas.
The wheels on top are just to weigh down the top to prevent air infiltration. I took some pains to make sure that the glazing was sealed to the insulation board and that there were no air leaks.
The idea is that the only thing that is different between the two collectors is the glazing material, so any performance difference is an indication of the efficiency of the glazing.
A logger is used to record the temperatures of the water in the bottles and also the air in each collector.
Red solid line -- temperature of the water in the bottle for the twinwall glazed collector
Black long dash -- temperature of the water in the bottle for the plexiglaz glazed collector
Red short dash -- temperature of the air in the twinwall glazed collector
Black short dash -- temperature of the air in the plexiglax glazed collector
The water in the plex glazed collector started out 7 F warmer than the water in the twinwall glazed collector, and over the course of the 3 hour collection period, the water in the twinwall collector caught up with and very slightly exceeded the water in the plex collector. So, the twinwall outperfomed the plex by this 7 F.
The water in the bottles went from about 64F up to about 132F, or a gain of about 68 F. So, the 7F better performance of the twinwall represents about a 10% gain over the plexiglas.
The air in the twinwall collector ran on average about 8F warmer than the air in the plexiglass collector.
The sun was good, but there were high thin clouds coming in and out, and you can see that in the up and down collector air temperatures.
Outside temperature started at about 24F and went up to about 28F over the course of the test. Wind was nearly calm.
It would have been nice to also test the Suntuf style corrugated polycarbonate, but I suspect that the corrugated Suntuf would do a little worse than flat plexiglas just due to the greater heat loss area caused by the corrugations. I will probably do a side by side comparison of twinwall and Suntuf on 4 by 8 ft solar air heating collectors later.
Note that this is a test done under cold weather conditions -- I would expect less difference under warmer conditions. The twinwall insulates better and reduces heat loss out the glazing, but it also absorbs more light than the Plexiglass (about 20% vs 10% for the plex), so under warmer conditions where heat loss to the outside is lower, the extra insulating value of the twinwall may be out weighed by its lower transmitance.
Gary November 20, 2011