The problem, put simply, is that plastic solar thermal panels, if built like traditional solar thermal panels, cannot turn themselves off if they start to overheat. You cant just paint a plastic panel black and slap some glazing on it. If you stop removing the heat from the panel, and the panel is built from plastic, it will soften or melt eventually. This was recognized by Rob in his instructable:
"because the whole collector is made of plastic, it is important that the temperature doesn’t get too high or it will soften and possibly spring a leak. 80 degrees C (176 degrees F) is about the limit. Don’t think it can get that hot? Think again. ...Therefore this may not be a practical design for residential installation"
The problem is figuring out how to make a solar panel turn off. If you can figure this out, then solar thermal panels could be built of plastic and the cost reduced to a point where it is affordable to everybody. (i.e. $1000 per panel to $200) Why is this important? Because heating our homes (air and water) accounts for the majority of energy we consume in our homes. If solar thermal panels were cheap, we could heat our house with them and save billions every year.
So this is where I am coming from. I want to find a way to build a solar thermal panel from commonly available materials and have it compete with commercial systems. The design I have come up with may be the answer, or it may not, but if the instructable community would work on this problem with me, the benfits to our society could be huge.
check out the blog I created for this idea for more info and for updates as I have them.