As an update to last week's post about the problem with the marking and deforming of the plastic mirrors, I am now even more certain that I have uncovered the cause of the problem.
I had been experiencing deterioration of the plastic mirror over time in the form of linear marks that appeared in the mirror and caused it to mark and then warp.
I'd shown a picture (similar to the one above) of the backs of the reflectors "lighting up" with the partially focussed light as occurs before the reflectors are properly tracking the sun or when they are left stationary and the sun passes overhead. When they are properly tracking of course, virtually all of the sun's light is focussed on the collector and virtually none of the light spills. If they are parked horizontally, no light is reflected onto the adjacent mirror.
Here is another picture of a badly deformed section of a mirror. It is very difficult to take a picture of a mirror, but I hope you will see that there are linear marks that severely distort the surface of the mirror.
I mentioned that I had left the array over the winter with the reflectors pointing at the western sky, thinking that this would allow the accumulated snow to fall off the reflectors thus reducing the heavy load of snow. That turns out to have not been that effective. Better to leave them horizontal since they seem able to take the load.
Also, on a sunny winter day the reflected light of the sun will move across the back surface of each mirror that has one beside it. The end reflectors did not show this effect and that was really the clue that I needed.
I mentioned that I have installed five new mirrors in the array after painting their backs with bright white paint. It has been over a month and none of these new mirrors have shown this effect.
Today, as a further test of the theory, I focussed the light from a magnifying glass in a clear undeformed section on the back of one of the mirrors that I had removed. It was a large magnifying glass. I tried not to focus the light right down to the smallest concentration that was possible, but rather, I tried to get about a 15:1 concentration (a disc of light about 3/4 of an inch across from a magnifying glass about 3" in diameter) to approximate the back-lighting that I thought that the mirrors might be receiving.
It didn't take more than a few seconds to hear a sort of a crackling sound. When I pulled the magnifying glass away and took off my protective eye glasses, I could not see anything unusual on the back side of the mirror. But looking at the other side, there was a mark that looked very much like the problem I have been fighting!
Here's picture of the damage I caused. See what you think.
What looks like double exposure is the dust on the front surface of the acrylic and it's reflection from the back surface mirror.
My hand was not steady enough to make a track like the sun does as it slowly moves across the sky, but the appearance of the spots that I made looks to me very much like smaller versions of the marks that I have seen far too many of.
With a solution in hand (painting the backs white) I am confident of the durability of the acrylic mirror.
Once again, this is not the only reflective material that can be used in the DIY Solar Heater, but it is the cheapest and easiest to use for this application.