Thursday, July 02, 2009

Mirror distortion may be due to back lighting

The reflectors of the DIY Solar Heater can be any thin sheet material 2x8 feet in size.

When I started using acrylic plastic mirrors in 2005, they seem to be a perfect candidate: readily available, low cost, lightweight, UV stable, strong and with excellent reflectivity. The mirror surface (a sputter coated aluminum film) is actually on the back surface so that it is well protected from scratches that might result from cleaning. The mirror material that I use is 0.060" thick and it bends nicely into a parabola shape formed by the ribs.

It wasn't long however, before I saw the first of these "marks" you can see in the picture (click to enlarge). The first mark was a curious thing. I just noticed it one morning streaking across a few inches of the otherwise perfect mirror, a bit in from the edge. It seemed to be a discoloration of the plastic or the mirror, I could not tell which and it seemed as if something hot had hit the mirror at an angle. Looking from the bottom, it seemed that the grey material covering the aluminum coating was cracked or crazed in a fine pattern.

I remember writing to a friend at NASA with a picture enclosed asking him if he thought a meteorite had bounced off the mirror? That's what it looked like.

Soon more marks started to appear. Many more.

In 2006 I had only four reflectors in place. Two of them were metal sheets, one was aluminum and the other was galvanized steel. The metal sheets did not show the effect.

This is what the array looked like this spring. Notice that the marks seem to have a common orientation. Interestingly, most of them were on the west side of the reflectors. A few had appeared on the east sides, but only a few.

By themselves the marks were a curiosity but might not have been that bad except that as they got worse, the acrylic sheet started to buckle in the area of the marks, pulling the sheet out of contact with the parabolic shaped ribs. It did not take much of this for the focus to deteriorate and for the heat captured by the collector to fall.

Here is what one of the sheets looked like along the edge and you will see clearly what was starting to happen.

By this time I was on the phone to the vendor Laird Plastics ( sending them pictures and asking what could be going on?

As a footnote, most of the components of the DIY Solar Heater are available from building material stores (like Lowes or Home Depot). When I have used special items, like the plastic mirror or the gear motor, or the solar sensor, I have chosen items from national distributors so that these should be readily available, at least in North America.

The local Laird reps were great. They showed concern and gave me reassurance that: 1) the acrylic was UV stable and was regularly used in glazing (windows) in commercial buildings, so the mirror should be stable also and 2) it was good for outdoor use as it was fully capable of the temperature range. They offered some helpful suggestions also like was the acrylic constrained by the mounting so that there was no room for expansion? I assured them that the metal strip along the edge (furring strip) was springy and that it had some "give" if the sheet was not exactly the right size. They also asking if there was some cushioning against the ribs where they bear on the sheets. I had used foam strips on about half of the reflectors on the edge of the ribs which would otherwise touch the backs of the sheets.

Since most of the reflectors had the problem, that was not the cause. Most of them, that is, except for two, the ones at each end of the array. These two DID NOT have the problem.

The Laird people were great, but they were as puzzled as I was. In 2007 I replaced all the sheets with new ones.

But by this spring however, all of those (except for the end ones) had shown the curious marks and deformation again. I was pretty desperate by this time and this was one of the reasons the project was dragging a bit.

One morning about two months ago I was out waiting for the array to swing east to catch the morning sun. This is what I saw.

The array does take a while to swing the entire 100 degrees from west to east, about 1/2 hour, since I have the RED ROCK sensor turned to a very low duty cycle to conserve battery power. At the time of the picture, the reflectors have not yet pointed at the sun.

Notice in the picture that each reflector throws light on the reflector in front of it, except that is, for the one at the very end! The light is not focussed well since the reflector ahead is far out of the focus, but it is a good blast of concentrated light nonetheless. It also has hot spots if the reflector causing the beam is already somewhat distorted.

I put my hand on the back of one of the reflectors at a hot spot. It felt uncomfortably warm. Could it be that this was the cause of the streaking marks and distortion deterioration?

I'd imagined a few different solutions to this problem. I decided first to replace a number of the mirrors (I had five spares left over from last year's batch) but this time before mounting I painted the entire backs of these new mirrors with bright white acrylic paint. Three coats in all.

I'm hoping that the white paint will dissipate most of the unfocused light (heat) from the adjacent reflector. I've installed these new mirrors with the painted backs about a month ago at the west end of the array and I am watching them closely.

This picture is of one of the new, white-coated mirrors receiving its daily blast of light from the one beside it as the array tracks from west to east in the morning.

So far, none of the new mirrors have shown the marks or deformation.

As a test of my theory, I blasted the back of one of the mirrors that I had removed with a heat gun for quite a few minutes, long enough for it to get too hot to touch. No deformation or marks formed. That was a bit disappointing. Perhaps the effect comes about because of radiant heat as opposed to convection heat? Another experiment to try is to blast the back of a mirror with a magnifying glass aimed at the sun. I will do this shortly.

I'd mentioned other possible solutions. One is to use another type of reflective sheet . Another is to speed up the solar sensor so that the array sweeps from west to east more quickly. Another is to "park" the array in the vertical position, rather than leaving it at the west. In this position, no light would hit the back of any of the reflectors.

However, if the white paint on the backs solves the problem, then none of these will be necessary.

Incidentally, I had left the array parked facing east this winter, thinking that the snow load would be shed more easily if it was off to one side. The east direction was really arbitrary. This may explain the huge predominance of the marks on the west sides of the reflectors. These ends were sticking up in the air receiving the periodic blasts of light on the few days that it was sunny. The array had been parked this way for about six months. This winter I will park them horizontally, for sure.

Thanks for your continued interest in my project.

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