I ran a quick experiment using different reflective materials and how they may improve the solar heat gain of a collector.
The reflective surfaces tested were:
Nothing-Have to have a baseline, ya know…
Bright satin white-to best emulate snow
Reflective mylar-from a thin emergency space blanket
A glass mirror
While others, including mclark, and Kevin, have tested reflectors by setting a them in front of a collector versus no reflector, I wanted to test the potential of different reflective surfaces that are likely to be used as reflectors. This test allows me to see just how much potential each surface has compared to others at the same time. It would not be practical to test this way using full sized reflectors in front of identical collectors. So...
I did the next best thing. On two different days with different weather conditions, I placed a sheets of flashing vertically with a assorted reflectors flat on a table in front of it. I allowed the setup to sit undisturbed for 30 minutes before testing.
Black Surface—10˚F, Hazy Sun
White-83.8+36.7% Temp increase
Brown Surface—40˚F, Clear Skies
White-110.6+10.2% Temp increase
Please note that in no way expect a reflector to suddenly increase my performance by the percentages listed above. But as mclark noted when he laid out some mylar in front of his collector, a 40% increase is certainly possible.
A couple of items of note are that the mylar film I used was about as thin as you can get, and thus, was unable to created a smooth, consistent reflection. So measuring the reflected surface was less consistent than the mirror. There were very inconsistent reflections. I think the mylar, if smooth, will perform as well as a mirror, at a fraction of the cost and weight. And mylar will likely need to be replaced every few years. Thicker mylar will last longer, and would be easier to create a mirror-like surface as long as the substrate was smooth.
The last thing that stands out to me is the mylar/mirror comparison is on a sunny day versus a hazy sun day. On a sunny day the mirror clearly outperforms the mylar. But on a hazy day, the results were very close. The hazy sky made the visible reflections somewhat hard to see. And this may have negated some of the disadvantage of the inconsistent mylar reflection. And while we have a few sunny days in Winter, there are likely to be a lot more partly cloudy days. So mylar may win out in less than clear, sunny conditions.
Any reflector is better than no reflector. It can allow you to either build a smaller collector for your heat needs, or increase the performance of existing collectors at a fraction of the cost of building a larger collector. Even the cheapest mirror costs about $1.25 per sf. A decent 2 mil mylar can cost less than $.10 per sf.
Greg in MN