Hi,
Well, I could be wrong about this, but ...
The selective coatings used on some collectors have a high absorbancy in the visible range (so they absorb solar well), but the have a low emissivity in the IR range, so they radiate heat poorly. This is good in that the absorbers lose heat by radiating it in the IR. Its not really that the selectively coated side is running cooler, the idea is actually that it can run hot but not radiate much.
But, The fact that it does not radiate well does not mean that it won't conduct heat to the air well. These are two different forms of heat transfer, and I don't believe that the conduction part is effected by the low emissivity coating. So, (I think) you can run air past the selectively coated side and still get good heat transfer between the air and the absorber.
I think that for any thin metal absorber (say 0.03inch or less) the front and the back are going to be very close to the same temperature -- metal is very conductive.
One reason for not running air between the absorber plate and the glazing is that you will have fast moving air scrubbing against the glazing, and that gives you a lot of heat loss out the glazing. The traditional solid metal absorber air heating collectors nearly all used the backpass design with all the airflow behind the absorber. The air passage is shallow so that the airflow behind the absorber is fast and turbulent, and this gives good heat transfer.
Gary
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