hot side of single absorb hot air collector?
 

In a single non perforated absorber layer/plate with air space in front and back will one side get hotter than other? Assume double glazed front/insulated back. In other words which is the best side to get hot air from?

The side that is receiving light will get hotter if it's just a black paint, but some solar thermal plates are coated with a coating that is not designed to re-radiate heat, they tend to be blue in color, if that is the case then the back is the side that you want to collect from.

So a DIY panel with a low tech black spray paint should heat up on the glazing side well?

If the plate is roll corrugated aluminum or similar, painted flat black on both faces then as aluminum is a good thermal conductor both sides will be equally hot. Thus allow the air to flow over both sides for max thermal efficiency in heating the air.

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