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Solar Air Heater
I built one like the angle air entry version on the left. It was loosely based on plans from Mother Earth News for a solar air heater. I decided to build it using a double paned french door, insulated the back and sides with 3/4" extruded polystyrene and stuck it in the window. Air intake on the bottom (opening facing down), exhausts on the upper half of the face. I put it in the window in 45 degree weather. Using a small probe thermometer in the exhaust opening, in about 15 minutes it was over 95 degrees. Another 15 minutes had it over 105 degrees. It was not facing due south but did get full sun through the glass.
The problem with mine would have been easy to fix if I'd tried. While the air was plenty warm, airflow was very disappointing. Since the airflow was so slow, I didn't feel a temperature improvement in my garage. A couple of small 12v fans that could have been powered by a small solar panel (like a car battery maintainer) would have increased flow with a corresponding decrease in temperature. As for question of a black piece on the inside, you absolutely want the black surface area. Want proof positive on that, on a cold sunshining day, go out to a parking lot and put your hands on a black car and a white car. The difference is dramatic.
Finally, be sure to either remove it or cover it up in the summer. My divider panel as two layers of extruded polystyrene and in the middle of this past summer, the polystyrene melted and ruined my collector. I had a lot of fun on the project though.
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