A fellow I know locally has two solar collectors (heat) he built himself from a magazine article he read.......... quite a while ago. Not unlike cold frames they're just black sheet metal insulated on the back in an "L" shape covered with glass conected to a thermostatically controled fan that pulls the cold air in. He claims it works very well to heat his shop in the spring and fall and suppliments it the rest of the year. I'll try to stop in and get some pictures this week. I wouldn't mind trying to build one myself.