passive solar heater1
One of Roe Snyder's passive solar heaters. The black cylinders are aluminum soda cans filled with sand and painted black. The cans provide a substantial amount of thermal mass, such that they can radiate their stored heat for hours after the sun goes down.
This device has openings and a fan in the rear that pulls in air, pushes it over the cans, and forces it back out (after being heated by the cans).
Mr. Snyder said this heater would approach 300F internal temperature after sitting in the sun all day, and then comfortably heat approx 1000 sq. ft. of interior space all night long.