Interesting...black painted cardboard for solar window heater. This simple device a traditional window & turnd it into a simple sun powered heater. Materials: Large piece of corrugated cardboard, Black paint, Paintbrush, Knife or scissors. South-facing window (east or west is okay but not as good). Blogger says zzz'air flowing in the bottom at 63 F and back out the top at a little over 100 F. Pretty hot for a piece of painted cardboard. The temperature outside is about 35 F."
How to Make a Passive Solar Water Heater. Following these directions you can build an inexpensive water heater that will heat water to 150 degrees.