The Solar Sponge

The solar sponge is an interesting idea. It is a very cost effective solar air heater that can be mounted on your roof. It is simpler than the models using cans and looks like it can be produced for much less money.

http://www.solarsponge.com/ for more info.

Don't forget to check out the graphs showing the solar sponge's performance. It seems that the already efficient solar sponge could have its performance increased.