I found this a few years ago (uses only a simple low-tech design, simple materials, and heat from the sun. I don't know how efficient it really is) http://www.

I found this a few years ago (uses only a simple low-tech design, simple materials, and heat from the sun. I don't know how efficient it really is) http://www.gabrielediamanti.com/projects/eliodomestico/ http://www.gabrielediamanti.com/projects/eliodomestico/