The design is simple: a glazed earthen ware pot nestled inside a larger, porous one with a layer of wet sand separating them. As the water evaporates through the surface of the outer pot, it draws heat from the inner one, keeping up to 12kg food fresh for as long as three to four weeks without using a single watt of electricity.
source : http://gizmodo.com/5935104/how-to-make-an-electricity+free-refrigerator
