A pot-in-pot refrigerator, clay pot cooler is an evaporative cooling refrigeration device which does not use electricity. It uses a porous outer earthenware pot, lined with wet sand, contains an inner pot (which can be glazed to prevent penetration by the liquid) within which the food is placed – the evaporation of the outer liquid draws heat from the inner pot. The device can be used to cool any substance. This simple technology requires only a flow of relatively dry air and a source of water.
Source: Ultra Facts