Chris Gupta of New Media Explorer reports:
This is Mohammed Bah Abba's Pot-in-pot invention. In northern Nigeria, where Mohammed is from, over 90% of the villages have no electricity. His invention, which he won a Rolex Award for (and $100,000), is a refrigerator than runs without electricity.
Here's how it works. You take a smaller pot and put it inside a larger pot. Fill the space in between them with wet sand, and cover the top with a wet cloth. When the water evaporates, it pulls the heat out with it, making the inside cold. It's a natural, cheap, easy-to-make refrigerator.
"The Coolgardie Safe was made of wire mesh, hessian, a wooden frame and had a galvanised iron tray on top. The galvanised iron tray was filled with water. The hessian bag was hung over the side with one of the ends in the tray to soak up the water.
Gradually the hessian bag would get wet. When a breeze came it would go through the wet bag and evaporate the water. This would cool the air and in turn cool the food stored in the safe.
It was usually placed on a veranda where there was a breeze. The Coolgardie safe was a common household item in Australia up to the mid-twentieth century. Safes could be purchased ready-made or fairly easily constructed at home. Some of the metal panel safes are very highly decorated, showing the great creativity of their makers."
Source Wikipedia
I would like to know how much cooler it makes the food. Does it drop the temperature more than 20 degrees?