WorldChanging: Another World Is Here: Un-Electric Fridge reports on Pot-in-Pot, the Rolex award winner:
The device owes its cooling powers to a simple law of thermodynamics. When moisture comes into contact with dry air, it evaporates, causing an immediate drop in temperature. When the water in the sand between the two pots evaporates, the inner pot is kept cool, preserving the goods inside.
Although this invention is new to Nigeria, it seems that the Quakers, Amish, et al. have been using similar “technology” for quite some time (only using wooden boxes rather than clay pots).