d-lab.mit.edu/news-blog/blog/g...refrigerator-mali
Clay pot coolers, also known as zeer pots, have been in use for centuries in many parts of the world and operate on a simple principle of passive evaporation. These storage devices, which operate without electricity, consist of two pots nested together with a layer of sand in between and covered with a wet cloth. As water evaporates from the surfaces of the device, evaporation cools the fruits and vegetables inside the pot.