A solar refrigerator is a self-contained refrigeration unit powered entirely by solar energy and evaporation. It was invented to improve living conditions in the developing world, where electricity is not widely available, and reduce the carbon footprint of the developed world by decreasing the amount of electricity needed to run the appliance.
The simplest solar refrigerators are made up of two cylinders. The outer cylinder can be made of any solid material, such as wood or plastic, with holes drilled into the side. The inner cylinder is solid and made of metal. The gap between the cylinders is filled with a material such as sand or soil that can be soaked with water. As the sun heats this material, the water evaporates. Heat is removed from the inner chamber through evaporation. The unit works by simply re-wetting the insulation material.
More sophisticated solar refrigerators use the compression methods of a traditional refrigerator. A refrigerant, usually water mixed with either lithium bromide or ammonia, is compressed by pressure from the sun's heat. Turning off the heat lowers the pressure, causing that liquid to evaporate into a gas and cool the unit's contents.
The efficiency of a solar refrigerator varies greatly depending on the level of engineering and cost, but the simplest forms can be made of inexpensive, widely available materials, making it a viable solution for the developing world's food-storage needs.