“Refrigerator” that does not consume electricity

  Camels living in hot desert areas keep their bodies cool by sweating; while the fur wicks away sweat, it also provides insulation to prevent the inflow of heat from the environment. The fur also has the function of controlling the rapid loss of water. Without fur, camels would sweat quickly and deplete their body of water, which is a very precious resource for camels.
  Inspired by camels, scientists have developed a material that cools things down without electricity and keeps them cool for days.
  The material consists of two layers of gel, an inner layer of hydrogel and an outer layer of airgel.
  Scientists have long been interested in hydrogels. This polymer material can absorb a large amount of water, and then remove the heat through evaporation, resulting in a cooling effect. But due to the lack of insulation and the inability to control the rate at which water evaporates, this effect cannot last long.
  Recently, scientists at the Massachusetts Institute of Technology, inspired by camels, combined hydrogels with aerogels. Airgel is a lightweight, porous silica material that has been dubbed “the lightest solid in the world”. Its structure is full of tiny pores, and more than 90% of its volume is air, making it both breathable and thermally insulating.
  This creates a double-layer gel, with the hydrogel layer acting like a camel’s sweat glands, allowing moisture to evaporate, providing a cooling effect. The role of the airgel layer is the same as that of a camel’s fur. On the one hand, it provides heat insulation while allowing water vapor to escape to cool down; on the other hand, it slows down the evaporation of water and controls the excessive loss of water to obtain a continuous cooling effect. The resulting gel bilayer was about 10 mm thick.
  The researchers tested it in an environment with controlled humidity and temperature. It is capable of cooling an object down to 7°C below the ambient temperature. At the same time, the addition of the airgel extended the cooling time by five times, up to 250 hours, compared to hydrogel alone.
  The gel bilayer could be used to keep food or medical supplies cool, and could also help cool buildings.
  Since the cooling effect of hydrogel comes from the evaporation of water, once the water evaporates, it will lose its cooling ability, so we need to replenish water regularly when using it.
  In order to reduce the useless loss of water, scientists plan to add a third layer outside the airgel layer as a switch. At certain temperatures, the third layer will allow moisture to evaporate, but when cooler temperatures do not require cooling, it will close, preventing unnecessary moisture loss.