Researchers at Saudi Arabia’s King Abdullah University of Science and Technology (KAUST) are working on a cooling system that requires zero electricity, something that would be highly valuable in many parts of the world that lack infrastructure but get a lot of sunlight.

The team’s strategy involves taking advantage of a natural “phase-change” phenomenon, where energy is absorbed as salt crystals dissolve in water. Simply put, this means that adding salt to warm water cools the water quickly as the salt dissolves.

The salt with the best cooling powers, according to the team, is ammonium nitrate. This type of salt is highly water-soluble, and its cooling powers are four times better than that of the next-best salt, ammonium chloride. Another advantage is that ammonium nitrate is quite affordable and is already widely used in fertilizer, making it quite accessible.

On top of being used to cool buildings, the system could be implemented in food refrigeration. In lab tests, water with added ammonium nitrate was placed in a metal cup, which was then placed inside a sealed polystyrene foam box. The dropping temperature of the cup also took the room temperature down within 20 minutes. The temperature went down from 25 degrees Celsius (77 degrees Fahrenheit) to 3.6 degrees Celsius (38 degrees Fahrenheit) and stayed below 15 degrees Celsius (59 degrees Fahrenheit) for over 15 hours.

The system is also circular. Once the salt dissolves, solar heat can be employed to evaporate the water, which leaves behind salt in the form of crystals which could then be gathered and reused in the cooling system.

Source study: Energy & Environmental Science—Conversion and storage of solar energy for cooling