Researchers from Stanford University in California have developed an optical surface for “radiative sky cooling” that could significantly reduce energy needed for air conditioning and refrigeration. The system cools water that flows through pipes connected to an air-conditioning system by radiating excess heat from water into cold space. The panels are covered in a mirror-like surfaces, which reflects about 97 percent of the sunlight while emitting the surface’s thermal energy through the atmosphere. In testing the panels could reduce the water temperature 3 to 5 degrees Celsius below ambient air temperature. The researchers modeled the cooling system of a two-story commercial building in Las Vegas-like climate with the cooling system. The model estimates it would save 14.3 megawatt-hours of electricity in the summer months, which is a 21 percent reduction in electricity use for cooling. The system would also reduce the water loss in cooling systems using evaporative cooling. Cooling systems consume 15 percent of electricity generated globally and account for 10 percent of global greenhouse gas emissions. The researchers have founded a company to further testing and commercializing the technology.