A thin-film coating has the potential to cool buildings by simultaneously radiating heat from inside and reflecting sunlight. The photonic radiative cooling material reflects incoming sunlight (the sun’s reflection seen on the panel) and sends heat from inside the structure directly into space as infrared radiation (reddish rays). The blue areas on the roof show the cooling effect. Courtesy of Nicolle R. Fuller/Sayo-Art LLC. Developed by a team at Stanford University, the 1.8-µm-thick photonic radiative cooling film is made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. The coating is a highly efficient mirror, preventing 97 percent of sunlight from striking the building and heating it up. It is also tuned to radiate at an IR frequency that can pass through the atmosphere without warming the air. “Every object that produces heat has to dump that heat into a heat sink,” said professor Dr. Shanhui Fan. “What we've done is to create a way that should allow us to use the coldness of the universe as a heat sink during the day.” Ideally the coating will lead to buildings that require less air conditioning, reducing demand for electricity. “Photonic radiative cooling makes off-grid cooling a possibility in rural regions, in addition to meeting skyrocketing demand for air conditioning in urban areas,” said research associate Dr. Aaswath Raman. A prototype radiator, which is about the size of a pizza, is nearly 9 °F cooler than the surrounding air during the day. The researchers said the process to make their film is scalable. However, as of now there is no system for conducting the heat from inside a building to the film so that it can be radiated away. The research was published in Nature (doi: 10.1038/nature13883). For more information, visit www.stanford.edu.