Imagine cooling a building on a hot day using little to no electricity. That’s exactly what a team at Stanford University is aiming for. The team, led by professor Shanhui Fan, has designed a photonic structure that could enable passive cooling during the hottest hours of the day. Using a technique called “daytime radiative cooling,” a photonic structure emits its heat as thermal electromagnetic waves to the cold of outer space, while simultaneously and strongly reflecting sunlight. The device exploits an atmospheric transparency window that exists between 8 and 13 µm that allows thermal waves to escape to outer space, effectively using it as a heat sink, the researchers said. In doing so, the structure can rest at an equilibrium temperature substantially below the ambient, providing a passively maintained cold surface. The design of a photonic structure enabling high-performance daytime radiative cooling uses multiple materials and layers and a photonic crystal. Courtesy Stanford University. Specifically, the structure design consists of two thermally emitting photonic-crystal layers of silicon carbide (SiC, with a thickness of 8 µm) and quartz (2.5 µm thick), with squares (5.4 µm width) etched in the layers with 6-µm periodicity that sit on top of a broadband solar reflector. They consist of three sets of five bilayers of magnesium fluoride and titanium dioxide, and have varying periods on a silver substrate to form the cooler. The 2-D, two-layer photonic crystal uses phonon-polariton modes to maximize emissivity in the atmospheric transparency window, while the chirped 1-D photonic crystal reflector below it minimizes absorbed solar radiation. The use of dielectric, rather than metallic, substrates allows for minimal solar absorption, the researchers said, while the 2-D photonic crystals atop the reflector emit selectively and strongly in the atmospheric window. This design has been shown numerically to reach an equilibrium temperature more than 30 degrees below the ambient air temperature, and to achieve a cooling power of 100 W/m2 at ambient. For more information, visit www.stanford.edu.