Infrared-Reflective Transparent Hyperbolic Metamaterials for Use in Radiative Cooling Windows

Passive multilayer coatings for windows have potential to improve energy consumption for indoor temperature regulation. The coatings should block the solar IR energy (800-2500 nm) while maintaining visible light transparency (400-700 nm) to prevent unwanted heating of the interior of a building or a vehicle. It should also efficiently radiate thermal energy to prevent excessive heating. Although solar energy management and radiative cooling techniques have been investigated individually, the combination of the two, a transparent radiative cooler, has emerged only recently. This study theoretically and experimentally demonstrates a transparent radiative cooling window using a combination of planar hyperbolic metamaterials and a uniform layer of polydimethylsiloxane, resulting in high visible transparency (>60%), IR reflectivity (>89%), and thermal emissivity (>95%). Daytime temperature experiments confirm that the cooling window efficiently lowers the interior temperature by as much as 7 degrees C.