Optical and thermal filtering nanoporous materials for sub-ambient radiative cooling

Emitting heat to Space is a passive, dry, and solid-state cooling approach with potential applications in thermal management of buildings, solar cells, and vehicles. Emitting in the 8-13 mu m atmospheric window while preventing heat from reaching the cold emitter is important to the performance. Here, we investigate the design of infrared-transparent thermally-insulating materials for radiative cooling at deep sub-ambient temperatures. Unlike surface-based designs that rely on selective emission, selective radiative heat transfer with Space is achieved via selective transmission through an insulating cover. We model the radiative and thermal transport characteristics of several nanoporous materials that are transparent in the 8-13 mu m range. We also study the effects of physical morphology and material composition on the scattering and absorption properties of the insulating cover, and its radiative cooling performance. By tailoring the nanostructure and material composition, one can define regions for short-wavelength scattering and long-wavelength absorption, and in turn, block solar and atmospheric heat. The combination of low thermal conductivity and selectively high transparency in the atmospheric window enable high-performance radiative cooling down to 35 K below ambient temperature.