(196a) Nanoporous Materials for Sub-Ambient Radiative Cooling

Passive cooling can be achieved using materials that selectively radiate heat to Space through the atmospheric transparency band. This solid-state approach shows promise in decreasing the electrical demand required in applications such as air-conditioning. Cooling at sub-ambient temperatures is achieved when solar and atmospheric heat is blocked from reaching the cold emitter radiating to Space via the 8-13 µm atmospheric window. Here, we investigate the design of infrared-transparent thermally-insulating porous covers for deep sub-ambient radiative cooling. Whereas surface-based designs rely on selective emission, heat transfer with Space is enabled by a selectively transparent cover in our approach. To understand radiative and thermal transport in the porous cover, we conduct a systematic study on the effects of structural parameters and material composition on the scattering, absorption, and thermal conductivity. The nanostructure and material composition can be tailored to define regions of short-wavelength scattering and long-wavelength absorption to block solar and atmospheric heating. The combination of selective and high 8-13 µm transparency with low thermal conductivity enables high-performance sub-ambient radiative cooling that can potentially reach temperatures 35 K below the ambient.