In search of the ideal radiative cooling material: the promise of porous ceramics*

Passive radiative cooling (PRC) of terrestrial objects is achieved by radiative heat loss into space through the long wavelength infrared (LWIR) atmospheric transmission window. Due to its passive nature and net cooling effect, it is a sustainable way to cool human environments. A major goal of radiative cooling research is to create designs with near-ideal spectral properties – i.e. selective emittance in the LWIR (λ~8-13 μm), and perfect reflectance elsewhere in the solar-thermal wavelengths (λ~0.2-40 μm). However, most PRC designs fall short in this regard, or else need multiple materials and complex architectures.

We propose porous ceramics with a smooth glazing as selectively LWIR emissive radiative coolers with near-ideal optical performance. The optical selectivity, spanning the thermal wavelengths, arises from a combination of the structural and intrinsic properties of the bilayer design. We theoretically show how this can lead to near-ideal spectral properties, and demonstrate real examples. Along with their robustness and resistance to weathering, this makes porous ceramics near-ideal materials for radiative cooling.