Simulations of micro-sphere/shell 2D silica photonic crystals for radiative cooling

Daytime radiative cooling has recently become an attractive passive approach to address the global energy demand associated with modern technologies, currently accounting for about 15 % of the worldwide energy consumption. By engineering surface properties to maximise the natural blackbody emission to radiate thermal energy in the spectral transparency window of the atmosphere (8 – 13 μm), it has been shown that thermal radiation can be transferred to outer space, resulting in surface cooling, without an external electrical input. One technique used is based on surface phonon-polaritons, i.e., thermally excited surface waves in polar dielectric materials which can be outcoupled into free space. Here, we theoretically investigate new surface morphologies in the form of silica micro-sphere and micro-shell photonic crystals (PCs) using rigorous coupled-wave analysis to achieve cooling of over 73 K below-ambient temperature. Additionally, the effect of impurities in silica is explored by simulating soda-lime glass micro-shells, which in turn, exhibit radiative cooling of 61 K below-ambient temperature.