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Radiative cooling is potentially one of the most innovative approaches to reducing energy density in buildings and industry, as well as achieving higher levels of energy efficiency. Several studies have reported the design of spectrally selective layered structures for daytime passive radiative cooling. However, a comprehensive design of such systems requires the spectral behavior of different materials and radiative heat transfer mechanisms to be addressed together. Here, we introduce a design methodology for daytime passive radiative cooling with thin film filters which accounts for the spectral tailoring at the visible and infrared spectrum. The major difference of this method is that it does not require a predefined target ideal emittance. The results show that higher cooling powers are possible compared to the previously reported thin-film structures, which were designed from a purely spectral perspective. The underlying mechanisms of the resulting spectral profiles, which give rise to improved performance, are investigated by wave impedance analysis. Cooling powers up to 100 W / m 2 are obtained with seven layers on Ag. The findings of this study indicate that structures with better performance in terms of cooling powers and temperature reduction rates can be obtained following the procedure discussed.

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Broadband thermal emitters/absorbers are of great interest for thermal management in high-temperature applications including thermophotovoltaics and hypersonics due to the dominance of radiative heat transfer. Several studies have been reported, which benefit from the effectiveness of photonic structures on wave coupling tuned by parametric sweeps and intuition. However, the higher emission/absorption potential of structures with nonintuitive geometries due to increased number of available design parameters has not been explored. To address this need, we have studied gradient-based topology optimization for the design of high-temperature thermal emitters/absorbers. By utilizing the adjoint method, which allows gradient calculation only with two simulations, more complex geometries that exhibit higher emission/absorption in the broadband spectrum are designed. Z r B 2 is chosen as the coating material, which belongs to the family of ultrahigh-temperature ceramics (UHTC), due to its very good thermomechanical properties. Emission/absorption rates reaching up to 85% levels in the broadband spectrum are achieved, which is around 40% levels in film form. Electromagnetic phenomena that give rise to elevated emission/absorption are also analyzed. Our findings demonstrate the potential of effectiveness of adjoint-based topology optimization in the broadband spectrum and high emission of Z r B 2 when patterned, which, to the best of our knowledge, was not previously explored.