Passive radiative cooling using temperature-dependent emissivity can sometimes outperform static emitters

Abstract

In passive sky-facing radiative cooling, wavelength-selective thermal emitters in the atmospheric transparency window of 8-13 $\hat{I}¼$m can reach lower temperatures compared to broadband emitters, but broadband emitters always have higher cooling power when the emitter is warmer than the ambient. Here, we propose a temperature-tunable thermal emitter that switches between a wavelength-selective state -- with high emissivity only in the atmospheric transparency window of 8-13 $\hat{I}¼$m -- and a broadband-emissive state with high emissivity in the 3-25 $\hat{I}¼$m range, thus maintaining high cooling potential across all temperatures. We also propose a realization of such a temperature-tunable emitter using the phase transition of vanadium dioxide (VO${}_2$), which can be tuned to the ambient temperature using a combination of doping and defect engineering.

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