Impact of parasitic heat fluxes on deep sub-ambient radiative coolers under variable pressure

Abstract

Passive daytime radiative cooling has proven to be a promising technology which allows objects to cool down by shedding heat to space, acting as a large thermal sink. However, the end equilibrium temperature of a radiative sky cooler (RSC) is limited by the parasitic heat fluxes. Operating RSC in vacuum suppresses the convective heat fluxes, but realizing commercial deep sub-ambient RSC system is rather challenging due to system complexity and sub-component material constraints. In this work, the behavior of the RSC under varying pressure and sub-component system parameters like thermal conductivity of medium and holder and back reflectors is studied. An analytical model is developed taking these parameters as input to analyze and predict RSC performance. A spectrally selective RSC with silver back reflector and polysilazane polymer as emitter having emissivity of 0.85 in the atmospheric window is fabricated to study its deep sub-ambient cooling performance. Through sequential measurements done under controlled environment using liquid nitrogen as cryogenic heat sink, the influence of individual parasitic heat fluxes on the RSC under variable pressure is comprehensively investigated. A sub-ambient cooling of 10 °C under ambient pressure, and 33 °C at a decreased pressure of 1.63 × 10-3 Pa was achieved. The experimental results are in good agreement with our modeled prediction. Subjected on intended usage and desired cooling performance from the RSC system, a rational design of sub-components and operational pressure regimes can be suggested from this study. We believe that these findings have multiple practical implications for designing future deep sub-ambient RSC systems and explores scopes for optimization.