Corrected radiative cooling power measured by equivalent dissipative thermal reservoir method

Abstract

A radiative cooler is an optical device which can spontaneously preserve its temperature below ambient by simultaneously reflecting solar radiation and emitting thermal radiation through the atmospheric window lying within 8–13 µm of the electromagnetic spectrum. In the radiative cooling process, the universe acts as the ultimate heat sink for heat dissipation. Cooling power is a key indicator of the radiative cooling performance, which can reach a typical value of 100 W/m2 under a clear sky. However, atmospheric transparency, which is usually hard to determine, affects the cooling power heavily. To fairly evaluate the cooling power under prescribed and controlled environmental conditions, we built an in-lab testing facility based on a hybrid refrigerative thermoelectric cooling system, which can artificially simulate the equivalent radiative cooling effect. We characterized the system performance and preserved the temperatures of the equivalent thermal reservoir at -19.9°C and -12.7°C steadily at ambient temperatures of 25°C and 45°C respectively. Under a reference atmospheric transmission spectrum featuring a high 8–13 µm transparency, we measure the cooling powers of a representative radiative cooler and compare them with the theoretical values for ambient temperatures between 25°C and 45°C. Measured cooling powers range from 108 W/m2 to 141 W/m2, which exceed the theoretical values by 17 - 33% due to excessive heat flow through the testing cavity and heat lost to the surroundings. The method can be extended to examine thermal and energy associated performances of derivative radiative cooling devices and systems.