Design and manufacture of a radiative cooler to measure the subambient cooling effect and cooling power.

The obscure theory of passive subambient daytime radiative cooling (PSDRC) was deduced in a more understandable way using an arithmetic formula rather than integro-differential equations. Based on two boundary conditions of the equations, an innovative radiative cooler was successfully developed to qualitatively observe PSDRC phenomena and quantitatively characterize the cooling effect and cooling power of radiative cooling coatings (RC coatings). The remarkable subambient temperature reduction over 4.0 °C was successfully achieved in a completely open environment without minimizing the parasitic conduction and convection from the ambient. Prominent PSDRC phenomena could even be observed in such an open environment on very cloudy days, which generally compromise the RC. A much more prominent subambient cooling depression of 10.0 °C was observed when a wind shield was employed to minimize the convection. With suppression of convection, the subambient daytime cooling effect on cloudy days was even more noticeable than that occurred on clear sunny days. The subambient cooling effect was still very remarkable even on clear sunny days in the winter. The average cooling power measured on a clear sunny day was 154.8 ± 9.7 W/m, corresponding to an average solar irradiance of 680 ± 90 W/m with a peak value of ∼820 W/m. Both the subambient RC effect and the cooling power measured under real weather conditions using the radiative cooler agreed excellently with the theoretical prediction, sufficiently demonstrating the great innovation, validity, and effectiveness of the device.