Droplet effect on the infrared transmittance of radiative cooler for direct water condensation

Accessibility to freshwater has become a global issue and many approaches have been developed to tackle this question. One promising way is using radiative coolers to condense dew from the air. This study aims to quantify systematically the effects of droplet diameters, spacial distribution, projected surface coverage ratio, and surface wettabilities on the normal-hemispherical transmittance and cooling performance. The Monte Carlo ray tracing method is employed to simulate the infrared light transfer through the droplets. For the first time, the normal-hemispherical transmittance and cooling performance of radiative cooler with different droplet contact angles were investigated theoretically. The weighted transmittance is found to be independent of droplet spacial distribution, but dependent on droplet (i)diameter, (ii)projected surface coverage ratio, and (iii)contact angle. Results show that the water condensed on the emitting surface can improve the cooling power. A super-hydrophilic PDMS film with a 90% projected surface coverage ratio can reach a cooling power of 109.3 W m⁻². In the process of water condensation, a cooling power loss of up to 23.4 W m⁻² will appear due to surface wettability, so super-hydrophilic surfaces are preferred. These results provide guidance on surface wettability preference for better performance of radiative cooler for direct water condensation.