Passive all-day freshwater harvesting through a transparent radiative cooling film

The freshwater shortage problem is a global problem that has raised great concerns. However, most solutions involve heavy energy consumption. Solar desalination and radiative cooling are two ways to gain freshwater without energy consumption. This study aims to design a transparent radiative cooling film by combining these two methods to achieve efficient all-day freshwater harvesting. The effects of doping particles, film thickness, and water droplets on the radiation transfer are quantified systematically through the combination of the Mie theory, Monte Carlo method, and Monte Carlo ray tracing method. For the first time, an evaluation method is established to assess the performance of solar transmissivity and radiative cooling simultaneously. The solar transmissivity and the ”sky window” emissivity of the polymer films (polymethyl methacrylate, PMMA and polydimethylsiloxane, PDMS) doped with silica (SiO2) microspheres are investigated numerically. The optimal transparent radiative cooling film can reach a solar transmissivity of 92.8% and a ”sky window” emissivity of 95.6%. Also, the effects of condensate droplets on the film at both day and night on the solar transmissivity and the ”sky window” emissivity are calculated. The results reveal that the super-hydrophilic surfaces have the highest solar transmissivity and ”sky window” emissivity among all the surfaces covered with droplets. This study provides guidance for designing high-efficiency transparent radiative cooling covers for all-day passive freshwater harvesting devices.