Role of Longwave Radiation in Solar Evaporation

Solar evaporation systems are typically evaluated based on solar energy, yet they also receive substantial energy input from longwave radiation (LWR), which varies with environmental conditions. This study demonstrates that LWR or atmospheric heat contributes significantly to the evaporation process. Controlled experiments under 1-sun illumination and dark conditions indicate that LWR measurably enhances vapor flux, and the amount of LWR absorbed depends on the design configuration. Unlike photovoltaics, which rely solely on solar energy, solar evaporators benefit from this additional energy source, increasing the total energy available for evaporation. We show that accounting for LWR can resolve instances of reported evaporation efficiencies exceeding 100% even when using the standard temperature-dependent latent heat of vaporization. By properly including LWR as an input in the energy balance, the calculated efficiency falls below the thermal limit, providing a more accurate benchmark for evaluating solar evaporation technologies. Addressing the role of LWR is essential for advancing solar thermal system performance standards.