Accelerating vapor condensation with daytime radiative cooling

Evolution has produced many survival techniques for animals living in extreme arid environments. One of them is the production of water from humid air. Darkling beetles in the Namib desert generate dew from the moisture-rich breeze blown in from the Atlantic. The key to dew generation is the beetle body, which radiatively sheds thermal energy, resulting in cooling that enables the condensation of water. This cooling mechanism has been utilized to develop radiative dew condensers that produce fresh water without consuming energy. However, even the state-of-art radiative dew condenser only works at night and around dawn, before the sun is at full strength, after which the absorption of sunlight swamps the radiative cooling power. Here, we develop daytime radiative condensers that create dew water even in sunlight. We design the thermal radiators to reflect almost all the solar radiation, which has not been previously realized in radiative dew condensers. Compared to state-of-art condensers, our daytime radiative condenser produces double the water over a 24-hour period. The integration of our daytime radiative condenser with solar water-purification systems can potentially increase water production from $0.4 L m^{-2} hour^{-1}$ to more than $1 L m^{-2} hour^{-1}$.