Abstract
Significance
Traditionally, the cooling for passive vapor condensation is mainly provided by convection and conduction. However, these mechanisms do not work when the vapor temperature is near the ambient air temperature. At such temperatures, radiative condensation remains effective. However, existing radiative vapor condensers must operate during the nighttime. Here, a novel radiative-cooling technique is applied to extend radiative condensation to both daytime and nighttime. The technique uses engineered thermal radiators that do not require lithography or other costly fabrication techniques and can be manufactured to cover large areas. When combined with traditional convective and conductive condensers, this passive technology can potentially double the water production rate for passive water technologies.
A radiative vapor condenser sheds heat in the form of infrared radiation and cools itself to below the ambient air temperature to produce liquid water from vapor. This effect has been known for centuries, and is exploited by some insects to survive in dry deserts. Humans have also been using radiative condensation for dew collection. However, all existing radiative vapor condensers must operate during the nighttime. Here, we develop daytime radiative condensers that continue to operate 24 h a day. These daytime radiative condensers can produce water from vapor under direct sunlight, without active consumption of energy. Combined with traditional passive cooling via convection and conduction, radiative cooling can substantially increase the performance of passive vapor condensation, which can be used for passive water extraction and purification technologies.