Sorbent-coupled radiative cooling and solar heating to improve atmospheric water harvesting

Atmospheric water harvesting (AWH) technology is a promising technology for addressing global water shortages and contributing to social development. Current AWH technologies, including fog collection, dew collection, and sorption-based AWH mostly focus on a single water harvesting mechanism, and this can limit their working conditions and overall performance. In this work, a composite hydrogel with a low phase change enthalpy of water (1695 kJ kg^-1) was coupled with radiative cooling and solar heating to improve passive AWH performance and working conditions. High thermal emittance ε¯_LWIR = 0.98 and solar absorptance α¯_solar = 0.93 were achieved for radiative cooling in the nighttime and solar heating in the daytime. During the night, radiative cooling could improve the water capture rate from 0.242 kg m^-2h^-1 (i.e., only sorbent) to 0.310 kg m^-2h^-1 (i.e., sorbent-coupled radiative cooling) in the outdoor experiment. In the daytime, solar interfacial evaporation improved the water release rate to 1.154 kg m^-2h^-1. Effects of meteorological parameters, such as relative humidity, ambient temperature, and solar intensity were also discussed theoretically and experimentally. It is indicated that the designed passive AWH device can work over a wide range of meteorological parameters. The outdoor all-day experiment indicated that the maximum water harvesting can be 2.04 kg m^-2 in a cycle work. This demonstrates that sorbent-coupled radiative cooling and solar heating provide a potential approach for future solar-driven AWH systems.