The scarcity of freshwater is one of the global challenges, which involves billions of the population nowadays. Passive radiative cooling technology, by emitting thermal radiation to the higher sky of lower temperature through the atmospheric windows, provides a green and sustainable solution to sub-ambient cooling. If the moist atmospheric air can be cooled below the dew point temperature, condensation is prone to happen, which can be observed in radiative cooling and radiative condensing. To cope with the water scarcity, it is of great need to ensure the condensation rate is high and stable. A theoretical analysis of how the condensation rate is affected by the environmental factors and the condenser design is thus necessary.

Herein, we propose a theoretical model of radiative cooling...[ Read more ]

The scarcity of freshwater is one of the global challenges, which involves billions of the population nowadays. Passive radiative cooling technology, by emitting thermal radiation to the higher sky of lower temperature through the atmospheric windows, provides a green and sustainable solution to sub-ambient cooling. If the moist atmospheric air can be cooled below the dew point temperature, condensation is prone to happen, which can be observed in radiative cooling and radiative condensing. To cope with the water scarcity, it is of great need to ensure the condensation rate is high and stable. A theoretical analysis of how the condensation rate is affected by the environmental factors and the condenser design is thus necessary.

Herein, we propose a theoretical model of radiative cooling in different climates and radiative condensation in a system with flow rate control. The cooling performance of a radiative cooler is affected by various environmental factors including humidity, aerosol, and cloud. In our model, the parameters, such as ambient temperature and ambient relative humidity at the ground-level atmosphere are correlated to the atmosphere at higher altitudes, which can be used to evaluate how different environmental factors affect the cooling performance of a radiator. In particular, it is found that the vertical variations of both vapor concentration and temperature in the atmosphere have a significant impact on radiative cooling, which is often missed in previous models. Moreover, it is found that aerosol scattering and cloud coverage, which are both closely correlated to the humidity, greatly weaken radiative cooling. Based on these findings, the design strategies of radiative coolers in different climates are explored.

Further, different factors influencing radiative condensation on a cooling surface are studied, including the cooling temperature, cooling power at the stationary temperature, and surface properties. Thereafter, a theoretical model that can predict the behavior of water harvester based on radiative cooling according to the environmental conditions was developed. Parasitic heat management is found to be critical to achieve radiative condensation. In addition, the air flow rate is revealed to be a key factor to affect the stationary temperature of the condensation system and thus affects the condensation performance. The optimal air inflow rate for maximizing water harvesting rate is attained through analysis. This model is verified by the indoor experiments using Peltier coolers in controlled environments. Strategies to design an atmospheric water harvester based on radiative cooling are discussed and the potential of using radiative condensation for atmospheric water harvesting in different seasons and different geographic locations is evaluated and mapped.