Radiative Cooling And Its Enhancement For Dew Harvesting In Extreme Climates

Sun,as a hot source,is one of the most important thermodynamic resources for human.The hot sink and resource are equally important from the viewpoint of thermodynamic,so it also has great potential to harvest energy from outer space with the temperature of-270℃.The atmosphere has the transparent nature between 8 to 13 micros,which is called atmospheric window.The radiative cooling technology radiates heat from the Earth surface into outer space through the atmospheric window for passively cooling.As an extension of radiative cooling,dew harvesting technology utilizes the ultracold outer space to radiatively cool a surface below the dew point and condensates the water vapor from the atmosphere.This passive technology holds great potential for freshwater harvesting as a significant amount of water vapor is stored in the atmosphere.So far,most of the existing experiments were conducted under low humidity.The radiative cooling in hot and humid climates has not been quantitatively clarified and experimentally explored.Analogous to concentrated solar power technology,concentrated radiative cooling can amplify both the cooling power and temperature reduction.However,a widely used theoretical model to analyze such a system has neglected a fundamental constraint from reciprocity,which leads to an overestimate of concentrating effect,and unclarified optimization and application for the system.The existing theoretical model of dew harvesting technology does not take spectral-directional emissivity into account,or neglects double effects of water vapor on radiation transfer obstruction and condensation supply.As a result,the fundamental limits of this technology have not been properly clarified.Moreover,lacking of the optimization for selective emitter with dew harvesting purposes,this technology can only harvest fresh water at the humidity of higher than 50%at present.For the discussion above,this paper starts a series of theoretical analyses and experimental verifications regarding radiative cooling and dew harvesting.The radiative cooling performance was analyzed under the ambient temperature widely ranging from 0-40℃and the relative humidity from 0-100%.Our analysis reveals an interesting crossover:for higher(lower)humidity,lower(higher)ambient temperature results in better cooling performance.Experimentally,we explored the radiative cooling at high humidity of53%-100%,and achieved a temperature reduction of 5℃below ambient under ambient temperature of 29℃and humidity of 100%.We pointed out the key mistake of Smith’s traditional theory of concentrated radiative cooling,and then developed a framework addressing the shortcoming.Modelling suggests the optimized shape and geometric dimensions of concentrators,as well as the limiting cooling performance.Experiments verified our model and the optimization for concentrators.Using the optimized concentrator,we experimentally amplify radiative cooling by 26%.Building upon radiative cooling model that takes spectral-directional emitter emissivity and spectral-directional atmospheric transmittance into account,and water vapor effects for radiation transfer obstruction and condensation supply,we developed a theoretical model coupling heat and mass transfer for dew harvesting in a wide ranging of parameters including ambient temperature,humidity,convection,cloud,and emitter inclination,and then calculate fundamental limits of the dew harvesting technology.Our analyses reveal an optimal convection for the highest dew harvesting rate.We designed a dew harvesting setup with a controllable air convection to permanently keep optimal convection,as well as the highest dew harvesting rate.We provide an approach to optimize emitter spectra,which produces dew of15 g/(m~2-hour)even at humidity of 40%,under which condition the blackbody emitter cannot harvest any dew.A selective emitter,consisting of six layers,was designed for real-world dew-harvesting purposes,which can also harvest fresh water theoretically at humidity of 40%.