Sustainable access to fresh water has been recognized as one of the grand challenges for engineering in the 21^st^century. Traditional technologies, such as distillation, reverse osmosis, and waste water recycling, are energy-consuming and expensive.  It is therefore difficult to widely apply them especially in developing countries where the crisis to access fresh drinking water is the most serious. Recent passive techniques utilizing solar energy, on the other hand, are constrained either by the exotic and expensive water absorbing material or by the bulky solar concentrating components.

Dew-harvesting technology utilizes the ultracold outer space to radiatively cool a surface below the dew point and condenses the water vapor from the atmosphere. This passive technology holds great potential for fresh water harvesting due to the fact that a significant amount of water vapor is stored in the atmosphere.  The theoretical analysis on dew-harvesting has a long history. Following the pioneering work by Lewis on the evaporation of a liquid into a gas, Hofmann developed an analogous mathematical framework to analyze the heat and mass transfer in the process of the dew formation.  Since then, several theoretical works have been conducted to quantitatively explore the dew-harvesting.  However, none of these works takes the spectral atmospheric transmittance into account, and very few of them considers the spectral emissivity of the condenser. As a result, the fundamental limits of this technique have not been properly clarified, making it hard to evaluate the performance of the experiments and to determine whether or not this technology is applicable under various conditions, in particular in relatively arid areas.

In this paper, building upon the recent developments of the radiative cooling technology, we develop a theoretical framework to analyze the nighttime dew-harvesting mass flux (*m''*) of a blackbody and a selective emitter in a wide range of parameters including the ambient temperature (*T~ambient~*), the relative humidity (RH), and the convection coefficient (*h*). This analysis should serve as the upper bound for daytime dew-harvesting under corresponding conditions.  We point out that the selective emitter surpasses its blackbody counterpart, in particular under high *T~ambient~*in relative arid areas, where the needs of the potable water are more demanding but impossible to be met using a blackbody emitter.  For example, at *T~ambien~*~t ~= 20℃with RH = 40%, a blackbody emitter fails in harvesting water by any means; in contrast, the selective emitter could have a *m''* of 13 gm^-2^hr^-1^. To realize the potentials of a selective emitter, we design a simple multilayer stack to demonstrate its dew-harvesting abilities.  We also quantify the bifunction of the convection, supplying water vapor vs. weakening the performance of radiative cooling, and propose a setup to adjust *h*, for the purpose of maximizing *m''*.


Fundamental Limits of the Dew-Harvesting Technology

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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