Thermodynamic Analysis of Vapor Transport and Photomolecular Effect in Interfacial Porous Evaporators and Solar Still Design

Reliable production of potable water for water-scarce communities is a humanitarian challenge due to warming climates. Solar still desalination, using interfacial porous evaporators, is a viable technology for many of these areas due to its low capital costs and high purity of water produced from the vapor distillation process. Recently, many groups have reported evaporation efficiencies of over 100% from solar energy input and have hypothesized that a reduced latent heat of intermediate water inside of interfacial evaporators results in enhanced efficiencies. In this work, we conducted a re-assessment of this hypothesis through a combination of experiments and simulations and found that differences in natural evaporation rates are likely due to experimental error from recessed evaporation surface heights rather than from reduced latent heat or increased surface area from microporosity. The results also show that a reduced latent heat alone cannot increase natural evaporation rates significantly because evaporation rates are intimately tied with vapor kinetics, which depend mostly on the airside properties. Furthermore, we present computational simulations to study water cluster kinetics and break-up from the photomolecular effect: a hypothesis that visible light can interact with the air-water interface and evaporate clusters of water molecules. The validity of these results is illustrated by comparisons of predicted airside temperature profiles with experiments. Finally, we conducted thermodynamic analysis on solar stills and found that simple re-designs taking advantage of environmental characteristics can enhance water production efficiencies.