Heat-localized solar evaporation: Transport processes and applications

Solar evaporation by heat localization has drawn intensive research interest recent years, because this potential desalination technology is free of fossil fuel consumption and carbon dioxide emission. With various materials and innovative structures being explored, many recently-proposed systems could achieve a solar-thermal evaporation efficiency (solar-to-vapor energy efficiency) higher than 90 %. However, the upper limit of the solar-thermal evaporation efficiency is not bounded by 100 %. In this work, the thermodynamic limit of the solar-thermal evaporation efficiency is analyzed, which is much higher than all reported evaporation efficiencies in previous works. There is still much room for further improvement in the evaporation efficiency, which would count on system-level regulation of the energy and substance transport processes. At the same time, taking advantage of the solar-driven transport of energy and substances, some hybrid devices that integrates new functions like power generation into the heat-localized solar evaporators have been emerging. However, the complexity of coupled heat and substance transports have imposed great challenges for optimization of both heat-localized solar evaporation and their hybrid systems. To inspire strategies for improving the performance of heat-localized solar evaporation and their hybrid systems at a system level, this article critically reviews heat-localized solar evaporation from the perspective of energy and substance transport. This review first discusses the energy transport processes including solar-heat absorption, energy conversion in the phase transition, heat dissipation to the ambient, and the corresponding strategies for higher solar energy utilization efficiency. We then discuss the substance transport processes including vapor, water, and salt in those devices, which reveals the importance of several structure parameters including the surface area, thickness and pore diameter of the evaporator. Hybrid applications beyond water desalination are discussed, including electricity generation, solar photocatalysis, and superheated steam generation. Finally, we give directions for further performance improvement of the heat-localized solar evaporation and their extended hybrid systems, as well as unresolved challenges, such as efficient vapor-power co-generation and large-scale vapor condensation for fresh water.