Highly Efficient Solar Vapor Generation Based On Heat Localization

With increasing of population and the rapid development of industry,the shortage of traditional energy and demands for freshwater is surging.The environmental pollution caused by traditional energy and the scarcity of freshwater resources threaten the survival of hundreds of millions of people around the world.In recent years,the field of solar vapor generation based on heat localization has undoubtedly become a research hotspot of science and technology,and will provide huge technological opportunities for improving the environment and human life.The design of materials/structures for solar vapor generation based on heat localization is a multidisciplinary study of optics,thermodynamics and fluid science.It not only solves the basic problems of solar thermal energy conversion,but also has broad application prospects in water distillation,seawater desalination and sewage purification.Based on this research background,from the aspect of material/structure design,this paper focused on the following three parts:（1）The influence of two-dimensional Janus structure on water transport,and the performance of Janus structure based solar vapor generation has been studied.Starting from water transport,the two-dimensional Janus structure with surface energy gradient is constructed by taking hydrophilic fabric as the substrate and unilaterally loading hydrophobic polymer nanoparticles.Polystyrene foam is selected as the support material and thermal insulation design,so that the two-dimensional Janus structure floats on the water surface and realizes heat localization.By constructing different surface energy gradients,reasonably controlling the water transport rate,balancing the water supply and evaporation rate,effectively reducing the heat loss,and achieving the water evaporation rate of 1.33 kg m^-2 h^-1 under 1 sun,the efficient solar vapor generation is realized.This work has great inspiration for the design of the solar vapor generation system in the aspect of the material selection of the absorber and the regulation of water transport in the future.（2）The influence of three-dimensional Janus bionic leaf structure on the water cluster and the diffusion of water vapor has been studied,and its performance of solar vapor conversion has been explored.In order to improv solar absorption and accelerate water vapor diffusion,a three-dimensional Janus bionic leaf structure was constructed with hydrophilic sponge structure as the substrate and hydrophobic porous membrane as the surface.Inspired by the transpiration of the leaves of the tropical rain forest,the bionic leaf structure was first used in the field of solar vapor generation.Hydrophilic sponge is selected as mesophyll tissue to realize continuous water transport,change the state of water cluster and reduce the enthalpy of evaporation.Photothermal polymers were selected as absorbers to mimic chloroplast function.Hydrophobic porous membrane is selected to imitate the stoma to accelerate the diffusion of water vapor.The influence of hydrophobic porous membrane with different pore size distribution on the diffusion rate of water vapor was explored,and the optimal structure design was obtained.The water evaporation rate reached 3.09 kg m^-2 h^-1 under 1 sun.This work has great inspiration for the design of the future solar thermal steam conversion system in terms of material/structure selection and accelerating water vapor diffusion.（3）The effect of three-dimensional sandwich structure on anti-salt blockage in long-term solar desalination has been studied,and its performance of solar vapor generation and seawater desalination has been explored.In order to solve the problem of salt accumulation and subsequent deterioration of evaporation performance in the process of solar desalination,focusing on avoiding salt crystallization,a three-dimensional sandwich structure was constructed by hydrophobic modification on the top and bottom of the hydrophilic sponge to avoid salt crystallization.The bottom hydrophobic/middle hydrophilic design of this sandwich structure provides a water/salt ion channel,and the middle hydrophilic/top hydrophobic design prevents salt crystals from separating out of the absorber surface during long-term operation.The photothermal polymer is selected as the absorber,and the hydrophilic sponge is used as the substrate to achieve continuous water transport while reducing the enthalpy of evaporation of water inside the sponge.The water evaporation rate reaches 2.93 kg m^-2 h^-1 under 1 sun.In addition,a portable solar desalination device weighing less than 100 g is designed,and the desalination rate reaches 0.52 kg m^-2 h^-1 when the solar radiation is below 0.4 k W m^-2.This work provides a solution for the water demand in the field survival,individual combat and other scenarios,and has certain inspiration for the hydrophobic/hydrophilic design of the future solar seawater system in salt resistant crystallization.