The Study Of Enhancing The Energy Utilization Efficiency Of Solar Still By Thin Liquid Film And Heat Localization

As one of the solar desalination technologies,solar still has many advantages,such as small scale,simple principle and convenient maintenance.Recently,in order to further improve the performance of solar still,researchers tried to use micro/nano technologies,which improve the solar absorption and heat transfer process in solar still.Many progresses have been made by this new approach.However,most of the current researches focus on material design,and is lack of discussion on heat and mass transfer.This leads to many problems,such as the high cost of materials,the lack of thermodynamic and heat transfer optimization of the system and so on.Therefore,it is very important to study the solar still systematically by combining micro/nano technologies and heat and mass transfer theory.In this paper,enhancing the energy utilization efficiency of solar still by thin liquid film and thermal localization is studied,based on carbon-based micro/nano particles and heat and mass transfer theory.First of all,the feasibility of using suspended graphite microparticle layer to improve the productivity of solar still is studied.Solar energy is absorbed near the evaporation surface by the particle layer.Therefore,the heat is localized near the evaporation surface,hence the heat loss is reduced and the temperature and evaporation rate are increased.Experimental results show that the productivity of solar still is increased by about 50% after using 1 wt% suspended graphite microparticle layer.Further research shows that adding phase change materials to the solar still with graphite particles can increase the productivity by 65%.Secondly,a floating solar evaporation setup is proposed.In the setup,graphite particles float above water surface instead of suspending under water surface.A thin liquid film is created under the evaporation surface,which further enhance the heat localization effect and increase the solar energy utilization efficiency.Experimental results show that the energy efficiency of the floating evaporation setup can reach 78% under the solar radiation of 1 k W/m2.Meanwhile,it is found that the evaporation energy efficiency increases slightly with the decrease of particle size.But the evaporation energy efficiency increases first and then decreases when the concentration of graphite particles increases.The experimental results provide guidance for the rational use of micro/nano particles in solar evaporation setup.Meanwhile,in order to provide theoretical guidance for further improvement of the floating evaporation setup,the effect of graphite particles and other evaporation parameters are analyzed through the thin film evaporation theory.The theoretical analysis shows that the size and wettability of graphite particles have slight effect on the evaporation,which is consistent with the experimental results.The main factors affecting evaporation are:(1)the distance between graphite particles on the evaporation surface;(2)the evaporation coefficient of water on the evaporation surface;(3)the superheat of the evaporation surface.Moreover,the analysis of salt rejecting shows that,the high concentration salt solution on the evaporation surface is mainly diluted through the natural convection of salt water.Furthermore,it is found that when the evaporation unit of the evaporation setup has a small width-height ratio,it has both good performance of salt rejecting and thermal efficiency,which is suitable for solar still.Finally,an airing evaporation setup is proposed,which improves the vapor diffusion near the evaporation surface while thin liquid film and heat localization are assured.Experimental results show that under 1 k W/m2 solar irradiation,the evaporation energy efficiency of the airing evaporation setup can reach 87%.The total evaporation rate of airing evaporation setup is about 20% higher than that of floating evaporation setup.Based on the theoretical analysis of natural convection evaporation,it is found that the performance of airing evaporation setup can be improved by increasing the evaporation area if the vapor diffusion isn't affected.The research of this paper shows that,based on thin liquid film and heat localization,a high solar energy utilization efficiency can be achieved through a low cost and a simple way,which provides guidance for the efficient use of micro/nano materials to improve the performance of the solar still.