Design And Properties Of Hydrogel Solar Evaporator Based On Chitosan

Freshwater scarcity is an important global issue,with a quarter of the world’s population living in areas of extreme water stress（the Middle East and North Africa）where demand exceeds availability.To meet the growing demand for fresh water,solar-powered desalination has received a lot of attention.In particular,solar-powered interface evaporation,where light absorbers are combined with water evaporators to limit heat loss and concentrate heat at the water/air interface,makes efficient use of solar energy.Solar-driven interfacial evaporation avoids volumetric heating,minimizes the amount of photothermal material used,and thus has higher energy conversion efficiency.Based on this,this paper designs two kinds of solar evaporators for efficient water evaporation,and explores the water evaporation performance of the evaporator.1)A hydrogel evaporator was prepared by dispersing copper sulfide-reduced GO nanocomposite material as a light absorber into chitosan-sodium carboxymethyl cellulose dual network hydrogel.The effects of chitosan concentration,sodium carboxymethyl cellulose concentration and photothermal material concentration on water evaporation performance were investigated.The absorbance of the gel was characterized by UV-visible near-infrared spectrometer.The absorbance of the gel reached more than 90%in the vision-near-infrared light region.The surface temperature of wet and dry gel was monitored in real time by infrared thermal imager.It was found that the surface temperature of wet gel rose about 20℃within 6 minutes.Xenon lamp and AM 1.5 filter are used to simulate the solar light source to carry out the water evaporation test on the evaporator.The evaporation rate of the evaporator can reach 1.52 kg m^-2 h^-1 under one sun,which can adapt to the weak light environment and different water environment.Therefore,it can be used in the solar driven seawater desalination.2)The gel precursor solution was prepared by using chitosan,gelatin and copper sulfide-reduced GO,and the internal morphology of the gel was regulated by controlling the cooling rate of the solution.The layered and porous hydrogels were obtained respectively,and the evaporation performance of the two structures of the hydrogel evaporators was explored.It is found that the evaporation performance of the layered evaporator is better than that of the porous evaporator.The reasons are as follows:1)In the layered evaporator,the internal structure enhances the light refraction,the light absorption efficiency is higher,and the photothermal conversion efficiency of the evaporator is improved.2)The gel side surface of the layered structure is easier to escape steam;3)The internal structure of the layered evaporator has strong capillary action and provides sufficient water transport channel.At the same time,since the surface temperature of the evaporator side is lower than the ambient temperature,the gel height is designed in this experiment to increase the surface area of the evaporator side to absorb the ambient heat and improve the evaporation performance of the evaporator.The evaporation efficiency of 3 cm gel prepared by controlling the morphology and height can reach 3.62 kg m^-2 h^-1 and the photothermal conversion efficiency can reach 97.7%under the solar irradiation of 1 k W m^-2.It is found that the evaporator still maintains a good evaporation rate under different water environments.The evaporation of brine with different concentrations can maintain a high evaporation rate and there is no salt deposition on the surface of the evaporator,which shows long-term stability and salt resistance,and has great development potential in seawater desalination.