Construction And Performance Research Of Solar Evaporation Systems With Salt Resistance Capacity

With the development of economy and the increase of population,the demand for water resources is increasing,and the unreasonable exploitation and utilization of water resources lead to the shortage of fresh water resources.Traditional water purification methods mainly use thermal distillation and membrane separation technology,which are limited by high energy consumption,complex infrastructure and environmental emissions.Solar steam generation(SVG)using photothermal materials is a green and feasible solution,and abundant solar resources provide an energy source for the SVG system.SVG based on air-water interface greatly improves the rate of water evaporation,but the evaporators used have some problems,such as poor salt resistance,poor solar absorptivity and low heat utilization during the evaporation process.In view of the above problems,this paper designed,prepared and explored the water evaporation behavior and salt resistance of several evaporators with different wettability.The results are as follows:(1)The traditional hydrophilic evaporator has poor salt resistance and low solar absorption.In order to solve this problem,a PPy/PVDF film(PFM)with hydrophobic surface was prepared on the basis of hydrophobic PVDF membrane.The hydrophobic surface of PFM showed good self floating ability in water evaporation experiments.In order to improve the light absorption capacity of PFM,the inverted pyramid structure was introduced on the surface of PFM by means of imprinting.Compared with the planar PFM,the PFM with inverted pyramid structure has better light absorption capacity(>97%).The PFM with an inverted pyramid structure has a water evaporation rate of 1.42 kg m^-2 h^-1 and a photothermal conversion efficiency of 88.7%under 1 sun irradiation.The hydrophobic surface can ensure that the brine will not enter the inside of the PFM during the evaporation of the brine,thereby inhibiting the salt pollution of the PFM.The simulated stability experiment shows that PFM has good stability and salt resistance.(2)Because the hydrophobic film can not be wetted in the evaporation process,evaporation only occurs at the bottom of the evaporator,which leads to a lot of heat loss.Therefore,the super hydrophilic hydrogel was used as the substrate to prepare the evaporator.However,the solar evaporator based on the hydrogel substrate still needs excellent toughness to ensure continuous and stable utilization while achieving fast evaporation rate.In this research,a solar evaporator based on dual network hydrogel was fabricated by double chemical crosslinking,consisting of polyacrylamide(PAAm),carboxymethyl cellulose(CMC)and CuS.By utilizing N,N'-methylenebisacrylamide(MBA)and Fe³⁺for double chemical crosslinking agent,PAAm and CMC formed an interpenetrating dual network structure CPM,which is helpful to improve the mechanical properties of the photothermal membrane.The breaking elongation of CuS/CPM membrane is 480%.Meanwhile,the hydrophilic network ensured the high efficiency of water transmission and water activation.The evaporation rate reached 1.61 kg m^-2 h^-1 at 1 sun,and the photothermal conversion efficiency reached 79%.In the process of simulated desalination experiment(10%wt NaCl solution),the evaporation rate did not decrease after 10 cycles,indicating the self-cleaning ability the good durability of CuS/CPM.(3)The hydrogel substrate adsorbs a large amount of water during the evaporation process.Excessive water will reduce the surface temperature,resulting in increased heat loss,which is not conducive to the rapid evaporation of water.This problem can be solved by adjusting the water content reasonably through material design.Based on the PDMS foam with good flexibility,the evaporator NPSF,which can adjust the water content during the evaporation process,is prepared by modifying the PDMS foam by hydrophilic,surface grafting PNIPAM,and blending PPy.Due to the porous structure with high surface roughness,NPSF shows excellent light absorption capacity(>97.5%).Under light conditions,the grafted PNIPAM on the surface of NPSF becomes hydrophobic,and the heat generated can be used for water evaporation more efficiently.Moreover,the hydrophobic surface weakens the interaction with water molecules,which is more conducive to the escape of water molecules.Under 1 sun irradiation,the evaporation rate of water is 2.0 kg m^-2 h^-1,and the photothermal conversion efficiency is 86.9%.Due to the large pore structure of NPSF,the evaporation rate can be further increased to 2.15 kg m^-2 h^-1 by adjusting the height of the evaporator properly.And because of the change of surface wettability,During the simulated desalination experiment(10%wt NaCl solution),the evaporation rate remained stable after multiple cycles.And it shows good salt resistance in long-term brine evaporation experiments.