| Drinkable fresh water resources,as the natural resources on which human beings depend for survival,are facing serious shortage.Desalination of seawater(brackish water)is becoming an important source of fresh water.Existing desalination processes such as thermal and membrane processes have the problem of consuming fossil fuels and producing large amounts of greenhouse gases.Compared with traditional fossil energy,solar energy,as an inexhaustible sustainable green energy,can provide a new energy-support for seawater desalination.As a direct use of solar energy to obtain distilled water,solar evaporation seawater desalination is gradually getting widespread attention.At present,the solar evaporation technology based on interface heating has some issues,such as poor solar energy absorption rate,low steam generation flux and untreatable salt crystallization.Hence,this study designed and prepared a series of efficient and economical photothermal interfacial evaporators through a variety of strategy sets,so as to ensure the spectral absorption of sunlight and good energy conversion,thus realizing the improvement of the overall efficiency of solar desalination.The research results obtained so far are as follows:Firstly,since the air trapped on the surface of the hydrophobic film makes it difficult for water to adhere,the surface tension of water can be reduced to facilitate evaporation at the evaporation interface.Therefore,a hydrophobic film with a cheap graphite thin layer combined with fiber paper is designed for solar desalination.By optimizing the graphite content,a photothermal film with a high spectral absorption rate(77%)was obtained.Due to its hydrophobic properties,the film could be selffloating on the water surface for interfacial heating,and the evaporation rate was 1.5 times that of direct illumination water.After adding the insulation layer for thermal management,the heat loss of the system was reduced,the evaporation rate of the interface was increased,and the energy conversion efficiency reached 70%.After the folding optimization,the repeated absorption of light between the two walls weakens the reflection loss and further improves the energy conversion efficiency,with the evaporation rate of 1.16 kg m-2 h-1.Due to the simplicity of the preparation method,the membrane can operate efficiently without the assistance of energy sources other than solar energy,which makes it extremely suitable for seawater desalination and emergency water supply under emergency conditions.However,it is worth noting that during the evaporation experiment for several consecutive days,as the evaporation rate exceeds the return rate of the salt,salt crystallization pollution will occur on the surface of the hydrophobic film and gradually accumulate with the increase of time.Salt crystal contamination increases light reflection and obstructs water transport through the membrane,leading to a gradual decrease in evaporation rate.Because hydrophobic film faces serious salt crystallization problem and low evaporation flux in the process of interfacial evaporation seawater desalination,super-hydrophilic photothermal fiber film was prepared to explore the influence of wettability and structure on evaporation and salt crystallization process.First,the super-hydrophilic fiber membrane PCF was prepared,which was based on the superhydrophilic fiber membrane and coated with the polymer polypyrrole(PPy)for photothermal conversion.After optimizing the polymerization time,the absorption rate of PCF reached 96%.In the process of solar interfacial evaporation,the evaporation flux is 1.52 kg m-2 h-1,and the corresponding energy conversion efficiency is 90%,which is significantly improved compared with the hydrophobic film.Moreover,since the superhydrophilicity provides continuous water channels and the high porosity provides abundant free water,PCF can dissolve crystals formed on the membrane surface and transport them back to the original seawater in a short time after the light is absent.With the help of good self-cleaning ability,PCF can run stably within 25 days(with an average speed of 1.42 ± 0.03 kg m-2 h-1),showing good long-term stable operation ability.At the same time,the superhydrophilic photothermal fiber membrane PDA/PEI/PPy@PI was prepared,which took the electrostatic spinning PI membrane as the hydrophobic core,PPy as the light absorption coating,and the coating with super hydrophilicity and corrosion resistance PDA/PEI as the shell.During the optimized polymerization time,the PDA/PEI/PPy@PI membrane had a low surface roughness and a broad spectrum solar energy absorption rate(93%).During the solar evaporation process,the evaporation flux was 1.43 kg m-2 h-1,and the corresponding energy conversion efficiency was 87%,which was significantly improved compared with the hydrophobicity membrane.Due to the low adhesion of the smooth interface to salt crystallization,the crystallization position will change with the change of water transportation direction.In the case of unilateral water transport,salt crystals will crystallize at the farthest end of the water transport direction.The salt crystals do not cover the film surface and are easy to clean.The membrane was operated stably(with an average rate of 1.21 kg m-2 h-1)in a continuous 7-day desalination process(10wt% brine),and the average collection rate of salt crystals was up to 60.2%,showing a good recovery capacity of salt resources.The super-hydrophilic film evaporator can improve the interface evaporation flux to a certain extent.However,even if the conversion efficiency of 2D evaporation of solar energy reaches 100%,the evaporation flux is only about 1.6 kg m-2 h-1 without light concentration.By extending the hydrophilic interface into three-dimensional space,the upper limit of evaporation flux can be raised by utilizing solar energy and heat energy accumulated in the environment.Therefore,3D photothermal evaporator(PMS)with low tortuosity was prepared in the following experiment.The evaporator is based on corn straw as substrate and PPy as the photothermal conversion coating.After polymerization,PMS has excellent vertical water lifting capacity and broadband spectrum solar absorption(97.5%).In the process of solar evaporation,the evaporation rate of the single 3D evaporator sticking out of the water surface into the three-dimensional space is directly proportional to the evaporation height.The optimized evaporation rate of the 3D evaporator is 6.4 times of that at the plane state.After the array is composed,the optimized maximum evaporation rate is 3.0 kg m-2 h-1,which is much higher than the array 3D evaporator reported earlier.In seawater evaporation applications,PMS can balance salt ion transport without salt crystallization due to its high watercontent.After continuous operation for 5 days,the 3D evaporator can maintain a stable and continuous flux,which proves the continuity and durability of operation.In this paper,the flux of solar distillation water is improved by constructing different wettability interfacial evaporator and improving dimension.At the same time,the technical problems of salt pollution and salt resource recovery in the process of brine separation are solved.In the following research process,the physical properties of evaporator can be further optimized to realize high-flux solar desalination,which provides the possibility for its practical application. |
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