Study On Solar Driven Water Evaporation Efficiency Based On Composite Nanomaterials

Solar driven water evaporation is a simple and green method to realize the water purification.However,the efficiency of solar driven water evaporation is quite low.Therefore,an expensive optical focusing system is needed to improve its photothermal conversion efficiency.Recently,nanomaterials have made this technology a reality,enabling it to use sunlight to locally heat the whole body in a normal environment,so that it can quickly evaporate.Using nanomaterials to make solar driven water evaporate can achieve high efficiency water evaporation without additional energy input.It has the characteristics of small heat loss,high conversion rate,and flexible structure and so on,and has the potential of large-scale application.In this thesis,the reduced graphene calcium alginate gel film was successfully prepared.The porous structure of calcium alginate gel was used to realize the selftransport of water.Meanwhile,the reduced graphene was dispersed uniformly in the gel,which increased the absorption of incident light,and the absorbance rate of the pure oxygen graphene film increased by 83.5% to 90%.The evaporation efficiency of the gel film increased from about 60% of pure oxygen graphene membrane to 77%.For the distilled water after evaporation and desalination,the salt ion concentration is far lower than that of the original simulated seawater,which meets the ion concentration standard of sanitary drinking water.After repeated seawater desalination test,the gel membrane can restore the initial efficiency after cleaning,and the stability of the gel film is good.In the continuous desalination process,we observed that the salt crystallization covered the light absorbing surface of the evaporation film and blocked the diffusion channel of water molecules,resulting in a rapid decline in the evaporation efficiency of the evaporation film.The initial evaporation efficiency can be restored by cleaning.Based on the thermal control characteristics of thin film to sunlight,we consider the tortuous distribution of porous media pores and the characteristics of large cracks.Based on the fractal scale of porous media pores and the bifurcation structure of cracks,we build a dual porous media model.According to the established model,we use the fractal theory to give the expression of the effective thermal conductivity of dual porous media.The expression does not contain empirical parameters,which are structural parameters of dual media,and each parameter has a clear physical meaning.The results are in good agreement with the existing experimental data.In a word,the solar driven water evaporation is a very complex process.We should consider not only the light absorption performance of the absorber,but also the water absorption characteristics of the absorber,and also the insulation of the absorber.In this thesis,the physical properties of an ideal evaporation material are studied,and only one heat conduction model is tested.The solar desalination is not limited to the above topics.However,the current results provide the understanding for solar desalination and the basis for further research.