| Photothermal conversion is one of the most important green technologies for solar energy utilization.In the field of photothermal conversion,solar-thermal evaporation is widely involved,which is a typically thermophysical process by heating water and then evaporating water.However,solar energy has intrinsic intermittent characteristics,which suffer from low and unstable radiation intensity.And in the traditional small solar-thermal evaporation systems,volumetric heating causes a large amount of thermal energy to dissipate into the bulk water,limiting its evaporation performance and leading to low energy utilization efficiency.Solar-driven interfacial evaporation based on heat localization upgrades the evaporation process from bulk water to only occur at the interface between liquid water and air,enabling rapidly heating and vaporizing in a local region,thus significantly improving the energy utilization efficiency of photothermal evaporation.Such a unique photothermal evaporation approach with high energy efficiency opens a new perspective for small water treatment systems and meanwhile,presents great application prospects in many fields,such as seawater desalination in remote island areas,sewage purification in urban suburbs,and steam sterilization and medical disinfection in restricted areas.Solar-driven interfacial evaporation process involves the conversion of light energy to heat energy,the heat transfer from solid phase to liquid phase,the transport of water molecules and ions at the solid-liquid interface,the diffusion and condensation of water steam,etc.An in-depth study on the mechanism of the above-mentioned mass and heat transfer process related to the interfacial photothermal evaporation is of great significance to guide the improvement of localized-evaporating structures,the preparation of solar-thermal materials,the performance optimization of solar-thermal evaporation and the design of photothermal systems.This thesis focuses on the optical and thermodynamics issues involved in the solar-driven interfacial evaporation based on the heat localization effect,in which carbon-based materials with miniaturized structures are applied.By using finite element simulation methods,combined with material characterization and experimental testing,the mechanisms of the orientation structure,microscopic size,and surface wettability of the photothermal material on the optics and thermodynamics processes are detailly investigated,such as light absorption,photothermal conversion,solid-liquid interfacial heat transfer,and water transport and vapor diffusion within the heat-localization systems.More importantly,the method to optimize the energy and heat management of the photothermal evaporation system for achieving efficient energy recovery and multi-level utilization is deeply explored.First,the finite element simulation method is used to probe the graphene-water interfacial heat transfer properties with different scales.The results show that the nano-scale graphene coating has a much stronger enhancing effect on the heat transfer at the solid-liquid interface than that of the micro-scale coating.Based on the enhancing mechanism of surface structure on the heat transfer at the solid-liquid interface,the microstructure and morphology of the photothermal materials with high interfacial heat transfer coefficient are rationally optimized,which is developed to conduct evaporation test.During interfacial evaporation,when the surface structure size of the solar-thermal materials is reduced from micro-scale to nano-scale,the energy utilization efficiency is improved by nearly 5%.Then,the simulation results find that based on the honeycomb structure,the biomass material can use its abundant low thermal conductivity sheets to block the heat transfer between the pores and achieve excellent thermal insulation performance.Subsequently,the natural bamboo is chosen to serve as a three-dimensional multifunctional membrane with a black node surface for photonic absorption,a hydrophilic property for water transport,a honeycomb structure for thermal insulation,sufficient micropores for vapor permeation,and a bamboo chamber for vapor condensation,succeeding in highly-efficient evaporation and allowing the heat recovery.Under the light radiation of 1 k W m-2,the membrane achieves a photothermal conversion efficiency as high as92.5%.More importantly,based on the excellent heat storage ability of the bamboo chamber,the combination of thermoelectric conversion and interfacial evaporation for clean water and electricity generation is achieved in an as-designed system,in which a thermoelectric supercapacitor is placed in the bamboo chamber to recycle the energy released by the hot vapor.In the practical application of interfacial photothermal evaporation,stable light energy input and efficient steam collection are the keys to obtain a high water production rate,and prolonging the life of photothermal materials is the key means to realize real-world application.From the perspective of optimizing energy management,we come up with a built-in latent heat storage strategy.Phase change materials are used to store the waste heat during evaporation and then release its stored heat into the light absorber when temporary diminishing of the incident light exposure,thus providing an effective buffer mechanism against the clean-water productivity fluctuations.The realization of the unique combination of photothermal conversion and thermal energy storage relies on the integration of paraffin wax independent of a graphene-based light absorber.Under the light radiation of 1 k W m-2,based on the heat localization effect,the fabricated dual-layer architecture achieves an evaporation rate of 1.36 kg m-2 h-1 and high energy efficiency of 90.0%.When the light source is turned off,due to the heat storage ability,the architecture obtains an evaporation rate of 0.70 kg m-2 h-1 and energy efficiency of 46.5%,2.5 fold higher than that in the conventional unit(an evaporation rate of 0.28 kg m-2 h-1 and energy efficiency of 18.6%).To overcome the light blocking and vapor collection issues,a free-standing light absorber consisting of a graphene array decorated on nickel foam is custom-designed,which enables a thin water layer with self-suction through graphene nanochannels from the bulk water to realize locally heating and minimize thermal loss.Deliberately,the porous membrane and graphene architecture are separated to form a vapor gap that can avoid the salt/oil-based contaminant in direct contact with the membrane,thus addressing the fouling issue.Meanwhile,the vapor diffuses from the absorber to the gap opposite to incident light and condenses in the distillate flow.With the synergistically combined advantages of photothermal membrane distillation and heat localization,high solar-water energy efficiency as high as 82.3%is achieved.In addition,by introducing the PEDOT-PSS coating to regulate the surface wettability of vertically-oriented graphene,underwater superoleophobicity and anti-salt-clogging property are realized,which could prevent the accumulation of salt particles and reject the adhesion of oil contaminants on its surface.Especially,due to the super-hydrophilicity,the ions can spontaneously diffuse from the high-concentration region（light-absorbing region）to the low-concentration region（bulk water region）through the water pathways,establishing the process of salt deposits to redissolve,thus realizing self-cleaning ability.Finally,to resolve the problems of membrane fouling and membrane wetting in photothermal membrane distillation,fluorine-containing groups are introduced into the carbon black photothermal layer coated on the commercial membrane to modify its surface wettability,thus obtaining a bifunctional photothermal membrane that can not only converts sustainable solar energy into thermal energy but also presents good water and oil-repellent property.During 60-hour continuous treatment of complex and low-surface tension feed water such as 16.70 wt%salty water and 1 g L-1 oil-in-water emulsion,the photothermal membrane achieves a stable permeate flux with a very small fluctuation and an excellent purification performance.During distillation,membrane fouling and wetting problems are alleviated under severe conditions,and the durable membrane further deepens the potential application prospects of interfacial photothermal evaporation. |
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