太阳能驱动水蒸发发电系统的研究进展

Research Progress on Solar-Driven Steam-Electricity System

太阳能驱动水蒸气产生(SSG)可应用于海水淡化、污水处理等领域,对获取纯净水和解决水资源短缺问题具有重要意义。通过提高光热材料的光热转换性能、水传输速率和系统的热量管理,可实现超高的太阳能蒸气转换效率。太阳能驱动水蒸发发电系统拓展了SSG的功能性,进一步提高了系统的能量利用率,为小型电子器件、柔性穿戴设备和偏远地区供电提供了创新模式。太阳能驱动水蒸发发电系统主要分为在SSG基础上集成发电模块和自供电2类发电方式。本文总结了太阳能驱动水蒸发发电系统的研究进展,分析了不同发电模式和材料类型的优缺点,对太阳能驱动水蒸发发电系统面临的挑战和未来的发展进行了分析和展望。

With the rapid growth of population and the development of industry,the lack of clean water resources becomes increasingly prominent.For the shortage of clean water resources,China as a country rich in seawater resources vigorously promotes seawater desalination technology,which has gradually developed into a strategic industry.Compared with the existing desalination technologies such as multi-effect distillation,multi-stage flash evaporation,reverse osmosis membrane,electrodialysis and so on,the desalination technology based on the principle of solar photothermal conversion has unparalleled advantages,i.e.,environmental-friendly,high efficiency and low cost.Solar-driven steam generation(SSG)could be widely applied in seawater desalination,sewage treatment and other fields,which is of great significance in acquiring pure water and solving the problem of water resource shortage.Ultra-high solar steam conversion efficiency could be achieved by increasing photothermal conversion efficiency,water transfer rate and heat management of the SSG system.In recent years,the evaporation models underwent a gradual transition from using photothermal particles dispersed in volume water to interface water evaporation with porous insulation layer or 1D or 2D water transport channels to the hydrophilic gel evaporation structure.It is reported that TiN hyperbranched nanowires have a better solar light absorption and a higher specific surface area to evaporate water rather than TiN nanoparticles and nanotubes.Its solar water evaporation rate and solar thermal conversion efficiency are 1.5252 kg·m^(-2)·h^(-1)and 94.01%,respectively,under 1 kW·m^(-2)simulated solar irradiation.Zhu et al.demonstrated that a carbon foam with a three-dimensional interconnected porous structure enables a sufficient diffusion of vapor with a convective flow and realizes an evaporation rate of 10.9 kg·m^(-2)·h^(-1).Solar-driven steam-electricity system expands the functionary of SSG,providing an innovative pattern for the electricity supply for tiny electronic devices,flexible wearable devices and remote area.Solar-driven steam-electricity system is mainly divided into integrating electricity generation module and self-powered electricity generation module.In SSG and PV cell integration modules,Xu et al.experimentally demonstrated that a prototype hybrid tandem solar device with WTIL can generate electricity with a power output of 204 W·m^(-2) and purify water at 0.80 kg·m^(-2)·h^(-1) under 1-sun illumination.Deng et al.realized that the evaporation rate of the prepared TEC in seawater electrolyte is 1.1 kg·m^(-2)·h^(-1),and the solar vapor conversion efficiency of the TEC is 60%with a peak output power of0.5 mW·m^(-2).In self-powered electricity generation modules,Guo et al.presented a device prototype for enhanced power generation from ambient humidity,the device to ambient humidity can produce voltages of 0.78 V and a current of 7.5μA,both of which can be sustained for more than 10 d.In this review,we summarized recent research work on the solar-driven steam-electricity system,and analyzef the advantages and disadvantages of different electricity generation modules.Summary and prospects Selection of solar absorbers:Three categories of solar-absorbing materials including carbon materials(such as carbon nanotubes,carbon nanoparticles,and graphene),plasmonic metals(such as Al,Au,Ag,Cu)and ceramic nanomaterials(such as transition metal nitrides and carbides)and semiconducting nanomaterials(such as Ti O_(2-x),MoS_(2))were developed.The present research focuses on the preparation of high-light-absorption photothermal absorbers with a low-cost and a superior chemical stability.1)Structural design of solar steam generation system:The solar-vapor conversion efficiency(η)can be calculated by η=Q_(c)/Q_(s)=H_(v)×dm/dt where Q_(s) is the incident light power(1 kW·m^(-2));Q_e is the water evaporation power;H_v is the evaporation heat of water(~2260 kJ·kg^(-1));m is the evaporated water mass;t is time.Based on Eq.(1),the solar steam conversion efficiency is directly proportional to water evaporation enthalpy and water evaporation rate.It is proved that the latent heat of water vaporization in a porous nanostructure is lower than the standard value.However,it is still unclear how to obtain a low water evaporation enthalpy by regulating the porous structure of the solar photothermal materials.It is crucial to obtain hierarchical nanostructures and evaluate its water vaporization latent heat.In order to increase the specific surface area of solar photothermal materials,porous foams and gels are designed and prepared,but the preparation of these artificially hierarchical materials is a time-consuming and costly process.It is thus imperative to explore naturally hierarchical plants that are low-cost and easy to obtain.2)Efficiency and mechanism of VOCs photodegradation in water:Solar-driven steam generation is utilized to produce fresh water by removing salts,heavy metals,micro-organisms,and most organic pollutants in raw water.However,it also accelerates the volatility of the volatile organic compounds(VOCs)and enrich them in the distilled water if the water source is contaminated by VOCs because of their similar boiling point with water,which poses a serious threat to the atmospheric environment and human-being health.VOCs such as alcohols,aldehydes,ketones,olefins,and aromatic compounds tend to migrate and accumulate in different environmental media such as soil,water,and air.The World Health Organization and China's sanitary standards for drinking water quality both have established limit values for VOCs.Consequently,it is crucial to efficiently remove VOC during the solar-driven interfacial evaporation.3)The relative merits of different electricity generation systems:Solar-driven steam-electricity system is mainly divided into integrating electricity generation system and self-powered electricity generation system.The integrating electricity generation module integrates a solar water evaporation module and an electricity generation module by rationally connecting the solar water evaporation module with photovoltaic device harvesting light energy,thermoelectric module recovering heat energy from water vapor,membrane structure utilizing salt concentration gradients and triboelectric nanogenerator based on steam condensation process.The electricity generation capacity of an integrated electricity generation system typically depends on the electrical output performance of the power generation module itself.Though the integrated electricity generation system can be commercially produced and has a stable performance,it has some problems such as complex structure,high cost and non-adaptability of different modules,which limit its practical application.The self-powered electricity generation system does not connect solar water evaporation module with other auxiliary equipment for electricity generation.It generates electricity directly by a hydrovoltaic effect,and its evaporation-electricity generation performance mainly depends on the microstructure of the photothermal material,such as the size of micro-/nano-pore channels and the zeta potential.The currently reported output currents range from nanoamps to microamps,and exploring current output in the milliamp range is a further research direction.