基于传热传质分析的热局域化太阳能蒸汽发生系统优化研究进展

Research advances on optimization of thermal localization solar steam generation system based on heat and mass transfer analysis

太阳能蒸汽发生系统能够利用太阳能局域加热水体产生蒸汽,在海水淡化和污水处理等领域具有较大的应用潜力.太阳能蒸汽发生系统主要包括光吸收系统和热局域化系统两大部分.由于能够实现低聚光下的高效太阳能光热蒸汽转化,且具有结构简单、成本低、蒸发效率高、易于推广应用等特点,太阳能蒸汽发生系统受到了广泛关注.近几年,越来越多的热局域化太阳能蒸汽发生系统被相继提出.然而由于能量损失和制备限制等原因,该类系统光-热-蒸汽转化效率还达不到100%.对于效率接近甚至大于100%系统的理论探索还比较匮乏,系统效率的提升仍有较大潜力.本文主要聚焦热局域化太阳能蒸汽发生系统,在总结前人工作的基础上,首先阐述了热局域化太阳能蒸汽发生系统的传热传质机理,然后从能量守恒的角度对转化效率的系统结构和外界环境依赖性进行了分析.相关机理的总结及能量分析的结果可以为系统结构优化、材料选取以及环境控制等提供参考.目前,太阳能蒸汽发生系统的大规模应用还存在较多问题,主要包括如何克服盐分在设备表面的沉积,以及蒸汽如何有效收集等.最后,对热局域化太阳能蒸汽发生系统未来的改进方向进行了展望.

The solar steam generation system has attracted tremendous research attention owing to its high conversion efficiency of solar energy.Currently,solar steam generation systems have been widely studied.Initially,the nano-fluid has been used to achieve solar steam generation,and then the thermal localization solar steam generation system is emerging as a new way because of its potential high evaporation efficiency.In this review,we firstly introduce the basic heat and mass transports in the thermal localization solar steam generation system,then sort out the underlying physics in the existing technology like system structure design and material selection from the perspective of energy conservation,finally point out what else could be further carried out to further improve the performance of the system.The main progresses of related researches at present include:(1)The heat and mass transfer mechanism in the thermal localization solar steam generation system was clarified and modeled for mathematical description,which could provide a theoretical basis for the further improvement of the system structure design and also selection of suitable materials.(2)For the light-to-heat conversion layer(the first layer),light-absorbing materials are selected mainly from two aspects such as high light absorption in a wide spectral band and low heat emissivity in long wavelength band,thus to achieve high light absorption performance and low heat radiation losses.At the same time,the size and spatial distribution of materials like nanoparticles on the surface of the first layer should be optimized to achieve a high light-to-heat conversion efficiency.(3)The porosity and thickness of the thermal insulation supporting layer(the second layer)will affect the heat loss,capillary and water supply rate.Therefore,there is an optimal porosity and thickness to minimize heat loss while ensuring the proper water supply rate.(4)In addition to the structure,environment can also have an important impact on the evaporation efficiency,including air flow rate,air relative humidity,air and water temperature,and the relative temperature difference between environment and bulk water.As the air velocity increases,the evaporation rate first increases because of disturbance from the air then becomes stable.As the relative humidity increases,the evaporation rate decreases almost linearly.The evaporation efficiency of the system is approximately linearly related to the ambient air temperature and also bulk-water temperature,besides higher ambient temperature and water temperature could improve the evaporation performance of the system.Some bottlenecks that limit the practical application of the thermal localization solar steam generation system were also summarized to highlight the potential focus of future work.Although the light-heat-steam conversion efficiency has been continuously increased,there is still much room for further enhancement.For example:(1)The material involved in the system should be further improved for meeting the requirement of large-scale application,long working life and good environmental adaptability;(2)it is still necessary to further examine the optimization of the first layer of microstructure and the selection of materials from the perspective of near-field radiation to achieve high light-heat-steam conversion efficiency;(3)the evaporation efficiency is expected to be further improved by methods like absorbing heat from environment and also by recycling utilization of vapor latent heat.These methods are expected to give a much high evaporation efficiency,even much larger than 100%.Besides evaporation efficiency,there are still some technical limitations that should be overcome,including precipitation of salt on surface,large-scale application related problems,and water-supply,solar-energy matching issues,etc.