【作者】 熊珍琴；

【导师】 王如竹； 代彦军；

【作者基本信息】 上海交通大学 ， 制冷及低温工程， 2009， 博士

【摘要】 溶液除湿系统是基于除湿溶液在一定浓度范围内具有强吸湿特性建立的一种空调系统,采用低品位热源驱动,具有独立控制湿度、节能和环保等突出特点。所使用工质除湿溶液具有强大蓄能能力,在太阳能利用方面具有显著优势。然而,溶液除湿系统的蓄能与保证空气除湿效果之间存在矛盾。为了保证空气含湿量达到需要范围,实际系统溶液浓度变化小(小于1%,约0.2%),导致蓄能能力未得到有效开发利用,同时也限制了溶液除湿系统的能量利用效率进一步提高。基于此,本文开展了以下研究工作:首先提出了溶液浓度变化范围大、除湿/再生之间温差小的热致浓度差理想溶液循环及采用此理想循环的两级双溶液大浓度差除湿系统,具有再生后浓溶液和除湿后稀溶液浓度差较大的特点。热力学分析结果表明理想溶液循环系统的热力性能系数、有效蓄能密度得到大幅提高,不可逆损失也显著减小。典型工况下,再生后浓溶液与除湿后稀溶液浓度差从0.16%增大到4.2%,使得系统的热力性能系数提高了142%,有效蓄能密度增加到385.5 MJ/m3。若采用氯化钙溶液预除湿降低溶液循环中除湿/再生过程溶液温差,系统热力性能系数进一步提高了25.9%,火用效率增加了23.0%。其次,搭建两级双溶液大浓度差除湿系统实验台,具有氯化钙溶液预除湿、再生后浓溶液与除湿后稀溶液浓度差大(大浓度差)两大特点。实验验证了系统在大幅降低溶液比再生热方面的优越性,氯化钙溶液的比再生热甚至低至2.0 kJ/g。系统热力性能系数和电力性能系数分别可达0.84和11.1。在不同典型室外环境下系统均能处理空气绝对含湿量达到ARI室内控制标准。氯化钙溶液表现出较好的预除湿效果,承担湿负荷比例在20~60%之间。空气的流速和溶液的初始浓度对除湿性能影响最为显著,而再生性能则对再生温度变化最敏感。第三,建立了基于波纹结构填料的空气-溶液热质传递三维模型,计算结果与实验结果吻合较好。利用该模型研究了双溶液除湿和大浓度差除湿的可行性与双溶液再生和大浓度差再生的节能性,深入分析了两级双溶液大浓度差除湿过程和再生过程的热质传递特性。研究发现,在给定工况下(空气与溶液进口温度分别为34 oC和30oC)溶液除湿过程中约有70~100%的除湿释放热量被溶液所吸收。对表征除湿过程和再生过程传热和传质情况的气液界面Nusselt数和Sherwood数进行分析,结果表明采用两级除湿/再生能够增强气液间传热传质效果。空气侧Nusselt数和Sherwood数分别在4~15和3~14之间,主要沿着空气流动方向逐渐下降。除了进口段有小幅增大外,溶液侧的Nusselt数在除湿/再生过程中均稳定在2.3。溶液侧Sherwood数在1~4之间,入口段效应范围比Nusselt数大,主要是因为溶液侧传质边界层的发展速度比热边界层慢。论文还分析了两级双溶液大浓度差(5%)太阳能除湿空调系统的能量调节特性和季节蓄能特性。借助除湿溶液的蓄能能力,系统不仅保证了24小时连续除湿,而且通过季节蓄能,显著提高了太阳能保证率和系统热力性能系数。与小浓度差运行模式(0.2%)相比,两级双溶液大浓度差除湿系统平均热力性能系数提高了73%,太阳能保证率增加了11~45%。与单溶液大浓度差除湿系统相比,两级双溶液除湿系统热力性能系数提高了20.3%,而太阳能保证率提高了5~14%。本论文提出的两级双溶液大浓度差除湿循环,解决了蓄能和良好的除湿效果之间的矛盾,对太阳能等低品位热能驱动溶液除湿系统的广泛应用具有积极作用。更多还原

【Abstract】 Liquid desiccant dehumidification systems work based on the strong moisture- absorption ability of liquid desiccants at certain concentration range. Such systems utilizes low grade heat source, and are effective in independently handling air moisture loads, in an environmental friendly manner. Besides, liquid desiccant solutions have large energy storage capacity, which is beneficial in solar driven systems. However, the concentration variance between the strong desiccant solution after regeneration and the weak desiccant solution before dehumidification is usually small to assure a low absolute humidity ratio of process air after dehumidification. Hence, the energy storage density is small in real application. The main objective of this thesis is to improve the energy efficiency of the system, and achieve a large energy storage density, as well as a good dehumidification effect.Firstly, an ideal desiccant cycle with a large concentration variance between strong desiccant solution after regeneration and weak desiccant solution after dehumidification, and a small temperature difference between desiccant solution in the regeneration process and in dehumidification process is proposed. Moreover, a two-stage dehumidification system using two kinds of desiccant solutions is designed and studied. Comparison with the conventional desiccant cycle with small desiccant concentration variance and large temperature difference is made, which shows significant improvement in the performance of the proposed dehumidification system. The thermal coefficient of performance (TCOP) increases by 148% with the large desiccant concentration variance. The TCOP can further be improved by about 25.9% by adding CaCl2 pre-dehumidification, and the exergy analysis shows that the exergy efficiency was lifted by 23.0%.Secondly, experimental investigation of the proposed two-stage dehumidification system using LiCl solution and CaCl2 solution has been made. The test results show that thermal coefficient of performance (TCOP) and electrical coefficient of performance (ECOP) can reach 0.84 and 11.1, respectively. Furthermore, the system not only could meet the absolute humidity ratio requirement (ARI standard) under three different typical outdoor conditions, but also had reduced specific regeneration heat dramatically (as low as 2.0 kJ/g for CaCl2 solution). The pre-dehumidification section of CaCl2 solution can handle 20~60% of the total moisture load. Parametric analysis is also done and the results indicate that the dehumidification performance is highly influenced by the air velocity and the desiccant concentration, while the regeneration performance is most sensitive to the regeneration temperature of desiccant solutions.Thirdly, a three dimensional mathematical model of the dehumidifier/regenerator filled with the waveform packing material is built to simulate the heat and mass transfer between air and liquid desiccant solution. The simulation results agree well with the experimental results. Using this model, the feasibility of dehumidification and the energy saving potential during regeneration process using two kinds of desiccant solutions in various concentrations is analyzed. Under given concdition, it is found that 70~100% of the heat rejected during dehumidification process can be absorbed by desiccant solution. The Nusselt number and Sherwood number profile on the liquid-gas interface are investigated. The heat and mass transfer between the air and the liquid desiccant solution is improved by the application of the two-stage unit. The Nusselt number and Sherwood number of the air side are in the range of 4~15 and 3~14, respectively. There is a significant decrease along the air flow direction due to the increasing thermal and mass boundaries. For the desiccant side, Nusselt number is stable at 2.3 except at the inlet for desiccant of the dehumidifiers/regenerators, and the Sherwood number ranges between 1~4. Nusselt number decreases faster than Sherwood number along the desiccant flow direction for the desiccant side since the thermal boundary layer of liquid desiccant solution developes faster than the mass boundary layer.Finally, the daily energy shift and seasonal energy storage performance of a solar driven two-stage dehumidification system using dual desiccant solutions with large concentration variance (5%) is studied. The all-day dehumidification is assured by storing the excess strong desiccant solution regenerated in the daytime for the dehumidification at night. Also, the excess strong desiccant solution regenerated during sunny days can be stored for use during the rainy days. Hence, the performance of the system improves significantly in terms of the solar fraction and average TCOP around the dehumidification season. The average TCOP of the two-stage dehumidification system is improved by 73% and the solar fraction is increased by 11~45% by lifting the concentration variance from 0.2% to 5%. Compared with the dehumidification system using LiCl alone with large concentration variance (5.0%), the TCOP and the solar fraction of the proposed system is higher by 20.3% and 5~14%, respectively.It is expected that the studied work here can be used widely in future to harvest the low grade thermal energy, such as solar thermal or waste heat, as the technology can well address the problems to ensure both good dehumidification and large energy storage density.更多还原