Theretical And Experimental Study On A Thermally Driven Two-Stage Desiccant Dehumidification System Using Dual Desiccant Solutions With Large Concentration Variance

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.