Numerical And Experimental Investigation Of A Heat Pump Driven Hollow Fiber Membrane-based Liquid Desiccant Air Dehumidification System

Liquid desiccant technology can adjust air humidity of people’s life and it is an effective manner to achieve independent temperature and humidity control. A compression heat pump driven and membrane-based liquid desiccant air dehumidification system is presented. Simultaneous heating and cooling of the salt solution are realized with a heat pump system to improve energy efficiency. Both the dehumidifier and the regenerator are made of hollow fiber membrane bundles packed in shells. In such modules, the semi-permeable membranes are used to separate the desiccant solution from the process air. Water vapor can permeate through these membranes effectively, while the liquid desiccant droplets are prevented from cross-over. Hence, the technology provides a solution for air dehumidification with energy conservation and environmental protection. In the present work, the heat and mass transfer process of the system is investigated and the mathematical models of the system are set up. Therefore, the thermodynamic mechanisms of the system are disclosed and methods for system design and optimization are proposed. The main works are summarized as following:(1) The hollow fiber membrane module is converted to a parallel-plates cross-flow heat mass exchanger and the influence of bundle arrangement can not be considered. The model is validated experimentally and it is simplified for engineering applications. The results show that the heat transfer process changes faster than the mass transfer process. Packing density is a dominant factor influencing performance, thus increasing the packing density is an efficient way to improve performance. But bundle arrangement has little effect on the performance of the system. Therefore, staggered arrangement is recommended for the bundle for higher heat and mass transfer efficiency.(2) A steady-state mathematical model for the whole system is developed and validated experimentally. The results indicate that the SDP(specific dehumidification power) are all greater than 100 gh-1m-2 under different operating conditions. The dehumidification efficiency is in the range of 0.3-0.5, which is comparable to the direct contact packed bed dehumidifier. Both the EER(energy efficiency ratio) and COP(coefficient of performance) increase with increases in air flow rates, air temperature and relatively humidity. As the liquid desiccant circulates faster, EER is improved but the COP is sacrificed. EER of the heat pump is all greater than 3.75. The COP is varied from 0.4 to 0.9. Therefore, the energy performance for the dehumidification system performs well.(3) A dynamic mathematical model for the system is also developed and validated experimentally. The dynamic model can predict the heat and mass transfer processes during the start-up time and describe the transient performance of the system in responses to the variations of the outside conditions. The results found that the initial solution concentration and the volume of solution in the container are the key factors influencing the start-up performances. As the initial solution concentration is close to the solution concentration at the equilibrium state and the volume of solution in the container is reduced, the start-up time of system decreases sharply. The initial solution temperature has a weak influence on the start-up time. Under the hot and humid conditions, adjusting the compressor revolving speed is a feasible way to satisfy the weather and load variations.(4) In order to avoid the corrosion of metal heat exchanger caused by liquid desiccant, air heat exchangers are used. Meanwhile, the multi-stage cooling-dehumidification and heating-regeneration process is introduced to improve performance of the system. A multi-stage mathematical model for the system is developed and validated experimentally. The results show that with the help of multi-stage dehumidification system, the solution concentration from the dehumidifier is lowered; the mean temperature difference of condensers is decreased and condensing temperature is reduced. Thus, COP can be increased by about 20% and the optimal operation parameters are also presented.(5) The mass entransy dissipation is introduced to evaluate mass transfer irreversibility in liquid dehumidification. Results indicate that as air flow equals to desiccant flow, mass resistances are small. The minimum mass resistance factors for cross flow are at the points with a larger partial vapor pressure difference from the iso-concentration line. Moreover, design parameters are optimized to reduce irreversible loss of mass transfer and parameters are adjusted to achieve energy matching and performance improvement.