Performance Optimization Of A Hybrid Connection Two-stage Fresh Air Dehumidification System Based On CaCl₂/LiCl Double Solutions

Liquid dehumidification technology has huge energy saving potential due to its high dehumidification efficiency and low-grade heat source requirements.This paper proposed a hybrid connection two-stage liquid dehumidified fresh air system based on CaCl2/LiCl double solutions(HTLDFA).In this system,the ambient air is adopted as the regeneration air of the CaCl2 solution while which of the LiCl solution is prepared in the second CaCl2 dehumidifier by a part of concentrated CaCl2 solution.The remaining concentrated CaCl2 and all concentrated LiCl solutions are used to dehumidify the fresh air in turn to achieve the purpose of deep dehumidification.Firstly,the performance of this system is analyzed through simulation,and the results show that it can more efficiently use low-temperature heat sources.When processing the outdoor air with a moisture content of 30.5g/kg to 9.4g/kg,the energy efficiency ratio and exergy efficiency of the system are 0.57 and 0.47,respectively.Besides,the optimal NTU of the main components of the system locate between 2-4 when the fresh air flow rate is 0.56kg/s and the required supply air moisture content is 9.67g/kg;the two-stage regeneration air flow rate should not be less than 1.6kg/s and 0.8kg/s,and the hot water flow rate should be greater than 1.9g/kg and 0.8g/kg.In addition,under the hybrid connection mode of CaCl2 and LiCl solution,the maximum dehumidification rate can be reached when the flow of CaCl2 solution entering the firststage dehumidification DehI accounts for 60%of the total flow.Secondly,analogous to the temperature entransy dissipation number of a heat exchanger,the definition of enthalpy entransy is proposed from the perspective of coupled heat and mass transfer in solution dehumidification/regeneration process,and is nondimensionalized to the enthalpy entransy dissipation number.Based on it,an analytical model of the counter-flow and parallel-flow dehumidifier/regenerator and cooling tower is established.The results show a good agreement with the numerical model and the experimental data that the errors do not exceed±8%.Finally,the effects of the irreversible degree of enthalpy transfer on the performance of dehumidifier/regenerator,cooling tower and system is analyzed based on the above analytical model.Results show that the enthalpy entransy dissipation number varies from 0 to 1 with NTU and enthalpy efficiency,and the change trend is consistent with the traditional exchanger;it is found that when the enthalpy entransy dissipation number is at a small level,the degree of dehumidification/regeneration is deep,while the heat and mass transfer between the solution and the air tends to stop when the enthalpy dissipation number tends to 1;when the enthalpy entransy dissipation number is constant,changing the equivalent heat capacity ratio(C*)of the air and the solution is equivalent to changing the flow rate of the air or the solution.In the case of known fresh air flow,increasing the C*of the first-stage dehumidifier and second-stage regenerator is beneficial to increase the EER,while increasing C*of the second CaCl2 dehumidifier,second-stage dehumidifier and CaC12 regenerator will lower EER.The enthalpy entransy dissipation number and analytical model proposed in this paper provide a new method to characterize the irreversible degree of dehumidification/regeneration of a solution and predict its performance.