Modeling and performance analysis of a hybrid-connected two-stage liquid dehumidification fresh air system based on CaCl2/LiCl double solution

Abstract

In this paper, a novel hybrid-connected two-stage liquid dehumidification fresh air (HTLDFA) system by combining CaCl2 with LiCl aqueous solutions is proposed, in which the ambient air is adopted directly as the regeneration air of CaCl2 solution and the air pre-dehumidified by part of the strong CaCl2 is used for regenerating the LiCl solution. In this way, regeneration air of lower humidity for LiCl solution can improve the regeneration efficiency of the LiCl solution and reduce its regeneration temperature. The NTU optimization of main units and the action mechanisms of other fluid parameters on the dehumidification performance of that system are examined. Numerical simulation results show the proposed system is more efficient in utilizing the low-temperature heat sources and the energy and exergy efficiencies can reach about 0.57 and 0.47, respectively. For processing humid fresh air of 0.56 kg/s by using this system, the NTUs of main components are in the optimal range of 2–4 and the flow-rates of two-stage regeneration air are not less than 1.6 kg/s and 0.8 kg/s as well as their driven hot water flow-rates should be more than 1.9 kg/s and 0.8 kg/s. In the HTLDFA system, the water flow-rate of the second-grade cooling tower by using the indoor exhaust air as evaporative air only needs about half that of the first-grade cooling tower of using surrounding air as evaporative air. As for the hybrid-connected mode of CaCl2 and LiCl solutions, the flow-rate of CaCl2 solution entering the first-grade dehumidification shares 60 percent of its total flow-rate to achieve the optimal system performance. Performance comparison with the previous two-stage liquid dehumidification system shows that the proposed system has greater efficiency in using low-grade heat sources and achieves higher EER and exergy efficiency.