Performance evaluation of an innovative liquid desiccant air-conditioning system directly integrating a heat pump with two membrane energy exchangers

Abstract

For high moisture removal applications with improved indoor air quality, the use of liquid-to-air membrane energy exchangers (LAMEEs) in liquid desiccant (LD) air-conditioning systems has been suggested in the literature. This study introduces a novel configuration for a heat pump LD system to improve its dehumidification and energy use. The evaporator and condenser coils of the heat pump (HP) are embedded in the LD channels of two LAMEEs; one is an internally cooled dehumidifier (ICD), and the other is an internally heated regenerator (IHR). These LAMEEs are designed to directly use the HP condenser coils for the IHR's regeneration process while also effectively controlling the local temperature rise of the LD solution inside the ICD. An integrated CFD model for both the ICD and the IHR is simultaneously solved, considering the coupling of the HP components and the closed loop of the LD solution. This model, which counts all existing balances of energy, mass, and salt content in various components, determines the condensing temperature employed in the regeneration process at each operational condition. This system-level model, which requires few inputs, is novel and very reliable for evaluating the overall performance of the proposed system in terms of specific dehumidification rates and COP. Case studies were carried out using the annual dehumidification design conditions of five Middle Eastern cities known for their hot and humid climates. For the cities of Sharm El Sheikh and Dammam, respectively, the system achieved promising results with specific dehumidification rates of 130.3 and 172.2 g/(m2.h) and system COPs of 6.38 and 5.67, when the temperature and specific humidity of outside fresh air of (30 °C, 19 g/kg) and (32.8 °C, 23.2 g/kg) were cooled and dehumidified to (20.18 °C, 6.52 g/kg) and (21.0 °C, 6.61 g/kg). Using the regeneration air flow rate as high as possible boosts the performance of the proposed HPLD system, concluding that the system is preferable for all-fresh-air handling units.