Heat and mass transfer in a counter flow parallel plate membrane-based absorption heat pump (PMAHP)

Abstract

Membrane-based absorption heat pumps (MAHP) operate based on the working principle of the absorption refrigeration system (ARS), which could be used to recover and transform low-temperature waste heat into useable heat sources with higher temperatures. This work focuses on the performance study of a counter-flow parallel-plate membrane-based absorption heat pump (PMAHP) to recover low-temperature waste heat from used cooling water at 40 °C. It consists of refrigerant (water) and absorbent (LiCl solution) streams flowing in neighboring channels formed by hydrophobic microporous membranes with air gaps sandwiched in between these channels. Air-gap design is incorporated to minimize the sensible heat loss through conduction between the two streams. Water vapor molecules travel from the water stream to the solution stream through the membrane and air gaps. As the water molecules are absorbed by the solution, they condense to release the latent heat of absorption and dilution. The recovered heat can be used subsequently for fluid heating or air-conditioning purposes. A three-dimensional, steady-state model based on the finite element method is used to study the conjugate heat and mass transfer mechanisms. Model validation results agree with experimental data with a general discrepancy of within 10%. Parametric studies on the performance of the PMAHP are carried out. Scaling analysis is applied to study the effects of geometrical parameters on the heat and mass transfer dimensionless parameters, fluid flow behavior, heat, and mass transport within the PMAHP. The optimal air gap width is determined to achieve maximum solution temperature lift, which shows an improvement of 99.6% compared to the base case. The findings of this study provide an insight regarding the potential aspects to be focused on for further enhancement in the PMAHP heat and mass transfer performance.