An analytical solution of heat and mass transfer in a counter/parallel flow plate membrane module used in an absorption heat pump

A plate membrane module is applied in an absorption heat pump for heating to provide domestic hot water. In the membrane module, water and salt solution are used as refrigerant and absorbent, respectively. The water and the solution streams, in a counter flow or a parallel flow arrangement, are separated by the membranes, which only allow transmission of water vapor, while prevent the water and the solution from permeating. The water vapor evaporated from the water stream is transferred across the membranes and the air-gaps and then absorbed by the solution stream. Absorption heat is then generated to heat the solution stream. In the membrane module, an element, including two plate membranes, an air-gap, and two neighboring channels, is selected as the calculation zone. A heat and mass transfer model is established in an element to study the coupled heat and mass transfer. An analytical solution is obtained and experimentally validated for the normalized equations governing the momentum, heat and mass transports, which provides a fast, accurate, and convenient tool for the structural design and optimization of the module. Influences of the dimensionless parameters on the solution temperature lift are studied. It is concluded that the solution temperature lift first increases and then decreases with a decrease in the solution mass flow rate. The solution inlet mass fraction (Xs,in) of 0.55, the solution inlet temperature (Ts,in) of about 25 °C, and the aspect ratios of the air-gaps (W/da) of about 50 may have the best performances of the membrane modules. Both the sensible and latent heat transfer parameters are the dominate factors affecting the solution temperature lift.