Analysis of heat and mass transfer in membrane-based absorbers with new working fluid mixtures for absorption cooling systems

Abstract

Absorption refrigeration technology, which has the ability to utilize heat directly for cooling purposes, has been one of the most widely used technologies for refrigeration and cooling applications since the early stages of refrigeration technology. Working fluid mixtures employed in the absorption cooling systems are environmental friendly and do not contribute in green house gas emission when compared to vapour compression systems which also use costly mechanical energy input. However, high initial costs and bigger size are some of the main obstacles that impede their wide use in small scale residential buildings and transport sector. In order to overcome these obstacles, design and configuration of the system and its components need to be reinvestigated in order to achieve compact components and reduce the size of the system. Use of membrane contactors in the form of hollow fiber membrane module or plate-and-frame membrane module is one of the alternatives to achieve compact components. Absorber is an important component of the absorption refrigeration system and plays a critical role in the overall performance, size, and capital cost of the system. In this study, numerical analyses are performed to evaluate the performance of a plate-and-frame membrane contactor based absorber employing water/(LiBr + LiI + LiNO3 + LiCl) and water/(LiNO3+KNO3+NaNO3) working fluid mixtures for air cooled absorption cooling systems and multi-stage high temperature heat sources applications, respectively. CFD tool ANSYS/FLUENT 14.0 is used to perform the simulation and investigate in detail the heat and mass transfer mechanisms and the fluid dynamics behaviour at local levels in the channels. Moreover, a MATLAB code is developed to investigate the effect of membrane material characteristics and operating conditions on the absorption performance of the absorber. This study recommends optimum operating and design parameters to effectively utilize the membrane based absorber.