Year
2022
Degree Name
Doctor of Philosophy
Department
Sustainable Buildings Research Centre
Abstract
Liquid desiccant air conditioning (LDAC) systems, which can independently deal with sensible and latent heat loads, have attracted increasing attention. In LDAC systems, regeneration is a key process for maintaining the dehumidification function of the liquid desiccant solution, which consumes a large part of the system energy demand. Most previous studies have been focused on direct contact regeneration, whilst this regeneration method has a major challenge of liquid desiccant droplets carryover. Membrane distillation has been recently recognised as an attractive alternative to regeneration as it can nearly eliminate the carryover issue in the regeneration process by using a microporous membrane and can generate dual products, i.e. concentrated liquid desiccant solutions and freshwater. However, the research development of membrane distillation for liquid desiccant regeneration is still in the early stage and further investigations are needed. This thesis aims to investigate and optimise membrane distillation for liquid desiccant regeneration and develop a membrane distillation regeneration-assisted LDAC system driven by solar energy for air dehumidification, cooling, and freshwater production. The novelties of this work include evaluating the membrane distillation performance by considering concentration polarisation and membrane fouling and, for the first time, analysing the feasibility of integrating membrane distillation into an LDAC system.
This study first experimentally investigated the performance of a flat plate direct contact membrane distillation (DCMD) module for regenerating a 25-30 wt.% lithium chloride (LiCl) solution. The results showed that the DCMD regenerator can provide stable regeneration capacity and water flux, and over 99.99% of LiCl salt rejection rate with low thermal efficiency due to the high feed concentration. The temperature difference between the feed and distillate sides should be higher than a threshold to avoid negative flux in the DCMD regenerator. The interactive effects of key operating parameters, i.e. initial feed concentration, feed inlet temperature, distillate inlet temperature, and volumetric flow rate, were studied by means of the response surface method (RSM). The RSM models showed good model fitness with R2 of 0.9820, 0.9787 and 0.9689 for the regeneration capacity, water flux and thermal efficiency, respectively. The predicted regeneration capacity showed a similar trend to that of water flux. In terms of regeneration capacity, the feed inlet temperature was the most significant influencing variable, whilst an interaction between the initial feed concentration and the volumetric flow rate was observed. In terms of thermal efficiency, the feed inlet temperature and distillate inlet temperature showed a significant interactive effect, whilst increasing the volumetric flow rate had a negative effect on thermal efficiency.
Recommended Citation
Liu, Jingjing, Experimental and Numerical Investigation of Membrane Distillation for Desiccant Regeneration in Liquid Desiccant Air Conditioning Systems, Doctor of Philosophy thesis, Sustainable Buildings Research Centre, University of Wollongong, 2022. https://ro.uow.edu.au/theses1/1580
FoR codes (2020)
4017 Mechanical engineering, 4012 Fluid mechanics and thermal engineering
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.