Evaluation of Heat Rejection Strategies for Liquid Desiccant Air-Conditioning Systems
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The increased demand for space cooling and the resulting summer peak in electrical demands are motivating research into thermally-driven air-conditioning systems that can use renewable or waste heat. One promising technology is the use of a Liquid Desiccant Air-Conditioning (LDAC). An evaporative cooling tower (ECT) was used as a cooling device to reject the heat from a LDAC at Queen’s University. Investigation of the ECT showed significant power consumption and ineffective operation in humid conditions which contributed to the low electrical and thermal coefficients of performance (COPE and COPT). Therefore, the present study was undertaken to identify and evaluate alternative heat rejection strategies that can provide a better thermal and electrical system performance. The relationship between the relative humidity and the ambient air temperature at a variety of cooling water temperatures was modelled using TRNSYS, a transient simulation program, as the first stage of the study. To find the most promising heat rejection technologies, available cooling methods were reviewed and two were selected (based on the results of the first phase of the study), namely, (i) a cooling water storage (CWS) system and (ii) a ground-source heat exchanger (GSHX) system. These were simulated and evaluated in detail. Finally, an experimental investigation of a stratified cooling water storage tank (SCWS) and night cooling water storage (NCWS) was undertaken to confirm the results of an additional simulation on the use of a stratified cooling tank. From the simulation results and the overall comparison of the selected heat rejection strategies of the LDAC, it was concluded that GSHX system with 25 boreholes showed better COPE than the CWS system whereas the COPT improvement of both systems was almost same. The improved performance of the GSHX however, is expensive as the capital cost for boreholes is higher than the other systems. When comparing the CWS system (with mixed tank) and the SCWS (with stratified tank) it was observed that the stratified tank had the better potential of improving the COPT, depending upon the water flow rate and initial tank temperature.