A design protocol for enhanced discharge exergy in phase change material heat battery

Abstract

Thermal energy storage using phase change materials (PCMs) is finding a plethora of emerging applications, yet it suffers from the detriment of asymmetric discharging where the temperature of the heat transfer fluid (i.e. water) often drops significantly below the melting temperature of the PCM. This phenomenon significantly impacts the servicing value of the stored thermal energy and would eventually result in over-reliance on the auxiliary fossil fuel input. In this paper, we have studied the heat discharge issue using a validated PCM model (shell-n-tube heat exchanger) and characterized three distinctive heat discharge regimes; each is featured with a unique thermal barrier profile which constrains hot water production within a certain temperature range. These heat discharge regimes are attributed to the formation of a solid PCM layer around the water pipe throughout the discharge phase. Such layer completely isolates the water flow in the tube from the molten PCM in the annulus and acts as a thermal barrier that constrains the maximum achievable temperature. The development of thermal barrier could be minimized by changing the PCM design parameters/vairables. In this work, we have proposed a computational design framework that does not rely on the traditional trial-and-error approach while capable at generating a PCM system design with minimal thermal barrier development. For a solar water heater (SWH) case-study, the result shows a significant improvement in the PCM system exergy efficiency and translates to 18% reduction in fossil fuel consumption for a PCM integrated solar hot water system.