Exergy analysis of particle dispersed latent heat thermal storage system for solar water heaters

Abstract

Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device if the energy source is intermittent and time dependent. However, the performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. The addition of high conductivity particles is proposed as one of the promising performance enhancement techniques for LHTS. In our study, an attempt is made to investigate the performance enhancement of a shell and tube storage unit applicable to solar water heaters due to the addition of high conductivity particles (copper) through exergy analysis. The enthalpy based two dimensional-transient equations are solved numerically for discharging process using FLUENT—a commercial computational fluid dynamics code. The numerical results show good agreement with experimental results available in the literature. The thermal behavior and performance of particle dispersed PCM unit in terms of exergy efficiency and total exergy recovered are compared with that of pure PCM unit. It is observed that the discharging time is reduced by around 28% by adding particles of volume fraction 0.1 and by around 85% with 0.6 volume fraction. However, the decrease in discharging time is not much appreciable beyond the particle volume fraction of 0.4. Similarly, the exergy efficiency and total exergy recovered are increased by 12% and 15%, respectively, with 0.1 volume fraction and by 50% and 60%, respectively, with 0.4 volume fraction. Different Reynolds numbers (i.e., Re=200–10 000) are considered in the numerical trials to investigate the influence of mass flow rate of heat transfer fluid (HTF) on the performance of the system. The results reveal that the role of HTF mass flow rate in increasing the overall thermal performance of LHTS unit is insignificant.