The Effect of PCM Capsule Material on the Thermal Energy Storage System Performance

Phase change material (PCM) based thermal energy storage (TES) systems are gaining increasing importance in recent years in order to reduce the gap between energy supply and demand in solar thermal applications. The present work investigates the effect of PCM capsule material on the performance of TES system during charging and discharging processes. The TES unit contains paraffin as PCM filled in spherical capsules and is integrated with flat plate solar collector. Water is used as sensible heat material as well as heat transfer fluid (HTF). The PCM capsules are of 68？mm diameter and are made using three different materials, namely, (i) high density polyethylene (HDPE), (ii) aluminum (Al), and (iii) mild steel (MS). The experimental investigation showed that the charging and recovery of stored energy are less affected by the spherical capsules material. The variables, like charging time and discharging quantity, are varied around 5% for the different capsule materials. Even though aluminum thermal conductivity is much higher than HDPE and mild steel, its influence on the performance of TES system is very low due to the very high internal heat resistance of PCM material stored in the spherical capsules. 1. Introduction Solar energy is the most promising inexhaustible heat energy source for the present and future needs of mankind. Compared to the power generation from the solar energy, utilization of solar energy for moderate temperature heat applications is more efficient and economical. The increasing cost of fossil fuels in the recent years is making solar energy utilization more economical for heating applications. One of the major problems with the solar energy is its intermittent nature. So to balance the energy supply and demands, a heat energy storage system is necessary. There are three main methods of thermal energy storing systems, that is, sensible, latent, and combined sensible and latent heat storage systems. The thermal energy storage systems using both sensible and latent heat storage methods are gaining a lot of importance now a days, due to their high thermal energy storage capacity per unit volume and isothermal behavior during charging and discharging processes. In this direction, a lot of research is going on throughout the world for improving the performance of TES systems in the recent years. Some of the important contributions related to TES system using sensible and latent heat are presented. Weislogel and Chung [1] reported the effect of condensation heat transfer in small arrays of PCM filled spheres on the charging process of