Heat Transfer Characteristics of Solar Thermal Storage System using Phase Change Materials

Abstract

This research is concerned with finding a correlation describing the heat transfer process occurs inside the sensible as well as the latent thermal energy storage. The study was done on a thermal energy storage that is based on the concept of the fully-mixed tank. The studied thermal energy storage contains a spiral coil for charging placed at the bottom part of the tank and phase change modules (PCMs). When the hotter heat transfer fluid passes inside the spiral coil, the heat transfers by the natural convection currents to the PCM modules inside the tank. The PCM modules store the energy in the form of latent heat. To describe the intricate heat transfer process of the studied tank, scale analysis was used to find a correlation between Nusselt number and Rayleigh number. Eight CFD simulations were performed and validated for eight configurations of the studied tank to get closure coefficients for the deduced correlation. The studied configurations vary in the heat transfer fluid flow rate, the heat transfer fluid inlet temperature and PCM volume fraction inside the tank. The novel correlation produced a curve of Nusselt number which perfectly fits the resulted Nusselt data of simulations. The effect of changing the heat transfer fluid flow rate, the heat transfer fluid inlet temperature and PCM volume fraction is discussed in detail. Increasing mass flow rate or temperature of coil fluid leads to increase in the storage medium temperature while increasing PCM volume fraction leads to increase the rate at which the storage medium decreasing. However, for those who will build their studies based on this study, more geometric and thermophysical properties should be studied to be included in such correlation. Other configurations and cases should be investigated, too, and the constants of the correlation have to be modified accordingly.