Advanced Thermal Energy Storage Heat Transfer Study with Use of Comsol and Matlab

Abstract: The interest in storing latent energy in phase change materials has risen over the last years as the need grows for more energy efficient systems. By storing energy, free chilling and heat can be saved for later use during high load hours. Thus the gap between supply and load can be overcome. It is an efficient way to provide both cooling and heating to buildings using phase change material (PCM) as they take up much less volume compared to a corresponding water-cistern with the same amount of stored energy. Low thermal conductivity of most of the PCMs can be compensated with advanced heat transfer design, however impact of different heat transfer mechanisms is not explicitly studied. In this work, a heat transfer study has been made on a finned cylindrical PCM heat exchanger with focus on determining the heat transfer effect of convection in non-gelled PCMs and the different ways to model such a system in a two dimensional axis-symmetric plane. The first and simpler numerical model of the two was built using Matlab, where the convection effect was simulated using an enhanced-conduction factor based on empirical equations. The other model was built in a CFD environment and simulates the convection with more complexity and more realistic behavior. The results show that the convection may contribute to 65% of the total heat transfer in non-gelled PCMs at a certain time and that using empirical equations for simulating convection is a fast and easy way to estimate the heat transfer, though not a recommended method for high accuracy results. The study also showed that because of the gravity-induced convection, the angle of the cylindrical finned heat exchanger affects the heat transfer and that more fins, while increasing the overall heat transfer rate, inhibits the effect of convection in a vertically positioned heat exchanger.