Numerical analysis for the optimal designs for thermal energy storage systems using phase change material (PCM)

Abstract

Using the latent heat of phase change materials (PCM) in latent heat thermal energy storage (LHTES) systems can effectively increase the thermal energy storage capacity while avoiding occupying large volume when using sensible heat of water in stratified water storage (SWS). However, the storage effectiveness of an LHTES can be hindered by low thermal conductivities of PCMs. Previous studies have shown that the insertion of expanded graphite into PCMs could improve their effective thermal conductivity by 50 times (Haillot et al. 2008). Built from the highly developed heat transfer enhancement technique, the work reported in this Thesis systematically analyzed the effect of heat transfer enhancement in PCM and other parameters on the optimal design for the basic unit of a tube-in-tank LHTES system. Firstly, an index of effective energy storage ratio, Est, was proposed to characterize the effective energy storage capacity of an LHTES system with reference to an ideal SWS system at the same volume. Secondly, using a CFD software, the effect of various parameters on Est of an LHTES system was tested. Thirdly, an analytical technique of required heat transfer length was developed from the effectiveness-NTU theory and used to predict the optimal designs for an LHTES system under various conditions. Finally, the effect of charging conditions on the subsequent discharging performance was tested in the optimal designs. The results of the current study demonstrated that the improvement in effective thermal conductivity of a PCM would remarkably enhance the effective energy storage of an LHTES system if other parameters, such as compactness factor for PCM, mass flow rate, tube length of a heat transfer fluid (HTF) and charging conditions, were well designed. Moreover, by enhancing heat transfer in both PCM and HTF, an LHTES could obtain both a high Est and a high COP at little cost of tube materials.