A 3D resolved-geometry model for unstructured and structured packed bed encapsulated phase change material system

Abstract

A 3D numerical model is developed to simulate melting in packed bed encapsulated phase change material (PCM) energy storage systems. The main novelty of the model is that it resolves the arrangement of capsules inside the system for both structured and unstructured packing and thus accurately captures the fluid flow, heat transfer and phase change in the capsules. To create the unstructured arrangement of capsules in the domain a novel dropping and rolling algorithm is implemented. The flow of heat transfer fluid (HTF) outside the capsules as well as the natural convection in the PCM within the capsules is also simulated. Initially, a single capsule study is performed, which shows that both natural convection inside the liquid PCM and forced convection outside the capsule affect the melting characteristics. Subsequently, the model is extended to simulate phase change in multiple capsules in a cubical domain arranged with structured and unstructured packing. The analysis of the effect of capsule arrangement on the melting and energy storage characteristics for both structured and unstructured packing of capsules is performed. Additionally, parametric studies are performed to analyze the effects of capsule size, fluid temperature, and fluid velocity for both types of systems. Simulation results show that the total melting time for unstructured packing is reduced on an average by 23.56% for the 2.5 mm capsules and by 10.14% for the 5 mm capsules as compared to structured packing. The effect of HTF temperature change is more prominent for structured cases, with an increase of 65.8% melting time as compared to 58.4% for the unstructured cases when the HTF temperature is reduced by 20 K. In contrast, the effect of velocity is less significant for the structured cases with a reduction of HTF velocity from 0.1 cm/s to 0.05 cm/s causing an increase of 27.3% melting time for the structured cases and 42.6% for the unstructured cases. The developed model can be used to capture the effect of different arrangements of capsules in encapsulated PCM energy storage systems and thus obtain effective designs for such systems.