Dynamic melting of encapsulated PCM in various geometries driven by natural convection of surrounding air: A modelling-based parametric study

Encapsulated phase change material (PCM) has a great potential to reduce the fluctuation of indoor air temperature and subsequent building energy consumption through ceiling applications in air-conditioning systems. However, the effects of capsule geometry on the dynamic melting performance remain unknown when the melting is driven by natural convection. In this work, we modelled the melting behavior of PCM encapsulated in six different capsule shapes subject to natural convection of surrounding air, which was validated by our measurement results. The model considers the conjugate heat transfer through the surrounding air, capsule shell and PCM, enabling the prediction of a three-stage dynamic melting process. The melting process of the encapsulated PCM were further studied for varying inclination angles (with respect to the vertical axis), temperature differences (between the surrounding air and PCM), and capsule sizes. The results show that the capsule geometry plays a dramatic role in the time of phase change. At the ambient temperature of 22°C, the short cylindrical capsule with a horizontal axis takes the longest time to complete phase change (162 min), whilst the pyramidal capsule with a horizontal base takes the shortest time (120 min). We also quantified the heat transfer between capsules and surrounding air for varying temperature difference and capsule size, and found that the pyramidal and tetrahedral capsules with a horizontal base melt faster than capsules of other shapes due to the higher heat transfer rate caused by larger surface area. For cases with a medium capsule size and low temperature difference, early termination of melting at 80% liquid fraction is recommended since more than 30% of melting time can be saved with only 20% reduction in cold storage capacity. This work provides a validated model that is able to predict the dynamic melting process of PCM of different geometries, which will enable informed design of PCM panels for improving thermal management and comfort in buildings.