Influence of PCM thermal conductivity and HTF velocity during solidification of PCM through the free cooling concept – A parametric study

Abstract

Abstract The thermal performance of a latent heat thermal energy storage (LHTES) system can be enhanced by incorporation of fins, metal matrices, lessing rings in the phase change material (PCM) encapsulation and by dispersion of high conductive nanomaterials in the PCM itself. Similarly, to increase the heat transfer to PCM, the surface convective heat transfer coefficient (‘h’) can be enhanced by increasing the heat transfer fluid (HTF) velocity (‘u’). However, it is important to know how much increase in PCM thermal conductivity (‘k’) and ‘h’ will be beneficial during the charging of PCM under real-time ambient conditions and when will we reach the margin of diminishing returns. Understanding these are the motivation of the present work which includes the parametric study of the impact of ‘k’ & ‘u’ on the charging of PCM (RT28HC) through free cooling concept under different operating conditions (two different HTF inlet temperature). The numerical results are validated using the experimental data and they both show good agreement with each other. The major inferences from the results are, i) increasing the PCM thermal conductivity reduces the charging duration of the LHTES system for both lower and higher HTF velocity (1 m/s and 8 m/s). However, reduction in charging duration while increasing ‘k’ is higher when the HTF inlet temperature is lower, ii) increasing the HTF velocity is beneficial only when the inlet HTF temperature is higher. For the case of lower HTF inlet temperature, the effect of increasing the HTF velocity is suppressed by the higher temperature driving potential between the HTF inlet temperature and the PCM phase change temperature. It is inferred from the parametric study that, among the three parameters considered (PCM thermal conductivity, HTF velocity, and HTF inlet temperature), HTF inlet temperature has the most influence on the charging of the PCM, followed by the PCM thermal conductivity and HTF velocity.