Synergistic effects of nano-enhanced phase change material (NePCM) and fin shape on heat storage performance of a finned shell-and-tube unit: An experimental study

Adding a fin or using a nano-enhanced phase change material (NePCM) can both improve the performance of shell-and-tube thermal energy storage (TES) units, but the combination of them may not be simple to achieve synergistic effect. In this paper, an experimental study was conducted to reveal the combined effects of fin shape and nano-additive loading on the heat storage performance of a finned shell-and-tube TES unit filled with NePCM, with nine cross-portfolio experimental configurations, i.e., three loadings of the nano-additive, graphene nanoplatelets (GNP) in this work, and three fin shapes including annular, helical and longitudinal fins. It was shown that the changes of GNP loading and fin shape could have either enhancement or deterioration effect on the various mechanisms of heat transfer during melting, such as conduction, convection, and close-contact melting mode, and that their combined effects on the heat transfer and heat storage rates of the TES unit are very complicated. The fin with a given shape should be combined with a suitable GNP loading to achieve synergistic effect for different application scenarios. For example, in pursuit of higher heat transfer rate (fast charging and discharging), the configuration of a certain GNP loading combined with the annular fin can be adopted. Considering the manufacturing cost, the annular fin with a gap between the fin and the shell wall can also be used to achieve the same heat transfer enhancement effect as that of the helical fin. In case of the longitudinal fin, the GNP loading has a great negative impact on the overall melting rate and the instantaneous heat storage rate, so it should be used in combination with pure PCM without adding any nano-additives.