Numerical analysis of the effect of the iso-surface fin redistribution on the performance enhancement of a shell-and-tube latent heat thermal energy storage unit for low-temperature applications

Abstract

In the present study, the melting and solidification processes of a Phase Change Material (PCM) are numerically carried out in eight geometrical configurations of horizontal Latent Heat Thermal Energy Storage (LHTES) units for low-temperature applications. The PCM considered in this study is water undergoing solidification below its density-temperature inversion point (T < 4 °C) at atmospheric pressure, and the Heat Transfer Fluid (HTF) considered is a water/glycol mixture solution. Among the eight geometrical configurations studied, four are made of longitudinal finned tubes and a shell and the other four are made of radial finned tubes and a shell. All the geometrical configurations studied have the same PCM volume, compactness and total heat transfer surface. The objective of the study is to compare their thermal performances in order to highlight the most efficient configurations during melting and solidification processes. Effects of fin redistribution at a constant total heat transfer surface on the local melting and solidification mechanisms are investigated. Variations in the solid volume fraction during melting and solidification and the associated kinetics are compared and subjected to quantitative analysis. Natural convection and its effect on the melting and solidification mechanisms are studied and analysed using the commercial CFD code Star CCM + V12.02. This study shows that increasing the number of fins, at a constant PCM volume, compactness and total heat transfer surface, reduces the thermal performance of the heat storage unit when complete melting or solidification is desired. The opposite behaviour is observed at low melting/solidification rates. Among the configurations studied, the best thermal performances at the end of the melting/solidification phases are obtained in heat storage units with a smaller number of fins. A solidification rate higher than 82 % was reached with the 4 longitudinal fins configuration, compared with 75.7 % with the 8 longitudinal fins configuration, 63.3 % with the 16 longitudinal fins configuration and 55 % with the 32 longitudinal fin configuration after 4 h of solidification process. A similar succession of solidification rates was observed with radial fin configurations. During the melting process, the same trends that had been observed during the solidification process were noted: better performance from configurations with 4 longitudinal or radial fins than from the other configurations with larger numbers of fins after 4 h of the melting process. Conversely, at the very beginning of the melting/solidification process, better performances were obtained in the configurations with the largest number of fins. Therefore, for given values of heat transfer surface augmentation and PCM volume, in shell-and-tube storage units used for low-temperature applications, it is recommended to prioritize finned-tube configurations with small numbers of tall fins rather than those with large numbers of short fins when complete melting or solidification is desired.