Numerical analysis of the performance enhancement of a latent heat storage shell and tube unit using finned tubes during melting and solidification

The present study deals with the numerical analysis of the melting and solidification processes in three horizontal storage units for Latent Heat Thermal Storage (LHTS) using water, around its density-temperature inversion point as phase change material (PCM) and a water/glycol solution as the Heat Transfer Fluid (HTF). One of these storage units, made of a plain tube and a shell is used in this study as the reference against which the other two configurations, made respectively of longitudinal and radial finned tube and shell are compared at constant heat transfer surface, compactness and PCM volume. The objective is to improve the thermal performance of the reference unit, and therefore the charge and discharge rates using surface extension. The effect of adding longitudinal or radial fins to the tube on the local mechanism of melting and solidification is studied. The major phenomena involved in these mechanisms such as natural convection are highlighted and their effects during melting and solidification processes are analysed using the CFD code Star CCM + V12.02. Quantitative analysis including the temporal evolution of the PCM average temperature and the solid volume fraction are analysed and compared for the three configurations. This study exhibits a complex natural convective flow structure in the three storage units due to the inversion of the buoyancy forces in the PCM for the considered temperature range -4 degrees C to 4 degrees C. During the solidification process, natural convection intensively develops at the earlier stages of the process and, as the time goes, its effect on the solidification becomes negligible as compared to the one due to conduction. During the melting process, heat is transferred to the PCM first by conduction and later by natural convection. The use of fins significantly increases the solidification and melting rates, especially for the configuration with longitudinal fins for which performance are found to be superior to the two other configurations.