Second Law Optimization of a PCM Based Latent Heat Thermal Energy Storage System with Tree Shaped Fins

The aim of this paper is to perform a thermodynamic optimization of a Y shaped fin design used to improve thermal performance of a latent heat thermal energy storage (LHTES) unit. The investigation is performed through a CFD model that takes into account the thermal behavior of the system. Temperature and phase fields are obtained to characterize the heat transfer phenomenon and to compute the entropy generation rate within the system. Global entropy generation and energy flux are adopted as objective functions to perform a shape optimization of the Y shaped fins with angles and branches lengths that can vary freely. The optimization results indicate that a higher energy transfer is achieved by a fin configuration with long secondary branches with an orientation angle of 30°. This design allows one to increase PCM solidification rate of about 30%. Furthermore, Y-shaped fins allow to increase the exergy flux released by the PCM, thus Second-law efficiency is not affected although entropy generation increases. This work represents the first detailed thermodynamic optimization of a system involving an unsteady process. This aspect is particularly important since a clear tendency of many energy systems is toward transient operation, thus design optimization methods should evolve accordingly.