Second law optimization of Y-shape fins for the solidification process in a PCM based storage system

The aim of this paper is to perform a thermodynamic optimization of a Y shaped fin design used to improve thermal performance of a cylindrical latent heat thermal energy storage (LHTES) unit. The investigation is performed by means of a CFD model that takes into account of the thermal behavior of the system. Temperature and phase fields are obtained to characterize the heat transfer phenomenon and to compute entropy generation rate within the system. Global entropy generation and energy flux are then adopted as objective functions in order to perform a shape optimization of the Y shaped fins with angles and branch lengths that can vary freely. The optimization results indicate that higher energy transfer is achieved by fin configuration with long secondary branch with an orientation angle of 30°; this design allows to increase PCM solidification rate of about 30% with respect to radial fins. Furthermore that Y-shaped fins allow also to increase exergy flux released by the PCM thus Second-law efficiency is not affected although entropy generation increases. In the authors’ knowledge this work represents a 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.