The aim of this work is to investigate design improvements of a shell-and-tube latent heat thermal energy storage unit using an approach based on the analysis of entropy generation. The study is conducted by means of a computational fluid-dynamic (CFD) model which takes into account phase change phenomenon by means of the enthalpy method. Thermal-fluid dynamic problem is solved both for the phase change material (PCM) and heat transfer fluid (HTF). The different contributions to the local entropy generation rate are computed and presented for both un-finned and finned systems. Fin arrangement is then modified according with the analysis of entropy generation distribution in order to increase the efficiency of the system. The results show that the improved system allows to reduce PCM solidification time and increase Second-law efficiency. The present paper constitutes a first detailed investigation of time evolution of entropy generation occurring during an unsteady process. (C) 2013 Elsevier Ltd. All rights reserved.