Entropy generation analysis for the design improvement of a latent heat storage system Elisa Guelpa, Adriano Sciacovelli * , Vittorio Verda Energy Department, Politecnico di Torino, Torino, Italy article info Article history: Received 16 September 2012 Received in revised form 7 January 2013 Accepted 11 February 2013 Available online 16 March 2013 Keywords: PCM Thermal energy storage CFD Entropy generation analysis abstract 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 con- ducted 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 en- tropy 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. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Imbalance between energy demand and energy supply affects several kind of technologies. Such issue is especially relevant for energy systems; typical examples include solar energy utilization, thermal power generation, combined cooling, heating & power system and air conditioning. As a consequence energy storage plays a fundamental role when it is necessary to fill the gap between energy availability and the need. The most common thermal energy storage technologies are sensible heat storage and latent heat storage. Latent heat thermal energy storage (LHTES) systems have received great attention of several researchers due to their high energy storage density, compactness and the possibility of storing and delivering energy at near constant temperature. In LHTES units phase change materials (PCM) are utilized which undergo melting or solidification process when energy transfer occurs between the PCM and the working fluid. In the last 25 years various typologies of PCM have been investigated; similarly several geometrical configurations have been studied for the PCM enclo- sures, such as spherical shell, cylindrical pipes and shell-and-tube configuration. For a comprehensive review of LHTES system applications and PCM distinctive characteristics the reader can refer to papers by Agyenim et al. [1] and Zalba et al. [2]. Despite the relative merit of latent heat energy storage the main disadvantage of such technology is related to poor thermal prop- erties of PCMs, in particular low thermal conductivity of phase change materials often leads to unacceptable low melting and so- lidification rates. As a consequence overall effectiveness of the system is strongly affected. Therefore the advance of LHTES re- quires the understanding of thermal behaviour of PCM during the phase transition and the performance assessment of LHTES units. A great amount of investigations, both numerical and experi- mental, have been performed to enhance thermal performance of LHTES units and in particular the shell-and-tube configuration since it is closer to real PCM heat exchanger applications. Such configuration is characterized by the PCM filling a cylindrical shell while heat transfer fluid (HTF) flowing through inner tubes and therefore heat transfer takes place between the HTF and the PCM. Several strategies have been proposed to improve thermal perfor- mance of LHTES shell-and-tube units including use of high thermal conductivity metal foam [3], encapsulation of the PCM [4] and use of highly conductive PCM-graphite composites [5]. However ma- jority of heat transfer enhancement techniques are based on the use of fins or extended surfaces in order to increase heat transfer area between PCM and HTF. Early investigations were conducted by Choi and Kim [6] who determined heat transfer characteristic for finned and un-finned pipe units during solidification of * Corresponding author. Tel.: þ39 011 564 4478; fax: þ39 011 564 4499. E-mail addresses: adriano.sciacovelli@polito.it, adriano.sciacovelli@gmail.com (A. Sciacovelli). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.02.017 Energy 53 (2013) 128e138