Thermal energy storage improves the efficiency and eliminates the mismatch between the energy supply and energy demand of solar thermal energy applications. Among the different types of thermal energy storage, a phase change material (PCM) thermal energy storage exhibits superior efficiency and dependability due to its high storage capacity and nearly constant thermal energy. The present work experimentally investigates the use of a triplex tube heat exchanger with internal-external fins as thermal energy storage. The experiment examined the PCM charging process under steady and non-steady heat transfer fluid (HTF) inlet temperature and the influence of the mass flow rates on the PCM melting. The PCM solidification process under different mass flow rates was also investigated, and the PCM temperature gradients in the radial and angular directions were analyzed. The results indicated that the HTF inlet temperature has more influence on the PCM melting process than the HTF mass flow rate. The charging time is reduced to 58% for HTF mass flow rate and 86% for HTF inlet temperature. (C) 2013 Elsevier B.V. All rights reserved.