This study focuses on heat transfer enhancement in double pipe energy storage system. Enhancement is achieved by use of metal screens/spheres placed inside the phase change material (PCM), which is paraffin wax and results in increasing the effective thermal conductivity of the combined media of PCM and metal screens/spheres. The experiments are conducted as a function of the diameter and number of spheres inserted in the phase change material. Also, the experiments investigate the effect of increasing the temperature of the heat transfer fluid (HTF). Results are presented in terms of variations in the PCM Nusselt number and the melting Fourier number. Results indicate three-fold decrease in the Fourier number and similar increase in the Nusselt number. Replacing 2-volume percentage of the wax material by the metal spheres results in this large enhancement. (C) 2004 Elsevier Ltd. All rights reserved.