Experimental investigation of palmitic acid as a phase change material (PCM) for energy storage has been conducted in this study. The performance,and heat transfer characteristics of a simple tube-in-tube heat exchanger system were studied, and the obtained results were compared with other studies given in the literature. The present study included some parameters, Such as transition times, temperature range and propagation of the solid-liquid interface, as well as the heat flow rate characteristics of the employed cylindrical tube storage system. The experimental results show that the melting front moves in the radial direction inward, as well as in the axial direction from the top toward the bottom of the PCM tube. It was observed that the convection heat transfer in the liquid phase plays an important role in the melting process. The flow rate and inlet temperature of the heat transfer fluid to the PCM tube in the experimented range has an insignificant effect on the phase change processes. On the other hand, the melting and solidification times of the PCM can be reduced significantly by placing the tube containing the PCM in a horizontal position rather than a vertical one. The heat storage capacity of the PCM tube is not as good as we expected in this study, and the average heat storage efficiency (or heat exchanger effectiveness) is 53.3. It means that 46.7% of the heat actually is lost somewhere. (C) 2002 Elsevier Science Ltd. All rights reserved.