This study focuses on heat transfer characteristics of phase change material (PCM) during energy storage and release in vertical double pipe configuration. Paraffin wax with an average melting temperature of 52 degreesC is used as the energy storage material and water is used as the heat transfer fluid (HTF). The heat transfer fluid flows in the inner tube and the wax is stored on the shell side. Eighteen thermocouple wires are placed in the wax to provide a detailed measurement for the temperature field during energy storage and release. Measurements are made as a function of the flow direction, flow rate, and inlet temperature of the heat transfer fluid. Results indicate that natural convection dominates the melting process for upward flow of the heat transfer fluid. On the other hand, the solidification process is dominated by conduction. During melting and upward flow of the HTF, the density difference of hotter and cooler molten wax layers initiates natural convection cells. Further mixing within the melt is also caused by descent of the higher density solid wax.