Abstract: A cylindrical phase change energy storage system was modeled numerically using the finite element method. The model system was composed of a cylindrical container into the center of which is inserted concentrically a copper pipe carrying hot water. Either copper fins or compressed expanded natural graphite (CENG) were placed as heat-conducting fillers between the container and the pipe for comparison. The former is radially positioned while the latter is randomly filled. The remaining space was filled with paraffin as a phase change energy storage material. Fluid flow, heat conduction and convection, and the paraffin phase change were simulated simultaneously. It was found that the heat transfer performance of the system increases with the number of copper fins when they were used as the filler. The time required to reach heat equilibrium is shortened significantly by increasing the density of the CENG when it was used. CENG is much more effective than copper fins to promote paraffin melting.