In this paper, melting performance of phase change materials (PCMs) in a horizontal concentric-tube thermal energy storage (TES) unit was numerically investigated with consideration of natural convection. Porous media were employed to enhance the thermal response of PCMs. Performances of different porous configurations were compared to optimize the location of porous insert, and the optimal filling ratio of porous insert was determined based on a new criterion proposed in this study, which is called TES rate density. This new criterion was proved to be effective to comprehensively evaluate the melting performance, including melting time, TES capacity, and total mass of materials. Furthermore, the effects of pore size and porous materials were discussed. The results showed that partially locating the porous media in the lower part has the best enhancement on melting performance of PCM and the optimal filling height ratio of porous media is 0.7. In this case, the TES rate density can be significantly increased by more than 6 times compared with the none-porous case. More importantly, compared with the full-porous case, 3% better comprehensive performance with about 28% less porous material can be achieved. Porous insert with high thermal conductivity, large pore size, and high porosity is recommended to enhance the melting performance of PCMs. From the point of view of practical utilization of the porous material, silicon carbide is recommended due to its relatively high conductivity, chemical inertness and low cost. (C) 2017 Elsevier Ltd. All rights reserved.