For latent thermal energy storage (LTES) systems, performance is limited by the suboptimum structures of heat exchangers and the low thermal conductivities of phase change materials (PCMs). In this work, a rapid charging tube-in-tank LIES system is developed by applying a compact spiral coil tube and paraffin/expanded graphite composite PCM with a high thermal conductivity simultaneously. Numerical simulation coupling heat transfer at the solid-liquid interface is used to investigate the thermal energy storage performance of the LIES system and the results show a good agreement with the experimental data. It is found that heat transfer temperature difference has a great influence on performance of the LIES system, while Reynolds number has a relatively little effect especially when Reynolds number is higher than 8700. Besides, the optimal ratio of the helix radius of the spiral coil tube to the radius of the PCM tank is confirmed to be between 0.53 and 0.64. Moreover, increasing bulk density of the composite PCM can bring about a shorter thermal energy storage time. In summary, the results are valuable for the design of a similar LIES system. (C) 2016 Elsevier Ltd. All rights reserved.