The startup process of a high temperature latent heat thermal energy storage system assisted by finned heat pipes was studied numerically. A transient three-dimensional finite volume based model was developed to simulate the charging process of phase change material with different configuration of embedded heat pipes. The melting of the phase change material was modeled by employing enthalpy-porosity technique. A eutectic mixture of sodium nitrate and potassium nitrate with melting temperature of 220 degrees C enclosed by a vertical cylindrical container was used as the phase change material. The effects of different heat pipe arrangement and the heat pipe quantities as well as the influence of natural convection on the thermal behavior of the latent heat thermal energy storage system were studied. The results indicate that the heat pipe configurations and the quantities of heat pipes integrated in a thermal energy storage system have a profound effect on the thermal response of the system. Employing more heat pipes decreases the thermal resistance within the system, leading to the acceleration of charging process and the decrease of container base wall temperature. It was also found that the inclusion of natural convection heat transfer in the charging process of the system renders to higher melting rate and lower base wall temperature. (C) 2015 Elsevier Ltd. All rights reserved.