In this paper, a new effective thermal conductivity model is proposed and used to numerically investigate a whole tank designed for latent-heat thermal energy storage (LHTES). The tank was filled with phase change materials (PCM) inside 9 x 9 x 20 spherical capsules. Previous studies have focused on the performance of only one capsule under the assumption that one capsule could represent the performance of the whole tank. The analysis of phase change involves much complexity, even for one capsule; thus it is challenging to analyze a whole tank due to the tremendous amounts of calculation time and memory capacity required. The new effective thermal conductivity model includes the effect of the natural convection in molten PCM, and estimates charging/discharging performance in the full scale system with reduced calculation time. This new effective thermal conductivity model can appropriately predict dissimilar melting behavior depending on the unique position of each capsule in the tank. The model was validated by comparison with experimental data, and also by rigorous numerical analysis including natural convection. (C) 2018 Elsevier Ltd. All rights reserved.