This paper presents a mathematical model of a thermal energy storage (TES) system filled with phase change material (PCM) encapsulated in spheres. Using the epsilon-NTU method, a suitable two dimensional representation was developed capable of predicting the heat transfer during phase change. Previous research using this method in other configurations has assumed one dimensional phase change, which is not applicable to packed bed PCM systems. To determine the NTU, an appropriate definition of the thermal resistance between the heat transfer fluid and PCM is required. To account for the varying phase change process, the thermal resistance was defined in terms of isothermal and parallel path heat transfer. Comparison of model data to experiment has revealed that the phase change within a PCM tank tends to occur simultaneously as defined by parallel heat transfer. A suitable correlation was developed, demonstrating the validity of the epsilon-NTU method for packed bed PCM systems. (C) 2014 Elsevier Ltd. All rights reserved.