Thermal energy storage in the form of latent heat of fusion of phase change material gained considerable attention in solar energy applications since it significantly increases the energy density and reduces the storage tank size compared to the sensible heat storage system. Several numerical and experimental studies have been conducted to enhance the performance of the system. In this study, 2-D continuous solid phase and effective packed bed models are developed to study the behavior and performance of a thermal energy storage system for high temperature applications, which is composed of spherical capsules encapsulated by phase change material (Sodium nitrate) and high temperature synthetic oil (Therminol 66) as heat transfer fluid. Temperature distribution, fluid flow, melting, solidification and thermocline behavior of the system are predicted and the influence of capsule size on the performance of the system is studied.Copyright © 2014 by ASME