Thermal energy storage (TES) systems based on phase change materials (PCM) have a specific place in industry and daily life. The TES systems store energy using PCM's latent heat which are packed within sealed container in the system. Due to the increasing development of these systems, the optimization issue in them is vital. In the present work, a spherical capsule containing RT-50 as a PCM was simulated and its constrained melting process was investigated. To improve heat transfer during the melting process, a circumferentially fin with a ?-shaped configuration inside the spherical capsule was used. The configuration of the fin was defined with several geometric characteristics and their change effect on the fin performance were investigated. After that, the optimal fin configuration in terms of complete melting duration (CMD) of the PCM was obtained by using constructal design theory and a special introduced algorithm. The advantage of this algorithm is a significant reduction in computational costs that can be used in optimization studies. Numerical solution was validated using experimental data, and the result was in good agreement with them. The results revealed that by using the optimized configuration fin with a volume percentage of 1.5%, the CMD decreases more than 41% compared to the unfinned capsule, which is equivalent to the efficiency of 1.68 for this configuration. In addition to this, the CMD decreases further by increasing fin volume. So that the fins with volume percentage of 2.5%, 3.5% and 4.5% reduce the CMD by 44.3%, 46% and 48%, respectively. However, due to the fact that the total volume of the capsule is constant, increasing the amount of fin reduces the volume of the PCM and the storage capacity, consequently. Hence, according to the aim of TES system which is faster performance or more storage capacity, optimal fin percentage to be selected. Keywords: Thermal energy storage system, Phase change materials, constrained melting, Constructal theory, Fin, Heat transfer optimization, spherical capsule
