In the present study, geometry optimization of a phase change material (PCM) heat storage system is presented. The existing PCM-fins heat exchanger system works at the back side of a solar receiver in order to minimize the effect of the solar radiation fluctuations inside the cavity. As initially designed, the system does not accomplish the expected design purposes and thus optimization is needed. Optimization is usually time-consuming and some algorithms need a starting point, therefore one suitable method is geometrical optimization which aims to find the optimal shape of a system for a given criteria and providing a rough optimal geometry. Here, constructal theory, 'point to volume', is applied to find the optimum shape factor of the elemental volume of the presented PCM-heat exchanger. With this methodology, an optimum ratio of the PCM and fin width and length is found and beyond that the method is extended to 'surface to volume' problem. Results have been numerically validated using a CFD software and demonstrate that it gives a very good approximation of the real optimum which can be used as initial configuration for further optimization through CFD simulation or other optimization methods that require a starting point. (C) 2018 Elsevier Ltd. All rights reserved.