In this paper, irreversibility of a thermal energy storage system is numerically investigated. The system consists of two concentric cylinders. The outer cylinder is filled with phase change material (PCM), while working fluid flows inside the inner pipe. The system works periodically. The related governing equations are solved by a control volume-based finite difference method. The effects of different parameters such as PCM size and melting point temperature are examined on the irreversibility of the system. The results show that the irreversibility of thermal storage module is strongly affected by the size of PCM ( diameter and length of the external cylinder) and melting temperature. Based on the obtained results, the irreversibility of the system can be reduced by proper selection of PCM size and melting temperature.