ABSTRACT Through the present study, the governing parameters of the thermal energy storage system involving water; as Phase Change Material (PCM) are experimentally investigated. The PCM is encapsulated in five different copper capsules (sphere, cylinder, pyramid, cone, and cuboid), for investigating the effect of geometrical configurations on the characterization of the freezing, and melting processes. All of these capsules have the same internal volume. The capsules are immersed in Calcium Chloride solution with 20% concentration by weight at 266 K during freezing, and 285 K during melting. It is found that the spherical capsule showed the best thermal energy storage performance among the five test configuration. For that aspect, the effect of the heat transfer fluid (HTF) temperature, and the mass flow rate on the thermal performance of the spherical capsule are further studied. A mathematical model is proposed, and solved for the spherical capsule. The model results are verified through comparison with the experimental results of the spherical capsule. Good agreement is found between the theoretical results, and the experimental data. "KEYWORDS: Phase change material; Thermal energy storage; Capsules; Heat transfer".