in this work we have studied numerically the charging and the discharge of solar thermal energy stored using latent heat from a storage unit contains PCMs. We have also studied the improvement of the heat transfer between fluid which is the water and the PCM by adding the fins to our storage unit with various configurations, including in-line fins and staggered fins. Then, we studied the assembly of two PCMs in our storage unit. In addition, an experimental work has been validated. The ANSYSFLUENT 15 code was used who is based on the enthalpy method to solve the formulation of the fusion / solidification processes, where three users define have been developed to describe the thermo-physical properties of the PCM.