Phase change materials (PCM) are widely used in thermal energy storage systems due to their high heat storage properties. However, due to the low thermal conductivity of PCMs, different surface areas are employed in order to increase the amount of energy. One of these methods is to use fins with high thermal conductivity. This study numerically investigated the thermal behavior of different PCMs (paraffin, paraffin wax, polyethylene glycol 6000) during the melting process in a thermal energy storage system with 15 fins. A FOX 50 heat flow meter was used for thermal conductivity measurements of these PCMs, and TA DSC Q200 (Differential Scanning Calorimetry) devices were used for specific heat measurements. The thermal property data of these measured PCMs were used in a time-dependent analysis. With the PCM data obtained, time-dependent thermal analyses were carried out using the Ansys-Fluent program based on the Computational Fluid Dynamics (CFD) method. The effect of these different PCMs on the melting processes was investigated by using water at 75oC in a 15-fin thermal storage system by observing their thermal behavior in the thermal energy storage system. In addition, cost analyses were conducted by determining the required amount of PCMs for the thermal storage system.