This paper presents the numerical analysis of a novel thermal energy storage (TES) system using phase change material (PCM) for direct steam solar power plants. The energy storage system consists a preheater, steam generator and superheater in a cascade arrangement. The performance of the integrated system that constitutes a novel concept of thermal storage system is analyzed, numerically. The numerical model is verified against experimental data and the realistic effects of the operating conditions on the energy storage system performance are considered. The effects of different design parameters on the performance of the system are investigated. The effects of thermal conductivity of PCM, heat transfer fluid (HTF) flow rate and the diameter of heat exchanger tubes are analyzed during the entire thermal cycling of the evaporator. The effects of HTF flow rate and temperature on the exergy efficiency of TES system are analyzed. The results indicate that thermal conductivity of PCM is the most effective parameter, and increase of this parameter from 0.5 to 5 W K-1 m(-1) leads to decrease of charging time from 25 to 4.5 h and increase of output steam quality from 0.2 to 0.5 during the discharging process. It is observed that cascade arrangement in preheater and superheater heat exchangers results in lower temperature gradient of the output HTF.