An innovative concentrated solar power system driven by high-temperature cascade organic Rankine cycle

Abstract

Direct steam generation (DSG) solar power systems eliminate synthetic oils and molten salts in the solar field and enable efficient heat collection. Commercial DSG solar plants usually have a steam generation temperature of 250–285 °C to reduce the technical challenges of wet steam turbines and the costs of high-pressure water storage tanks. The power conversion efficiency is relatively low due to the limited steam generation temperature. This paper proposes a high-temperature solar power system driven by the cascade organic Rankine cycle (CORC). It has three features: water/steam for solar heat transfer, water and phase change material (PCM) for heat storage, and CORC for power conversion. It is the first time that the storage tank temperature is independent of the steam generation temperature in a DSG. Steam can be generated in the solar field at a temperature of 310 °C or even 370 °C. The fundamental of the innovative system is illustrated. The thermodynamic performances during the normal operation and discharge processes are investigated. The results show the maximum thermal efficiency of the CORC system in the normal operation mode is 32.85% at a steam temperature of 311 °C, while the top and bottom cycle efficiencies are 15.38% and 20.86%, respectively. The efficiency increases to more than 37% at 370 °C. Combining the two-tank storage and the PCM unit can overcome the problems of decreasing the heat release rate from PCM during heat discharge while maintaining the CORC system's power output and prolonging the heat storage time. The proposed system is potentially more cost-effective than the existing DSG solar plants.