Thermodynamic and exergy analysis of a combined pumped hydro and compressed air energy storage system

renewable energy sources such as wind and solar. In this paper, a comprehensive thermodynamic and exergy model is developed to study the thermal characteristics of a combined Pumped-Hydro and Compressed-Air (PHCA) energy storage system. The effect of key parameters, including storage pressure, pre-set pressure, aircompression mode and pump/hydroturbine efficiency on system performance is investigated. The results showed that an optimum pre-set pressure existed to maximize energy storage level for a specific storage pressure. The storage pressure also showed a large effect on the energy storage level and work output. As the storage pressure increased from 4 to 16 MPa, the energy storage level and work output increased remarkably. Furthermore, the performance of a PHCA system was largely influenced by the air compression/expansion mode in the energy storage vessel. The PHCA system stored 10% more energy through an isothermal compression process than that through an isentropic air compression process. It generated 14% more work output through an isothermal expansion process than that through an isentropic air expansion process. The exergy analyses showed that exergy destruction in pump was about 15% higher than that in hydroturbine.