Off-design characteristics and operation strategy analysis of a compressed carbon dioxide energy storage system coupled with a combined heating and power plant

To advance renewable energy development, it is crucial to increase the operational flexibility of power plants to consume renewable energy. Supercritical compressed carbon dioxide energy storage (SC-CCES) system is considered as a promising solution. This paper develops thermodynamic and off-design models for system components to formulate the system off-design model. The round-trip efficiency (RTE), system power efficiency (SPE), total exergy efficiency (TEE), and energy storage density (ESD) are defined to analyze the off-design performance of the system under two operation strategies. The results indicate that the system achieves the RTE of 32.44 %, SPE of 67.58 %, TEE of 43.26 %, and ESD of 1.73 kWh/m3 under the rated operating condition. The input power, output power, heat transfer rate, and mass flow rate during the charging and discharging process are proportional to the load level for both operation strategies. The RTE and TEE of the system vary inversely with the charging load level and directly with the discharging load level. The ESD is independent of the charging load level and increases with the discharging load level. The speed regulation-throttling regulation (SRTR) operation strategy demonstrates superior system performance and a broader range of operating conditions.