Constructing a high-temperature pumped thermal energy storage system by employing CO2 heat pump and multi-stage extraction steam Rankine cycle

To address the challenges posed by the intermittency of renewable energy and the demand for long-duration energy storage, a high-temperature pumped thermal energy storage (PTES) system has garnered significant attention due to its advantages of fewer geographical constraints and high energy storage density. Given this, to provide a potential retrofit pathway for retired coal-fired power plants, a novel PTES system is proposed by integrating a CO 2 heat pump with a multi-stage extraction steam Rankine cycle (SRC). The heat pump employs a regenerative reverse Brayton cycle, while the SRC features an extraction-regeneration structure characterized by three high-pressure stages, four low-pressure stages, and one deaeration process. The low-temperature heat source for the CO 2 heat pump originates from atmospheric-pressure thermal storage water, comprising the condensation heat and the fifth-stage low-pressure extraction heat from the SRC. The high-temperature heat released by the CO 2 heat pump is stored in molten salt, serving as the driving heat source for the SRC. For this PTES system, a steady-state theoretical model based on mass, energy, and exergy conservation is established at an energy storage capacity of 100 MW × 5 h. At the design point, the heat pump coefficient of performance (COP) reaches 1.32, the power-generation efficiency of the SRC is 43.08%, and the overall round-trip efficiency (RTE) achieves 56.86%. Exergy analysis of key components indicates that the SRC heater before the high-pressure turbine incurs the highest exergy loss, followed by the regenerator and compressor of the heat pump, while the condensate pump of SRC exhibits the lowest exergy loss. Additionally, the exergy efficiency is 57.02%. On this basis, the influence patterns of compressor inlet pressure and temperature, molten salt temperatures, turbine inlet pressures, and storage water temperatures on system performance are revealed. The sensitivity analysis shows that the hot storage water pool temperature has the most significant impact on system performance. Furthermore, targeting the maximum RTE, a genetic algorithm is employed to synergistically optimize the aforementioned operating parameters. It indicates that the optimal RTE can reach up to 61.27%. The study provides a thermodynamic framework to evaluate the performance of the proposed PTES system, and offers guidance for component design, techno-economic assessment, and pilot-scale validation.