Thermo-economic assessment of a pumped thermal compressed carbon dioxide energy storage system: An approach to enhanced energy density

Compressed carbon dioxide energy storage (CCES) has certain advantages in addressing the intermittency of renewable energy sources. However, traditional gas-liquid carbon dioxide energy storage (GL-CCES) systems suffer from low volumetric energy density (VED) and heat transfer mismatch in the reheater near the critical region. To alleviate these issues, this paper proposes a novel pump-thermal compressed carbon dioxide energy storage system (PT-CCES), which combines CCES with a quasi-Carnot battery subsystem. For the proposed system, an evaluation framework covering energy, exergy, and economic performance is developed, on which multi-objective optimization is carried out using the NSGA-II algorithm. The proposed PT-CCES reduces excessive exergy destruction in the key reheater of baseline GL-CCES by adjusting the outlet temperature and pressure of the supercritical turbine, shifting the CO2 state away from the near-critical region. The sensitivity analysis further reveals that, in the absence of external thermal demand, comprehensive performance improves as the mass flow difference between the thermal storage water supply and demand sides approaches zero. Under optimal trade-off conditions, the PT-CCES system achieves a round-trip efficiency of 67.55%, a VED of 0.1366 kWh/m3, and a levelized cost of storage of 0.1255 $/kWh. The VED advantage remains robust across a wide range of key operating parameters. Furthermore, at maximum VED, the total engineering volume is reduced by 10.97% compared to the baseline system, equivalent to saving approximately 7.08 × 104 m3 of physical space. Overall, the proposed PT-CCES system is suited for large-scale energy storage deployment in applications where available space is limited.