Comprehensive thermo-economic analysis of an isobaric compressed CO2 energy storage system: Improvement of the thermodynamic pathway

Mitigating fluctuations across multi-time scales is crucial for the large-scale integration of renewable energy, and compressed carbon dioxide energy storage (CCES) is one of the most promising technological pathways. To enhance the performance of CCES systems, this paper proposes an isobaric CO₂ storage device and constructs various charge–discharge pathways. Thermodynamic analyses are conducted from both process and system perspectives, and economic performance under different pathway combinations is investigated. The results indicate that the energy consumption for isobaric storage is significantly lower than the exergy destruction under isochoric storage. The compressed heat evaporation path can effectively reduce evaporation loss and achieves a charging efficiency of 73.56 %. The discharge path (D-path) coupled with waste heat can significantly increase system capacity, with discharge efficiency improving from 62.73 % to 70.68 %. Utilizing the ambient heat evaporation without external heat source, the optimal roundtrip efficiency is 53.42 % and the corresponding low storage pressure is 4.2 MPa. Moreover, under the combination of flash evaporation and external heat source, the system roundtrip efficiency decreases with promotion of the low storage pressure, and the optimal energy and exergy efficiencies of 42.83 % and 75.95 % can be achieved near the critical pressure. The integration of compression heat evaporation and external heat source offers the competitive economic performance, the levelized cost of unit electricity and payback period are 0.144$/kWh and 7.07 years by coupling with low-temperature waste heat at 400 K.