Comparative study of operating modes on a gaseous two-stage compressed carbon dioxide energy storage system through energy and exergy analysis based on dynamic simulation

This paper conducts a thermodynamic analysis on up to 8 operating modes, including various pressure and water storage settings, of a gaseous two-stage compressed carbon dioxide energy storage system. Additionally, the potential of CO2 binary working fluids in stabilizing the system condition is also discussed through the comparison with pure CO2 cases. According to the results, the system efficiency ranges from 48.48 % to 56.70 %, depending on the operating mode. The constant pressure mode exhibits an 89.1 % variation in pressure ratio on compression than the sliding pressure mode at the cost of reduced system efficiency. Similarly, a reduction of up to 17.3 % in the pressure ratio on expansion is found in constant pressure discharge process despite the expense of a nearly 1 % decrease in system efficiency. A lower HX1 outlet temperature leads to more balanced water consumption and thus improved system efficiency. Almost doubled energy density is observed when highpressure tank (HPT) volume decreases to 1000 m3. The application of binary working fluids results in a more stable system performance compared to the sliding pressure mode, and higher system efficiency than the constant pressure mode. The work in this paper offers insights and guidance for future practical system designs.