Optimal design and thermodynamic performance analysis of a novel isothermal reservoir supercritical compressed CO2 energy storage system

Supercritical compressed CO 2 energy storage system (SC-CCES) is a novel physical energy storage technology derived from compressed air energy storage (CAES) and S-CO 2 Brayton cycle. In particular, SC-CCES is characterized as a closed-type energy storage system, with low and high-pressure chambers interacting mutually during charging/discharging process. However, there is rare research of optimal design for SC-CCES, specifically achieving system architecture and eight initial storage parameters of low and high-pressure chambers, and its thermodynamic analysis and unsteady operating characteristics are also limited. To this end, a novel isothermal reservoir supercritical compressed CO 2 energy storage system is first proposed herein, for which high-pressure chamber maintains isothermal throughout the whole charging/discharging process. Furthermore, S-CO 2 release mechanism of high-pressure chamber (adiabatic/isothermal) and low-pressure chamber is achieved, and then optimal initial storage parameters of SC-CCES are obtained with the objective of maximizing storing/releasing S-CO 2 mass. Thermodynamic performance of SC-CCES based on adiabatic/isothermal mode of high-pressure chamber (SC-CCES-ASC and SC-CCES-ISC) are comprehensively compared. Results show that there is an optimal initial storage temperature of high/low pressure chambers when releasing maximum S-CO 2 mass, which rises with increasing initial storage pressure. Optimal eight initial storage parameters of low and high-pressure chambers for SC-CCES are achieved, specifically there is smaller optimal initial storage temperature (380 K) for SC-CCES-ISC with S-CO 2 releasing mass increasing by 39.8%. Furthermore, S-CO 2 compressor's speed and consumed power increase with improving the pressure ratio, while its mass flow rate decreases. By contrast, S-CO 2 turbine's speed, generated power and mass flow rate decrease with expansion ratio declining. Compared with SC-CCES-ASC, SC-CCES-ISC's energy charging period (1100 s versus 750 s)/energy discharging period (1809 s versus 1653 s) are longer, meanwhile S-CO 2 turbine's output power increases by 9.7%. It highlights that the round-trip efficiency of SC-CCES-ASC (achieving the maximum value of 69.7%) decreases with increasing initial storage temperature of high-pressure chamber, while its energy storage density presents the opposite trend. In particular, the round-trip efficiency of the SC-CCES-ISC first increases and then decreases with increasing initial storage temperature of high-pressure chamber, and its energy storage density gradually increases. Specifically, the round-trip efficiency of SC-CCES-ISC (24.3%) is lower than that of SC-CCES-ASC (69.7%), while SC-CCES-ISC presents higher energy storage density over SC-CCES-ASC (0.63 kWh/m 3 versus 0.36 kWh/m 3 ).