Design and performance analysis of a multi-level compressed carbon dioxide energy storage system for a wider charge/discharge power range

Nowadays, the overuse of fossil fuels has resulted in serious environmental problems. Efficiently utilizing renewable energy is a proper solution. As for the energy storage system, because the input power and power load may greatly vary, a multi-level compressed carbon dioxide energy storage system with a wider power range is proposed in this paper. It has three charge processes and three discharge processes by matching different power ranges. The system performance is analyzed based on the thermodynamic model. The result shows that the suitable pressure for low-pressure storage tank (LST), medium-pressure storage tank (MST), and high-pressure storage tank (HST) are 3 MPa, 6.27 MPa, and 24 MPa, respectively. The charge power range of the three charge levels are 1.16 MW–1.73 MW, 1.73 MW–2.57 MW, and 2.89 MW–4.29 MW, respectively. The ratio of the lowest to the highest charge power is 0.271 which is much lower than the traditional one-level system. And it has a power gap of 0.324 MW. The discharge power range of the three discharge levels are 0.198 MW–0.912 MW, 0.912 MW–1.33 MW, and 1.33 MW–2.24 MW, respectively. The RTE varies from 0.301 to 0.713 and the ED changes from 0.753 to 6.14 kWh/m³ during operation. In the power gap, the utilization of an electric heater will greatly reduce the RTE and slightly raise the ED. To achieve higher energy conversion efficiency, the result shows that the higher power gap harms the system performance. A smaller power gap is suitable for the proposed system.