Stable Operation Characteristics of the Pressurized Water Thermal Storage Tank System Based on Nitrogen Pressure Control

Abstract To address the challenge of frequent pressure fluctuations in insulated compressed air thermal storage systems, this study develops a pressurized water thermal storage double-sphere tank solution with nitrogen pressure control as the key technology. By computational fluid dynamics modeling and process simulation, the flow process and energy conversion between pressurized water and nitrogen during the operational cycle were thoroughly investigated. Additionally, long-term dynamic simulations were conducted to extensively explore the dynamic response characteristics of the temperature and pressure field in tanks under different thermal storage temperatures, flowrates, and cycle numbers. The research results are as follows: first, the pressure changes of the system follow a trend of first decreasing, then remaining constant, then increasing, and finally decreasing during a cycle. No evaporation occurs because the minimum pressure remains above the saturation. The obvious temperature decrease is observed at the second static stage, while the cold tank temperature remains constant. Second, the differences in the resistance coefficient result in the nonlinear relationship between heat storage flowrate and gas pressure. At a high flowrate of 105.79 kg/s, the pressure reduction remains moderate. Finally, both the gas temperature and pressure field have reached equilibrium after three cycles, with the maximum pressure (1.42 MPa) occurring at the end of the heat release stage. The initial temperature and pressure of the next cycles rapidly increase because of heating, which is the main difference between them. In summary, the stable operating temperature for the pressurized water thermal storage system under this model should be less than 458 K, and the design pressure should be 1.6 MPa.