Modeling and Performance Analysis on an Integrated System Combining Supercritical Carbon Dioxide Brayton Cycle With Leakage Feedback Utilizing a Heat Pump or Direct Compression

Abstract CO2 leakage due to the coexistence of pressure differentials and clearances within the integrated turbine-alternator-compressor (TAC) of the kW-level sCO2 Brayton cycle is a key issue affecting cycle efficiency and long-term stable operation. To analyze the impact of leakage and feedback, this study examined the integrated system of the sCO2 Brayton cycle with leakage feedback and compared the system performance differences between heat pump feedback and direct compression feedback. Thermodynamic models for the Brayton cycle and feedback unit were established, and the effects of varying key parameters were analyzed. A genetic algorithm was employed to further optimize the system. The selection suggestion charts for the feedback unit were obtained. It is recommended to choose a direct compression feedback unit for conditions with a cavity pressure higher than 6.41 MPa, whereas a heat pump feedback unit is recommended for other conditions. The heat pump feedback unit has advantages particularly at high rotational speeds, low leakage factors, and low cavity pressures. The efficiency of the optimized integrated system is improved by up to 12.53%. The net power accounted for only 53.6% of the turbine output power, the main cycle compression power accounted for 32.1%, the TAC losses accounted for 10.7%, and the total compression power of the feedback unit accounted for 3.7%. Ignoring the internal leakage and losses of TAC can overestimate the power generation capacity of the Brayton cycle, resulting in it being only 79% of the ideal case after considering losses and power consumption.