Study on the Influence of Key Parameters of Supercritical Carbon Dioxide Brayton Cycle System on Cycle Efficiency
The supercritical carbon dioxide(S-CO2)Brayton cycle has the advantages of high thermal energy utilization,compact structure and simple layout.Coupled with reactor,it is easy to achieve integration and miniaturization,and is suitable for the development of small modular reactors.As the key devices in the cycle system,the selection of turbomachinery inlet parameters and its own efficiency greatly affects the efficiency of the overall cycle,especially the changes in device efficiency after miniaturization further affect the cycle efficiency.In order to study the influence of device parameter selection on the efficiency of the system cycle,the thermodynamic model was established for four types of S-CO2 Brayton cycles:the Simple Brayton Cycle,the Recompression Brayton Cycle,the Reheat-Recompression Baryton Cycle and Intercooling Recompression Brayton Cycle.The effects of key parameters on cycle efficiency were analyzed in depth,such as the inlet enthalpy value of the main compressor and turbine,and inter-compressor inlet pressure and reheat pressure.The impact of changes in device efficiency under different operating conditions and power levels on the efficiency of the overall cycle are also studied in this paper.The results show that the four cycle layouts have similar trends in parameter changes under different operating conditions,with the Reheat-Recompression Brayton Cycle having the highest efficiency,followed by the Intercooling Recompression Brayton Cycle,the Recompression Brayton Cycle,and the Simple Brayton Cycle.Among them,the inlet temperature and pressure of the main compressor should be slightly higher than the CO2 critical point to reduce compressor power consumption.The turbine inlet temperature is limited by the outlet temperature of the core circuit,and exist an optimal value for its inlet pressure to achieve the highest cycle efficiency.The inlet pressure of the intermediate compressor and the inlet pressure of the reheat turbine also exist the corresponding optimal values for the highest cycle efficiency,and the optimal values increase linearly with the increase of the highest system pressure,it is independent of the efficiency of the devices.In view of the situation that the isentropic efficiency of devices is taken as a fixed value in the current study on the system efficiency of the S-CO2 Brayton cycle,the efficiency calculation method for dimensionless devices is adopted to calculate the device efficiency under different power levels and working pressure ratios,and then the impact of device efficiency on the efficiency of the overall cycle is analyzed when the operating conditions change.Taking the Recompression Brayton Cycle as an instance,the variation of the cycle efficiency with the system pressure ratio under different device efficiencies and power levels is given,and compared with the calculation results using constant equipment efficiency.The results using the efficiency calculation method for dimensionless devices can more comprehensively and objectively reflect the cycle characteristics of the S-CO2 Brayton cycle under different operating pressure ratios and power levels.
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