Off-design performance analysis and comparison of a coal-based semi-closed supercritical CO2 cycle under different operational strategies

The coal-based semi-closed supercritical carbon dioxide (sCO2) power generation system is a superior choice to service the peak-shaving power station to meet the fluctuation of the renewable energy power due to its high efficiency, flexibility and near-zero carbon emission. Studying the off-design performance and operational strategy are crucial to achieve an efficient system operation during power load adjustment. In this study, the detailed design and off-design models of the coal-based semi-closed sCO2 system are developed. A novel series-parallel method is employed to model the off-design model of multi-stream regenerator. Subsequently, based on the temperature resistance of the regenerator, the strategy of constant turbine inlet temperature (TIT) and the strategy of constant turbine outlet temperature (TOT) are proposed successively, and the safe operation range and the part-load performance are compared in detail. Finally, by introducing a partial admission turbine, an optimal strategy (the hybrid strategy) is innovatively proposed from the perspectives of flexibility and high-efficiency for the coal-based semi-closed sCO2 system. The results show that the suitable strategy depends on the load of the system. When the load drops to 82.64 %, the regenerator inlet temperature reaches the upper limit under constant TIT strategy. However, the constant TOT strategy can effectively address the issue of super-temperature, enabling the system to safely drop the load from 100 % to 20 % without a phenomenon of compressor surge. For the hybrid strategy, the optimal partial admission degree allows the relative efficiency of the system only decreases to 69.73 % at 20 % load, which exhibits better off-design performance in terms of wide load-following range and outstanding thermodynamic efficiency.