Thermodynamic and economic analysis of a SRCO2–MEOFC–HTSC multi-cycle coupled system for power and steam cogeneration utilizing medium- and high-temperature waste heat

To enhance the utilization efficiency of medium- and high-temperature flue-gas waste heat, a novel power-steam cogeneration system integrating a supercritical CO2 recuperative cycle (SRCO2), a modified organic flash cycle with a two-phase expander (MEOFC), and a high-temperature steam heat pump cycle (HTSC) is proposed. A thermodynamic model was established and optimized using a particle swarm optimization algorithm with net electric output as the objective to determine operating conditions and working fluids. On this basis, the thermodynamic performance of the proposed SRCO2-MEOFC-HTSC system was compared with three existing configurations: SRCO2-organic flash cycle-HTSC (SRCO2-OFC-HTSC), SRCO2-regenerative organic flash cycle-HTSC (SRCO2-REOFC-HTSC), and SRCO2-organic rankine cycle-HTSC (SRCO2-ORC-HTSC). The evaluation was conducted from the perspectives of energy, exergy, economic impact. The results indicate that, using O-xylene as the working fluid for the organic cycle in all four coupled systems, the proposed system yields 3.3%-4.4% higher net power at an expander inlet temperature of 440 °C. After optimization, the optimal working fluid for each coupled system was identified, and under identical heat source conditions, an improvement of 7.0%-13.2% in net power output was achieved. From an economic perspective, the unit cost of energy generation is reduced by 1.8%-5.3% as the heat source temperature increases, while annual primary-energy savings are improved by 3.69%-9.49%. Regarding adaptability to heat source fluctuations, net power output increases by 68.5% to 82.4% over the heat source temperature range of 450-600 °C, demonstrating operational stability. These results confirm that the SRCO2-MEOFC-HTSC system exhibits significant advantages in energy efficiency, performance stability, and cost control.