Performance Analysis and Optimization of an SCO₂ Recompression Brayton Cycle Coupled with a Transcritical CO₂ Cycle Using CO₂ Mixtures

Introduction: Based on the patents review, the exhaust temperature of the CO2 is significantly higher than the ambient temperature in the Supercritical CO2 Recompression Brayton Cycle (SCRBC), indicating that the SCRBC system still has a huge potential for waste heat recovery. A combined thermal system is proposed to recover the waste heat of the SCRBC system and improve the thermal and economic performance. Methods: The transcritical CO2 (tCO2) cycle is coupled at the waste heat end of the SCRBC system to utilize waste heat at low temperatures, using the organic fluid/CO2 mixture as working fluid. Parameter sensitivity analysis has been performed on the combined system, with respect to the compressor pressure ratio, turbine inlet pressure and temperature, organic fluid mass fraction, and condensation temperature. Furthermore, a multi-objective optimization process is performed to find the optimal operating parameters for the combined system, using the nondominated sorting genetic algorithm II (NSGA-Ⅱ). Results: The results indicate that the CO2 mixture can not only improve the thermal efficiency, but also reduce the total product unit cost. There exists an optimal pressure ratio, turbine inlet pressure and temperature, and organic fluid mass fraction for the combined system. The R32/CO2 mixture achieves the least total product unit cost, which is reduced by 5.75% when compared with the SCRBC system. However, the R41/CO2 mixture obtains the highest thermal efficiency, which is increased by 10.48% when compared with the SCRBC system.. Conclusion: This study can provide valuable guidance for the system design and parameter selection in some waste heat recovery thermal systems.