Multi-objective optimization of a SCO₂ Brayton/zeotropic ORC combined cycle based on machine learning regression and classification

This study introduces an innovative Supercritical CO₂/Zeotropic ORC Combined System (SZOC) that substantially improves supercritical CO₂ (SCO₂) cycle performance through utilization of zeotropic working fluid temperature glide characteristics. The proposed system features an intermediate heat exchanger (IHX) as its central coupling mechanism, and integrates a recompression SCO₂ Brayton cycle with a zeotropic organic Rankine cycle (ZORC) to achieve superior thermodynamic performance. The analysis methodology combines a thermodynamic modeling approach incorporating 1D radial turbine simulations with customized printed circuit heat exchanger (PCHE) models specifically designed for both SCO₂ and ZORC applications. To recognize the system’s inherent nonlinear complexity, an advanced machine learning framework is implemented to develop a highly accurate surrogate model (R2 > 99.6 %) in conjunction with a specialized classification algorithm (97.8 % accuracy) for nonlinear constraint identification. The latter significantly enhances the optimization workflow. Through multi-objective optimization using the NSGA-II algorithm, the working fluid mass fraction ( R_m ) and split ratio ( R_s ), with thermal efficiency ( η_c ) and area-to-power ratio (APR) serving as primary performance metrics are simultaneously optimized. The present analysis reveals that R32/R245fa mixtures ( R_m ≈ 85 % R32) combined with a split ratio of R_s ≈ 0.2 demonstrate exceptional thermodynamic compatibility. The TOPSIS decision analysis has identified R32/R600a ( R_m = 88.2 %, R_s = 0.203) as the optimal configuration, delivering a remarkable combined cycle efficiency of 45.62 % − representing a 2.18 % absolute enhancement over conventional SCO₂ systems (43.44 %), while increasing APR from 0.3965 to 1.3476 m/MW. These results demonstrate the advantages of zeotropic mixtures for advanced thermal system optimization and establish important foundations for next-generation SCO₂ combined cycle development and provide actionable design guidelines for practical implementation.