Off-design characteristics of supercritical CO₂ Brayton cycle near the critical point considering heat-exchanger thermal inertia

The supercritical carbon dioxide (sCO₂) Brayton cycle has attracted much attention due to its potential to meet the flexibility demands of future power systems with high renewable penetration. However, the off-design behavior under compressor inlet conditions near the critical point received limited investigation, despite its importance for enhancing system flexibility. Recuperators, as key components, involve substantial heat transfer, resulting in strong thermal inertia. While rotational speed and mass flow rate can be adjusted rapidly, the temperatures of the heat exchangers respond much more slowly, making cycle flexibility highly dependent on thermal inertia. This effect has been inadequately considered in early studies, causing inaccuracies in off-design performance predictions. In this work, a comprehensive one-dimensional model of an sCO₂ split-flow recompression Brayton cycle is developed, explicitly accounting for multi-parameter coupling and heat-exchanger thermal inertia. The off-design performance characteristics and governing sensitivities near the critical point are investigated. The results indicate that increasing the main compressor inlet temperature near the critical point reduces both cycle efficiency and net output power, with a slower degradation as the temperature deviates from criticality. Sharp variations in thermophysical properties near the critical pressure cause pronounced performance fluctuations, whereas system performance stabilizes above 7.7 MPa. Increased heat-exchanger wall thickness prolongs system stabilization time without affecting steady-state performance. Genetic algorithm–based off-design optimization further improves cycle efficiency by 0.66–1.26%, providing guidance for flexible operation of sCO₂ Brayton cycles.