Off-design performance of a supercritical CO2 propulsion system for nuclear-powered merchant vessels under variable load and seawater temperature

This study presents a preliminary analysis of the steady-state off-design performance of a nuclear-powered supercritical CO 2 (sCO 2 ) system, focusing on part-load operation, seawater temperature variations, and coordination strategies for a single-reactor dual-loop configuration. The results show significant differences in thermal efficiency among the seven operating strategies. Compressor shaft-speed control provides the highest efficiency among stand-alone strategies but is limited to 100%–50% full power of a loop (FP loop ). Inventory control maintains high efficiency over the full 100%–20%FP loop range but requires extraction of more than 50% of the working fluid, corresponding to a storage tank volume of 36.4 m 3 . A combined shaft-speed and inventory control strategy enables full-range operation while improving efficiency and reducing tank volume to 59.6% of that required by inventory control alone. Owing to the strong nonlinear variation of CO 2 properties near the critical point, the initial tank temperature significantly affects system design and increasing it from 26°C to 38°C substantially reduces required storage volume at 20%FP loop . The dual-loop configuration improves redundancy and enhances low-load efficiency. Compared with a single-loop system, thermal efficiency increases by up to 7.36 percentage points (PPs) over the 100%–10%FP sys range and by 6.75 PPs under realistic propulsion profiles. Regarding seawater temperature, a 12°C rise above the design value reduces thermal efficiency by 1.48 PPs and lowers shaft power output to 82.38%FP sys under non-active control. Shaft-speed regulation mitigates power degradation by increasing compressor speed within the allowable pressure limit, achieving 90.44%FP sys — an 8.06 PPs improvement with respect to non-active control — at the cost of an additional efficiency penalty of about 1.45 PPs.