System design and hierarchical mode switching energy management strategy development for a heat pipe-cooled reactor-battery hybrid energy system

Heat pipe-cooled microreactors (HPRs) offer a promising low-carbon power solution for marine vessels due to their inherent safety and compactness. However, their substantial thermal inertia limits their ability to track rapid, large-amplitude ocean load fluctuations. To address this, this work proposes a hybrid energy system integrating an HPR with a Li-ion Battery Energy Storage System (HPR-BESS). To evaluate its performance, a multi-physics dynamic model was developed, coupling reactor neutronics and thermal-hydraulics with the transient thermodynamics of a supercritical CO2 (sCO2) Brayton Energy Conversion System (ECS) and battery dynamics. The work employed a multi-objective genetic algorithm (MOGA) to co-optimize the sCO2 ECS design, ESS capacity, and parameters of the Energy Management System (EMS), balancing system compactness with stability. Furthermore, this study formulated an hierarchical mode-switching control strategy, which employs a moving time-window integral trigger to switch between Stable Mode and Active Load-Following Mode. It drives the reactor power to locate and stabilize at the median of fluctuating load under marine conditions, preventing oscillations in the reactor's operating state. Simulation results under the representative ocean mission profile demonstrate that by leveraging the battery ESS for instantaneous compensation, the hybrid architecture achieves a sub-second load response, masking the inherent thermal delay of the HPR. Additionally, it minimizes unnecessary control actions, reducing total control drum travel to 1.51°. Compared to benchmark filtered PID and fuzzy controllers, the proposed strategy reduces HPR fuel temperature fluctuation coefficients by 59% and 49%, respectively, yielding a minimum coefficient of variation (CV) of 0.0018. These results validate the HPR-BESS as a robust, responsive solution for dynamic marine energy demands.