A hierarchical sizing and energy management framework for hybrid ship power systems

The maritime sector is under increasing pressure to improve ship energy efficiency and reduce operational greenhouse gas emissions under tightening international and regional regulations. Hybrid ship power systems combining diesel generators and battery energy storage can improve fuel efficiency; however, their performance strongly depends on the coupling between generator sizing and energy management. This study proposes a hierarchical framework that integrates diesel-generator configuration selection with a multi-layered energy management strategy for hybrid ship power systems. The framework is evaluated using a 1392 h operational load profile from a container vessel. The proposed energy management strategy combines low-pass load smoothing, proportional state-of-charge feedback, cascaded generator dispatch, hysteresis-based unit commitment, and physical operating constraints. Candidate generator configurations are evaluated through time-domain simulation and selected using a multi-criteria formulation that accounts for equivalent fuel consumption, generator-type diversity, and startup-related operational penalties. Four cases are compared: the baseline diesel configuration, baseline with battery, optimized sizing without battery, and optimized sizing with battery-assisted energy management. The results show that battery integration alone reduces average specific fuel consumption from 195.94 to 193.90 g/kWh, corresponding to a 1.04% improvement. Generator sizing alone provides a 0.72% improvement, while the integrated sizing–EMS configuration reduces average SFC to 190.82 g/kWh, achieving a total fuel saving of 2.57% relative to the baseline. This corresponds to approximately 20.2 t of fuel and 64.8 t of operational CO 2 reduction over the analyzed period. The findings demonstrate that generator sizing and energy management should be treated as a coupled design problem rather than independent optimization tasks.