In this paper, a diesel/battery hybrid electric propulsion system (HEPS) of a polar icebreaker is proposed to achieve local low noise, and reduce fuel consumption and exhaust emission. The HEPS model is established with the inverse simulation method and the parameters are optimized with two indexes, annual fuel consumption and total cost during the entire life cycle, by NSGA-II algorithm. Compared with the single-objective optimization solutions, the bi-objective optimal design has better comprehensive performance with 2.78% less fuel consumption and 29.07% cost saving respectively. Further, the Pareto-solution set are concentrated in two areas and generate a "gap" depending on the number of battery modules. The battery capacity of the solutions before the "gap" could be fully utilized, while the battery of the solutions after the "gap" is out of service in the open water region. Additionally, the solution subset after the "gap" leads to rapid increasing fuel consumption.


Currently, polar transportation and scientific research activities have been frequently carried out (Parsons et al., 2011; Zhang et al., 2019). The icebreaker is the key equipment to complete polar navigation, and plays a pivotal role in icebreaking, escorting and emergency rescue. However, it encounters various problems such as high fuel consumption, exhaust emissions and noise (Lindstad et al., 2015). The propulsion system of a polar icebreaker in service is mainly diesel electric propulsion system, of which the diesel generator sets continuously provide power during the mission voyage, resulting in that zero emission and ultra-low noise operation is difficult to achieve in the port and polar region and affecting the ecological environment and riding comfort (Xu et al., 2017).

The environmentally friendly hybrid propulsion system of ships with energy storage battery has been developed rapidly due to its great potential in energy saving and emission reduction. While the high cost of rechargeable battery should be considered, the better economic and environmental benefits could be achieved with optimization method. At present, the research on HEPS design aiming at fuel consumption and cost is more common in the small and medium ships like ferry and tugboat. It was noted that (Zhu et al., 2020) the fuel consumption and emissions of short haul RORO ferry were taken as the optimization objectives, and the Rule-Based control and Grey Wolf Optimization were adopted to achieve 2.91% and 7.48% fuel consumption reductions respectively. Afterwards, a bi-level optimization method with multi-objective particle swarm optimization as the upper level and an adaptive equivalent consumption minimization strategy as the lower level was proposed (Zhu et al., 2018). The method was applied in an anchor-handling tug supply vessel, and resulted in 3.37% less fuel consumption and 13.95% less net present cost compared to the conventional optimization solutions. However, to the best of the authors' knowledge, the optimal design of diesel/battery HEPS for icebreaker has not been reported in literature due to the complexity of polar navigation, and the existing research focuses on the traditional diesel electric propulsion system. The electric propulsion system of the icebreaker 'Shirase' was combined with the industrial motor control and intelligent power network control technologies to improve the economy and fuel efficiency of the propulsion system when the ice load changed rapidly (Ajioka and Ohno, 2013). Apart from that, the propeller and the diesel engine of icebreaker could operate in the high efficiency zone by optimizing the sizing and speed of propeller (Yang et al., 2011). The working mode of the engine can be switched to improve the operational efficiency with the uncertain load.

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