This paper reviews different hybrid-electric propulsion modes, i.e., series hybrid, parallel hybrid, and series-parallel hybrid. It then focuses on traditional propeller propulsion, shaftless rim propulsion, and pod propulsion. It mainly distinguishes and compares various propulsion methods, from advantages and disadvantages, power range, to technical issues, and analyzes the applicable ships and propulsion efficiencies for each type of propulsion method. Although shaftless rim propulsion and pod propulsion have various key technical challenges, their propulsion efficiencies are higher than traditional propeller propulsion. And they are promising to be used in hybrid power ships in the near future.
The IPCC reported that various human activities had caused global warming of the terrestrial, atmospheric, and marine environments. The global surface temperatures will continue to rise unless greenhouse gas emissions are dramatically reduced in the next few decades, otherwise, global warming in the 21st century will exceed 1.5°C or even 2°C (Preston, 2021; Huy et al., 2017; Gao et al., 2017b). It drives the development of the energy sector toward zero emissions, and it is essential for energy, shipping, and other industries to carry out rapid and far-reaching transformations (Park, 2020; Shao et al., 2018). In recent years, with the development of ships in the direction of high-speed and large-scale, the shipping industry's emissions have become higher and higher, and the share of global emissions has gradually increased. The IMO issued an agreement to determine the carbon emissions of the worldwide fleet in 2030, around 40% lower than in 2008 (Wang et al., 2020, 2021c). It is urgent to promote the development of new energy ships, especially electric propulsion ships.
Electric propulsion is an ideal clean energy source due to its high efficiency, low emissions, and good adaptability to inland rivers and oceans (Jin et al., 2019). Electric propulsion ships are mainly divided into pure-electric and hybrid-electric propulsion ships. Pure-electric propulsion ships use a single battery pack as an energy supply, and it is an ideal propulsion method to achieve zero emissions. There are deficiencies, such as the high price of lithium batteries, poor compatibility, low energy density, and other issues that have not been resolved (Nuchturee at al., 2020). Hybrid-electric propulsion ships thus play an essential role nowadays. It uses multiple energy sources as power, effectively reducing the limitations of battery packs and improving power redundancy, which can be used as an alternative propulsion method before 2030 when battery technology is not mature (Jafarzadeh and Schjolberg, 2018). The current hybrid-power systems mainly include series hybrid-power systems, parallel hybrid-power systems, and series-parallel hybrid-power systems. The propulsion system of electric propulsion ships is closely related to the power system. Ships using electric propulsion provide greater possibilities for propulsion due to the advantages of saving space and eliminating part of the shafts (Inal et al., 2022). There are three main propulsion methods for electric propulsion ships: direct shaft propulsion, shaftless rim propulsion, and pod propulsion. The power systems can be integrated with various propulsion systems onboard, and the matching method has a significant effect on the propulsion efficiency of the ship.
