Review of maritime defossilisation regarding regulation, energy efficiency, alternative fuels, and technologies

The International Maritime Organization has outlined a strategy for the shipping sector to reach net-zero greenhouse gas emissions by 2050. As ongoing research continues to evaluate new technologies and energy sources, their potential, effectiveness, costs, and challenges evolve year by year. Conducting a comprehensive review of existing research and past efforts can provide a solid basis for upcoming research for the maritime sector. This review updates the current state of research on maritime defossilisation, focusing on promising alternative fuels, propulsion systems, their associated challenges and cost ranges, as well as regulatory frameworks, and energy efficiency and emission reduction measures. It also identifies gaps in the existing techno-economic literature, particularly regarding cost elements that remain insufficiently addressed. The maritime sector's energy transition is analysed from a systemic perspective, and potential solutions to overcome bottlenecks are assessed. The findings reveal that, in the long term, e-ammonia is commonly viewed as the most promising alternative fuel for achieving net-zero targets, while liquefied natural gas may take a transitional role, and biofuels may play a more limited role due to their limited supply potential. During the past decade, research interest in marine fuels has shifted from fossil fuels to alternative fuels such as ammonia, hydrogen, and methanol. These fuels together comprised approximately 40% of all fuel-focused studies in 2020-2024. The review highlights that scaling up new energy sources and technologies remains a major bottleneck in the maritime energy transition, and that carbon pricing and taxes can facilitate a shift towards a defossilised shipping sector.