Towards sustainable design of zero-emission ships

The maritime sector faces increasing pressure to transition towards zero-emission propulsion systems as part of global decarbonization efforts. Regulatory frameworks, technological advancements, and economic constraints create a complex landscape in which sustainable ship design requires a systematic and data-driven approach. This PhD thesis addresses this challenge by developing an optimization framework aimed to minimize greenhouse gas emissions while ensuring cost-effectiveness in hybrid propulsion system design and operation. The research introduces a nested optimization methodology to tackle decarbonization at both operational and system design levels. The inner layer focuses on power-split optimization, significantly reducing fuel consumption and emissions. The outer layer addresses system-sizing optimization, determining the optimal propulsion system configuration to achieve a trade-off between environmental performances and economic feasibility. These optimization layers provide a decision-support tool for designing next-generation sustainable vessels. Moreover, a novel index, the Greenhouse Gas Intensity Indicator, has been developed to address the limitations of existing regulatory indices. Unlike conventional Tank-to-Wake approaches, which evaluate only site-generated emissions, this index adopts a Well-to-Wake perspective. The results demonstrate that the GII produces significantly different evaluations compared to existing indices, providing a more comprehensive and technologically relevant measure of emissions. This is particularly important to fairly evaluate the impact of emerging low-carbon technologies such as alternative fuels, hybrid and full-electric systems, and fuel cells, which are approaching the maritime sector. The findings of this research highlight the critical role of optimization in reducing maritime emissions. They show that a properly optimized power-split strategy leads to substantial GHG reductions, while system sizing optimization identifies optimal trade-offs between environmental impact and cost-effectiveness. Furthermore, integrating alternative propulsion technologies is highly effective, reinforcing the need for a holistic approach that considers fuel production methods and onboard energy management.