Energy, environmental and economic investigations of cruise ships powered by alternative fuels

In recent years, methanol, ammonia and methane–hydrogen blends are currently receiving most of interest as green fuels enabling energy transition in the maritime sector. In this work, the energy, environmental and economic performances offered by these alternative fuels applied to modern large-size cruise ships are investigated. Overall, 5 different routes, three different operating conditions (maneuvering, port and navigation) and summer–winter seasons have been accounted for. Due to the enhanced fuel flexibility of gas turbines, attention is here focused on power configurations integrating combined gas electric and steam plants with reciprocating engines. First, a cogeneration efficiency optimization study has been carried out considering variable ship operating conditions. Interestingly, combined gas steam power plants resulted to cover up to 68% of the overall thermal power request and to provide energy savings when electrical power demand exceeds 15 MW during navigation and maneuvering. Methane–hydrogen blends enabled the highest cogeneration efficiencies (i.e. ≈82%). Successively, environmental impact of power plants is assessed by means of the Energy Efficiency Design Index and Carbon Intensity Indicator, whose attained values have been shown to comply with regulation limits up to 2025 and 2050, respectively, under ammonia and methane–hydrogen feeding conditions. On the contrary, methanol appears a valuable option only for a short-term energy transition perspective, since compliance with regulations lapses before 2028. Finally, investments and operational costs for power plants have been computed to assess their economic viability. In the current scenario, comparable costs result for ammonia and methanol fueled cruise ships, while 40–60 M€increase in operational expenditure is achieved at 25-th year by methane–hydrogen blends due to the high price still affecting green hydrogen.