Modelling, analysing and optimising ship energy systems

Ship design and operation today focus largely on reducing the environmental impact of shipping. This is supported by the regulatory framework with an ambition to steer shipping towards net-zero greenhouse gas emissions. The current focus is on regulating ship carbon intensity and energy efficiency. Ship decarbonisation has a holistic impact on the entire ship design, and ship machinery and energy systems are at the heart of the change. This thesis explores ship energy system development towards decarbonised shipping by focusing on ship energy system modelling, analysis and optimisation through case examples. The context of the study lies in the methodology and questions that are relevant to early-stage ship design, where key decisions about ships and their systems are made. Optimisation was studied for two different ship design purposes: machinery selection and exploring the optimal set-up of waste heat recovery processes and battery system dimensioning. While selecting the main machinery topology for a cargo ship and a passenger ship, linear and non-linear optimisation were compared. Although this relatively straightforward task might favor the fast and efficient linear optimisation approach, in practice, machinery selection involves more dimensions than merely optimising fuel consumption. Therefore, as a secondary target, multi-objective optimisation was explored. This enabled comparison of the trade-offs between achieving the highest reductions in fuel consumption and the space required. While the machinery study involved defining a special algorithm for the process, a cruise ship energy system study took an existing ship energy simulation model as a starting point for optimisation. The objectives for the optimisation were fuel saving, investment cost, and engine running hours. The largest energy and fuel saving potential was recorded at close to 4% with the studied variables. Furthermore, the optimisation framework and result visualisation allowed for the examination of interesting sensitivities and relationships between the design variables and optimisation targets. The largest reduction in ship carbon emissions and energy consumption was witnessed in a conceptual case study of a cruise ship operating on a Mediterranean route. The application of several technologies, such as ultrasound antifouling, shore power, battery hybrid machinery, waste heat recovery and air lubrication led to combined fuel savings of 18,7% for the selected operational profile. By combining these technologies into a machinery powered by hydrogen, the total ship energy consumption was reduced by 25%. Synergies in ship energy system, such as machinery efficiency and ship heating requirements, contributed to this result. Nevertheless, even further development potential for the Mediterranean cruise ship was identified through the heat utilisation efficiency analysis. The heat system analysis relied on entropy generation calculation for selected ship heat systems, especially those related to waste heat recovery processes. Energy system modelling and simulation methods are an important part of ship design today, as they also help validate the rule compliance in the early design stages, where improvements can still be made to the ship at moderate costs. Optimisation methods enable an expansion of the solution exploration area in practical design tasks. Furthermore, integrating the optimisation into ship energy modelling reduces the need to perform additional model validation and development process. Nevertheless, supporting analysis methods - such as the heat utilisation efficiency indicator, used in parallel with other indicators - guide the designer in searching for new areas of optimisation and establishing the quality of the results.