Throughout current literature, different combinations between fuel cell type, heat engine type and fuel have been considered. Most studies focus on one combination, and operating conditions (fuel cell temperature, fuel cell pressure, cell voltage, fuel utilization factor, etc.) vary throughout these studies. Due to these variations in operating conditions, as well as differences in integration strategy, it is difficult to select a single combination to be the most suitable for marine applications. Additionally, current literature primarily focusses on maximizing system efficiency. Other factors such as system size, weight, IR signature and acoustic emissions are less frequently discussed in current literature. System size and weight are important factors for the feasibility of the combined cycles in current vessel designs, and the IR signature and acoustic emissions are important criteria for naval applications.

In this thesis, a solid oxide fuel cell (SOFC) is combined with an internal combustion engine (ICE) or a gas turbine (GT), and the system is fuelled by diesel or methanol (MeOH). This results in the following four combined cycles:
• Diesel-fuelled SOFC-ICE
• Diesel-fuelled SOFC-GT
• MeOH-fuelled SOFC-ICE
• MeOH-fuelled SOFC-GT
These combinations are compared for naval surface combatants based on system efficiency, annual energy consumption, annual CO2 emissions, exhaust gas temperature, total system weight and total system volume. Additionally, the combined cycles are compared to the current power generation system of a SIGMA surface combatant to determine the relative performance of the combined cycles to a conventional heat engine.

To determine the annual energy consumption, CO2 emissions and exhaust temperature, the combined cycles are modelled in Cycle-Tempo. The SOFC, ICE and GT are modelled as individual models and validated with available reference data. These individual models are then combined into a single model for each combined cycle, which is validated by comparing the results with results from current literature. The size and weight of the system is then obtained by selecting readily available components and summing their dimensions and weights.

For each comparison criteria, a different combined cycle performs the best. To minimize annual energy consumption, total system weight and total system volume, the diesel-fuelled SOFC-ICE combined cycle performs the best. To minimize exhaust gas temperature, the diesel-fuelled SOFC-GT combined cycle is the best choice. To minimize annual CO2 emissions, the MeOH-fuelled SOFC-ICE combined cycle is preferred. The exact performance of the combined cycles for these comparison criteria, depends on the power split between the SOFC and the heat engine. The annual energy consumption for the diesel-fuelled SOFC-ICE is between 4.4% (for a power split of 4.6-95.4) and 18.2% (for a power split of 25-75) lower than the current system of the SIGMA. The weight of this system is between 1.5% lighter (for a power split of 4.6-95.4) and 51.6% heavier (for a power split of 30-70) compared to the current system. For both the diesel-fuelled combined cycles, the exhaust temperature at silent speed can be reduced by 212 ◦C, which is a reduction of 58.3% compared to the current system. At cruising speed, the diesel-fuelled SOFC-GT can reduce the exhaust temperature with up to 207 ◦C at a power split of 30-70, which is a reduction of 65.8% compared to the current system. When using the MeOH-fuelled SOFC-ICE combined cycle, the annual CO2 emissions can be reduced by up to 25.9% compared to the current system for a power split of 25-75.

Depending on the preferred comparison criteria (annual energy consumption, emissions, exhaust temperature or system weight and volume), a different combined cycle and power split is preferred. There is no single solution that performs best for all criteria. It is therefore up to the designer to choose which criteria is most important and select the combined cycle accordingly. For this thesis, annual energy consumption and total system weight and volume are considered most important. For the combined cycles, a trade-off needs to be made between these criteria. A higher power split results generally in a lower annual energy consumption, but this also leads to a higher system weight and volume. To select the best power split for the SIGMA, a utopian solution is defined. This solution consists of the lowest annual energy consumption and the lowest system weight. This solution is impossible to reach, and the combined cycle and power split that lies closest to this utopian solution is deemed the best. This best solution is the diesel-fuelled SOFC-ICE combined cycle with a power split of 8.4-91.6 in favour of the ICE.

The diesel-fuelled SOFC-ICE combined cycle is implemented in the general arrangement plan of the SIGMA to assess the feasibility of this system in terms of system volume. The current power generation system (ICEs and diesel gensets) are removed, and the new components for the combined cycles are placed in the newly available space in the vessel. In the current general arrangement of the vessel, a maximum power split of 7.8-92.2 in favour of the ICE can be placed without having to change the design of the vessel. This power split is slightly lower than the ideal power split of 8.4-91.6. This lower power split results in a slightly higher annual
energy consumption and slightly lower system weight and volume compared to the ideal power split. Compared to the current system of the SIGMA, the diesel-fuelled SOFC-ICE combined cycle with a power split of 7.8-91.2 can reduce the annual energy consumption and CO2 emissions by about 6%. The exhaust temperature at silent speed reduces by about 212 ◦C (about 58%) and by about 80 ◦C (about 25%) at cruising speed. This reduction in energy consumption, CO2 emissions and exhaust temperature comes at the cost of a 4% increase in total system weight.

Overall, fuel cell combined cycles can help in reducing the fuel consumption and GHG emissions of the marine sector. Nevertheless, the implementation of fuel cell combined cycles alone is insufficient to reach the net-zero targets set by the IMO as outlined in [2]. For naval surface combatants, the combined cycles only reach a small reduction in fuel consumption and CO2 emissions (6%). The main benefit for these vessels lies in the reduction in exhaust gas temperature. A significant reduction in exhaust gas temperature (58% at silent speed and 25%
at cruising speed), and therefore a reduced IR-signature, is possible with a system small enough to still fit in the current design of the SIGMA.