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.

There is currently a resurgence of interest in nuclear propulsion within the maritime sector, which is reflected by the ambition, as mentioned in the Dutch maritime sector report "No Guts, No Hollands Glorie", to develop a standardized, modular nuclear reactor for ship integration within 10 years. Nuclear energy has the potential to reduce the maritime sector’s contribution to climate change, as it does not emit CO2 during operation. Additionally, in contrary to many other renewable energy sources, nuclear energy offers a high-energy-density power source capable of providing sufficient energy for longer periods of operation. Not only would this improve the strategic autonomy of the Royal Netherlands Navy, but it would also be a solution for the increasing energy demands of additional unmanned systems or advanced combat systems, like high power radars and rail guns.

Implementing nuclear propulsion in future (naval) vessels presents challenges, particularly regarding the dynamic power profile of ships during operation. Land-based nuclear reactors typically operate as stable power sources, which contrasts with the fluctuating power demands of ships, especially naval vessels. This research aims to find a solution for these dynamic power requirements, without the use of energy storage capabilities, like batteries, for peak shaving capabilities.

Based on the expected implementation of a small modular reactor (SMR) by 2034, the Future Air Defender and the Amphibious Transport Ship were selected as potential vessel types of interest for the Royal Netherlands Navy to implement an SMR. Additionally, the high-temperature gas-cooled reactor (HTGR) and the supercritical carbon dioxide (sCO2) recompression power conversion cycle were selected for the nuclear power plant. A dynamic model of the selected SMR and its energy conversion system has been developed to compare its ramp rate with those of conventional naval prime movers, such as diesel engines and gas turbines.

The simulation results indicate that the reactor dynamics alone are insufficient to meet common ramp rates of naval vessels, demonstrating that relying solely on reactor control is not a viable control strategy. However, the implementation of the turbine bypass valve, while operating the reactor at a constant load, provides dynamic power behaviour comparable with diesel engines and even gas turbines. Potential drawbacks include reduced cycle efficiency at part load, as well as significant pressure and temperature gradients within the heat exchangers. The unacceptable temperature increase at the reactor’s inlet was addressed by incorporating a dump cooler into the primary circuit. This research therefore concludes that an HTGR, in combination with an sCO2 cycle and a bypass valve, is capable to provide the dynamic power requirements of a naval vessel.

Environmental and operational challenges associated with an excessive fuel consumption have led to the need for future naval vessels to be less dependent on fossil fuels. The chosen configuration is combined diesel-electric and diesel (CODLAD). However, there is a performance gap regarding the manoeuvrability between the previously implemented gas-turbine propulsion and the diesel-electric hybrid propulsion. In order to reduce this performance gap between a gas-turbine propulsion system and a diesel-eclectic propulsion system a new optimized control system needs to be developed.
Adaptive pitch control avoids the angle of attack peak, reducing the torque peak that can potentially overload the diesel engine. Adaptive pitch control is the only control strategy that operates at a low angle of attack, and therefore is less perceptible for cavitation. However this peak in angle of attack is responsible for the peak in thrust. Adaptive pitch control therefore needs large shaft speed fluctuations to be able to create a large thrust. Adaptive pitch control is therefore either slow or forces the diesel engine to rapidly make large changes in its shaft speed. The rapid changes in shaft speed could increase the amount of wear inside the engine, reducing the time between maintenance.
During the simulations torque control for the diesel engine performs extremely well during acceleration. Using torque control for the diesel engine the static power distribution of the propeller load was varied. During these variations it was concluded that a large acceleration can be achieved with a heavy loading on the diesel engine in steady state, supported by a large torque production during acceleration by the induction machine. A heavy matching of the diesel engine was possible because the diesel engine only produced the power needed for the steady state propeller power as matched via the combinator. The induction machine is made responsible for keeping the speed and coping with disturbances. The induction machine is inherently more suited than the diesel engine, due to its fast response as well as it being able to produce full torque or maximum power without any negative adverse effects. However torque control is not available from manufacturers.
This research includes a novel implementation of a dynamic power split, which produced very promising results. The dynamic power split uses the diesel engine with speed control parallel with the electric drive in dynamic torque control. The dynamic torque control for the induction machine allow for optimal usage of the inherent characteristics of the electrical machine. Using dynamic torque control the induction machine is responsible for the high frequency load changes such as disturbances as well as the first part of the acceleration. The diesel engine governed by a slower integral dominated PI controller accounts for the low frequency load changes, including the higher load at a new steady sate. The diesel engine with speed control parallel with the electric drive in dynamic torque control produces excellent acceleration characteristics, making it a viable option to improve the acceleration characteristics of a diesel-electric hybrid system.