The tugboat of the future will run on batteries... or on fuel cells... or both? And the ferry with combustion engines using sustainable fuels... in combination with batteries and electric motors... or also with fuel cells? "The question of the propulsion system of the future is complex," says Tobias Kohl, Director Application Engineering Marine at Rolls-Royce. That's because the possibilities are many. While until a few years ago a diesel-mechanical propulsion system was the norm and a diesel-electric propulsion system was modern, today many new combinations of different energy sources, converters and storage systems are already available or under development.
The only thing that is clear is that in order to meet the International Maritime Organization's (IMO) target of reducing CO2 emissions from shipping by 50 percent by 2050 compared with 2018, new fuels will be needed, and with them new propulsion concepts. And these will vary depending on the application.
When energy density is in demand: methanol as a fuel
Internal combustion engines will continue to play an important role. In the future, however, these will often no longer be powered by fossil diesel, but increasingly by sustainable fuels such as HVO, which is already available at present, or e-methanol in the future. "We are clearly focusing on methanol as a marine fuel for the future and want to be a pioneer here," confirms Denise Kurtulus, Vice President Global Marine at Rolls-Royce. The energy density of this fuel is high compared to other sustainable fuels, and its liquid state makes it easy to store and refuel at ambient temperatures. Even existing infrastructure can continue to be used in many cases. The methanol tanks can be flexibly arranged in the ship. In addition to reduced complexity, the more attractive investment costs are another advantage of the methanol tank system. Rolls-Royce Power Systems is initially developing methanol engines on the basis of the mtu series 4000 and will launch them on the market from 2026.
CO2-free and extremely flexible: fuel cell systems
But internal combustion engines will not be the only energy converters on large ships, as they have been in the past. Fuel cells will also be used. This is because hydrogen-powered fuel cells produce no harmful emissions whatsoever, neither CO2 nor nitrogen oxides or particulates. Fuel cells powered by green methanol emit only small amounts of CO2 due to the methanol reformer required, but no more than was previously bound in the e-methanol and are thus operated in a CO2-neutral manner. Introducing hydrogen as a fuel for ships poses a number of challenges due to the physical properties of hydrogen, for example with regard to infrastructure, refueling and on-board bunkering.
Further advantages of fuel cells are their low noise emissions and vibrations and their very high efficiency (approx. 50 - 60 %). Fuel cell systems offer new, very flexible possibilities in the design of the propulsion or energy system through the introduction of an electrical energy transmission and an electrical drive of the propellers: A variable number of batteries and fuel cells is possible, flexibly expandable with combustion engine driven units. All-electric drives are also conceivable, in which the energy is stored exclusively in batteries.
Rolls-Royce Power Systems is already working on the development of fuel cell systems for marine applications and will launch its own mtu fuel cell systems for main propulsion and on-board power generation from 2028.
The future: hybrid systems with different components
"All these individual components for propulsion, energy supply and storage - be they fuel cells, batteries or internal combustion engines powered by sustainable fuels - will enable us to develop entirely new propulsion concepts," says Tobias Kohl. For example, combustion engines are increasingly being integrated into a hybrid system in which batteries feed electric motors and combustion engines or fuel cells supply drive energy and charge the batteries.
Another trend: combining different energy systems on board a ship. Until now, the standard has been for the propulsion system to be separate from the on-board power system. But with the increasing use of electrical components on board, the two systems are merging. This means that the electricity generated by fuel cells can be used both for the hotel load on board and for the electric motors of main and auxiliary propulsion systems.
Application decides on ship architecture
But how will the tugboat or ferry of the future operate? Purely electric with batteries...or with fuel cells, or with combustion engines...or with a combination of everything? "It depends on the application of the vessel and the infrastructure provided," explains Tobias Kohl. Certainly, tugs that only operate in port and thus do not have large range requirements are more likely to be able to be operated electrically than fast ferries that are underway all day on longer routes. "And it's also certain that they won't make it across the ocean with purely battery-electric drives," Kohl said. For that, the energy storage systems would have to be huge, and there isn't that much space on board. Ferries in coastal areas will rely on internal combustion engines with sustainable fuels such as methanol, integrated into an electric, mechanical or even hybrid propulsion system, and later on fuel cell systems.
Rolls-Royce is currently developing a complete solution architecture that can be adapted according to customer needs. The basis is mtu hybrid solutions in a modular system that can be varied according to customer requirements in terms of performance and range, for example with additional batteries. This modular system is designed in such a way that future technologies such as methanol combustion engines or fuel cells can be subsequently integrated as soon as they become available.
Automation system mtu NautIQ brings together complex technologies
All forms of propulsion have one thing in common: they are becoming much more complex than before. This also increases the importance of automation systems that can monitor both the propulsion system and the entire ship as a kind of neural system. "With mtu NautIQ, we are well positioned here and can offer our customers reliable, economical and future-proof monitoring and control solutions from the bridge to the propeller," Tobias Kohl summarizes. He is confident that the shipping industry will be able to meet its major goal of reducing CO2 emissions by 50 percent by 2050 compared to 2018. Denise Kurtulus also confirms this: "I have received more inquiries about climate-friendly solutions in the past 12 months than in all my previous professional years combined. Manufacturers and customers are now looking specifically at ways to reduce CO2 emissions. The urgency to act is recognized. That's a good, hope-giving sign," she says.