Will it be hydrogen, methanol, synthetic diesel or ammonia? So far, the fuel scenario of the future has been difficult to estimate. But slowly, the picture is starting to come into focus. One thing is clear however: with IMO rules stipulating that by 2050, CO2 emissions in shipping have to be 50% lower than in 2018, the call is for new fuels, and with them new propulsion concepts. mtu experts at Rolls-Royce have now come up with various configurations for tugboats.
The fuels of the future will be manufactured using a method known as Power-to-X. The basic principle is as follows: Renewable energy sources such as wind or solar are used to generate electrical power. This is used in electrolysis, in which water is broken down into its components (hydrogen and oxygen). The hydrogen part can be directly used as a fuel or synthesized to produce other fuels such as e-Methane, e-Methanol and E-diesel. If the CO2 needed to synthesize these fuels is derived from the atmosphere or biomass, their combustion can be said to be carbon-neutral because no additional CO2 is being released. Hence they are set to play a decisive role in meeting the IMO's climate targets.
Strong need for dialog in the tech revolution
But which fuel is most suited to marine propulsion and in which applications? “We’ve been talking to our customers about this in detail over the last few months and have now developed various propulsion concepts for tugs,” said Peter Gommeringer, marine applications engineer specializing in new technologies at Rolls-Royce Power Systems. The concepts are based on a hypothetical tugboat of 29m length and 13m width that requires 5,000 kW of driving power and 100 kVA for on-board power.
Practical advantages of methanol for tugs
One solution currently favored by many customers is the methanol internal combustion engine. The energy density of methanol is high compared with other sustainable fuels, and since it comes in liquid form, it is convenient to store and tank at ambient temperatures. In many cases, it is even possible to use existing infrastructures with methanol. Methanol tanks can be flexibly accommodated within vessels and demand much fewer safety measures than hydrogen or ammonia. Due to the safety aspect and fewer complications, the lower investment cost of the methanol tank system presents another advantage.
Innovative battery/fuel cell combinations
With another concept, the hypothetical tugboat is powered by a combination of fuel cells and batteries, whereby the energy is generated in hydrogen fuel cells and temporarily stored in the batteries. The energy source is liquid hydrogen, which offers high energy density and is stored in a cylindrical tank below deck. “The big advantage that a hydrogen-powered fuel cell has over an internal combustion engine is that it does not generate harmful emissions – neither CO2 nor nitrous gases, nor particulates.” explained Benjamin Oszfolk, fuel cell expert from the mtu PowerLab at Rolls-Royce Power Systems.
The fuel cell/battery concept offers great flexibility, which is one of its main advantages over the internal combustion engine. With the latter, the driving power needed is delivered solely by the engines, whose output depends on the maximum power requirement of the tug. Fuel cells combined with batteries, however, open new possibilities. The amount of energy placed at disposal can be tuned to the tug's power requirement at any one time, thereby providing scope to save fuel. If the tugboat mode does not require maximum power or speed, only the number of fuel cells needed is actually switched on. If maximum power output is called for because the tugboat has to travel at high speed or is in tugging mode, all fuel cells are switched on. The energy stored in the batteries will also be used. “That gives our experts the scope to work out the ideal combination of fuel cells and batteries for each application and thus deliver customized drive system solutions,” explained Oszfolk.
Methanol fuel cells
Engineers at Rolls-Royce Power Systems are also considering the use of methanol for fuel cells. Using a ‘methanol reformer’, hydrogen is produced from the methanol which is then used in the fuel cell to generate power. The big advantage of this concept is the fact that methanol has a much higher energy density than hydrogen and therefore permits fuel tanks of more compact dimensions.
Concepts based on other alternative energy carriers
Experts are even contemplating an electric-only tug in which the power is taken solely from batteries and not generated by on-board fuel cells. “The electric-only system scores the most points for low operating costs,” pointed out Gommeringer, further explaining that it does suffer from a major disadvantage – the low energy density of the batteries, which leads to a very short range. “Electrically powered harbor tugs are certainly an option to be considered, but they will not be able to cover long distances,” he concluded.
The task with each concept is to find the right balance in terms of range, operating and procurement costs and technical feasibility.
In the near future, mtu experts at Rolls-Royce Power Systems will be engaging with operators, shipyards and owners to discuss these and other concepts in more detail and establish which will be the most efficacious. A concrete study project is currently underway in collaboration with the renowned marine engineering company Robert Allan and towage specialist Svitzer. Other studies are to follow: “The task with each concept is find the right balance in terms of range, operating and procurement costs and technical feasibility,” explained Gommeringer, who is clearly inspired by his mission. “This is a steep challenge, but it's also an opportunity for us to help drive the energy turnaround in the maritime industry and change the face of shipping for good with our innovative propulsion concepts.”