STORY Commercial Marine

Technology turnaround: Which fuel for the tugboats of the future?

Posted on May 19, 2021 by Lucie Maluck, Images by Rolls-Royce Power Systems

mtu experts from Rolls-Royce show the way forward to sustainable tugboat operation.
Hydrogen, methanol, ammonia, synthetic diesel – there's a lot of uncertainty surrounding the fuels of the future. But one thing is clear: 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? “That question hasn't been finally answered,” said Peter Gommeringer, marine applications engineer specializing in new technologies at Rolls-Royce Power Systems. “A technological change of this magnitude depends on us and all the key players – shipyards, operators and owners – pulling together to work out the best propulsion concept in each case for the given requirements.” Gommeringer and his colleagues have developed different propulsion concepts for a harbor tug. Their subject is a hypothetical tugboat of 29m length and 13m width that requires 5,000 kW of driving power and 100 kVA for on-board power.

Innovative battery/fuel cell combinations

The initial concept for powering the hypothetical tugboat is based on a combination of fuel cells and batteries. The required energy for this is generated in hydrogen fuel cell systems and temporarily stored in 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 virtue of fuel cells is that they can be deployed very flexibly, which is what essentially sets them apart from other drive and propulsion systems today. To date, the propulsion power for the tugboat has been solely derived from diesel engines. That has meant that their power rating has to correspond to the maximum power requirement of the vessel. 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.  

Experts from Rolls-Royce Power Systems know how to calculate the ideal combination of fuel cells and batteries so that a tugboat is operated with maximum efficiency.

Concepts based on other alternative energy carriers

Experts at Rolls-Royce are simultaneously studying other propulsion concepts based on fuels such as methanol. “Methanol does not have the power density of diesel fuel. However, of all the new fuels, it is the one that most resembles diesel fuel and thus demands the fewest modifications to the engine room,” explained Peter Gommeringer. Configurations involving the combination of internal combustion engines fueled by methanol with fuel cells and batteries therefore present other exciting possibilities.

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.

Peter Gommeringer, Applications Engineer for new technologies at Rolls-Royce Power Systems

Driving the maritime energy transition

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.” 

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