With growing pressure on ports to reduce emissions, tug owners need to decide how to cut their environmental footprint
Tug owners and ports need to decide how to reduce emissions, in particular which fuels they will use in future.
While there have been a few technology testers, most tugs being built and delivered have diesel main engines and diesel generator sets rated to IMO Tier II environmental requirements.
More tugs are being built with hybrid propulsion, a few fleets powered by LNG and a few tugboats with batteries for electrical power to replace diesel during transits. Some operators are preparing to trial methanol, ammonia and hydrogen, and owners such as Crowley are beginning to use biofuels.
In the long-term, owners need to reduce emissions of greenhouse gases to zero or seek methods to become carbon neutral. These drivers come as IMO has taken another major step towards its goals to cut carbon emissions and the European Union (EU) has set out its plans.
IMO’s Marine Environment Protection Committee (MEPC 76) adopted amendments to Marpol Annex VI in June which require all ships to calculate their Energy Efficiency Existing Ship Index (EEXI) following technical means to improve their energy efficiency and to establish their annual operational carbon intensity indicator (CII) and CII rating.
These measures are intended to meet the IMO carbon intensity reduction target of 11% from 2023 to 2026, but do not directly affect tugboats. However, shipowners may need to change their operations to improve their CII rating.
Of more interest to tug owners is the EU Commission formally supporting the Waterborne Technology Platform through a memorandum of understanding for co-operation.
The partnership’s main goal will be to provide and demonstrate zero-emissions solutions for all main ship types and services before 2030 to meet EU decarbonisation goals and enable zero-emissions waterborne transport before 2050.
Waterborne will consider other challenges such as digitalising the sector or reducing shipping emissions in ports, developing clean refuelling solutions and recharging infrastructure.
According to the European Tugowners Association, the European Commission will invest up to €530M (US$627M) on actions within the scope of the co-programmed European partnership. This will be supplemented with up to €3.3Bn (US$3.9Bn) for the period 2021-2030 by the other partners, which will be invested in research, innovation and other activities.
Tackling propulsion options
These rulings challenge tug owners and port authorities to decide which fuels will be used to power future tugboat fleets.
Rolls-Royce Power Systems marine applications engineer specialising in new technologies Peter Gommeringer says technical issues remain the biggest challenge.
“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,” he says.
Mr Gommeringer and his colleagues have developed propulsion concepts involving MTU engines or batteries for a harbour tug. When it comes to liquid fuel, he is in favour of methanol, with hydrogen derived from renewable energy sources.
“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 engineroom,” says Mr Gommeringer.
He says configurations combining internal combustion engines fuelled by methanol with fuel cells and batteries present possibilities for tug propulsion.
Hydrogen fuel-cell systems could also be installed and batteries used for temporary energy storage, says Rolls-Royce Power Systems fuel-cell expert from the MTU PowerLab Benjamin Oszfolk.
In this design, the energy source would be liquid hydrogen, which offers high energy density. It would be stored in a cylindrical tank below deck.
“The big advantage a hydrogen-powered fuel cell has over an internal combustion engine is that it does not generate harmful emissions – neither CO2 nor nitrous gases (NOx), nor particulate matter (PM),” Mr Oszfolk explains.
Fuel cells can be deployed flexibly, but their power rating should correspond to the maximum power requirement of the vessel. This may be achieved using batteries as the energy output could be tuned to the tug’s power requirement.
If the tugboat mode does not require maximum power or speed, only the number of fuel cells needed is switched on. If maximum power output is required for the tugboat to travel at high speed or 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 customised drive-system solutions,” says Mr Oszfolk.
Propulsion experts are also contemplating an electric-only tug with batteries. “The electric-only system scores the most points for low operating costs,” says Mr Gommeringer.
But there are disadvantages, such as low energy density of the batteries and a short range of operations. “Electrically powered harbour tugs are certainly an option to be considered, but they will not cover long distances,” he concludes.
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