Mobility at sea

Cleaning up on the high seas

04 June 2020

Whenever more sustainable mobility is discussed, shipping largely sails under the radar of public perception. And yet, most of the world’s goods are shipped across its oceans, nor is there an end in sight to the growth of maritime traffic. The intention is for alternative fuels, more efficient logistics and the rediscovered power of wind to clean up shipping in the years to come.

Around 90 percent of the world's trade in goods is moved by sea. International shipping accounts for 2.6 percent of global CO2 emissions. This may not sound very much - but it’s more than Germany releases into the atmosphere in a whole year. And shipping is set to continue to grow in the coming years. According to the UN maritime agency IMO, climate-damaging emissions from shipping could increase by up to 250 percent between 2012 and 2050. The industry, through the IMO, has actually set itself the goal of reducing CO2 emissions by at least 50 percent by 2050 compared to the 2008 figures.

It’s with this in mind that cruise operator Aida Cruises is increasingly relying on LNG - liquid natural gas. Unlike conventional marine diesel, the combustion of liquefied gas does not emit sulphur dioxide or particulate matter and produces significantly fewer nitrogen oxides and less soot.

Cruising on liquefied gas

In 2018, Aida Cruises launched the world's first cruise ship which can be powered exclusively with liquid natural gas, the “Aida Nova”. According to the company, nitrogen oxide emissions are reduced by 80 percent compared to conventional heavy fuel oil, while CO2 emissions are cut by 20 percent. Two further LNG ships are expected to set sail in 2021 and 2023. Since spring 2020, the world's first liquefied natural gas car carriers have been shipping Volkswagens from Emden in Germany’s East Frisia to the east coast of the United States. And before this year is out, the “CMA CGM Jacques Saade” will be freighting containers between Asia and Northern Europe in a more environmentally friendly manner than has previously been the case. The 400-metre-long and 61-metre-wide container giant is the first of nine 23,000-TEU-class vessels to have been commissioned by French shipping company CMA CGM.

According to Statista, 170 liquefied-natural-gas vessels were plying the seas worldwide in 2019, with another 70 on order and 123 LNG-ready – that is, designed for conversion to the lower-emission propulsion system. When compared to the global fleet of some 60,000 large ships, this is a pretty modest number. And the infrastructure for supplying the liquefied gas, although it has grown in recent years, is still far from being universally available.

LNG under fire

As LNG slowly picks up speed, criticism of the alternative fuel is becoming widespread. “Liquefied natural gas is revealing itself as a harmful aberration,” says the environmental organisation NABU. It cites a study by the Inter­national Council on Clean Transportation (ICCT) which claims that LNG is more harmful to the environment than conventional marine diesel. The cause is methane, which escapes into the atmosphere during the extraction, transport and use of LNG – and heats the earth even more than CO2. Instead of pumping money into liquefied natural gas, ICCT researchers recommend investing in energy-saving technologies, carbon-neutral fuels, batteries, fuel cells, and wind-assisted propulsion.

The Maritime LNG platform, a lobbying organisation, disputes these findings. It claims that the ICCT study does not take into account the current state of engine development, which is significantly reducing methane loss. And, contrary to what the researchers have assumed, 50% of LNG is no longer produced in the United States by fracking, which releases a particularly high amount of methane.

"The use of green hydrogen as a fuel makes equal sense. Technologically speaking, this is the next logical step."

Ralf Brauner, shipping expert

One thing is clear, though: even if the methane problem can be solved, the CO2-saving potential of LNG remains limited. “As a transition technology, liquefied natural gas is still a sensible way forward,” says shipping expert Ralf Brauner. Not least, explains the professor of seafaring and logistics at Jade University in Elsfleth, because it will be possible at a later date to burn artificial and regeneratively produced natural gas in gas engines. “But the use of green hydrogen as a fuel makes equal sense. Technologically speaking, this is the next logical step,” says Brauner. If the hydrogen is produced using electricity from solar, wind or hydropower, the ships will only release water vapour into the air: in other words, they will be completely emission-free.

Fuel cells on the Baldeneysee

The leisure boat „MS Innogy“ , which is equipped with a fuel cell and electric motor, has been ferrying passengers across the Baldeneysee in Essen since 2017. But, of course, hydrogen technology is primarily intended for use on somewhat larger water bodies. Aida Cruises aims to trial the use of fuel cells on board the “Aida Nova” in 2021. The Swedish-Swiss ABB Group is planning to team up with French hydrogen specialist Hydrogéne de France (HDF) to develop fuel cell systems for heavy sea-going vessels – so that even containers can be transported across the ocean without generating emissions. ABB is working for French shipping company Compagnie Fluviale de Transport (CFT) to come up with a fuel cell system to power a pusher boat on the Rhône in 2021. The EU-funded project aims to demonstrate “that zero-emission operation is both feasible and economically viable,” says Matthieu Blanc, COO at CFT.


Green hydrogen is still expensive and currently only available in small quantities. It is also associated with major investments. “This requires the complete transformation of supply chains – including in ports,” says shipping expert Brauner. The use of organic-based or synthetic fuels, on the other hand, would not require any changes to the existing fuelling infrastructure. SynFuels are produced using green hydrogen and CO2 – ideally from the ambient air – and can already be combusted in today’s engines. However, they are even more expensive than green hydrogen and are currently only being produced in small pilot plants. With the advent of larger production facilities, prices are expected to fall in the coming years.

For shipping expert Brauner, one thing is clear: There cannot be one optimal alternative propulsion system for all types of ship, nor does there need to be. “Which solution is best for which ship depends on the purpose and route for which it’s used,” says the marine researcher and meteorologist from Jade University. The OECD's International Transport Forum (ITF) also considers the key to the sustainability of shipping to lie in a bundle of different drive systems and measures. According to a study by the IT, if implemented together, these could wean shipping off fossil fuels almost completely by 2035.

Battery-powered leisure craft

Small ships which call in at their home port once or several times a day are especially good candidates for battery operation, Brauner stresses. A solar-powered boat has been ferrying people across the Spree in Berlin since 2020. Two electrically operated excursion craft known as “Suncat 120” can each accommodate 180 passengers. It should soon also be possible to admire Spitsbergen, the Oslo fjord and the Niagara Falls without wreathing their breath-taking natural beauty in choking fumes.

Even larger commuter boats are already being powered by batteries. The trailblazers of electric propulsion on the water are for the most part Scandinavian shipping companies. Since 2019, the electric ship “Ellen” has been transporting up to 198 passengers and 31 cars or five trucks along the 40-kilometre-long route between the Danish islands of Aar and Als. This is rendered possible by a huge battery pack of 4,300 kilowatt hours. By way of comparison, the electric car with the longest range – the Tesla Model S Long Range – has a battery of 100 kilowatt hours, on which it can travel 600 kilometres.

Hybrid ferries and the rediscovery of wind power

Larger ferries with longer routes, on the other hand, are increasingly using hybrid propulsion systems: The batteries on board are charged by diesel generators during the crossing. The electric motors support the diesel engines during port manoeuvres and can also power the ship unaided over shorter parts of the route. The hybrid ferries “Berlin” and “Copenhagen” have been travelling between Rostock and the Danish city of Gedser since the end of 2016. These 8,800-tonne ships, designed for 1,300 passengers and 460 cars, can travel under full electric power for some 30 minutes, making them quieter and emission-free when they come into port. The “Copenhagen” is expected to save even more emissions in the future, thanks to what looks like an oversized advertising pillar, of the kind common in German cities, which will be installed on the ship in mid-2020 (picture at the top).

In fact, the 30-metre-high cylinder is what is known as a Flettner rotor. This rotary sail takes advantage of the Magnus effect, which was discovered 150 years ago: When the wind blows against the rotating cylinder, the air on one side is accelerated – and a vacuum is created. On the other side of the rotor, the air flow is slowed and excess pressure generated. The interaction of these two forces causes lift, like the wing of an aircraft: The ship gets a powerful boost, which is about ten times stronger than the force that could be generated by a sail of the same size. “This will enable us to reduce CO2 emissions on the Rostock-Gedser route by four to five percent,” said Scandlines CEO Søren Poulsgaard Jensen.

“The Flettner rotor will enable us to reduce CO2 emissions on the Rostock-Gedser route by four to five percent.”

Søren Poulsgaard Jensen, Scandlines CEO

Since 2014, Finnish start-up Norsepower has equipped two ferries and a tanker with rotor sails which, according to the operators, have reduced fuel consumption by five to ten percent. The French company Airseas goes even further, promising fuel savings of up to 20 percent, depending on the route and area of operation. This is made possible by a giant towing kite developed by the Airbus spin-off. In favourable winds, the kite sail attached to the bow is deployed automatically – and all the ship’s officer in charge has to do is press a button. Airbus has already equipped a freighter of its own with this “Seawing”. Now, Japanese shipping company K-Line wants to test the towing kite intensively over a period of two years.

The idea is not new: About 15 years ago, Hamburg-based company Skysails launched a similar system. However, the towing kite didn’t take off at the time – in part because low fuel costs meant that it would not be profitable for the shipping companies. "Of course, such systems have to be not just efficient, but also easy to use,” says shipping expert Brauner. “And it’s here that Flettner rotors have advantages over towing kites.” After all, a kite sail must first unfurl itself and then be held in position in front of the ship, whereas a Flettner rotor can unleash its additional thrust at the push of a button. The rotor sails are also known to be weatherproof and low-maintenance but are not suitable for every ship and route. They take up space on deck, and their use ideally requires lateral winds to get the best performance out of them and to save as much fuel as possible.

Speed limit at sea?

Fuel consumption and the associated emissions could also be reduced with a comparatively simple measure: cutting speeds. Just by travelling ten percent slower, a ship will cut its fuel consumption by some 20 percent. For the experts of the International Transport Forum, it follows that lower speeds are a sensible measure for environmentally friendly shipping, by which energy savings of up to 60 percent achieved. For this reason, NGOs and environmental associations have been calling for a speed limit at sea for years.

“Unlike roads, however, a general speed limit on the high seas might be counterproductive.”

Ralf Brauner, shipping expert

Ralf Brauner from Jade University agrees that, in principle, lower speeds could bring about major potential savings. These could be realised if logisticians were to think ahead more effectively. Goods that were not so urgently needed could be sent off earlier and reach their destination at a leisurely pace. “Unlike roads, however, a general speed limit on the high seas might be counterproductive,” says Brauner. It wouldn’t allow ships to accelerate if need be to avoid rough seas.

Waves and wild weather slow ships down appreciably and drive up fuel consumption. For this reason, the shipping expert advocates consistent weather routing, i.e. a forward-looking sailing practice that adapts to the weather conditions. “If you travel faster for a day to make better progress in rough seas, you can then slow down or reach the port a day earlier, which significantly improves the carbon intensity of the journey," Brauner explains. Depending on the ship and the journey profile, weather routing could save between five and 25 percent of emissions. “Many shipping companies are already doing this,” says Brauner. Weather routing and lower speeds don’t have to be mutually exclusive. The researchers from the ITF have also proposed an average speed limit as a possible part of the solution. Under this idea, the average speed on a given route would not be permitted to exceed a certain upper limit. This would still give the captains enough leeway to adjust their sailing speed to suit the weather conditions.

Clean ships sail cleaner

Ships are slowed down not only by wind, waves and weather, but also by the biotope on their hulls: over time, mussels, algae, mud and other deposits accumulate below the water line. Ralf Brauner describes the braking effect of this as “like having an aerodynamic car with up-to-date technology but holding an umbrella out of the sunroof when you’re driving.” The additional consumption due to significant fouling on the hull can be up to 20 percent, the expert claims. A large container ship might thus burn 40 tons of additional fuel for no good reason – every single day. Clean ships are thus a good investment – for shipping companies and the environment alike. “There's still a lot to be done here,” says the expert.

Shore power for small floating towns

But it isn’t only at sea that the floating giants can become more environmentally friendly. To date, most shipping companies have had to run their ships’ diesel engines in port to generate electricity. And a docked cruise ship consumes as much energy as a small town. In the future, ships will be increasingly supplied with electricity generated on shore. This is good both for the environment and for the health of the inhabitants of densely populated port cities. “Shore power has enormous potential, especially if it comes from renewable sources,” Brauner stresses.

In Hamburg, cruise ships have been able to plug in to recharge their batteries since 2016. Starting in 2023, the Hanseatic city will be the first European port to supply large container ships with environmentally friendly energy. However, there are still few shore power plants in Europe, and many shipping companies are reluctant to convert their vessels, an undertaking that can cost anywhere between € 300,000 and € 2 million. And the electricity also costs more than energy from a diesel generator. After initial hesitation, shipping company Color Line only made up its mind to use the shore power facilities in Kiel for its two Norway ferries in summer 2019. “This costs us a five-figure sum per month,” says Dirk Hundert­mark, Managing Director of Color Line in Germany, of the price difference to diesel. To generate an incentive for the use of shore power, particularly environmentally friendly ships get a discount in Hamburg of up to € 3,000 euros off their mooring fees.

Californian ports created new facts on the ground some six years ago: Since then, the use of shore power has been mandatory. Starting in 2014, 50 percent of the fleet had to use shore power in port, a figure which has climbed to 80 percent this year. China plans this year to expand the shore power facilities in its major container ports to nearly 500. From 2021, the use of shore power will be mandatory in Chinese ports – first for cruise ships, and, from 2022, also for ferries and container ships.


Ralf Brauner is professor of seafaring and logistics at Jade University in Elsfleth. The shipping expert focuses on marine meteorology, maritime technology and sustainable shipping.