Energy efficiency series: can aviation and maritime transport be electrified?

Battery-driven electric motors are much more efficient vehicle drives than hydrogen-fuel cells and internal combustion engines. This was demonstrated in the third part of the energy efficiency series using passenger cars as an example. This fourth instalment explores the modes of transportation in which this combination is helpful and economically feasible as well as other ways to improve energy efficiency in transportation.

Whether travelling on land, water or sea or even through the air, electric drives are the ultimate authority in energy efficiency. Imagining that storage and conversion losses are minimised because the electricity comes directly from aerial lines, efficiencies could reach record-setting dimensions. Some electric locomotives transfer far more than 80 percent of the electricity they receive from wind turbines and solar panels via the grid to the drive wheels.

However, most means of transportation are designed to carry their energy supplies ­ usually in tanks or batteries. Comparisons on this basis show that batteries are more energy efficient no matter how big or heavy the vehicle may be. Nevertheless, batteries have physical limitations. After all, even lithium-ion cells, a fairly light variant, have a weight problem.

Energy density determines the application

Mid-sized EV car batteries that have a range of about 500 kilometres easily weigh in excess of 500 kilogrammes. These batteries often have a displacement of approximately 200 litres, which reduces trunk space. But this is manageable for most applications.

Batteries can be an economically viable option for lorries designed for short hauls, e.g. to travel the last mile from the warehouse to the business or customer, for vehicles used on construction sites and for garbage trucks.

By contrast, batteries push the limits of economic viability. An eighteen-wheeler would probably require a power storage unit with a capacity of 750 to 1,000 kilowatt hours (kWh) to travel over 500 kilometres, a distance that is commonplace for long hauls. Such a lithium-ion battery would weigh around five metric tons ­ which is quite hefty even for a 40-tonne lorry, given that the average payload of a diesel-powered tractor truck is approximately 25 metric tons.

This is why many experts believe that hydrogen drives stand a greater chance of prevailing above all in long-haul traffic, even if the energy efficiency of this fuel is not all that great. Similar considerations apply to ocean and air travel.

Battery-powered ships are already profitable on short hauls

Norway is a pioneer of the electricity transition at high sea. The world’s first electric ferry went into operation in 2015. It takes cars, cyclists and pedestrians across the Sognefjord about every 30 minutes. Several electric ferries have been commissioned since then.

And since April of 2022, Yara Birkeland has been the world’s first fully electric container ship. It transports mineral fertiliser from its production site in Porsgrunn to the nearby Port of Brevik, which is not far from Oslo. The 80 metre-long ship and the less-than-ten-sea-mile route are a far cry from Panamax-class ships, which can be as much as 366 metres long and 50 metres wide and travel half way around the globe.

However, based on calculations made in an article in renowned science journal ‘Nature’ battery electric ships are already more economical than conventional drives running on heavy oil on routes of up to about 1,000 kilometres. According to the data, on ships, battery volume, which reduces cargo space, is more important than weight. In the current scenario, researchers compute a volumetric energy density of 470 watt hours per litre (Wh/l) and costs of 100 US dollars (USD) per watt hour.

The authors surmise that these figures could rise and fall to 1,200 Wh/l and USD 50 per Wh in the near future. This would make conventional freightliners more cost-effective on routes in excess of 3,000 kilometres than conventional ocean giants. This factors in the USD 100 CO₂ tax per metric ton of heavy oil. Environmental costs have not been considered in either of the scenarios.

But there are other ways to increase the energy efficiency of ships. For instance cleaning the hull regularly reduces consumption – independent of the drive. This also applies to towing kites which are being developed, e.g., by Airbus subsidiary AirSeas. At the end of October, the startup announced it managed to achieve about 16 percent in fuel savings in a field trial.

The skies have their own rules

European aircraft manufacturer Airbus is also among the companies that are engineering hydrogen airplanes. Hydrogen is basically considered the sole alternative to kerosene. Of course, in a carbon-neutral economy, this fuel would have to be produced sustainably. However, common methods are extremely energy intensive: production, transport and storage of hydrogen almost waste 50 percent of energy input.

Carbon-neutral fuels synthesised from hydrogen even waste more than half of the energy put on the system as electricity by wind turbines and solar panels. This also holds true for sustainable aircraft gasoline, usually referred to by its acronym SAF (sustainable aviation fuel).

Nevertheless, these are apparently the only two ways to ensure zero-emissions air travel. “Small drones and air cabs can run on a battery, but no one really believes that electric airplanes are conceivable ,” says aviation expert Michael Santo from business consultancy H&Z. This would still hold true if the energy density of batteries improved considerably as lithium-ion variants with a capacity sufficient for a medium-haul flight would weigh several times more than the rest of the aircraft.

However, Santo claims there are numerous other approaches to reducing aircraft fuel consumption: “Technically, use of better materials can reduce weight and aerodynamic drag, and engines can be optimised further.” He says that, in addition, airplane capacity utilisation could be improved and flight routes could be planned more carefully. At the same time, Santo points out that these areas have constantly been fine-tuned in the past because “after all, fuel consumption has long been one of the largest aviation cost variables.”

In fact, according to figures published by the German Aviation Association (BDL), member companies of the BDL reduced consumption per passenger kilometre by 43 percent between 1990 and 2019. Despite this, come aviation experts claim that, theoretically, additional fuel savings of 30 percent are possible. However, Santo views rapid leaps in innovation with scepticism: “As evidenced by the Boeing 787 Dreamliner, innovation can be to the detriment of reliability and safety. And this is not a trade-off that customers and passengers are happy to settle for ­ especially in air travel.” The expert adds that this means that major additional fuel savings will take a long time to achieve.