Batteries' struggle with jet propulsion

It's clear aviation needs to use green fuel, or at least find an alternative way to power aircraft through the sky that does not emit CO2, NOx, contrails, and particulates.

In this day and age, nothing can be ruled out – the tech that enables us to fly cleanly in the future might not have been conceived yet – and as things stand, there are three strong candidates to replace the polluting fuels that currently power the aircraft:

Green hydrogen fuel derived from Power-to-X
Lithium-ion batteries
Hydrogen-electric engines

Green fuel is a nice idea, but the Power-to-X technology converting alternative energy into liquid fuel is still at a nascent stage. Denmark's target to produce enough green hydrogen to fuel all its domestic flights by 2030 illustrates the gigantic scale that would be needed to power all aircraft in the future.

Likewise, lithium-ion batteries have a long way to go. Over the last decade they have enabled us to increasingly drive electric cars, scooters and bicycles – to the extent they power 1 percent of all passenger cars in Europe – but in order to power aircraft they need to be incredibly heavy. Testing began in 2019, but no major strides have been made yet.

Considerable progress, however, has been made with electric engines. Last year, Canadian-based company Duxion Motors reported a successful test of its eJet Motor, the world’s first rim-driven jet propulsion motor.

Also last year, engineers at MIT created a 1MW motor of a weight and size comparable to current small aero-engines. The creation would be best suited to hybrid propulsion systems: electric and traditional, or electric and hydrogen.

Good vibrations on maiden flight

The potential of hydrogen-electric engines is so great that an ambitious timeline has already been drawn up.

'A hydrogen-electric engine can harness 60 times more energy than lithium-ion batteries – and for a significantly lower cost too.

In September this year, a 40-seater aircraft powered by a hydrogen electric engine took to the skies in the US state of Washington – a successful 15-minute flight carried out by Universal Hydrogen that reached a height of 3,500 metres and performed with greatly reduced noise and vibrations.

Universal Hydrogen, a Californian-based company founded in 2020, is one of several making serious headway in the sector – and they already have an order to convert 75 aircraft for US carrier Connect Airlines, which commented after the flight:

“We are committed to being North America’s first zero-emission airline. This historic flight – taking hydrogen, which can be made with nothing but sunshine and emitting only water – is a key milestone on our journey.”

Germany heads trials with hydrogen aircraft

Not to be outdone, Europe also staged hydrogen-electric trials this year – H2FLY, a German company based in Stuttgart, oversaw four flights taking off in Maribor, Slovenia.

The H2FLY aircraft uses liquid hydrogen to power a hydrogen-electric fuel cell system. Using hydrogen gas, the aircraft had a range of 750 km, but this doubled using cryogenically-stored liquid hydrogen (LH2), which enables lower tank weights and volume, thus increasing the range and useful payload.

A HY4 demonstrator aircraft was fitted with a hydrogen-electric fuel cell propulsion system powered by LH2. The longest of the four flights lasted three hours, prompting H2FLY co-founder Professor Josef Kallo to comment:

"This achievement marks a watershed moment in the use of hydrogen to power aircraft. Together with our partners, we have demonstrated the viability of liquid hydrogen to support medium and long-range emissions-free flight. We are now looking ahead to scaling up our technology for regional aircraft and other applications, beginning the critical mission of decarbonising commercial aviation.”

Some pretty major German players are funding the development, including the federal ministries for economic, digital and transport affairs.

Timeline for hydrogen aircraft extends over next two decades

Nevertheless, the timelines outlined by hydrogen-electric aircraft developers do not envisage the aircraft replacing current fleets anytime soon.

For example, ZeroAvia, a British-American developer founded in 2017, has plans to start retrofitting existing aircraft with its hydrogen-electric powertrains in 2025 – starting with aircraft of 9-19 seats and a range of 300 nautical miles.

These will be followed by larger models in 2027 (40-80 seats; 1,000 NM), 2029 (100-200 seats; 2,000 NM) and 2032 (200 seats; 3,000 NM). But it is only by 2040 that it expects to retrofit an aircraft capable of intercontinental travel (200+ seats; 5,000 NM).

Still, continental travel could be very different within a decade if development continues at the same rate as today.

Other airlines taking major strides in the sector include: Ecojet, Alaska Airlines, Avolon, Delta Airlines, EasyJet, GOL, Icelandair, Japan Airlines, JetBlue, SAS, United Airlines, Vertical Aerospace, Virgin Atlantic, Viva Aerobus and Wideroe.

Engines power aviation’s greener future

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Hydrogen's coming strong, but is it for the long-haul?