Hydrogen is likely to be the main fuel of choice for future air travel with the prospect of driving aviation towards a 2050 net zero.
Batteries have established themselves as the primary means of delivering zero carbon road vehicles – for now at least.
But for other forms of transport, marine and air, the jury is still out on the best pathways for their decarbonisation, not least because a main fuel option, green, renewable generated hydrogen, is still to come. While batteries are likely to be an option for maritime applications with green hydrogen becoming a ‘competitor’ as well as for road transport, it is green hydrogen and other synthetic e-fuels that are likely to win out for much of aviation.
The reasons are simple. Batteries are both bulky and heavy and as such are best suited to the smaller, specialised and shorter-distance applications. In Brazil, for example, EDP Brasil and Embraer are testing electrification in a small single-engine aircraft utilised for aerial agricultural applications such as crop dusting.
The World Economic Forum, which identified electric aviation as one of the top 10 technologies for 2020, anticipates that a distance of up to 800km should be within the range of battery-powered electric aircraft by 2025.
But time will tell. Airbus, which launched its largest plane demonstrator E-Fan X in 2017, a hybrid in which one of the jet engines on a BAe 146 was replaced with an electric motor, canned the programme in 2020 before the planned maiden flight.
The reason cited was a refocussing of efforts on the technology bricks for decarbonisation and while work continues on other electric flight applications, the company states hydrogen as one of the most promising zero emission technologies to reduce aviation’s climate impact.
The hydrogen plane
The question arises how a plane or a jet engine can run on hydrogen.
One option is that it could be used as a replacement for traditional jet fuel for combustion in a modified jet engine, i.e. similar to the operation of current jets. The other option is for the hydrogen to power a fuel cell, which in turn creates electricity to power the aircraft, i.e. similar to the development of hydrogen-powered vehicles.
A further option is to use the hydrogen to create other synthetic e-fuels, which similarly could be used as a jet fuel replacement. Indeed, such alternative fuels are already being used with a synthetic biomass-based fuel (SAF) certified for use in up to a 50% blend in today’s aircraft.
Hydrogen offers high potential as a fuel, with an energy per unit mass three times greater than traditional jet fuel. However, with a lower volumetric energy density, it requires greater storage capacity, which means that the design and appearance of future aircraft will likely change.
With its creation from renewable energy, the fuel is green. And the only byproduct is water, thus achieving the emission reduction goal.
Hydrogen planes today
The first flight of a hydrogen powered plane was demonstrated as far back as 2008 when Boeing introduced a fuel cell modified one-man motor-glider, which undertook three flights of approximately 20 minutes duration.
Since then developments have continued apace, leading to the first commercial scale flight, which took place in 2020. In the UK, California startup Zeroavia demonstrated a hydrogen powertrain retrofitted six-seater Piper Malibu aircraft, which flew to a height of 300m.
Zeroavia envisages its technology timeline developing to the first commercial offering of an up to 20 seat plane with 500 nautical mile range becoming available in 2024 and an up to 200 seat, 2,000 nautical mile range plane by 2030.
Boeing seems to have dropped off the zero carbon fuel development radar, publicly at least, leaving it to Airbus of the big manufacturers to lead the way.
The company is working on three concepts, codenamed ‘ZEROe’, with turbofan, turboprop and blended wing designs.
The turbofan design is powered by modified gas turbine engines running on hydrogen through combustion. The liquid hydrogen is stored and distributed via tanks located behind the rear pressure bulkhead.
The turboprop design uses two or in one design option six turboprop engines powered by hydrogen fuel cells in a design most likely suited to shorter haul trips, up to 1,000 nautical miles.
The blended wing body design in which the wings merge with the main body of the aircraft also is powered by turbofan engines. The wide fuselage opens up multiple options for hydrogen storage and distribution as well as for cabin layout.
Airbus is targeting 2035 for the market readiness of its first zero-emission commercial aircraft, although what design that will take is still open. In part it will depend on the combination of hydrogen technologies, on which the company expects to decide by 2025.
As part of that plan in March a 100% synthetic fuel project was launched with the first test flight of a 100% SAF fuelled A350 to investigate issues such as engine performance and emissions.
A key factor in the growth of hydrogen-fuelled aviation is the availability and cost of hydrogen and Airbus says that projections are in line with its ZEROe programme.
To kickstart the infrastructure requirements in February the company in partnership with Air France-KLM and others launched a call for interest in hydrogen development in Paris airports. The call covers not only storage and distribution but also the wider use of hydrogen for transport and other applications at airports.
Glenn Llewellyn, Airbus VP of Zero-Emission Aircraft, describes decarbonising aviation as a team sport, with Airbus in it to win it.
“There’s no ‘silver bullet’ solution to decarbonising aviation. A mix of new energy pathways and fuels, including hydrogen, will be vital to achieving the disruptive CO2 reductions the industry is targeting over the medium to long terms,” he says.
“We’re collaborating closely with a network of partners – including engine manufacturers, airports, technology partners and energy suppliers – to push the development of the various technologies to support their introduction and scale-up.”