Aircraft fuel efficiency has continually improved over the years, and a conventional flight today produces half the carbon greenhouse gas of a flight in 1990. But the industry has been growing so rapidly that, by 2020, aviation emissions were 70% higher than in 2005, and they could triple by 2050 if no action is taken.
At the same time, climate science says that we need a net-zero global economy by 2050 to limit temperature rises to 1.5 C above pre-industrial times. Aviation will have to play its part and is already facing increasingly stringent legislation globally.
There are a number of potential solutions, including batteries, biofuel and hydrogen.
Although some small aircraft have flown with batteries, the technology is limited by the power-to-weight ratio of current devices – batteries powerful enough to get a large aircraft off the ground are just too heavy to do so. Biojet fuel could be a short-term option to improve the industry’s carbon footprint, says Professor Pericles Pilidis, head of Thermal Power and Propulsion Engineering at Cranfield University in the U.K., but it still emits greenhouse gases into the atmosphere.
Percentage of new commercial aircraft Boeing has committed to operating on sustainable aviation fuel by 2030
The year by which Airbus believes the first zero-emission commercial widebody aircraft be carrying passengers
Boeing has recently committed to having all of its new commercial aircraft operating on 100% sustainable aviation fuel (SAF) by 2030, says Tim Boon, Aviation Analyst at research group IBA. “Boeing has stated that further studies on the feasibility of hybrid electric and hydrogen propulsion systems continue to be carried out, but SAF is the main focal point for the airframer over the next decade.”
Deutsche Post DHL Group is pushing for the development and use of fuels produced from renewable energy sources. By 2030, at least 30% of fuel requirements in aviation and line haul are to be covered by sustainable fuels. “As the world’s largest logistics company, it’s our responsibility to lead the way and guide the logistics industry into a sustainable future,” says Frank Appel, CEO, Deutsche Post DHL Group.
Hydrogen to help reduce carbon
Many others in the industry believe that hydrogen holds real promise, not least as the “hydrogen economy” starts to be rolled out in other sectors, with much of it being produced using renewable energy.
“As recently as five years ago, hydrogen propulsion wasn’t even on our radar as a viable emission-reduction technology pathway,” says Glenn Llewellyn, Vice President, Zero-Emission Aircraft, Airbus. “But convincing data from other transport industries quickly changed all that. Today, we’re excited by the incredible potential hydrogen offers aviation in terms of disruptive emissions reduction.”
The European aircraft manufacturer has unveiled three concept designs for hydrogen-powered planes – a turboprop, a turbofan and a futuristic “blended wing” design. The company believes the first zero-emission commercial widebody aircraft could be carrying passengers by 2035. To meet this ambitious target, Airbus says it will need to launch the ZEROe aircraft program by 2025.
The ZEROe aircraft all use hybrid-hydrogen, meaning they have not only modified gas turbine engines that burn liquid hydrogen, but also hydrogen fuel cells that create electric power.
At the other end of the scale, startup ZeroAvia is looking to build hydrogen powertrains and has already flown a six-seater aircraft powered by a hydrogen fuel cell. It is now looking to scale up its technology to fly a 19-seater vehicle by the end of 2023. “The six-seater is a demonstration project to show that it is safe to fly with hydrogen,” says Sergey Kiselev, head of Europe for the company. “The 19-seater is the smallest commercially relevant aircraft. In time we hope to produce a powertrain big enough for planes that seat 40-70 passengers, both with a range of up to 500 nautical miles [926 kilometers]. More than half of flights are below this range.”
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Changing the anatomy of aircraft
The company hopes that its powertrain will initially be retrofitted to existing aircraft, but it is talking to manufacturers about integrating it into new aircraft as well.
At the longer ranges required for long-haul flights, the simplest answer is to burn liquid hydrogen, but this still produces non-carbon emissions such as nitrogen oxides, which are harmful to human health.
Another challenge of using liquid hydrogen is that it needs to be stored below –252.8 C, but this also creates the opportunity to introduce superconductivity into electrical systems, which would make those systems much more efficient and lighter, Pilidis says. In turn, this brings “opportunities to change the anatomy of the aircraft to make it less energy-intensive.”
The blended wing design integrates the wings, fuselage and engines, which are traditionally separate elements of the aircraft, so that the entire aircraft creates lift and minimizes drag. This shape helps to increase fuel economy and creates larger payload (cargo or passenger) areas in the center of the aircraft, according to NASA.
Pilidis believes that some of the first users of hydrogen wide-body aircraft will be logistics companies like DHL. “Many aircraft are tested first as cargo aircraft and then go into passenger service. I would expect history to repeat itself.”
There is still a long way to go, but the rapid development of the hydrogen-fueled aviation sector opens up the possibility of making zero-carbon flying a reality before the middle of the century. — Mike Scott
Published: June 2021
Images: Airbus; ZeroAvia