Aircraft and jet engine manufacturers pride themselves in their legacy of technology innovation, and deservedly so. But commercial aviation will be technically challenged as never before to help their airline customers cut greenhouse-emissions in half by 2050 from where they were in 2005, as mandated by governments.
To succeed, they will need to think out of the proverbial box, which is why OEMs are exploring all technology options, such as electrification, alternative fuels and more aerodynamic aircraft designs. What may come as a surprise is the rapid emergence of hydrogen propulsion as the preferred fuel.
Airbus is leading the charge, with plans to deliver a zero-emissions commercial aircraft by 2035, backed by massive investment in hydrogen research from French and German governments.
The European manufacturer recently unveiled three different concepts: a 120 to 200-seat aircraft powered by a modified gas turbine engine powered by hydrogen rather than jet fuel, with a range of more than 2,000 miles; a 100-passenger aircraft powered by turboprop engines, rather than turbofan, powered by hydrogen, with a range of 1,000 nm; and a blended-wing body aircraft using a turbofan engine and which would be able to store hydrogen with the wider fuselage.
The use of liquid hydrogen as a jet fuel isn’t exactly new. In 1957, hydrogen was used to power a U.S. Air Force Martin B-57 over Lake Erie. The U.S. National Advisory Committee for Aeronautics experiment demonstrated for the first time the feasibility of using liquid hydrogen in flight for air-breathing jet propulsion.
Like Airbus, Boeing also sees the long-term advantages of commercial aviation transitioning to a hydrogen ecosystem. “There’s a lot of infrastructure and regulatory framework that has to evolve with the technology, and that is not a fast process,” said Mike Sinnett, vice president of product development and future airplane development at Boeing Commercial Airplanes.
Boeing has been working on hydrogen-powered aircraft for more than a decade. It has developed technology demonstrators and flown prototypes on hydrogen fuel cell technologies and hydrogen combustion technologies on small unmanned vehicles. On the military side, Boeing’s Phantom Eye is a high-altitude, long-endurance unmanned aircraft powered by liquid hydrogen.
All of the major engine manufacturers are working on research to adapt their gas turbines to burn hydrogen, although no leader in this technology has emerged yet. While hydrogen offers the potential to effectively decarbonize single- and twin-aisle commercial aircraft—the biggest contributors to aviation’s climate footprint—the challenges are formidable.
From production to consumption, major technical, operational and economic obstacles must be overcome to bring hydrogen to commercial aviation. And let’s not forget the public’s image of hydrogen, which is commonly associated with the conflagration that engulfed the airship Hindenburg as it attempted to moor at Naval Air Station, Lakehurst, N.J., in 1937.
Today, the safety of hydrogen—real and perceived—is an unavoidable challenge to its use in commercial aviation. The public image doesn’t reflect the research reality. Nonetheless, public perceptions will have to be addressed.
“Every single hydrogen flight demonstrator has tackled the safety issue, and in every case they’ve come out with the fact that hydrogen is safer than jet fuel, said Paul Eremenko, CEO of startup Universal Hydrogen. “The reason that it’s safer than jet fuel is that it doesn’t pool; it burns quickly and it burns up.”
One of the biggest unknowns associated with hydrogen’s rapid rise to the top of industry’s agenda on sustainability is the implications for the electrification of propulsion.
Technologists among the engine manufacturers believe the two are not mutually exclusive and there’s actually room for both. At the very least, they point out, the appeal of hydrogen means commercial aviation will have more options than ever in striking a balance between performance and emissions, with each option offering its own advantages and disadvantages.
For example, Hybrid-electric propulsion combining batteries with a piston or turbine engine enables longer-range flights and larger aircraft but doesn’t eliminate emissions. Battery electric propulsion, on the other hand, has zero inflight emissions but is limited by battery energy density to smaller, shorter-range aircraft.
Then there’s hydrogen-electric propulsion using fuel cells. This combination of technology would enable larger, longer-range aircraft with zero inflight emissions. However, fuel cells are not expected to reach the power density needed to power the single- and twin-aisle aircraft responsible for most of aviation’s emissions that are harmful to the environment.
Long term, hydrogen almost certainly will play a major role in the decarbonization of aviation as propulsion engineers figure out how electric and hydrogen technologies can be combined in a hybrid powerplant. In the nearer term, sustainable aviation fuels, combined with continued improvements in turbine engine fuel efficiency, are considered the best option for reducing aviation’s carbon footprint.
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