The 19-seater Dornier 228 propeller plane that took off into the cold blue January sky looked ordinary at first glance. Spinning its left propeller, however, was a 2-megawatt electric motor powered by two hydrogen fuel cells—the right side ran on a standard kerosene engine—making it the largest aircraft flown on hydrogen to date. Val Miftakhov, founder and CEO of ZeroAvia, the California startup behind the 10-minute test flight in Gloucestershire, England, called it a “historical day for sustainable aviation.”
Los Angeles–based Universal Hydrogen plans to test a 50-seat hydrogen-powered aircraft by the end of February. Both companies promise commercial flights of retrofitted turboprop aircraft by 2025. French aviation giant Airbus is going bigger with a planned 2026 demonstration flight of its iconic A380 passenger airplane, which will fly using hydrogen fuel cells and by burning hydrogen directly in an engine. And Rolls Royce is making headway on aircraft engines that burn pure hydrogen.
The aviation industry, responsible for some 2.5 percent of global carbon emissions, has committed to net-zero emissions by 2050. Getting there will require several routes, including sustainable fuels, hybrid-electric engines, and battery-electric aircraft.
Hydrogen is another potential route. Whether used to make electricity in fuel cells or burned in an engine, it combines with oxygen to emit water vapor. If green hydrogen scales up for trucks and ships, it could be a low-cost fuel without the environmental issues of batteries.
Flying on hydrogen brings storage and aircraft-certification challenges, but aviation companies are doing the groundwork now for hydrogen flight by 2035. “Hydrogen is headed off to the sky, and we’re going to take it there,” says Amanda Simpson, vice president for research and technology at Airbus Americas.
Are hydrogen-powered planes possible?
The most plentiful element, hydrogen is also the lightest—key for an industry fighting gravity—packing three times the energy of jet fuel by weight. The problem with hydrogen is its volume. For transport, it has to be stored in heavy tanks either as a compressed high-pressure gas or a cryogenic liquid.
ZeroAvia is using compressed hydrogen gas, since it is already approved for road transport. Its test airplane had two hydrogen fuel cells and tanks sitting inside the cabin, but the team is now thinking creatively about a compact system with minimal changes to aircraft design to speed up certification in the United States and Europe. The fuel cells’ added weight could reduce flying range, but “that’s not a problem, because aircraft are designed to fly much further than they’re used,” says vice president of strategy James McMicking.
The company has backing from investors that include Bill Gates and Jeff Bezos; partnerships with British Airways and United Airlines; and 1,500 preorders for its hydrogen-electric power-train system, half of which are for smaller, 400-kilometer-range 9- to 19-seaters.
By 2027, ZeroAvia plans to convert larger, 70-seater turboprop aircraft with twice the range, used widely in Europe. The company is developing 5-MW electric motors for those, and it plans to switch to more energy-dense liquid hydrogen to save space and weight. The fuel is novel for the aviation industry and could require a longer regulatory approval process, McMicking says.
Next will come a 10-MW power train for aircraft with 100 to 150 seats, “the workhorses of the industry,” he says. Those planes—think Boeing 737—are responsible for 60 percent of aviation emissions. Making a dent in those with hydrogen will require much more efficient fuel cells. ZeroAvia is working on proprietary high-temperature fuel cells for that, McMicking says, with the ability to reuse the large amounts of waste heat generated. “We have designs and a technology road map that takes us into jet-engine territory for power,” he says.
Universal Hydrogen, which counts Airbus, GE Aviation, and American Airlines among its strategic investors, is placing bets on liquid hydrogen. The startup, “a hydrogen supply and logistics company at our core,” wants to ensure a seamless delivery network for hydrogen aviation as it catches speed, says founder and CEO Paul Eremenko. The company sources green hydrogen, turns it into liquid, and puts it in relatively low-tech insulated aluminum tanks that it will deliver via road, rail, or ship. “We want them certified by the Federal Aviation Administration for 2025, which means they can’t be a science project,” he says.
What are the challenges for hydrogen-powered aircraft?
The cost of green hydrogen is expected to be on par with kerosene by 2025, Eremenko says. But “there’s nobody out there with an incredible hydrogen-airplane solution. It’s a chicken-and-egg problem.”
To crack it, Universal Hydrogen partnered with leading fuel-cell-maker Plug Power to develop a few thousand conversion kits for regional turboprop airplanes. The kits swap the engine in its streamlined housing (also known as nacelle) for a fuel-cell stack, power electronics, and a 2-MW electric motor. While the company’s competitors use batteries as buffers during takeoff, Eremenko says Universal uses smart algorithms to manage fuel cells, so they can ramp up and respond quickly. “We are the Nespresso of hydrogen,” he says. “We buy other people’s coffee, put it into capsules, and deliver to customers. But we have to build the first coffee machine. We’re the only company incubating the chicken and egg at the same time.”
This rendering of an Airbus A380 demonstrator flight (presently slated for 2026) reveals current designs on an aircraft that’s expected to fly using fuel cells and by burning hydrogen directly in the engine. Airbus
Fuel cells have a few advantages over a large central engine. They allow manufacturers to spread out smaller propulsion motors over an aircraft, giving them more design freedom. And because there are no high-temperature moving parts, maintenance costs can be lower. For long-haul aircraft, however, the weight and complexity of high-power fuel cells makes hydrogen-combustion engines appealing.
Airbus is considering both fuel-cell and combustion propulsion for its ZEROe hydrogen aircraft system. It has partnered with German automotive fuel-cell-maker Elring Klinger and, for direct combustion engines, with CFM International, a joint venture between GE Aviation and Safran. Burning liquid hydrogen in today’s engines is still expected to require slight modifications, such as a shorter combustion chamber and better seals.
Airbus is also evaluating hybrid propulsion concepts with a hydrogen-engine-powered turbine and a hydrogen-fuel-cell-powered motor on the same shaft, says Simpson, of Airbus Americas. “Then you can optimize it so you use both propulsion systems for takeoff and climb, and then turn one off for cruising.”
The company isn’t limiting itself to simple aircraft redesign. Hydrogen tanks could be stored in a cupola on top of the plane, pods under the wings, or a large tank at the back, Simpson says. Without liquid fuel in the wings, as in traditional airplanes, she says, “you can optimize wings for aerodynamics, make them thinner or longer. Or maybe a blended-wing body, which could be very different. This opens up the opportunity to optimize aircraft for efficiency.” Certification for such new aircraft could take years, and Airbus isn’t expecting commercial flights until 2035.
Conventional aircraft made today will be around in 2050 given their 25- to 30-year life-span, says Robin Riedel, an analyst at McKinsey & Co. Sustainable fuels are the only green option for those. He says hydrogen could play a role there, through “power-to-liquid technology, where you can mix hydrogen and captured carbon dioxide to make aviation fuel.”
Even then, Riedel thinks hydrogen will likely be a small part of aviation’s sustainability solution until 2050. “By 2070, hydrogen is going to play a much bigger role,” he says. “But we have to get started on hydrogen now.” The money that Airbus and Boeing are putting into hydrogen is a small fraction of aerospace, he says, but big airlines investing in hydrogen companies or placing power-train orders “shows there is desire.”
The aviation industry has to clean up if it is to grow, Simpson says. Biofuels are a stepping-stone, because they reduce only carbon emissions, not other harmful ones. “If we’re going to move towards clean aviation, we have to rethink everything from scratch and that’s what ZEROe is doing,” she says. “This is an opportunity to make not an evolutionary change but a truly revolutionary one.”
This article appears in the April 2023 print issue as “Hydrogen-Powered Flight Cleared for Takeoff.”
Prachi Patel is a freelance journalist based in Pittsburgh. She writes about energy, biotechnology, materials science, nanotechnology, and computing.