Pilots celebrate after a successful flight in an aircraft powered by liquid hydrogen at Maribor Airport in Slovenia on September 7, 2023.

Pilots celebrate after a successful flight in an aircraft powered by liquid hydrogen at Maribor Airport in Slovenia on September 7, 2023. Peter Santini / Bloomberg via Getty Images

Flying Green: The Pursuit of Carbon-Neutral Aviation Revs Up

To eliminate global aviation’s sizable carbon footprint, researchers are working on a range of alternatives to fossil jet fuel. Recent test flights powered only by hydrogen or biofuels have been successful, but steep challenges remain before aviation can become carbon-free.

Last September, a small white aircraft with an unusual design took off from Maribor Airport, in Slovenia. The two pilots were not seated front and center; instead, they steered the plane in a capsule attached far out on the right wing. The plane had some other unusual features. On the far-left wing, another slender capsule contained a tank of so-called cryogenic hydrogen, cooled to minus 253 degrees Celsius, or minus 423 degrees Fahrenheit. A fuel cell inside the plane caused the liquid hydrogen to react with oxygen, producing water and enough electricity to power an electric motor with the propeller attached. The plane flew not with fossil fuel, but with hydrogen.

The “HY4,” as the aircraft is called, circled the southeastern foothills of the Alps for a total of more than three hours that day — a major success, according to Josef Kallo, a professor of electrical engineering at the University of Ulm in Germany and CEO of H2Fly, a startup based in Stuttgart. “We have flown with hydrogen many times before, but now we were able to demonstrate all the processes required for a flight of more than 1,000 kilometers,” Kallo says. His goal is to make so-called “green hydrogen,” produced by wind and solar farms, the new standard fuel in aviation. “Hydrogen is the most efficient fuel that can be made using renewable energies,” he says.

The aviation industry finds itself in the midst of a struggle over how to clear its reputation as a leading climate polluter.

The small plane is only a first step. Kallo is also working on an experimental version of the world’s first hydrogen passenger plane in partnership with Deutsche Aircraft, a manufacturer of medium-sized Dornier aircraft, and the German Aerospace Center. “We want to demonstrate this technology in a 40-seater powered with liquid hydrogen by 2026,” he says. His dream for the 2030s is that hydrogen propulsion will be able “to truly decarbonize at least half of global air traffic.”

The successful test flight is one of many signs that aviation is going through a deep technological shake-up. Mitigating CO2 emissions and climate impact has become one of the top priorities in the industry, experts say. Governments, private companies, and scientists across the world are ramping up sustainable aviation research, trying to find new, more efficient designs for planes and engines and to use recyclable materials. Fuels are at the center of this shake-up. Jet fuel made of fossil kerosene needs to be replaced. But with what?

H2Fly’s Josef Kallo and other experts are convinced that in order to achieve carbon neutrality by 2050 a deep technological transformation is needed — away from oily liquids like kerosene to hydrogen, a carbon-free fuel so far used only in space flights. At least two other startup companies, U.S.-based Universal Hydrogen and U.K.-based ZeroAvia, are pursuing this goal, too. ZeroAvia calls hydrogen-electric propulsion systems “the only alternative propulsion system that can deliver the range, payload, and emissions elimination required by the market.”

An Emirates ground crew fills an Airbus A380 with sustainable fuel for a demonstration flight at Dubai International Airport on November 22, 2023.

An Emirates ground crew fills an Airbus A380 with sustainable fuel for a demonstration flight at Dubai International Airport on November 22, 2023. Giuseppe Cacace / AFP via Getty Images

But hydrogen’s future role is far from clear. Airlines would need to buy hydrogen planes at a massive scale, and airports would have to make huge upfront investments in new hydrogen infrastructure, including pipelines, tanks, and filling stations for the frigid liquid.

In fact, a strong faction within the global aviation industry outright rejects that hydrogen can become a meaningful alternative to kerosene within the next 25 years. “It is arithmetically impossible to replace the world’s fleets with hydrogen-powered airplanes in time” to meet the industry’s climate-neutrality target, Christopher Raymond, chief sustainability officer of Boeing, claims. The U.S.-based company is the archrival of the world’s largest manufacturer of airliners, Europe-based Airbus. Raymond argues that, in a best-case scenario, “hydrogen-powered aircraft may make a small contribution to moderating emissions in 2050.” For now, the aviation industry finds itself in the midst of a struggle over how to clear its reputation as a leading climate polluter.

Globally, aviation contributes about 2.5 percent to energy-related CO2 emissions, a sizeable share. Contrails and nitrous oxides, byproducts of burning fossil fuels at high altitudes, also contribute to heating Earth’s atmosphere and oceans, and they could even triple aviation’s climate impact, according to a widely cited 2021 study by Manchester Metropolitan University in the U.K.

An experimental Boeing 787 flew from London to New York with fuel made from waste fats and waste plant sugars.

Yet despite Paris Agreement pledges to pursue efforts to limit planetary warming to 1.5 degrees Celsius, compared to preindustrial levels, air travel and transport are booming. Studies project a further increase driven mainly by travelers and airlines from newly prosperous countries in the Global South.

Instead of introducing hydrogen as a new fuel, Boeing’s Christopher Raymond and many others in the aviation industry are betting on a much less disruptive strategy for achieving climate neutrality. “Sustainable Aviation Fuels,” or SAFs, are chemically almost identical to fossil kerosene, but they are made from today’s animal waste and plant biomass. Boeing claims that SAF “lowers carbon emissions over the fuel’s life cycle by up to 80 percent, depending on the feedstock.” Another advantage of SAFs is that they are “drop in” fuels, meaning they can be easily incorporated into existing airport fueling systems. In a sign of progress, an experimental Boeing 787 run by Virgin Atlantic flew from London to New York last November with 100 percent fuel made from a mix of waste fats and waste plant sugars. It was the first flight of its kind and a major milestone.

Rolls-Royce tests an engine run on sustainable fuel for Virgin Atlantic.

Rolls-Royce tests an engine run on sustainable fuel for Virgin Atlantic. Rolls-Royce

But overall, rollout of SAF has been slow. In 2023, the aviation industry purchased only 500,000 tons, according to the International Air Transport Association (IATA), which represents 380 airlines. That’s twice as much as in 2022, but still only a miniscule 0.2 percent of the 286 million tons of fossil fuel combusted in planes that year.

Regulators currently allow planes to burn fuel mixtures that contain a maximum of 50 percent SAF, though engineers are retrofitting engines to burn a larger share. Boeing has said that all its new planes will be able to fly with pure SAF by 2030. Deutsche Aircraft, a manufacturer of medium-sized planes, plans to introduce a SAF-only plane, the Dornier 328eco, to the market in 2026 according to Riaan Myburgh, the company’s chief engineer for research and technology. “SAF really is the short-term and even medium-term solution,” he says.

Two problems cast a big shadow: SAF’s availability and its carbon footprint. While most SAFs are currently derived mainly from animal and industrial waste, IATA has called for algae, waste biomass from forestry, agriculture, and municipal waste to be added to the feedstock of refineries as fast as possible. With such a diverse feedstock, however, achieving and proving carbon-neutrality will be difficult. Any kind of biomass feedstock will generate CO2 emissions, for example when energy-intensive fertilizer or diesel tractors and trucks are used in industrial agriculture.

By 2028, says an engineer, the industry must make “its most important decisions on how to become climate neutral.”

That’s also true for so-called “electro-SAF,” which is made by splitting hydrogen from water using renewable energy. This hydrogen is then combined in refineries with carbon from biomass or from industrial exhaust gases to form synthetic kerosene. The process, also called “Power to Liquid,” can only become truly climate-neutral when the necessary carbon is captured directly from the atmosphere using wind or solar power, a technology not yet tested or available at scale.

Against this backdrop, proponents of “green hydrogen” fuel like H2Fly’s Josef Kallo argue that the best way to move forward is to take carbon out of the equation altogether and simply use pure hydrogen made with renewable electricity as fuel.

Björn Nagel, a 47-year-old aviation engineer and director of the Hamburg-based Institute of System Architectures in Aeronautics, has looked at aviation’s big existential questions from every possible angle in the past years. As head of a team of 60 scientists, he spends his days zipping between virtual reality animations of future planes, experimental robotic assembly lines, rooms crammed with expensive machinery, and meetings with scientists and aircraft manufacturers. His institute is part of the German Aerospace Center, the country’s equivalent of NASA, a research agency with 12,000 employees.

Rendering of aircraft powered by liquid hydrogen and an airport equipped to service them. Hydrogen tanks are visible in the rear of the plane in the foreground.

Rendering of aircraft powered by liquid hydrogen and an airport equipped to service them. Hydrogen tanks are visible in the rear of the plane in the foreground. German Aerospace Center

From his research lab on the banks of the Elbe River, Nagel can see the global aircraft production chain in action. Regularly, huge cargo planes appear on the horizon and descend toward a nearby airport, carrying fuselages, wings, and other large plane parts built elsewhere across Europe for his next-door neighbor, Airbus. About half of all the company’s new A320 jets are assembled here.

This proximity increases Nagel’s sense of urgency. “By 2028,” he says, “the global aircraft industry must have made its most important decisions on how to become climate neutral.” It takes about seven years to develop a new type of plane, and after becoming operational, these planes will remain in service for 30 to 40 years. “It’s a race with time,” he says.

Nagel led one of the world’s most comprehensive studies of aviation’s climate strategies, a project called “EXACT,” in which 165 scientists from 20 institutes compared, among other things, how airports around the world — Tokyo, Dubai, and Frankfurt among them — could be supplied with either SAF or hydrogen and how their environmental impacts would differ. They examined how to reduce contrails and nitrous oxide emissions most efficiently, and they came up with futuristic new plane designs, including one with a staggering 10 electric engines and propellers.

Sustainable fuel might not be available in large enough quantities, and harvesting it could harm vulnerable ecosystems.

The studies underscored the challenges of using hydrogen as aviation fuel. “It’s obvious that you can’t just put a hydrogen tank in an airplane and fly off,” Nagel says. New materials are needed to build the planes because tiny hydrogen atoms easily leak and tend to make anything they touch brittle. Nagel acknowledges that deep changes to aircraft architecture are necessary. Computer-simulated hydrogen planes developed in the EXACT study don’t have their fuel tanks spread out in the wings, as current planes do. Instead, they are designed to be nearly spherical, to reduce their surface area and the weight of the insulation needed to keep hydrogen fuel in its cryogenic state. One option under consideration is to place them in the plane’s tail, which would affect the plane’s overall weight balance.

Fuel cells must also become much lighter and more efficient than they are today. In terms of climate impact, the most important open question is to what extent hydrogen — which only produces water vapor as an exhaust — causes contrails. A project is under way in cooperation with Airbus, in which research planes fly behind passenger jets to determine the effects. For SAF, a considerable reduction could be shown compared to fossil jet fuel, since due to their higher chemical purity, SAFs emit much less of the soot that causes contrails.

But SAF still has strong downsides. “Using hydrogen to first synthesize kerosene” — as in the Power to Liquid process, says Nagel — “consumes up to 45 percent more primary energy than using hydrogen directly as a fuel, a huge number.”

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Also, the biomass needed for SAF might simply not be available in large enough quantities in the future, and harvesting and collecting it could harm vulnerable ecosystems. In terms of cost, liquid hydrogen fuel will require higher initial investments, Nagel says, but it should come out on par or cheaper than SAFs when their biomass demand, and the high cost of direct air capture of carbon, are figured in. When critics point out that hydrogen requires four times the volume per unit of energy as kerosene, Nagel answers that it weighs three times less. “According to our calculations, this largely cancels each other out,” he says.

Nagel admits that many challenges remain. “The devil is in the details” is a phrase he often uses. Overall, he has concluded that hydrogen, if made with electricity from renewable sources, has enough advantages to become the fuel of the future, with SAF and “electro-SAF” filling in for some time to come and for certain demands.

Airbus has committed to introducing a hydrogen-powered plane fit to transport passengers to the market by 2035.

Nagel is convinced that hydrogen’s technological challenges can be resolved. With that goal, his institute is turning a ground-based A320 jet into a “Hydrogen Aviation Lab” at Hamburg airport to test components and create a filling station for hydrogen fuel made by the region’s wind farms, in a project run by Lufthansa Technik. The German Aerospace Center is also involved in turning a Dornier 328 into a flying test bed for hydrogen-electric propulsion, and it is testing direct hydrogen combustion engines in Cologne.

But like all big players, the research agency is hedging its bets on aviation’s future. It is also researching Power to Liquid technologies, for which a pilot plant will be built in Eastern Germany. And in the U.S., Boeing is pursuing research on hydrogen propulsion. “We know it will take an ‘SAF and’ approach and not an ‘SAF or’ approach to achieving net-zero by 2050,” Boeing’s Raymond concedes.

Given Boeing’s current massive quality problems with its existing fleet, it is safe to say that hydrogen propulsion will either be developed in Europe, or nowhere. “We’d like to work with all aircraft manufacturers on hydrogen but it is clear that Boeing has a different focus right now,” Nagel says. In contrast, Airbus has committed to introducing a hydrogen plane fit to transport passengers to the market by 2035. At the Dubai air show last December, Airbus’ chief sustainability officer, Julie Kitcher, radiated optimism when she said that hydrogen propulsion is moving towards “technological readiness.” In January, the company opened a brand-new R&D center for hydrogen technologies in Stade, some 40 kilometers north of Hamburg down the Elbe River.