Drive To Decarbonize Aviation Raises Green Hydrogen Challenges

Sustainability has returned rapidly to the agenda for an aviation industry still struggling to survive the COVID-19 pandemic. And hydrogen has suddenly and unexpectedly risen to the top.

Airbus is leading the charge, with plans to deliver a zero-emissions commercial aircraft by 2035 backed by massive investment in hydrogen research from the French and German governments. The European manufacturer on Sept. 21 unveiled three different concepts for liquid-hydrogen-fueled airliners (AW&ST Sept 28-Oct. 11, p. 16).

• Fuel cells offer a near-term path to hydrogen
• Long-range flying favors direct combustion
• Short-haul flights could use hybrid approach

The reasons are understandable. Hydrogen offers the potential to decarbonize single- and twin-aisle commercial aircraft, the big contributors to aviation’s climate footprint, while also tapping into wider investment in hydrogen infrastructure because of its broad appeal as a zero-emissions energy carrier. This is particularly true in Europe, which is blazing the trail toward a hydrogen economy.

But the challenges are not to be underestimated. From production to consumption, significant technical, operational and economic obstacles must be overcome to bring hydrogen to aviation. But from industry leader Airbus to disruptive startup ZeroAvia, there is a belief that hydrogen propulsion’s time has come.

Not everyone is as convinced. Boeing sees the longer-term advantages of switching to a hydrogen ecosystem but believes the process will take longer than the schedules suggested by Airbus. “We do see promise in a transition to more hydrogen-based fuels over time. However, I don’t believe it’s something that’s right around the corner. There’s a lot of infrastructure and regulatory framework that has to evolve with the technology, and that is not a fast process,” says Mike Sinnett, Boeing Commercial Airplanes vice president of product development and future airplane development.

“We know a lot about how kerosene is burned, how it can be stored safely and how it can be transported, and how engines use that fuel in all environments from the Arctic to the desert,” he says. “As we transition to more sustainable fuels, we have to ensure there’s no backpedaling in those levels of safety. And that means there’s still a lot to learn about how you would create, transport and use hydrogen in all the operational environments, ranging from a Siberian winter to an Algerian summer.”

There are three main ways to use hydrogen in aircraft: in fuel cells to generate electricity, direct combustion in turbine engines, and in the production of synthetic sustainable aviation fuels (synfuels). Fuel cells can be used indirectly, to produce emergency and auxiliary power, and directly for propulsion. Turbine engines can burn hydrogen with some modifications to the combustion and fuel systems. Synfuels are drop-in replacements for fossil-based kerosene and require no significant aircraft or engine changes.

According to the “Hydrogen-Powered Aviation” report released in May by Europe’s Clean Sky and Fuel Cell and Hydrogen research programs, fuel cells can reduce the climate impact of aircraft by 75-90%, direct combustion by 50-75% and synfuels by 30-60%. Hydrogen does have the potential to increase contrails, which can adversely impact climate change, but more research is required.

Fuel cells are in widening use in the automotive sector, with producer Ballard projecting a $100 billion market over the next 10 years to power medium- and heavy-duty trucks. But the power density of fuel cells needs to be increased by two to three times to make them suitable for aircraft. Direct combustion requires liquid hydrogen storage, which has a major impact on aircraft design and demands the development of lightweight cryogenic tanks.

Synfuels, also called e-fuels, are produced by combining carbon captured from the atmosphere with green hydrogen, which is produced by electrolysis of water using renewable electricity. These fuels emit CO₂ when burned in flight, but can be considered carbon neutral because they recycle atmospheric carbon. The technology is still in its infancy, however, though large synfuel projects are planned in Denmark and Norway.

The report foresees fuel-cell propulsion being used in new-design commuter and regional airliners within 10-15 years, and sooner in retrofitted or modified aircraft using gaseous hydrogen storage. Medium- and long-range twin-aisle aircraft powered by hydrogen-fueled turbines could enter service within 20-25 years, the report projects.

Between these two categories, the report foresees the emergence within 15 years of a new class of short-range airliner with hybrid hydrogen propulsion—a 165-passenger single-aisle aircraft with reduced, 2,000-km (1,100-nm) range and slower, Mach 0.72 cruise speed but a 70-80% lower climate impact.

Using liquid-hydrogen storage, this concept combines hydrogen-fueled turbofans with a powerful, 11-megawatt fuel-cell system driving electric motors mounted on the main turbine shafts. Both systems work for takeoff and climb, and the fuel cell drives the fans in cruise when the turbines are turned off.

“In our view the bookends of the aircraft range spectrum are fairly well-established,” says Alan Newby, Rolls-Royce director of aerospace technology and future programs. “We have all-electric at one end, for general-aviation-type operations. And in the medium to long range, we think sustainable aviation fuel is the only credible solution at the moment.

“So where we come back to is this middle ground with commuters, regionals and then maybe shorter-haul midmarket operations where hydrogen may offer a solution and potentially either in combination or in competition with a hybrid-electric solution,” he says.

Rolls sees clear potential in fuel cells for smaller aircraft. Already developing electric propulsion for aircraft, the company is now gaining experience with fuel cells through an agreement with a Daimler-Volvo joint venture to use systems developed for heavy trucks for standby power at data centers.

“We’ve seen companies like Zero-Avia looking at what fuel cells might do for smaller aircraft, and for us that sector of the market is quite attractive because we have both parallel and series hybrid-electric solutions,” Newby says. Today those systems use batteries, “but it is quite conceivable as fuel cells get developed that you could swap those in and essentially keep the same electrical system,” he adds.

Despite their zero-emissions promise, hydrogen fuel cells are not expected to power long-haul aircraft. “For longer ranges, the power density of fuel cells becomes a challenge. It is more likely that such aircraft would use liquid hydrogen and burn it directly in the gas turbine,” Newby says.

“The gas turbine has a long way ahead of it for medium- and long-range transport, whether powered by hydrogen or by sustainable aviation fuels—there’s nothing to displace it for the longer-range mission,” he says. “So we need to keep investing in the gas turbine to minimize its use of whatever fuel is chosen.”

Whether it is consumed in a fuel cell, burned in a turbine or converted to synfuel, production and distribution of enough green hydrogen to meet the expected demand for decarbonization coming from all sectors of industry and society could prove one of the biggest challenges facing aviation.

“In 2019, around 90% of the hydrogen produced in the planet was from fossil sources. In 2019, the commercial fleet burned just shy of 100 billion gal. of fuel,” says Michael Winter, Pratt & Whitney senior fellow for advanced technology. “If you were to replace 10% of that enthalpy with hydrogen, you’ve used 90% of the world’s hydrogen production in 2019. So the question is, if we’re going to put money into infrastructure, where would we rather put it as a society: synthetic aviation fuel or hydrogen?”

Winter questions whether there will be enough renewable energy to make all the green hydrogen Europe is talking about producing, and he points out that a modern turbofan in cruise is already 10% cleaner than the world’s electrical power grid on a CO₂/kWh basis. “Will there be an abundance of clean water to use for electrolysis in the future?” he asks, noting that if desalination is required, it is an energy-intensive process.

Newby echoes the concerns about scaling up green hydrogen production but points to Rolls’ work on small modular nuclear reactors as one potential path. While countries are expanding their wind, solar and other renewable energy sources, small nuclear reactors could also feed electricity to the grid as a zero-carbon source, drive electrolyzers to produce hydrogen, or use hydrogen as a feedstock for synfuels.

Hydrogen may hold out hope for decarbonizing aviation, but a sustainable way to scale up production is essential. “That will need investment, and it will need the appropriate incentives, because we’re going to need hydrogen to decarbonize the whole world, not just aviation,” Newby says. “One of the particular challenges we face as an industry is how that adoption will be prioritized among various sectors.”