Aerospace manufacturers agree that the ability to produce hydrogen in vast quantities using only renewable forms of energy such as solar and wind power will prove the most critical factor in the aviation industry’s effort to develop the element as a viable, non-CO2-emitting fuel. In interviews with AIN, at least five major OEMs stressed the need for massive investment to develop the complex infrastructure required to produce hydrogen on an environmentally sustainable basis, and to distribute, store and disburse it as a fuel safely, all of which must happen alongside an effort to develop hydrogen propulsion systems for large and small aircraft.
Only if the development of green hydrogen production and the infrastructure needed to get it into aircraft fuel tanks safely occurs in parallel with OEMs’ development of primary and hybrid-electric forms of hydrogen propulsion can aviation meet its 2035 estimate of being able to replace Jet-A as a fuel for large transport aircraft, say Airbus, Boeing, CFM International/GE Aviation, GKN Aerospace, and Rolls-Royce.
Many activities need to take place in parallel before 2035 to allow for the use of hydrogen—in differing forms depending on aircraft size and mission—as a fuel throughout the aviation industry. Development of hydrogen-propulsion systems and the so-called green hydrogen ecosystem “won’t be a serial process,” said Brian Yutko, Boeing’s vice president and chief engineer for sustainability and future mobility.
“I think a lot of parallel activities will happen,” said Yutko. “What these airplanes will all need once we make them is a supply of renewable energy [for fuel-making processes] and green hydrogen.” Within the same timeframe, aviation must conduct extensive R&D and scientific study regarding questions of non-CO2 effect on climate, he added.
The industry needs to understand whether—or in what circumstances and weather conditions—the water vapor produced as exhaust by combusting hydrogen might create persistent contrails that at night could warm the air to foster climate change, according to Glenn Llewellyn, vice president of the Zero Emissions project at Airbus.
Contrails that disperse during the day would not create those “radiative forcing” conditions, but combusting hydrogen produces about 2.6 times as much water vapor as does burning jet fuel, said Alan Newby, director of aerospace technology and future programs for Rolls-Royce. So an important question centers on whether that additional water vapor could create hydrogen-propulsion contrails persisting overnight.
However, whether or not the additional water vapor produced by hydrogen combustion would turn into contrails at all requires equal clarity, said Arjan Hegeman, general manager of advanced technologies for GE Aviation. Hegeman is deeply involved in the joint CFM-Airbus ZeroE project, which will use an Airbus A380 to flight-test a hydrogen-fueled GE Passport turbofan engine to study any contrails it produces.
Contrails from aircraft today develop when water vapor in the exhaust gases of hydrocarbon-based aviation fuels creates ice crystals in the air. The crystals form round nuclei provided by hydrocarbon-derived sub-particles also emitted in the engines’ exhaust gases, Hegeman explained. Because hydrogen-derived exhaust gas contains no hydrocarbon sub-particles, contrail formation by engines burning hydrogen might prove very minimal.
But current scientific knowledge on hydrogen-produced contrail formation remains incomplete, as does knowledge of exactly how ambient air temperatures, humidity, and pressures at different altitudes affect contrail formation, said Hegeman. Scientists do know that changes in altitude, winds, and routing can reduce it, however.
Changing the routing or altitude of a jet fuel-powered aircraft can also cause increased CO2 emissions—but a hydrogen-powered aircraft wouldn’t create any. So changing its routing and altitude to reduce contrail formation would make “further mitigation [of climate-changing effects] available,” said Llewellyn.
Making hydrogen a viable alternative to jet fuel will require other areas of research and development, according to Yutko. For instance, combusting hydrogen creates NOx formation (and emissions), so researchers must find ways to minimize NOx formation in hydrogen-burning turbine engines’ redesigned combustors. However, hydrogen combustion’s characteristics do “allow you to reduce the amount of NOx produced,” noted Llewellyn.
How the cryogenically cold temperature at which liquid hydrogen gets stored affects the materials the hydrogen contacts also need study, said Yutko. Liquid hydrogen—the only form with enough energy density suitable for powering large, long-range aircraft—causes “material embrittlement” in various materials. How embrittlement will affect OEMs’ choices of materials for aircraft fuel tanks and fuel-handling systems, and its effects on part lives, must be understood.
Boeing and GKN Aerospace are cooperating with government and industry partners to develop fuel tanks large enough to hold the amounts of hydrogen required by large aircraft for long flights and made of materials not prone to hydrogen embrittlement. Carbon fiber composites appear particularly suitable.
In partnership with DARPA and NASA, Boeing has developed an all-composite, reduced-mass, cryogenic rocket fuel tank the OEM said is one of the biggest ever made and tested. GKN Aerospace, meanwhile, has started three hydrogen fuel-tank projects, said Max Brown, v-p of technology for GKN’s future advanced technologies initiative.
One, in the UK, centers on thermoset-composite fuel tanks. Another, in the Netherlands, involves thermoplastic-composite tanks. The third, in Sweden, focuses on metal fuel tanks. GKN plans to compare the results to see which material appears best for storing liquid hydrogen.
Airbus based its 2035 estimate for entry into service of hydrogen-powered large transport aircraft on its ability to mature and flight-test the required technologies sufficiently by 2026 to begin designing hydrogen-powered aircraft. Aircraft development, production, and certification would then take another nine years. “We think that timeline is challenging but achievable,” said Llewellyn.
However, by using modular, fuselage-wide tanks and powertrains developed in-house to drive hydrogen fuel cell-powered electric motors, well-backed start-up Universal Hydrogen reckons it will certify modified, hydrogen-powered, seat/mile cost-competitive ATR and Dash 8 regional turboprops for service by 2025.
“The vision of Universal Hydrogen…is to influence the conversation around the use of hydrogen as a fuel source, particularly for the single-aisle market,” said Rod Williams, Universal Hydrogen’s chief commercial officer. “We believe that when our products are in commercial service by 2025 it will be uncontroversial and irrefutable that the next-generation single-aisle aircraft will be powered by hydrogen.”