How May Hydrogen Propulsion Affect Hybrid-Electric Engine Development?
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What is your view on hydrogen-based engines making their way to production platforms and what is the potential timing? And what is going on with investments in hybrid-electric engines: Who is leading the way there?
Three Aviation Week editors teamed up to answer this question: Executive Editor, Technology Graham Warwick, Senior Propulsion Editor Guy Norris and France Bureau Chief Thierry Dubois.
The rapid (re)emergence of hydrogen propulsion as an option for reducing the climate effect of aviation, fueled by Airbus’ drive to develop a zero-emission aircraft, has inevitably raised questions about what this means for the electrification of propulsion.
The broadest answer is that there is room for both. There is also overlap between both, and who will win where will play out over the next decade.
Instead of the largely binary choice of propulsion available to aviation for decades—piston engines or turbine engines—the industry now has a spectrum of options to choose from that offers different combinations of performance and emissions.
Battery electric propulsion has zero inflight emissions but is limited by battery energy density to smaller, shorter-range aircraft. Hybrid-electric propulsion combining batteries with a piston or turbine engine enables longer ranges and larger aircraft but does not eliminate emissions.
Hydrogen-electric propulsion using fuel cells enables larger, longer-range aircraft with zero inflight emissions. But fuel cells are not expected to reach the power density needed to power the single- and twin-aisle aircraft responsible for the vast majority of aviation’s emissions.
Most in industry believe that hydrogen needs to be used in a different way in order to reduce emissions from medium- and long-range commercial aircraft, either by direct combustion in turbine engines or by combining hydrogen with captured carbon to produce drop-in synthetic fuels.
The latter seems to be the preferred route for the major engine manufacturers, who already believe that sustainable aviation fuels—combined with continued improvements in turbine engine fuel efficiency—are the best near-term option for reducing aviation’s effect on climate.
It gets interesting where electric and hydrogen technologies could come together in a hybrid form of propulsion. Airbus has hinted at this, and a report by Europe’s Clean Sky and Fuel Cell and Hydrogen research programs has explicitly detailed this potential option.
The “Hydrogen-Powered Aviation” report describes a liquid-hydrogen-fueled (LH2) 165-seat aircraft—an Airbus A320/Boeing 737-class type, but slower and with shorter range—that uses both fuel cells and turbine combustion for propulsion. Both operate for takeoff and climb, but in cruise only the fuel cells are used.
Airbus is considering hybridization the other way round: fuel cells would be used for takeoff only, as a complement to turbine engines. That way the fuel cells would remain reasonably small, while the turbine engines would be sized for cruise and could be made smaller.
What that means is the ongoing development of electric propulsion and emerging work on hydrogen could prove to be complementary, and not only competitive, in certain aircraft classes.
So who is leading what? In hybrid-electric, the pioneers are startups: Ampaire in the U.S. and VoltAero in France. Both are flying propulsion testbeds based on modified Cessna 337 Skymasters. Ampaire plans to certify a hybrid-electric conversion of the 19-passenger de Havilland Canada Twin Otter as its next step. The Cessna Caravan is another potential target. VoltAero is developing a 4-10-seat family of clean-sheet aircraft called the Cassio 2.
In hydrogen-electric, the leaders are also startups. ZeroAvia is flying a six-seat Piper M350 modified to hydrogen fuel-cell propulsion and has plans to certify a conversion for 19-passenger regional aircraft, such as the Dornier Do 228, as its first product. Universal Hydrogen is developing a fuel-cell power train for retrofit into the De Havilland Canada Dash 8-300.
With Airbus planning to use LH2 and direct combustion in its zero-emission airliner, all the major engine manufacturers are working on research to adapt their gas turbines to burn hydrogen. But no leader has emerged yet.