Aviation is one of the most polluting businesses, thus it aspires to become emission-free and fly on hydrogen. However, the issue remains as to whether this is technically feasible. Aircraft design as we know it will have to undergo a dramatic transformation.
When Airbus announced the ZEROe project in September 2020, it drew these types of reactions. The goal is to have a commercial hydrogen-powered aircraft accessible by 2035. That’s only thirteen years, but it’s also thirteen years. That is pretty rapid in the aviation sector, where things are normally done step by step.
As a result, Boeing, a rival, is skeptical. “Hydrogen isn’t a viable option before 2050.” We’ll have to make do with sustainable aviation fuels till then,” CEO David Calhoun stated in June of last year. Guillaume Faury, an Airbus colleague, has a different take on sustainable aviation fuels (SAFs): “They will undoubtedly be required in the next years in order to significantly cut emissions.” Our first hydrogen plane will arrive in 2035, though it may come a bit later. It is critical that all stakeholders in the aviation industry continue to take action.” Because there are a number of obstacles to clear before the first hydrogen Airbus with passengers takes off.
Taking Bold Steps
If aviation is to meet the targets of the Paris (2015) and Glasgow (2021) climate accords, it must take drastic measures. It presently accounts for 2% to 3% of all CO2 emissions. Prior to the Corona crisis, it was predicted that this would treble by 2050, with annual aviation growth of 4 to 5%, and without any restrictions. According to the International Association of Airlines (IATA), this will range from 1.5 to 3.6 percent every year through 2040.
By 2050, Europe must be the first climate-neutral continent, according to the European Commission’s Green Deal. However, by 2050, aviation will have reduced emissions by up to 60% thanks to a package of initiatives from the supplementary Fit for 55 plan, such as emission rights, sustainable fuels, and technical innovation. As a result, there are still some further measures to be completed. IATA and aircraft manufacturers agreed in October of last year to achieve aviation climate neutrality by 2050. In a paper, Clean Sky, a European aviation industry research initiative, institutes such as NLR (Dutch Aerospace Laboratory), its German and French counterparts, and the European Commission have plotted the road in 2020. Aviation requires completely new fuels and engine technology to cut pollution.
According to the IATA, SAFs now account for barely 0.1 percent of all fuel utilized. They can only be blended with kerosene up to 50%; the laws don’t allow for any more. Extensive testing is currently underway with 100 percent SAF, which cuts CO2 and NOx (nitrogen oxide) emissions by 80 percent, according to measurements. In early December, United Airlines was the first airline to fly people only on SAF. Synthetic fuels, such as synfuels and e-fuels, begin with SAFs made from residual waste, frying fat, or green waste.
Captured CO2 and green hydrogen are combined to create a clean fuel. This costs a lot of electricity and is only environmentally friendly if the electricity is generated entirely sustainably with wind or solar energy. SAFs will be the fuels for CO2-neutral long-haul aircraft until 2050, according to Clean Sky and other experts. Electric flying is a possibility for trips of up to 1000 kilometers (which are responsible for two-thirds of CO2 emissions from aviation, according to Clean Sky) – either with batteries or fuel cells that create energy using hydrogen.
Both of these options are hefty, making them unsuitable for bigger aircraft or longer flights. Because when kerosene or SAFs burning makes an airplane lighter, batteries and fuel cells do not. “Battery development is not progressing quickly enough to achieve zero-emission aircraft by 2035,” said Glenn Llewellyn, vice president of Airbus’ Zero Emission (ZEROe) Aircraft Program.
Wings that fly
Hydrogen is the solution, according to Clean Sky, which continues as the new Clean Aviation initiative at the end of last year. At Airbus, they believe the same thing. ZEROe is researching four aircraft designs through 2023, with a favorite selected around 2025 following more research. After the project’s launch in 2027, one of them might be operational in 2035.
The initial design is a 100-passenger high-decker (a propeller-driven aircraft with the wing on top of the fuselage) with a range of 1,850 kilometers. The second concept is a low-decker with turbofan engines that can carry 200 passengers and has a range of around 3,700 kilometers. The most groundbreaking are two variants of a ‘flying wing,’ both of which can carry 200 passengers but have a greater range due to the aircraft’s V-shape, which allows for additional fuel tanks. Because fuel storage is one of the challenges. Hydrogen has three times the energy value of kerosene and is three times lighter, but it occupies up to four times the space in gaseous form.
Tanks this large would make an airplane far too large. Liquid hydrogen is more practical (LH2). It’s been tested in space, but it’s only liquid at -253 degrees Celsius. The hydrogen tank must be stored in the fuselage since the wings are too tiny and unsuitable for storage. As a result, the gadget becomes longer and heavier. It’s no surprise that the ZEROe low-decker looks like the existing A350, but because of the tank in the back, it can only carry a few more people than the smaller A320neo.
As a result, compact and light tanks are a primary focus. MTU Aero Engines, a German aviation engine manufacturer, is investigating this and sees two possibilities. “The first is a tank that can hold both liquid and gaseous hydrogen at a pressure of around 10 bar. Without the need for extra pressure devices, the gaseous hydrogen may be retained at the necessary pressure for usage in fuel cells. “For smaller fuel systems, this is ideal,” explains MTU communications director Markus Woelfle. “The second is an active/passive tank, which is best suited to bigger systems.” It comprises a low-pressure LH2 storage tank (3 bar), which feeds ice-cold hydrogen into a smaller high-pressure tank for direct combustion. The benefit of a smaller storage tank is counterbalanced by the high-pressure system’s increased complexity.
Airbus is looking at whether and whether cooling technologies are required to maintain the hydrogen at -253 degrees, but Woelfle believes they will not be required. “The hydrogen is kept isolated (in a thermos flask, for example) and remains liquid for a long period. In principle, you should let some of it evaporate after a while to avoid too high a pressure in the tank, but because the hydrogen is used up, this will seldom happen during regular flights.”
Because of its low density, the ice-cold LH2 has a tendency to leak around couplings and attack metal pipes, therefore the whole tank and piping system must be solid. The certification of hydrogen aircraft will pay special attention to this.
Airbus has its doubts
Another conundrum: should hydrogen be used in fuel cells or injected straight into engines? Since 2017, the American-British business ZeroAvia has been working on the first alternative, in which hydrogen and oxygen are electrolyzed to produce energy, which may then be utilized to power electric motors via batteries. In September 2019, ZeroAvia began flying a modified six-seat Piper Malibu, however, the plane was damaged beyond repair during a landing in April 2021. The business is presently working on a bigger prototype in a Dornier 228 with two 600-kilowatt engines. In four to five years, a 2-megawatt version should be accessible. De Havilland Canada and ZeroAvia have reached an agreement.
Using hydrogen directly in the engine necessitates a number of changes. For example, due to its increased flammability, hydrogen is more unstable than kerosene, and there is a potential for the flashback, in which burning hydrogen returns to the tank, posing an explosion risk. Of course, this is undesirable, thus further study is needed, according to TU Delft, which is developing the four-seater Aero-Delft Phoenix. Direct injection with a fuel cell electric motor for added thrust is the third option.
Airbus has yet to be released. In an interview in November 2020, then-chief technical officer Grazia Vittadini expressed reservations regarding the best solution: “If you already have hydrogen on board, why would you contemplate batteries?” That’s something we need to look into.” That is already being done by the DLR (Deutsches Zentrum für Luft- und Raumfahrt). “Direct combustion of LH2 in the turbines appears to be the optimum choice for short and medium-haul aircraft, i.e. 1,850 to 3000 kilometers,” explains DLR director Björn Nagel. “The reduced weight compensates for the turbine engines’ slightly inferior energy efficiency as compared to fuel cells.” During combustion, however, NOx is still emitted.”
Despite the fact that the German research institute Bauhaus Luftfahrt designed a 400-passenger hydrogen aircraft for long-distance travel in 2019 and the British FlyZero presented a concept for such a plane with 279 seats in December 2021, mass and weight, according to Nagel, are decisive in this category. “Synthetic kerosene, manufactured from green hydrogen and CO2, appears to be the best alternative for this sector.” The emissions are comparable to those of oil-based kerosene, however, synthetic kerosene made using collected CO2 can be climate neutral. The benefit is that a huge hydrogen tank is not required. For short- and medium-range aircraft, synthetic kerosene is also a possibility, although hydrogen is preferable because of its higher energy efficiency.”
In its analysis, Clean Sky predicts that hydrogen aircraft with 325 passengers and a flying range of 10,000 kilometers will be achievable, although Airbus estimates that the ZEROe project will have a range of 3700 kilometers. MTU Aero Engines specifies a limit of 6500 kilometers for an optimal flying range. “The greater volume of LH2 necessitates a larger tank,” Woelfle says. Either take fewer passengers or luggage or opt for a bigger tank in a broad, double hull, which comes with the drawback of increased air resistance.
There is a point at which flying long distances with hydrogen is less appealing than flying long distances with SAFs, depending on weight, speed, and necessary engine power.” The limits of the increased tank capacity appear to be solved only with a novel design like the flying wing.
A chilly challenge
There’s still a lot to learn. The manufacture of hydrogen is difficult, and airport infrastructure must also adapt. According to a Schiphol representative, “the current fuel system at the airport may be utilized for drop-in fuels (bio and synthetic fuels mixed with kerosene, ed.).” “For hydrogen, we’ll need a different system.” That’s something we’re working on right now. In the first case, one alternative is to cooperate with tankers.”
At airports, Nagel does not anticipate any big issues. “At the airport, hydrogen can be delivered in gaseous form and subsequently liquefied. You may also generate hydrogen on-site. However, because LH2 has a larger volume, transporting it to airplanes needs greater tanker capacity. I don’t anticipate a significant impact on ground operations.”
It’s unclear how the ice-cold hydrogen will react once it reaches the airliner. This triggered a two-year field investigation by the DLR, Lufthansa Technik, the ZAL (Zentrum für Angewandte Luftfahrtforschung), Hamburg Airport, and the Hamburg Municipality. The test item is a Lufthansa Airbus A321 equipped with hydrogen systems. “Handling liquids at -253 degrees is quite difficult. On the one hand, to avoid evaporation, the temperature must be kept as consistent as possible,” argues Nagel. “On the other hand, the aircraft’s structure must be safeguarded against low temperatures and temperature variations as much as feasible.” Small hydrogen molecules can pass through materials and disperse. This generates a significant heat demand.”
Under the effect of ice-cold molecules, the brittleness of metals and other alloys rises, increasing the risk of breaking, according to the DLR director. As a result, the chance of leakage rises. “That’s why it’s critical to have a strong integration of the cryogenic H2 systems with the aircraft itself, as well as the interface with other systems.” Because these effects are difficult to calculate with a computer, measurements within the A321 are required. The data is analyzed using computer models and a database. They must give insight into the maintenance and repair implications. “LH2 systems have the potential to significantly reduce an aircraft’s operational expenses. As a result, while creating a new gadget, they must be considered,” adds Nagel.
The climatic influence of hydrogen on the atmosphere is also being researched. LH2 produces bigger and heavier ice crystals when burnt directly in the engine, however, study shows that they swiftly dissolve and have little influence on the greenhouse effect. The consequences of hydrogen emitted by fuel cells are less known. To avoid the production of extremely long condensation trails, the water vapor might be released at a specific temperature and in a regulated way.
Is it becoming too difficult?
Airbus is working quickly on ZEROe, over a year and a half after the introduction, and the business is analyzing all potential stumbling obstacles. “It’s our number one priority.” “We’re at various stages of research and development, looking for partners and learning about hydrogen,” Guillaume Faury explains. He believes that five years of research will be sufficient and that actual development will begin in 2027, with the hydrogen Airbus ready for commercial usage in 2035. According to Lufthansa, it will probably definitely be the first customer.
More time is needed, according to Boeing, to develop the technology and manufacture enough hydrogen.
The Dornier 228, with which ZeroAvia is planning phase two, is a type with which the DLR also expects to perform testing in 2026. Start-ups By 2025, Deutsche Aircraft and H2FLY propose to fly a Dornier D328eco with a 1.5-kilowatt fuel cell installation. Universal Hydrogen, based in the United States, is creating a modular hydrogen system that can be integrated into current turboprop aircraft, with hydrogen stored in exchangeable capsules in the rear fuselage. The business presently has a number of clients. United Engine Corporation is researching hydrogen engines in Russia, where a Tupolev was already flying on hydrogen in 1988.
In a nutshell, hydrogen is hot and at the center of every sustainability argument. In Europe, in particular, there is a widespread assumption that hydrogen will finally solve the aviation climate problem, and efforts are being made to speed up research and manufacturing. The hydrogen drivers in the United States are ZeroAvia and Universal Hydrogen.
Boeing (as well as Brazil’s Embraer) does not, at least not in the immediate run, believe in it. More time is simply needed to develop the technology and have enough hydrogen accessible, according to the American aircraft maker. “We are not denying hydrogen and are clearly doing our study,” Mike Sinnett, Boeing vice president of product development, told Reuters in July 2021. We’ve done a lot of study on hydrogen already, but we don’t want to set the assumption that this is the solution until we’re certain.”
