The use of hydrogen in aviation has received sudden and strong attention in Europe. It began in early June, with the French government setting environmental goals for aviation (after consulting with the industry), including switching to hydrogen as a primary fuel.

Two weeks later, McKinsey and Co. issued a report on the same topic, essentially concluding a target of 2035 is within reach, albeit challenging. The six-month study was commissioned by two European Joint Undertakings, Clean Sky 2 and Fuel Cells and Hydrogen 2. The European Commission expressed support for the still-to-be structured project, linking it to the “hydrogen strategy” it is to adopt in July.

The well-funded, converging moves mark the start of a tectonic shift, at least in research and perhaps, in a few years, for the entire industry. Powering aircraft with hydrogen will benefit from the maturity of the technology in the automotive field, decreasing production costs, as well as from the recent discovery of natural hydrogen sources.

Nevertheless, many obstacles lie ahead. Choices will have to be made that may shape the future of the industry—between gaseous and liquid hydrogen as well as between fuel cells and turbine engines, among others. Every decision will have implications both for aircraft and the entire ecosystem—from airports to air traffic management (ATM).

The respective merits of gaseous and liquid hydrogen are being thoroughly assessed. Views are beginning to coalesce.

“A propulsion system made of a high-pressure tank for gaseous hydrogen, a fuel cell, an electric motor and a propeller would easily reach a high level of technology readiness, up to the size of an ATR 72 or even a 100-seater,” says Luis Le Moyne, director of France’s automotive and transportation academy (ISAT). The sector has experience with in-service vehicles such as the Toyota Mirai. An aircraft’s range might be limited to 1,000 km (540 nm), however, due to the heavy storage systems, he estimates.

He even suggests greater storage pressure should be adopted. In automotive, 700-bar tanks are standard. “They make hydrogen competitive with hydrocarbons, in terms of range,” Le Moyne says. “In aviation, as hydrogen propulsion will be more expensive, increasing the pressure to more than 900 bar (10,000 psi) could make sense, as it would give a better cost-to-performance ratio.”

McKinsey’s report focuses on liquid hydrogen, which it deems better suited to most of aviation. But it does not rule out gas. “Gaseous storage can be suitable for shorter flights and is commercially available,” it notes. Development of an evolutionary hydrogen aircraft design and tests with gaseous hydrogen tanks might enable faster time to market, it adds, although capping the aircraft size at 19 seats.

Two members of research and technology organizations concur. Gaseous hydrogen could be suitable only on a small-size aircraft, says Philippe Novelli, program director for propulsion and environment at France’s aerospace research office (Onera). Refueling with gaseous hydrogen would be prohibitively lengthy for an aircraft with a capacity greater than 19 passengers, adds Jean-Francois Brouckaert, Clean Sky’s chief scientific officer.

There are caveats against the use of liquid hydrogen. “Using liquid hydrogen would be very difficult in aviation,” says Le Moyne. One reason would be the complex system required to maintain a temperature of -253C (-420F), he explains.

A major problem appears at the liquefaction stage. “The process is exothermic, meaning you lose energy if you want to use liquid hydrogen instead of compressed gaseous hydrogen,” says Isabelle Moretti, a researcher at the University of Pau in France and former chief scientific officer at Engie, a major energy supplier in Europe.

But liquid hydrogen seems to be increasingly perceived as the way to go. Using gaseous hydrogen in a 700-bar tank is inconceivable because the tank’s weight would be prohibitive, Novelli contends. The relevance of using gaseous versus liquid hydrogen can be measured using the gravimetric index—the ratio between the fuel’s mass and the combined mass of the fuel and its tank.

In that regard, liquid hydrogen is believed to be a better fit than its gaseous form, though progress is needed with liquid hydrogen tanks. A state-of-the-art tank has a gravimetric index close to 20%, while the target is 35%. “So we are talking about halving the tank’s weight,” says Brouckaert.

The choice for liquid hydrogen may be assisted by expertise at space launcher manufacturer ArianeGroup. The Hyperion research program will evaluate risks for hydrogen aircraft propulsion, including the cryogenic system. The French government is funding the program, which is starting this year.

What about the fuel cell and turbine engine options? A fuel cell is more efficient and would be the best option to reduce an aircraft’s climate impact. But practicality depends on aircraft size.

A fuel cell’s efficiency, at 55-60%, compares favorably to that of a gas turbine—40-45%. Its power density (per weight unit), however, is lower.

Combining a fuel cell and a propeller is well-suited to aviation propulsion, according to ISAT’s Le Moyne. Fuel-cell power density has made great progress in recent years.

There is another environmental advantage. Hydrogen emissions do not contain soot, reducing the formation of climatically harmful contrails. This is counterintuitive, as more water vapor is created. But droplets most often form around soot particles.

The benefit would be even stronger with a fuel cell as opposed to burning hydrogen in a turbine engine. “Water vapor emitted by a fuel cell is cooler and fully controllable inside the aircraft. It could be conditioned, depending on the state of the atmosphere in which the aircraft is flying,” according to the McKinsey report.

Minimizing contrails could imply optimizing cruise altitude, thus involving ATM.

As a result, fuel-cell propulsion could reduce climate impact by 75-90%. This would be more than hydrogen combustion, which would cut it by 50-75%, according to McKinsey’s estimate.

Which option does Airbus CEO Guillaume Faury prefer? Although he sees “many more constraints” with a fuel cell, he and his company’s engineers do not know yet.

“They probably don’t have the same time frame, not the same complexity, not the same investment,” Faury says. “That’s why today we say that we [are looking] at different routes. We accelerate [the process] by looking at all of them at the same time. There is more investment going into innovation now, by the way, not only in aviation. There is cross-fertilization with other means of transport. We are in the hydrogen council like many other industries, including cars, shipping, energy—everybody is there.”

“While a fuel cell would be suitable up to the size of a regional aircraft, burning hydrogen in a turbofan would better suit short-to-medium-range aircraft such as the [Airbus] A320,” says Onera’s Novelli. The maximum power available from a fuel cell has yet to increase to meet an A320-size aircraft’s need of about 10 megawatts, adds Clean Sky’s Brouckaert. “Such an aircraft could use a combination of a fuel cell and turbofans, the latter being only used at takeoff.”

For widebody, long-range aircraft, the most suitable option is not hydrogen due to the spiraling complexity and size of the hydrogen systems, according to McKinsey. Rather, they would rely on synfuel, also known as power-to-liquid. The process uses renewable energy to convert CO2 and water into jet fuel. Hydrogen is therefore a major part of, but not the entire, solution, says McKinsey.

Hydrogen vs. Kerosene

Aviation Fuel Costs and Emissions in 2040

In terms of schedule, researchers and Faury agree that 2035 is realistic. “We are talking about the entry into service of the first decarbonized plane by 2035,” says Faury. “It is really something I believe in because it means launch of the program in 2027 or 2028. We have to have completed maturity of the technologies by 2025. Then you have two years for the preparation of the launch, consulting the suppliers, defining the general architecture [and] doing the business case.”

“With a few years of research ahead of us and some specific technologies to mature, we can do it. But we shall not underestimate [the] technical challenges, including safety,” says Brouckaert.

In terms of infrastructure, a favorable factor is the intent expressed by a number of airport managers—such as in Toulouse—to begin using hydrogen for ground transportation vehicles in the 2020-25 time frame. This will familiarize them with the technology, says Glenn Llewellyn, Airbus vice president for zero emissions technologies.

Liquid hydrogen will drop from four times the cost of kerosene today to roughly the same cost by 2050, according to McKinsey. This will be part of a lower-than-expected extra cost for the passenger (see graph). While it may make air travel more expensive, it would remain affordable, especially if consumers keep in mind what is at stake for the Earth’s environment.

Overall, McKinsey’s report is in agreement with studies released over the last 12 months by competing consultancies Roland Berger and Oliver Wyman, thus strengthening the conclusion that hydrogen-powered commercial air transport is feasible.

Finally, environmentally friendly hydrogen may come from natural sources (and not necessarily from water electrolysis with renewable power). Local hydrogen flows at the Earth’s surface—as opposed to underground pockets—are being discovered, notably in volcanic areas. “What happened with natural gas, which replaced gas obtained from coal, will happen with hydrogen,” says the University of Pau’s Moretti.

Source: AviationWeek