Airbus ZEROe concepts will help explore the design and layout of the world’s first climate-neutral, zero-emission commercial aircraft, which they aim to put into service by 2035. Credit: Airbus.

It likely won’t happen in my lifetime, but according to one German aviation expert, commercial aircraft powered by hydrogen technology may be as common as gas turbine engines are today, EFahrer.com reported.

Under pressure to reach net-zero greenhouse gas emissions by 2050, airlines are experimenting with alternatives, including so-called sustainable aviation fuels, or SAF, as well as all-electric aircraft and hydrogen.

All of them are an integral part of the decarbonization roadmap.

Some companies argue that business commuters of the 2040s will take short hops on six-seater electric vehicles that take off and land vertically.

But these vehicles — VTOLs, for “vertical takeoff and landing”— can go only so far, and the most successful VTOL prototypes have barely gotten off the ground.

For medium or longer distances, this just won’t fly, as they say.

Which brings us to hydrogen.

Professor Tobias Grosche from the University of Worms in Germany sees electric aircraft only to a limited extent as an alternative. In his opinion, hydrogen has a better potential to power future aircraft.

How exactly does it work?

Compressed hydrogen gas can be used in two ways for aviation: for direct combustion in a turbine and in a fuel cell to generate electricity.

During combustion, modified engines could use the hydrogen together with the ambient air for propulsion.

Only small amounts of water vapor and other combustion gases are produced, but no harmful CO₂, according to the professor.

It is unlike anything seen on today’s runways: the “pod” configuration — one of several being conceptualized as part of ongoing R&D on the ZEROe concept aircraft— features a series of stand-alone propulsion systems based on hydrogen fuel cell technology. Credit: Airbus.

In order to convert it into electricity, an aircraft needs a fuel cell that uses hydrogen gas to generate electricity. By doing without turbines and using electric motors, the aircraft would be quieter and no longer generate contrails.

In Grosche’s view, however, this technology is expensive and the fuel cell to be carried would add additional weight. In general, there are still technical challenges for both concepts that would have to be solved.

On the one hand, the infrastructure at the airport, i.e. tanks, lines and the refueling itself, would have to be converted to hydrogen.

In a transition period, airport operators would most likely have to have two systems, one for kerosene and another for hydrogen — an expensive option.

On the other hand, hydrogen requires more volume than kerosene and has to be either heavily compressed in special tanks or cooled to below minus 200 degrees.

“In the end it will be a question of space,” supposes Grosche, because the tanks will be spherical or cylindrical due to the higher loads.

 In terms of combined fuel and tank weight, aviation fuel has the advantage over hydrogen by a factor of 1.6, says Rajesh Ahluwalia, a hydrogen and fuel- cell researcher at Argonne National Laboratory.

Whereas aviation fuel constitutes about 78% of the combined weight of tank and fuel, liquid hydrogen accounts for just 18% of the total in current storage designs. To compete with fossil fuels, the fuel weight fraction has to reach at least 28%, he says.

Today’s internal combustion engines in aircraft can be modified to run on alternative fuels for improved environmental performance. Now, hydrogen combustion — either via gas or liquid — is emerging as one of the most promising options in this respect. Credit: Courtesy Airbus.

In today’s design models, the kerosene tanks are located in the wings to allow more space for cargo and passengers.

With the “ZEROe” concept, Airbus is already planning aircraft with hydrogen tanks, but their range is limited by the smaller amount of fuel carried.

Of the three concept designs, the first is a turboprop (propeller) driven aircraft capable of carrying around 100 passengers about 1,000 nautical miles (1,850km). The second, a turbofan (jet), could carry 200 passengers twice as far.

The third concept is a futuristic-looking blended-wing design that is a striking departure from commercial models today.

Airbus says this third design could be capable of carrying more passengers over longer distances than the other two, but has not released more detail at this stage.

All three designs are envisaged as hydrogen hybrids, which means they would be powered by gas-turbine engines that burn liquid hydrogen as fuel, and also generate electricity via hydrogen fuel cells.

Hydrogen “is one of the most promising technology vectors to allow mobility to continue fulfilling the basic human need for mobility in better harmony with our environment,” says Grazia Vitaldini, chief technology officer at Airbus.

But hold the phone, says aerospace giant Boeing, which has not yet committed itself to a specific technology.

Shortly after the ZEROe reveal, the company’s commercial vice-president and general manager of product development Michael Sinnett threw cold water on the near-term prospects for hydrogen aircraft.

Says Sinnett, hydrogen-fuel production and storage challenges will take significant time to work through.

Additionally, for hydrogen propulsion to become reality soon, government regulators must work at a pace matching technological development — also no sure bet.

“I don’t think it’s something that’s right around the corner,” Sinnett says.

Likewise, the industry understands how kerosene-burning turbofans operate in all variety of conditions — a crucial point that cuts hydrogen off at the knees.

“Battery-powered propulsion to fuel larger aircraft over longer distances is not possible with today’s technology,” explains Matthieu Thomas, ZEROe Aircraft Lead Architect. “Hydrogen fuel cells could be a great alternative because they can generate — with zero emissions —significantly more power and energy for a given weight. This makes fuel cells an extremely interesting technology to achieve our ambitions.” Credit: Courtesy Airbus.

The same cannot be said of hydrogen power, Sinnett says.

“We have to ensure there [is] no back-peddling on those levels of safety,” Sinnett says. “That means, there is a lot to learn” about hydrogen propulsion.

Aircraft manufacturers and airlines are therefore relying on sustainably produced fuels, Subsonic jetliners such as the Boeing 737 are powered by turbofans, which use mechanical energy derived from a gas turbine to accelerate air rearward by means of a ducted fan.

Today’s gas turbines could burn hydrogen with relatively few modifications. 

Phillip Ansell, director of the NASA-funded Center for High-Efficiency Electrical Technologies for Aircraft at the University of Illinois at Urbana-Champaign, says “You can almost just drop hydrogen into today’s engines.”

He compared it to the conversion process to adapting a propane grill to operate with natural gas.

But though it would produce no CO2 or soot, hydrogen will produce pollutant nitrogen oxides (due to nitrogen’s presence in air) and water vapor. 

Grosche also raises concerns that even if Airbus launches its first hydrogen aircraft in 2035, it will be a long time before all airlines have converted their fleets.

I’ll be high up in the sky, in copy editor heaven (or hell) by the time that happens, and so will many of you reading this.

Normally an aircraft has a lifespan of 20 to 30 years, so it is questionable whether aviation can really become CO2 neutral by 2050.

In addition to investing in new machines, there is another obstacle: hydrogen is currently expensive and only available to a limited extent.

Before the pandemic grounded most flights, commercial aviation accounted for about 2.5% of global emissions of carbon dioxide.

It sounds like a small proportion of the whole, but it is more than those of Germany (2.2%), and this is not the whole story.

Carbon dioxide accounts for about half of aviation’s contribution to what is known as its effective radiative forcing – that is, its total contribution to the factors that actually drive a rise in global average temperature.

Contrails – water vapour trails from aircraft – are aviation’s largest other factor.

The World Wildlife Fund describes aviation as “one of the fastest-growing sources of the greenhouse gas emissions driving global climate change.” It adds that air travel is “currently the most carbon intensive activity an individual can make.”

All these are factors that make eco-activist Glum Greta, well … glum.

The good news is that commercial aviation has an excellent track record in improving efficiency.

Carbon dioxide emissions per passenger flight have fallen more than 50% since 1990 thanks to improved engines and operations.

The bad news is that these gains have been overwhelmed by rising volumes of air traffic.

This has increased by at least a fifth over the past five years, and is predicted to reach 10 billion passengers a year by 2050.

Source: EFahrer.com, Wall Street Journal, The Atlantic, BBC News, FlightGlobal.com, Airbus.com, CNBC, Physics Today