Sustainable aviation: NASA ups bet on liquid hydrogen

Can airplanes fly without polluting the sky? NASA and its sustainable aviation partners think so, as liquid hydrogen project enters phase 2.
16 August 2023

Future clean machine? Air travel comes at an environmental price, but sustainable aviation research by NASA and its partners wants to change that.

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In 2019, NASA funded a project to sow the seeds for an all-electric aircraft platform that it hoped would put commercial aviation on a more sustainable flight path. At the time, analysts were forecasting that air travel was on track to increase by 90% in the US over the next 20 years. And the aviation industry’s contribution to global emissions – based on Boston Consulting Group analysis – could balloon from single-digit percentages up to as much as 20% by 2050. Sustainable aviation needed some bright ideas.

What’s the flight path to sustainable aviation?

Four years later, the USD $6 million NASA project – dubbed CHEETA, reflecting how its members form a Center for Cryogenic High-Efficiency Electrical Technologies for Aircraft – has cleared the bar set by phase one’s concept stage. And it’s now time for the partners – which include nine academic institutions, Boeing, GE, and the Air Force Research Lab, all based in the US – to develop prototypes of their designs.

At the core of CHEETA’s sustainable aviation plan is the use of cryogenic hydrogen. “Because hydrogen takes up a lot of volume, it’s best kept cold in a liquid state,” said Phillip Ansell, lead researcher on the NASA program and director of the new Center for Sustainable Aviation at the University of Illinois Urbana-Champaign. “Instead of thinking of that as a barrier, we saw it as an opportunity to leverage those unique characteristics.”

Superconductivity at room temperature has had a rough ride recently, but the approach, which reduces the energy losses in electric circuits, already has an industrial track record at low temperatures. “We saw a great deal of promise in using the 20 Kelvin cryogenic temperature of liquid hydrogen to enable the use of superconducting technology,” Ansell goes on to explain.

And it’s not just organizations in the US that seem keen on this idea. European aviation giant Airbus is also looking to combine liquid hydrogen and the efficiency gains of superconducting low-temperature circuits. Airbus announced a three-year demonstrator program (ASCEND) in 2021, which has the goal of increasing power density in the propulsion chain while keeping the mass of the distribution system low.

The power and stored energy requirements of air travel, particularly over long-haul routes, make sustainable aviation a tough problem to solve. If you plot the maximum power versus stored energy requirements of different modes of transport on a graph, EVs such as the Tesla Model S and Toyota’s Prius would be on the bottom left – with parameters that can be achieved using batteries.

However, wide-bodied airliners such as the Airbus A380 – which need to carry large numbers of passengers to make the economics of flying add up – are all the way up on the top right. Sustainable aviation needs to address both the need for large amounts of power for take-off and cope with the large distances currently traveled without refueling.

CHEETA still has to prove itself on the scale of a commercial airliner, but the design concept – which uses hydrogen-fed electricity-generating fuel cells to power electric rotors – is off to a promising start. “We were able to reduce the electrical system losses to below 2 percent, so the whole system is over 98 percent efficient from the output of the fuel cell to turning the rotor of the electric machine,” Ansell reports.

There’s a raft of projects globally looking at hydrogen as a replacement for jet fuel. Hydrogen burns without the carbon emissions of current jet fuel, and produces just water as a product of combustion. But it’s a very small molecule that’s difficult to contain, and critics may point to how fuel cells in vehicles have been slow to take off.

That being said, there are multiple reasons to believe that sustainable aviation based on hydrogen will succeed despite the struggles experienced in the automotive sector. As mentioned, power density and energy storage requirements make it hard to see how batteries will suit all but the smallest of aircraft, making hydrogen-powered fuel cells a more compelling prospect in aerospace.

And the number of refueling stations required are orders of magnitude less than for road transport. There are in the region of 100,000 gas stations for vehicles in the US, which would be a massive infrastructure challenge for hydrogen. However, considering major airports, you only have around 100 facilities to focus on.

Plus, as reported on by TechHQ, airports can use ground vehicles as a pathfinder for building out hydrogen refueling operations. The area around Toulouse in France, which includes a major Airbus manufacturing facility, has a pilot-scale hydrolyser to supply airport buses. And the longer-term plan is to scale this up to provide green fuel for light aircraft.

The global pandemic hit the aviation industry hard, but it is now bouncing back in a leaner and more fuel-efficient form. Older planes were retired as part of cost-cutting during Covid. And the industry has long been adept at making marginal gains on performance that have dampened its emissions despite rising numbers of passengers. But it won’t be able to decouple emissions from a boom in air travel without ditching fossil fuels for good.

There are lots of good news stories about sustainable aviation fuel (SAF), which can even be made from food waste, and gives airlines a drop-in replacement. But the use of SAF by airlines globally is currently tiny, and it’s not a zero-emission solution. SAF is said by the International Air Transport Association to reduce CO2 emissions by 80%, as the CO2 absorbed by plants is recycled – for example, when SAF is derived from biomass.

Also, there’s a chicken and egg problem of aviation customers not wanting to pay high prices, but SAF suppliers needing to have a larger market to bring down costs. Given the relatively quick win that SAF offers, it feels likely that biofuels will provide some form of bridge until new technologies, such as those being pursued by CHEETA and other projects, are commercialized.

China, India, and growth in SAF

Experts point to the feasibility of China and India becoming major suppliers of SAF, as the demand for airline travel is expected to grow dramatically in both countries. Producing SAF could utilize concentrations of municipal waste and agricultural residues, as well as creating jobs and driving economic growth.

Considering China, the country is a global force in renewable energy, and producing biofuels to support a path towards sustainable aviation – as well as bolstering energy security – would be a logical move.

What’s also clear is how demand for aviation has weathered developments in communications. Video conferencing capabilities have never been as accomplished as they are today, yet business travel – a profitable revenue stream for airlines – has rebounded. And researchers have shown that the appetite for travel has remained strong over the past 200 years despite the various breakthroughs in communication, such as growth of the postal service, fax machines, and the internet.

This appetite for travel gives confidence to developers of sustainable aviation that passengers will be ready and waiting for future net-zero flights, which cannot come soon enough for the planet.