Sustainability

Hydrogen Fuel For Aircraft — Could It Work Commercially?

From production through to transport and storage is just the start — what about cryogenic fuel and engines?

The aviation industry, a vital contributor to global transportation, is at a crossroads. With approximately 1,300 daily takeoffs and landings at Heathrow Airport alone, meeting the industry’s colossal demand for jet fuel — around 20 million litres daily — has become an intricate logistical challenge. The need for a transition to more sustainable fuel sources is pressing, driven by ambitious global targets to achieve net-zero carbon emissions.

While Sustainable Aviation Fuel (SAF) is already in use, its scalability and cost-effectiveness are questioned. Hydrogen, with its potential to store vast amounts of energy without producing carbon or nitrous oxides, emerges as a promising alternative. However, challenges ranging from infrastructure to safety and cost must be addressed for hydrogen to become a mainstream aviation fuel.

The scale of the challenge

Heathrow Airport is one of the world’s busiest, handling some 1,300 flights daily. It illustrates the magnitude of the aviation industry’s fuel consumption, supplying staggering 20 million litres of jet fuel every day, equivalent to filling up a car around 400,000 times. Managing this colossal operation involves a complex supply chain, with fuel piped from refineries and stored at facilities known as fuel farms.

Despite not directly buying or selling fuel, Heathrow plays a crucial role in infrastructure planning, ensuring adequate space for storage and pipes, and facilitating the needs of airlines and fuel suppliers.

The aviation industry, particularly in the UK, the US, and the European Union, is under increasing pressure to align with environmental goals. Commitments such as the UK’s Jet Zero plan aim for net-zero carbon emissions by 2040. The US and the EU have set similar targets by 2050. To achieve that is a huge task.

Sustainable Aviation Fuel (SAF) as an intermediate step

Sustainable Aviation Fuel (SAF), derived from non-fossil fuel sources, presents an immediate alternative to traditional jet fuels. Airlines are already incorporating SAF into their fuel blends, often mixed with regular jet fuel. The simplicity of supplying SAF to airports, delivered via existing pipes, makes it an attractive option.

However, doubts linger regarding the cost-effectiveness and scalability of SAF to meet the soaring demands of the aviation industry. As the industry grapples with SAF’s limitations, attention turns to hydrogen as a potential game-changer.

The promise and challenges of hydrogen

Hydrogen, with its potential to produce energy without emitting CO², emerges as a compelling alternative to JP1. However, to be utilised in aviation, hydrogen must be in its liquid form, requiring cooling to an extreme -253⁰ C. This presents immense challenges, including pressurised containers and the safety risks associated with handling a liquid with the potential to “boil-off” and escape as a gas, posing hazards.

France’s Air Liquide, drawing from its extensive experience supplying cryogenic hydrogen for rockets, has been at the forefront of exploring hydrogen’s potential in aviation. Over 50 years of supplying cryogenic hydrogen to the European Space Agency’s Ariane rockets has equipped Air Liquide with significant technological know-how.

Collaborating with Airbus and Group ADP, the company is investigating hydrogen’s viability in aviation. The partnership includes participation in the H2Fly consortium, which successfully flew an aircraft using liquid hydrogen, marking a milestone in testing systems for fueling hydrogen aircraft.

Liquid hydrogen safety considerations

Liquid hydrogen, a key player in this aviation transition, requires careful consideration due to its unique characteristics. Its storage and distribution at airports entail significant costs, with some estimates from consultancy Bain & Company suggesting it could reach a billion dollars per airport. Safety concerns, especially regarding handling a liquid at extremely low temperatures, further complicate the equation.

Handling liquid hydrogen at -253⁰ C requires advanced technologies and safety measures. Air Liquide, confident in its ability to handle cryogenic hydrogen, asserts its capability to deliver several tonnes of liquid hydrogen in a matter of minutes — a crucial factor for the aviation industry, which values quick turnaround times comparable to conventional fueling processes. Time is big money in the aviation industry.

Innovative solutions: Universal Hydrogen’s approach

Innovative start-ups like Universal Hydrogen are proposing solutions to address the challenges associated with hydrogen. Universal Hydrogen’s approach involves handling the complexities of hydrogen away from airports, potentially at the facilities where the gas is produced. The company has developed special tanks or modules to transport liquid hydrogen, eliminating the need for pipes, hoses, and pumps. These well-insulated modules can keep hydrogen in its liquid form for four days.

The company’s modular system presents a novel way to transport and integrate hydrogen into aircraft. Capable of holding 180kg of liquid hydrogen at -253⁰ C, two modules would provide enough fuel for a 500-mile flight with an extra 45 minutes of reserve flight time. The modules can be seamlessly integrated into aircraft, offering a streamlined and efficient solution.

Universal Hydrogen are teamed up with H2Fly in the fuel supply chain. H2Fly has built a demonstrator aircraft.

The economic challenge: Cost of transition

While innovative solutions like Universal Hydrogen’s modular approach offer promise, the economic challenge of transitioning aviation infrastructure remains significant. The cost of installing the equipment required to store and distribute hydrogen at airports is a substantial barrier. Addressing this challenge is crucial for the widespread adoption of hydrogen as an aviation fuel.

Large airports have ‘fuel farms’, acres of fuel tanks storing fuel at ambient temperatures. Imagine having to replace all that infrastructure with refrigerated tankage.

Hydrogen’s role in aviation’s future

The question of whether hydrogen will become a mainstream aviation fuel remains unanswered. Aircraft powered by hydrogen are still in the early stages of development, facing challenges such as space constraints in fuselages and the availability of environmentally-friendly green hydrogen. Hydrogen’s specific energy density, a crucial metric for aviation, should be examined in comparison to traditional jet fuels.

Technological Frontiers

Liquid hydrogen’s specific energy density is approximately 8.5 MJ/L (megajoules per litre). Comparatively, JP1, a conventional jet fuel, has a specific energy density of around 35 MJ/L (43 MJ/kg). This significant difference underscores the challenge of adapting existing aircraft to accommodate the voluminous storage requirements of liquid hydrogen for long-haul flights.

The integration of hydrogen as a mainstream fuel has a big impact on engine technologies. This shift requires a departure from conventional gas turbines designed for traditional jet fuels, as hydrogen combustion presents unique challenges.

Hydrogen Combustion Dynamics: Adapting existing aircraft engines or developing entirely new propulsion systems optimised for hydrogen is a critical technological frontier. Hydrogen combustion differs from traditional fuels, with different combustion dynamics, flame stability, and overall engine performance.

Efficient Propulsion Systems: Hydrogen’s lower energy density by volume requires innovative propulsion systems to maintain or exceed the efficiency levels of conventional engines. The development of engines that can extract maximum energy from hydrogen while ensuring reliability and safety is fundamental to the success of the concept.

Material Compatibility and Engine Design: Hydrogen causes hydrogen embrittlement in certain metals and requires a meticulous reassessment of materials used in engine construction. Engine designs must account for these compatibility challenges to ensure the structural integrity and longevity of components. Metals at cryogenic temperatures are easily fractured. Because of this, designers are focused on hydrogen fuels cells.

Integration of Hydrogen Fuel Cells: As part of the technological evolution, integrating hydrogen fuel cells into aircraft becomes a key consideration. This involves not only modifying existing engines but also developing new propulsion systems that can convert liquid hydrogen to electricity, driving electric motors that power the aircraft.

In 1932, English engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its inventor, is one of the most developed fuel cell technologies, which NASA has used since the mid-1960s (Wikipedia)

Progress: In an interview reported in verticalmag.com, Joseph Kallo, founder of German company H2Fly said:

We developed a couple of generations of powertrains together with DLR and Ulm University, also driven by us. Crucially, we have all the required technology bricks from liquid hydrogen storage, the hydrogen fuel cells themselves, fuel cell cooling technology, the inverter, and the electric motor. This is very important as we have a complete powertrain solution, which differentiates us from some of our competitors. Today, our target is a 40-seater aircraft with hydrogen fuel cell propulsion, using liquid hydrogen.

The H2Fly demonstrator has flown:

I just love that engine cowling shape, reminiscent of a WW2 Typhoon.

California-based Joby Aviation sees potential in the technology, reportedly acquiring the German startup in April 2021. While the eVTOL developer is currently working on getting its S4 eVTOL aircraft certified for urban air mobility mission, the acquisition shows that Joby has one foot in the door on hydrogen development. (verticalmag.com, ibid.)

Future prospects and uncertainties

The aviation industry stands at a critical juncture, recognising the imperative of transitioning to more sustainable fuels, pushed by governments. Hydrogen, with its potential to revolutionise aviation by eliminating CO2 emissions, offers a complex but expensive way forward. The challenges of handling liquid hydrogen, ensuring safety, and addressing economic considerations are formidable.

Start-ups like Universal Hydrogen and H2Fly demonstrate innovative solutions, but the industry must address the economic hurdles of infrastructure transition and a whole host of yet-to-be-written rules and regulations. The specific energy density comparison between liquid hydrogen and traditional jet fuels highlights the volumetric challenges associated with hydrogen storage on the ground and, critically, in the aircraft themselves.

The question

Could it work commercially?

Japan Airlines and Universal Hydrogen are proposing to convert ATR and Dash 8 regional airliners to use its hydrogen propulsion system.

To work commercially it would have to be at least marginally profitable at each stage of the value chain and be competitive with the current alternatives. Ultimately, excluding subsidies, the end user — Joe Public would have to pay.

Against that has to be balanced the huge industry and fleet changes required without affecting the overall health of the airline industry, which does itself generate taxation revenue from all the jobs it supports, at the very least.

Also to be considered is the effect on global warming. How does one carry out a cost/benefit analysis on that? I know people have tried.

Governments can use fiscal policies (i.e. taxation) to make the alternative JP1 less attractive to airlines and that fits nicely with paying lip-service to a zero carbon policy. They can also use policy announcements and regulations to prod industry obedience, as consumers start to worry about the cost.

But as I write, strong pushback against net-zero timelines is becoming apparent, particularly in the auto industry.

One final point: The commercial production of hydrogen gas using ‘traditional’ methods has a significant carbon footprint. Alternative methods (such as electrolysis with ‘green’ electricity) and biomass gasification also have associated, though lower, carbon footprints.

Nothing is ever straightforward.