The Scale of the Challenge

Heathrow Airport, among the busiest in the world, illustrates the vast fuel consumption in aviation, with a staggering 20 million liters of jet fuel supplied daily — equivalent to filling up a car about 400,000 times. The management of this extensive operation relies on a complicated supply chain, with fuel transported from refineries and stored at designated facilities known as fuel farms.

Even though Heathrow does not directly buy or sell fuel, it plays a crucial role in infrastructure planning, ensuring there is enough space for storage and piping, and accommodating the requirements of airlines and fuel suppliers.

The aviation sector, especially in regions like the UK, US, and EU, is increasingly pressured to meet environmental objectives. Initiatives such as the UK's Jet Zero plan aim for net-zero carbon emissions by 2040, with similar targets set by the US and the EU for 2050. Achieving these goals represents a considerable challenge.

Sustainable Aviation Fuel (SAF) as an Intermediate Step

SAF, which is derived from non-fossil sources, offers an immediate substitute to conventional jet fuels. Airlines are already blending SAF with traditional jet fuel, benefiting from the straightforward delivery of SAF via existing pipelines.

However, questions remain about the cost-effectiveness and scalability of SAF to satisfy the soaring demands of the aviation sector. As the industry confronts SAF's limitations, attention is increasingly turning to hydrogen as a potential breakthrough.

The Promise and Challenges of Hydrogen

Hydrogen emerges as a viable alternative to JP1 due to its ability to generate energy without CO? emissions. However, for aviation use, hydrogen must be in liquid form, necessitating cooling to an extreme -253°C. This requirement leads to significant challenges, including the need for pressurized containers and the safety risks associated with managing a liquid that can "boil off" and release gas, creating hazards.

France’s Air Liquide, leveraging over 50 years of experience in supplying cryogenic hydrogen for rockets, is at the forefront of investigating hydrogen's potential in aviation. Collaborating with Airbus and Group ADP, Air Liquide is assessing hydrogen's feasibility in aviation, participating in the H2Fly consortium, which successfully completed flights of an aircraft powered by liquid hydrogen, marking a significant milestone in testing hydrogen aircraft fueling systems.

Liquid Hydrogen Safety Considerations

The use of liquid hydrogen in aviation requires meticulous attention due to its distinct properties. The storage and distribution of hydrogen at airports could incur substantial costs, with estimates from Bain & Company suggesting expenses could reach up to a billion dollars per airport. Safety concerns surrounding the handling of a liquid at extremely low temperatures further complicate the scenario.

Managing liquid hydrogen at -253°C necessitates advanced technologies and rigorous safety protocols. Air Liquide asserts its capability to deliver multiple tonnes of liquid hydrogen within minutes — an essential factor for the aviation industry, which demands rapid turnaround times comparable to traditional fueling methods. Time is a critical aspect in aviation economics.

Innovative Solutions: Universal Hydrogen’s Approach

Start-ups like Universal Hydrogen are developing strategies to tackle the challenges associated with hydrogen. Their approach focuses on managing hydrogen complexities away from airports, potentially at production facilities. Universal Hydrogen has created specialized tanks or modules for transporting liquid hydrogen, circumventing the need for pipes and pumps. These well-insulated modules can maintain hydrogen in its liquid state for up to four days.

This modular system provides an innovative method for transporting and integrating hydrogen into aircraft. Capable of holding 180 kg of liquid hydrogen at -253°C, two modules would supply enough fuel for a 500-mile flight plus an additional 45 minutes of reserve time. The modules can be seamlessly incorporated into aircraft, presenting a streamlined and efficient solution.

Universal Hydrogen is collaborating with H2Fly in the hydrogen supply chain, with H2Fly having developed a demonstrator aircraft.

The Economic Challenge: Cost of Transition

Despite the promise offered by innovative solutions like Universal Hydrogen’s modular system, the economic challenge of transitioning aviation infrastructure remains significant. The expenses associated with installing the equipment needed for hydrogen storage and distribution at airports pose a formidable barrier. Addressing this challenge is vital for the broad adoption of hydrogen as an aviation fuel.

Major airports currently feature ‘fuel farms’ — extensive areas of fuel tanks storing fuel at ambient temperatures. Transitioning to refrigerated storage systems would require a complete overhaul of existing infrastructure.

Hydrogen’s Role in Aviation’s Future

The question of whether hydrogen will become a common aviation fuel remains unresolved. Hydrogen-powered aircraft are still in their developmental stages, grappling with issues such as space limitations in fuselages and the availability of environmentally-friendly green hydrogen. The specific energy density of hydrogen, a critical factor for aviation, must be evaluated against traditional jet fuels.

Technological Frontiers

Liquid hydrogen’s specific energy density is roughly 8.5 MJ/L (megajoules per liter), while conventional jet fuel (JP1) boasts a specific energy density of about 35 MJ/L (43 MJ/kg). This notable disparity highlights the difficulties of modifying existing aircraft to accommodate the substantial storage requirements of liquid hydrogen for long-haul flights.

The integration of hydrogen into aviation fuels will necessitate significant advancements in engine technologies. This transition requires a departure from traditional gas turbines designed for conventional jet fuels, as hydrogen combustion introduces unique challenges.

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

Efficient Propulsion Systems: The lower energy density of hydrogen requires innovative propulsion systems to maintain or surpass the efficiency levels of conventional engines. Developing engines that can extract maximum energy from hydrogen while ensuring reliability and safety is fundamental to the success of this concept.

Material Compatibility and Engine Design: Hydrogen can cause embrittlement in certain metals, necessitating a thorough reassessment of materials used in engine construction. Engine designs must consider these compatibility issues to ensure the structural integrity and longevity of components, as metals are prone to fracture at cryogenic temperatures. As a result, designers are focusing on hydrogen fuel cells.

Integration of Hydrogen Fuel Cells: Integrating hydrogen fuel cells into aircraft becomes a key consideration in technological evolution. This process involves not only modifying existing engines but also developing new propulsion systems capable of converting liquid hydrogen into electricity to power electric motors driving 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, stated:

> 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 successfully flown:

I find the engine cowling shape reminiscent of a WWII Typhoon appealing.

California-based Joby Aviation sees promise in this technology and reportedly acquired the German startup in April 2021. While Joby is currently focused on certifying its S4 eVTOL aircraft for urban air mobility missions, the acquisition indicates its interest in hydrogen development.

Future Prospects and Uncertainties

The aviation industry stands at a crucial crossroads, recognizing the need to transition to more sustainable fuels, driven by governmental policies. Hydrogen, with its potential to transform aviation by eliminating CO2 emissions, presents a complex and costly path forward. The challenges of managing liquid hydrogen, ensuring safety, and addressing economic factors are significant.

Innovative start-ups like Universal Hydrogen and H2Fly illustrate promising solutions, yet the industry must confront the economic obstacles of transitioning infrastructure and establish a comprehensive set of regulations. The comparison of energy density between liquid hydrogen and traditional jet fuels underscores the volumetric challenges related to hydrogen storage, both on the ground and critically within the aircraft.

The Question

Could it work commercially?

Japan Airlines and Universal Hydrogen are exploring the conversion of ATR and Dash 8 regional airliners to utilize its hydrogen propulsion system.

For hydrogen to be commercially viable, it must demonstrate at least marginal profitability at each stage of the value chain and compete effectively with existing alternatives. Ultimately, excluding subsidies, the end user — the public — would need to bear the cost.

This must be weighed against the significant changes required within the industry and fleet without compromising the overall health of the airline sector, which contributes to tax revenues through the jobs it supports.

Additionally, the implications for global warming must be considered. Conducting a cost/benefit analysis in this context poses challenges, and attempts have been made to address this.

Governments can employ fiscal policies (e.g., taxation) to make traditional JP1 less appealing to airlines, which aligns with a zero-carbon policy. They can also use policy announcements and regulations to encourage industry compliance, especially as consumer concerns about costs rise.

However, there has been notable resistance to net-zero timelines, particularly in the automotive sector.

One final point to consider: The commercial production of hydrogen gas through 'traditional' methods carries a significant carbon footprint. Alternative production methods, such as electrolysis using 'green' electricity, and biomass gasification, also incur carbon footprints, though generally lower.

Nothing is ever straightforward.