The pursuit of sustainable aviation through green hydrogen as a fuel source is in its early stages, with a broad spectrum of potential solutions under exploration. Some of these ideas may already be recognized, while others have yet to emerge. What remains certain is that the transition away from fossil fuels in this sector will yield benefits in our lifetime, creating a legacy of environmental stewardship for future generations.
The global transport sector is responsible for emitting approximately 8 gigatonnes of CO₂ annually. Nearly half of these emissions come from passenger vehicles, with trucks, buses, and lorries contributing just over a quarter. Shipping and aviation each account for about one-eighth of this total. Within this landscape, aviation CO₂ emissions, at roughly 1 billion tonnes per year, mirror the total emissions of industrialized nations like Germany or Canada. Additionally, aviation emissions are increasing at a faster rate than other modes of transport, underscoring the urgency of decarbonizing the sector.
Green hydrogen stands out as a promising clean energy vector, produced using renewable electricity. Its potential to drive sustainability in aviation lies not only in its zero-carbon credentials but also in its versatility for various propulsion technologies.
Hydrogen in Propulsion Systems
Hydrogen’s initial foray into aviation began with long-duration drones used in applications such as agricultural surveillance, pipeline monitoring, and military operations. By storing compressed hydrogen in lightweight Type 4 composite cylinders and channeling it through fuel cells, these drones achieve ranges far beyond their battery-powered counterparts.
The next wave of innovation involved turboprop aircraft equipped with powerful fuel cells to drive propellers. Various organizations have experimented with this approach, favoring high-temperature PEM (Proton Exchange Membrane) fuel cells for their superior power density. These fuel cells can produce over 1 MW per power pod, enabling substantial performance gains.
In past years, there have also been successful tests of hydrogen-fired jet engines. For example, notable tests confirmed the viability of running regional jet engines on green hydrogen in controlled environments, marking significant progress toward adopting hydrogen for larger commercial aircraft.
Challenges in Hydrogen Storage
The critical barriers to hydrogen adoption in aviation stem from storage limitations. Unlike land-based applications, where the mass of fuel containers is often inconsequential, aviation demands a balance between volumetric and gravimetric energy density due to space and weight constraints.
Both gaseous and liquid hydrogen have high gravimetric energy density, making them viable candidates for aviation fuel. However, traditional hydrogen storage methods, such as Type 1 steel cylinders filled to 200 bar pressure, exhibit poor gravimetric efficiency—less than 1% of their weight is hydrogen. Advanced Type 4 cylinders operating at 700 bar improve this efficiency to around 5%, but this is still insufficient for large-scale aviation applications.
Liquid hydrogen, while more energy-dense, requires cryogenic storage solutions. Even with state-of-the-art cryogenically compatible aluminum tanks, gravimetric efficiency is capped at 30%. Developing reusable composite tanks could potentially increase this efficiency to over 50%, paving the way for practical adoption in commercial aviation.
Cryo-Compressed Hydrogen: A Niche Solution
A more innovative approach involves cryo-compressed hydrogen, which combines cooling and compression to achieve higher density than standard liquid hydrogen. For instance, a cryo-compressed hydrogen tank at 300 bar can store 40 grams of hydrogen per liter, compared to 36 grams for conventional liquid hydrogen. This enhanced density could prove vital in aviation, where weight savings translate directly into performance gains.
Efforts to scale this technology include collaborations between companies and startups exploring cryo-compressed hydrogen storage for aircraft. This approach not only improves gravimetric energy density but also reduces the energy required for hydrogen liquefaction, further enhancing its sustainability profile.
Reducing Boil-Off Losses with Advanced Cooling
Hydrogen’s cryogenic nature presents unique challenges, such as boil-off losses during storage and transport. Innovations like helium-cooled sub-liquid hydrogen (sLH₂) offer solutions by reducing heat leakage and vaporization. Although this approach demands additional power for cooling, it minimizes hydrogen loss and optimizes overall energy efficiency, making it a promising avenue for aviation fuel supply chains.
Scaling Infrastructure for Liquid Hydrogen
Liquid hydrogen infrastructure is pivotal for scaling aviation applications. Currently, North America leads in global hydrogen liquefaction capacity, catalyzed by programs that prioritized space exploration. Expanding this capacity to support aviation requires investment in hydrogen liquefiers and distribution systems. For instance, some facilities have already achieved daily liquefaction capacities of 30 tonnes, serving industries beyond aerospace.
Europe, historically reliant on compressed gaseous hydrogen and pipeline networks, is now witnessing a shift. New hydrogen liquefiers under construction are expected to double the region’s capacity, aligning with the growing demand for aviation-grade green hydrogen.
Simplifying the Hydrogen Supply Chain
The absence of airport-based liquid hydrogen refueling infrastructure remains a bottleneck for adoption. Innovative concepts, such as modular and portable liquid hydrogen tanks, offer a practical interim solution. These tanks can be pre-filled, transported, and loaded onto aircraft like conventional luggage carts, bypassing the need for extensive on-site refueling facilities.
Exploring Hydrogen Derivatives for Aviation
Given the logistical challenges of storing and transporting hydrogen, alternative derivatives are gaining attention. Green ammonia and synthetic aviation fuels (e-SAF) are leading contenders.
Green ammonia, which liquefies under low-pressure conditions, offers a higher volumetric energy density than liquid hydrogen. While its gravimetric energy density is lower, ammonia’s storage efficiency approaches that of hydrogen, making it a viable alternative for certain applications. Additionally, ammonia can be partially cracked to produce hydrogen and burned in jet engines, mimicking traditional fuel storage configurations.
E-SAF, a sustainable hydrocarbon fuel produced by combining green hydrogen with CO2, offers compatibility with existing aircraft and infrastructure. By consuming CO2 during its production, e-SAF provides a circular carbon solution, reducing reliance on fossil-derived kerosene without requiring radical changes to the aviation industry.
The Road Ahead for Hydrogen Aviation
The integration of green hydrogen into aviation represents a transformative opportunity to reduce the sector’s carbon footprint. However, the path forward requires continued investment in technology, infrastructure, and alternative fuel solutions. From drones to commercial aircraft, the potential applications of hydrogen are vast, each contributing to a future where air travel aligns with global sustainability goals.
By overcoming storage challenges, scaling infrastructure, and exploring derivative fuels, the aviation sector can position itself as a leader in the clean energy transition. With green hydrogen at its core, sustainable aviation is no longer a distant dream but a viable reality within reach.