Comment by Florence Lee, Renewable Energy Systems Engineer at Energy Systems Catapult. 

The growth of the aviation sector has enriched our lives; the increased accessibility to international markets has facilitated global trade and commerce, and provided opportunities for foreign travel, fostering happiness and breaking down cultural barriers. In 2022, aviation accounted for approximately 2% of global anthropogenic carbon dioxide emissions – those caused or influenced by human activity. Although this is a small proportion, if trends continue, it will account for 39% of the UK’s carbon emissions by 2050. This pathway is clearly an economically and environmentally unviable option for the aviation industry which will need to decarbonise by 2050 to remain relevant.

Between now and 2050, the aviation sector can likely only afford one fleet refresh, meaning that to achieve its Net Zero targets, large zero-carbon aircraft will need to enter service by 2035. This establishes a narrow window for the development of engineering solutions to reach technical readiness level (TRL) 6 by 2028, which is necessary for consideration in the upcoming fleet and for securing support from stakeholders such as airlines, global operators, and airports.

The UK is a world leader in aerospace engineering, with capabilities in developing fuel systems, gas turbines, and thermal management. Currently the aviation sector plays a significant role in the UK’s economy, employing 116,000 people and generating £11 billion of gross value added (GVA) which amounts to roughly 1% of the country’s GDP. This sector provides valuable benefits to the UK economy, such as facilitating business investments and enabling transportation. Furthermore, the aviation sector has been experiencing growth, driven by the increasing demand resulting from globalisation. Given this context, in addition to the breakthroughs in low carbon fuels and advanced aircraft technologies, it may be worth implementing supportive policies focused on demand management within the aviation sector.

The transition towards zero-emission aviation presents both a threat to the UK’s place in the industry and a unique opportunity for the UK to increase its market share. If the UK can capitalise on the transition by delivering Net Zero engineering solutions, it could generate £36.5 billion GVA per annum in 2050 and lead to the creation of an additional 38,000 jobs in the industry. To achieve this, the UK must leverage its existing strengths and prioritise innovation in zero carbon engineering solutions.

Recent research efforts have focused on efficient designs and the replacement of kerosene with sustainable aviation fuels, known as SAFs. In 2023, a historic milestone for clean aviation was achieved with the world’s first transatlantic flight using 100% SAFs.  However, achieving Net Zero in aviation through SAFs would require external abatement mechanisms, such as green feedstocks and direct air carbon capture storage (DACCS). SAFs generally produce the same carbon tailpipe emissions as conventional kerosene aviation fuel.

In contrast, hydrogen fuel produces no tailpipe CO₂ emissions and has gained traction despite the commercial and technological readiness levels being far lower than SAF. Analysis from the FlyZero programme indicates that liquid hydrogen fuel is the most feasible option for zero-carbon commercial carriers, and by the mid-2030s, liquid hydrogen will become greener and cheaper to produce than SAFs which will require hydrogen to be produced at scale. Despite the uncertainties surrounding the long-term place of SAF in the industry, they serve as a vital stepping stone in the transition to Net Zero emission aviation. Manufacturers like Norsk e-Fuel are actively investing in expanding SAF production to mitigate the carbon intensity of the sector in the immediate future.

Hydrogen technologies

There are three ways in which hydrogen fuel can be a substitute for fossil fuels in aviation applications, these are via hydrogen combustion, fuel cell technology, or a hybrid solution combining both approaches, potentially with the inclusion of battery energy. The technical selection depends on the specific application and the requirements of the aircraft; fuel cells offer potential use cases as the primary propulsion system or the Auxiliary Power Unit (APU). Fuel cell powered auxiliary power units (APUs) present an opportunity to decarbonise operations on the ground where stringent policies from key stakeholders in the field, i.e. large airports and international transit hubs, may drive zero-carbon solutions.

Fuel cells have the potential to serve as the primary propulsion for aircraft, particularly up to the regional market level. Generally, fuel cell propulsion systems are anticipated to be utilised in aircraft at the sub-regional and regional levels, while hydrogen combustion engines are likely to be employed for narrow-body and midsize aircraft. Nonetheless, the transition point from fuel cell propulsion systems to hydrogen gas turbines remains undefined and is a current focus of research. Zeroavia has recently made substantial progress through the HyFlyerII program, achieving a milestone with the 10th flight of their hybrid fuel-electric 19-seater aircraft. Furthermore, as part of the ZEROe programme, Airbus is in the process of developing four prototypes, each utilising either fuel cell or combustion propulsion systems.

Research and development areas

Hydrogen fuel cells are electrochemical devices in which hydrogen is reacted with oxygen to produce electricity. Polymer electrolyte fuel cells, known as PEMFC or PEFC, have a wide application of use and have seen significant implementation in the transportation sector, particularly in the automotive sector. They have several desirable characteristics that make them ideal candidates for aviation powertrains, notably high-power density, scalability, rapid response times, and fast and start-up/shutdown times. Their largest technological barrier is the purity of the hydrogen required (99.9%) as impurities will poison the catalyst and degrade the performance of the fuel cell. However, the aviation industry is no stranger to tight regulation around fuel and should be better equipped to tackle this challenge than other users.

There are significant technological challenges that PEMFC must overcome to reach commercial readiness. H2Gear is a £54 million project aimed at addressing these challenges and developing a fuel cell powertrain for aviation. Led by GKN Aerospace in collaboration with fuel cell manufacturers Intelligent Energy, it is the largest ATI project to date.

The current TRL of PEMFC for aviation is TRL 6 which is significantly lower than the TRL 8-9 of PEMFC for land-based applications this is due to the operational system constraints for aerospace. Reducing fuel burn in aviation requires maximising propulsion system efficiency, whilst minimising aircraft weight and drag.  This translates to fundamental areas of research for PEMFC; increasing the system power density, reducing the system drag, and managing the impact of altitude.

Innovation in fuel cell design is key to reducing the parasitic load of the thermal management systems and the stack weight. The FlyZero project has identified the key technological developments for fuel cells and the balance of plant in their Fuel Cell Roadmap for net zero aviation, one such area is high-temperature PEMFC fuel cells or HT PEMFC. HT PEMFC are a key technological breakthrough for the aviation industry, operating at elevated temperatures (180-220°C vs 80°C-120°C LT PEMFC), they eliminate liquid phase water in the fuel cell and reduce the challenges associated with thermal management by increasing the temperature differential between the ambient air and the fuel cell stack. Despite their potential, crucial research is needed to enhance the durability of the membrane electrode assembly (MEA) to increase the operational life span to over 20,000 cycles and to improve the polarisation curve and power densities.

Innovation in accelerating power system density is complemented by research in weight reduction strategies; the fuel cell stack endplate contributes to 15-45% of the overall stack mass and thus the ATI has prioritised research into lightweight endplate materials, intending to eliminate their use by 2035. Optimal sizing of the fuel cell system is also key to minimising excess weight, which is a trade-off between ensuring sufficient power delivery during take-off, while maximising cruise efficiency without incurring a detrimental mass penalty.

In addition to refining fuel cell design, overcoming several hurdles in the balance of plant is essential. One of these challenges involves the use of liquid hydrogen, which is favoured over gaseous fuel due to its more compact storage requirements. However, these advantages come with their own set of challenges in material selection, storage technologies, and distribution. Particularly, the phase change from liquid to gas poses intricate challenges, such as managing multi-phase flows within the pumping system.

Furthermore, thermal management presents a significant engineering obstacle when dealing with cryogenic fluids at -253°C that must be heated to < 160°C at entry to the fuel cell stack. While the wide range of fuel temperatures in the system presents a significant challenge, the need to increase the temperature of the cryogenic hydrogen also presents an opportunity to use the fluid as a thermal sink. The development of systems that utilise the cryogenic hydrogen in this manner, to improve the efficiency of fuel cell air precooling, is an active area of research.

Hydrogen fuel offers a unique opportunity for decarbonising the aviation sector which will face challenges in achieving Net Zero without the integration of disruptive technologies. Both fuel cells and hydrogen combustion engines offer substantial potential to provide the required power density and will be essential to address the diverse needs of the market. The UK possesses relevant expertise and is poised to seize this opportunity for the aviation industry, however doing this requires a coordinated, cross-sectoral approach. Deploying supportive policies to nurture supply-side innovation is paramount for competitiveness against incumbent technologies and for sustainable demand management.

For more information on hydrogen in aviation visit the ATI, FlyZero, Intelligent Energy, ZeroAvia and Airbus ZEROe.