The use of sustainable aviation fuels (SAF) to decarbonize aviation is a promising prospect but high production costs and low supply has limited its adoption to date. There are two potential energy sources for zero-emission planes (ZEP) – hydrogen and electricity. Due to the weight of current batteries, electric aircraft will be limited to short-range commuter missions in the near term. Hydrogen-powered aircraft, on the other hand, are promising but not without their own technological challenges.
The Promise of Hydrogen
As a fuel source, hydrogen provides three times the energy per unit mass than fossil jet fuel and produces zero carbon emissions on usage. It can be used to generate electricity through fuel cells or combusted to power gas turbine-based aircraft propulsion systems. Both energy pathways, fuel cell and combustion, are carbon-free. Additionally, by not producing any soot, hydrogen might be able to address the warming impact of contrail cirrus, the clouds produced by plane exhaust, which could be aviation’s greatest contributor to global warming.
On the fuel-cell side, smaller companies like ZeroAvia and Universal Hydrogen are working to retrofit existing 40- to 90-seat aircraft with hydrogen fuel-cell propulsion. These companies are focusing on using compressed hydrogen as a fuel and the retrofitted aircraft are expected to service routes of 750 km or less (like Los Angeles to San Francisco, for example).
Hydrogen combustion aircraft are being pursued by Airbus with their ZEROe initiative. A recent study by ClimateWorks partner, the International Council on Clean Transportation (ICCT), supported by the Heising-Simons Foundation, suggests that liquid hydrogen combustion designs could service a majority of short- and medium-haul routes, representing one-third of passenger aviation demand, starting in 2035. The study also suggests the fuel costs will be higher than fossil jet fuel but lower than e-kerosene, positioning hydrogen as a potential solution for the deep decarbonization of aviation.
Where challenges emerge
Several challenges need to be addressed before hydrogen can succeed as aviation fuel. In order for this technology to be successful, it is key to ensure hydrogen is produced through sustainable methods with low life-cycle carbon emissions. Without proper life-cycle accounting, hydrogen fuel will not provide the necessary carbon reductions and, under certain conditions (e.g. “blue” hydrogen produced with significant methane leakage and low carbon capture rates), could be worse than using fossil jet fuel. Usage of green hydrogen (that is, hydrogen produced using additional renewable energy) must be incentivized to achieve the deep decarbonization required from the aviation sector.
An engineering challenge is hydrogen’s low density. It is necessary to compress or liquefy hydrogen to carry enough to power a flight. To produce the same energy as a unit of jet fuel, it would require seven times that volume of compressed gaseous hydrogen and four times that volume of liquid hydrogen. Compression or liquefaction of hydrogen requires more energy and suitable storage tanks which are heavy and difficult to integrate with the aircraft structure. For the time being, this will limit the use of hydrogen to flights less than 3,400 km, the ICCT report finds.
Finally, bringing hydrogen-compatible planes to market will also require significant investment in the research and development of key hydrogen-related technologies such as liquid hydrogen storage tanks and hydrogen combustion turbines. Infrastructure investments in hydrogen production, delivery, and refueling would improve the economic case for these aircraft. This can be supported with research to determine where and how to invest in hydrogen infrastructure to maximize the CO2 mitigation potential while minimizing cost.
Areas of philanthropic support
Zero-emission planes are an important complement to sustainable aviation fuel (SAF) in decarbonizing aviation. They can address commuter, short- and medium-haul markets while the limited SAF supply can be focused on the long-haul market that requires higher density liquid fuels. There are a few ways the philanthropic community can support this progress:
- Seed research that looks into international standards to support the development of hydrogen-powered aircraft as well as hydrogen plane designs.
- Support advocacy to the International Civil Aviation Organization’s (ICAO) environmental committee to set and enforce sector-wide net zero targets.
- Promote cooperation between the government, industry, civil society, and other research organizations that will help bring these aircraft to the skies.
- Endorse policy levers that favor the development of this industry.
To learn more about the viability of hydrogen-powered aviation, explore the International Council on Clean Transportation’s study and follow along as ClimateWorks continues to expand its aviation portfolio.
