Fueling greener aviation with hydrogen

"While there is a long way to go for hydrogen aviation to be realized at scale, we hope that our analysis of both onboard system design and enabling infrastructure will be used to prioritize development efforts," says Dharik Mallapragada, one of the study's coauthors.

The aviation industry's energy-related CO₂ emissions have grown faster than those of rail, road and shipping in recent decades, according to the International Energy Agency. To reduce the potential climate impacts of this growth, scientists are improving aircraft design and operation, and developing low-emission fuels such as hydrogen, which is used for direct combustion or to power electric fuel cells. Hydrogen's appeal as a fuel source is that its use produces no CO₂ and provides more energy per pound than jet fuel. To understand the potential impact of switching from traditional jet fuel to hydrogen fuel in aviation, Anna Cybulsky, Mallapragada and colleagues modeled its use in the electrification of regional and short-range turboprop aircraft.

The researchers calculated that the extra bulk of a hydrogen fuel tank and fuel cells retrofitted to an existing plane would need to be offset by weight reductions elsewhere, such as reducing the aircraft's payload (cargo or passengers). This could mean that more flights would be needed to deliver the same payload. The team's model suggested, however, that improvements in fuel cell power and the fuel system's gravimetric index (the weight of the fuel in relation to the weight of the full fuel tank) could eliminate the need to reduce payload, thus eliminating the environmental impact of additional flights. At the same time, they noted that shifting to hydrogen-powered flight may reduce the aviation industry's CO₂ emissions by up to 90%.

A bigger challenge than switching aviation fuel types may be providing the infrastructure needed to generate and distribute hydrogen in a low-carbon and cost-effective manner. One low-carbon production method uses natural gas reforming (extracting hydrogen from methane gas) coupled with carbon capture, but it requires access to CO₂ infrastructure and sequestration sites. Another green option is electrolysis, which splits water into hydrogen and oxygen, and could be done by using electricity from a nuclear plant or renewable resources. But this would add substantial demand to electrical grids. Cybulsky and colleagues noted that because grid electricity prices can be highly variable across a region, it may be more cost-effective to transport hydrogen from a low-cost production facility to end-users.

For these reasons, the researchers suggest that the rollout of hydrogen-based aviation might start at locations that have favorable conditions for hydrogen production, such as Hamburg, Germany, or Barcelona, Spain. The infrastructure required to support hydrogen use in aviation would also benefit decarbonization efforts in other industries, including road transportation and shipping, by making hydrogen fuel more available.

The authors acknowledge funding from the Massachusetts Institute of Technology Energy Initiative Low-Carbon Energy Centers for Energy Storage and Future Energy Systems Center.