Performance Analysis of Evolutionary Hydrogen-Powered Aircraft
EXECUTIVE SUMMARY
Aviation is a hard-to-decarbonize sector of the transport industry due to the stringent mass and volume requirements for aviation fuel. The high energy content of liquid jet fuel, both per unit mass (specific energy) and per unit volume (energy density), makes it difficult to replace. Significant emphasis has been placed on drop-in Sustainable Aviation Fuels (SAFs) to reduce emissions without sacrificing aircraft performance. But SAFs emit carbon dioxide (CO2) when combusted (carbon capture during production reduces life-cycle emissions) and their uptake has fallen short of expectations due to their high cost, limited supply, and concerns about the land-use impacts of biofuels.
Interest is growing in hydrogen, particularly liquid hydrogen (LH2), as a potential alternative to SAFs. LH2 emits no CO2 during combustion and can be produced with near-zero carbon emissions if made using renewable electricity (“green hydrogen”). However, its low energy density and heavy cryogenic tank requirements incur performance penalties when compared to Jet A-powered aircraft.
This study explores the potential performance characteristics, fuel-related costs and emissions, and replaceable fossil fuel market of LH2-powered aircraft entering service in 2035. In keeping with aviation’s conservative approach to new aircraft design, only evolutionary advances in design parameters that are feasible by 2035 are considered. Two LH2 combustion designs are assessed: a smaller turboprop aircraft targeting the regional market, and a narrow-body turbofan aircraft suitable for short and medium-haul flights. These designs are benchmarked against the ATR 72 and the Airbus A320neo, respectively.
Both hydrogen-powered designs will require an elongated fuselage to accommodate LH2 storage behind the passenger cabin. Gravimetric indices (GI), which denote the ratio of the fuel mass to the mass of the full fuel system including the cryogenic tank, are investigated, at values between 0.2 and 0.35. Seating pitch (SP) values of 29 and 30 inches, mimicking the seating density of low-cost and regular airliners, are used. The potential market coverage of LH2-powered aircraft families, which include variants of the baseline design with different range and passenger capacities, are analyzed as well.
Overall, we find that LH2-powered aircraft entering service in 2035 could contribute to aviation’s 2050 climate goals but with performance penalties relative to fossil-fuel aircraft. Compared to fossil-fuel aircraft, LH2-powered aircraft will be heavier, with an increased maximum takeoff mass (MTOM), and less efficient, with a higher energy requirement per revenue-passenger-kilometer (MJ/RPK). They will also have a shorter range than fossil-fuel aircraft. Nevertheless, we estimate that evolutionary LH2-powered narrow-body aircraft could transport 165 passengers up to 3,400 km and LH2-powered turboprop aircraft could transport 70 passengers up to 1,400 km. Together, they could service about one-third (31 to 38%) of all passenger aviation traffic, as measured by revenue passenger kilometers (RPKs) (ES 1). This represents 57% to 71% of all RPKs serviced by narrow-body aircraft and 89% to 97% of all RPKs serviced by turboprops. Aircraft with lighter fuel systems (GI of 0.35) and tighter seating density (seating pitch of 29 inches) would provide larger market coverage.
Our analysis finds that fuel costs for a green LH2-powered aircraft will be higher than for Jet A-fueled aircraft, but cheaper than “blue” LH2 generated from fossil fuels with carbon sequestration or synthetic e-kerosene (ES 2). Taxes levied on CO2 emissions, represented by the hatched bars, will be needed to make green LH2 cost-competitive with fossil jet fuel. We estimate that a carbon price of about $250/tonne-CO2e would be needed for fuel price parity for LH2-powered aircraft in the United States in 2035, falling to $100/tonne-CO2e in 2050. Europe, where renewable hydrogen is expected to be more expensive, may require a higher CO2 price to reach cost parity with Jet A. Other benefits from using hydrogen, including reduced air pollution and non-CO2 climate impacts, are not valued in this calculation.
These air-traffic analyses have been carried out with airline route data for 2019. We also project the CO2e mitigation potential of each LH2-powered design running on green hydrogen from 2035 to 2050. ES 3 shows the mitigation potential for these aircraft assuming that fleet renewal and growth is sufficient for LH2 designs to cover between 20% and 40% of the addressable market in 2050. The maximum possible coverage (100%) is also shown in green.
The 20% to 40% cases yield 126-251 million tonnes (Mt) of CO2e mitigated in 2050, representing 6-12% of passenger aviation’s CO2e inventory that year. Deployed to their maximum potential (the 100% case), evolutionary LH2-powered aircraft running on green hydrogen could cap aviation emissions at 2035 levels; other technologies and policies would be needed to further reduce emissions. The 100% cases yield 628 Mt of CO2e mitigated in 2050, representing 31% of passenger aviation’s CO2e inventory that year.
In summary, while LH2-powered aircraft do not perform as well as their jet fuel counterparts, they could service one-third of all passenger aviation traffic. Their CO2e mitigation potential is maximized when fueled by green hydrogen, which is expected to cost more than fossil jet fuel but less than blue hydrogen and e-kerosene. If deployed to their maximum potential, these aircraft could cap aviation emissions at 2035 levels, although a 6-12% reduction in CO2e emissions, relative to 2050 levels, is more realistic. Finally, to the extent that manufacturers need to prioritize aircraft development, we recommend a focus on narrow-body LH2 designs since they would provide the highest potential emissions coverage.
by Jayant Mukhopadhaya, Ph.D. and Dan Rutherford, Ph.D. ICCT International Council on Clean Transportation
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