Liquid Hydrogen Aircraft: A Flight into the Future or a Detour?
This article was published on LinkedIn in August 2023.
Several European research programs are delving deep into hydrogen aircraft technology, with projections indicating a potential entry into service by 2035. However, this is not the maiden voyage of such ambitious plans surrounding liquid hydrogen (LH2) aircraft.
Source: G.D. Brewer et al. (1975) Study of the application of hydrogen fuel to long-range subsonic transport aircraft.
A 1976 NASA-funded study explored airport requirements for LH2 with potential introduction around 2000 [1]. Interestingly, the driving force then was not climate change but concerns over the future availability of jet fuel. Spearheading this research was G. D. Brewer from Lockheed Martin, who later penned the book “Hydrogen Aircraft Technology” in 1991 [2]. Brewer’s insights suggest that retrofitting a major airport like SFO for LH2 aircraft is feasible, albeit at a hefty price tag of around 2 billion dollars (adjusted for today’s inflation) and a whopping 300 MW of electric power for the liquefaction plant. I remain skeptical about witnessing commercially viable applications before 2050. Brewer’s book, however, offers a comprehensive primer on the subject, touching upon several environmental facets:
Air Pollution
Traditional aircraft engines, powered by hydrocarbon fuels, release a cocktail of pollutants, including unburned hydrocarbons, CO, NOx, and soot particles. With LH2, the carbon component is eliminated, leaving only NOx emissions, which arise from the high-temperature reaction of atmospheric nitrogen and oxygen in the combustion chamber. NOx emissions can form smog and ozone at the ground and deplete ozone in the stratosphere. Some research suggests that LH2 aircraft might have a lower NOx emission index than jet fuel [3]. However, this needs real-world validation (see footnote). On the bright side, the potential for improved air quality is promising. Yet, concerns linger about nanoparticles from condensed vented oil vapor, which could also have climatic implications [4,5].
Climate Impact
Non-CO2 emissions, notably contrails, account for up to two-thirds of aviation’s climate effects [6]. Efforts are underway to develop contrail avoidance strategies [7]. Exhaust soot serves as ice nucleating particles, and with cleaner burning sustainable aviation fuels (SAF), the soot emissions and thus the contrail’s optical density can be reduced [8]. The effect is equivalent to using a thinner blanket at night that traps less heat.
While LH2 combustion is soot-free, it releases about seven times more water vapor per kg of fuel burned (or 2.5 times more per distance flown). This increased water vapor, in conjunction with oil particles or other nucleating agents, could lead to more persistent contrails, potentially negating some of the climate benefits of reduced CO2 and NOx emissions [9, see footnote]. A holistic assessment of the climate impact would need to weigh the pros and cons of these factors.
The Way Forward
While speculating about the future is enlightening, our immediate focus should be on pragmatic, swift, and cost-effective solutions to achieve net-zero emissions. Given the longevity of aircraft and their engines (a program lasting 50 years is not an exception), it is unrealistic to expect operators to discard their assets prematurely or invest billions in hydrogen infrastructure. The answer lies in drop-in SAFs that are compatible with both current and next-gen engines. Thankfully, some nations are recognizing this and are seizing the opportunity.
I am excited about SAF developments and intend to play my part in supporting SAF scaling and environmental impact assessments.
Footnotes
Emission index of NOx
Brewer [2] suggests that the emission index of NOx for LH2 (amount per kg fuel burned) will be 2.80 times lower than for jet fuel because LH2 has 2.8 times higher energy density and the fuel flow is accordingly lower. I think it is exactly the opposite. If we assume that the NOx concentration in the exhaust will be the same for both fuels (worst case), the emission index for LH2 will be 2.80 times higher. Simply put, burning 1 kg of LH2 produces 2.8 times more exhaust (and NOx) mass than jet fuel.
There are currently no real-world measurement data for a viable LH2 combustor for an aircraft engine.
Vented oil aerosol as ice nucleating particles
Depending on the engine design, aircraft engines vent air with residual oil vapor from the air/oil separator into the hot exhaust, cold bypass, or overboard (a vent at the bottom of the engine nacelle). The oil consumption varies, typically around 0.1 – 0.5 liters per hour for a large turbofan engine. Increased oil consumption is a maintenance problem, and oil emissions have not been regulated or reported.
Bier et al. [9] estimate that if 1% of vented oil (assumed consumption of 1 l/h) enters the plume, the number of ultrafine particles and thus the number of ice crystals could be similar or higher compared to contrails formed from jet fuel combustion.
References
[1] LH2 Airport Requirements Study, G.D. Brewer, ed., NASA, 1976. https://ntrs.nasa.gov/api/citations/19770003090/downloads/19770003090.pdf
[2] Brewer, G.D. (1991). Hydrogen Aircraft Technology (1st ed.). Routledge. https://doi.org/10.1201/9780203751480
[3] Agarwal, P. et al. Injector Design Space Exploration for an Ultra-Low NOx Hydrogen Micromix Combustion System. Proceedings of the ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition. Volume 3. Phoenix, Arizona, USA. June 17-21, 2019. https://doi.org/10.1115/GT2019-90833
[4] Ungeheuer, F. et al. Nucleation of jet engine oil vapours is a large source of aviation-related ultrafine particles. Commun Earth Environ 3, 319 (2022). https://doi.org/10.1038/s43247-022-00653-w
[5] Ponsonby, J.et al. Jet aircraft lubrication oil droplets as contrail ice-forming particles. In EGUsphere 2023, pp. 1-25. DOI: 10.5194/egusphere-2023-1264.
[6] Lee, D. S. et al. The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Atmospheric Environment, Volume 244, 2021, 117834, ISSN 1352-2310, https://doi.org/10.1016/j.atmosenv.2020.117834.
[7] EU Project BeCoM – Better Contrail Mitigation https://www.becom-project.eu/
[8] Voigt, C., Kleine, J., Sauer, D. et al. Cleaner burning aviation fuels can reduce contrail cloudiness. Commun Earth Environ 2, 114 (2021). https://doi.org/10.1038/s43247-021-00174-y
[9] Bier, A. et al. Contrail formation on ambient aerosol particles for aircraft with hydrogen combustion: A box model trajectory study. In EGUsphere 2023, pp. 1-39. DOI: 10.5194/egusphere-2023-1321.
