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Unsolvedopen · Research Frontier · Today (unsolved as of Oct 2026)

Energy & Industry / Transport & Mobility

Zero-Emission Aviation

Jet fuel stores over 40 times as much energy per kilogram as today's batteries; Airbus has pushed its hydrogen airliner beyond 2035.

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Aviation causes about 2-3% of global CO2 emissions plus climate effects from contrails. Batteries suit short trips with few passengers; for long haul only synthetic or biogenic fuels (SAF) or hydrogen, which needs large, heavy tanks, remain. Aircraft fly 25-30 years - what is ordered today is still flying in 2050.

As of October 2026

IATA expects sustainable aviation fuel (SAF) to be only about 0.8% of aviation fuel in 2026 (2.4 million tonnes); the UK mandate is 3.6% in 2026. In February 2025 Airbus said its ZEROe hydrogen aircraft would arrive later than the planned 2035 because hydrogen technology is progressing more slowly than expected, and it has since focused on fuel cells. Electric passenger aircraft remain limited to small machines and eVTOLs.

What is missing

  • Batteries with several times higher energy density at system level (cells around 250-300 Wh/kg today, less in an aircraft)
  • Green hydrogen in large quantity and at low price, plus airport infrastructure
  • Synthetic kerosene (e-fuel) from hydrogen and CO2 in millions of tonnes at bearable cost
  • Development and certification of new aircraft types, which takes 10-15 years
  • Understanding and avoiding non-CO2 effects such as contrails

Becomes possible once solved

  • Climate-neutral long-haul flights
  • Regional electric flights to small airports
  • Air travel compatible with climate goals

Open steps

  • Contrail avoidance High AI leverageForecast contrail-forming regions and reroute flights routinely without large fuel penalties.
  • Aircraft-grade batteries Medium AI leverageReach several times today's system-level energy density with safety and cycle life suited to aircraft.
  • Cheaper e-kerosene synthesis Medium AI leverageMake synthetic jet fuel from hydrogen and CO2 at far lower cost with better catalysts, reactors and heat integration.
  • Hydrogen aircraft system design Medium AI leverageDesign lightweight cryogenic tanks, fuel-cell or turbine propulsion and airframes with safe fueling.

Where AI could help

Low AI leverage. The limits are battery energy density, green hydrogen and fuel supply and 10-15 year certification; AI helps mainly with contrails and efficiency.

  • Forecast contrail-forming regions so flights can avoid them
  • Screen battery, fuel-cell and e-fuel catalyst materials with machine learning
  • Optimize aerodynamics and flight trajectories for lower fuel burn
  • Use simulation to cut wind-tunnel and test iterations during design

Shown so far

  • In August 2023 Google Research, American Airlines and Breakthrough Energy reported that AI contrail forecasts let pilots on 70 test flights cut contrails by 54%, at about 2% more fuel on those flights. source

Prerequisites

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Sources

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