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Glossary · Fuels & hydrogen

E-fuels (power-to-X)

Synthetic fuels made from electrolytic hydrogen, sometimes combined with captured CO2 or nitrogen, such as e-methane, e-methanol, e-kerosene and e-ammonia.

Power-to-X converts electricity into chemical energy carriers. The first step is always green hydrogen; further synthesis yields:

  • E-methane: Sabatier reaction CO2 + 4 H2 → CH4 + 2 H2O
  • E-methanol: CO2 + 3 H2 → CH3OH + H2O; a shipping fuel already used by Maersk vessels
  • E-kerosene and e-diesel: reverse water-gas shift plus Fischer-Tropsch synthesis
  • E-ammonia: Haber-Bosch with nitrogen from air separation; no carbon needed (see ammonia as fuel)
Overall efficiency ≈ ηelectrolysis × ηsynthesis × ηCO2 capture ≈ 0.65 × 0.75 × 0.9 ≈ 40–50%

Making 1 litre of e-kerosene requires roughly 20–25 kWh of electricity. Used in a combustion engine at 25–35% efficiency, the power-to-wheel efficiency of e-fuels is 10–15%, versus 70–80% for a battery electric vehicle, which is why e-fuels are mostly targeted at aviation, shipping and chemicals rather than cars.

For e-fuels to be low-carbon, CO2 must come from biogenic sources or direct air capture, and the electricity must be renewable. Production costs in 2024–2025 studies were EUR 2–5 per litre of e-kerosene, several times the price of fossil jet fuel. The EU mandates 1.2% synthetic aviation fuel in 2030 and 35% in 2050 under ReFuelEU.

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Related terms

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  • Green hydrogen

    Hydrogen produced by splitting water in an electrolyser powered by renewable electricity, with near-zero direct CO2 emissions.

  • Sustainable aviation fuel (SAF)

    Drop-in jet fuel made from biomass, waste or captured carbon and hydrogen, certified to blend with fossil Jet A-1 and cut lifecycle emissions.

  • Ammonia as fuel

    Use of ammonia (NH3), a carbon-free hydrogen carrier, as a fuel for ships, power plants and long-distance hydrogen transport.

  • Direct air capture (DAC)

    Engineered systems that extract CO2 directly from ambient air using chemical sorbents or solvents, producing a concentrated stream for storage or use.

  • Electrolyser

    An electrochemical device that uses electric current to split water into hydrogen and oxygen; the core equipment of green hydrogen production.

  • Well-to-wheel emissions

    Greenhouse gas emissions of a vehicle from producing and delivering its energy (well-to-tank) plus using it in the vehicle (tank-to-wheel), per unit distance.