Power-to-X: Science and limits

Where half the electricity disappears on the way to fuel

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

Power-to-X builds low-carbon hydrogen into molecules that are easier to store, ship or burn: ammonia (with nitrogen from air), methanol and methane (with CO2), and jet fuel or diesel (with CO2, via Fischer-Tropsch). Only about 40-60% of the electricity ends up in the fuel.

The conversion, step by step

Electrolysis
Electricity plus water → Hydrogen · 65-70% LHV for large commercial electrolysers¹CO2 or N2 supply
Heat and electricity → Pure CO2 (from fermentation, flue gas or air) or N2 (air separation) · Air separation and compression for ammonia: about 500 kWh per tonne NH3. Air capture needs about 4-10 GJ of heat per tonne of CO2¹⁴Synthesis
H2 plus CO2 or N2 → Ammonia, methanol or Fischer-Tropsch syncrude · H2 to ammonia 88%, to methanol 80%, to Fischer-Tropsch syncrude 57% (LHV); a jet-optimised refinery then gets about 75% kerosene¹End to end
Electricity → Liquid or gaseous fuel · 40-60% (IEA); roughly half (ICCT)¹²

The science

These syntheses are catalytic reactions that give off heat, so once hydrogen exists the chemistry runs downhill. Haber-Bosch joins nitrogen and hydrogen over iron at high pressure (N2 + 3H2 → 2NH3). Methanol forms over copper (CO₂ + 3H2 → CH₃OH + H₂O); the Sabatier reaction over nickel makes methane (CO₂ + 4H2 → CH₄ + 2H2O). For jet fuel, CO₂ is first turned into carbon monoxide (reverse water-gas shift), then Fischer-Tropsch synthesis at about 200 C and 20-30 bar on cobalt or iron links CO and H2 into chains that a refinery sorts into kerosene, diesel and naphtha. In the carbon routes some hydrogen leaves as water, a loss no catalyst can avoid. Ammonia has no oxygen to remove, so it is the most efficient carrier, then methanol, with Fischer-Tropsch last.

Limits set by science

Limit	Value	Why it exists	Best today
Ammonia energy ceiling	About 88% of hydrogen LHV kept in NH₃ (calculated: 0.176 t H2 per t NH₃)¹	Synthesis gives off heat, so ammonia holds less energy than the hydrogen in it.	IEA assumes 88%, close to the ceiling; air separation and compression add about 500 kWh/t
Carbon-route ceilings	About 88% for methanol, 83% for methane, 84% for Fischer-Tropsch fuel (calculated from heating values and 0.19, 0.5 and 0.43 t H2 per t of product)¹	Part of the hydrogen ends up in water, carrying its energy away as heat.	80% H2-to-methanol and 57% H2-to-syncrude in IEA modelling; e-methane only at pilot scale (6 MW)
Air capture minimum	About 0.44 GJ per t CO₂ (calculated from the free energy of separating 0.04% CO₂ from air)⁴	CO₂ in air is so dilute that real sorbents need many times this, mostly as heat to release the CO₂.	About 4-6 GJ of heat per t for solid sorbents (80-120 C) and 6-10 GJ for liquid solvents (about 900 C)
Grid carbon threshold	Below 160-190 g CO₂-eq/kWh for e-methanol; below 95-140 g for e-methane and e-kerosene³	Above these levels the e-fuel emits more over its life than the fossil fuel it replaces.	Only met with dedicated renewable or nuclear power

Sources

The Role of E-fuels in Decarbonising Transport (revised January 2024) International Energy Agency, 2024
Current and future cost of e-kerosene in the United States and Europe International Council on Clean Transportation (ICCT), 2022
Global Hydrogen Review 2024: GHG emissions of hydrogen and its derivatives International Energy Agency, 2024
Direct air capture of CO2: from insights into the current and emerging approaches to future opportunities (Zeeshan, Kidder, Pentzer, Getman, Gurkan), Frontiers in Sustainability 4 Frontiers, 2023