Production by electrolysis, storage, transport, derivatives and applications of green hydrogen in industry, transport and the power system.
What is green hydrogen?
Green hydrogen is hydrogen produced by water electrolysis using electricity from renewable sources (solar, wind or hydro).
Unlike grey hydrogen, made from natural gas with CO₂ emissions, or blue hydrogen, which captures part of those emissions, green hydrogen produces no greenhouse gases during production.
It also has a very high energy density: 1 kg of H₂ holds the same energy as 2.6 kg of natural gas or 3 kg of petrol.
How is it produced?
Electrolysis splits water molecules (H₂O) into hydrogen (H₂) and oxygen (O₂) using electricity. When that electricity is renewable, the process is carbon-free. There are several technologies:
- AEL (alkaline electrolysis): the most mature, but the least efficient.
- PEM (proton exchange membrane): more efficient and able to respond quickly to variable renewable supply. Water is split into oxygen, protons and electrons, which then recombine to form hydrogen.
- SOEC (solid oxide electrolysis): still in development; promises high efficiency and lower energy use.
After electrolysis, the hydrogen is purified and compressed for storage and transport.
Storage and transport
It is usually stored under pressure (around 30 atm), although higher-pressure solutions are under development. It can be stored as:
- Compressed gas at high pressure.
- Cryogenic liquid, cooled to −253 °C.
- Chemical carriers, such as ammonia or methanol.
It can be used directly or transported through adapted pipelines, tankers carrying compressed or liquefied hydrogen, or converted into derivatives that simplify logistics.
Green hydrogen derivatives
- Green ammonia (NH₃): via the Haber-Bosch process powered by renewable electricity. Used for fertilisers, as a shipping fuel and as a carrier for storing and transporting hydrogen.
- Green methanol (CH₃OH): from green hydrogen and captured CO₂. A chemical feedstock, a marine and road fuel, and the basis for synthetic petrol (Methanol-to-Gasoline).
- Synthetic natural gas (SNG): through methanation (the Sabatier reaction), combining hydrogen with CO₂ to produce methane. It replaces fossil gas in grids, boilers and thermal plants.
- Synthetic fuels (e-fuels): petrol, diesel and sustainable kerosene made from hydrogen and CO₂. An alternative for aviation and heavy transport without major engine changes.
- Liquid organic hydrogen carriers (LOHC): hydrogen is stored in liquid molecules such as benzyltoluene, allowing safe transport without extreme temperatures or high pressure.
Why does it matter?
Green hydrogen is a versatile energy carrier for decarbonising industry, transport, power generation and energy storage.
Industry
- Green steel: direct reduced iron (DRI) using hydrogen replaces coal and eliminates CO₂ emissions. Companies such as ArcelorMittal and the HYBRIT project are already investing in this route.
- Fertilisers: green ammonia makes carbon-free fertilisers possible.
- Refining and chemicals: replacing fossil hydrogen in refineries and in methanol and plastics production significantly cuts emissions.
Transport
- Fuel cell electric vehicles (FCEV): cars and long-haul trucks with greater range and faster refuelling than batteries.
- Shipping: the International Maritime Organization is promoting its adoption, and carriers such as Maersk and CMA CGM are already testing vessels running on green methanol and ammonia.
- Rail and aviation: hydrogen trains already operate in Germany, and synthetic kerosene is being researched for aviation.
Power generation and storage
- Long-term storage: surplus renewable energy can be stored and released when needed, for example by injecting hydrogen into the gas grid (Power-to-Gas).
- Hydrogen turbines: manufacturers such as Siemens Energy and General Electric are developing turbines that generate emission-free power when there is no sun or wind.
- Industrial heat: it can replace natural gas in high-temperature processes such as glass, cement and ceramics.