Green hydrogen production: how does it work?

By ENGIE - 27 October 2021 - 10:51

How to produce renewable hydrogen How much does it cost? What are the challenges? We explain everything, from renewable electricity to green hydrogen and electrolysis.


Green electricity is the first step of the production of green hydrogen!

There are three main sources of carbon-free electricity: water, the wind and the sun. Hydroelectricity is the primary source of renewable electricity in France and worldwide. This technique transforms the power of water into electric current in hydro power plants installed on natural water courses or dams. In second place is wind power (onshore or offshore), which is produced by transforming the kinetic power of the wind into mechanical energy, and then electricity, using wind turbines. Finally, solar electricity finishes in third place, thanks to farms of photovoltaic panels that convert light into electricity. This renewable energy can then be used to produce renewable hydrogen. OK. But how?


>> See also: What is hydrogen? <<


Producing green hydrogen from water...

Indeed! Water molecules (H2O) contain hydrogen (H). The H2 is separated from the O in a process called the electrolysis of water. Electrolysis is THE technique used to produce hydrogen that consists of “breaking” the water molecules using an electric current in an electrolyzer in order to extract the dihydrogen H2. The electricity must itself be carbon-free in order to consider this hydrogen as green or renewable. 


Key figure

A hydrogen-fuelled car consumes 1 kg per 100 km.


Is the electrolysis of water the only way to produce green hydrogen?

No! There is another technique. Pyro-gasification consists of heating organic matter (wood biomass, end-of-life wood, crop residue, etc.), plus various forms of carbon-containing waste (solid recovered fuel, old tyres, non-recyclable plastic, dried sludge from wastewater treatment plants, etc.) to temperatures between 900°C and 1,200°C in the presence of a small quantity of oxygen. This process extracts a complex gas containing hydrogen. Since this solution is still emerging, electrolysis currently remains the most common technique. 


What is the difference between green hydrogen and low-carbon hydrogen?

The difference lies in their carbon weight, or the quantity of greenhouse gases emitted when they are produced, which in turn depends on the energy used. When exclusively renewably-sourced electricity is used (hydraulic, wind power, or solar) for electrolysis, then the hydrogen is green, also said “renewable”.

On the other hand, low-carbon hydrogen is produced either by:

  • vaporeforming natural gas (which consists of “breaking” the methane molecule at 700°C using catalysts) with CO2 capture and storage
  • electrolysis with electricity from the grid, if it is decarbonised enough.


What are the challenges of producing green hydrogen? 

The main obstacle to the development of renewable hydrogen is its production cost. How can this cost be reduced? Through industrialisation and the large-scale deployment of electrolysis. This is the reason why the French Environment and Energy Management Agency (ADEME) recommends “supporting the development of renewable sources of electricity (hydraulic, wind power or solar) by deploying electrolyzers to produce hydrogen, especially in the industrial and heavy transport sectors”. The French State’s hydrogen strategy plans to promote this industrialisation by supporting the production of electrolyzers by small and medium-sized companies and major groups throughout the country.


The impact of electrolyzers on costs

Electrolyzers play a key role in the development of hydrogen, because their design and build have a direct impact on costs. According to the 2020 report of the International Renewable Energy Agency (IRENA)1, “increasing the size of electrolyzers to 20 MW (compared with 1MW currently) could cut costs by more than one third”. Increased production of cells and the optimisation of systems could also have a significant impact on costs. Still according to the IRENA, “the road map required to limit global warming to 1.5°C could cut the cost of electrolyzers by about 40% by 2030”.



1: “Green hydrogen cost reduction”, IRENA, December 2020