Green hydrogen, blue hydrogen, grey hydrogen, purple hydrogen, turquoise hydrogen is all identical (see panel below). It’s just hydrogen, a colourless, odourless gas. The only difference is the way it’s produced.

Grey hydrogen derived from unabated natural gas or coal produces nine to 12 tonnes of CO2 for every tonne of H2 produced. Green hydrogen produced from splitting water molecules into H2 and O2 inside an electrolyser powered by renewable energy has zero greenhouse gas emissions.

Green hydrogen can help save the planet. Grey hydrogen is helping to destroy it.

So how can a buyer of hydrogen be sure that their gas has a renewable origin? After all, grey hydrogen is currently a lot cheaper to produce than green, so any unscrupulous producer or distributor could make a tidy profit by passing off grey H2 as green.

Spanish conglomerate Acciona has come up with the first solution — a digital platform that guarantees the renewable origin of green hydrogen.

GreenH2chain — which is based on blockchain technology that makes it impossible to tamper with — allows users to track and verify the transportation of green hydrogen from its production site to final delivery.

It is a similar system to the Guarantees of Origin used to verify renewable energy on the grid.

A screenshot of Acciona's GreenH2chain digital platform. Photo: Acciona

“This technological solution will allow renewable hydrogen consumers to quantify, record and monitor the decarbonisation process of their own energy supply,” said Acciona.

“In the future, GreenH2chain will be complementary to any official systems dedicated to certifying the renewable origin of hydrogen, once these are established.”

The system will first be used to guarantee the origin of green hydrogen produced at Acciona’s Power to Green Hydrogen pilot project on the Spanish island of Mallorca, which will produce more than 330 tonnes of green H2 per year when it becomes operational in July.

The colours of hydrogen

Green hydrogen : produced by splitting water (H2O) molecules into hydrogen and oxygen inside a machine called an electrolyser that is powered by renewable energy. It is also known as renewable hydrogen. If the electricity used is not 100% renewable, then it is not green hydrogen, it is electrolytic hydrogen.

Grey hydrogen : produced by splitting methane (CH4) molecules using a process called steam methane reforming (or a similar process called autothermal reforming). The remaining carbon reacts with air to form CO2. For every tonne of H2 produced, nine to 12 tonnes of CO2 are produced. Grey hydrogen can also be produced from coal gasification, which is common in China. This is sometimes known as brown or black hydrogen.

Blue hydrogen: this is produced the same way as grey hydrogen, but the CO2 emissions are captured and stored (or used). Strictly speaking, this is low-carbon hydrogen as only 95-99% of the CO2 released be captured.

Turquoise hydrogen: produced by heating methane to high temperatures inside pyrolysis ovens (ie, in the absence of oxygen, so CO2 cannot be formed in the process). This technology is still at the experimental stage.

Purple hydrogen: produced by gasification of municipal waste (heating it to temperatures of more than 1,000°C), which produces zero greenhouse gas emissions. This is an unofficial term disliked by the hydrogen-from-waste industry, which prefers to call its product “green hydrogen”.

By Leigh Collins