Green Hydrogen

work in progress

Overview

Falling costs for hydrogen produced with renewable energy, combined with the urgency of cutting greenhouse-gas emissions, has given clean hydrogen unprecedented political and business momentum. Its potential role for the energy transition and the road to net zero has raised high expectations over the last years and months.

This article sheds light on (green) hydrogen, its use, market and costs, as well as its advantages and disadvantages.

What is Hydrogen?

Hydrogen (H₂) is the simplest and lightest element as well as the most abundant element in the universe. Each atom of hydrogen has only one proton and one electron. Hydrogen occurs naturally on earth only in compound form with other elements in liquids, gases, or solids. Hydrogen combined with carbon (hydrocarbons) exists in petroleum, natural gas, and coal. Hydrogen combined with oxygen is water (H₂O).

As a secondary source of energy, hydrogen carries energy produced from other resources like fossil fuels, water, or biomass. To produce hydrogen by separating it from other elements (electrolysis), it needs more energy than hydrogen provides when converted to useful energy. Hydrogen has the highest energy content of any common fuel by weight (about three times more than gasoline), but it has the lowest energy content by volume (about four times less than gasoline).

It is very clean-burning and when combined with oxygen in a fuel cell, hydrogen produces heat and electricity with only water vapor as a by-product.

Production of Hydrogen

There are different methods of hydrogen production, which determines how climate-friendly the hydrogen is. There is a color scheme used to distinct these different production methods:

• Grey hydrogen is produced by splitting fossil fuels like methane (CH₄) (sourced from natural gas) or coal with steam into H₂ and CO₂. Every ton of hydrogen produced from natural gas also produces 8–10 tons of CO₂. Since hydrogen produced from coal produces significantly higher CO₂ emissions (20-35x), it is sometimes also called brown or black hydrogen instead of grey.
• Blue hydrogen is produced as well by splitting methane or coal with steam, but unlike grey hydrogen, most (not all) CO₂ emissions are captured and stored for reducing its climate impact.
• Turquoise hydrogen: using processes such as pyrolysis, the methane is split into H₂ and solid carbon black which can be more easily stored. At the moment, turquoise hydrogen is still at the pilot stage.
• Green hydrogen is produced by splitting water into hydrogen and oxygen using renewable electricity. No emissions occur.

Besides this color scheme, sometimes the wording “low-carbon hydrogen” is used. This refers to both green hydrogen and blue hydrogen, see IEA (2022): Africa Energy Outlook.