How clean is hydrogen for the energy transition?

In a joint study, researchers from Leiden University and the Paul Scherrer Institute have calculated the environmental impact of hydrogen production from today to 2050. For the first time, nine different production processes were considered in one study and extrapolated globally. The result: hydrogen, yes, but only green, please!

Romain Sacchi and his colleagues at Leiden University have analysed the life cycle of nine different hydrogen production processes and extrapolated them globally for the first time. © Paul Scherrer Institute/Markus Fischer

All hydrogen is not equal. It comes in many colours – from black to green. This does not refer to its physical colour but rather to a terminology identifying its origin (see grey box below). When we talk about green hydrogen, for example, we mean that it has been produced using water electrolysis that relies on renewable energy and water. We call it black, like coal, when it is produced using hard coal.

Currently, hydrogen is mainly required for chemical conversion processes, such as ammonia production using the Haber-Bosch process, which is used as a fertiliser component. In industrial processes, hydrogen is used as a protective gas and is required in metal and glass production, for example. The steel industry is also dependent on large quantities of this light gas. And hydrogen can be converted directly into electricity via fuel cells, which can be used in vehicles.

Hydrogen demand is increasing...

Since hydrogen releases almost no direct emissions, it can be crucial for the transition to a net-zero energy system, explains Romain Sacchi, scientist at the Laboratory for Energy Systems Analysis at PSI. The transportation sector and heavy industry in particular could benefit greatly.

The catch: today's production itself releases a lot of carbon dioxide (CO2) – on average, around 14 kilograms per kilogramme of hydrogen. Natural gas is currently the most efficient and most important raw material for hydrogen. This is referred to as grey hydrogen. Low-carbon production processes account for less than one per cent of the global market. And demand is growing. We assume that hydrogen production will increase by a factor of four to eight by 2050, explains Sacchi.

To obtain a complete picture, Romain Sacchi and his colleagues at Leiden University calculated the environmental impact of global hydrogen production and analysed possible future scenarios in a joint study. For the first time, they analysed the entire life cycle of nine different production processes and included them in a single study. They reported their results in the current issue of the scientific journal Energy & Environmental Science.

... and so are CO2 emissions in its production process

The researchers analysed three different scenarios from the International Energy Agency. Their finding: between 2020 and 2050, the cumulative greenhouse gas emissions from hydrogen production could amount to 39 to 47 gigatonnes of CO2 equivalents. The latter figure refers to the net-zero scenario, the most ambitious target with net-zero emissions by 2050. In this case, by 2050 hydrogen production could emit up to 12 per cent of the remaining carbon budget available to prevent global warming of more than 1.5°C, says Bernhard Steubing, an associate professor at the Leiden Institute of Environmental Sciences.

That may sound surprising, says Sacchi. Despite the use of low-carbon technologies, the quantities to be produced in the net-zero scenario are significantly higher than in less ambitious scenarios with grey hydrogen. The resulting emissions may seem high, but the use of hydrogen replaces fossil fuels, whose utilisation would make it impossible to achieve the 1.5°C target.

One reason for this is that less greenhouse gases are released during the production and utilisation of hydrogen than during the extraction and combustion of fossil fuels. Secondly, some of the hydrogen produced is to be utilised in electric motors via fuel cells, which are significantly more efficient than conventional combustion engines.

Green hydrogen versus blue hydrogen

Therefore, the seemingly most obvious solution would be to cover all consumption with green hydrogen. We have to be careful, counters Sacchi. If we blindly convert renewable energy into hydrogen, it may then be lacking for applications that could be electrified directly. In water electrolysis, only around 50 per cent of the energy used can be converted into hydrogen. That's why it makes more sense to use the electricity directly wherever possible or to store it in batteries.

Therefore, hydrogen production that is exclusively green would only be beneficial with a massive expansion of new renewable energy production. And we find that such an expansion is in turn associated with water, metals, and land consumption – there are trade-offs to be made, adds Sacchi.

The authors also follow the scenarios of the International Energy Agency, which assume that blue hydrogen will be used on a large scale. The production of blue hydrogen uses natural gas for what is known as steam methane reforming. The resulting CO2 is captured and stored. Shijie Wie, a doctoral candidate at the Institute of Environmental Sciences at Leiden University and first author of the study, finds the strategy risky: With this technology, we are sticking with fossil fuels. Capturing CO2 is very expensive and involves technical challenges. We assume that hydrogen production would still lead to annual carbon emissions of up to one gigatonne of CO2 by 2050.

There is no perfect solution. Green hydrogen makes sense – but only with the necessary expansion of new renewable energy sources. The study by Romain Sacchi and his colleagues can help better understand the environmental impacts of various hydrogen technologies to develop an appropriate roadmap for the desired energy transition. 

Text: Paul Scherrer Institute/Benjamin A. Senn based on a press release from Leiden University

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Nomenclature of hydrogen

Depending on how hydrogen is produced, it is given different names in the energy industry. Here is the common "colour code":

  • Green hydrogen: produced with electricity from renewable energy
  • Turquoise hydrogen: produced by so-called methane pyrolysis. In this process, natural gas is thermally split into hydrogen and solid carbon
  • Orange/yellow hydrogen: produced from organic materials such as biomass, biogas, and biomethane
  • Purple/red hydrogen: produced with electricity from nuclear energy
  • Blue hydrogen: produced from natural gas by capturing and geologically storing carbon dioxide
  • White hydrogen: naturally occurring hydrogen
  • Grey hydrogen: produced from natural gas (often all fossil fuels are grouped under "grey")
  • Brown hydrogen: from lignite gasification
  • Black hydrogen: from hard coal gasification


Dr. Romain Sacchi
Laboratory for Energy Systems Analysis
Paul Scherrer Institute, Forschungsstrasse 111, 
5232 Villigen PSI, Switzerland
+41 56 310 57 64 [English, French]

Original publication

Future environmental impacts of global hydrogen production
Shijie Wei, Romain Sacchi, Arnold Tukker, Sangwon Suh and Bernhard Steubing, 
Energy & Environmental Science, 22.02.2024
DOI: 10.1039/D3EE03875K 

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