Green hydrogen has become one of the big names in the energy transition. Because it can replace fossil fuels in industries that are hard to decarbonize (steel, cement, fertilizers, chemicals, mining and high-temperature industrial processes), projects have been launched practically all over the world.
The logic seems straightforward: if hydrogen is made by electrolyzing water with renewable electricity, the result is “green” hydrogen.
Environmentally, though, the answer is not that straightforward.
What is starting to matter is not only how the hydrogen was made, but how much greenhouse gas was actually emitted to produce a kilogram of it.
This is where Life Cycle Assessment (LCA) comes in.
Green does not necessarily mean zero emissions
An electrolyzer does not produce CO₂ when it splits water into hydrogen and oxygen. But that does not mean the hydrogen automatically has a zero carbon footprint.
To know its real climate impact, the boundaries of the analysis have to be widened. Not all green or renewable energy sources are 100% free of associated CO₂, and without a Life Cycle Assessment (LCA) that claim cannot be confirmed.
The electricity that powers the electrolyzer has a carbon footprint. So do building and installing the infrastructure, treating and supplying the water and, depending on the boundaries set for the study, compressing, storing, conditioning and transporting the hydrogen.
As a result, two plants that each produce exactly one kilogram of H₂ by electrolysis can get significantly different environmental results.
The unit that is becoming especially important is:
kg CO2e / kg H2
That is, kilograms of carbon dioxide equivalent emitted per kilogram of hydrogen produced.
With this indicator, hydrogen is no longer classified mainly by its origin or “color” but assessed quantitatively on its climate performance.
The source of the electricity can change everything
Electricity is normally one of the main drivers of the carbon footprint of electrolytic hydrogen.
An electrolyzer fed directly by solar or wind power can have a very different carbon intensity from one that runs on grid electricity with a large share of fossil generation.
This is why installing an electrolyzer does not in itself guarantee hydrogen with a low carbon footprint.
Where the electricity comes from, when it is generated, how its origin is certified and what emissions are associated with it all matter too.
This distinction will have growing consequences for projects that intend to sell hydrogen or its derivatives (ammonia, methanol or synthetic fuels, for example) on international markets.
From colors to numbers
In the early years of the industry, a fairly simple classification became popular: gray, blue, green, pink and turquoise hydrogen, among others.
It is useful for communicating which technology was used, but not enough to compare actual environmental performance.
Two projects classed as “green” do not necessarily have the same carbon footprint.
For that reason, several markets are moving toward schemes in which the key parameter is greenhouse gas emissions intensity calculated over the life cycle.
Brazil is an especially interesting case for Latin America.
Its legal framework initially defines low-carbon hydrogen as hydrogen whose emissions intensity, determined through life cycle assessment, is 7 kg CO2e per kg of H2 or less.
In addition, under the Brazilian Hydrogen Certification System, certificates are to report the emissions intensity associated with the product’s supply chain.
The point is an important one: it is no longer enough to state that hydrogen is renewable. Its carbon intensity now has to be measured, documented and eventually certified.
There is no single universal number
It is also important not to oversimplify.
The carbon footprint of a kilogram of hydrogen depends on the methodology used and, in particular, on the system boundaries.
- Is only production at the plant being measured?
- Are the emissions from generating the electricity included?
- Are water production and treatment considered?
- Is the infrastructure included?
- How much of the compression and storage is covered?
- Is transport to the user included?
These questions are essential because two values apparently expressed in the same unit (kg CO2e/kg H2) are not necessarily comparable if they were calculated with different boundaries, assumptions and rules.
That is precisely why LCA is so important.
LCA as a design tool, not only for certification
Measuring hydrogen’s carbon footprint should not be seen only as a regulatory requirement or a way to obtain certification.
Used properly, LCA makes it possible to identify environmental hotspots and compare alternatives before major investments are made.
For example, it can help evaluate different renewable electricity sources, electrolysis technologies, capacity factors, water treatment systems, compression or storage options and logistics scenarios.
It also helps answer a question that is particularly important for an industry seeking to decarbonize:
How much CO₂ are we actually avoiding by replacing the current fuel with hydrogen?
Because the ultimate aim is not to use hydrogen for its own sake. The aim is to reduce emissions.
A new commercial variable
Carbon intensity may also become a commercial feature of the product.
A kilogram of H₂ with a lower carbon footprint may have access to markets, incentives or customers that another kilogram of hydrogen (chemically identical) will not necessarily have.
The United States, for example, set a benchmark of 4 kg CO2e/kg H2 in its Clean Hydrogen Production Standard, calculated on a well-to-gate life cycle basis. The European Union also uses methodologies that consider life cycle emissions to determine the climate performance of renewable and low-carbon fuels.
The international direction seems clear: carbon traceability will be increasingly important in the hydrogen market.
So how green is green hydrogen?
Increasingly, the answer should be expressed as a number rather than a color.
A hydrogen project genuinely focused on decarbonization should be able to answer at least three questions:
- How many kg of CO₂e are emitted per kg of H₂ produced?
- Which stages of the life cycle generate those emissions?
- How many emissions are actually avoided when that hydrogen replaces the conventional fuel or feedstock?
Answering them requires data, methodology, traceability and Life Cycle Assessment.
Hydrogen will continue to be an important tool for the energy transition. But as the industry matures, simply saying that a project produces “green hydrogen” will probably no longer be enough.
The next step will be to show how green it really is.