Color theory: Only green is green

Hydrogen (H) is mainly available in chemically bound form, for example in water as H2O or ammonia NH3. Hydrogen as a gas (H2) and energy source is produced using various processes. Depending on the type of production, this results in different levels of emissions. The coloring serves to represent the production of hydrogen.

Green – only climate-neutral through renewable electricity

Hydrogen is only “green” if it is produced in an electrolyzer using renewable electricity. The sources can be wind and solar energy, but also bioenergy, hydropower or geothermal energy. Water (H2O) is broken down into its components hydrogen (H2) and oxygen (O2). The energy source hydrogen is therefore produced using green electricity without the use of carbon-containing substances.

Evaluation: Since there is currently hardly enough green electricity in Germany, “green” electricity or “green” hydrogen will initially have to come largely from abroad in order to cover the forecast high demand. There are a number of studies about which regions of the world are predestined for manufacturing; including the “PtX Atlas” from Fraunhofer IEE (see link on our Homepage). Some of the export countries are regions in which climate change could worsen water shortages. In those countries, seawater desalination plants would be required in excess of the number of such plants currently available.

Blue – Production from natural gas coupled with CCS

When natural gas (methane, CH4) is broken down, the result is called “blue” hydrogen. The CO2 that escapes does not reach the atmosphere, but is separated, collected and stored. This happens, for example, in underground caverns, exploited gas and oil deposits or in the seabed.
This process is called “Carbon Capture and Storage” (CCS). So the CO2 has not been eliminated, it is just out of sight and no longer appears in the balance sheet.

The “Law for the Demonstration of Permanent Storage of Carbon Dioxide” (Carbon dioxide storage law, KSpG) requires that the stored CO2 remains permanently and completely in the storage. The maximum storage volume in Germany is four million tonnes per year (§2 KSpG). The KSpG dates from 2012 and was approved in 2018 by the Federal government evaluated.

Scientists assume that underground storage can permanently keep 65 to 80 percent of the “waste product CO2” out of the atmosphere. According to the Federal Environment Agency (UBA), however, it is not yet clear whether potential users of the technology can keep this promise. There are a number of research and pilot projects underway. What is particularly problematic is “the enormous additional energy required for capture, transport and storage”.

Evaluation: According to the UBA, “no negative effects on human health are generally expected” during normal operation. However, health risks “could arise as a result of accidents (e.g. escape of CO2) or through a gradual release from the storage complex”. Under “unfavorable conditions”, salty groundwater can reach sweet groundwater near the surface and the earth’s surface, which leads to damage (salinization) in groundwater, soil and surface water. The use of CCS technology for hydrogen production increases the consumption of limited fossil raw materials by up to 40 percent. This makes natural gas for hydrogen production not a sustainable source.

Turquoise – Produced by methane pyrolysis

Hydrogen called “turquoise” is produced from fossil methane (CH4) using methane pyrolysis. The methane is broken down into the components hydrogen (H2) and carbon (C) in the absence of oxygen. Hardly any carbon dioxide (CO2) is produced during production. Granular solid carbon is a raw material that the chemical industry, among others, processes in its process chains.

Evaluation: Methane pyrolysis has been little researched. One Fraunhofer study According to him, methane pyrolysis “could play an important role in the short term if it can be brought to the required technological maturity and proven to be economically viable.” It is crucial to “use only renewable energies to split the methane and to ensure that the carbon produced during pyrolysis does not subsequently enter the atmosphere.”

Gray – production with high CO2 emissions

“Gray” hydrogen is produced by industry from fossil hydrocarbons such as natural gas (methane) using steam reformation. It is an endothermic process, meaning heat is supplied. The methane reacts with oxygen in the water vapor. The CO2 is released into the atmosphere unused.
Hydrogen is also “gray” when electricity from fossil fuels is used for electrolysis. Strictly speaking, “turquoise” hydrogen is also “gray” if it is not produced using “green” electricity.

Evaluation: According to experts, around ten tons of CO2 are released per ton of hydrogen produced in this way, which escapes and increases the greenhouse effect - not sustainable.

Orange – bioenergy with a carbon footprint

The attribute “Orange” is used for the production of hydrogen from bioenergy, i.e. carbon-neutral energy obtained from organic substances such as biomass, biofuel, biogas and biomethane, according to a short study “Hydrogen color theory“ of the Berlin Institute for Climate Protection, Energy and Mobility e.V. (IKEM). The organic materials would usually be obtained from waste and residues from agriculture, forestry, households and industry.

Evaluation: After use, the carbon that was once stored by the organic matter is released back into the environment. However, hydrogen from these sources still has a (low) carbon footprint because the greenhouse gas released was previously sequestered. Therefore, “the hydrogen produced from bioenergy is not given the color green,” says IKEM.

Red, pink, purple – uranium is not a renewable source

Hydrogen produced using electricity from nuclear power plants is classified as “red,” “pink,” or “purple.” Although electricity production does not produce CO2 emissions on site, uranium mining, along with transport and processing, does. According to the IKEM, the carbon footprint of nuclear power plant decommissioning and nuclear waste are also difficult to estimate due to a lack of solutions for long-term storage. In contrast to renewable energies, the costs of infrastructure have not decreased in recent years and could even increase “in the future due to more extreme weather conditions, for example as more extensive safety measures need to be integrated”. Therefore, “red” hydrogen is “not viewed as sustainable.”

Evaluation: The ore is not a renewable source. A nuclear power plant does not emit any CO2. Uranium mining, transportation and the uncertain future of fuel storage make nuclear power an unsustainable source for hydrogen production.

So colorful – more colors

"Brown“Hydrogen is produced by gasifying lignite, in which “black“Hydrogen is used as a basis for hard coal. “Yellow” hydrogen is created from an energy mix of renewable and fossil fuels. Hydrogen is “white” when it is – rarely – obtained from natural deposits using fracking.

Andreas Lohse

Editor's note: Power-to-X usually uses the coloring for hydrogen in the articles on the website as an attribute and without quotation marks. The quotation marks in this post are for emphasis.