What is green hydrogen?

Green hydrogen is making headlines as the latest big trend in the energy revolution, but do we know what it is, what it is used for and how it is produced?

We are facing a real technological challenge to curb the impact of human activity on the planet. Our dizzying economic and social development —industrial revolutions— has gone hand in hand with a dangerous increase in our carbon footprint, threatening the climate balance and, therefore, the ecosystems and life on our planet. A reality of which, fortunately, we are becoming more aware every day —perhaps not enough— and which, as a society, we want to put a stop to through different solutions that allow us to stop polluting and damaging our planet, while ensuring an improvement in the quality of life of all people.

Thus, in recent decades, investment in innovation and technology has allowed us to develop different solutions for the generation of green energies, the reduction of pollution or the suppression and/or substitution of emissions of harmful components into the atmosphere for other, more innocuous ones. In short, human beings are trying, albeit not yet with the necessary global consensus, to curb a very important problem for both humanity itself and for the planet. Thus, over the last ten years we have been experiencing revolutionary changes in sectors such as transport, in which two protagonists have been (re)born: the electric vehicle (it should be remembered that the first vehicles were powered by electric motors and not combustion engines) and the hydrogen vehicle. It is precisely on this last point that we would like to focus. Do we know what hydrogen or, even more complex, green hydrogen is and how it can change the transport sector? Let’s get to it.

As we explained, every activity leaves a footprint; something that is evident in our current model of mobility based on the combustion of fossil fuels. Whether it is diesel, petrol or kerosene (mainly in aviation), engines based on fossil fuel combustion obtain mechanical energy through the chemical energy of a fuel burning in a combustion chamber. This means that huge quantities of polluting gases and particles such as nitrous oxides, carbon monoxides, carbon dioxides, as well as other volatile organic compounds and micro-particles of different types end up entering our atmosphere; causing more than evident damage to our health and environment.

For this reason, for years we have been trying to find alternatives to these fossil fuels in order to use them in our vehicles. A search that in recent decades has resulted in the classification of these alternative technologies into two main groups:

• Electric motors: Their operation is based on one or more electric motors which, powered by batteries, generate resistance and transform the kinetic energy (movement) into alternating current, which passes through the converter again to become direct current and, in turn, is stored in the battery to be used once more.
• Hydrogen combustion engine: This works with a fuel cell, which is located at the front of the car, and which, through a chemical reaction with oxygen obtained from outside the vehicle, produces electricity to drive. The surplus produced by this process is just water vapour. How this hydrogen is produced and how much we pollute in this production process is another matter, hence the importance of the type of hydrogen used. This is where the famous green hydrogen comes in again; now we will see why  .

Hydrogen and its particular rainbow

As we said, the production and/or generation of hydrogen is so important in this process that it totally determines the carbon footprint that we bring about with the use of each technology. First of all, we must explain that hydrogen is not a compound that can be captured freely in our natural environment. It is present, but not in the quantity or form necessary for its capture, as it is what in chemistry is called an “energy vector”, being also light and easily stored. This basically means that in order to obtain it, a process subject to the use of energy is necessary; in other words, depending on the source of energy used, hydrogen can generate a carbon footprint of a different nature. The greener the process of obtaining hydrogen, the greener the hydrogen itself will be.

Perhaps, knowing this, the most important thing is to know the differences between the different types of hydrogen that exist depending on the way in which it is produced. Thus, we can speak, as if it were a martial arts belt, of eight classifications by colour, although hydrogen itself has no colour as such, with the following differences:

Black/brown hydrogen

It is produced as a result of the gasification of carbon through the burning of different carbonaceous minerals such as bituminous coal (black coal), hard coal, or lignite (brown coal). Being based on combustion alone, as part of the chemical process various pollutant emissions, including carbon dioxide, are released into the atmosphere. This is why it is considered the most environmentally damaging type of hydrogen.

Grey hydrogen

This is the most common and easiest hydrogen colour to produce (and therefore the cheapest), but also one that releases the most carbon dioxide into the atmosphere. Grey hydrogen is produced by what is known as steam methane reforming (SMR) by reaction of fossil fuels, especially natural gas.

Yellow hydrogen

Yellow hydrogen is one in which the electricity used for electrolysis comes from a variety of generation sources, including both those based on renewable energy and those using fossil fuels. The peculiarity is that yellow hydrogen also refers to hydrogen produced using solar energy, although this would fall under green hydrogen as a whole; in fact, we could argue that yellow hydrogen is a shade of green hydrogen.

Blue hydrogen

Blue hydrogen is defined as hydrogen produced as a result of the use of natural gas as a feedstock. This process is particularly exciting since, as a result of this use of gas, carbon dioxide is separated and captured to be stored in deep geological formations (cavities in the earth’s crust) for subsequent use in the production of eco-fuels. It is a low-emission hydrogen, but it cannot be described as clean.

Turquoise hydrogen

It is obtained through a revolutionary method, disclosed by the Japanese industrial company Ebara, which allows the methane contained within natural gas and biogas to be extracted by pyrolysis of the methane. As a result, the carbon produced in the process ends up in a solid state and is not released into the atmosphere. It does not need to be recaptured and stored and can be used in the production of a range of other useful carbon-based commodities, such as fertilisers.

However, this production process is still under development and turquoise hydrogen cannot be evaluated and generated on a par with other hydrogen colours.

Pink hydrogen

This is a type of hydrogen that is produced by electrolysis of water, breaking the water molecule to obtain hydrogen and oxygen, with a great peculiarity: the electrical energy used in the process is nuclear. It is an almost sustainable hydrogen, since its environmental footprint is only related to the nuclear energy itself.

Green hydrogen

This type of hydrogen is our main character today. It is produced by the electrolysis method, breaking the water molecule to obtain hydrogen and oxygen with a special feature: it only uses electricity from renewable sources. In other words, green hydrogen is the only one that is obtained with 100% clean energy such as photovoltaic (yellow), wind or hydroelectric energy, and does not produce any direct emission of carbon dioxide into our atmosphere.

White hydrogen

When we talk about white hydrogen, we are talking about hydrogen that is naturally occurring, usually in gaseous form in the atmosphere and sometimes in underground reservoirs. The big problem is that this type of hydrogen does not have an associated technology that would allow us to exploit it on a large scale, making it useless for our purposes.

How is green hydrogen produced?

As we have seen, there are a multitude of processes that generate hydrogen as a result, although not all of them can be considered sustainable as such. This is why, in an effort to clarify how H₂ is produced, we are going to focus on trying to unravel how the main character of this article —green hydrogen— is generated.

Let’s see, hydrogen is a chemical element of the periodic table —specifically the first in the list— which, in this case, is obtained through the separation of the molecules that form water (H₂O) through a process of dissociation of these molecules by the contribution of electricity. This process, called electrolysis, separates the hydrogen molecules from the oxygen molecules, and in the case of green hydrogen it is done thanks to the electrical energy generated by any renewable energy source (mainly wind and/or photovoltaic energy).

In this way, the electric current is applied continuously inside the electrolyser, a process for which we must first convert the current from alternating current to direct current thanks to power electronics and devices called rectifiers. In order for these rectifiers to operate at the right levels of alternating current and voltage coming from the grid, they must be protected against possible alterations, which is why we use transformation centres, equipped with protection relays, as well as transformers to step down voltages. This makes them key elements for correct operation and requires a high level of technology and innovation. However, there are two dilemmas in this process:

1. In the event that the electrolysis process is carried out by connection to the electricity distribution grid, the coupling shall be carried out by means of connection and isolation centres to the public distribution grids.
2. In the event that the electrolysis process is based on electricity transmission networks, we require electrical substations, as we must transform the electricity from high to medium voltage, guaranteeing the safety of the process at all times, as well as its correct operation.

In either case, the basic scheme would be that of a quantity X of water stored and/or transported to a hydrogen generation plant that passes through an electrolyser to be subjected to a molecular separation process, using electrical energy from renewable sources, which breaks down its initial molecular composition. It is after this separation that the oxygen is stored for industrial or medical use and/or expelled from the equation via the atmosphere, while the hydrogen is sent to storage tanks where it is kept as a compressed or liquefied gas for industrial use or to produce hydrogen fuel cells.

This is the journey that allows a simple drop of water to be converted, through renewable energy and electricity infrastructure, into a green fuel with zero emissions. This is why the development of this industry is so important.