Hydrogen

Hydrogen is believed to be one of the three elements produced in the Big Bang. Hydrogen can be found in stars that use it as fuel to produce energy, and in the "empty" spaces between stars. We owe most of our planet's energy to hydrogen, because the Sun's nuclear fusion process converts hydrogen to helium, releasing vast amounts of energy.

On Earth, hydrogen usually does not exist by itself in nature and must be produced from compounds that contain it, for example from water (H₂O). The only exception is a very small percentage that exists in the Earth's atmosphere. This reduced percentage is due to its low density; Earth's gravity is not able to hold it and it floats in space. The lost flow is about 95,000 tons of hydrogen per year.

Hydrogen is a gas at room temperature, but turns liquid at -252.8˚C, and from liquid to solid state at -259.2˚C. About 10% of the weight of living organisms is made up of hydrogen - mainly water, proteins and fats.

Electrolysis is a process generating hydrogen for the production of carbon-free electricity from water. Its principles have long been taught in schools: it involves harnessing an electric current to break down a molecule of water (H₂O) into hydrogen (H₂) and oxygen (O₂) gases. More specifically, water is injected at the positive electrode (anode) where it is first broken down into oxygen, H+ ions and electrons. The H+ ions then migrate to the negative electrode (cathode) where they recombine with electrons to form hydrogen. The membrane is used to let the protons migrate while blocking the electrons to circulate them to the anode.

For hydrogen to be green, the electricity used to generate the chemical reaction of molecule separation must itself come from renewable energies, namely solar, wind or hydroelectricity.

Hydrogen can be stored in gaseous form in cylindrical tanks under pressure, most often at 200 bars for industrial uses, these tanks then being mounted on racks to facilitate their transport in larger numbers. In the automotive industry, current tanks allow compression up to 700 bars. For buses and trucks, as in the maritime sector, the current standard is storage at 350 bars, but it is possible to go up to 700 bars, and even beyond, in suitable tanks. The greater the compression, the greater the volume of hydrogen stored in the same space. However, the storage capacity does not quite double if the pressure is doubled, because the walls of the tanks, generally made of composite, are then thicker to withstand these high pressures.

Another storage method is to make the hydrogen liquid. To do this, it is necessary to bring it to -253°C and to maintain this temperature, which requires a lot of energy. The interest of liquid hydrogen lies in the small volume occupied in comparison with the energy it carries. However, this solution is only of interest when the use of hydrogen to power a fuel cell is as continuous as possible: the hydrogen used to power the cell comes from what is called "boil off" , that is to say the change of state from liquid hydrogen to its gaseous form when it heats up and becomes volatile.

There are other more or less technologically mature storage processes, which make it possible to store hydrogen at ambient temperature and pressure using receptacles which will trap the hydrogen molecules before releasing them as needed. These chemical solutions are called LOHC (Liquid Organic Hydrogen Carrier). There are also storage solutions based on metal and inorganic hydrides, the objective of which is similar: to make it possible to solve the problems of pressure and volume of hydrogen storage, the difficulty being to limit the quantity of energy necessary for these methods so as not to lose too much energy efficiency over the entire storage operation.

Currently, 95% of the hydrogen produced in France is of fossil origin, like almost 99% of hydrogen produced in the rest of the world. It is most often obtained from the steam reforming process of methane, the main component of natural gas. Each kg of hydrogen thus produced emits 12 kg of CO2, and its price varies from 1 to 2.5€ per kg. Nearly 45% of world production comes from this technique.

About 25% of hydrogen production comes from "co-production" of refined products from hydrocarbons, which is then called "fatal" hydrogen. Its production cost is variable since it is a "residue" from the production of other chemical elements, and therefore its carbon footprint is too.

A third method uses coal, burned at a very high temperature (1200 to 1500°C) to separate hydrogen – which should be called dihydrogen H2 – from CO2, in the form of gas. This production, about 30% of the total, makes it possible to obtain hydrogen whose price per kg varies between 1.5 and 3€, but releases 19kg of CO2 per kg of hydrogen into the air.

These are industrial models that produce "grey" hydrogen. "Green" hydrogen, which only contributes less than 1% of global production (about 5% in France), comes from the use of carbon-free or renewable energies (solar, wind, etc.). Water electrolysis, which allows a zero carbon footprint, represented only 0.1% of global hydrogen production in 2019, due to its relatively prohibitive production cost compared to other methods, one kg of hydrogen costing between €3 and €12 for its production alone (excluding the cost of transport, distribution, etc.).

To enable the large-scale deployment of "green hydrogen" in the future, electrolysis from a renewable energy source is one of the paths, and it is clearly one of the options selected by the 2020 recovery plan to make France and Europe champions in the production of "green" hydrogen.

It is inexhaustible: On Earth, the most abundant source of hydrogen is water. During the electrolysis and electrochemical conversion processes with a fuel cell, the only by-products are oxygen and water. Its availability is therefore infinite.

It is full of energy: Although its density is very low, which makes it necessary to compress or liquefy it, hydrogen has an exceptional energy density: 1 kg of hydrogen releases 4.1 times more energy than 1 kg of coal, 2.8 times more than 1 kg of petrol and 2.4 times more than 1 kg of natural gas.

It is the best ally of renewable energies: Hydrogen makes it possible to store surplus renewable energies in the long term so that they can be reused later.

It is light: Despite a lower theoretical yield than battery storage, hydrogen storage proves to be up to 10 times lighter. This lightness saves volume and reduces the mass necessary for energy storage, even taking into account the mass of the tanks intended to store it. This is why fuel cell-battery hybridization takes on its full meaning when the autonomy of a means of transport is an essential objective, on land as well as at sea. This is why "heavy mobility", such as trains, buses, trucks and ships, which are fuel-intensive and need just much power to cover long distances on a single tank of energy, are the ideal target market for hydrogen in the energy transition to low carbon solutions.

It is clean: When it comes from renewable sources, the production of hydrogen is carbon neutral. Its use in a fuel cell does not emit CO2, NOx or fine particles. It only rejects pure water, without any minerals, and heat. We can even state that the ambient air used by the cell to carry out the chemical reaction comes out much purer than it entered the fuel cell, because it is filtered upstream of the process.

It recharges quickly: In the mobility sector, it is possible to refuel in a few minutes compared to several hours for its battery equivalent. A major benefit for tomorrow's electric mobility.

During the electrolysis of water, the molecules of hydrogen are separated from the molecules of oxygen. The fuel cell proceeds in reverse, and will use oxygen from the air and hydrogen, from which it captures the electrons during the chemical reaction, before they are transformed into electricity. The principle of an electro-hydrogen generator is to be able to replace a diesel generator to power an electric motor, for uses related to mobility, or directly supply carbon-free electricity for any other application. There is no such thing as a hydrogen engine, even if the possibility of operating internal combustion engines with hydrogen, in part, also exists, and the objective is to have them run only on hydrogen. In most of the current innovations, it is the production of carbon-free electricity via the combination "hydrogen + fuel cell" which is targeted, both as an alternative to fossil fuels and to overcome the disadvantages of heavy and expensive batteries.

Mainly used until now as a raw material for chemicals and oil refining, hydrogen is increasingly identified as the energy vector of the future due to its ability to store energy and the fact that its use does not emit CO2. It is presented today as a possible substitute for hydrocarbons, and an effective way to facilitate the integration of renewable energies. Although 95% of the 75 million tonnes of hydrogen produced every year in the world come from fossil fuels, new technologies for producing carbon-free hydrogen continue to develop. The production of hydrogen from biomass or by electrolysis is supported by the emergence of a new demand for “green hydrogen”.

In many industries, the use of carbon-free hydrogen should be used in processes that traditionally use fossil hydrogen, such as ammonia production and oil refining, but also in new processes as a substitute for other fossil materials. Projects to experiment with new ways of integrating carbon-free hydrogen or recovering fatal hydrogen have thus multiplied in recent years, and the 2019 energy-climate law set a target of up to 40% of renewable hydrogen production by 2030.

In the mobility sector, hydrogen-powered vehicles represent a good alternative to meet the challenges of sustainable mobility. They only discharge water, have an autonomy equivalent to a vehicle equipped with an internal combustion engine, and recharge quickly. In addition to the multiplication of the number of hydrogen-powered car models, 2019 witnessed the acceleration of the dynamics of "hydrogen for trains" with an increased in the number of orders for the train developed by Alstom, and by the growing interest of local authorities for the deployment of hydrogen-powered buses.

As part of an ever more renewable future electricity mix, the hydrogen energy vector makes it possible to compensate for the intermittency of renewable energies by storing, in gaseous form, the excess electricity produced during periods of high production and low consumption (Power to Gas). The energy storage made possible by hydrogen also makes it possible to extend the prospects of self-consumption to the scale of a house, a building or a village.

Achieving zero emissions by 2050 will require a wide range of technologies and the transformation of infrastructures. Energy efficiency, paradigm shifts, renewable energies and new technologies will have to take their part.

Already acclaimed for its many advantages and supported by many States which are investing strongly to democratize its uses and achieve their low-carbon objectives in the years to come, hydrogen represents a real opportunity to accelerate the energy transition.