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Hydrogen

Hydrogen

It fuels the stars and has a particularly high energy density. It doesn’t produce polluting emissions and could help us in dealing with the energy challenge facing our planet. There is just one flaw, however: it isn’t easy to produce. However, advances in the technology to produce it cleanly, with the help of renewable sources, are paving the way towards a new future.

What is it?

The energy of the universe

Among the many elements that make up matter, hydrogen is the lightest and the most abundant. It forms almost 90% of the visible universe, mostly in gas form. It consists of a two-atom molecule (H2), which, in an atmosphere rich in oxygen like ours, burns in a similar way to methane. Compared with conventional fuels, it has the highest energy content per unit of weight, three times greater than gasoline.

But hydrogen is also the propellant in the nuclear fusion reactions that power the stars. It is this extraordinary element, therefore, that is the origin of the renewable energy that the Earth receives each day from the sun. Moreover, when used in fuel cells, it combines with oxygen to produce electric energy and water. For all of these characteristics, hydrogen carries our hopes of being able to produce the sustainable, non-polluting energy we need for heating and for powering electric appliances in our homes and industry. In particular, it can play a decisive role in the decarbonization of energy-intensive industries, like steel production or the chemical industry.

Green hydrogen

Green and zero impact

There’s just one problem. In spite of its abundance on Earth hydrogen does not exist naturally on its own. It can be found only linked to other elements, like water (molecules of hydrogen and oxygen) or in hydrocarbons (hydrogen and carbon). To separate it from other elements found on Earth it is necessary to extract the hydrogen and this separation process requires energy and, therefore, economic, and often environmental costs.

Given that it is not found in its pure form in nature, and producing it requires the contribution of another type of energy, hydrogen is considered an energy carrier rather than a source of energy, such as solar or wind power.

Only the so-called “green hydrogen,” which is obtained by separating it from water through a process of electrolysis powered by renewable energy, is fully zero impact, without polluting emissions and without consuming precious natural resources. The scientific and technological communities have been working for some time on making green hydrogen easier and cheaper to produce and, thanks to the enormous progress made in recent years, this goal now seems almost within reach. This is why many experts believe a new energy era of hydrogen is dawning and that the age of oil is drawing to a close.

History

From alchemists’ laboratories to spaceships

How green hydrogen is produced

Electrochemistry meets renewables

Today around 95% of the hydrogen used on the Earth, most of which is used in industry, is obtained by reforming methane or through the gasification of coal, processes that generate substantial emissions of carbon dioxide but which are currently the cheapest methods available. There are, however, other ways to obtain hydrogen, for example through thermochemical processes, and, above all, through the electrolysis of water. This involves systems called electrolyzers that require a certain amount of electrical energy and which therefore, in order to be sustainable, must be powered by renewable sources such as wind power or photovoltaic.

At the moment, plants for producing hydrogen on a large scale are not yet competitive with traditional plants from a cost point of view. However, the expected fall in the cost of electrolyzers, the enormous progress in the efficiency of photovoltaic cells and wind generators and the consequent reduction in costs of kWh from renewable sources, are rapidly changing the scenario.

How do plants like this work? The heart is the electrolyzer or electrolytic cell, where the separation of water into its constituent elements – hydrogen and oxygen – occurs. The water is brought into contact with two electrodes, a positively charged anode and a negatively charged cathode. The electrical current detaches the molecules into H+ hydrogen ions and OH- hydroxide ions. At the cathode the hydrogen ions acquire electrons through a reduction reaction and become gaseous hydrogen. At the anode the hydroxide ions give off electrons through oxidization, leading to the formation of oxygen.

If the electrolytic cell is located in proximity to a renewable energy plant, part of the electricity production (for example, the electricity produced in excess of the capacity that can be fed onto the grid) can be used to power it. In this way the hydrogen produced serves as a chemical energy storage facility that can later provide the energy when required, either as a raw material in the process for steel production or as a fuel to provide high-temperature heat.

Benefits

Extremely efficient and does not produce emissions

  • Hydrogen is the fuel with the highest energy density: 1kg contains the same energy as 2.4 kg of methane or 2.8 kg of gasoline.
  • Thanks to the facility with which electrical energy can be converted into hydrogen, it is the most efficient energy carrier available to us for storing surplus electricity production from renewable sources
  • Another precious feature of hydrogen is the high conversion efficiency. In a car powered by hydrogen fuel cells, up to 60% of the chemical energy of the hydrogen is converted into motive power for the vehicle, while the mechanical yield of combustion engines using petrol or diesel varies from 20% to 35%.
  • Hydrogen is widely used in industry, as it’s easy both to store and transport, for example in pipes like those used for gas.
  • Hydrogen is the only fuel that, however it is used (whether in combustion engines or in fuel cells), does not produce polluting emissions, just water.
Did you know?

Hydrogen pipelines? Yes, but smart ones

An idea recently proposed to promote the capillary distribution of hydrogen is to take advantage of the pipelines already used to transport gas around the city. The idea consists of mixing hydrogen with methane to produce a blend to use in homes for heating or cooking.

A solution of this type does, however, pose two problems: firstly, it would reduce only in part the climate-altering emissions produced from burning methane (taking up 10% of the space of the tubes with hydrogen, the emissions would be reduced by only 1%). In this way many of the benefits of hydrogen would be lost. Secondly, there is also the problem of safety concerning the effective capacity to control the exact composition of the gas blend in all stretches of the network.

A solution that is more effective to promote the capillary distribution of hydrogen, however, could be to initially use short pipelines, to connect the green hydrogen production plants to final users in the vicinity. This would take advantage of all the benefits of this extraordinary energy carrier and thus significantly boost the decarbonization process. Having thus achieved a substantial reduction in the demand for gas, it would then be possible to consider extending these distribution islands to include a greater number of users, both domestic and otherwise.

Sources: for data on hydrogen 
https://www.sciencedirect.com/science/article/pii/S1364032119308275

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