The hydrogen used for the production of energy is gas molecule hydrogen, identified with the formula H2. It is a molecule that has been known for over 200 years, and when it burns it frees energy, producing only water. However on the Earth there are no molecule hydrogen deposits, as in the case of fossil fuels. It is a molecule that is found in great quantities in nature, but only in combination with other atoms to form, for example, water or methane. Due to the simple fact that on the Earth there are none, hydrogen is not a primary source of energy, in order to use it, it must be produced, using energy. In any case, only when it will be produced in a sufficiently economic manner will it be used as an energy vector, and only after having solved the problems tied to the fact that it is a gas that is difficult to transport, store and use.
Natural hydrogen is a colourless, odourless and non-toxic gas. It is very light, 14.4 times lighter than the air. Consequently, natural hydrogen alone cannot be found on the Earth since it disperses into the outer space. Researchers say that hydrogen only represents 0.9% of the Earth’s crust components. It can be found in elementary state in volcanic emissions, fumaroles, and oil springs. However, it is present combined with other elements in many compounds such as water, mineral substances, hydrocarbons such as oil and methane, coal, animal and vegetal organisms, and organic substances. Therefore, if natural hydrogen is to be found, it will be necessary to consume much energy to extract it from the substances that contain it. This is why hydrogen is not a primary energy source, but an “energy carrier”, i.e. a form of energy that is not naturally available (as the case is with natural gas, oil or coal).
In the gaseous state it is a good fuel, albeit less dense than natural gas. When it is burnt, it produces a quantity of heat expressed in Joules per kilogram 2.6 times greater than the energy produced by burning natural gas. Another feature is that it tends to form hydrides, i.e. solid compounds, when it gets in contact with most of elementary metals, thus making them more fragile.
If it is cooled at a temperature of –253°C, hydrogen becomes liquid and in that state it does not react chemically with metals. This is the reason why it is difficult, in the gaseous state, to use pipelines to transport it, whereas it is easier to transport it when it is liquid.
Production from fossil sources
The technologies for the production of hydrogen from fossil fuels are mature and widely used, even though they should be improved from an economic, energetic and environmental point of view.
These processes are bases on the production of hydrogen through different refining stages and fractioning of hydrocarbon molecules until the complete elimination of carbon is obtained.
Production from biomass
With regard to the production of hydrogen from biomass, no process has yet reached a mature level from an industrial point of view. One of the most used techniques to obtain hydrogen from biomass is pyrolysis, that is a process based on thermal decomposition, which breaks complex molecules of organic substances into simple and separate elements.
Production from water
Hydrogen can be produced from water by splitting the molecule into its components (hydrogen and oxygen) through different processes, among which the most consolidated is electrolysis.
Electrolysis is the scission of water through the use of electric energy according to the following reaction: water plus electric energy equal hydrogen plus oxygen.
A bit of history
The use of hydrogen as fuel was already known around the 1950s. At that time in big Italian cities, and still today in some European cities, the so-called “city-gas” was distributed for household heating purposes. That gas was a mixture of hydrogen (approximately 50%) and carbon oxide, achieved through the reaction of carbon and water steam.
Hydrogen not only needs to be produced, but it also needs to be transported and stored in the place of consumption. Such activities are particularly difficult due to the characteristics of hydrogen. It is flammable, has a low density and is easily dispersed into the air. Preservation and transport still make a widespread use of hydrogen quite difficult. In the last few years different storage systems have become more and more efficient and suitable for many applications.
Hydrogen can be built-up and transported in gaseous, liquid state or absorbed by special materials. Each way has some pros and cons. Anyhow it is necessary to make significant progress in terms of R&D in order to make hydrogen more reliable and economically competitive (i.e. to build a suitable network for car supply).
Storage of compressed hydrogen
The easiest and most economical way to accumulate hydrogen, and use it, is in the form of compressed gas at a pressure of 200-250 bar (and over). Tanks containing compressed gas are the simplest mode of transportation, however this method is limited by the fact that hydrogen needs very large containers, up to three times the size of the tanks used for methane and ten times the size of those used for petrol.
Storage of liquid hydrogen
In order not to use big containers, it is possible to make use of liquid hydrogen, which occupies less volume than methane. But also this method causes some difficulties: hydrogen becomes liquid at –253°C and in order to keep it in this state special tanks and lots of energy are needed.
Other technologies exploit the ability of hydrogen to bind to chemical compounds or metals in order to facilitate its storage and transport. Hydrogen can chemically bind to different metals and metal alloys by forming hydrides, i.e. compounds that are able to trap the hydrogen at relatively low pressures (the gas penetrates inside the crystal lattice of the metal) and release it at high temperatures.
Storage in carbon nanostructures
An extremely recent experimental technology for hydrogen accumulation consists of using carbon nanostructures (carbon nanotubes and nanofibres), i.e. microscopic structures made of carbon fibres inside which it is possible to store a certain quantity of hydrogen.
According to the quantities, hydrogen can be transported through tankers and hydrogen pipelines. The two possibilities significantly differ in costs and therefore only technical-economic assessments can determine the best solution for each case. The long experience in the gas sector can be directly used to create hydrogen distribution networks, which are quite similar to natural gas existing networks.