An example in this sense is the space that is given in the media to hydrogen and the potential benefits for its use as an alternative fuel to petrol and diesel with a view to a path of decarbonisation of transport and operating machines envisaged between now and 2050.
In this space, on the other hand, an analysis of hydrogen linked, in the current state of technology , to objective advantages and disadvantages is proposed .
Builders and hydrogen
As far as hydrogen is concerned, in the first place, there is no doubt that there are several manufacturers who have started experiments and research in this sense and it is no doubt that the use of propulsion based on hydrogen is better suited than full electric to accommodate applications that require high autonomy , fast supplies and on-board storage that do not penalize tares and dimensions.
The first element of Dmitrij Ivanovich Mendeleev 's table responds well to these needs, also being detached from polluting emissions if used in fuel cells. Without forgetting the possibility of using it to power endothermic engines if suitably modified with a view to smooth transition from the endothermic to the fuel cells themselves. This is thanks to a highly exothermic reaction with oxygen induced by a high calorific value, twice as high as, for example, that of natural gas with the same mass. An optimal carrier therefore, as well as being available in large quantities and therefore perfect as a "carbon free" solution. But how is hydrogen produced?
Hydrogen: how it is produced
Unfortunately, as a well-known proverb says " There is no rose without thorns" and in this specific case the sharp points are constituted by the fact that in order to have this element , one must currently resort to technologies that have nothing "carbon free" resulting on the contrary strongly penalizing for the environment .
hydrogen: how is it produced?
In practice, it happens that hydrogen has to be "produced" as there are no deposits that allow its direct use . It is in fact an unstable element on a chemical level and which, as such, tends to bind quickly to other elements . Just think of water or the hydrocarbons themselves, which are nothing but more or less long chains of carbon and hydrogen atoms.
It follows that the only way to dispose of "pure" hydrogen is to extract it from molecules in which it is present, a process which however requires a consumption of energy in greater quantities than that which is then released using the hydrogen obtained as fuel.
Hydrogen for clean energy could be produced from seawater
Let's be clear, even the extraction and refining of fossil fuels requires energy , but the total balance between that absorbed and that yielded is more advantageous if analyzed in the context of the entire chain, i.e. from the production to the use of the element as an energy source for an engine. All the more reason if we consider that the production of hydrogen on an industrial level is now advanced using fossil fuels , either to give rise to gasification processes or to carry out "steam reforming" processes.
How hydrogen is produced: the production processes
In the first case, hydrogen is extracted from water by bringing it to a high temperature and making it react with coal or biomass, in the second case it is extracted directly from hydrocarbons , in most cases consisting of methane .
In both cases , therefore, fuels must be burned to produce a fuel, incurring non-secondary energy losses which give rise to efficiencies oscillating around 70 percent. In both cases, the processes also give rise to significant quantities of carbon monoxide and carbon dioxide , precisely those gases that a "carbon free" approach would like to ban.
The production processes linked to electrolysis and based on the use of electricity deriving from renewable sources to break the chemical bonds existing between the atoms of water molecules would be decidedly more sustainable . Such processes are already in use, but at the moment they do not allow for the production of large quantities of hydrogen.
Management issues
To further complicate and penalize the possibility of using the element there are also the problems induced by the needs of storage and transport, two management aspects that give rise to further losses in yield in the well-to-wheel process. Although the energy density per unit mass of hydrogen is high, 143 megajoules per kilo, in standard atmospheric conditions it has an extremely low density and therefore in order to have adequate reserves it is necessary to compress it through dedicated systems inside tanks that are equally dedicated.
To better highlight the terms of the problem, a comparison can be made with LPG, a gas which at ambient pressure has a density of two kilos per cubic meter and can be liquefied by subjecting it to a pressure of only ten bars, giving rise to stocks of 500 kilos per cubic metre, equal to energy storage with a density of 25 megajoules per litre. Hydrogen at ambient pressure instead has a density of 90 grams per cubic meter , about 20 times less than LPG, and in order to have an energy density of the order of five megajoules per litre, five times less than LPG under the conditions indicated above , it must be brought to pressures above 200 bar. For this reason it happens that in the refueling phase the hydrogen is also cooled, another process whichit requires energy and therefore reduces the final efficiency of the well-to-wheel chain.
Alternatively, to avoid compression, one can opt for liquefaction , which can however be obtained by bringing the fuel to temperatures of the order of 252 degrees Celsius below zero. In this case it is possible to reach energy densities of around ten megajoules per litre , less than half of those proposed by LPG. Apart from the energy needed to start the process, it happens that it is then necessary to keep the tank at such temperatures with significant technical difficulties.
Hydrogen: advantages and disadvantages
Assuming, however, that all the above problems are overcome and that usable hydrogen is available to produce useful work, the efficiency of the processes changes depending on whether it is burned in a heat engine or used inside a fuel cell to produce electricity to be stored and stabilized inside a battery before transferring it to electric motors .
According to the most up-to-date studies on the subject, it happens that the energy losses calculated starting from the sources and arriving at these batteries are just under 95 percent with greenhouse gas emissions of around 110 grams for each megajoule of energy produced or five tenths of gram per kilowatt hour. Discouraging data if one considers that the production of diesel makes it possible to store even more than 70 percent of the energy at the source with emissions of about 15 grams per megajoule of greenhouse gas emissions, about eight times lower than those of hydrogen, and equivalent five hundredths of a gram per watt hour.
However, in favor of hydrogen are the efficiencies of fuel cells which oscillate between 40 and 60 per cent and the higher yields of electric motors, between eighty and ninety per cent, compared to the latest generation diesels which travel over 40 percent.
Ultimately it happens that for now, and it is emphasized for now , the use of hydrogen as an energy vector in the transport segment , in the light of the existing production and storage problems, is still far from being functional for the reduction of gas emissions greenhouse.
