DYNA JOURNAL ENGINEERING

There is as yet no common, accepted nomenclature, so that there may be some variations and a great deal of confusion in this respect. The reason for this designation of a fuel, considered as an energy vector of the future, is to be found in the more or less energy used in its production, from a "renewable" or "climate-friendly" point of view.
The particularity of hydrogen, which requires a significant amount of energy to obtain it, means that its use always results in a reduction in energy efficiency. Let us remember the figures: the optimal electrolysis of water requires between 40 and 50 kWh to obtain 1 kg of hydrogen, which needs about 15 kWh more to store it at transportable pressures. This kg of hydrogen contains a potential energy of less than 35 kWh, so we only obtain around 60% of the energy used for hydrolysis. This figure will decrease depending on the means used to exploit it: the use of hydrogen is a strong energy waster, only justified when it is surplus in non-regulated processes or for uses that do not allow other means, hence the convenience of classifying it according to its means of production.
Black or brown hydrogens, obtained by reforming coal or lignite with steam. It gives off CO2, other gases and particles.
Grey hydrogen, obtained by reforming methane (natural gas) with steam: this is the most common industrial method. It releases CO2.
Blue hydrogen, obtained by the same process from natural gas but with capture and storage of the CO2 released.
Pink hydrogen, obtained by electrolysis of water with energy generated in nuclear fission units. The purple or red variants are sometimes cited, depending on whether heat or various catalysts are used more intensively.
Turquoise hydrogen, recently proposed to be obtained by pyrolysis of methane leaving solid carbon as a residue.
Green hydrogen, obtained by electrolysis of water with electrical energy generated by renewable means. Sometimes, if this energy is solar, it has been called yellow.