Hydrogen : from production to storage

Molecular hydrogen_H2 (which is practically non-existent in nature, but whose combustion energy is 3 times greater than that of a liter of petrol) is seen as a possible energy carrier, particularly for the development of renewable energies that do not emit_CO2. But we still need to know how to produce, transport and store it properly. This lecture attempts to answer this question by giving a panoramic view of production/transportation and storage processes in terms of principle and technological maturity, in order to better highlight the associated difficulties and challenges.

Hydrogen is produced from abundant hydrogen sources on earth, such as fossil fuels, biomass and water: 40% from natural gas, 30% from oil, 18% from coal and 4% from water electrolysis. It should be noted that the production of_H2 from fossil fuels via steam-reforming or partial oxidation processes, although leading to yields of 60-70%, is associated with massive_CO2 emissions. The same applies to thermochemical conversion of biomass for_H2 production. On the other hand, for identical yields, the production of_H2 from water is less polluting, on one condition: that the primary electricity used is clean (i.e. from nuclear power plants, not thermal power plants). Nevertheless, it is obtaining_H2 from_H2Ovia photoelectrolysis or photosynthesis, or even artificial photosynthesis, that is the most attractive in terms of sustainable development. However, in this case, yields are still too low, which justifies the intensity of current research in these fields.

The hydrogen produced is transported by pipeline or, more commonly, in liquid or compressed form, hence the importance of storing it. This is done either by conventional means (compressed gas and cryogenic liquid), or by solid-state storage via adsorption (physical process) or absorption (chemical process). A review of these different forms of storage reveals their advantages and disadvantages in terms of energy losses, storage safety and convenience. As a result, the choice of a system is not universal, but depends on the application: a stationary application will be less constrained by the weight of the system, space and aeronautical applications will be less constrained by economic cost, and electric vehicle applications will be more constrained by volume. Metal hydrides are highly coveted as a solution to these volume problems, but once again, the lack of suitable materials is delaying their use.

We can't conclude this presentation without mentioning the risk factor, which is also a source of stagnation in the development of hydrogen technologies. Hence the importance of demonstration projects to convince the public and public authorities of the safety of_H2.