The future: the direct production of solar hydrogen

One of the main problems in a future sustainable energy scenario will be the large-scale production of energy at competitive costs and without the production of greenhouse gases, which will be served by transmission vectors such as electri­cal energy and hydrogen. Once the potential of ‘energy transfer’ through electrical interconnection is used up, the new hydrogen vector will allow transferring, over long distances, the potential of the primary sources from the production areas to the consumption areas, as it currently happens for the fossil fuels. Obtaining large quantities of energy to vector as hydrogen without the emission of greenhouse gases means using water as a raw material and as a primary energy source, which does not produce greenhouse gas. The hydrogen production from solar concentra­tion systems, by means of thermochemical processes at high temperatures, prom­ises achieving high earnings in terms of conversion, which are necessary for the process effectiveness. In fact, if presently the hydrogen production by electrolysis is the more mature process to obtain hydrogen from solar source, this process is characterized by a global yield (from hydrogen energetic content radiant energy, passing through the collection and radiation concentration, the conversion into electricity and electrolysis) of the order of 27%. Using photovoltaic conversion for electricity production, followed by electrolysis of water, we do not obtain higher yields, but we typically reach a global yield of the order of 12%. Except the costs, which are currently hard to evaluate, from the energetic point of view they are more useful than those methods in which the heat solar conversion in hydrogen happens in a direct way, based on the scheme represented in Fig. 97; in this way, theoretically it is possible to obtain global conversion yields of the order of 46%.

The thermochemical cycles, comprising oxidation-reduction reaction series that involve different natural intermediate substances, allow the cleavage of the water into hydrogen and oxygen starting from relatively elevated temperatures of heat (800-1500°C), but in solar concentration systems these temperatures are, however, achieved using high concentration systems such as towers or parabolic disk systems. This typology of the process is known since the 1970s, but only dur­ing the last few years it has become the object of renewed interest, driven more and more by the impelling environmental problems. The possibility to thermally feed such cycles by solar energy makes these production systems completely renewable and so perfectly compatible with a sustainable development strategy [45].

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