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Efficiencies of electrolytic and thermochemical hydrogen production


Two methods not involving fossil fuels for manufacturing hydrogen have been suggested as a basis for an eventual hydrogen economy: first, electrolysis of water using nuclear (or ultimately solar, geothermal or thermonuclear) electricity and, second, thermochemical cycles using nuclear heat, in general from a high temperature reactor2,3. In the thermochemical processes, a series of chemical steps is envisaged that will allow the differential splitting of the H2O molecule by means of reactions at different temperatures and with appropriate thermodynamic requirements to give high efficiency. The sequence of reactions, ideally of gas–solid type, acts as an adsorption–desorption-cycle heat engine. For economic hydrogen production, high thermal efficiency combined with low capital investment cost is necessary. The question is whether a system constructed on the basis of a sequence of chemical reactions will be more efficient (and ultimately less costly) than one based on a heat engine producing electricity, followed by electrolysis. Here I shall examine briefly some aspects of the efficiency of both approaches.

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APPLEBY, A. Efficiencies of electrolytic and thermochemical hydrogen production. Nature 253, 257–258 (1975).

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