Efficiencies of electrolytic and thermochemical hydrogen production

Abstract

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 reactor^2,3. 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.