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Two-Step Thermochemical Cycles for High-Temperature Solar Hydrogen Production

This paper reviews two-step water splitting cycles by redox iron-based metal oxides to produce hydrogen utilizing concentrated solar high-temperature heat as energy source. The concentrated solar radiation is focused upon a windowed solar reactor where maximum temperatures can exceed 1500 ̊C in the greatly insolated “sun-belt” regions such as the South-western United States, southern Europe, north Australia, etc. The concentrated solar high-temperature heat has the potential to produce hydrogen from water thermochemically. A two-step thermochemical water splitting by iron-oxide redox pair is one of the most promising cycle. However, the cycle requires heat above 1500 K minimum. This provides a challenge to “windowed” solar reactor concepts, such as a monolithic device reactor, a foam-device reactor, a Counter-Rotating-Ring device reactor, and an internally-circulating fluidized bed reactor. In the reactors, the kinetics of the reactions must be an important key factor. Thus, further development of reactive redox materials and devices will have a very large impact on the improvement of these reactors. This paper reviews the working redox materials, the reactive devices and the reactors developed for the two-step thermochemical water splitting cycle by a redox iron-based oxide.