Theory of Direct Solar Thermal Hydrogen Generation

In the high temperature gas phase equilibrium of water, in addition to H2O, H2 and O2, the atomic components H and O must be considered. These components are relatively insignificant at temperatures below 2500 K, as the pressure equilibrium constants for either diatomic hydrogen or oxygen formation from their atoms are each greater than 103 at T < 2500 K. However, the atomic components become increasingly significant at higher temperatures. The pressure equilibrium constants of the water dissociation reaction are summarized in Table 1 for water splitting at temperatures at which significant, spontaneous formation of H2 occurs:

Kogan has calculated that, at a pressure of 0.05 bar, water dissociation is barely discernible at 2000 K.1 By increasing the temperature to 2500 K, 25% of water vapor dissociates at the same pressure. A further increase in temperature to 2800 K under constant pressure causes 55% of the vapor to dissociate.1 These basic facts indicate the difficulties that must be overcome in the development of a practical hydrogen production by solar thermal water splitting:

(a) attainment of very high solar reactor temperatures,

(b) solution of the materials problems connected with the construction of a reactor that can contain the water spitting products at the reaction temperature, and

(c) development of an effective method for in situ separation of hydrogen from the mixture of water splitting products.

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