Cascading pressure thermal reduction for efficient solar fuel production

•Vacuum-pumped thermal reduction analyzed in two-step solar fuel production.•Novel approach examined—a cascade of reduction chambers at diminishing pressure.•Over an order of magnitude pressure decrease possible using cascade approach.•Decreased reduction pressure results in substantial solar-to-fuel efficiency gains.•A moving packed particle bed reactor is ideally compatible with the cascade concept.

Efficient two-step solar-thermochemical fuel production requires vacuum pumping or inert gas sweeping to lower the oxygen pressure in the thermal reduction step. Pumping is hampered by large oxygen volumetric flows, whereas sweeping is energy-intensive, requiring heat recovery at high temperature, and a dedicated inert gas purification plant. A novel pumping approach—using a cascade of chambers at successively lower pressures—is analyzed and shown to lead to over an order of magnitude pressure decrease compared to a single-chambered design. The resulting efficiency gains are substantial, and represent an important step toward practical and efficient solar fuel production on a large scale.

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