(559c) An Efficient and Scalable Approach to Solar Thermochemical Fuel Production Via an Iron Aluminate-Based, Pressure-Swing Redox Cycle

Technologies like solar thermochemical hydrogen (STCH) production, which leverage the oxygen-exchange properties of metal oxides to split water (and/or carbon dioxide), are attractive because they can harness the entire solar spectrum [1], whereas photovoltaic-driven alternatives (e.g., water electrolysis) must rely only on specific wavelength ranges. Generally, this approach involves alternating between two steps namely, (1) the endothermic removal of oxygen from an oxide and (2) the exothermic oxidation of the reduced oxide with water (and/or carbon dioxide) to produce hydrogen (and/or carbon monoxide). The endo/exothermic nature of each reaction implies that opposite changes in temperature are required to drive the cyclic conversion of, for example, water into hydrogen. However, in practice, implementing a temperature swing between reduction-oxidation (redox) regimes imposes significant irreversibilities that arise from sensible heating of the solid, thus resulting in inefficient operation unless the heat rejected during cooling is recovered via ultra-high temperature thermal energy storage – the feasibility of which continues to remain uncertain. Therefore, in this work, a scalable fluidized bed reactor – situated within an extremely well-insulated cavity receiver – was designed and operated such that the oxidation (or fuel producing) step was intentionally initiated at temperatures near where reduction is favorable, thereby avoiding the complications associated with solid-solid heat recuperation. To mitigate the undesired thermodynamic consequence of performing the exothermic oxidation reaction at such higher temperatures, iron aluminates, a low-cost class of materials with an exceptional isothermal (and near-isothermal) capacity for fuel production [2], were considered for this prototype demonstration. To further boost performance, the oxidation step was additionally performed at elevated pressures, as doing so has recently been shown to improve both the equilibrium extent and rate of the reaction [3], despite it being equimolar. Our state-of-the-art experimental results, obtained under concentrated radiation from a 45 kW_e high-flux solar simulator, provide compelling motivation for the commercial viability of solar thermochemical routes for the production of fuel.

[1] Warren, K. J., & Weimer, A. W. (2022). Solar thermochemical fuels: Present status and future prospects. Solar Compass, 1, 100010. https://doi.org/10.1016/j.solcom.2022.100010

[2] Warren, K. J., Tran, J. T., & Weimer, A. W. (2022). A thermochemical study of iron aluminate-based materials: A preferred class for isothermal water splitting. Energy & Environmental Science, 15(2), 806-821. https://doi.org/10.1039/D1EE02679H

[3] Tran, J. T., Warren, K. J., Mejic, D., Anderson, R. L., Jones, L., Hauschulz, D. S., Wilson, C., & Weimer, A. W. (2023). Pressure-enhanced performance of metal oxides for thermochemical water and carbon dioxide splitting. Joule, 7(8), 1759-1768. https://doi.org/10.1016/j.joule.2023.07.016