Session: 17-02: Symposium Steinfeld - Solar fuels via two-step cycles + the addition

Paper Number: 138490

138490 - An Efficient and Scalable Approach to Solar Thermochemical Fuel Production via an Iron Aluminate-Based, Pressure-Swing Redox Cycle 

Abstract: 

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

Presenting Author: Kent Warren University of Colorado Boulder

Presenting Author Biography: Kent Warren is a research associate in the Department of Chemical and Biological Engineering at the University of Colorado Boulder. He has a background in mechanical engineering, earning his BS in 2015 from Valparaiso University and his PhD in 2019 from the University of Florida. His research focuses on fundamental aspects of thermal and fluid sciences, chemical thermodynamics and kinetics, and solid-state ionics in relation to the development of commercially viable sustainable energy conversion and storage technologies. In particular, his undergraduate, graduate, and now postgraduate work has involved examining the use of concentrated solar energy as a source of high-quality heat for the production of value-added commodities. Warren also recently served as an Adjunct Professor in the College of Engineering at Valparaiso University.

Authors:

Kent Warren University of Colorado Boulder
Liam Taylor University of Colorado Boulder
Robert Anderson University of Colorado Boulder
Dragan Mejic University of Colorado Boulder
Justin Tran University of Colorado Boulder
Deepak Dileepkumar University of Colorado Boulder
Alan Weimer University of Colorado Boulder