A critical perspective and analysis of two-step thermochemical fuel production cycles

Two-step thermochemical fuel production cycles powered using concentrating solar systems offer a route to convert solar energy to chemical fuels. In this work, we offer a critical assessment of the state of the art, a detailed technical analysis of this technology in terms of theoretical limitations and potential performance, and potential paths forward in the development of these processes. The state of the art for demonstrated reactor systems is analyzed using key performance indicators including energy efficiency, feedstock conversion extent, power output, and volumetric power density. The technical analysis first looks into the theoretical limitations on the cycles’ process conditions and the role of the redox material. This is followed by a detailed thermodynamic analysis of the state-of-the-art ${\text{CeO}}_2$-based cycle, based on fixed bed mixed flow reactors, which closely represent the reactor designs used in demonstrations. Finally, a scale-up analysis is performed for the ${\text{CeO}}_2$-based cycle. The results from the theoretical analysis agree well with trends seen in experimental demonstrations of the concept. From the analysis, the low power density of the ${\text{CeO}}_2$-based cycle is highlighted as a critical design limitation that will seriously restrict further scale-up of this technology. We share perspective on this and other issues, and offer some outlook for future development.