A strategy for optimizing efficiencies of solar thermochemical fuel production based on nonstoichiometric oxides

Thermochemical cycling (TC) is a promising means of harvesting solar energy. Two-step TC with a redox active metal oxide (e.g., ceria, a benchmark material) serving as a reaction intermediate for dissociating steam or carbon dioxide, has attracted much attention recently. However, further improving the energy conversion efficiency of this process remains a major challenge. In this work, we propose an innovative modification to the heat recovery approach as a means of enhancing efficiency. Specifically, a variable amount of oxidant (e.g., steam) is injected to actively assist the cooling of thermally reduced metal oxide, achieving both in-situ heat recovery and potentially faster cooling rates than conventional approaches. Our analysis, based on a thermochemical heat engine model, shows that the solar-to-fuel efficiency using ceria under typical solar TC operating conditions could be significantly improved (the efficiency of the new strategy can reach 24.36% without further gas or solid heat recovery when the reduction temperature is 1600 °C) whilst temperature swing be reduced simultaneously compared with conventional methods. Exergy efficiency is also analyzed for thermochemical splitting of water and CO₂. This new strategy contributes significantly to the simplification of solar reactor design and to potential enhancement in both fuel productivity and energy conversion efficiency on a temporal basis.