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

Abstract

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.