Efficiency of two-step solar thermochemical non-stoichiometric redox cycles with heat recovery

Improvements in the effectiveness of solid phase heat recovery and in the thermodynamic properties of metal oxides are the most important paths to achieving unprecedented thermal efficiencies of 10% and higher in non-stoichiometric solar redox reactors. In this paper, the impact of solid and gas phase heat recovery on the efficiency of a non-stoichiometric cerium dioxide-based H2O/CO2 splitting cycle realized in a solar-driven reactor are evaluated in a parametric thermodynamic analysis. Application of solid phase heat recovery to the cycling metal oxide allows for lower reduction zone operating temperatures, simplifying reactor design. An optimum temperature for metal oxide reduction results from two competing phenomena as the reduction temperature is increased: increasing re-radiation losses from the reactor aperture and decreasing heat loss due to imperfect solid phase heat recovery. Additionally, solid phase heat recovery increases the efficiency gains made possible by gas phase heat recovery.

► Both solid and gas phase heat recovery are essential to achieve high thermal efficiency in non-stoichiometric ceria-based solar redox reactors.
► Solid phase heat recovery allows for lower reduction temperatures and increases the gains made possible by gas phase heat recovery.
► The optimum reduction temperature increases with increasing concentration ratio and decreasing solid phase heat recovery effectiveness.
► Even moderate levels of heat recovery dramatically improve reactor efficiency from 3.5% to 16%.