Isothermal versus two-temperature solar thermochemical fuel synthesis: A comparative study

Metal-oxide based, two-step thermochemical cycling is a promising means of harvesting solar energy, in which water or carbon dioxide is dissociated to synthesize fuels via successive reduction and oxidation half-reactions driven by concentrated solar heat. Isothermal thermochemical cycling has recently emerged as an important special case of two-step solar TC with distinct advantages of easier heat recovery and potentially high efficiencies, and criteria for the design of oxide materials of isothermal thermochemical cycling are talked about recently. However, the pros and cons of isothermal versus two-temperature (i.e. two-T) TC are under debate. In this work, the two approaches are compared by exploring the influence of temperature, reduction pressure and thermodynamic properties of materials on solar-to-fuel efficiencies. Through the analysis, isothermal cycling is shown to work much better on CO2 splitting for easily reducible materials than two-T cycling; even for materials conventionally considered good for two-T cycling, isothermal cycling could still excel (over two-T cycling) under certain operating conditions. General principles for materials screening are also explored. There are many factors that favour isothermal thermochemical cycling over two-T cycling, such as high temperature, high heat recovery rate, small reduction enthalpy or splitting of CO2 instead of water. In addition, materials with large specific heat, which are unfavorable for two-T cycling, may be suitable for isothermal cycling. At high temperatures or with high heat recovery rates, isothermal thermochemical cycling exhibits excellent performance even for H2O splitting. The theoretical solar-to-fuel efficiency of ∼28% (for CO2 splitting at 1650°C and pO2=10−5 atm, without heat recovery) of isothermal cycling may indicate a meaningful route to effective solar thermochemical fuel production.