Thermodynamic and efficiency analysis of solar thermochemical water splitting using Ce–Zr mixtures

Abstract

We demonstrate the evaluation of the solar-to-fuel conversion efficiency of ceria-zirconia mixtures in a solar thermochemical water splitting system. Energy balance calculations were performed to study the effect of different process conditions on the thermodynamic efficiency of the system. Zirconium-doped ceria showed an enhanced efficiency compared to pure ceria when used in thermochemical water splitting system. A significant enhancement in solar-to-fuel efficiency was shown in case of isothermal redox cycles, at temperature approaches 1800K with 90% gas heat recovery efficiency, 5% Zr-doped ceria gives an efficiency of 0.032% compared to an efficiency of 0.005% given by pure ceria. However, negative effects were observed upon their use in two-temperature redox cycles at normal oxidation temperatures (900–1200K). Higher or lower oxidation temperatures resulted in significant enhancement in the conversion efficiency. At oxidation temperature approaching 1600K and reduction temperature approaching 1773K with 90% and 80% gas and solid heat recovery efficiency respectively, an efficiency of 0.762% is obtained upon using 5% Zr-doped ceria compared to an efficiency of 0.34% when using pure ceria. At oxidation temperature of 600K and at the same reduction temperature and heat recovery efficiencies to the previous case, an efficiency of 6.4% is obtained upon using 15% Zr-doped ceria compared to an efficiency of 5.2% when using pure ceria. The optimum conditions for operating a thermochemical water splitting reactor using 5%, 10%, 15% and 20% Zr-doped ceria were investigated and identified.