Design of a Hemispherical Reactor for Solar Thermochemical Cycling with Spectrally-selective Window for Efficiency Enhancement

Two-step solar thermochemical cycles (STC) for H₂O and CO₂ splitting is a promising technology to produce solar fuel. Reaching up to higher solar-to-fuel efficiencies (e.g., >10%) relies on not only the selection of efficient redox materials, but also devising effective solar reactors. We propose a hemispherical STC reactor design with spectral-selective window for solar flux homogenization and solar collection efficiency enhancement. The new geometry effectively increases the percentage of redox oxide between 1500-1700 °C from 89.8% (for a cylindrical geometry) to 97.6% at 15 kW irradiation. A thermodynamic model is developed, taking into account the geometry, reactions, conservations, spectral selectivity and all the major energy losses. Results show that for a typical ceria-based temperature-swing STC in the range of 900-1600 °C combined with inert gas purging and vacuum pumping, the spectral-selective transmissive coating significantly reduces the re-radiation loss by 69.56%. With solar concentration ratio of 5263, the reactor heating rate increases from 172 °C min^-1 to 194 °C min^-1, decreasing the incident solar energy demand from 703 J g^-1oxide to 621 J g^-1oxide. More importantly, the solar-to-fuel efficiency increases from 10.58% to 12.14%. Spectral selectivity lowers re-radiation loss (>70%) and effectively maintains high collector efficiency and solar-to-fuel efficiency at low concentration ratios (C<3000).