Design of a hemispherical reactor with radiation regulation for efficient solar thermochemical fuel production

Solar thermochemical cycle (STC) for CO2 splitting offer a promising pathway for sustainable fuel production. The key performance indicator is solar-to-fuel efficiency, which is highly dependent on regulating incident solar radiation and minimizing re-radiation loss during the temperature swing of two-step solar thermochemical cycle. In this study, we propose a hemispherical fixed-bed reactor design incorporating a spectrally-selective transmissive window and a variable iris. This configuration shall facilitate rapid redox cycle with reduced energy losses, thereby enhancing solar-to-fuel efficiency, feedstock conversion, and power output. A comprehensive thermodynamic model is developed to evaluate the reactor's performance under practical operating limits, including low concentration ratios and fluctuating direct normal irradiance (DNI). For a typical ceria-based solar thermochemical cycle operating between 900 °C and 1600 °C, results show that the new window design could reduce re-radiation loss by 72.09 %, increasing solar-to-fuel efficiency from 11.09 % to 12.10 % under a 1700 nm cut-off wavelength and a 40 mm aperture radius (concentration ratio of 3500). The application of the spectrally-selective transmissive coating to quartz window would enable solar-to-fuel efficiencies exceeding 11 % at moderate concentration ratios (1000−3000). The selective coating would also reduce solar mirror area by 38.00 % at concentration ratio of ∼1400, lowering the total cost of solar concentrating system by 33.18 %. Furthermore, multi-objective optimization using NSGA-II identifies optimal trade-offs, achieving simultaneous enhancements in solar-to-fuel efficiency, feedstock conversion, and power output to approximately 11 %, 15 %, and 1 kW, respectively. This innovative reactor window design provides a viable strategy for achieving efficient and rapid solar thermochemical cycle under real on-sun scenarios.