An improved engineering design of a solar chemical reactor for the thermal dissociation of ZnO at above 2000K is presented. It features a rotating cavity receiver lined with ZnO particles that are held by centrifugal force. With this arrangement, ZnO is directly exposed to concentrated solar radiation and serves simultaneously the functions of radiant absorber, chemical reactant, and thermal insulator. The multilayer cylindrical cavity is made of sintered ZnO tiles placed on top of a porous 80%Al2O3–20%SiO2 insulation and reinforced by a 95%Al2O3–5%Y2O3 ceramic matrix composite, providing mechanical, chemical, and thermal stability and a diffusion barrier for product gases. 3D computational fluid dynamics was employed to determine the optimal flow configuration for an aerodynamic protection of the quartz window against condensable Zn(g). Experimentation was carried out at PSI’s high-flux solar simulator with a 10kW reactor prototype subjected to mean radiative heat fluxes over the aperture exceeding 3000suns (peak 5880suns). The reactor was operated in a transient ablation mode with semicontinuous feed cycles of ZnO particles, characterized by a rate of heat transfer—predominantly by radiation—to the layer of ZnO particles undergoing endothermic dissociation that proceeded faster than the rate of heat transfer—predominantly by conduction—through the cavity walls.
This article provides a comprehensive overview of the work to date on the two‑step solar H2O and/or CO2 splitting thermochemical cycles with Zn/ZnO redox reactions to produce H2 and/or CO, i.e., synthesis gas—the precursor to renewable liquid hydrocarbon fuels. The two-step cycle encompasses: (1) The endothermic dissociation of ZnO to Zn and O2 using concentrated solar energy as the source for high-temperature process heat; and (2) the non-solar exothermic oxidation of Zn with H2O/CO2 to generate H2/CO, respectively; the resulting ZnO is then recycled to the first step. An outline of the underlying science and the technological advances in solar reactor engineering is provided along with life cycle and economic analyses.
A novel high-flux solar simulator, capable of delivering over 50kW of radiative power at peak radiative fluxes exceeding 11,000 suns, is operational at the Paul Scherner Institute. It comprises an array of ten Xe arcs, each close-coupled with ellipsoidal specular reflectors of common focus. Its optical design, main engineering features, and operating performance are described. The Monte Carlo ray-tracing technique is applied to optimize the geometrical configuration for maximum source-to-target transfer efficiency of radiative power. Calorimeter measurements indicated an average flux of 6800kW∕m2 over a 60-mm-diameter circular target, which corresponds to stagnation temperatures above 3300K. This research facility simulates the radiation characteristics of highly concentrating solar systems and serves as an experimental platform for investigating the thermochemical processing of solar fuels and for testing advanced high-temperature materials.
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