Pilot-scale solar reactor operation and characterization for fuel production via the Zn/ZnO thermochemical cycle

Abstract

Successful demonstration and promising characterization of a solar reactor pilot plant for thermal reduction of ZnO as part of a two-step water and CO2 splitting cycle has been accomplished at the 100kWth scale in a 1MW solar furnace. The solar reactor pilot plant was operated for over 97h and achieved sustained reaction temperatures well above 2000K, while demonstrating ZnO dissociation rates as high as 28g/min totaling over 28kg of processed reactant during 13 full days of experimentation. In-situ, high temperature, flow visualization of the quartz window enabled the unimpeded operation of the solar reactor. As many as three consecutive full day experiments were conducted without complication. Solar power delivered to the reaction cavity ranged between 90 and 128kWth, at peak solar concentrations as high as 4671kW/m2. The products Zn and O2 were quenched with Ar(g) and recovered in a filter battery, where collected particles contained molar Zn-content as high as 44%. During experimentation, switching between product collection filter cartridges resulted in 54 unique experiments, where a maximum solar-to-chemical efficiency of 3% was recorded for the solar reactor. Robust characterization of the product quenching device revealed inherent limitations in its effectiveness, and thus solar-to-fuel energy conversion efficiency was limited to 0.24% if it would have been possible to supply 4640Ln/min of Ar(g). Further, only a limitation on available experimental time prohibited the demonstration of significantly higher dissociation rates, achievable with higher ZnO reactant feed rates. While the use of large volumes of quenching Ar(g) to separate the reaction products remains a significant obstacle to achieving higher solar-to-fuel efficiencies, demonstration of solar reactor technology at the pilot-scale represents significant progress toward the realization of industrial-scale solar fuels production.