The thermodynamic implications of conducting the solar-combined ZnO reduction and CH4-reforming under stoichiometric and non-stoichiometric conditions are examined. For a solar flux concentration raton of 5000 and for a solar cavity-receiver operating at 1300 K, the solar thermal conversion efficiency is 55 percent for a stoiciometric molar ratio of ZnO and CH4, and decreases by 50 percent when using excess methane by a factor 10 over the stoichiometric molar amount. A technical solution for operating a gas-solid vortex-flow solar reactor under stoichiometric conditions was established by using a pulsed-feed of methane to carry out the particles of ZnO. Using this technique, nearly stoichiometric operation was demonstrated,vith a prototype reactor in a high-flux solar furnace, thereby opening up a means for efficient conversion of sunlight into chemical fuels.