Cavity solar reactors (Fig. 20.3) were introduced in the work of Trombe et al. (1973), for melting of oxides, Flamant et al. (1980) for calcite decomposition, and Steinfeld et al. (1998b) and Haueter et al. (1999) for oxide reduction. As the name suggests, concentrated radiation enters a cavity aperture via a window. In Steinfeld et al. (1998b), a 5 kW continuous-feed, ‘vortex’ cavity reactor was designed for co-producing Zn and syngas, while in Haueter et al. (1999), a 10 kW rotating-cavity reactor, similar to the aforementioned design, was used for the decomposition of zinc oxide.