Solar energy material processing involves affecting the chemical conversion of materials by their direct exposure to concentrated solar energy. For this purpose, we use solar furnaces made of high-concentration, hence, high-temperature, collectors of the parabolic dish or heliostat type. Solar energy can also assist in the processing of energy-intensive, high-temperature materials, as in the production of solar aluminum, the manufacture of which is one of the most energy-intensive processes. It also includes applications related to the production of high-added-value products, such as fullerenes, which are large carbon molecules with major potential in commercial applications in semi- and superconductors, to commodity products such as cement (Norton, 2001). None of these processes, however, has achieved large-scale commercial adoption. Some pilot systems are described briefly here.
A solar thermochemical process developed by Steinfeld et al. (1996) combines the reduction of zinc oxide with reforming of natural gas, leading to the co-production of zinc, hydrogen, and carbon monoxide. At equilibrium, chemical composition in a blackbody solar reactor operated at a temperature of about 1000°C, atmospheric pressure and solar concentration of 2000, efficiencies between 0.4 and 0.65 have been obtained, depending on product heat recovery. A 5 kW solar chemical reactor was employed to demonstrate this technology in a high-flux solar furnace. Particles of zinc oxide were introduced continuously in a vortex flow, and natural gas contained within a solar cavity receiver was exposed to concentrated insolation from a heliostat field. The zinc oxide particles are exposed directly to the high radiative flux, avoiding the efficiency penalty and cost of heat exchangers.
A 2 kW concentrating solar furnace was used to study the thermal decomposition of titanium dioxide at temperatures of 2000-2500°C in an argon atmosphere (Palumbo et al., 1995). The decomposition rate was limited by the rate at which oxygen diffuses from the liquid-gas interface. It was shown that this rate is accurately predicted by a numerical model, which couples the equations of chemical equilibrium and steady-state mass transfer (Palumbo et al., 1995).
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