Materials Processing

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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