Solar thermal collectors and applications
Solar furnaces
Solar furnaces are made of high concentration and thus high temperature collectors of the parabolic dish and heliostat type. They are primarily used for material processing. Solar material processing involves affecting the chemical conversion of materials by their direct exposure to concentrated solar energy. A diverse range of approaches are being researched for applications related to high added-value products such as fullerenes, large carbon molecules with major potential commercial applications in semiconductors and superconductors, to commodity products such as cement [172]. None of these processes however, have achieved large-scale commercial adoption. Some pilot systems are shortly described here.
A solar thermochemical process has been developed by Steinfeld et al. [173] which combines the reduction of zinc oxide with reforming of natural gas leading to the co-production of zinc, hydrogen and carbon monoxide. At the equilibrium chemical composition in a black-body solar reactor operated at a temperature of 1250 K at atmospheric pressure with solar concentration of 2000, efficiencies between 0.4 and 0.65 have been found, depending on product heat recovery. A 5 kW solar chemical reactor has been employed to demonstrate this technology in a high-flux solar furnace. Particles of zinc oxide were introduced continuously in a vortex flow natural gas contained within a solar cavity receiver exposed to concentrated insolation from a heliostat field. The zinc oxide particles are exposed directly to the high radiative flux avoiding the inefficiencies and cost of heat exchangers.
A 2 kW concentrating solar furnace has been used to study the thermal decomposition of titanium dioxide at temperatures of 2300-2800 K in an argon atmosphere [174]. 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 [174].
