(254b) Solar Thermochemical Production of Solar-Grade Silicon: Thermodynamic and Economic Analyses

Solar thermochemical production of solar-grade silicon:
thermodynamic and economic analyses

Ronald
Michalsky ¹, Peter H. Pfromm ¹, Bryon Parman
², Vincent Amanor-Boadu ²

¹ Department of Chemical Engineering, ²
Department of Agricultural Economics,

Kansas State
University, Manhattan, Kansas, USA

Availability of
low cost solar-grade silicon will improve the sustainability of electricity
generation from photovoltaic thin-film-Si cells. A precursor for manufacturing
solar-grade Si, polycrystalline Si is conventionally produced via carbothermal
reduction of silica above 1700 °C in an electric arc furnace with carbon
electrodes. Besides the use of electricity as a costly energy source, the
process emits fossil CO₂ at least equimolar to the amount of Si
produced. Sustainability would be greatly improved if the release of fossil
carbon as CO₂ indirectly due to electricity generation and directly
from silicon reduction could be reduced or eliminated.

Solar-thermal H₂
production via H₂O hydrolysis oxidizing a Zn reactant and sequential
ZnO thermal dissociation at up to 2000 °C using concentrated solar radiation is
a well studied solar thermochemical approach. Materials such as Al, Si₃N₄
and SiC have been produced from their metal ores via solar carbothermal
reduction.

To reduce the
energy expenses during Si production and to avoid associated CO₂
emissions, this work proposes solar thermochemical reduction of SiO₂
at high temperatures using sulfur as reducing agent. Production of elemental
sulfur is outpacing demand in the marketplace raising issues with sulfur
disposal. Based on Gibbs free energy computations, a process sequence for
producing polycrystalline Si and sulfuric acid from SiO₂, elemental
sulfur, H₂, air and water at near ambient pressure and high
temperatures is proposed here. Technical considerations such as temperature requirements
and the avoidance of the cumbersome handling and mixing of two solids will be
discussed. To assess sustainability and economic competitiveness the mass and
energy balance for the conceptual process will be presented along with an economic
analysis determining the price of sulfur-based solar-grade Si required for
breaking even with plant investment and operational costs.