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.