Thermal Technology, Santa Rosa, Calif., has developed a process that produces solar-grade silicon using rice hulls as the raw material. Matt Mede, Thermal Technology’s president and CEO, presented his research at the Materials Challenges in Alternative & Renewable Energy Conference in Florida.

According to Mede, producers of high purity photovoltaic silicon are expanding production to meet growing market demands. New technologies coming online are intended to produce photovoltaic-grade silicon at lower cost.

“Advanced carbothermic reduction of SiO2 is a key enabler to the photovoltaic solar industry's quest for cost reduction,” Mede says.

Thermal Technology combined the process of carbothermic reduction with biomass pyrolysis technology to convert rice hulls to solar-grade silicon.

“Using the combination of pyrolysis and carbothermic reduction, it is possible to make material with sufficient purity for the solar application,” Mede says. “The pyrolysis makes the perfect blend of SiO2 and C suitable for the reduction process, with a morphology that facilitates cleaning to remove tramp elements.”

According to Mede, more than 95 percent of domestic solar-grade silicon (SG-Si) is produced using the Siemens process, which consumes up to 200 kWh/kg of SG Si. In contrast, when carbothermic reduction is combined with energy-producing pyrolysis, the energy requirement drops to 50 kWh/kg of SG Si.

Thermal Technology’s method of creating SG Si consumes up to four times less energy, reducing CO2 production as a result, and eliminates the methane produced when rice hulls are plowed into fields or taken to landfills.

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