Country-specific carbon footprint and cumulative energy demand of metallurgical grade silicon production for silicon photovoltaics

Abstract

The photovoltaic (PV) industry requires high-quality silica sand to produce metallurgical-grade silicon (MG-Si) for silicon PV (Si PV). However, high-quality deposits are scarce, and using lower-quality resources may increase the carbon footprint and cumulative energy demand (CED) of Si PV modules. The environmental impact of quartz mining and silica sand extraction for PV has not been updated in over 15 years. It's not representative of current methods used for low-quality sand and is not country-specific. As a result, PV production's environmental impact might be underestimated. We used life-cycle assessment to evaluate the carbon footprint and CED of quartz mining, silica sand extraction, and MG-Si production for high-quality (> 98% silica), industrial-grade (95% silica), and low-quality (65% silica) quartz deposits, which are necessary to calculate Si PV's energy and carbon payback time. The carbon footprint per metric ton of silica sand extraction increased from 22.7 kg CO2eq for high-quality quartz to 47.9 for industrial-grade and 86.7 for low-quality. China currently uses foreign industrial-grade quartz but plans to use low-quality domestic resources, which could increase the carbon footprint of producing one kg MG-Si from 12.1 to 16.5 kg CO2eq. The CED could increase from 188 to 286 MJ. We also investigated illegal mining's environmental impact. The carbon footprint and CED of MG-Si production decreased by 26 to 60% for illegal mines compared to legal mines. Illegal MG-Si production can be cheaper and have a lower carbon footprint, and therefore, third-party-supply-chain verifications are essential to ensure that PV doesn't contribute to the problem.