An international research team led by the U.S. Department of Energy’s National Renewable Energy Laboratory has emphasized the importance of the R&D effort aimed at recovering high-purity silicon from end-of-life solar modules.
The authors of the paper Research and development priorities for silicon photovoltaic module recycling to support a circular economy, published in Nature Energy, stressed the recovery and reuse of silicon should be prioritized. The researchers said current panel recycling efforts rarely recover silicon of the purity required for reuse in modules, with the situation exacerbated by cracks at the solar cell level. With today’s cells made from ever thinner, more fragile silicon wafers – and, therefore, more prone to cracking – introducing a direct silicon reuse strategy is becoming even harder.
“Silicon pure enough for producing solar cells, but in the form of whole cells or broken cell fragments, may not be immediately usable in processes designed for chunks of virgin polysilicon,” wrote the academics.
The value of silicon recovery, according to the research group, may be higher if the recycling process is able to deliver solar-grade silicon and not metallurgical-grade silicon, with the lower-purity silicon typically recovered from used panels at present worth around $2/kg whereas solar-grade material commands around $10/kg.
Recovering and reusing solar-grade silicon also significantly reduces the environmental impact of PV panels as the material accounts for around half the carbon footprint of devices. “Second, the current rates of recovery and re-use of solar grade Si is low and, therefore, have significant scope for improvement,” the NREL team stressed.
However, the industry has low tolerance of impurities in silicon intended for reuse and there is no exhaustive list of potential impurities, the researchers added. “Impurity control is a challenge throughout the supply chain,” wrote the authors of the paper.
Purification and crystal growth processes offer the potential to improve the purity of recovered silicon, stated the researchers, who added: “The challenge and research opportunity are in re-optimizing existing processes or developing new processes for the impurity profile and physical form of recovered silicon, all at reasonable cost.”
Although there is not sufficient volume of end-of-life panels to justify large scale recycling infrastructure, the researchers said, the R&D effort to improve recycling approaches should be undertaken now so the technology is in place when bigger quantities of solar modules begin to need replacement.
“The environmental and economic impacts of recycling practices should be explored with techno-economic analyses and life cycle assessments to optimize solutions and minimize trade-offs,” wrote the paper’s authors. “Another challenge today is scale, which translates to not enough modules reaching end of life to warrant investment in PV-specific recycling infrastructure and what we call integrated, high-value recycling systems,” research co-author Garvin Heath told pv magazine. “But R&D is needed to be ready once the scale is there, which is predictable and will come.“
More research and technological improvement is necessary, in particular, to develop systems-based analytical tools for recycling process designs which consider trade-offs among cost and revenue and life cycle assessment, stated the researchers, as well as for making recycling infrastructure flexible enough to handle an increasing variety of panels.
“In addition, a broader context – considering policy, logistics and data – must be addressed to create economically and environmentally robust c-Si recycling systems,” the academics added.
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