Image: Fraunhofer ISE
Each year, around 10,000 tons of silicon from discarded PV modules end up on Germany’s recycling market. This number will increase to several hundred thousand tonnes per year by 2029. The aluminum, copper, and glass of the discarded modules can be reprocessed, but the silicon solar cells cannot. In order to be able to reuse the silicon, researchers from the Fraunhofer Center for Silicon Photovoltaics CSP and the Fraunhofer Institute for Solar Energy Systems ISE, together with German PV module recycling company Reiling GmbH & Co. KG, have developed a solution, in which the silicon in the discarded modules was recycled on an industrial scale and reused to produce new PERC solar cells.
The majority of German PV systems were installed during the first wave PV expansion in 2009.
“This expansion will foreseeably be followed by a first wave of disposal twenty years later, around 2029, when the feed-in tariff for the installed PV modules expires,” explains Prof. Dr. Andreas Bett, institute director of Fraunhofer ISE. “Therefore, it is necessary to establish adequate processes and procedures for recovering the silicon material from the discarded modules at an early stage.” Already in 2021, the total installed quantity of PV modules in Germany was about five million tons, with a silicon content of 150,000 tons. As a semiconductor material, silicon is the main component of solar cells.”
A working group at Fraunhofer CSP, together with Reiling GmbH & Co. KG, has therefore developed a process for recovering the silicon material with funding from the German Federal Ministry for Economic Affairs and Climate BMWK (formerly BMWi). This process can be used to recycle all crystalline solar PV modules, regardless if they are made by the same manufacturer.
“If this were not the case, then this would be far too much work for the recycling companies,” explains Prof. Dr. Peter Dold, project manager at Fraunhofer CSP. “It was important for us to develop a scalable process that makes economic sense. A lot is possible in the lab, but our new process should prove itself in the practice for the recycling industry.”
Solar cell fragments are separated from the by-products of the mechanical recycle process. This is done to speed up the process. Fraunhofer CSP first removes cell fragments ranging in size from 0.1 to 1 mm from glass and plastic using various sorting processes. The backside contact, silver contacts, anti-reflective and emitter are then removed step by step by Fraunhofer CSP. The silicon cleaned in this way is processed into monocrystalline or quasi-monocrystalline ingots in standard processes and then into wafers.
Crystallization takes place with 100% recycled silica without the addition commercial ultrapure. The wafers made of recycled silicon were fabricated into PERC solar cells at Fraunhofer ISE’s PV-TEC. The first trial showed a conversion efficiency of 19.7%.
“This is below the efficiency of today’s premium PERC solar cells, which have an efficiency of around 22.2 percent, but it is certainly above that of the solar cells in the old, discarded modules,” says Dold.