Thesis title: RECOVERY OF SECONDARY RAW MATERIALS FROM EXHAUSTED PHOTOVOLTAIC PANELS
Due to the increasing demand for energy and the fact that photovoltaic (PV) systems produce renewable and clean energy, installed PV capacity is expected to increase significantly in the coming decades. As a result, both the consumption of raw materials required for the production of PV systems and the environmental impact of end-of-life (EoL) panels due to the presence of heavy metals will increase. Consequently, the recovery and reuse of materials such as copper, aluminium, silver, glass, silicon, and polymers from PV waste is very important to reduce the use of resources that are being depleted and also to promote the increased installation of solar power or the use of these materials in other production chains. The European Commission has included PV waste in the WEEE Directive (2012/19/EU), indicating the minimum targets for recovery (85%) and recycling (80%) of Waste Electrical and Electronic Equipment (WEEE). More than 90% of the installed PV modules are made with crystalline silicon (c-Si) cells, while the remaining 10% are manufactured with thin-film technologies of various kinds (mainly CdTe, amorphous Silicon and CIGS, copper indium gallium di- selenide). Currently, treatment techniques for recycling PV modules are in the pre-industrial experimental phase due to low economic viability. Indeed, large-scale experimentation is not yet possible due to the small volume of EoL PV panels sent for recycling. Thus, the treatment of EoL-PV panels is still an open field that requires much research. The EoL photovoltaic panel recycling process involves the steps of pre-treatment, shredding and material recovery by physical, thermal and chemical processes. A PV module consists of about 80% glass, 10% Al (frame), 5% silicon cells, 4% Tedlar (conductive backsheet), 1% of encapsulant (EVA polymer film), Cu and other components. By shredding the panel, the screened separation between the glass powder and other materials can be achieved. Plastics are rather difficult to recover because of the presence of metal components.
The thesis work was carried out in cooperation with the company Nike srl which has been recycling WEEE for a long time and also photovoltaic panels for the past few years. The company treats this waste using only physical processes to keep recycling costs and environmental impact low. However, the physical characterization carried out in this study on the materials obtained from the mechanical treatments employed by Nike revealed some critical issues to the point that this process in any case is not very profitable for the company since most of the materials that make up the panel are disposed of or burned. In an effort to ensure a greater economic return to the company while pursuing a low environmental impact of the process, the choice made in this work was to continue to use manual and physical operations to recover high-value materials while also introducing chemical operations. The process developed is based on a new mechanical treatment of which three different variants (referred to as method S, method B and method F) were studied to determine which was the best performing. The new process involved manual removal of the junction box and cables (2% w/w PV) and removal of the frame with a framing machine and a blade crusher to obtain the aluminium fraction. The newly developed mechanical operation allowed complete removal of the backsheet (called backsheet fraction, 4.7% w/w PV) and obtained both a glass fraction with EVA (polyethylene-vinyl acetate) fragments and a powder (cell fraction) composed of EVA, cells, and electrical contacts. The latter fraction was sieved (Ø < 1 mm) to separate the electrical contacts from the rest of the components. Removal of EVA from the glassy fraction, using a tumbling machine, resulted in the recovery of an easily marketable high quality pure glass (73% w/w PV). In addition, two other important fractions were produced that can be valorised: the cell fraction and the backsheet fraction. The cell fraction included 5.8% EVA, 3.8% Si, and all the metals used to make the cell (Al, Ag, Zn, Pb, Sn, Cu, Fe). Four approaches were chosen to separate the metal and silicon components from EVA in the cell fraction: separation by density, electrostatic separation, thermal degradation and solubilisation/precipitation using the solvent/non-solvent technique. After an evaluation from the point of view of efficiency, a qualitative assessment of the methods was carried out from the point of view of environmental and economic impact and investment costs. The thermal process was found to be the best compromise among the parameters considered, which strongly could influence the economic feasibility of the industrial PV treatment process. Finally, with the complete removal of the backsheet, consisting mainly of PET (polyethylene terephthalate) and small amounts of PVDF (polyvinylidene fluoride), the monomers terephthalic acid and ethylene glycol could be recovered by basic hydrolysis.
In conclusion, the newly process developed in this work, directed at increasing the company's economic return in processing PV panels, mainly uses mechanical operations for the main treatment steps of EoL photovoltaic panels and then chemical operations to refine the recovered materials. Despite the use of chemicals, the process still has a low environmental impact compared to other industrial methods.