| Description |
Photovoltaic (PV) modules are becoming a larger part of the global energy portfolio, with the global installed PV capacity growing from 40 GW in 2010 to 227 GW in 2015. The use of PV modules is expected to continue growing rapidly and as the installation of PV modules grows, so does the amount of end-of-life (EOL) PV modules. These modules have a long life of 25 to 30 years, making the current PV waste volume low. However, this is expected increase to 1.7 to 8 million tons by 2030. There currently is not an economical process for the recycling of these PV modules, resulting in the current waste modules being disposed of in landfills. These modules contain valuable and recoverable materials such as aluminum, silver, copper and high-purity silicon. Modules also may contain lead that could be exposed to the environment if not properly disposed. In this work, current recycling and manufacturing methods were evaluated, as well as the potential value of the recyclable materials. This gave an understanding of the desire to recycle the materials, especially the silicon wafer. The high-purity silicon needed in the PV modules has a high embodied energy due to the energy-intensive refining processes. Knowing the current recycling processes available revealed the high energy and chemical consumption used. This allowed for the development of potential recycling processes that reduce economic and environmental costs. By attempting to develop a recycling process that reduces the use of chemicals and recovers the valuable materials using dry mechanical processes, the costs of recycling the PV modules could be reduced. The PV panels were first manually dismantled before the nonrecoverable materials (polymers), could be separated. A mechanical, chemical and a thermal approach was used for the separation of the polymers. The thermal approach allowed for the removal of the polymers, while the chemical and mechanical approaches were not successful in the separation of the recoverable materials from the polymers. The remaining materials, glass and the silicon wafer, were then analyzed to determine mechanical methods for separation. The glass material has a slightly higher density than the silicon and a lower conductivity. These traits were not significant enough to allow for successful separation of the materials at a large scale. The silicon wafer was significantly thinner and more brittle than the glass material causing the wafer to reduced in size more than the glass particles. The difference in size of the glass and the silicon wafer allowed for separation of the materials with mesh and slotted sieves. The initial grade of the silicon wafer in the mixture was 4.5%, and this was then increased to 56.7% using the mesh sieves to remove the larger size fractions that contained mostly glass. The slotted sieve separation resulted in a recover of 88.2% of the silicon wafer at a grade of 86.1%. Size separation is a relatively low energy consumption, at 26.51 kWh/ton, compared to the energy required to produce the silicon wafer, 390 MWh/ton. By increasing the grade of the material, the amount of material that needs to be processed further is reduced as well as the associated energy of processing. The size reduction of the recovered material is likely needed for further processing techniques. The materials' unique composition makes the analysis of the particle size distribution and the energy required to reduce the particle size difficult. These materials have different properties and can be mixed in different ratios, greatly affecting the size distribution and the breakage properties of the materials. The breakage of the glass and silicon wafer were evaluated separately, as well as together, to understand the difference in the breakage of a mixture of the materials. This showed that the silicon wafer broke at a much higher rate than the glass particles. Several different population balance models were applied to the particle distributions to determine a method that could accurately represent the silicon wafer. Population balance models that fit the smaller fractions of material, where the silicon wafer was located, resulted in more accurate representation of the size reduction based on the energy input. A purely mechanical method for the recovery of silicon wafers from EOL PV modules was not developed, but a method that greatly increases the grade of the material with little energy consumption was developed. Using the material with the increased grade of the silicon wafer to refine the silicon reduces the amount of material that needs to be processed, thus reducing the total energy and chemicals need to recycle the silicon. Knowing the energy required to reduce the size of the particles recovered helps with understanding the energy that will be needed to complete the refining of the silicon. These are potentially valuable processes that can be used to recover the silicon wafer from EOL PV modules. . |
