*2.2. Modern Technologies of PV Panel Recycling*

The modern industrial algorithm for recycling silicon-based photovoltaic panels begins with their disassembly, during which aluminum (frame) and glass parts (coating) are separated. Almost 95% of the glass is recyclable, and all external metal parts can be reused to form new frames for solar modules [38]. The remaining materials are heat-treated at a temperature of 500 ◦C, during which the encapsulating plastic evaporates, leaving the silicon elements ready for further processing. In modern recycling plants, evaporating plastic is not released into the environment but used as a heat source for further heat treatment (energy recovery technology), which partially reduces adverse environmental effects [15,16,39].

After heat treatment, 80% of the PV modules (by mass) can be reused, while the rest is subject to additional cleaning. Silicon particles contained in cracked and scratched wafers are etched with acid and then melted for reuse to produce new silicon modules, resulting in an 85% level of recovery of the raw silicon material. The reuse of silicon can significantly reduce the adverse environmental effects of silicon photovoltaic panel manufacturing by saving, in addition to silicon sand itself, a significant amount of energy and water used in the production stage of metallurgical silicon. Based on the data of Huang and co-authors [16], it is possible to estimate the energy savings in the manufacturing of 1 kW of a photovoltaic panel due to the reuse of silicon as 76.2 kWh and water savings as 76.2 L. It avoids an average emission of 123.34 kg of CO2, 3.36 kg of SO2, and 0.73 kg of NOx.

The recycling of thin-film photovoltaic panels is more complicated [21,40]. In the first step, the panel is crushed to a particle size of not more than 4–5 mm, which allows the removal of the lamination that holds the internal materials. Unlike silicon panels, the remaining substance consists of both solid and liquid materials, which need to be separated. Liquids pass through a precipitation and dehydration process to allow the recovery of semiconductor materials. Separation of semiconductor metals is carried out in various ways, depending on the technology used in the manufacture of panels; however, an average recovery rate of 95% is achieved. Solids contaminated with so-called interlayer materials are cleaned by vibration. Furthermore, the material undergoes washing, as a result of which there remains clean glass that must be melted and reused [39].

There are some new technologies for recycling photovoltaic silicon modules at the stage of laboratory research: technology for dissolving a laminating film in an organic solvent (tetrahydrofuran, o-dichlorobenzene or toluene), technology for dissolving with additional ultrasonic treatment, hot cutting, pyrolysis, technology for dissolving in nitric acid, and some others [41,42]. For thin-film modules, in addition to organic solvent dissolution and hot cutting technologies, laser exposure, vacuum processing, and flotation technologies are also being developed [43,44].

Previous studies have shown that the recycling of silicon modules becomes feasible at a scale of at least 19,000 tons per year [45]. Only when such volumes are achieved, the cost of processing decreases due to the economies of scale. The economic feasibility of recycling an entire PV panel is much higher than the expediency of recycling PV modules only, since the metal frame and electrical cables are easier to recycle, and the recovered materials (aluminum, copper) are valued higher [46,47]. The economic feasibility of recycling silicon panels substantially depends on the price of new modules, which has fallen significantly in recent years. For example, for Europe, the commercial attractiveness of silicon recycling is also determined by the lack of its mining (Figure 4). Of all the European countries, only Norway has a share of more than 4% in the global silicon production market.

#### *2.3. Modern Technologies of Blade Recycling*

The simplest method of recycling a composite material is mechanical (cutting, crushing, crumpling). However, it damages individual fibers, which leads to a decrease in mechanical characteristics. According to the data [49], the tensile strength of recycled fiberglass compared to new is not more than 78%; for carbon fiber, it is less than 50%, with the fiber recycling ratio (recyclate yield rate) of 55–58% of the initial mass. The material recycled in this way has a low cost and cannot be a substitute for the virgin material. Typically, such a recycled composite material is used for less demanding mechanical applications, for instance, aggregates for artificial wood or asphalt and concrete in the construction industry [50,51]. Recycled carbon fiber (CF) is also suitable for short-fiber nonwoven composites used in aircraft and vehicle interiors [52]. Manufacturers can use recycled material as a filler or strength enhancer in new products, reducing the cost and environmental impact of waste disposal. In addition, recycled CF can be mixed with a polymer to produce thermally and electrically conductive materials used to improve the durability of paint, cement, and other construction materials [36,53].

Other actively developed methods for processing composite materials are pyrolysis, oxidation in a fluidized bed, and chemical decomposition of polymer resins (binders) [54]. During pyrolysis, the composite is heated to 450–700 ◦C in the absence of oxygen. The polymer resin is evaporated, and the fibers remain intact and can be restored entirely [36]. The recyclate yield rate of both carbon and glass fibers during pyrolysis is 67–70% of the original mass. The residual tensile strength is 52% for fiberglass and 78% for carbon fiber [49].

During oxidation in a fluidized bed, the polymer component (resin) is burned in a stream of hot (450–550 ◦C) air enriched with oxygen. The carbon fiber recyclate yield rate is higher and reaches 86–90% of the initial mass, while for fiberglass, it is 42–44% [49]. The residual tensile strength is 50% for fiberglass and 75% for carbon fiber. The chemical influence on the binder polymer element, selected directly for a specific combination of fiber and polymer matrix, helps to get the maximum possible amount of fiber suitable for reuse with minimal time and resources. The recyclate yield rate of both carbon and fiberglass is almost 100% [49], the residual tensile strength for fiberglass is 58%, and for carbon fiber, it is 95%.

To the best of our knowledge, the economic characteristics of the various processes of recycling of composite materials are absent in the literature because all of the methods mentioned above are still at the stage of laboratory research. Meanwhile, the data on the current energy intensity of these methods are known, which in combination with data on recycling efficiency, allow us to judge the competitiveness of methods from an economic point of view indirectly (Table 1). However, economic feasibility depends not only on the cost of the production process but on the availability and size of the market for recycled materials. Therefore, the prospects of the above methods for recycling blades of wind turbines are also determined by the fact that they are suitable for the recycling of composite wastes from other sectors of the economy (aircraft, automotive). Therefore, the cost of their development and practical application can be reduced due to economies of scale.


**Table 1.** Comparison of end-of-life (EoL) treatment methods for wind turbine blades. Source: based on data of [49].

The possibility of incinerating composite materials in municipal waste plants for energy recovery depends on the ratio of polymer to carbon fiber in its composition [50]. A significant disadvantage of incinerating composite material is that approximately 60% of the initial mass remains in the form of ash. Ashes must also be disposed of in some way. At the moment, it is either subject to disposal at landfills (which also contradicts the legislation in the field of waste management in some countries) or to reuse in construction materials [55].

Another possibility for recycling wind turbine blades is to reuse them as load-bearing structures in the construction of buildings, technical infrastructure (e.g., bridges), or to create artificial reefs [55,56]. However, cases of such reuse of the blades are more likely experimental than industrial methods of disposal.

As can be seen from the analysis of the currently existing technologies for processing used capacities for wind and solar energy, the final stage of the life cycle of renewable energy facilities can impact the environment significantly. Therefore, the choice of electricity generation technology in planning the development of the energy system in a specific territory (region, municipality) should take into account the total environmental effects throughout the life cycle.
