**1. Introduction**

The transition to a circular economy can significantly contribute to the achievement of the Sustainable Development Goals (SDGs), in particular, Goal 12 ("ensuring sustainable consumption and production patterns"). The foundations for the successful circulation of resources are laid long before the manufacturing of the products, namely, at the design stage of the production systems. A better design can make a product more durable, more suited for repair, modernization, or restoration. Thus, a better design of manufacturing processes will reduce the environmental burden throughout the entire life cycle of the product.

In the last decade, there has been a growing interest in the issues of optimal choice of energy technologies in academic literature. For instance, Turconi and co-authors [1] compared the main technologies of electricity generation based on hard coal, lignite, natural gas, oil, nuclear power, and several renewable sources by the amount of emissions. Poinssot and colleagues [2] weigh environmental footprints of a closed cycle of nuclear energy against the environmental footprint of an open cycle. Paper [3] examined carbon emissions and water consumption of electricity generation

from pulverized coal, wind power, and solar PV, while [4] did the same kind of comparative analysis for a wider mix of technologies, including oil, natural gas, hydropower, biomass, and nuclear energy. Chang and coauthors [5] tried to choose a more ecologically friendly technology from coal and shale-gas-fired power generation, using the amount of greenhouse gas emissions and water consumption as the criteria. Paper [6] compared the environmental impacts of different PV-technologies, while [7] extended the analysis to include wind power and biogas electricity generation for consideration. The interest in this research topic has been determined by the rapid development of renewable energy (RE), which has a lower negative impact on the environment than traditional hydrocarbon energy throughout the entire life cycle [1,8–10]. However, the issues of decommissioning and end-of-life (EoL) treatment of renewable energy capacities, such as solar and wind plants, have not been thoroughly studied [11]. Disposal at landfills or incineration at waste processing plants remains the most common EoL treatment technologies for used photovoltaic modules and blades of wind turbines. Various mechanical, thermal, and chemical technologies of photovoltaic (PV) panel recycling, as well as recycling technologies for composite materials used in blades of wind turbines, are just at the beginning of their industrial applications. Therefore, the estimates of their environmental impact are quite different and not widely understood. There is no sufficient evidence for the comparison of available renewable energy technology by environmental impact throughout the entire life cycle, including the EoL stage. These knowledge constraints make the problem of a better eco-design of energy systems difficult to solve.

The significant growth of economic feasibility of solar and wind energy, achieved in recent years, opens up new prospects for even more rapid development of RE around the world, including the countries previously quite skeptical about renewables. This fully applies to Russia, which in recent years has been trying to reduce its lag from leading countries in the development of RE [12]. The hasty introduction of reactive innovations in the field of RE as a necessary response to the lag in technological development cannot be considered as an advantage from an economic point of view. However, it allows taking the experience and knowledge of other countries into account and making better decisions in the design of energy systems from an environmental point of view. In other words, the existing lag in technological development can be turned into a competitive advantage in the eco-efficiency of energy systems.

The environmental impact of the EoL stage is still a challenging area in the field of life cycle assessment. Slowly growing expertise in large-scale decommissioning of RE plants built 20–25 years ago as demonstration projects [13,14] brings new data and allows managers and policy-makers to formulate and solve the problems of optimal eco-design of energy systems. Monitoring of new technologies of EoL treatment of wind and solar plants is critical for effective regulation of the scale and type of RE installations, the method of recycling, and the location of recycling plants. The solution to such problems will contribute to the development of a circular economy.

The aim of this study is to extend current knowledge of the environmental impacts of most common RE technologies throughout the entire life cycle. It examines recent literature data on the life cycle assessment (LCA) of various technologies for the recycling of wind turbines and PV panels and develops recommendations for the eco-design of energy systems based on renewables. The rest of the paper is organized as follows: Section 2 presents the results of the systematic literature review of state-of-art recycling technologies for PV panels and wind turbines. Section 3 describes the features of inventory and life cycle assessments for the case of RE's recycling. Section 4 investigates the results of life cycle inventory and assessment of RE's recycling obtained under different assumptions and within different system boundaries. This section also presents general conclusions regarding the possibilities for improvements in the eco-design of the energy technologies under consideration. Section 5 discusses the practical applications of the study for the eco-design of energy systems in Russia. Section 6 debates the main contribution of the study to the existing pool of scientific literature, as well as its limitations and possibilities for future research.

#### **2. Modern Recycling Technologies for PV Panels and Wind Turbines: A Systematic Literature Review**

#### *2.1. Ecology Issues of Renewable Energy Sources (RES)-Capacity Recycling*

At the end of 2018, the global installed PV capacity reached 486 GW (REN21, 2019; Figure 1). Given that the average life of modern solar panels is 25 years [15], and the average weight of the most common 200 Wp multi-Si PV model is 16.8 kg [16], the world may face the need to dispose of more than 24 million tons of expired multi-Si PV models by 2035. In the future, these volumes will only increase due to the rapid development of solar energy in all countries [17,18]. According to expert forecasts, PV panels will reach 4500 GW of cumulative installed capacity worldwide by the middle of the century [15].

**Figure 1.** Historical development of annual new grid-connected photovoltaic (PV) installations (in GW) and anticipated decommissioning until 2035. Source: based on [19].

The PV solar panels contain lead (Pb), cadmium (Cd), and many other harmful chemicals. So far, the most common EoL treatment technology for PV models remains their disposal at landfills. [15]. It can be quite dangerous since harmful chemicals can leak into the ground, causing drinking water contamination [20]. Incineration is also used for PV models, as with regular municipal waste. It is crucial to understand that incinerating all kinds of electronic waste, including PV modules, can release toxic heavy metals into the atmosphere [21]. It is also known that some of the materials in PV modules are persistent and accumulative when released. This can cause long-term adverse ecology effects. Incineration also abolishes the opportunity of recovering raw materials. The only advantage of this method is that PV modules do not need to be separated from other commercial or industrial waste [22].

Industrial recycling capacities of used or defective PV models are currently limited and represented only by the Czech company Retina, several factories of First Solar in the United States, Germany, and Malaysia, Toshiba Environmental Solutions [15], and Veolia's new factory in France. At the legislative level, only the EU has established the rules for the collection and processing of solar panels in Waste of Electrical and Electronic Equipment (WEEE) Directive (Directive 2012/19/EU). Some countries with developed solar energy, such as South Korea, Japan, and the USA, are actively working on the problem of organizing the recycling of solar waste, while others, including China, are only exploring ways to solve this problem [23].

As has been previously reported in the literature, existing industrial technologies for recycling solar panels make it possible to achieve 95–97% recovery of cadmium and tellurium for thin-film solar cells [24], 90% recovery of glass [15,24], and 80% recovery of silicon for multi-Si PV models [25]. They also achieve 67–81% recovery of aluminum and 80% recovery of ethylene-vinyl acetate (EVA),

which is used to fix individual PV models into a single panel. The use of recovered materials in the production cycle reduces resource consumption. It can achieve a drastic reduction in waste; however, it is associated with enormous expenditures of energy and/or chemical materials, as well as the emission of significant amounts of harmful substances into the atmosphere. Therefore, according to the data [16], the recycling of all the components of 1-kW multi-Si PV panels by modern industrial thermal and chemical methods emits 6.32 kg SO2, 23.4 kg NO2, 13.8 kg CO2, as well as 0.97 kg of ammonia (NH3), 2.59 kg of ethylene, and 4.26 kg of methane.

At the end of 2019, the worldwide installed capacity of wind turbines reached 621 GW and 29 GW for onshore and offshore wind energy, respectively [26] (Figure 2). Considering the historical pace of development of wind energy in the world and an average 20-year lifetime of wind turbines, one can expect that in the coming years, up until 2030, the volumes of decommissioning of wind turbines will grow exponentially.

**Figure 2.** Historical development of annual new wind turbines installations (in GW) and anticipated decommissioning until 2035. Source: based on [26].

A closer look at the literature on the EoL of wind turbines reveals that the most difficult components for recycling are the blades and the concrete foundation [27,28]. The blades are currently simply landfilled or, in some cases, incinerated at municipal waste plants. As they are made from composite materials, their recycling is very complicated. In cases where it is technologically possible, the recovered materials have much lower quality than virgin materials [29,30]. It does not allow us to reuse them for the same purposes. In addition, the large sizes of the blades, which can reach 80–90 m in length on modern turbines, make it difficult to transport them to the place of recycling.

The landfill of large volumes of wind turbine blades is a severe environmental and economic problem [31]. The mass of the blades of modern wind turbines reaches 12.37–13.41 tons per 1 MW, and for turbines ending their life in the coming years, it is about 8.34 tons per 1 MW [32]. It means that if in 2020, it is necessary to landfill more than 31,000 tons of blades, in 2025, this amount will be already more than 142,000 tons, while in 2030, this number will be more than half a million tons. Landfilling of such large objects will require their preliminary energy-intensive shredding or cutting.

The EoL treatment of offshore wind turbines is an even more significant problem. Although their share in the total volume of wind turbine installations is much lower (Figure 3), they are more susceptible to adverse environmental effects and have a shorter lifetime. Thus, for example, in 2015, the first 10 MW Yttre Stengrund Swedish wind farm was decommissioned after only 15 years of operation [33]. In 2018, Utgrunden I Marine Wind Park (10.5 MW, Sweden) was decommissioned, which had operated for 18 years. In 2013, the first marine park built in the UK by Blyth (4 MW) ceased to operate after 13 years. Due to the lack of a clear demolition program, it is still abandoned in its original location. The Beatrice Demonstration Project (10 MW, UK) met the same fate.

**Figure 3.** Onshore and offshore wind turbine installations in global wind capacity [26].

From a logistical point of view, the process of dismantling a marine wind park is much more complicated. Besides, there is currently no clear understanding of whether it is necessary to completely dismantle the foundation of wind turbines or they must be left in the same place [34]. Several methods are being developed in the literature to extend the lifetime of marine parks, which are based on the common idea of their more or less large-scale re-equipment. Some academicians have suggested replacing the engine, gearbox, and turbine blades and reusing the old tower, foundation, and power grid [33]. Others have suggested replacing all wind farm equipment except the foundation, which can last up to 100 years [35]. The implementation of such re-equipment is expected to reduce costs compared with the construction of new offshore wind farms [33]. Still, it does not entirely solve the disposal problem.

From an economic point of view, the extraction of valuable materials from used blades is of more interest. For example, Vestas' blades are made up of glass fiber, carbon fiber (CF), epoxy resin, and polyurethane [36]. Carbon fiber (CF), in addition to wind energy, is also actively used in aircraft, automotive, shipbuilding, and other industries. Carbon fiber is produced from light fractions of crude oil, propane, and propylene through a multistage chemical treatment that requires significant energy costs. The production of one kg of carbon fiber requires 165 kWh, while the recovery is only 8.8 kWh. According to [37], the energy intensity of the glass fiber recycling is 10 times less than that of its production.

Some European countries, especially the ones the most committed to the concept of developing a circular economy, prohibit the disposal of unprocessed waste with a high organic component at the legislative level. Due to the presence of carbon fiber in the composition of blades material, the organic component in them reaches 30%. It obliges energy companies to look for other ways to treat old blades from decommissioned wind turbines.

Thus, the issues of optimizing the entire life cycle of the most common renewable energy technologies are far from their final solution. The last stages of the life cycle of solar and wind power plants represent the most prospective area for improvements and development of new technologies.
