**3. Methodology and Data**

In the last decades, life cycle assessment (LCA) methodology is commonly used in the literature for evaluating the environmental performance of the entire life cycle of the products, processes, or production systems. The LCA algorithm is described in the series of ISO 14040–14043 standards and consists of the following four main stages:

(1) Goal and scope definition. At this stage, a researcher has to determine the research objective, define the functional unit, and set the system's boundaries. When determining the system's boundaries, most researchers use the "cradle-to-grave" or "cradle-to-gate" approaches. In a "cradle-to-gate" approach, life cycle stages such as extraction of raw materials, transportation, processing of materials, and manufacturing of the product are taken into account. When using the "cradle-to-grave" approach, steps such as the usage and final disposal of the product are additionally taken into account.

(2) Life cycle inventory (LCI). At this stage, a complete map of the studied life cycle is constructed as a sequence of production and transportation processes. The inputs of each production/transportation process (such as raw materials, water, and energy consumption), intermediate processes, and outputs (such as main product, by-product, wastes, and emissions to the air, water, and soil) are determined. As a rule, a specific production process that occurs at a particular enterprise, including its supply chain, is investigated. If some stage of the life cycle exists so far only as a laboratory (experimental) process and has various scenarios, then averaged data from various sources can be used for calculations. If the

processes are not yet profoundly understood, their inventories are incomplete. In such cases, the results of inventory analysis often complement sensitivity analysis.

(3) Life cycle impact assessment (LCIA). At this stage, the identified potential environmental impacts of the production system are translated into such categories as global warming, acidification, ecotoxicity (human, marine, or terrestrial), ozone depletion, abiotic depletion, and eutrophication. Indicators for measuring these environmental impact categories are determined by one of the following mature methods of LCIA: CML 2001, Cumulative Energy Demand (CED), eco-indicator 99, EDIP, ILCD, ReCiPe, IPCC, or IMPACT 2002+. Some methods (for example, CML 2001) allow the translation of all the adverse environmental effects of the product life cycle into physical units, such as kg CO2-eq, kg NOx-eq, kg PO4-eq, and kg 1.4-DCB-eq. Others (for example, ReCiPe endpoint) allow the aggregation of all the negative effects in all categories into a single dimensionless quantity). The choice of methodology mainly depends on the preferences of the researcher and the objectives of the study.

(4) Interpretation. At this stage, a researcher summarizes the LCI and LCIA results, identifies critical points of the life cycle with the most considerable adverse effects, and makes recommendations for possible improvements.

If the life cycle of the product also includes the recycling stage, the modeling and calculation of the total adverse environmental effects are complicated by the fact that the recycled product can enter the production cycle again. In this case, it reduces the need for some raw materials. In modeling the processes of recycling, two main approaches are used: the "cut-off approach" and the "end-of-life approach". When using the first approach, the recycling efforts are economically allocated among the treatment process and all the recovered materials with a positive economic value. In the second approach, the potential benefits from the usage of recycled materials are calculated by awarding credits for the avoided environmental impacts caused by the primary production of replaced products. At the same time, some researchers use a simplified approach called open-loop recycling (OLR), in which further flows of secondary materials are not taken into account [57]. More details about differences in allocation approaches can be found in [38].

When choosing the technology for generating electricity from several possible options by environmental parameters, we are faced with the need to take into account the full life cycle of the electricity plant, including the recycling of massive generating equipment. The first stages of the life cycle of wind and solar energy are relatively well studied using LCA. For example, [6,39,58] compare a broad spectrum of environmental categories of photovoltaic technologies, [59] studies a life cycle of organic photovoltaics, and [60–62] compare GHG emissions of several photovoltaic technologies. Other studies [63–65] give life cycle assessments of solar panels manufactured in specific locations. Ecological aspects of the disposal of used equipment have also been studied (e.g., [66]). In contrast, accounting for recycling as a primary route for end-of-life treatment is a complex problem [38].

First, as a rule, when conducting an LCA for an electricity plant, 1 kWh of energy produced over the entire life of the equipment is used as a functional unit. Such a choice of a functional unit is convenient because it allows one to compare the results of LCA for different technologies and make a choice in favor of the most environmentally friendly. In LCA of the recycling of used generating equipment, the mass of the equipment itself (PV panel or wind turbine) usually is considered as a functional unit. Therefore, it is impossible to directly aggregate the LCA results of the first and last stages of the life cycle.

The second difficulty is the lack of a universal approach to determining the system's boundaries for recycling processes. Some researchers include the collection and transportation of used PV panels and wind turbines, while others suggest that recycling is done directly at RES plant location. The third difficulty is that, as noted above, most of the recycling processes of RES capacities are at the stage of laboratory research, so the data on the inputs and outputs of these processes are minimal and have great uncertainty.

This study systematizes the LCA results of various methods of EoL treatment of used PV panels and wind turbines, brings them to single units of measurement, and converts them to functional units used to evaluate the earlier stages of the life cycle of electricity plants. The conversion to the functional unit of 1 kWh of all estimations from the literature was carried out under the assumption that the capacity factor of PV plant is 14% [12], solar irradiation in the location of PV plant is 1200 kWh/m2/year [12], the weight of the 200 Wp c-Si module is 16.8 kg [16], the weight of 1 m<sup>2</sup> of the module is 13.2 kg [67], the weight of 1 m<sup>2</sup> of the CdTe module is 14.63 kg [67], and the module efficiency is 10.5% [67].

The primary difference of the present research in comparison with similar ones is that we did not restrict ourselves with a certain LCIA method and considered a maximally wide spectrum of impact categories. The purpose of our comparative analysis of LCA results is to identify common conclusions that remain valid even with different approaches, assumptions, and system boundaries and can serve as guidelines for the eco-design of energy systems.

#### **4. Results and Discussion**
