Integration & assessment of recycling into c-Si photovoltaic module’s life cycle

Abstract

Photovoltaic (PV) energy generation devices have experienced a vigorous growth in production over the last decade in all major industrialised countries. In this research paper, the assessment of recycling and subsequent production of new crystalline silicon (c-Si) PV modules taking advantage of the recovered Si in terms of industrial symbiosis is being examined. With the aid of life cycle analysis method, the potential payback is thoroughly examined through a scenario application in the second useful phase of the PV module. Furthermore, key environmental performance indicators have been implemented in order to evaluate power and resource demands.

Keywords:

• Life Cycle Assessment
• photovoltaics
• environmental performance indicators
• recycling
• grave to cradle approach

1. Introduction

Photovoltaics (PVs) remain one of the most dominant technologies related to renewable energy sources (RES). The last decade we experienced a fast growth of PV installations including a lot of small size ones. By the end of 2015, the cumulative PV installed capacity achieved was 233 GW, whereas for 2019 the estimation is approximately 450 GW (SolarPower Europe 2015). The substitution of fossil fuel technologies with PVs provides reduction of greenhouse gas (GHG) emissions that impact the global climate, as well as reduction of pollutant emissions such as nitrogen oxides (NO_x), sulphur dioxide (SO₂) and carbon monoxide (CO), which cause health-related issues and burden the environment.

There are numerous PV module technologies; however, the basic categories currently are crystalline silicon (c-Si) PVs (mono-crystalline (m-Si) and poly-crystalline (p-Si)) and thin film PVs (which include amorphous silicon PVs, cadmium telluride PVs and copper indium selenide PVs amongst others). Nowadays, c-Si PVs is the most common and mature technology, having a market share of approximately 85% globally (Brenner and Adamovic 2017).

PV modules contain substances such as glass, aluminium, copper and semiconductor materials that can be successfully recovered and reused, either in new PV modules or in other products. Life expectancy of dominant PV technologies is estimated at around 25–30 years, so current total PV waste is not significant because the majority of installed PV systems is still in service. However, in a few years it is expected that there will be a critical part of the waste stream, as a large number of end-of-life (EOL) modules will become available for recycling. Regarding c-Si PV modules, which represent the 90% of global PV waste until now (Brenner and Adamovic 2017), there is a number of companies that excelled in the field of PV recycling, including Deutsche Solar AG (Müller, Wambach, and Alsema 2005), SolarWorld AG (SolarWorld 2016), Solar Cells Inc. (SCI) and Pilkington Solar Int. (PSI) (Bohland et al. 1998; Wambach 1998), amongst others.

The necessity of massive and efficient recovery of non-functional PV modules has been indicated by numerous researchers (Choi and Fthenakis 2014; Corcelli et al. 2016; Fthenakis and Kim 2011; Kang et al. 2012; Katsigiannis et al. 2014; Klugmann-Radziemska et al. 2010; Rentoumis et al. 2015) and recently the European Directive 2012/19/EU (EU 2012) on waste electrical and electronic equipment (WEEE) dictated the management of PVs EOL. At a global level, policy action is needed to address the new challenges that arise, with enabling frameworks being adapted to the needs and circumstances of each region or country (Sinha 2017).

PVs are considered one of the cleanest electricity generation technologies during their operation. However, their whole life cycle includes a number of stages that may emit GHGs and pollutants: solar cells manufacturing, material transportation, PV module assembly, PV system installation, maintenance and system disposal or recycling. In order to assess their impacts to the environment, a macroscopic approach of their life cycle has to be performed. This can be implemented through life cycle analysis (LCA), which is a framework for considering the environmental inputs and outputs of a product or process from cradle to grave (Fthenakis and Kim 2011).

This paper examines the performance of a concept c-Si PV module recycling plant, situated in Greece, which is utilising a novel recycling procedure. Recycled c-Si PV cells can reach maximum efficiency up to 18% (Derbouz Draoua et al. 2017). Plant operation is evaluated through conventional and custom environmental performance indicators (EPIs), in the context of LCA. The above-mentioned evaluation is implemented according to the requirements per functional unit (1 kg of reusable Si in our case).

The paper is organised as follows; Section 2 presents the c-Si module macro-structure and some of the established c-Si PV recycling methods starting from 2003. Moreover, Section 3 provides information on the applied LCA framework, assumptions, details on the selected EPIs and the results of the analysis with supporting illustrations, whereas Section 4 concludes the paper.

2. Materials & methods: end of life of c-Si PV modules

2.1. The c-Si PV module structure

The crystalline structured PV cells that utilise Si as semiconductor compose the first generation of PV technologies. Mono-crystalline (m-Si) and poly-crystalline (p-Si) cells belong to this generation. Approximately, 65% of the total cost of the PV module concerns the actual manufacturing cost of the PV cell (Choi and Fthenakis 2014; Kang et al. 2012; Klugmann-Radziemska et al. 2010). The typical c-Si module composition is presented in Figure 1.

Figure 1. A typical c-Si material composition.

The main components of both those technologies include (see also Figure 2):

1. The Si wafer of about 200-μm thick, which is the key component, essential for the production of such modules. This substrate of semiconductor material undergoes numerous microfabrication steps before being integrated into the cell and efficiently convert sunlight into DC electricity.

2. Anti-reflective coating (ARC), which covers the p-n junction. ARC is usually composed of a single layer of silicon nitride (SiN_x), in an effort to cut down the optical losses due to Si’s high reflectivity.

3. Silver (Ag) as a material of surface electrical contacts, and Cu ribbons for solar cells connections.

4. A double layer of ethylene–vinyl acetate (EVA), which concludes the encapsulation. Apart from protection issues, the EVA layer is also used as an adhesive between the glass and the Si wafer.

5. ARC encapsulation and composite glass, which protect the module from damage. Sometimes, blackout material can be implemented to prevent pre-energising of the cells. On the other hand, when solar activation is not of any significant concern, a transparent protective film can be used. Moreover, the use of a UV enhancement material may increase the efficiency of the cell in the UV region.

6. Plastic backing that consists of polyvinyl fluoride (PVF) and polyethylene (PET), in order to make the module weatherproof alongside with the Al framing.

7. The junction box for the electrical connection of different PV panels.

Figure 2. Expanded view of a typical c-Si module (1.6 m × 1 m, 215 W_p).

2.2. Established recycling methods for c-Si PV modules

In 2003, a recycling method of c-Si PV panels was introduced by Deutsche Solar (Müller, Wambach, and Alsema 2005), which can be applied in a variety of PV types and sizes. The recycling procedures include the treatment of (i) waste arisen during Si wafer production, (ii) non-functional Si wafers and (iii) PV modules that failed at quality testing after production or reached their end-of-useful life due to low efficiency or damage.

In the first stage of the procedure, PV modules are placed into a furnace that facilitates the manual disassembly, leading to the separation of metallic parts from the cover glass and the Si wafers. The metallic parts and the glass are then forwarded to third parties that have such recycling facilities, whereas the wafers remain for further ‘in house’ chemical treatment. The first stage is etching which includes application of several acidic solutions that remove the ARC and the p–n junction. The Si wafers are then isolated and melted into Si ingots and by extension, integrated into evolved and more efficient Si PV technologies.

SolarWorld (2016) presented its pilot recycling program, also in 2003, separating itself from any previous techniques using a pyrolytic method in order to remove the organic parts before the chemical treatment of Si wafers. The recovered Si was then utilised from the company itself, while the other recovered materials were sold to partners. Furthermore, SolarWorld had established its own ‘bring-in’ system for the collection of PV waste.

Estimations have shown that 84% of the initial input weight could be recovered while preserving the purity of the materials. Glass could be recovered by over 90%, while the percentage of semiconductors re-used reached 95%. It is also worth mentioning, that after the thermal treatment, 80–96% of the PV cells is being recovered intact. To conclude, chemical treatment can bring up to 98% recovery of cells, depending on the level of prior inflicted damage (SolarWorld 2016).

Moreover, Solar Cells Inc. (SCI) and Pilkington Solar International (PSI) c-Si PV recycling procedures shared many similarities, thus they are presented together (Bohland et al. 1998; Wambach 1998). They included recovery techniques for both Si fractions and functional PV cells from the respective modules.

SCI specialised in fraction treatment and not in whole modules. Its method recovered most of the backing film and a portion of the functional PV cells. The procedure begins with the gradual increase of the temperature and the manual removal of the plastic backing. At 500 °C, the EVA encapsulation is being pyrolysed. As a result, new cells were produced based on recycled materials with a minor loss, in comparison with the initial efficiency.

PSI also used pyrolysis for the removal of the organic components with a successful recovery of 60% of the PV cells imported (Wambach 1998). Yet, the method was not realised in a noble gas environment and consequently, surface Ag contacts were being oxidised, hampering the procedure. It’s been estimated that PSI pyrolysis method lasted about four hours due to potential surface charring, whereas SCI’s technique took no longer than an hour and a half.

Recent advances in PV modules recycling include combination of physical and chemical technologies in order to follow the ‘zero waste concept’ and to enter with PV recycling the Circular Economy. These technologies related with light, water and biodegradable auxiliaries, and they use an optical nanotechnology in order to open the composite without destroying the glass of PV module (Palitzsch and Loser 2017). Moreover, special care is given to the effective recycling and purification of silicon powder from kerf loss during the whole process (Halvorsen et al. 2017; Moen et al. 2017). A number of new technologies have been developed in order to reduce the kerf losses and thus increase the silicon efficiency (Jungbluth et al. 2012).

3. Life Cycle Assessment

Life cycle assessment (LCA) is a proven method for the evaluation, compilation of the inputs, outputs and environmental impacts of a certain production system during a product’s lifetime from raw material extraction and acquisition through production process, use, EOL treatment, recycling/recovery and final disposal. LCA methodology has been documented and standardised through the International Organization for Standardization (ISO), through ISO 14040 Environmental Management – Life Cycle Assessment – Principles and Framework, 2006 (ISO 2006a) and the latest addition to it – ISO 14044 Environmental Management – Life Cycle Assessment – Requirements and Guidelines: 2006 (ISO 2006b). LCA studies are divided between conventional (environmental LCA) and nonconventional LCA (social LCA, cost LCA, etc.) (Gerbinet, Belboom, and Léonard 2014).

Recent state of the art reviews in the context of LCA on PV technology have shown a significant decrease in the environmental footprint over the last 40 years (Louwen et al. 2016). Regarding energy payback time (EPBT) indicator, a significant decrease of almost two orders of magnitude over the last four decades has been observed, as material use, energy use and efficiencies have been constantly improving (Fthenakis 2016). The GHG footprint had been calculated around 20 g CO₂-eq/kWh for p-Si PV systems, and around 25 g CO₂-eq/kWh for m-Si PV systems down from 143 g CO₂-eq/kWh for p-Si in 1992 (Schaefer and Hagedorn 1992) and 409 g CO₂-eq/kWh for m-Si in 1990 (Kreith, Norton, and Brown 1990). A significant portion of GHGs impact is included in the processes of solar cell sandwich layer incineration, post-incineration treatments and transport (Latunussa et al. 2016).

Following an analysis of 232 references regarding EPBT and Energy Return on Investment (EROI) indicator analyses, Bhandari et al. (2015) described that an EPBT range of 1.0–4.1 years; from lowest to highest, the module families ranked in the following order: CdTe, CIGS, a-Si, p-Si and m-Si. Additionally, CdTe modules present lowest global warming potential (GWP) amongst these module families (Leccisi, Raugei, and Fthenakis 2016). The average EROI varied from 8.7 to 34.2. High-concentration PV systems, which belong to third-generation PVs may present even lower EPBT (Peng, Lu, and Yang 2013). Regarding CdTe PV modules, crucial parameters in their EPBT estimation include electricity usage in manufacturing, glass content in PV modules and metal content in modules and balance of systems. The transition to a larger, lighter and more efficient version of current thin film modules is expected to reduce the product environmental footprint of CdTe PVs to a factor of 4 below that of an average PV module (Sinha and Wade 2017).

3.1. Goal and scope

In order to measure the impact of utilised materials and processes, the amount of collected materials per functional unit is calculated. For this reason, it is assumed in our case that the product unit is equal to 1 kg of reusable Si. The calculations include reference flows (in kg of material/kg of reusable Si), energy indices for every phase of the product’s lifecycle (in kWh/kg of reusable Si), as well as the EPIs that are mentioned in sub-Section 3.3.

The analysis followed an approach, starting from the PV waste input to the recycling unit assessing the impacts occurring during each phase of treatment and the final production of the recovered Si. In addition, our research was extended to include one-step further and evaluate the total impact of a second production phase as a stepping stone to observe the total impact of the recovered materials when incorporated in modern PV technologies. As a result, our approach resembles a ‘grave to cradle’ philosophy. Current maximum efficiencies are 25.6% ± 0.5 for m-Si PV cells and 20.8% ± 0.6 for p-Si PV cells (Green et al. 2015).

Our research did not examine the processing of the recovered copper, glass, Al, Ag, as this is taking place through third parties and data regarding their procedures are not available to us in detail enabling us to confidently incorporate them within our LCA. In Figure 3, it is shown in detail the recycling procedure together with the relevant quantities that is used in our analysis.

Figure 3. System boundaries of the proposed ‘grave to cradle’ approach.

3.2. Life Cycle Inventory

The proposed process aims at the treatment of c-Si PV waste modules. Information on machinery and waste data have been based on contemporary recycling industry standards.

3.2.1. Proposed c-Si PV recycling method

The first step regarding the EOL process focuses on the transportation of the input material. It is considered that will be realised via diesel-fuelled trucks with a maximum capacity of 3.3 tns, covering the entire mainland. The main purpose of the transport is to go through collection points and pick-up the EOL PV modules. Due to the high distribution of the PV installations across the Greek territory, the collection points are scattered accordingly in order to serve the respective needs.

The proposed procedure (Rentoumis et al. 2015), with capacity of 88 tns/month, concerns both fractions and whole c-Si PV modules and commences with the physical dismantling of the input waste by removing the Al framing, the junction box and the cabling. Subsequently, the delamination stage follows, with the thermal treatment of the PV modules and the parallel removal of the organic EVA in a noble gas environment. In case of shattered glass, the thermally treated module is shredded, contrary to the intact cover glass case, where after thermal treatment the physical disassembly is facilitated and the glass is easily removed. The shredding step leads to a desired granulometry, which is necessary for the mechanical separation of the glass and the Si fractions. Finally yet importantly, the chemical treatment includes the adjusted use of various acidic solutions (EU 2016) in accordance with the type of input panels through which Ag and Al are recovered whilst ARC and p-n junction are removed. As a final stage, the post-processing of the recovered Si into ingots has been fostered. Note that the Al, Ag, glass fragments and junction boxes will be forwarded to external partners.

The thermal treatment/delamination is realised with the presence of a pyrolytic reactor in continuous operating mode. Additionally, an afterburner, cyclones, wet scrubbers and proper piping have also been integrated. In terms of shredding, the process includes the utilisation of two different set-ups of shredding machinery. The first one, Twin Shaft Shredder, is a common machine found in recycling sites that allows the decrease of size of PV waste through rotating motion. The second type of shredding machinery, the Granulator, is commonly found in plastic shredding. The mechanical separation is realised with the use of an inclined Cascade-Screening machine.

The chemical treatment of the c-Si wafers is adjusted according to the quality criteria set by the production standards of the c-Si cells. In order to achieve 4 N quality (99.998% Si), the wafers have been placed inside C₂H₄O₂, BR, HF, HNO₃ solutions. In addition, an industrial furnace has been installed for the remelting of wafers into ingots for further use. To conclude, forklifts, roller conveyors and belt conveyors were implemented in order to facilitate loading and unloading procedures within the recycling site. After the recovery of the silicon ingots, the plan dictates the re-utilisation of them along with additions of materials (Al, Ag, glass fragments and Cu) that have been extracted in raw form and have been forwarded to partners for further treatment.

As a final stage, two (2) scenarios regarding the second useful life of recovered Si were included in the assessment. The first one is focused on fixed PV technology implementation, while the other one on solar tracking. Both of them are being described in detail, below.

The LCA was implemented with Gabi 6.0 software. Furthermore, the following assumptions were considered:

1. In order to calculate the average consumption of fuel required for the vehicle fleet, the mean distance was assumed to be 200 km for the collection to EOL phase route and 400 km for the EOL to production route. These estimations were based on the fact that only one plant for PV module recycling would be sustainable to be situated in the Greek mainland, according to the expected recycling needs (Panagiotopoulos 2013). For the time being, 12 recycling collection points exist in Greece regarding WEEE.

2. The estimation of annual produced electricity of c-Si PVs per module area, which is needed for LCA analysis, is implemented as follows. For year 2015, electricity production of PV installations in Greece equal to 1486 kWh/kW_p (annual capacity factor equal to 17%) (Operator of Electricity Market 2016). Given the fact that c-Si technology represents more than 96% of total installed PV power in Greece (Katsigiannis et al. 2015), and considering 215 W_p average c-Si PV module power, 1.6 m² average PV module area, 22 kg weight and 90% PV derating factor due to ageing and dust, the resulting index is equal to 180 kWh/m².

3. In addition, in order to evaluate the different scenarios that concern the potential installation onto which the new PV modules will be implemented, some more assumptions have been made. We consider that the modules are installed onto fixed and tracking PV systems with 95.5 and 4.5%, respectively (SPIEF 2016).

• In the first case (installation onto fixed PV systems), it is assumed that the average value of energy generated, in Greece, will reach 1400 kWh/(kW_p year) or 1.40 kWh/(W_p year) for a c-Si module of 215 W_p. It is then calculated that an average value for a useful life of 25 years is 1260 kWh/(kW_p year) or 1.26 kWh/(W_p year).

• In the second case, based on the fact that tracking systems present a 40% increase in the amount of energy generated it is assumed that the corresponding figures will be 1764 kWh/(kW_p year) or 1.764 kWh/(W_p year).

3.2.2. Comparison between proposed and established recycling methods for c-Si PV recycling

The comparison of the recycling methods described in Section 2.2 shows that there are sharing a large number of common characteristics. In order to clarify the differences between Deutsche Solar, SolarWorld, SCI-PSI and proposed PV recycling methods, Table 1 lists the basic similarities and differences between these five (5) procedures.

Table 1. List of common and unique factors between Deutsche Solar, SolarWorld and proposed PV recycling methods.

Factor	Deutsche Solar	SolarWorld	SCI	PSI	Proposed method
PV module type	c-Si	Si (various types)	Si (various types)	Si (various types)	c-Si
Type of PV modules that can be recycled (fraction/whole)	Both	Both	Fractions of modules	Complete modules	Both
Heating process	Furnace	Pyrolysis	Inert Atmosphere Pyrolysis	Atmosphere Pyrolysis	Inert Atmosphere Pyrolysis
Use of noble gas environment	No	No	Yes	No	Yes
Re-melting	No	No	No	No	Yes
Configurability	No	No	No	No	Yes
Recycled materials	Glass, Aluminium, Silicon, Copper	Glass, Aluminium, Silicon, Copper	Glass, Aluminium, Silicon, Copper, Backing film	Glass, Aluminium, Silicon, Copper, Silver	Glass, Aluminium, Silicon, Copper, Silver

3.2.3. Life Cycle Inventory data

The data used for the LCA of the current recycling procedure were the product of research conducted during the Operational Program ‘Competitiveness and Entrepreneurship’ of the National Strategic Reference Framework (NSRF) – Research Funding Program: ‘Cooperation 2011 – Redesign and Recycling of PV Panels’.

Data regarding standard recycling procedures were extracted from the EcoInvent 3 database, while the appropriate sizing and configuration of machinery in terms of capacity and energy consumption was based on recycling industry standards.

With regard to electricity consumption, Greek energy mix has been taken into consideration.

3.3. Life Cycle Impact Assessment – selection of proper environmental performance indicators

EPIs are concerned with the assessment of the environmental impact of materials, energy sources, activities, processes, products, hardware equipment and services (Carlson 2002). In the standard ISO – 14031 Environmental management–Environmental performance evaluation – Guidelines (ISO 1999), EPIs are described and they are used to accurately illustrate the ambiguous environmental data of a company in a comprehensive manner. Usually, they are applied to relate material and energy data to other variables in order to increase the informational value harvested from quantitative data. EPIs have the following purposes (Jasch 2000):

• Comparison of environmental performance over time and between different operational scenarios

• Highlighting of optimisation potential and weak point analysis

• Clarification of environmental targets

• Identification of market chances and cost reduction potential.

EPIs are a useful tool in the hands of top management, environmental managers and other departments as comprehensive and concise key data-sets in a vast sea of environmental information. They provide decision makers, with an overview of relevant progress, but also highlight areas worth of attention. On this basis, environmental goals can be backed up with concrete figures, which make the definition and pursuit of environmental targets manageable and verifiable. In addition, their connection to conventional indicators allows for the identification of potential monetary benefits.

In this paper, for the assessment of the environmental impacts, several crucial categories have been selected according to Indicators’ Theory. Regarding RES technologies such as PVs, two commonly used EPIs are EPBT (1) and the Energy Return Factor (ERF) (2) (Espinosa et al. 2012). ERF is also found in the literature as EROI. EPBT refers to the time required by the PV system to generate the amount of energy consumed during its life cycle, whereas EROI depicts the total amount of energy saved per unit of energy dedicated to the investment. EPBT and EROI are calculated as follows (see also Table 6):

(1)

(2)

where E _EMB stands for the total energy required during the module’s life cycle, E _GEN represents the energy savings due to the yearly power generation by the PV module, while L is the lifetime of the PV device. In our case, L is considered equal to 25 years, which is the period of PV power purchase agreement in Greece. Table 2 (Wong, Royapoor, and Chan 2016) presents values of EPBT that can be found in the literature. It has to be noted that in the majority of cases, no material recycling has been considered.

Table 2. List of EPBTs used as reference for our research.

Reference	Location	Mounting type	Irradiation (kWh/m^2/yr)	Module efficiency (%)	Lifecycle (years)	EPBT (years)	Emissions (g CO2-eq./kWh)
Leccisi, Raugei, and Fthenakis (2016)	Central-Northern Europe	Ground Mounted	1000	17	30	2.8	~63
Leccisi, Raugei, and Fthenakis (2016)	Central-Southern Europe	Ground Mounted	1700	17	30	1.6	~37
Leccisi, Raugei, and Fthenakis (2016)	Southwestern United States	Ground Mounted	2300	17	30	1.2	~27
Bhandari et al. (2015)	Mean value of multiple studies*	Roof or ground mounted*	1700*	13*	30*	4.1*	N/A
Peng, Lu, and Yang (2013)	Hong Kong	Roof Mounted	1600	13.3	20–30	7.3	671
Jungbluth et al. (2012)	Switzerland	Roof Mounted	1117	14	30	3.3	N/A

* Mean value or value list.

Moreover, numerous additional EPIs related to PV recycling process have been taken into account in the LCA calculations. The whole assessment has been realised in Gabi 6.0. (see also Figure 6):

• Climate change (GWP) (in kg CO₂ – eq.) – 100 years

• Ozone Depletion (ODP) (in kg R11 – eq.)

• Human Toxicity, cancer effects/non-cancer effects (in CTUh)

• Particulate matter/Respiratory organics (kg PM2.5 – eq.)

• Ionising Radiation (kg U235 – eq.)

• Photochemical Ozone formation (POPC) (in kg NMVOC – eq.)

• Acidification (in moles of H⁺ eq.)

• Eco-toxicity (CTUe)

• Resource depletion, water (kg)

• Resource depletion, mineral, fossil (kg Sb eq.)

3.4. Life Cycle Interpretation and discussion of the results

The first step of the assessment was the calculation of the input PV module/product unit analogy. We discovered that, according to our proposed recycling methodology, for every kg of reusable Si produced, almost 1.66 PV modules are required (see also Table 3). Note that even though in Table 3 only the reusable components are presented, EVA, plastic backing, potting compounds and adhesives have been taken into consideration when the total weight of the input panel was computed.

Table 3. Required reference flow of functional unit (1 kg Si).

Materials	Input quantity required (kg/kg Si)
Al frame	3.66
Glass cover	26.47
Solar cells	1.14
Cu (PV module, cabling)	0.016
Total PV module weight	36.66

As a second step, energy consumption calculations were implemented. Table 4 shows the energy requirements for each type of transportation considered. Moreover, based on our machinery designs and the market standards amongst respective resellers and manufacturers, a list of power requirements for the needed equipment has been compiled. In addition, Table 5 includes the estimated energy demands required per PV module input (see also Figure 4) and per functional unit.

Table 4. Energy requirements for the transportation from collection to production phase.

Transport	kWh/module	Input of energy required (kWh/kg Si)
Collection to EOL (Αvg: 200 km)	2.129	3.548
EOL to Production (Avg: 400 km)	0.116	0.193

Table 5. Power and energy requirements distribution analysis across PV module recycling and production phase per functional unit.

Implemented machinery	Power req. (kW)	kWh/module	kWh/kg Si
Loading/unloading	–	24.96	41.60
Physical disassembly	7.2	2	3.3
Thermal treatment	26.7	26.7	44.5
Shredding	84.7	7.06	11.76
Granulator	34.9	11.64	19.4
Inclined cascade screens	16.3	16.3	27.16
Furnace/melting	10	10	16.66

Figure 4. Energy requirements distribution across PV module recycling.

Table 6 shows the estimations of EPBT and EROI. Regarding EPBT, it is concluded that a PV module (≈275.5 kWh/y) needs approximately 1.45 years in order to generate the amount of energy consumed during transport, recycling and production, phase (see also Figure 5). Moreover, according to EROI it is expected that the module will save at least 17.24 times the energy dedicated to the initial investment (average annual sun Irradiation for Greece 1800 kWh/m², vis-à-vis 1285 kWh/m² for Germany).

Table 6. EPBT & EROI performance indicators calculations.

Index	Value
EPBT	1.45 Years
EROI	17.24 Times

Figure 5. Energy required for the ‘grave to cradle’ approach: transport, recycling and production.

As illustrated in Figure 6, a complete performance assessment has been realised regarding the way the power demands of transport, recycling and production affect the conventional environmental indicators based on the functional unit (1 kg/Si).

Figure 6. Conventional indicators performance (Gabi 6.0).

As expected, power requirements for disassembly roughly does have any serious impact on any of the indicators (<2%) due to the non-energy strenuous activities. Thermal treatment affects the sum of indicators equally, in the range of 10%. Moreover, shredding processes, mechanical separation and re-melting compose the ≈8% of the total impact in every indicator category.

On the other hand, electricity required for the production phase seems to greatly affect the majority of the indicators, where in some cases represents the 75–85% of the total environmental impact.

To conclude, transport and loading/unloading activities within the framework are responsible for about the ≈9% of the total impact. Complementary to Figure 6, Table 7 shows the total impact for every indicator category that has been presented.

Table 7. Quantification total impact for every conventional performance indicator for 1 kg Si.

Index	Value
GWP (kg CO2-eq.) – 100 years	679.84
ODP (kg R11/CFC-eq.)	5.50
Human Toxic (Cancer) (CTUh)	2.70
Human Toxic (Non Cancer) (CTUh)	3.90
Particulate Matter (kg PM2.5-eq.)	0.54
Ionising Radiation (kg U235-eq.)	0.26
POCP (kg NMVOC – eq.)	1.35
Acidification (moles of H+eq.)	5.95
Eco Tox (CTUe)	35.65
Resource depletion, water (kg)	406.11
Resource depletion, mineral, fossil (kg Sb eq.)	1.549

4. Conclusions

In addition to the presentation of the unified recycling methodology concerning c-Si PV module recycling, an extended assessment has been realised with the aid of conventional and custom EPIs. Our approach was based on the probable paths that a recycling unit could follow as far as the management of recovered material is concerned. In addition, two parallel scenarios have been evaluated in the context of a second useful life after the manufacturing of the new PV module. To summarise, the results of the research suggested EPBT to be 1.45 years. In addition, the calculation of EROI yielding 17.24 times the initial energy dedicated to the investment renders the endeavour environmentally and commercially viable.

These values are lying within the standards set by previous researchers. More specifically, the comparison of the obtained results with those given in Table 2 shows that the proposed recycling method leads to optimal values of EPBT, by taking into account solar irradiation: EPBT = 1.45 with annual irradiation 1800 kWh/m² for the recycled c-Si PV modules in Greece, compared to (minimum) EPBT = 1.2 with annual irradiation 2300 kWh/m² for the c-Si PV modules in Southwestern United States. On the other hand, the very high values of GWP in our methodology (679.84 kg CO₂-eq.) can be explained by the fuel mixture of Greek power system, which is based mainly on coal and natural gas. As a result, this study shows the positive performance of the proposed methodology. The selection of different electricity grid parameters can lead to a significant GWP reduction.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work has been co-financed by the European Union (European Regional Development Fund – ERDF) and Greek national funds through the Operational Program ‘Competitiveness and Entrepreneurship’ of the National Strategic Reference Framework (NSRF) – Research Funding Program: ‘COOPERATION 2011 – Redesign and Recycling of PV Panels’.

Notes on contributors

Athanailidis V. Ilias completed his diploma in Production Engineering and Management from Technical University of Crete in 2013, followed by a MS degree from National Technical University of Athens in System Automation. His research areas are Production Systems Automation, Technologies for Product Design CAD/CAM, Recycling Processes and Product Life Cycle Management. He is member of the Technical Chamber of Greece.

Rentoumis G. Meletios , PhD candidate, was born in Eleusis, Greece, in 1990. He received his Diploma, from the Technical University of Crete, in Production Engineering and Management in 2013 and his MS degree in Production Systems in 2015. His research areas are CAD/CAM and PLM systems, Renewable Energy Technologies and Manufacturing Infrastructure. He is member of the Technical Chamber of Greece.

Katsigiannis A. Yiannis was born in Athens, Greece. He is an assistant professor in the Department of Environmental and Natural Resources Engineering at Technological Educational Institute of Crete. His research areas are related to renewable energy sources and their integration to power systems, artificial intelligence and smart grids. He is member of the Technical Chamber of Greece.

Bilalis Nikolaos is a professor of Computer Aided Manufacturing and director of CAD Laboratory in School of Production Engineering and Management at the Technical University of Crete. His research interests are on Technologies for Product and Process Development and Integration, Tools for Product Design CAD/CAM/CAE/CAPP, Technologies for Product Development Rapid Prototyping and Rapid Tooling and Virtual Environments, Product Innovation, Tools and methodologies for Product Evaluation Design for Disassembly and Manufacturing Excellence.