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Resources, Conservation & Recycling 127 (2017) 29–41

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Resources, Conservation & Recycling journal homepage: www.elsevier.com/locate/resconrec

Photovoltaic waste assessment in Mexico

T

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Adriana Domínguez , Roland Geyer Bren School of Environmental Science and Management, University of California at Santa Barbara, Santa Barbara, CA 93106, United States

A R T I C L E I N F O

A B S T R A C T

Keywords: Photovoltaic PV waste Mexico Solar energy PV recycling

Large growth in renewable energy technology is required to combat climate change. Photovoltaic (PV) is the most promising technology with the largest potential, and Mexico has one of the best locations to exploit solar resources. During 2015, the Mexican government approved 7.8 GW of PV projects. This PV deployment is linked to a great generation of PV waste once the PV systems reach their end-of-life. Considering 30 years as average module lifetime, around 2045, Mexico will have 1.2 million mt of PV waste, 691 thousand mt of which are PV modules waste (31 millions PV modules). Since PV modules represent only 55% of the material contained in PV systems, this paper presents an assessment of the future PV-waste volumes in Mexico, including not only the PV modules but the balance of system (BOS). In total, near to 1 million mt of different metals will be contained in the PV-waste stream (42% Fe, 26% Al, 26% Si, 5% Cu). Fortunately, assuming the best available recycling technology, around 920 thousand mt of PV waste could be recycled. Precious and valuable metals (e.g. 271 mt of silver, 10 mt of gold, 17 mt of gallium, 10 mt of indium, 139 mt of cadmium and 100 mt of tellurium) can be recovered. This study analyzes the PV-waste generation under different scenarios such as: market share in PV modules technology, recycling yields for precious and critical metals, metal composition of transformers and thin film panel development.

1. Introduction Photovoltaic energy is a reliable and sustainable source of electricity. This renewable energy allows the increasing demand for electricity to be satisfied worldwide, without generation of greenhouse gases during its operation. During 2015, photovoltaic solar energy experienced a 28% growth rate, the highest growth rate of renewable energy capacity, followed by wind energy at 17% (Lins, 2016). The total installed capacity of solar photovoltaic at the end of 2015 was 227 GW, or 1.3% of the world's electricity generation (Masson and Brunisholz, 2015). According to the International Renewable Energy Agency (IRENA), it is expected to reach 4500 GW by 2050. High cumulative deployment rates are anticipated for some countries: China (1731 GW), India (600 GW), the United States (600 GW), Japan (350 GW) and Germany (110 GW). Latin America is still far from these levels of development, however, during 2015 solar PV increased 166% in this part of the world. Chile and Honduras contributed with 78% of the new solar capacity installed in 2015 (see Fig. 1 (Weckend et al., 2016). Chile is leading PV in South America, although countries like Mexico, Brazil and Peru have adopted policies that could favor the development of PV in the forthcoming years (Masson et al., 2014). The first PV projects development in Mexico were off-grid

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1

installations for rural electrification in the 1990s. Today, rooftop installations are being introduced in the commercial and residential sector. Distributed solar PV systems can yield energy cost-savings for commercial and residential sectors subjected to the DAC1 tariff. An overview of the cumulative installed capacity of solar PV in Mexico is displayed in Fig. 2. According to the Mexican Energy Secretariat (SENER), the first solar PV installations in Mexico were used mainly for rural electrification, supply of energy in the residential sector, water pumping, and exterior lighting in the commercial and industrial sectors. The year 2013 saw the greatest growth in solar capacity, up to 82 MW, in large part due to the first large-scale solar power project, Aura solar I (39 MW) (SENER, 2015a). The Energy Outlook published by SENER for the years 2015 to 2029 can be seen in Fig. 2. Although these are the best predictions available, there is a lack of data consistency. For instance, the energy outlook of 2015 shows 170 MW, however the cumulative installed capacity was 234 MW during 2015 (see Fig. 1). According to this document, Mexico has projected 1822 MW of additional solar PV projects in the public service and 1273 MW as distributed generation, by 2028 (SENER, 2015b). Furthermore, the survey Initiative for the Development of the Renewable Energy in Mexico performed by PricewaterhouseCoopers (PwC), estimates that starting in 2017, it will become economically

Corresponding author. E-mail address: [email protected] (A. Domínguez). DAC (Doméstico de Alto Consumo) tariff is the highest electricity tariff paid in the residential sector. For those consumers with high electricity requirements.

http://dx.doi.org/10.1016/j.resconrec.2017.08.013 Received 3 February 2017; Received in revised form 11 August 2017; Accepted 13 August 2017 Available online 31 August 2017 0921-3449/ © 2017 Elsevier B.V. All rights reserved.

Resources, Conservation & Recycling 127 (2017) 29–41

A. Domínguez, R. Geyer

Fig. 1. Cumulative PV capacity installed in Latin America in 2015. Data from Whiteman and Esparrago (2016), Werner et al. (2015).

Fig. 2. Overview of the cumulative installed PV capacity in Mexico and projections 2016–2030.

linked to the generation of photovoltaic waste once the PV systems reach the end of their lifespan (25–30 years). As PV installations increase, the PV waste will rise as well. Hence, it is important to draw up a plan for recycling future PV waste, since recycling has great benefits. The recovery and reuse of secondary materials has become an important issue due to the fact that PV modules use valuable metals (e.g. gold, silver, tellurium, indium, gallium, etc.) and other materials (e.g. glass) capable of being recovered, recycled and reused, sometimes within the same PV industry, contributing to the circular economy of this industry. Furthermore, recycling can lead to a decrease in resource depletion, a reduction of environmental impacts associated with mining and processing of valuable and limited virgin natural resources and energy savings. The growth of PV is just beginning in Mexico as well as in Latin America, and there is no regulation in regards to the disposal and treatment of this kind of waste. As a result, it is necessary to look for solutions to the PV waste challenge ahead of us in the coming decades. By planning the management of PV waste, the exports of these kind of waste to developing countries could be lessened, thereby reducing the environmental and human health impacts derived from improper recycling. Additionally, the development of a new PV waste recycling industry will generate jobs and make a significant contribution towards a sustainable renewable-based energy future. This paper aims to analyze the PV waste that will be generated in Mexico from end-of-life PV systems in the following years. It is very important to know the kind and amount of metals that are in the PV waste as well as other recoverable materials (e.g. glass), in order to propose a recycling plan that ensure that enough and the right type of recycling technology and capacity can be built. To accomplish the PV

feasible for the deployment of solar PV in residential rooftop, reaching 6.4 GW by 2020 (PwC, 2012). Moreover, by December 2015, the Energy Regulatory Commission of Mexico (CRE) had approved 275 projects distributed in 28 of the 31 states of the Mexican Republic, that together account for 7.8 GW. The projects under construction during 2016 account for 3.1 MW with the other 4.7 MW planned to begin construction between 2016 and 2018. This means that the target of 5.6 GW predicted by SENER by 2030 will be surpassed more than 10 years earlier. Due to Mexico's great solar potential, PV could potentially contribute closer to 30 GW of power capacity in 2030, according to IRENA's Roadmap 2030 for a Renewable Energy Future (REmap). This means that solar PV energy predicted by SENER should be increased five times. This 30 GW scenario will include 60% utility-scale and 40% rooftop installations, and will require an average annual installation rate of 1.5 GW/year (IRENA, 2015). The REmap also envisions 860 MW of solar PV rural electrification systems, in order to provide electricity to households without access to grid power, street lighting, agricultural pumping, mobile phone towers, etcetera. In an effort to reduce greenhouse gas emissions, Mexico is committed to achieve a target of 25% of renewable energy by 2018, 30% by 2021 and 35% by 2024. The development of solar PV in Mexico would contribute to the fulfillment of these targets. It is important to mention that Mexico has one of the best locations to exploit solar resources, with high solar irradiation levels, averaging 5.5 kWh/m2 up to 10 kWh/m2 per day, especially in the northwestern region during spring and summer (SENER, 2016). These levels are similar to the southwestern U.S. region, where many utility-scale solar projects are under construction (SEIA, 2016). However, the development of solar PV energy is 30

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A. Domínguez, R. Geyer

waste assessment in Mexico, a model to estimate the material content in PV installations has been developed. In Section 2, the material composition of each kind of PV module technology (c-Si, a-Si, CdTe and CIGS) is presented, taking into account the market share of PV modules by technology as well as the specifications of modules according to several companies. Besides, the metals used in the BOS systems (without batteries) was included in the model. In Section 3, the results of PV waste projections in Mexico are presented. This includes a detailed accounting of material contained in the PV systems in order to obtain the metal inventory of PV installations in Mexico. Then, a deep survey of recycling technologies (see Appendix A) was carried out, and the PV waste recycling model was introduced. The estimated amount of metal capable of being recycled was calculated. The model was developed for a specific market share of PV modules by technology, recycling yields for precious and critical metals, metal composition of transformers and thin film panel development, therefore a sensitivity analysis for these factors was included.

Table 1 PV technology groups.

2. Material and methods

where the useful lifetime of a module was considered as 25 years (BIO, 2011). The second is an IRENA/IEA Report from 2016 where the average lifetime of a PV panel was considered as 30 years (Weckend et al., 2016). The highlighted data for year 2014 was considered as the typical market share for the projects that will be developed in Mexico for the years 2016 to 2018.

A photovoltaic system is composed of several components such as: (1) PV modules, which are integrated by interconnected solar cells, (2) inverter, to convert the electric current from DC to AC, (3) transformer, (4) mounting structure, required for some PV systems like rooftopmounted systems or utility-scale power stations, (5) cabling, (6) tracking systems, to improve the caption of sun light and (7) batteries, in the case of off-grid PV systems that will require energy storage. All additional components to the PV panels are known as balance of system (BOS).

Silicon-based (c-Si)

Thin-film based

Other

Single or Monocrystalline (sc-Si) Poly or multicrystalline (mc-Si) Ribbon

Copper indium gallium (di)selenide (CIGS) Cadmium telluride (CdTe) Amorphous silicon (aSi) Emerging Thin-film Copper zinc tin sulphide (CZTS) Perovskite Organic PV (OPV) Dye-sensitised (DSSC) Colloidal quantum dot PV (QDPV)

Concentrating solar PV (CPV) Crystalline silicon (advanced c-Si)

Mat. content (kg) =

Install Cap. (W )*Mat. composition(kg/m2)*Market Share(%) Module Nominal Power (Wp/ m2) (1)

In Table 2 other technologies include concentrating solar PV (CPV) and organic PV/dye-sensitised cells (OPV/DSC). Due to lack of material composition of the modules identified as other technologies, its percentage of market share was added to the c-Si technology because it is currently the most used kind of photovoltaic panels. Hence, a market share of 91% for c-Si is used; it includes 55% of multicrystalline and 45% of monocrystalline silicon panels. The market share of a-Si was considered as 2% because trends show that it has been discontinued due to low efficiency (Weckend et al., 2016; Jean et al., 2015). The technical data for each kind of PV module was taken from the Ecoinvent database 3.3, additionally some commercial panels are presented as well in Table 3. The efficiency is calculated assuming the standard condition of 1000 W/m2 of irradiance and 25 °C. The unit process raw data for the photovoltaic panel c-Si is 1 m2 of PV panel, with 60 solar cells (156 × 156 cm2) with a capacity of 224 Wp. However, as no data about weight was given, 23 kg was taken from the average weight of this kind of c-Si modules according to the technical specifications of the solar panel producers enlisted in Table 3 for c-Si. A PV panel contains an average of 10 metallic elements such as silver, gold, copper, nickel, zinc and aluminum. These elements are profusely used with recycling yields ranging from 20% (i.e. molybdenum and tantalum) to 100% (i.e. aluminum and copper), depending on the metal. PV systems contain up to 20 different elements,

2.1. Methodology to estimate material content in PV waste Extensive research on material contained in PV modules as well as the BOS system (inverters, transformers, cabling, mounting and tracking) was performed and applied to Mexican PV installations. Since the Ecoinvent database 3.3 has the most recent information about materials used in each technology utilized in PV systems, data on material content of different PV module technologies and the BOS is mainly based on the Photovoltaics Report of the Swiss Centre for Life Cycle Inventories (Jungbluth et al., 2009). This report shows the unit process raw data for different PV technologies production. In order to ensure that the material content data taken from this report corresponds with the material composition of actual PV panels, the total mass was calculated and benchmarked against the weight of commercial PV modules. Additionally, several information sources were consulted and are also given for comparison. 2.1.1. PV modules There are several photovoltaic module technologies which are usually named according to their main light-absorbing material. In this paper, PV modules are grouped in three categories: silicon based, thinfilm based and other, as shown in Table 1. Frequently, crystalline silicon modules are often known as first generation (G1) technologies, thin film modules are classified as a second generation (G2) technologies, and emerging thin-film technologies and any other modules are known as third generation (G3) technologies. Additionally, PV technologies can be classified in accordance with their structure in two categories: wafer-based (fabricated on semiconductor wafers, e.g. c-Si) and thin-film cells (semiconducting films deposited onto a substrate) (Jean et al., 2015). In order to calculate the material content of the PV systems, Eq. (1) is applied. It is important to know the market share of PV technologies. The kind of PV module technology used for a specific project determines characteristics such as material composition, area, and module nominal power. Table 2 shows the market share of PV modules according to two sources. The first is an European Study from 2011,

Table 2 Market share of PV modules by technology. Installation

Source

PVWaste

Market share (%) c-Si

2000 2005 2010 2014 2020 2030

BIO (2011) BIO (2011) BIO (2011) Weckend et al. (2016) Weckend et al. (2016) Weckend et al. (2016)

2025 2030 2035 2044 2050 2060

90 95 80

a-Si

10 3 2 92 82 70.4

CdTe

CIGS

Other

2 17 5 5.2 4.7

1 2 5.8 15.7

1 7 9.2

The data marked in bold numbers were used to perform the calculations.

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A. Domínguez, R. Geyer

Table 3 Photovoltaic panels specifications. Source

PV modules

Area [m2]

Power [Wp]

Weigh [kg]

Efficiency [%]

Ecoinvent 3.3 Perseidsolar First Sunergy Ecoinvent 3.3 Sharp Sunelec Kaneka Ecoinvent 3.3 First Solar GE Ecoinvent 3.3 Xsunx Stion tsmc

c-Si

1.46 1.69 1.59 2.3 1.4 1.42 1.22 0.72 0.72 0.72 0.72 1.6 1.09 1.09

224 225 230 128 130 100 110 65 85 78 80 160 135 145

23 25 23 18.86 26 24 18.3 12 12 13 12.6 28 16.8 17.5

15.3 13.3 14.4 5.6 9.3 7 9 9 11.8 10.8 11 10 12.3 13.3

a-Si

CdTe

CIGS

The data marked in bold numbers were used to perform the calculations.

Fthenakis, 2014; Latunussa et al., 2016; Radziemska et al., 2010). The Ecoinvent database 3.3 has the most comprehensive material composition data and was used to generate the material inventory of each PV module technology. The material inventory is a comprehensive compilation of the materials (e.g. metals, EVA, glass) contained in a photovoltaic solar module. Depending on the PV technology, some modules can contain hazardous (e.g. Cd used in a-Si, CdTe and CIGS or Pb used in c-Si and CdTe) and critical materials. For instance, Table 5 shows the requirements of Gallium (in CIGS), Indium (in a-Si and CIGS), Magnesium (in c-Si, a-Si and CIGS) and Tellurium (in a-Si and CdTe), which are critical raw materials according to the European Union and the U.S. Department of Energy (Ad-hoc Working Group, 2010; DOE, 2011). The specific amounts of all materials contained in each kind of solar panel are presented in Table 5. Glass is widely used in thin film PV panels because glass is the solid and low-cost backing where the thin layer of semiconductor material is applied. An exception to this is a-Si which uses aluminum (42%) and steel (40%) as back sheet. The encapsulant material EVA (Ethyl Vinyl Acetate) accounts for 4 to 16% of the material composition of the PV panels. C-Si contains 10% steel whilst CdTe uses only 1%. Silicon represents 0.8% of material composition in c-Si technology because it is used for the solar cells, and although a-Si uses Si as well, it is used in a thin layer that only represents 0.0026%. Aluminum is another metal broadly used in PV panels, because the frame of modules is made of aluminum alloys, accounting for

as shown in Table 4. Many are valuable, such as precious and special metals, four of them are identified as critical raw materials, and some are catalogued as hazardous. The composition of the waste stream coming from PV systems changes with the development of new technologies and the search of environmentally friendly materials. Metals used in PV systems can be classified in five categories of materials: base and special metals including ferrous metals (iron and steel, which accounts 45% of the total weight), precious metals, hazardous or toxic metals, critical metals (metals essential for high-technology and green applications, but their supplies are susceptible to economical and political issues) and other materials (e.g. glass and EVA). Critical materials identified in Table 4 represent a key source needed to manufacture products for the clean energy economy worldwide. Hence, it is assumed that Mexico or any other country will be affected by the supply chain disruptions and price fluctuations related to these valuable resources. Furthermore, Mexico is not a producer of critical metals, except for the production of 44,000 mt of magnesite per year. Any attempt to recycle critical metals will support the global market for secondary raw materials. The average material composition of the different PV modules technologies analyzed in this paper, is presented in Table 5. Additional material composition data can be found in (Weckend et al., 2016; BIO, 2011; Corcelli et al., 2016; Katsigiannis et al., 2015; Choi and

Table 4 Metal used in photovoltaic systems. Component

Precious metals

Base and special metals

Toxic/hazardous metalsa

Other metals

Critical materialsb

Other materials

PV technology c-Si

Ag

Al, Cu, Ni Fe, Ti, Sn, Zn Al, Cu, Fe Cr, Mn, Zn Al, Cu, Fe, Ti Cr, Sn, Zn Al, Cu, Mo Sn, Zn Al, Cu, Fe Mn, Ni, Ta Sn, Zn Al, Cu, Fe Cu Al, Fe, Zn Al, Cu, Fe, Zn

Pb

Si

Mg

Glass, EVA

Cd

Si

Mg, Te, In

Cd, Pb

Si

Te

Glass, EVA

Cd, Se

Mg, Ga, In

Glass, EVA

Pb

Mg

a-Si CdTe CIGS Inverter

Transformer Cabling Mounting structure Tracking a b

Ag, Au

According to U.S. Environmental Protection Agency (EPA, 2016). According to the European Union and U.S. Department of Energy's Critical Material Strategy (Ad-hoc Working Group, 2010; DOE, 2011).

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Table 5 Average material composition of PV modules per technology. Technology metal

Ag Al Cd Cr Cu Ga In Fe Mg Mn Mo Ni Pb Se Si Sn Steel Te Ti Zn EVA Glass Total

c-Si

a-Si

[kg/m2]

[%]

8.89E−03 2.54E+00

5.77E−02 1.65E+01

1.13E−01

7.31E−01

8.02E−02

5.20E−01

1.63E−04 7.20E−04

1.06E−03 4.67E−03

1.22E−01 9.02E−06 1.47E+00

7.91E−01 5.86E−05 9.51E+00

8.01E−07 1.20E−06 1.00E+00 1.01E+01 1.54E+01

5.20E−06 7.81E−06 6.50E+00 6.54E+01 1.00E+02

CdTe

CIGS

[kg/m2]

[%]

[kg/m2]

[%]

[kg/m2]

[%]

3.24E+00 4.00E−04 4.40E−05 7.00E−02

4.16E+01 5.13E−03 5.65E−04 8.99E−01

1.50E−02 2.00E−02 3.00E−03 5.00E−01

9.04E−02 1.20E−01 1.81E−02 3.01E+00

1.51E+00 3.00E−02

8.58E+00 1.71E−01

9.00E−04 5.80E−05 1.02E−01 7.30E−05

1.16E−02 7.45E−04 1.31E+00 9.37E−04

5.00E−02 1.00E−02 5.00E−03

2.84E−01 5.68E−02 2.84E−02

4.70E−02

2.67E−01

1.00E−02

5.68E−02

1.00E−02

5.68E−02

1.00E−02

5.68E−02

1.00E−02 9.00E−01 1.50E+01 1.76E+01

5.68E−02 5.12E+00 8.53E+01 1.00E+02

2.00E−04

2.57E−03

3.10E+00 5.00E−04

3.98E+01 6.42E−03

2.90E−05 1.24E+00 3.58E−02 7.82E+00

3.72E−04 1.59E+01 4.59E−01 1.00E+02

7.00E−04

4.22E−03

5.00E−02 2.30E−07 2.00E−01 2.00E−02 2.30E−08 3.00E−08 6.00E−01 1.52E+01 1.66E+01

3.01E−01 1.39E−06 1.20E+00 1.20E−01 1.39E−07 1.81E−07 3.61E+00 9.15E+01 1.00E+02

The total material composition of PV modules are highlighted in bold.

composition of this inverter. Steel accounts for 75%, followed by 17% copper and 7% aluminum. The inverters use an adaptable technology to acquire the maximum output from the solar panels. In an attempt to assure that the inverter will be operating at its maximum output, the PV arrays are usually oversized by using an inverter-sizing ratio around 1.15 (Mondol et al., 2006). This ratio is used in Eq. (2), which provides an estimation of the number of inverters needed for a specific PV capacity.

9–42% of mass. Magnesium is present in the three panels that use an aluminum frame because the aluminum alloy considered is AlMg3. The thin film CdTe solar modules do not have a frame. Copper is used for interconnectors accounting for 0.3–3%. 2.1.2. PV inverters Generally the solar PV system will be integrated into the grid, which means that the DC supply must be converted to grid frequency alternating current (AC). This is performed with an electronic device called an inverter. There are different types of inverters available to suit different applications. For instance, the micro inverter controls the power of an individual solar panel and is used often in domestic and commercial installations. Another type, is the string inverter which takes its input from a number of panels. Utility-scale solar PV power plants use large string inverters (Breeze, 2016). A string inverter of 500 kW-AC was set up as the inverter used in the PV installations analyzed in this paper. The material inventory of this kind of inverter was developed with data from the Ecoinvent database 3.3. Table 6 shows the material

Number of inverters =

PV Capacity (W )*Inverter Sizing ratio Inverter Capacity (W )

(2)

2.1.3. Transformers Conventional distribution transformers are widely used and represent a key element in any PV system (Testa et al., 2012). Eq. (3) is used to estimate the number of transformers needed for a specific PV capacity. Usually the power factor for the transformer is considered to be 0.8.

Number of transformers =

Table 6 Average material composition of the inverter 500 kW-ac.

PV Capacity (W )*Transformer power factor Transformer Capacity (W ) (3)

Metal

[kg/inverter]

[%]

Al Cu Steel Mn Ni Fe Ag Ta Sn Mg Pb Au Zn Total Electricity (kWh) NG (MJ)

131 339 1438 0.001 0.16 0.05 0.37 0.02 0.01 0.01 1.8 0.51 0.4 1912 36,344 0.1

6.9 17.7 75.2 0.0001 0.01 0.003 0.02 0.001 0.001 0.0004 0.1 0.03 0.02 100

The metal inventory for a 1.6 kVA copper transformer was taken from the Ecoinvent database 3.3 and is presented in Table 7. The units are kg of metal per kg of transformer. A 1.6 kVA transformer weights 3995 kg (Schneider, n.d.). Aluminum transformers weight 45% of equivalent copper-based ones (ABB, 2012). This means that a transformer uses 0.14 kg of aluminum instead of 0.32 kg of copper, as shown in Table 7. Table 7 Average material composition of the transformer (Units: kgmetal/kgtransformer). Cu transformer Cu Fe

Al transformer 0.32 0.63 Al Fe

The most used metals in the inverters are marked in bold.

33

0.14 0.63

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(GWh/year), and year of installation. CFE provides information about the capacity in MW, but not information about the number of modules or the kind of technology used in each project (CFE, 2015). Typical outputs for commercial PV modules vary between 175 Wp and 315 Wp. It was assumed that 250 Wp modules were utilized when no specific information is available in the website of each project. If the developers of an specific project indicates the number of modules that will be installed, then it can be determined what modules will be used among the solar projects in regards to its maximum power (e.g. 240 Wp, 245 Wp, 250 Wp, 290 Wp, 300 Wp, 315 Wp, and etcetera). A total of 277 solar installations were located among 28 of the 32 states of Mexico, as depicted in Table 9. Each project can be identified according to its phase of development: under construction or under development. The total installed capacity of 7.8 GW can be partially or totally operative between 2013 and 2018. Based on the project capacity, the number and capacity of modules (e.g. 245 Wp, 250 Wp, 350 Wp, etc.), inverters (500 kW) and transformers (1.6 kVA) needed for their operation were calculated, a summary of which is depicted in Table 9. Total amounts of PV modules indicate that for each GW installed, around 4 millions of PV waste panels will be released. Considering that the lifetime of PV modules has been estimated to be 25–30 years, on average, it can be assumed that the installed PV systems turn into PV waste around year 2045. Taking into account the number of total PV modules of Table 9, the market share by technology for 2014 of Table 2, and the weight of each kind of PV modules, it could be roughly calculated that in 2045 Mexico will have 690,907 metric tons of end-of-life PV panels. This is on the high end of another cumulative PV waste estimate for Mexico which states that between 2040 and 2050 there will be 55,000 to 630,000 mt of PV panels waste (Weckend et al., 2016).

Table 8 Average material required for open ground mounting. Metal

[kg/m2 of PV module]

Al Steel Zn

3.9 7.5 0.27

2.1.4. Cabling There is a wide variety of wires and cables used in photovoltaic power generation. Cables connect the solar panel to the combiner box, inverter and transformer. There are two common conductor materials used in PV systems, aluminum and copper. Copper has a greater conductivity than aluminum. In this study, it is assumed that copper is the metal used in cabling with a ratio of 0.64 kg of Cu per m2 of PV module (Sinha et al., 2011). For a typical PV nominal cable diameter of 5 mm, the weight of copper is around 35 kg per kilometer of cable. 2.1.5. Mounting structures The most common materials used in mounting structures are steel and aluminum. Aluminum extrusions have been widely utilized by the PV industry for PV frames and are usually used in both residential and commercial rooftop PV mounting structures. In regards to ground mounted PV installations, galvanized steel structures are preferred, and vary in design depending on geographical location and soil conditions. The fact that shipping aluminum is less expensive than shipping steel, and that lightweight aluminum components are easy to assemble, which reduce the installation costs, could lead to the use of aluminum even for the largest ground mount utility scale installations (Mascarin and Hannibal, 2009). For this study, the photovoltaic mounting system production for open ground modules includes the material listed in Table 8, based on Ecoinvent database 3.3. This dataset represents the installation of the mounting system needed for open ground mounting of 1 m2 of PV module, which uses both metals; aluminum and steel.

Table 9 PV projects approved in Mexico up to December 2015.

2.1.6. Tracking systems Solar tracking systems automatically move to follow the trajectory of the sun across the sky, maximizing the power output of photovoltaic systems. There are various different kinds of solar trackers, such a single or dual axis. Although advancements in technology and reliability of these systems have been accomplished, the main drawbacks include the higher cost and the maintenance compared with their stationary counterparts. Furthermore, the use of solar trackers depends on the location and site conditions of the PV installation. According to a recent study, solar tracking systems are not feasible for Sunbelt countries like Mexico, because the gain in energy does not compensate the energy required for running the tracking systems (Eldin et al., 2015). On top of that, the PV panels could overheat due to excessive exposure to solar irradiance in hot climate countries. Despite this fact, there are some PV installations in Mexico approved with tracking systems already. These installations are located in the states of Aguascalientes (150 MW), Baja California (60 MW), Chihuahua (243 MW) and Coahuila (100 MW). Tracking systems use 10% more aluminum, 50% more steel and zinc and 30% more copper than fixed mounting structures. In this study, the increase of metal was accounted for in the final metal inventory. 3. Results 3.1. PV waste projections in Mexico Beyond data provided in Section 1 in Fig. 2, a deeper analysis of the PV sector in Mexico has been carried out. For this endeavour, it was necessary to look into the CFE – Comision Federal de Electricidad (Federal Electricity Commission) database in order to identify data such as: state, city, place, installed capacity (MW), potential electricity

State

Capacitya [MW]

Modules [units]

Inverters [units]

Transformers [units]

Sonora Chihuahua Guanajuato Coahuila Durango San Luis Potosi Zacatecas Jalisco Baja California Sur Sinaloa Aguascalientes Nuevo Leon Tamaulipas Baja California Yucatan Chiapas Queretaro Estado de Mexico Colima Hidalgo Campeche Guerrero Puebla Tabasco Nayarit Tlaxcala Veracruz D.F. Michoacan Morelos Oaxaca Total

1698 1666 559 530 437 435 346 345 341 280 198 159 126 111 97 92 91 81 64 50 32 32 32 32 10 10 1 0 0 0 0 7856

6,710,364 6,653,655 2,235,728 2,079,812 1,748,800 1,740,000 1,385,120 1,380,786 1,366,618 1,125,000 670,182 635,680 504,000 443,560 388,760 368,000 365,600 325,232 256,000 200,000 128,000 128,000 128,000 128,000 40,000 40,000 4360 0 0 0 0 31,179,257

3906 3832 1286 1218 1006 1001 796 794 784 644 457 366 290 255 224 212 210 187 147 115 74 74 74 74 23 23 3 0 0 0 0 18,071

1327 1302 437 414 342 340 271 270 266 219 155 124 98 87 76 72 71 64 50 39 25 25 25 25 8 8 1 0 0 0 0 6138

a

34

Data from CFE (2015).

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A. Domínguez, R. Geyer

PV technology. Semiconductor materials such as silicon (26%) and zinc (1%) are used for the solar cells. Copper is mainly used for wiring, accounting 5%. It is important to mention that these calculations correspond to the 7.8 GW of cumulative installed capacity up to December 2015, and that any additional GW installed will increase these results.

3.2. PV waste recycling in Mexico Recycling of PV waste is crucial, not only to avoid environmental pollution but to conserve mineral resources. Mexico has no specific regulation in place regarding PV waste recycling. However, there are different legal instruments that could contribute to the PV waste management such as the Political Constitution of the United States of Mexico, the General Law for Ecological Balance and Environmental Protection (LGEEPA) and the General Law for Prevention and Comprehensive Management of Waste (LGPGIR). In this framework, the country commits to conduct an adequate management and disposal of the waste classified as “special handling”, where e-waste is including but is limited to specifics items and does not include solar panels. To estimate the amount of secondary materials that can be recycled from PV installations, extended research about the actual and future recycling yields of each metal identified in the metal inventory of Table 10 was performed. The recycling yields for twenty one metals used in PV systems are displayed in Table A13. These recycling yields were taken from twenty-three specialized sources in PV recycling. Some of these recycling processes are under experimental, pilot scale, patented or commercial status, and were described in Table A12. These recycling facilities are located mainly in the U.S., the European Union (Germany, Italy, Sweden, Poland and Slovakia), Asia (Korea, China and Japan) and Brazil. Since it is unknown if Mexico will achieve these recycling yields; because the PV recycling industry has yet to be established; this paper shows the best recycling scenario assuming the use of the best available recycling technology. Existing estimates show that for the twenty-one metals analyzed in this study, the recycling yield is around 90% for eleven metals, and for the other nine metals it is between 18% and 52%. The recycling yield reported for solar cells varies from 60% to 97% (Fthenakis, 2000; Kyu Yi et al., 2014; Tao and Yu, 2015). The total recycling yield for silicon modules is around 90% (Nieland et al., 2012; Anctil and Fthenakis, 2013). The same yield is reported for the BOS (Ravikumar et al., 2015).

Fig. 3. Metal inventory of PV components.

It can be observed in Table 9 that the states of Chihuahua, Sonora and Coahuila located in the northwest border region with the U.S., have the greatest installed capacity, accounting for 50% of the total capacity. Conversely, the states of Michoacan, Morelos, Oaxaca and Distrito Federal do not have any PV projects. Considering that 45% of the bulk of materials used in PV systems belong to the BOS system and 55% to the PV modules, as shown in Fig. 3, this study analyzes not only the PV modules but the balance of system (BOS) of PV installations. It includes inverters, transformers, mounting structures, cabling and tracking. Additional equipment such as electricity meters, fusebox, batteries and charge controller are not considered. Finally, the metal inventory for Mexico was developed and is shown in Table 10. It can be observed that Mexico will have around 1 million metric tons of metal waste contained in end-of-life PV systems, including BOS components (inverters, transformers, cabling, mounting and tracking) in a 10 years period between 2040 and 2050. The metal waste stream will be composed mainly of iron (e.g. used broadly as steel, for mounting structures) with 42%. Aluminum and magnesium will account for 26% and 0.4%, respectively, because the alloy AlMg3 is used mainly for the frames of c-Si technolog which is the predominant

Table 10 Metal inventory of PV installations in Mexico.

35

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A. Domínguez, R. Geyer

Table 11 Recycling yields and estimated amount of metal in PV waste in Mexico.

Table A13 shows the recycling yields selected for this study, however, these yields can be modified in the model to adapt the recycling scenario for the Mexican case study once more information becomes available. Table 11 shows the assumed recycling yields as well as the amount of metals that could be potentially recycled. Around 94% of the metals in the PV systems could be recovered. This is around 920 thousand mt of secondary metal. The recycling processes of aluminum, copper and iron are well known and achieve very high yields. These metals can be easily recovered from solar PV end-of-life components because the bulk of these materials are used as mounting structures, frames and cables. Nevertheless, there are metals that are contained in small amounts within the solar cells, transformers and inverters, which require more complex recycling processes, such as those presented in Table A12. Certainly, there are challenges in metal recycling (e.g. recycling technologies, product design, social behavior, etc.) that need to be addressed and factors (volumes involved, economic value of metals) that need to be considered. These factors should be contextualized for the Mexican case study once the PV recycling sector has been established and accurate information becomes available.

Fig. 4. Market share of different PV technologies. Data from Weckend et al. (2016).

be c-Si, 6.9% CdTe and 7.3% CIGS. The sensitivity analysis depicted in Fig. 5 shows the percentage by which each metal will vary depending on the change of PV module technology. For instance, an increment of 5% on the market share of CIGS will greatly increase the demand of gallium (265%), indium (170%) and selenium (265%). A growth of 2% on the market share of CdTe will intensify the use of cadmium (100%) and tellurium (36%). Since the market share of c-Si technologies will decline by 7%, the use of silicon (−6%) will diminish as well as the aluminum (−9%) and magnesium (−9%) used for the frames of this kind of modules. Iron and manganese will not be used any more because a-Si is the only PV technology that uses these metals.

3.3. Sensitivity analysis The model was developed for a specific market share of PV modules by technology type, recycling yields and metal composition of BOS (e.g. transformer) and PV modules, hence a sensitivity analysis for these factors was conducted. 3.3.1. Effect of market share in PV modules technology According to Fig. 4, the solar PV technology trend shows a reduction in silicon-based technologies, which includes mono-crystalline, poly or multi-crystalline, ribbon and amorphous silicon. The use of CdTe thin film will decrease slightly, while CIGS will increase. It is important to analyze how the material of PV waste will change, if these trends continue. As the metal composition of other technologies (e.g. organic PV, dye-sensitised cells, etc.) is currently not well known, it was assumed that by 2030, 44.1% of other technologies will be shared between three technologies (c-Si, CdTe and CIGS) proportionally to their market share in 2014, while a-Si technology will be discontinued. Hence, 85.8% will

3.3.2. Effect of recycling yields for target materials Recycling of metals contained in PV systems may be vital to enable further global PV deployment growth. Base metals like aluminum, copper and iron have reached high recycling yields. Likewise, toxic/ hazardous metals experience high recycling yields due to the specific restrictions related to their disposal. However, recent studies reveal that historical production of critical metals (e.g. Te, In, Ga) will be insufficient to satisfy the supply of some metals required to meet the rapid PV deployment (Anctil and Fthenakis, 2013; Kavlak et al., 2015). These 36

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A. Domínguez, R. Geyer

Fig. 5. Change in material inventory according to PV forecast technology change.

Fig. 6. Change in material recovery according to different recycling yields.

aluminum instead of copper is clear (USGS, 2016). The difference in price is around 41 million dollars.

materials face constraints in their supply because they are by-products of base metals. This paper investigates the metal recycling potential of some critical and precious metals (see Fig. 6). The recycling yields shown in Table 11, are susceptible to change depending on the recycling technology utilized. Although some metals such as silver, cadmium, gallium, indium, selenium and tellurium, mainly used in thin film technologies, show very high levels of recovery (89–95%), the fact is that these technologies are relatively new. The optimistic high recycling yields shown in Table 11 could be lower, and the amount of material recovered will be different, as shown in Fig. 6. The low recycling yields considered are: silver – 30%, cadmium – 27%, gallium – 30%, indium – 20%, selenium – 38%, and tellurium – 35% (Paiano, 2014; Goe and Gaustad, 2014; Buchert et al., 2009). On the other hand, valuable metals such as gold, or others like molybdenum, chromium, tantalum and zinc, with very low recycling yields (18–36%) may experience an improvement in their recovery (Goe and Gaustad, 2014; Sibley, 2011). For instance, if gold could be recovered with a recycling yield of 50% instead of the actual 36%, an additional 40% of gold would be recovered.

3.3.4. Effect of thin film panel developments The thin film technologies such as cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) are still under development. The advantage of these PV technologies is the great reduction of semiconductor material usage compared with c-Si, leading to lower production cost. Due to the increasing scarcity of mineral resources further reduction in the use of some metals. For instance, CIGS panels will experience a tiny reduction in copper (0.002%), indium (0.006%), gallium (0.002%) and selenium (0.01%), but an increase of 1% in aluminum. Hence, the most significant change on metal inventory is an increase of 3260 tons of aluminum. For the case of CdTe panels, the semiconductor material usage will decline almost by half (0.7%), while nickel, zinc and tin will increase to 0.41% (Weckend et al., 2016). The semiconductor material will decrease in 3.6 tons for Cd and 1.6 tons for Te. 4. Discussion

3.3.3. Effect of metal composition of transformers (Al vs. Cu) Copper and aluminum are the two conductors used in transformer windings. Depending on factors such as application, cost and availability, one or other can be selected. For instance, copper is frequently used in large power transformers (Olivares et al., 2010). The material inventory presented in Table 10, considers that transformers are made only of copper. However, the use of aluminum transformer is expected to increase because global experience suggests that the distribution transformers wound with aluminum are as good as copper, with the advantage of cost reduction (Paul et al., 2012). When aluminum is selected as winding material, the amount of copper used in the overall PV system will decrease in 7844 metric tons (13.7%), whilst aluminum will increase in 3530 metric tons (1.4%). Considering the price of copper (6.1 $/kg) and aluminum (1.9 $/kg), the economic advantage of using

As PV installations increase, PV waste will rise as well. It is important to analyze the development of PV in countries such as Mexico, that have little current PV deployment but a large impending increase in PV capacity. This analysis allows the government and stakeholders to become aware of the reality of the PV sector in Mexico, which generally is underestimated in reports made by worldwide experts. A deeper analysis of national databases was performed and accurate data were obtained. The quality, magnitude and efficiency required to have proper end-of-life management of PV systems relies upon this factual data. The present study excludes residential rooftop installations. These will increase the amount of future PV waste, but probably not significantly. Some studies have presented material composition of different technologies which include up to five metals (Weckend et al., 2016; 37

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A. Domínguez, R. Geyer

region with the U.S., have the greatest installed capacity, together accounting for 50% of the total capacity, while the states of Michoacan, Morelos, Oaxaca and Distrito Federal, do not report any project. This study reveals that for each GW of installed PV capacity, around 4 millions of PV modules waste will be generated. By 2045, Mexico will have 1.2 million mt of PV waste, 691 thousand mt of which are PV modules (31 millions PV modules). If the BOS components (inverters, transformers, cabling, mounting and tracking) are included, nearly to 1 million mt of different metals will be contained in the PV-waste stream (42% Fe, 26% Al, 26% Si, 5% Cu). Fortunately, assuming the best available recycling technology, around 920 thousand mt of PV waste could be recovered. Precious and valuable metals (271 mt of silver, 10 mt of gold, 17 mt of gallium, 10 mt of indium, 139 mt of cadmium and 100 mt of tellurium) can be recovered. Recycling of metals contained in PV systems may be vital to enable further global PV deployment growth. Metal recycling faces challenges in recycling technologies, product design and social behavior that need to be addressed. This survey aims to inform Mexican PV stakeholders to draw up a plan for recycling the future amounts of PV waste, because recycling has great benefits such as: reduction of energy use and environmental impacts associated with mining and processing of valuable and limited virgin natural resources avoiding mineral depletion, development of a new PV waste industry, lessen the exports of PV waste to developing countries, reduction of environmental and human health impacts from improper recycling and contribution to a sustainable renewable-base energy future. Furthermore, the development of the new PV waste recycling industry will create job opportunities in Mexico. Knowing nothing about the value of the extractable materials or benefit in any way from the recycling of PV waste, results in little to no incentive to recycle. With this lack of incentive, how and where PV panels and BOS are recycled depends greatly on regulatory policies and mechanisms (or lack thereof) of each country to deal with PV waste. Currently, Mexico does not have a specific regulation in regards to PV waste recycling. However, it is very important to know the amount and kind of metals contained in the PV waste in order to propose a recycling plan that ensures that enough and the right type of recycling technology and capacity is available, guaranteeing that hazardous metals will be properly disposed and that valuable metals will be recovered. This paper is the first analysis of this kind for Mexico. In future research not only technical but also environmental, social and economic dimensions should be studied.

BIO, 2011; Corcelli et al., 2016; Katsigiannis et al., 2015; Choi and Fthenakis, 2014; Latunussa et al., 2016; Radziemska et al., 2010). However, this study provides a more detailed model of the average material composition of PV modules per technology analyzing twentyone metals contained in the PV systems. Some of these metals (e.g. Ga, In) are used in such a small quantity that they are often neglected, but nevertheless, their importance make it imperative to analyze. Trends concerning the market share of PV module technologies show a reduction of silicon-based technologies and an increase of thinfilm based technologies (CdTe and CIGS). Accordingly, the material content of PV waste will change. The result is a significant growth in gallium (265%), indium (170%) and selenium (265%) demand for CIGS, as well as cadmium (100%) and tellurium (36%) demand for CdTe. All these metals are considered critical due to the supply chain disruptions and price fluctuations they have faced. The mining industry in Mexico is vital for the development of the country because it accounts for one quarter of government revenue. Mexico is the world leader in silver production and is among the world's top 10 producers of cadmium, copper, gold, lead, molybdenum and zinc, however, Mexico imports metals that could be recycled from PV waste, like aluminum and tellurium. In this regard, despite the materials used in PV, recycling is imperative to assure sustainable PV deployment. It is worth mentioning that different types of metals contained in PV modules face specific recycling challenges, which in turn influence their recycling yields. Actual recycling yields are strongly related to the economic value of the recovered metal. For instance, currently, a critical metal like Indium (largely used in LCD production) is not recovered from e-waste since it is not as valuable as the recovered precious metals (e.g. indium – 340 $/kg, gold – 40,831 $/kg, in 2016 U.S. dollar terms (USGS, 2016)). Also, indium is produced mainly from the residues generated during the zinc ore processing which is cheap (2.2 $/kg). Thus, while indium has high value, it is still sent to landfill or incineration (Götze and Rotter, 2012). This situation could be experienced by indium and other critical metals in PV recycling if the lack of regulations for the recovery of specific metals continue, leading to lower recycling rates than technically feasible. However, this work attempts to present a recycling scenario where recycling yields are driven by environmental concerns rather than economic. 5. Conclusions

Acknowledgements

This paper estimates and quantifies the generation of PV waste in Mexico across different PV technologies. The analysis involves a total of 277 solar installations located among 28 of the 32 states of Mexico, altogether accounting for 7.8 GW of installed capacity that should be partially or totally operative between 2013 and 2018. The states of Chihuahua, Sonora and Coahuila located in the northwest border

We would like to thank Parikhit Sinha from First Solar for his assistance in obtaining reliable industry data. The research leading to these results has received funding from the CONACYT (Consejo Nacional de Ciencia y Tecnologia - Mexico) grant no. 263709.

Appendix A. Recycling technologies for PV modules Recycling technologies for PV panels have already been researched for the past 15 years (Weckend et al., 2016). Photovoltaic companies such as First Solar® and SolarWorld ®, have constructed the first commercial recycling plants for thin-film modules and c-Si modules, respectively. Both companies are members of PV CYCLE. This association was founded in 2007 with the objective to make the photovoltaic industry “double green” by implementing the voluntary take back and recycling for end-of-life PV modules. Together with project partners such as PV manufacturing companies, research institutes, mining companies and government agencies, PV CYCLE supports R & D activities to undertake in-depth research in recycling PV modules. For instance, in Europe there are some research initiatives to investigate different PV recycling technologies. FRELP – Full Recovery End-oflife Photovoltaic, which test and develops innovative methods that can enable high-value recovery of materials and energy in an economical viable way (FRELP, n.d.). PV Morede – Photovoltaic panels Mobile Recycling Device focused on the development and industrialization of a patented mobile plant for the recycling of PV modules aimed at the recovery of raw materials and energy operating with an innovative mechanical process (Morede, n.d.). CU-PV – Cradle-to-cradle sustainable PV modules, which looks for reduction of silver in silicon PV technology through new metallization methods (CU, n.d.). Furthermore, several recycling technologies have been developed around the world in an experimental way, some of them have been patented and some other are under pilot scale, the technical details can be consulted in Table A12.

38

39

Commercial

CdTe

Crushing, acid leaching, separation, precipitation, decantation, filtration and electrowinning

Solar World: plastic components are removed by a thermal process at 600 °C before the silicon wafer is recovered through etching (Deustche Solar) First Solar: shredder, hammer mill, leaching, solid–liquid separation, vibrating screen to separate glass and EVA. The rising of glass allows the metal recovery by precipitation and the metal rich filter cake is recycled by a third party

NA Te: 92% Mo: 6% Glass, EVA, Se, In and Ga

CdTe, CdS CdTe CIGS

Silicon cell: 98% Glass: 90% Te: 80% Semiconductor material:95% Glass: 90%

c-Si CdTe

Glass, EVA and Te

Te: 95–98% Cd: 4% Cd: 16–20% Cu: 94–99% Se: 88–90%

CdTe CIS

Shredding and leaching with a nitric acid based lixiviant. Then, the lixiviant is electrolysed to precipitate the Te and decomposed to obtain the Cd. A subsequent decomposition and oxidation and distillation is required to obtain Cu, Se, In and Zn from CIS. ANTEC Solar GmbH: Mechanical disintegration, pyrolysis treatment at 300 °C, Dry etching at 400 °C and precipitation. Treatment with dilute hydrochloric acid under precipitation of hydrogen peroxide Crushing, acid leaching, skimming and filtration, precipitation and two phases of stripping and electrodeposition

Patented

Glass: 80–85% Se: 90–99% Ag: 94% Cd: 98% Te: 98% In is also recovered

c-Si, a-Si, CdTe CIGS c-Si

Silicon cell: 80% (very low efficiencies) Glass: 100% Silicon cell: 100% Silicon cell Cd: 99.99% Te: 99.99% Cu was partially extracted Si: 62% Cu:85% Glass: 100% Si: 86 % Glass: 100%

Material recovered

CdTe,CIS

Thermal dismantling at 500 °C followed by crushing/milling to reduce particle size, vacuum blasting and flotation. The last product is sent to 5N Plus.

c-Si c-Si CdTe

Dissolution of EVA with trichloroethylene at 80 °C for 10 days, and mechanical pressure to suppress the swelling of EVA Thermal treatment with a layer of SiO_2 A dilute aqueous solution of hydrogen peroxide and sulfuric acid was applied to leach out cadmium and tellurium. Then, cation-exchange resins were used to separate cadmium and copper form tellurium. Two steps heating were used in a thermal treatment process. Immersion in organic solvent to recover the tempered glass followed by heat treatment at 600 °C for 1 h to remove the EVA, then the PV cell is immersed in a chemical etching solution for 20 min Two blade rotors crushing followed by hammer crushing, thermal treatment (650 °C) and sieving Oxidation at 800 °C for 1 h and reduction by two options: 1) Riley reaction with an organic molecule and 2) sulphur dioxide Milling the modules, sieving, leaching in nitric acid and precipitating the leached solution using sodium chloride.

Pilot scale

c-Si

Pyrolysis in a conveyor belt furnace and the pyrolysis in a fluidised bed reactor

Experimental

c-Si c-Si

PV

Recycling process technology

Status

Table A12 Photovoltaic module recycling technologies.

Krueger (2010)

Larsen (2009)

Diequez et al. (2003) Palitzsch and Loser (2011) Rocchetti and Beolchini (2015) Rocchetti and Beolchini (2015)

Goozner et al. (1997) Goozner et al. (1997)

Berger et al. (2010)

Granata et al. (2014) Gustafsson et al. (2014) Dias et al. (2016)

Wang et al. (2012) Kang et al. (2012)

Doi et al. (2001) Radziemska et al. (2010) Fthenakis et al. (2009)

Frisson et al. (2000)

Reference

A. Domínguez, R. Geyer

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A. Domínguez, R. Geyer

Table A13 Metal Recycling Yields [%]. Source

Ag

Al

Latunussa et al. (2016) Paiano (2014) Tao and Yu (2015) Huang and Tao (2015) Redlinger et al. (2015) Katsigiannis et al. (2015) Goe and Gaustad (2014) Kyu Yi et al. (2014) Choi and Fthenakis (2014) Bergensen et al. (2014)a Marwede et al. (2013) Sinha (2013) Tobergte and Curtis (2013) Marwede and Reller (2012) Kang et al. (2012) Wang et al. (2012) Anctil and Fthenakis (2013) Sibley (2011) Held and Ilg (2011) Wambach et al. (2009) Buchert et al. (2009) Hahne and Hirn (2009) Larsen (2009) This study

95 30–50

100

a

Au

Cd

Cr

Cu

20–100

100 78–100 94–99

27

78 41

Fe

Ga

14 100

36

Mg

Mn

Mo

Ni

Pb

Se

88–90 80–99

22 100

In

Si

Sn

Ta

95 76–86 74 90

Te

Ti

Zn

80–95 80–100

80–99

Cell

97

80–99 85

33

18

96

38

32 100

97

85–96 85–96 95 90

85–96 85–96 90

100 80–95

80–97 86 85

32

36

29

95 14 95

62 95

20

30

33

37

33

41

63

21

52

27

95

100

78

85 40

15

10–20

35–90

21

80 95

78–100 95

100

36

95

20

100

73–85 90

90

90

33

37

18

41

96

89

100

32

52

27

BOS.

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