1. Introduction

Photovoltaic (PV) solar modules are designed to produce renewable and clean energy for approximately 25 years. The first substantial PV installations happened in the early 1990s and since early 2000s solar PV electricity distribution has grown extremely fast [1].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The cumulative worldwide PV generation capacity reached 302 GW in the end of 2016 [2] and the predominant technology (90% of the market) is crystalline silicon (c-Si) cells [3]. Also, during the last years there were several advances on renewable energy in general, including significant price decline and a constant increase in attention to environmental impacts from energy sources [4, 5]. Furthermore, the International Technology Roadmap for Photovoltaic (ITRPV) prediction for the installed PV capacity in 2050 is 4500 gigawatts [6].

and distributors of photovoltaic modules have been invited to provide information on the chemical substances contained in the product and to inform the waste disposal companies.

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In USA, some states go beyond the Resource Conservation and Recovery Act which regulates hazardous and non-hazardous waste management [13]. California, for example, has additional threshold limits for hazardous materials classification based on the Senate Bill 489 that categorizes end-of-life PV modules as Universal Waste (facilitating easy transport). This bill is cur-

In Australia, governments have recognized the significance of guaranteeing that regulations are in place to deal with the PV waste issue. Ministers agreed that the state of Victoria would lead innovative programs that seek to reduce the environmental impacts caused throughout the lifecycle of photovoltaic systems. These efforts are part of an industry-led voluntary product management arrangement to address the potential emerging risks of PV systems and their waste. PV modules are listed under the National Product Administration Act to signal the

The non-inclusion of PV residues in waste legislation in some countries is due to different reasons. Solar modules have a lifespan of up to 25–30 years [18] and so there has been limited interest in investigating the aspects of EoL so far. Moreover, the quantity of this type of waste is still considered insignificant compared to the quantity of other WEEE [19], which currently makes setting up specific recycling plants for solar modules uneconomical. In addition, the definition of mandatory requirements for EoL treatment could still be an obstacle to the effective acceptance of these recycling processes [20]. Because of that, there should be a continuous focus on scientific evidences on the potential impacts and benefits related to the

Furthermore, recycling processes for all the different PV technologies are not yet well developed. The processes are well developed for mono or multicrystalline silicon. FirstSolar [21] has an established recycling process for CdTe, but for other thin films there are still room for improvements. and are being tested and for generation 3 (new materials [22]) the recycling

Only about 10% of PV modules are recycled worldwide. The main reason for that is the lack of regulation. Actually, it has been shown that, for the current recycling technologies, siliconbased modules do not have enough valuable materials to be recovered and the cost of the recycling process is always higher than the landfill option (not considering the externalities), making recycling an economically unfavorable option [23]. However, the prediction for 2050 is that the recoverable value could cumulatively exceed 15 billion US dollars (equivalent to 2 billion modules, or 630 GW) [7]. In addition, the recycling of solar PV modules can ensure the sustainability of the long-term supply chain [24], thereby increasing the recovery of energy and embedded materials and, also, reducing CO2 emissions and energy payback time (EPBT)

For years, the PV industry and researchers have worked intensively in search of different types of efficient and cost-effective materials to manufacture solar PV modules and specific ways of

JPEA strongly recommend that industry follow the guidelines [15].

intention to consider a scheme to deal with such waste [17].

treatment of photovoltaic residues.

technologies are not well developed yet.

related to this industry.

rently pending United States Environmental Protection Agency approval [16].

As a result of the increase in the global market for PV energy, the volume of modules that reach the end of their life will grow at the same rate in the near future. At the end of 2016, the cumulative global PV waste reached 250,000 metric tons, while it is expected that by 2050 that figure will increase to 5.5–6 million tons [7].

Much PV waste currently ends up in landfill. Given heavy metals present in PV modules, e.g. lead and tin, this can result in significant environmental pollution issues. Furthermore, valuable metals like silver and copper are also present, which represents a value opportunity if they can be recovered. Hence, the landfill option cerates additional costs and it does not recover the intrinsic values of the materials present in the PV modules.

Hence, methods for recycling solar modules are being developed worldwide to reduce the environmental impact of end-of-life modules and to recover some of the value from old PV modules. However, current recycling methods are mostly based on downcycling processes, recovering only a portion of the materials and value, so there is plenty of room for progress in this area. Moreover, currently only Europe has a strong regulatory framework in place to support recycling, but other countries are starting to build specific frameworks related to PV waste. It's clear that sustainable development of the PV industry should be supported by regulatory frameworks and institutions across the globe, which is not the case at the moment. There must be adequate management policies for photovoltaic modules when they reach their end-of-life (EoL) or when they are not able to produce electricity any longer.

As mentioned above, the European Union (EU) provides a legislative framework for extended producer responsibility of PV modules in European scale through the Waste Electrical and Electronic Equipment (WEEE) Directive 2012/19/EU [8]. The main objectives of this policy are to preserve, protect and improve the quality of the environment, to protect human health and to utilize natural resources prudently and rationally. Since February 2014, the collection, transport and recycling of PV modules that reached their EoL is regulated in every EU country [8].

On the other hand, countries with fast expanding PV markets such as China [9], Japan [10], India [11], Australia [12] and USA [13] still lack specific regulations for EoL PV modules. These countries treat PV waste under a general regulatory framework for hazardous and nonhazardous solid waste or WEEE. However, there are some exceptions.

In 2012 the Japanese government introduced a "feed-in tariff" [14] that guaranteed the rate for electricity generated from renewable energy and exported to the grid, which supported rapid growth of solar module installation in the country. Once all the installed capacity starts reaching EoL (within 20–30 years) they will create a significant waste problem for Japan. In late 2017, the Japan Photovoltaic Energy Association (JPEA) has published voluntary guidelines on how to properly dispose of EoL photovoltaic modules. Also, manufacturers, importers and distributors of photovoltaic modules have been invited to provide information on the chemical substances contained in the product and to inform the waste disposal companies. JPEA strongly recommend that industry follow the guidelines [15].

The cumulative worldwide PV generation capacity reached 302 GW in the end of 2016 [2] and the predominant technology (90% of the market) is crystalline silicon (c-Si) cells [3]. Also, during the last years there were several advances on renewable energy in general, including significant price decline and a constant increase in attention to environmental impacts from energy sources [4, 5]. Furthermore, the International Technology Roadmap for Photovoltaic

As a result of the increase in the global market for PV energy, the volume of modules that reach the end of their life will grow at the same rate in the near future. At the end of 2016, the cumulative global PV waste reached 250,000 metric tons, while it is expected that by 2050 that

Much PV waste currently ends up in landfill. Given heavy metals present in PV modules, e.g. lead and tin, this can result in significant environmental pollution issues. Furthermore, valuable metals like silver and copper are also present, which represents a value opportunity if they can be recovered. Hence, the landfill option cerates additional costs and it does not

Hence, methods for recycling solar modules are being developed worldwide to reduce the environmental impact of end-of-life modules and to recover some of the value from old PV modules. However, current recycling methods are mostly based on downcycling processes, recovering only a portion of the materials and value, so there is plenty of room for progress in this area. Moreover, currently only Europe has a strong regulatory framework in place to support recycling, but other countries are starting to build specific frameworks related to PV waste. It's clear that sustainable development of the PV industry should be supported by regulatory frameworks and institutions across the globe, which is not the case at the moment. There must be adequate management policies for photovoltaic modules when they reach their

As mentioned above, the European Union (EU) provides a legislative framework for extended producer responsibility of PV modules in European scale through the Waste Electrical and Electronic Equipment (WEEE) Directive 2012/19/EU [8]. The main objectives of this policy are to preserve, protect and improve the quality of the environment, to protect human health and to utilize natural resources prudently and rationally. Since February 2014, the collection, transport and recycling of PV modules that reached their EoL is regulated in every EU country [8]. On the other hand, countries with fast expanding PV markets such as China [9], Japan [10], India [11], Australia [12] and USA [13] still lack specific regulations for EoL PV modules. These countries treat PV waste under a general regulatory framework for hazardous and non-

In 2012 the Japanese government introduced a "feed-in tariff" [14] that guaranteed the rate for electricity generated from renewable energy and exported to the grid, which supported rapid growth of solar module installation in the country. Once all the installed capacity starts reaching EoL (within 20–30 years) they will create a significant waste problem for Japan. In late 2017, the Japan Photovoltaic Energy Association (JPEA) has published voluntary guidelines on how to properly dispose of EoL photovoltaic modules. Also, manufacturers, importers

(ITRPV) prediction for the installed PV capacity in 2050 is 4500 gigawatts [6].

recover the intrinsic values of the materials present in the PV modules.

end-of-life (EoL) or when they are not able to produce electricity any longer.

hazardous solid waste or WEEE. However, there are some exceptions.

figure will increase to 5.5–6 million tons [7].

10 Solar Panels and Photovoltaic Materials

In USA, some states go beyond the Resource Conservation and Recovery Act which regulates hazardous and non-hazardous waste management [13]. California, for example, has additional threshold limits for hazardous materials classification based on the Senate Bill 489 that categorizes end-of-life PV modules as Universal Waste (facilitating easy transport). This bill is currently pending United States Environmental Protection Agency approval [16].

In Australia, governments have recognized the significance of guaranteeing that regulations are in place to deal with the PV waste issue. Ministers agreed that the state of Victoria would lead innovative programs that seek to reduce the environmental impacts caused throughout the lifecycle of photovoltaic systems. These efforts are part of an industry-led voluntary product management arrangement to address the potential emerging risks of PV systems and their waste. PV modules are listed under the National Product Administration Act to signal the intention to consider a scheme to deal with such waste [17].

The non-inclusion of PV residues in waste legislation in some countries is due to different reasons. Solar modules have a lifespan of up to 25–30 years [18] and so there has been limited interest in investigating the aspects of EoL so far. Moreover, the quantity of this type of waste is still considered insignificant compared to the quantity of other WEEE [19], which currently makes setting up specific recycling plants for solar modules uneconomical. In addition, the definition of mandatory requirements for EoL treatment could still be an obstacle to the effective acceptance of these recycling processes [20]. Because of that, there should be a continuous focus on scientific evidences on the potential impacts and benefits related to the treatment of photovoltaic residues.

Furthermore, recycling processes for all the different PV technologies are not yet well developed. The processes are well developed for mono or multicrystalline silicon. FirstSolar [21] has an established recycling process for CdTe, but for other thin films there are still room for improvements. and are being tested and for generation 3 (new materials [22]) the recycling technologies are not well developed yet.

Only about 10% of PV modules are recycled worldwide. The main reason for that is the lack of regulation. Actually, it has been shown that, for the current recycling technologies, siliconbased modules do not have enough valuable materials to be recovered and the cost of the recycling process is always higher than the landfill option (not considering the externalities), making recycling an economically unfavorable option [23]. However, the prediction for 2050 is that the recoverable value could cumulatively exceed 15 billion US dollars (equivalent to 2 billion modules, or 630 GW) [7]. In addition, the recycling of solar PV modules can ensure the sustainability of the long-term supply chain [24], thereby increasing the recovery of energy and embedded materials and, also, reducing CO2 emissions and energy payback time (EPBT) related to this industry.

For years, the PV industry and researchers have worked intensively in search of different types of efficient and cost-effective materials to manufacture solar PV modules and specific ways of keeping them adequately bonded to withstand several years of outdoor exposure. The modules are made to minimize the amount of moisture that can come in contact with the solar cells and their contacts while keeping manufacturing costs down. The current standard c-Si module is bonded using two layers of EVA to bond the layers together. Because of that, recycling solar modules is a relatively complex task, since these materials need to be separated. Once the materials/layers of a solar module can be separated, metals such as lead, copper, gallium, cadmium, aluminum and silicon can be recovered and reused in new products.

but the passivated emitter and rear cell (PERC) [31] is gaining importance in the world market and is expected to replace the Al-BSF technology in the future [3]. The heterojunction (HIT) cells are also expected to gain some space with predictions of 15% of the total market share by 2027 [7]. Besides that, Si-based tandem solar technologies are expected to appear in mass production

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There are different cell structures for crystalline silicon-based PV cells [32]. The cells are electrically interconnected (with tabbing), creating a string of cells in series (60 or 72 cells are standard numbers used) and assembled into modules to generate electricity (Figure 1).

A typical crystalline silicon (c-Si) PV module contains approximately 75% of the total weight is from the module surface (glass), 10% polymer (encapsulant and backsheet foil), 8% aluminum (mostly the frame), 5% silicon (solar cells), 1% copper (interconnectors) and less than 0.1% silver (contact lines) and other metals (mostly tin and lead) [33]. The rest of the components

The EU directive [8] established recycling targets in terms of module weight and also expresses the intention to increase the collection rates to allow the progressive recycling of more material and less to be landfilled. Even with targets aiming for 65% recycling product weight, some of the current studied recycling processes can recycle over 80% of the weight of a PV module (Figure 2). However there is still incentive to improve, considering that most of the weight is from glass and frame, which are relatively easy to remove, depending on the recycling process.

Thin-films represent less than 10% of the total PV industry [3]. The currently dominant technologies are cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si) with, approximately, 65%, 25% and 10% of the total thin-film market

have a small percentages of the module weight [29, 34].

after 2019 [7].

2.2. Thin-film technologies

share, respectively [35].

Figure 1. Silicon solar module basic structure [32].

Originally created by PV CYCLE in 2007 and commercially available in Europe, the process of recycling mono or multicrystalline silicon modules begins with the separation of the aluminum frame and the junction boxes and then a mechanical process is used for the extraction of the remaining materials of the module (a process similar to recycling of glass or electronic waste). The problems with this process are that the value of the material recovered is low (as it is a downcycling process) and that the maximum amount of recovered materials is about 80%, which is not sufficient for future requirements, and the value of recovered materials is smaller than the original [25]. Thin film processes are under development or near implementation in Italy, Japan and South Korea but costs are not yet competitive. Even up to 90% recovery of materials is not sufficient when compared to production costs [26]. Lastly for recycling processes aiming to generate new materials, the aim is to keep the materials intact for reuse or direct recycling, recovering the frame, glass, tabbing and solar cells without breakages and in good condition. The recovery rates can achieve up to 95% and the materials recovered have higher commercial value. However, these processes are complex and are currently just at laboratory scale, being studied by a few research groups [27].

Even with the difficulty of recovering rare, toxic and valuable materials from solar modules, the recycling process has a remarkable environmental advantage [28]. Nevertheless, the need to recycle this type of waste is imminent. The better knowledge of these technologies and growth on the waste amounts that could generate profitable outcomes has supported the development of the first PV recycling plants. Hence, PV manufacturing companies (e.g. First Solar, Pilkington, Sharp Solar, and Siemens Solar) are investing in the research on solar modules at EoL [29].

The challenges to design the ideal PV recycling process are many. The focus should be on the avoidance of damage to the PV cells and module materials, economic feasibility, and high recovery rate of materials that have some monetary value or are scare or are hazardous, that can be reused in the supply chain. Finally, the next step for the industry and researchers is to create module designs that are "recycling-friendly" [29].
