3. Photovoltaic recycling technologies

Thin-film solar cells were developed with the aim of providing low cost and flexible geometries, using relatively small material quantities. CdTe, CIGS and a-Si are the main technologies for thin-film PV modules [36]. CdTe is the most widely used thin-film technology. It contains significant amounts of cadmium (Cd), an element with relative toxicity, which presents an environmental problem that has been studied worldwide [37, 38]. CIGS has a very high optical absorption coefficient because it is a direct band gap material (can be tuned between 1.0 and 2.4 eV by varying the In/Ga and Se/S ratios [39]) and efficiency of approximately 15.7 0.5% for high bandgap [40]. A-Si has low toxicity and cost but also low durability and it is less efficient compared with the other thin-film technologies [41]. Current projections expect the a-Si module market to disappear in the near future, since they cannot compete on costs or

Figure 2. Total collection rate for WEEE in 2014 as a percentage of the average weight of EEE put on the market in the

Basically, thin-film modules consist of thin layers of semiconducting material (CdTe, CIGS or

a-Si) deposited on a substrate (glass, polymer or metal) (Figure 3).

Figure 3. Thin-film solar module basic structures [36].

efficiency [3].

three preceding years (2011–2013) [8].

14 Solar Panels and Photovoltaic Materials

PV modules are largely recyclable. Materials such as glass, aluminum and semiconductors can, theoretically, be recovered and reused. Hence it is vital that consumers, industry and PV producers take responsibility for the EoL of these modules. So far, the most common methods for recycling c-Si PV modules are based on mechanical, thermal and chemical processes.

Although thin-film solar cells use far less material than c-Si cells, there are concerns about the availability and toxicity of materials such as tellurium (Te), indium (In), and cadmium (Cd), for example. Furthermore, the production processes also generates greenhouse gases emissions during some reactor-cleaning operations. Because of these issues, it is very important to focus on the recycling of PV modules for all the technologies.

PV Cycle is a not-for-profit organization which goal is to manage PV waste through their waste management programme for solar PV technologies [42]. PV Cycle was the first to establish a PV recycling process and PV waste logistics throughout the EU. In 2016 their process of recycling PV achieved a record recycling rate of 96% for c-Si PV modules (fraction of solid recycled) [25], which is a percentage that surpasses the current European WEEE standards. The process begins with the removal of the cables, junction box and frame from the PV module. Then, the module is shredded, sorted and separated. The separation of the materials allows them to be sent to specific recycling processes associated with each material. The summary of this process is shown in Figure 4.

FirstSolar [21] developed a recycling process for CdTe modules. The company manages the collection and transportation of EoL modules to the recycling centre; however, the recycling process itself must be financed. This is made by setting aside funds by the company itself at the time of the module sale, which also happens with WEEE. The summary of this process is shown in Figure 5.

The recycling process starts with the shredding of the modules into large pieces and subsequently in to small fragments (5 mm or less) by a hammer mill. During the next 4–6 h the semiconductor films are removed in a slow leaching drum. The remaining glass is exposed to a mixture of sulfuric acid and hydrogen peroxide aiming, to reach an optimal solid–liquid ratio. After that process, the glass is separated again. The next step is to separate the glass from the larger ethylene vinyl acetate (EVA) pieces, via a vibrating screen. The glass is cleaned and sent to recycling. Sodium hydroxide is used to precipitate the metal compounds, after which they are sent to another company where they can be processed to semiconductor grade raw materials for use in new solar modules. This process recovers 90% of the glass for use in new products and 95% of the semiconductor materials for use in new solar modules [21].

Figure 4. Summary of PV cycle recycling process for c-Si modules [25].

Figure 6. Summary of ANTEC solar GmbH recycling process for CdTe modules [43].

Also, for recycling CdTe modules, ANTEC Solar GmbH designed a pilot plant with a similar technology to the First Solar process. It starts with a physical fragmentation of the modules. After that, these small pieces are exposed to an atmosphere containing oxygen at 300C. These conditions result in the delamination of the EVA. Subsequently, these fragments are taken to a 400C atmosphere containing chlorine gas which causes an etching process. This step of the process generates CdCl2 and TeCl4 that are condensed and precipitated afterwards [43]. The summary of this process is shown in Figure 6.

is burned and the CIS layer is grated. For the c-Si modules, the semiconductor materials are recovered as well as the glass cullet [45]. The summary of these processes is shown in Figure 8. In 2014 the Environment Ministry of Japan, through NEDO, together with private companies, began working on new technologies to pry the PV modules apart. The new technology appeared to solve a clear problem, the firm attachment of the glass and the cells to the EVA, and the consequent difficulty to separate them simply by smashing them to pieces and sorting

A Review of Recycling Processes for Photovoltaic Modules

http://dx.doi.org/10.5772/intechopen.74390

17

NPC incorporated is one of the companies that make solar module recycling equipment. The process, called the "hot knife method", can separate the cells of a module from the glass in about 40 seconds. It places the module between two rollers, which move it along and hold it steady until it runs into a 1 meter-long steel blade ("hot knife") that is heated to 180–200C and

In Japan, the scrap glass can be sold for 0.5–1 Yen/kg. At that price, the 10–15 kg of glass in a solar module is worth about 15 Yen (approximately 0.14 US D). Their goal was to develop a recycling technology that can cost less than 5 yen/watt (1000 yen for a 200-watt module, not including transportation cost) by the end of April 2018, which they already did by January 2018 [46].

Furthermore, some innovative treatment processes for recycling PV solar modules have been

Loser Chemie has some collection points from where they gather several types of photovoltaic systems (c-Si, CdTe, CIGS and GaAs). The company has developed and patented original

them out [46].

developed.

slices the cell and the glass apart (Figure 9) [46].

Figure 7. Summary of SolarWorld recycling process for Si modules [44].

Figure 8. Summary of NEDO recycling process for Si modules (pilot project).

Figure 9. Summary of "hot knife" recycling process for PV modules [46].

A company that has a well stablished c-Si recycling process is the SolarWorld [44]. This company started recycling in 2003 with a pilot plant using a thermal process. Today, the takeback of modules is organized via a "bring-in" system [44]. Their process is based on a thermal process, which starts by pyrolising the modules. During this process, the plastic components are burnt at 600C. The solar cells, glass and metals are separated manually after that. The glass and some metals are sent to other companies for recycling and the solar cells can be turned into wafers again. The outcomes of this process are the recovery of more than 84% of the module weight, being 90% of the glass and 95% of the semiconductor materials [44]. This process can recover up to 98% unbroken cells depending on the conditions of the module and the thickness of the cells. The summary of this process is shown in Figure 7.

A pilot project was funded by the Japanese Government via the New Energy and Industrial Technology Development Organization (NEDO). The recycling process for Si or CIS is based on pyrolysis of the polymers in a furnace. The process starts with the removal of the frames and the backsheet foil before the thermal process begins. After that, for CIS only, the EVA resin

A Review of Recycling Processes for Photovoltaic Modules http://dx.doi.org/10.5772/intechopen.74390 17

Figure 9. Summary of "hot knife" recycling process for PV modules [46].

Also, for recycling CdTe modules, ANTEC Solar GmbH designed a pilot plant with a similar technology to the First Solar process. It starts with a physical fragmentation of the modules. After that, these small pieces are exposed to an atmosphere containing oxygen at 300C. These conditions result in the delamination of the EVA. Subsequently, these fragments are taken to a 400C atmosphere containing chlorine gas which causes an etching process. This step of the process generates CdCl2 and TeCl4 that are condensed and precipitated afterwards [43]. The

Figure 6. Summary of ANTEC solar GmbH recycling process for CdTe modules [43].

Figure 5. Summary of first solar recycling process for CdTe modules [21].

16 Solar Panels and Photovoltaic Materials

A company that has a well stablished c-Si recycling process is the SolarWorld [44]. This company started recycling in 2003 with a pilot plant using a thermal process. Today, the takeback of modules is organized via a "bring-in" system [44]. Their process is based on a thermal process, which starts by pyrolising the modules. During this process, the plastic components are burnt at 600C. The solar cells, glass and metals are separated manually after that. The glass and some metals are sent to other companies for recycling and the solar cells can be turned into wafers again. The outcomes of this process are the recovery of more than 84% of the module weight, being 90% of the glass and 95% of the semiconductor materials [44]. This process can recover up to 98% unbroken cells depending on the conditions of the module and

A pilot project was funded by the Japanese Government via the New Energy and Industrial Technology Development Organization (NEDO). The recycling process for Si or CIS is based on pyrolysis of the polymers in a furnace. The process starts with the removal of the frames and the backsheet foil before the thermal process begins. After that, for CIS only, the EVA resin

the thickness of the cells. The summary of this process is shown in Figure 7.

summary of this process is shown in Figure 6.

is burned and the CIS layer is grated. For the c-Si modules, the semiconductor materials are recovered as well as the glass cullet [45]. The summary of these processes is shown in Figure 8.

In 2014 the Environment Ministry of Japan, through NEDO, together with private companies, began working on new technologies to pry the PV modules apart. The new technology appeared to solve a clear problem, the firm attachment of the glass and the cells to the EVA, and the consequent difficulty to separate them simply by smashing them to pieces and sorting them out [46].

NPC incorporated is one of the companies that make solar module recycling equipment. The process, called the "hot knife method", can separate the cells of a module from the glass in about 40 seconds. It places the module between two rollers, which move it along and hold it steady until it runs into a 1 meter-long steel blade ("hot knife") that is heated to 180–200C and slices the cell and the glass apart (Figure 9) [46].

In Japan, the scrap glass can be sold for 0.5–1 Yen/kg. At that price, the 10–15 kg of glass in a solar module is worth about 15 Yen (approximately 0.14 US D). Their goal was to develop a recycling technology that can cost less than 5 yen/watt (1000 yen for a 200-watt module, not including transportation cost) by the end of April 2018, which they already did by January 2018 [46].

Furthermore, some innovative treatment processes for recycling PV solar modules have been developed.

Loser Chemie has some collection points from where they gather several types of photovoltaic systems (c-Si, CdTe, CIGS and GaAs). The company has developed and patented original

Process Advantages Disadvantages Status Ref.

Electro-thermal heating • Easy removal of glass • Slow process Research [51]

• Delamination time depends on area • Harmful emissions and

A Review of Recycling Processes for Photovoltaic Modules

http://dx.doi.org/10.5772/intechopen.74390

• Expensive equipment • Harmful emissions and

• Other separation processes required for full removal of EVA

texturisation (damage to

Research [49]

19

Research [50]

Research [52]

[53]

[54]

Research (pilot)

Research (pilot)

Commercial [55]

Commercial [56]

Commercial [57]

wastes

wastes

• Slightly worse

cell surface)

• It can cause cell defects due to inorganic acid • Generates harmful emissions and wastes

cesses required for full EVA removal • Dusts containing heavy

• No removal of dissolved

• Use of chemicals Commercial [58–60]

• Harmful emissions • High energy requirements • Cell defects and degradation due to high tempera-

metals • Breakage of solar cells • Equipment corrosion

solids

ture

Organic solvent dissolution • Easy access to the EVA

Organic solvent and ultrasonic irradiation

Mechanical separation by hotwire cutting

Pyrolysis (conveyer belt furnace and fluidised bed

Solvent (Nitric acid) dissolution

Dry and wet mechanical

Thermal treatment (Two steps heating)

process

reactor)

• Less cell damage • Recovery of glass

• Low cell damage • Recovery of glass

glass sheets • Cost-effective industrial recycling process

wafer

• Separate 80% of wafers and almost 100% of the

• Complete removal of EVA and metal coating on the

• It is possible to recover intact cells

Physical disintegration • Capable of treating waste • Other separation pro-

• No process chemicals • Equipment widely available • Low energy requirements

• Full removal of EVA • Possible recovery of intact

• Economically feasible process

materials • Simple and efficient process

cell

Chemical etching • Recover high purity

Table 1. Silicon solar modules recycling processes.

• More efficient than solvent dissolution process • Easy access to the EVA

Figure 10. Summary of loser Chemie recycling process for PV modules (pilot project).

processes using mechanical and chemical treatment to recycle solar cells [47]. The first step is to crush and separate the materials mechanically. In the next stage, they use chemical treatment to recover the semiconductor metals. After that, the aluminum metallisation is also recovered and can be used for producing wastewater treatment chemicals as aluminum oxide [47]. The summary of these processes is shown in Figure 10.

Reclaim PV has teamed up with major solar module manufacturers who distribute in Australia and is refining its processes. The company is developing a process of reclaiming efficient cells from damaged solar modules. Their cell recycling system is able to extract efficient components (but not unbroken cells) from end-of-life solar modules in order to develop new green products or be reintroduced into the PV industry as new solar modules [48].
