Photovoltaics are already making a major contribution to energy production and this will continue to grow, because solar energy is considered a sustainable alternative to oil and gas. However, the photovoltaic modules also bring some environmental problems. The processing of the contained silicon costs a lot of energy, thin-film modules emit nitrogen fluoride during production, which is harmful to the climate, recycling has not yet really been ramped up and the efficiency of the solar modules has room for improvement.  

Photovoltaic systems use the photoelectric effect to convert light into electrical energy. A special feature is the silicon in the solar cell. This metal has the special property of being charged by the electromagnetic radiation of sunlight. The charge is bundled by a layer system and converted into usable electricity. 

The recycling industry is becoming more and more dynamic in the area of ​​used photovoltaic (PV) modules as huge quantities of used PV modules are expected to pile up in the near future. The report of the International Energy Agency (IEA) from 2022 shows that up to 80 million tonnes of old PV modules are expected to accumulate worldwide. For Germany, the number is  forecasted to be 1,000,000 tonnes in 2030 and 4.3 million tonnes by 2050. Most old PV modules are expected in the following countries: 1. China, 2. USA, 3. Japan, 4. India, and 5. Germany. The research company Rystad Energy estimates 27 million tonnes of old PV modules worldwide from 2040 onwards. 

Recycling of PV modules

Disused PV modules usually would have had a lifespan of around 20 to 30 years. However, solar systems are also being phased out due to reduced performance, weather damage and product guarantees.

In Germany, citizens have the option of handing in old PV modules to the recycling centre free of charge. The PV module manufacturers are legally obliged to take back PV modules free of charge and to transfer them to the material cycle. Many manufacturers have been using the industry-wide PV recycling system "PV Cycle" since 2010. Other take-back providers include PVEX and Take-e-way. The materials such as glass and aluminum are mainly recycled here. The recycling of metals from solar cells is not yet implemented. 

In Germany, a recycling rate of 80% must be achieved when recycling PV systems, which is determined by the WEEE directive of the EU. In practice, however, it has been shown that up to 95% of the materials are often recycled.  

Both mono- and polycrystalline PV modules can be easily recycled: They consist mainly of glass, aluminum and plastics Up to 95% of the PV module is made from it. The remaining 5% are metals like silicon, silver, copper, tin and lead. (Average composition of a silicon-based photovoltaic module is shown in the figure below)

Only the aluminum, glass and copper are currently recycled, but not the solar cell with the valuable metals. Since the production of metals, especially silicon, produces high CO2 emissions, the focus should also be on the recovery of these materials. 

The difficult thing about PV module recycling is breaking up the firmly bonded module components and recovering the materials they contain in high purity. Since the WEEE recycling quotas are easy to meet with the separation of the aluminum frame, cable and glass, there are no incentives to invest in other recycling technologies. Adjustments to the recycling requirements are expected, such as quotas per ingredient should apply. Therefore, the metals such as silver and silicon would also have to be recycled. 

Silicon recycling

One of the most important and valuable components is the semiconductor material silicon. 

The Fraunhofer CSP has developed a recycling process with which the silicon can be recovered on an industrial scale. After separating the aluminum frame, the junction box and the glass plates, the solar modules are shredded. The material fraction in the size range 0.1 – 1 mm is electrostatically separated in the hamos KWS  into clean metals and glass. 

The metal fraction contains silicon, silver, copper, tin and lead. In the next step, wet-chemical etching is used to remove backside contacts, silver contacts, anti-reflective layers and emitters. It is then possible to convert the silicon into standard processes. 

Furthermore, an additional material flow in the size range 1 - 4 mm is screened. This consists of busbars, which can also be separated from the glass in the hamos KWS electrostatic separator. The recovered glass is mainly used to make cellular glass (foam glass). Processes for better glass separation are currently being tested. 

Due to the large number of old PV modules expected,hamos recycling process could be used to recover the metals from old PV modules on an industrial scale. The special feature of the recycling concept is the high purity of the recyclates. 

How to boost recycling and reuse of PV modules 

In order to strengthen the recycling and reuse of PV modules, the German Environmental Aid DUH, together with partners from the solar and waste disposal industry, published a white paper with potential for improvement in 2021. The White Paper makes the following demands:

• Module design adjustment for better separation of materials
• No lead components
• Limits on silicon and silver usage
• Strengthen reuse and recycling
• Ensuring reusability through expert transport
• Expansion of collection and disposal structures 

Among other things, it is made clear that recyclers need an incentive, for example through recycling specifications per recyclable material and a plannable and secure material flow, in order to invest in new and high-quality recycling technologies and thereby achieve high revenues for the recycled materials. 

Selinda Sliz is the head of marketing at hamos GmbH with 14 years working experience helping the recycling industry to draw attention to innovative separation solutions.