As red-green at the turn of the millennium introduces the Renewable Energy Act, solar systems conquer the roofs of the Republic.

Now, more than 20 years later, the first-generation solar cells are nearing the end of their lives - and their remains will soon pile up to stately e-waste.

The International Renewable Energy Agency, Irena, estimates that nearly 100,000 tonnes of solar scrap will be generated in Germany by 2025. That's equivalent to nearly five million today's standard modules. By 2030, the amount of waste is expected to grow to around 400,000 tonnes.

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The first-generation solar systems will reach the end of their lifetime in the next few years

Precious metals in waste incineration

Behind the glass front and the aluminum frame, both of which are easy to recycle, there is even more in one module: The solar cells, at their heart, consist of several hundred grams of silicon, plus a few grams of lead, zinc, tin and sometimes even small amounts of silver - all valuable raw materials.

These materials are embedded in thin plastic films with which they are firmly fused. That makes it very difficult to recycle them.

"You first have to separate the recyclables cleanly before they can be reused, but this is extremely time-consuming," explains Ullrich Didszun, German representative of the PV Cycle recycling organization founded by the European solar industry. Therefore, most of them land today in the waste incineration plant - so the materials are irretrievably lost.

An oven should help with recycling

A research group around the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB and the disposer Suez want to change that. "We have developed a reactor in which the metals and silicon can be cleanly removed from the plastics so that they can be recycled," says project manager Oliver Grimm, Managing Director of Suez Süd.

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The system resembles a large oven in which the modules are pushed in like baking trays. In it, they are heated under exclusion of oxygen and high pressure, so that the plastics to gas - methane, propane and butane - dissolve. Pyrolysis is the name of this process.

What's left are the glass and the aluminum, the silicon and the metals. They can then be recycled using established electrochemical processes. "These are standard processes that have long been used in metal recycling," says Grimm.

The experts are currently testing their concept in a pilot plant in Knittlingen, Swabia. So far with success, so Grimm - he assumes, by the end of this year with the construction of a large-scale plant can start. It should be able to recycle 200,000 modules per year at the beginning. If significantly larger quantities occur in a few years, it could be extended accordingly.

Repair: Technically no problem - but uneconomical

If modules have to be discarded, often only one or two broken solar cells are to blame - but that is enough to noticeably reduce the electricity yield. Why not replace these cells instead of disposing of the entire module?

Whether this makes sense, researchers at the Technische Hochschule Mittelhessen (THM) in Giessen have examined. They used a saw wire to cut the plastic films on the back, remove the defective cells, insert new ones and connect them to the other cells. "Technically, this works very well," reports Harald Weigand, professor in the Life Science Engineering department at THM.

However, this requires a lot of manual work. That makes the repair expensive. "We are disillusioned when it comes to profitability," says Weigand. Also, because the Renewable Energy Act gives no incentive to replace defective cells. In addition, repaired modules would have to undergo a safety inspection if they were to be placed on the market - leading to additional costs, the researcher said.

"The market is flooded with cheap new modules from Asia," explains Weigand. "It simply can not compete with a repair, however useful it would be in terms of resource efficiency."