Solar power is growing at record rates, and the question is becoming ever more pressing: what to do with panels once they have served their time? By 2030, according to estimates, more than a million tonnes of such waste will have accumulated worldwide, and by 2050 more than 200 million tonnes. Inside the panels are valuable metals: they account for just 0.05% of the weight but two-thirds of the scrap value. Extracting them, however, is not easy.
The metals are sealed inside the panel by a very strong polymer called EVA (ethylene vinyl acetate - a plastic that for decades protects the cells from moisture and sunlight). To break open this "cocoon", conventional methods either heat the panel to 450-600 °C or douse it with caustic chemicals. Both require a lot of energy and produce dirty waste.
Researchers from the James Watt School of Engineering at the University of Glasgow have proposed another route. In a paper published in the journal Communications Materials, they described how, using a 1064 nm fibre laser (a nanometre is a billionth of a metre), they scanned cells obtained from Germany's Fraunhofer Center for Photovoltaics. The beam passes almost unhindered through the transparent EVA and is absorbed by the silicon beneath it, heating the boundary between the layers.
If the laser power is above a certain threshold, the EVA separates from the cell as an intact film. The metal busbars can be removed mechanically, leaving a powder dominated by silicon, aluminium and silver. The powder is first treated with a dilute sodium hydroxide solution to remove the aluminium, and then with a mixture of iron chloride and choline chloride, which dissolves the silver. In one type of panel, more than 99% of the silver was recovered - that is, almost all of it, and with high selectivity, without capturing extraneous substances.
The authors stress that their method relies on simple and mild reagents, whereas traditional silver leaching uses hazardous and polluting nitric acid. According to the researchers' estimate, the new approach cuts the energy cost of recycling by 84%.
The first author of the work was Bashayer Alsulami, a recent postgraduate of the James Watt School, building on her earlier research on recovering metals from old panels. A significant part of the work was led by Dr Joseph Cameron. He explained that the plastic encapsulating the cells is designed to last for decades outdoors - which is precisely why old panels are so hard to recycle. The laser, he said, makes it possible to remove the encapsulant quickly and in one piece, without burning or dissolving it, and to obtain a cleaner material for recycling.
The study was carried out as part of the APOLLO project with the participation of the REACT centre (Responsible Electronics and Circular Technologies Centre) under the leadership of the University of Glasgow. The university is now working with the Glasgow company Watt Laser Ltd. to assess the possibility of commercial production of the technology. Watt Laser representative Delan Brannagan noted that the method uses standard industrial fibre lasers, the same as those on production lines for marking and cutting. According to him, scaling from a single laser head to multi-head systems is expected to take less than a year, but an industrial partner-customer is needed to tune performance.
Professor Jeff Kettle, lead researcher at REACT and corresponding author of the paper, called the project an example of what the centre was created for: a global problem is turned into a process that a local laser company can develop into industrial equipment. For now, this is a laboratory method and scaling plans, not a finished product on the market.