The new recycling method makes solar cells even more environmentally friendly

For years, arguments against renewable energy focused on its high costs. But as the price of wind and solar fell, the arguments shifted. Suddenly, concerns about the waste left behind at the end of the life of solar panels became so common that researchers at the US National Renewable Energy Laboratory felt compelled to publish a commentary in Nature Physics discrediting them.

Some of the misinformation is pure nonsense. The main ingredients of most panels are silicon, aluminum and silver, none of which pose a major environmental threat. Solar panels also have a useful lifespan of decades, and the vast majority of those in existence are less than 10 years old, so waste has not yet become much of an issue. And, even after these panels become obsolete, recycling techniques are available.

Perhaps the only realistic concern is that existing recycling technologies rely on nitric acid and may produce some toxic waste. But a group of researchers from Wuhan University have found an alternative means of recycling that avoids producing toxic waste and is more energy efficient as a bonus.

Carving away from layers

As mentioned above, waste from solar panels is not yet a problem. The authors of the paper describing the new recycling technique note that, by the end of 2020, 18 percent of solar cells in use had been produced that year, and the pace of production has accelerated dramatically since then. And panels tend not to fail so much as to slowly degrade in efficiency to the point where installing a new panel makes economic sense.

That said, the number of cells ready for recycling will increase dramatically within a few decades, and it is expected that there will be 80 million tons of panels ready for recycling every year by 2050. So the methods to do this have already been invented. Most of the value in solar panels comes in the form of the silver used for wiring and the high-purity silicon in the cells. But there’s also an aluminum frame and stand, a glass cover with an anti-reflective coating, and welding that connects some of the wiring.

Current techniques dissolve the silver in nitric acid and use other acids to treat a layer of silicon nitride on the panel, as well as some of the small materials, such as solder. These techniques result in chemicals that are difficult to recycle or dispose of.

The new work, instead of focusing on the complete dissolution of the materials used in the construction of the panel, relies on a short chemical treatment that largely breaks the bonds between the individual layers. While this results in some chemical byproducts, most of the material ends up intact and in a relatively pure form.

The process begins with the physical removal of the aluminum frame and glass cover, both of which can be melted down and reused for manufacturing. This leaves the cells, which the researchers disassemble using a molten mixture of sodium and potassium hydroxide, which undergoes chemical reactions with most components it comes in contact with. This acts as an etching process, reacting away the material right on the surface of the cell.

The researchers tried different conditions, ranging from spraying in the NaOH/KOH mixture to soaking the cells in it, and a range of temperatures. They were placed in a two-second dip in the etching mixture, followed by a short period (one to two minutes) at 200° C. Longer treatments and elevated temperatures tended to result in the complete removal of some layers of material; the shorter exposure allowed these layers to separate while remaining largely intact.