For years, the arguments against renewable power focused on its high costs. But as the price of wind and solar plunged, the arguments shifted. Suddenly, concerns about the waste left behind when solar panels hit end-of-life became so common that researchers at the US's National Renewable Energy Lab felt compelled to publish a commentary in Nature Physics debunking them.

Part of the misinformation is pure nonsense. The primary ingredients of most panels are silicon, aluminum, and silver, none of which is 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 hasn't even become much of a problem yet. And, even once these panels age out, recycling techniques are available.

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

Etching away layers

As mentioned above, waste from solar panels really isn't a problem yet. The paper's authors describing the new recycling technique note that, at the end of 2020, 18 percent of the solar cells in use had been manufactured that same year, and the pace of manufacturing has accelerated dramatically since. And panels tend not to fail so much as slowly drop in efficiency to the point where installing a new panel makes economic sense.

That said, the number of cells ready for recycling will grow dramatically within a few decades, and there are expected to be 80 million tonnes of panels ready for recycling each year by 2050. So, methods for doing so have already been devised. Most of the value in the solar panels comes in the form of silver used for wiring and the high-purity silicon of the cells. But there's also an aluminum frame and backing, a glass cover with anti-reflective coating, and solder connecting some of the wiring.

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

The new work, rather than focusing on completely dissolving the materials used in constructing the panel, relies on a brief chemical treatment that largely severs the connections among 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 starts with physically removing the aluminum frame and glass cover, both of which can be melted 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 of the components it comes in contact with. This acts as an etching process, reacting away the material right at the cell's surface.

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