In December 2020, there were almost one million photovoltaic systems installed in Italy, with a capacity of over 21,000 megawatts, according to the data reported by the Gestore Servizi Energetici, or about three times the total capacity of coal-fired power plants in the country. But the number of installed systems is set to rise further, worldwide.
While the spread of photovoltaics can be seen as a step towards the transition to renewable energy sources and energy independence, disposing of large numbers of panels at the end of their operating life can be a problem.
Raising the alarm back in 2016 was a report by the International Renewable Energy Agency, an intergovernmental organisation for the promotion of clean energy. According to the agency, in the next few years the time will come to decommission many of the plants that were installed in the early 2000s, as their operating time is precisely around 25 to 30 years.
What happens today is that, at the end of their use, the panels are usually subjected to the crushing of materials to gradually reduce their size, so as to separate the recoverable components: an industrial process that involves the expenditure of energy and money. Researchers at the IMT School for Advanced Studies Lucca are working on an alternative method to recycle photovoltaic panels more efficiently.
Peeling the layers
A study recently published in the journal Engineering Fracture Mechanics by researchers from the IMT School's MUSAM laboratory highlights the advantages of another technique, the so-called peeling, which involves separating the different layers that make up the panels. The complexity of their recycling depends, in fact, on the composition of the modules, which are normally made up of different materials: a layer of tempered glass on the surface, silicon solar cells that convert sunlight energy into electricity, and a back layer. The solar cells are also covered on both sides by a further layer, usually composed of plastic material (ethylene vinyl acetate or EVA), which holds the different components together and protects them from external environmental conditions.
The researchers have developed a numerical simulation method that allows them to calculate the amount of energy required for the peeling process taking into account the environmental conditions. According to the simulations, the energy required to use this technique is about one hundred times less than the energy required for peeling. To come to this result, the researchers first used a new approach to calculate how the force holding the layers together decreases with humidity and temperature. They were thus able to see that adhesion decreases with increasing humidity, temperature and exposure time to these external agents. "The model allows us to predict the behaviour of the materials that make up the panels. In this way, we can virtually design new recycling models, obtaining accurate predictions without the need to conduct expensive experiments," explains Zeng Liu, a PhD student in the NewFrac project at the MUSAM research unit and one of the authors of the study. For an end-of-life panel in a hot and humid region, for example, peeling should be even more cost-effective, requiring less energy to separate the layers. In fact, the EVA polymer degrades its adhesive properties more rapidly over time under these kinds of environmental conditions. Numerical simulations have clearly shown that the peeling process, which is not cost-effective for a new panel and therefore not introduced in the recycling stages so far, is facilitated by the chemical-physical degradation of the EVA layer after 25-30 years of life.
The future of photovoltaics
Efficient recycling of photovoltaic panels can mean a major reduction in carbon emissions and energy consumption worldwide. "So far, much attention has been paid to the production of environmentally friendly photovoltaic modules, but efforts focused specifically on the treatment of end-of-life products are still limited," notes Marco Paggi, professor in construction science at the IMT School and director of the MUSAM laboratory. "Recycling is not only an effective way to prevent toxic and hazardous substances in photovoltaic products from entering groundwater and soil, thus causing negative biochemical effects on the environment, but it also allows us to preserve valuable metallic materials such as silver, germanium, cadmium as well as pure energy-intensive material such as silicon wafers. In perspective, then, photovoltaic recycling could give rise to a new industrial chain in Italy, as our country is among the top six in the world for photovoltaic plants, installations that will inevitably have to be reconditioned and recycled at the end of their life."
The mathematical simulation approach proposed by the researchers opens the way to new research on peeling and the ageing process of modules. For example, future studies could investigate the amount of energy required for peeling under dynamic environmental conditions, considering daily and seasonal temperature differences. In general, an approach such as the one proposed could be useful for the market when, as will soon be the case, the prospect of sustainably recycling a large number of installations arises. In general, an approach such as the one proposed could be useful to the market when, as will soon happen, the prospect of sustainably recycling a large number of plants arises.
Marco Maria Grande