The PV system TISO-10 (TIcino SOlare) was connected to the grid in 1982 on the roof of today’s SUPSI PVLab at the University of Applied Sciences Southern Switzerland in the Italian-speaking canton of Ticino, where it has been working almost continuously for almost 40 years.
The array’s performance was measured for around 35 years from 1982 to 2017 when the Swiss Federal Office of Energy (SFOE) commissioned two scientists – Alessandro Virtuani, Senior Researcher at the École Polytechnique Fédérale de Lausanne (EPFL) and Mauro Caccivo, Head of the SUPSI PVLab themselves – and their teams, to analyze the huge amount of data collected. “We had to go through an incredible amount of paper and it took almost two years to sort through all of the relevant information,” said Caccivio PV magazine.
40 year old panels
The array was built at a cost of around CHF 284,000 (around USD 309,000 today and USD 475,000 then), and the 288 modules used for the project cost around CHF 21 (currently USD 22.9 and around USD 37 then) each Watt. The glass backsheet products with an output of 37 W each were supplied by Arco Solar, which was first taken over by the German conglomerate Siemens and then in 2007 by the German solar panel manufacturer SolarWorld. “When the modules were bought in 1980”, Arco Solar was one of the largest manufacturers in the world and had an annual production capacity of around 1 MW, “says Caccivio.
Although the electrical design of the system changed several times, after the inverters were replaced, all modules were aged together, always exposed to the outside environment and sunlight and never renovated or rebuilt, with only a few exceptions in which junction boxes and bypass diodes were replaced.
“These modules have an impressive mechanical robustness,” emphasized Virtuani and Caccivio. However, their weight and mechanical dimensions, as well as the thickness of their solar cells, are not representative of what could happen to a solar system in terms of cracks to a PV system built with modules made today or in the last few years. “But these modules can tell us a lot about moisture ingress and yellowing,” they said. “Each panel was wrapped in a backing sheet of steel foil, which acts as a barrier against the ingress of water and is surrounded on both sides by layers of Tedlar,” they explained, adding that the product, despite it resembles the structure of a glass / glass panel rather than that of a conventional glass / backsheet panel, is closer to what we would define today as a glass-glass panel.
The 10% efficient modules have an open circuit voltage of 21.5 V, a short circuit current of 2.55 V and a fill factor of 68%. Each of them measures 121.9 × 30.5 × 3.8 cm, weighs 4.9 kg and is based on 35 monocrystalline cells with a diameter of 102 mm. “Today the cells are much more sophisticated and can contain surface passivation layers or more complex pattern structures,” said Virtuani. “This complexity can potentially make cells weaker and expose them to higher breakdown rates.”
The inverters were changed five times in total. The first devices provided by Abacus controls were replaced after 10 years by a new product from Invertomatic and the system design was also modified, with longer strings and a slightly reduced number of modules. SMA inverters were installed later and the system design with 288 modules was returned to the original configuration. The solar modules are the only components of the system that have never been changed.
The performance of the modules was not the same for all panels and the researchers were able to divide them into three groups, with the most powerful modules showing almost no signs of yellowing, while the other two groups had medium and strong degrees of yellowing. “In the third group, the yellowing was so intense that some panels were brown in color,” said Virtuani. “The long-term electrical performance and aging of the panels are strongly correlated with their respective groups and the behavior of the potting compound used to manufacture them. “
The chemical analysis carried out over the past few years confirmed that the three encapsulation materials are made from the same base polymers, but that their three respective suppliers used different additives in the encapsulation formulation, which explains the different performance. “As in many cases with PV systems, the devil is in the details,” says Virtuani. “Replacing a single element, in this case the potting material supplier, can affect the overall performance of a PV generator,” he added. “That shows that the parts list is important. Much!”
Since the extraction of the polymers from the module is a destructive technique, the researchers were only able to perform the chemical analysis on a limited number of modules, but excluded other causes of degradation, as yellowing is a problem that only affects the encapsulation Materials. “On the other hand, the modules showed no signs of moisture penetration,” explains Caccivio.
The three potting compounds are all based on polyvinyl butyral (PVB), a thermoplastic polymer that has been used to encapsulate PV modules since the early 1980s and has since been replaced by ethylene vinyl acetate (EVA). “A former manager of Arco Solar confirmed that it was probably PVB at the time and that the company uses three different PVB suppliers,” the scientists emphasized.
Around 21.5% of the modules showed an annual degradation of -0.2% per year, which should correspond to the value promised by the manufacturer, while another group with 72.9% of the panels showed an annual degradation of between -0.2% and -0.7%. per year. “Most of the panels in the second group have also performed well and met initial expectations,” said Caccivio, noting that this group can be further broken down into two subgroups in terms of the different encapsulation materials.
From 1982 to 2017, the modules in the first group were degraded by a maximum of 13% and those in the second group by up to 21%, with half of them not exceeding the 20% threshold. According to the Swiss company, around 70% of the modules used in the array would still meet a performance guarantee that module manufacturers are currently considering for the technology of tomorrow, which corresponds to a service life of 35 years.
The analysis also showed that 87.5% of the module had some type of slight front detachment and problems with multiple junction boxes, but these problems were evenly distributed among the three groups. However, the overheating of the junction boxes had less of an impact on the modules in the first group. In addition, some modules showed cracks, backing deterioration, internal circuit corrosion, hot spots, and burn marks, among other things.
The first and most important lesson from studying the PV system and modules is that “the parts list (BOM) matters,” said the researchers, adding that material selection is just as important today as it was forty years ago.
Modification or exchange?
When asked about the future of this PV system and the aging systems in general and whether revamping or repowering could be a better option than letting old systems continue to generate electricity, although with lower yields, the two scientists offered different perspectives.
“Economically, it may be preferable to revamp and repower, or eventually replace the old array with a completely new system,” said Caccivio. “However, the European Commission has set a 40-year lifecycle for the solar modules and we must reasonably use solar products until the end of their lifecycle or until 80% of their original output has been achieved.” It has lost 0.2% of its original efficiency and is still well above that 40-year threshold, he added.
According to Virtuani, it is possible to extend the life cycle of a PV system with proper maintenance. “If the PV system works well, it can run for 40 years,” he explained. “On the other hand, several business plans are currently being developed over a period of 30 years. But nothing prevents us from extending the PV system operation longer than planned if the systems and modules are working well. “
He also said that extending the life of a PV system beyond the 30-year limit may also depend on the uses of the PV system. For example, an array that powers a water pump doesn’t need to run at full capacity and can likely be used for much more than its owner expects.
The results of the two researchers were presented in two different papers: “35 years of photovoltaics: Analysis of the TISO 10 kW solar system, Lessons Learned in Safety and Performance – Part 1 and” 35 years of photovoltaics: Analysis of the TISO 10 kW solar system, Lessons Learned in Safety and Performance – Part 2 “, both of which were published in Advances in Photovoltaics.
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