Since 1982, a 10-kilowatt photovoltaic system has been supplying power to the Swiss grid. A research team investigated the performance of the array's initial 35-year lifespan and found that solar modules can be targeted—at least in temperate climates—with a lifespan of 35 years, and the bill of materials is important!

Photovoltaic system installed on the roof of SUPSI PVLab, Lugano University of Applied Sciences and Arts in southern Switzerland.

The TISO-10 (TIcino SOlare) photovoltaic system was connected to the grid in 1982 on the roof of the SUPSI PVLab of the University of Applied Sciences and Arts in Southern Switzerland. The laboratory is located in the Italian-speaking canton of Ticino, where it is almost uninterrupted. The ground has been in operation for nearly 40 years.

From 1982 to 2017, the performance of the array was measured for approximately 35 years, when the Swiss Federal Office of Energy (SFOE) appointed two scientists-senior researcher Alessandro Virtuani and Mauro Caccivo at the Federal Institute of Technology Lausanne (EPFL), SUPSI PVLab The person in charge-and his team analyze the large amount of data collected. Caccivio told pv magazine: "We had to read a lot of papers, and it took nearly two years to sort out all the relevant information."

The construction cost of the array was approximately 284,000 Swiss francs (approximately US$309,000 today and approximately US$475,000 at the time), and the cost per watt of the 288 modules used in the project was approximately 21 Swiss francs (currently US$22.9, approximately 375,000 US dollars at the time). Dollar). Each glass backsheet product with an output power of 37 W is provided by Arco Solar, which was first acquired by the German conglomerate Siemens in 2007, and then by the German solar panel manufacturer SolarWorld in 2007. , Arco Solar is one of the largest manufacturers in the world, with an annual production capacity of approximately 1 MW,” Caccivio pointed out.

Although the electrical layout of the system has changed many times, after the inverter is replaced, all the modules age together and are always exposed to the outdoor environment and sunlight. They have never been refurbished or modified. There are only a few exceptions. The junction box and bypass have replaced diodes.

"These modules have impressive mechanical robustness," Virtuani and Caccivio emphasized, however, they pointed out that their weight and mechanical dimensions and the thickness of solar cells do not represent what may happen to solar cell arrays today or in recent years. The photovoltaic system constructed by the module to be manufactured has cracks. "But these modules can tell us a lot about moisture penetration and yellowing," they said. They explained: “Each panel is encapsulated with a backplane made of steel foil, which acts as a barrier against entry of water, with a Tedlar layer sandwiched on both sides.” They added that although the product is more like a glass/glass structural panel, It is not a traditional glass/back panel, it is closer to the glass-glass panel we define today.

The open circuit voltage of the 10% efficiency module is 21.5 V, the short-circuit current is 2.55 A, and the fill factor is 68%. Each is 121.9 × 30.5 × 3.8 cm in size, weighs 4.9 kg, and is based on 35 single crystal cells with a diameter of 102 mm. "Today's batteries are more complex and may include surface passivation layers or more complex pattern structures," Virtuani said. "This complexity may weaken cells and expose them to higher degradation rates."

In total, the inverter has been replaced five times. The first batch of equipment supplied by Abacus was replaced by Invertomatic's new products 10 years later. The system design was also modified, strings were added, and the number of modules was slightly reduced. In the later stage, the SMA inverter was installed and the system design was restored to the original configuration with 288 modules. The solar module is the only component in the system that has never changed.

The module performance of all panels is not the same. The researchers divided them into three groups. The best performance groups showed almost no signs of yellowing, while the other two groups showed moderate and high yellowing. "In the third group, the yellowing was so severe that some panels eventually turned brown," Virtuani said. "The long-term electrical performance and aging of panels are highly correlated with the behavior of their respective groups and the sealants used to make them."

Chemical analysis conducted in recent years confirmed that these three sealants are all made of the same base polymer, but their three suppliers use different additives in the sealant formulation, which explains the different properties. "As in many cases of photovoltaic installations, details determine success or failure," Virtuani said. "Just changing one component, in this case the supplier of the sealant, will affect the entire performance of the photovoltaic array," he added. "This shows that the bill of materials is important. A lot!"

Since the extraction of polymers from modules is a destructive technology, researchers can only perform chemical analysis on a limited number of modules, but they ruled out other causes of degradation, because the yellowing problem is only related to the sealant material. "On the other hand, the modules showed no signs of moisture penetration," Caccivio explained.

All three encapsulants are 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). The scientists emphasized: “A former manager of Arco Solar confirmed that the sealant was probably PVB at the time and the company used three different PVB suppliers”.

Approximately 21.5% of the components showed degradation of -0.2% per year, which should correspond to the value promised by the manufacturer, while another group representing 72.9% of panels showed degradation of -0.2% to -0.7% per year. "Most of the panels in the second group also performed well and met initial expectations," Caccivio said, noting that this group can be further divided into two subgroups related to different encapsulants.

From 1982 to 2017, the modules in the first group were degraded by up to 13% overall, and the components in the second group were degraded by up to 21%, although half of them did not exceed the 20% threshold. According to the Swiss Group, about 70% of the modules used in the array will still meet the performance guarantees that module manufacturers are currently considering applying them to future technologies, which means a lifespan of 35 years.

The analysis also shows that 87.5% of the modules have some type of slight frontal delamination and multiple junction box problems, but these problems are evenly distributed in the three groups. However, the overheating of the junction box has little effect on the first group of modules. In addition, some modules have cracks, backplane degradation, internal circuit corrosion, hot spots, and burn marks.

The researchers stated that the first and most important lesson learned from studying photovoltaic devices and modules "is that the bill of materials (BOM) is important", adding that the choice of materials is as important today as it was forty years ago.

When asked about the future of this photovoltaic system and the future of general aging devices, if retrofitting or re-supplying may be a better option than letting the old array continue to generate electricity, despite the lower output, the two scientists offered different views.

"Economically speaking, it may be preferable to implement a retrofit and re-power supply, or eventually replace the old array with a brand new system," Caccivio said. "However, the European Commission has established a 40-year life cycle for solar modules. It is reasonable that we must use solar products until the end of their life cycle or provide 80% of their original performance." He added that the photovoltaic system studied showed , Part of which only lost 0.2% of its original efficiency, and still far exceeded the 40-year threshold.

Virtuani said that by implementing proper maintenance, the life cycle of photovoltaic systems can be extended. "If the photovoltaic system works well, it can run for more than 40 years," he explained. "On the other hand, some 30-year business plans are currently being developed. However, if the system and components are operating well, we can extend the operating time of the photovoltaic system to longer than planned."

He also said that the limitation of extending the life of photovoltaic systems beyond 30 years may also depend on the application of photovoltaic systems. For example, the array that powers the water pump does not need to be running at full capacity, and its use may be much more than what its owner expected.

The results of the two researchers were published in two different papers: "35 Years of Photovoltaics: TISO-10-kW Solar Power Plant Analysis, Safety and Performance Lessons-Part 1" and "35 Years Photovoltaics: TISO analysis-10 kW solar power plants, safety and performance lessons-Part 2," are published in Photovoltaic Progress.

This article was revised on 05/07/21 to reflect that the short-circuit current of the panel is 2.55 A instead of 2.55 V as mentioned earlier.

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More articles by Emiliano Bellini

If I conduct a test, I will report the trend of annual kWh or at least its kWh/kWp (specific annual production). This is the most important for investors. It's interesting to report only module downgrades and bin them, but it obscures (intentionally?) the fact that non-uniform downgrades cause mismatches at the system level, so from the owner's point of view, downgrades can get worse. In addition, I will run some samples through the UL list or equivalent sequence to see if these aging modules are safe enough for sale. If not, they are not safe to use regardless of the wishful thinking of the research supporters. The system should be isolated from contact hazards, but can be protected better than normal systems. This is a great study, a necessary study, but when all j-boxes and diodes fail, of course there are inverters, and there are common signs of delamination, corrosion and battery cracking, so It seems irresponsible to draw conclusions and communicate this. PV is a 40-year product. This is the last thing the industry should say to itself. Finally, Siemens Solar became Shell Solar before it became Solar World America, and I remember that even it was subsequently acquired by Sunpower.

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