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PV Longevity & Degradation

I have been wondering lately about the life span of solar-electric (photovoltaic; PV) modules. They are usually warranted for 20 or 25 years, but what actually goes bad, and when? Do they really have an infinite theoretical life span, but develop corrosion in the metal parts? Do they run out of electrons? Yours in anticipation,
Anton Berteaux • Winters, California

Although several types of photovoltaic modules exist, laminated crystalline modules are by far the most common and have the longest history in the field, dating from the 1950s, with mass-production beginning in the late 1970s. The information here relates to this type of module.

Crystalline modules are typically designed for a 30-year operational lifetime. Manufacturers perform accelerated life-cycle testing during the design phase to predict module longevity in the field. The actual silicon cells used in modules have an infinite life span and show no degradation after decades of use. However, module output can decrease over time. This performance degradation is the result of two main factors—the slow breakdown of a module’s encapsulant (typically ethylene vinyl acetate; EVA) and back sheet (typically polyvinyl fluoride films), as well as the gradual obscuration of the EVA layer between the module’s front glass and the cells themselves.

Module encapsulant protects the cells and internal electrical connections against moisture ingress. Because it’s impossible to completely seal out moisture, modules actually “breathe” to a very small degree. Moisture that enters a module is, in turn, forced back out on a daily basis, as module temperature increases. Because modules spend their lives out in the elements, sunlight slowly breaks down the encapsulation materials through ultraviolet (UV) degradation, and they become less elastic and more plastic. Over time, this limits a module’s ability to force out moisture. The trapped moisture eventually leads to corrosion at the cell’s electrical connections, resulting in higher resistance at the affected connections and, ultimately, decreased module operating voltage.

The second source for output degradation occurs as UV light breaks down the EVA layer between a module’s front glass and the silicon cells. This gradual breakdown of the material isn’t usually visible to the naked eye, but over time this obscuration limits the amount of sunlight that can hit the cell. A slight but incremental decrease in cell output current is the result.

PV warranties typically allow for 20 percent output degradation over the module’s 20- to 25-year warranty life. But measurements of many modules put into service in the 1980s show that it’s unusual to see even half that much degradation. Many of those earliest modules still perform to their original specifications. It is safe to say that modules carrying warranties of 20 years or more have a high probability of working well 30 years from now.

Windy Dankoff • Solar Pioneer
Joe Schwartz • Home Power

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