Researchers at NREL Find Fewer Failures of PV Panels and Different Degradation Modes in Systems Installed after 2000
Overall failure rates for photovoltaic (PV) solar panels have fallen dramatically when compared to installations prior to 2000, according to a comprehensive review by researchers from the Energy Department’s National Renewable Energy Laboratory (NREL).
Data on the failure of a PV panel, defined as one that had to be replaced, was culled from reports on 4,500 globally deployed panels and another 50,000 installed systems around the United States.
Installations between 2000 and 2015 exhibited a median failure rate of 5 panels out of 10,000 annually, while for those panels installed between 1980 and 2000 the rate was twice as high. The difference is not explained by a greater deterioration of the pre-2000 installations, because when grouped into systems of similar age, pre-2000 installations exhibited higher percentages of failures.
An article detailing the findings of the research, Photovoltaic Failure and Degradation Modes, appears in the journal Progress in Photovoltaics. The authors are Dirk C. Jordan, Timothy J. Silverman, John H. Wohlgemuth, and Sarah R. Kurtz from NREL and Kaitlyn T. VanSant from the Colorado School of Mines.
Well-built PV modules have demonstrated performance for 30 years outdoors, and warranties of 25 or more years are common, giving researchers a challenge: Indoor accelerated tests and certifications that quickly give confidence in the warranty are in demand by consumers, investors, and manufacturers. However, as degradation modes can have non-linear effects on performance, uncertainty is added to long-term lifespan predictions for new PV products. Reliability test standards for PV modules were initially developed in the early 1980s by the Jet Propulsion Laboratory including early mortality and extreme weather tests. Panels produced following these “Block V” test standards also exhibited reduced failure rates, compared to earlier installations.
NREL researchers and others continually further develop improved tests and work to include them in internationally agreed upon updates to IEC standards for PV quality certification. The findings suggest that quality-control measures have thus far kept failure rates low despite cost-cutting changes in materials and manufacturing.
“We analyze outdoor performance in detail. Accelerated tests can then be improved to better reflect the types and percentages of degradation mechanisms that are observed in real world performance,” said lead author Dirk Jordan.
While encapsulant discoloration was a common degradation mode in older PV panels, more recently installed crystalline silicon panel problems have been dominated by hot spots and internal circuitry discoloration when the types of degradation modes were weighted by severity. Additionally, two roof-mounted systems installed pre- and post-2000 in a hot and humid climate exhibited a higher rate of failures, and further research is needed. In the interim, researchers recommend utilizing high-quality, certified panels and installations allowing airflow underneath panels, rather than flush mounting them to roofs, especially in hot climates. Available thin film data was primarily qualitative; panels predominately had issues with glass breakage and corrosion in the absorber or transparent conductor layers. Hot spots can be a safety issue, so the findings are important as the authors and others from NREL’s Reliability and Systems Engineering group continually work to improve both safety and performance. John Wohlegmuth, co-author and a leader of international IEC test standards, stated that the recent updates to IEC qualification standards for crystalline silicon PV panels include more rigorous stress testing for hot spots.
Funding for the research came from the Energy Department’s Solar Energy Technologies Office.