The crystalline silicon photovoltaic (PV) modules under open circuit conditions typically degrade at a rate of about 0.5%
per year. However, it is suspected that the modules in an array level may degrade, depending on equipment/frame
grounding and array grounding, at higher rates because of higher string voltage and increased module mismatch over the
years of operation in the field. This paper compares and analyzes the degradation rates of grid-tied photovoltaic modules
operating over 10-17 years in a desert climatic condition of Arizona. The nameplate open-circuit voltages of the arrays
ranged between 400 and 450 V. Six different types/models of crystalline silicon modules with glass/glass and
glass/polymer constructions were evaluated. About 1865 modules were inspected using an extended visual inspection
checklist and infrared (IR) scanning. The visual inspection checklist included encapsulant discoloration, cell/interconnect
cracks, delamination and corrosion. Based on the visual inspection and IR studies, a large fraction of these modules were
identified as allegedly healthy and unhealthy modules and they were electrically isolated from the system for currentvoltage
(I-V) measurements of individual modules. The annual degradation rate for each module type is determined
based on the I-V measurements.
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