The DuPont Global Field Reliability Program is a highly developed field inspection and analysis program that tracks material degradation and its effect on module performance. This 2019 field analysis was compiled from inspection and analysis by DuPont teams of nearly 2 GW of PV installations around the globe. While our field analysis looks at all component materials, we focus special attention on backsheet durability, which plays a critical role in ensuring modules will last long enough to reach the financial objectives of their owners.
Module defect trends
While there were no defects in the majority of module materials, the following defects were observed at certain levels in fielded PV modules: Cell/Interconnect (corrosion, hot spot, snail trails, broken interconnect, cracks, burn marks); backsheet (outer-layer [air side] and inner-layer [cell side] cracking, delamination, yellowing); encapsulant (discoloration, browning, delamination); and other defects (glass defects, loss of AR coating, junction box).
After assessing the extent of defects located specifically within the backsheet level of these fielded PV modules, the following causes were recognized:
Backsheet defects by panel age
There was a sharp increase in backsheet defects after 4 years for competing module materials, yet Tedlar® defects stayed at a low 0.04%. In fact, Tedlar® PVF film-based backsheet maintains the lowest defect rates, even after 35 years in the field.
Backsheet defects by degradation mode
Cracking and delamination can compromise the electrical insulation of the module. Yellowing can lead to mechanical degradation and embrittlement of many backsheet polymers.
The results of the DuPont Global Field Reliability Program’s 2019 Study revealed many common problems in the backsheet materials market across a wide variety of landscapes:
PVDF backsheet failures include outer-layer cracking (allowing delamination, directly exposing the core layer to the environment) and inner-layer yellowing (an early sign of materials degradation and embrittlement).
Polyamide backsheet failures include widespread backsheet through-cracks, which are prevalent along busbar ribbons and can extend to cell gaps and other regions with continued weathering. These cracks can cause arcing and shorts (often leading to localized burn-through and sometimes full module fires) and inverter tripping and ground faults. There have been over 12 GW of field failures to date.
PET backsheet failures include inner-layer cracking (enabling moisture to enter, often leading to busbar corrosion) and outer-layer cracking (exposing PET core to environmental degradation and allowing moisture to enter). Exposing module interiors to moisture can lead to shorting, inverter trips, power loss and multiple module fires.
Glass module failures include delamination (appears to originate near edges of a module or at individual cells) and cracking (likely originates at scratches or chips on the glass surfaces and edges or at stress risers introduced by the racking system).
Materials Matter™ when it comes to backsheets
A large independent power producer (IPP) in Arizona discovered one of their sites was producing less energy than predicted. Upon reviewing the system for failures, they found certain backsheets had started to crack and delaminate – leading to high leakage currents tripping inverters and causing partial shutdowns and late starts. DuPont inspected the 7-year-old site through its Fielded Module Inspection Program and discovered widespread backsheet cracking and delamination on many of the modules. While the site was composed of modules from a single manufacturer and model, as many as three different backsheet types were identified, suggesting the module manufacturer used multiple bills of materials (BoMs) for the same project. The results were clear:
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1 David C. Miller, Michael Kempe, “Creep in Photovoltaic Modules: Examining the Stability of Polymeric Materials and Components,” 35th IEEE Photovoltaic Specialists Conference (PVSC ’10), Honolulu (2010)