Choice of PV backsheet material affects module life

June 20th, 2019, Published in Articles: Energize

DuPont Photovoltaic and Advanced Materials’ (DuPont’s) 2019 Global Field Reliability Study shows that the choice of backsheet material used on PV modules can result in significant degradation and negatively impact solar module performance.

The report lays out failures with PET, polyamide and PVDF backsheets and issues with glass-on-glass modules. The report also highlights trends in module failures and defects, chronicles backsheet defects by panel age, provides comprehensive data on backsheet defects by temperature and climate, and includes a case study.

This programme is one of the most thorough of its kind, guided by a multistep inspection protocol at sites in North America, Europe, Asia, and the Middle East and staffed with some of the most experienced scientists in the solar industry. This team of experts collected data from 6,5-million modules at 355 installations producing 1,8 GW of total power.

Key findings (compared to the previous year)

  • The number of site inspections in the report grew from 275 to 355.
  • The number of panels surveyed increased from 4,2-million to over 6,5-million (1,04 to 1,8 GW).
  • Overall module defect rates increased since 2018.
  • Total module defects: 34%.
  • Total backsheet defects: 14%.
  • Backsheet defects increased 47% from 2018.
  • Cracking comprises 66% of all backsheet defects.

Case study

A large independent power producer (IPP) in Arizona, USA, discovered one of its sites was producing less energy than predicted. Upon reviewing the system for failures, it was found that certain backsheets had started to crack and delaminate, which lead to high leakage currents which tripped inverters and caused partial shutdowns and late starts.

DuPont inspected the 7-year-old site through its Fielded Module Inspection Programme 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 had used multiple bills of materials (BoMs) for the same project.

Results from the 2019 study

The 2019 report was assembled from inspection and analysis by DuPont teams using a variety of criteria including component, material, mounting, time in service and climate. “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,” says Roy Choudhury, the company’s global reliability manager.

Module defect trends

While there were no defects in the majority of module materials, the following defects were observed at certain levels:

  • 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.
  • Other: Glass defects, loss of AR coating, junction box.

Fig. 1: Defects in PV modules (trends).

Backsheet defects by panel age

There was a sharp increase in backsheet defects after four 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 temperature

Higher temperatures cause backsheet defects to accelerate. These rates are 75% greater for roof installations than ground, because roof-mounted systems typically run 15°C higher than ground- mounted installations [1].

Fig. 2: Higher operating temperatures accelerate backsheet defects.

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.

Fig. 3: The sharp increase in backsheet defects over time, by backsheet material.

Types of backsheets

PVDF backsheet failures

  • Outer-layer cracking: Widespread cracks allow for delamination, directly exposing the core layer to the environment.
  • Inner-layer yellowing: Yellowing can be an early sign of material degradation and embrittlement.

Polyamide backsheet failures: Widespread backsheet through-cracks

  • These failures are prevalent along busbar ribbons, but with continued weathering can extend to cell gaps and other regions.
  • Arcing and shorts often lead to localised burn-through and sometimes full module fires.
  • Reported inverter tripping and ground faults.
  • Over 12 GW of field failures to date.

PET backsheet failures: Inner- and outer-layer cracking

  • Inner-layer cracks enable moisture to enter, often leading to busbar corrosion.
  • Outer-layer cracking exposes PET core to environmental degradation, also allowing moisture to enter.
  • Exposing module interiors to moisture can lead to shorting, inverter trips, power loss, and multiple module fires.

Glass-on-glass module failures: Delamination and cracking

  • Delamination appears to originate near the edges of a module or at individual cells.
  • Cracks likely originate at scratches or chips on the glass surfaces and edges or at stress risers introduced by the racking system.

Fig. 4: Comparison of two backsheets.

Conclusion

For nearly a decade, DuPont has collaborated with field partners, customers, downstream developers, universities, and national labs to perform field inspections to assess and understand the state of degradation of fielded PV modules.

The report highlights the fact that the choice of backsheet material on PV modules has a direct affect on the longevity of PV modules in the field and may result in significant loss of power from PV installations over time.

Reference

[1] David Miller and Michael Kempe: “Creep in Photovoltaic Modules: Examining the Stability of Polymeric Materials and Components,” 35th IEEE Photovoltaic Specialists Conference (PVSC ’10), Honolulu, 2010.

Contact Tara Stewart, DuPont, tara.c.stewart@dupont.com

 

 

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