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Hail Stress Sequence

Why HSS Testing Matters

The increase in PV sites being deployed to extreme hail prone regions is prompting concern for many in the solar industry. Aside from the hail risk, some of these locations are ideal for large utility-scale solar sites due to the excellent solar resource, available land and economies of scale. Both developers and investors are enticed by these benefits, but hail mitigation methods must be implemented to ensure successful long-term operations.

These mitigation strategies include selecting modules that have higher hail resistance and implementing weather monitoring and tracker stowing practices. They no longer include relying on big insurance payments if the site is severely impacted by hail. In the current insurance landscape for hail claims, insurers are requiring large deductibles, are implementing limits that significantly reduce payouts, and/or are adding exclusions to insurance policies such as not covering modules with cell cracks. This has essentially pushed hail damage risk on to the other project stakeholders.

PVEL’s field team has performed field EL on over 2 GW of sites with hail damages in the past four years. This testing was initially used to identify modules with underlying cell damage and helped size the insurance payment. But as insurance coverage for hail damages grew more restrictive, mitigating hail damage became increasingly important for site owners. PVEL’s HSS goes well beyond the minimum requirements of IEC 61215 and is providing critical data for site developers and investors in determining which modules should be considered to help lower hail damage risk.

Modules with hail damage. Those without broken glass may have extreme cell cracking, but it is no longer likely that insurance will pay for their replacement.

A module with over 25 cracked cells due to hail, which was not eligible for insurance coverage due to changes in insurance policies.

Front Encapsulant
Cells, Cell Interconnects

Materials Assessed

These materials determine how well or poorly a module can withstand hail impacts:

  • Cells
  • Encapsulant
  • Cell Interconnects
  • Glass
  • Frame
  • Module Size
Explore PVEL’s Test Methodology
Key Takeaways
Scroll through the key takeaways.

Glass//glass modules are more than twice as likely to break than glass//backsheet.

During 50 mm hail testing, 89% of 2.0 mm glass//glass modules experienced broken glass compared to 39% for 3.2 mm glass//backsheet modules. Glass//glass modules suffered breakage on the front glass, rear glass, or both.

Corner impacts were the most damaging.

For both glass//glass and glass//backsheet modules, the first impact directed at a module corner yielded the highest breakage rate. 62% of broken glass//glass modules and 43% of broken glass//backsheet modules cracked with this first shot.

Glass breakage reduced at lower hail diameters, but wasn’t completely eliminated.

Manufacturers can choose to retest at reduced impact energies using 40 or 35 mm hail. The glass//glass breakage rate decreased to 57% with 40 mm hail and 18% with 35 mm hail. The factors contributing to breakage versus non-breakage are under investigation.

Extreme hail is not a concern for many site locations.

While very important for some locations, many regions do not experience extreme hail. As such PVEL is not naming Top Performers in the HSS category or counting these as failures in the Scorecard failure statistics. Site stakeholders in areas with significant hail risk should contact PVEL for introductions to manufacturers who did well in HSS testing.

Test Procedure

The PQP hail stress sequence starts by shooting 11 labmanufactured hail balls of 50 mm diameter at terminal velocity (32 m/s). The hail is fired at targeted areas of the module at a 0° angle, per IEC 61215. These shots deliver an impact energy of approximately 31 joules, which is significantly higher than the 2 joules from the baseline 25 mm hail test in IEC 61215.

After receiving the hail impacts, modules without broken glass are subjected to dynamic mechanical load (DML) to propagate cell cracks, followed by 50 thermal cycles and 10 humidity freeze cycles to simulate environmental stresses that can accelerate cell crackrelated power loss.

PVEL also offers hail testing at different hail diameters and velocities for projects with varying degrees of hail risk.

Test Result Spotlight

Glass breakage due to hail impacts is significantly more common for glass//glass modules compared to glass//backsheet modules. But for modules without broken glass, cell cracking and associated power loss is more prominent in glass//backsheet modules than glass//glass modules, as shown in these examples.

HSS - glass//glass

glass//glass post-HSS with 0.4% power degradation.

HSS - glass//backsheet

glass//backsheet post-HSS with 4.9% power degradation.

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