Solar PV and Extreme Weather

August 4, 2022\Toronto, Ontario, Canada

In some parts of the country people are well into hurricane season, a particularly troublesome aspect of summer and fall. Hurricanes are nature’s wildest storms, bringing winds up to 160 mph, hail, flooding, and flying debris. If you live in an area affected by hurricanes, you’re probably asking yourself if a PV solar system can weather one. The answer? Oftentimes, yes!

Built to Withstand a Heavy Storm

A good quality rooftop or ground-mount PV system is especially designed to take a beating, often remaining intact in the face of extreme weather.

A key factor is the durability of the solar panel. The top wind speed for a Category 3 storm (or major hurricane) is 129 mph and most solar panels are built to weather that and more. Solar panels are made from extremely durable components, including fully heat-tempered glass with the ability to withstand extreme wind and snow loads. Silfab Solar’s panels undergo rigorous internal and third-party testing with a variety of commercially available fastening hardware, racking options, and mounting zone configurations.

Evaluated under the correct combination of system design conditions and choice of racking hardware, Silfab panels are rated to withstand snow loads (downward force) or extreme wind loads (uplift force) of 112 psf (5400 Pa) in both directions. In referencing the latest set of building code design calculations (ASCE 7-16), this extreme test load ensures that Silfab’s solar panels can be used in residential or commercial systems even in the most extreme coastal regions as can be seen in the ASCE wind contour map in Figure 1 (basic wind speed up to 180 mph). It is not straightforward to translate wind speeds to local pressures for structural design calculations, and this very important effort must be left to structural engineering professionals to determine on a system-by-system basis.

Map of wind loads for USA
Figure 1. Map of wind load provisions for the U.S. as published by ASCE 7-16.

Why the Mechanical durability of PV modules is an important issue?

In 2017, when Hurricane Maria ravaged through the Caribbean and reached a top speed of 175 mph, the impact on Puerto Rico was catastrophic. It was the largest blackout in U.S. history, as power lines and infrastructure were rendered essentially nonexistent, and millions of people went without electricity for months. In fact, the Puerto Rico Electric Power Authority (PREPA) wasn’t able to fully restore power to the island until 328 days (11 months) after the storm hit the island. However, at one Veterans Administration (VA) hospital in San Juan, their facility remained online post-Maria and ready to help those injured by the storm. Their 645-kW rooftop solar panel system was still operating at 100% capacity. The solar panels in this system were engineered to withstand 170 mph winds. In contrast, some PV systems suffered major damage or complete failure with airborne solar modules, broken equipment, and twisted metal racking.

Figure 2 shows the nighttime satellite images of Puerto Rico before and after Hurricane Maria, Category 4 storm, on September 20th. The electric grid was severely damaged, and reports indicated that the majority of transmission lines were destroyed. The impact is visible in a before-and-after night-time image comparison. Figure 3 shows two different systems after Hurricane Maria.

Night view of before and after hurricane Maria
Figure 2. Nighttime Satellite images of Puerto Rico (top) and (bottom) Hurricane Maria on September 20th.

The U.S. Department of Energy and Rocky Mountain Institute experts investigated the root causes of solar PV system failures in the wake of Hurricanes Irma and Maria in the fall of 2017. They reported several root causes of partial or full system failure through observation. The key output of their report is a list of recommendations, which are organized into two categories: (1) specifications and (2) collaboration.

Specifications:

  1. Module Selection, specify high test-load 112 psf (5400 Pa) PV modules.
  2. Review detailed PV module load ratings for all mounting design options in combination with racking/hardware specifications.
  3. Require structural engineering review of wind load calculations to the appropriate building code in force for that specific jurisdiction.
  4. Specify structural engineering wind tunnel report review and ensure module load does not exceed load rating.
  5. Specify hardware locking solution for example T clamps, bolts, torque range, perimeter fencing and racking frame materials/design.
  6. Specify all hardware sizes and dissimilar metal combinations based on 25 years (or project life) for corrosion.
  7. Do not recommend any self-tapping screws.

Collaboration:

Recommendations for interdisciplinary collaboration identify opportunities for increased resiliency of PV systems, which require multiparty involvement but do not represent the standard practice in the PV industry.

  • Collaborate with module suppliers for implementation of static and dynamic load tests representative of the highest uplift design loads that could be present in Cat. 5 hurricane winds.
  • Collaborate with racking suppliers and system designers/installers for full-scale mock-up and testing representative of max uplift design loads for Cat.5 winds.
  • Collaborate with equipment suppliers to document material origin and certificate of alloy/grade and coatings consistent with assumptions used in engineering calculations.

Left image, a solar PV ground mount in perfect condition after Hurricane Irma
Figure 3. (Left) solar PV system survived and powered on the next morning after Hurricane Irma, Necker Island, British Virgin Islands and (Right) solar farm damages during hurricane

According to previous failure analysis reports, PV modules mechanical stability under different stresses is a crucial factor for having a safe and reliable solar system. Therefore, for many customers who live in the regions that experience extreme weather like hurricanes, hail, thunderstorms, blizzards, heavy winds and etc., there is, understandably, significant concern for the durability of the solar panels. Coastal areas (including Caribbean islands) want to know if solar panels are worth the investment given the destructive nature of hurricanes that frequent the region.

When looking at hurricanes specifically, the most damage to PV systems originate from wind and water exposure. Heavy snow or high wind speeds can deform and damage solar panels, racking components, or even the underlying substrate if the realized mechanical loading exceeds the design limits of the PV system as a whole. The risk and degree of this damage is highly dependent on the quality of the solar panel and related system hardware components as well as the quality of the due diligence that is incurred in the design and installation phase. In general, PV panels are made with robust and highly engineered materials designed to withstand more than 35 years life even in the most extreme of conditions. Through the engineering design process, component materials are carefully selected, and the structure of the panel is rigorously optimized and tested. The result of this effort is a product that ensures long term performance and durability, even in the event of hurricanes.

Standard tests for PV module mechanical performance:

All solar panels, regardless of brand, style, shape or material, are built to withstand winds and snow loads to some degree. However, the ability of a module to withstand wind pressures varies greatly between manufacturers. Each new solar panel design or a new/untested combination of bill of materials (BOM) for an existing solar panel must undergo a series of sequential accelerated life testing including environmental, electrical, and mechanical stresses as well as performance evaluation according to the International Electrotechnical Commission (IEC) PV-related standards, IEC 63209, 61730, and 61215. In North America, there are harmonized versions of these international standards under Underwriter Laboratories (UL); namely UL 61730/61215 (61730 = safety, 61215 = performance). As per these standards, all PV modules shall be designed to withstand the electrical, mechanical, thermal and environmental (e.g. temperature, mechanical load, humidity, UV/weather, pollution, etc.) stresses occurring in their intended use. IEC 63209 goes one step further by extending the duration of these accelerated life testing sequences to properly address the full life cycle of a solar panel (>30 yr. expected lifetimes).

      I. Static mechanical-loading (SML) test:

A solar panel experiences various types of mechanical stresses:

  1. Before and during installation
    • Loading and transportation (direct impact, vibrations, and shocks)
    • Poor/rough handling during unloading and installation
    • Dropping panels, walking on panel surfaces
  2. After installation
    • Solar panels are exposed to various sources of mechanical loading (static as well as dynamic) by snow, wind, and hail
    • The static mechanical-loading (SML) test was designed to qualify and certify the reliability of the PV module with respect to continuous wind and static snow loads while the dynamic mechanical load (DML) sequence was designed to mimic repetitive/cyclic loads

According to the IEC 61215 standard, the minimum required test load for a solar panel is 50 psf (2400 Pa) which is carried out on a single panel mounted in the configuration of interest (e.g. two rails fastening the module along the long edge in the specified mounting zones). This test consists of three cycles of loading. Each cycling loads the front side of the panel for one hour, unloads it, flips it, loads the back side for one hour, unloads it, and flips it. During the loading as well as the one-hour dwell time in each cycle, the electrical continuity is monitored within the interconnected solar cell circuit to ensure there are no breaks or discontinuities.

Most solar panels can withstand up to 50 psf (2400 Pa) loading in both directions. However, if planning to install a PV system in regions that experience extreme weather like hurricanes, it is necessary to ensure the intended solar panels can withstand the highest possible hurricane-force winds. Silfab Solar works to ensure that all solar panel products offered into the market are specifically designed and third-party validated to withstand extremely high wind loads translated to a static test load of 75 psf (5400 Pa) which serves 99% of the most extreme coastal markets for both residential and commercial systems. Figure 4 graph shows the maximum static mechanical test loading ranges for Silfab solar panels compared with the average maximum static mechanical test loading ranges of the products from competitors. As can be seen, Silfab modules performed significantly higher under wind loads compared with other products in the market. Figure 5 shows the mechanical loading test setup at Silfab solar.

Silfab Solar mechanical loading tests of solar panels.
Figure 4. Shows the mechanical loading test setup at Silfab Solar.
Sand bag testing on Silfab front and back side of module.
Figure 5. Silfab SML test setup. Testing at 5400 Pa which needs 101 sandbags (10 kg per bag).

II. Dynamic mechanical-loading (DML) test:

The IEC proposes an additional mechanical test (IEC 62782) for solar panels which is employed as part of a testing sequence in the IEC 61215 test standard. This test involves a dynamic mechanical loading (DML) stress to characterize a solar panel’s ability to withstand frequent and cyclic loading conditions such as what might be experienced in a wind event. Unlike the static mechanical load (SML) test, designed to simulate static snow and ice loads, DML stress testing simulates dynamic push-pull loads associated with hurricanes, typhoons, and other high-wind events. As part of the DML test, modules are subjected to 1,000 cycles of +/- 21 psf (+/- 1000 Pa) test loads at a frequency of three to seven cycles per minute. Afterward, modules are placed in an accelerated stress environmental chamber and subjected to extreme thermal cycling (TC), (‑40 °C to 85 °C) and humidity-freeze (HF) cycles.

DML testing is effective in initiating defect sites in both the cell and solar panel construction (e.g. cell microcracks, structural defects in the glass, interconnect ribbons, and solder joints) while the subsequent TC and HF chamber test conditions work to propagate the defects to properly evaluate a solar panel’s robustness against outdoor stresses (such as dynamic wind loading). Silfab executes the full DML sequential testing at Renewable Energy Test Center (RETC), a third-party laboratory with reports readily available upon request (Figure 6). Silfab’s product have all performed well in the DML test sequence with minimum observed degradation which confirms that Silfab’s solar panels are well engineered for high wind environments.

RETC test for DML testing
Figure 6. RETC test setup for DML test.

      III. Hail durability test:

Hail can cause major damage to solar panels for electrical performance, reliability, and safety (Figure 7). Therefore, in hail-prone regions it’s important to purchase a solar panel that has been tested under worst-case hail impact conditions and ensure that the durability is in line with the conditions expected in the region of interest.

Silfab’s PV modules are tested for hail impact at Intertek labs in Lake Forest, California using a pneumatic ice ball launcher (certified to the relevant IEC and UL PV test standards), which can launch ice balls at specified speeds and weights. Ice balls strike the PV module at over 10 specified spots with controlled speed, diameter, and consistency. Simulated hail has a diameter of 25 mm and a speed of 83 km/h. Silfab solar panels perform well under these specified test conditions and show no visible damage from the simulated hail. If more extreme conditions become prevalent in the changing climate conditions, Silfab will continue to elevate the requirements to ensure compatibility of our products with higher speeds and larger ice balls.

Hail testing on a module that failed.
Figure 7. Examples of failure of the module after hail impact test (these photos are from RETC lab and are not related to Silfab products).

Why Silfab Panels Are Stronger:

Silfab panels are one of the strongest panels in the solar PV industry for the following reasons:

  • As a North American manufacturing leader, Silfab Solar’s PV modules are specifically designed for, and dedicated to, the North American market with superior reliability and performance.
  • Silfab PV modules are made with high quality glass that features tempering, texturing, and high-lifetime antireflective coatings. The tempering treatment provides specific mechanical durability and safety features for PV applications.
  • The engineered frame profile of Silfab’s panels offers more beam strength and minimal mechanical deflection/deformation under load than comparative products. A detailed analysis and assessment of the frame profile through extensive research and development has enabled the deployment of a new frame design to dramatically increase the module’s load-bearing strength. The reduced deflection of the module reduces the susceptibility of the solar cells and solder joints to be damaged, which increases the module’s long-term reliability.
  • Multi-busbar technology decreases the size of the interconnect wires while increasing redundancy with additional wires/ribbons interconnecting cells. The smaller wires attached at more locations with smaller solder joints, decreases the overall residual stress between the cell and the interconnecting wire/ribbon. The decrease in residual stress leads to an increased resilience to microcracks that might be caused by wind or snow loads.

Generating energy with solar PV is a cost-effective and reliable solution for power for consumers in the Caribbean, the Florida Keys, and other remote areas which experience high risk of power outages. There are certainly steps you can take to prepare for the possibility of extreme wind events in these regions, but if choosing to build a PV system to enhance power reliability, it is crucial to select solar panels and system components that are designed for enhanced mechanical load applications. It is highly advisable to work with a structural engineering firm to review the system designs as well as the specific selection of components and racking/mounting configurations to ensure your system is ready to “weather the storm”. You can find peace of mind from the fact that a properly engineered PV system designed with tested and validated components will provide a reliable source of power through any severe climatic event. Silfab Solar’s product offerings are designed for extreme weather, whether its hurricane-level winds, severe hail impact, or high snow loads. Silfab Solar designs and develops their solar panels with advanced 3D mechanical load modelling followed by iterative prototyping and internal qualification. Final designs are validated and verified by a 3rd party test lab utilizing a battery of simulated test conditions (e.g. SML, DML, and hail impact) to ensure survivability in all possible extreme weather conditions.

References:

https://chariotenergy.com/blog/can-solar-panels-withstand-hurricanes/

Minimum Design Loads and Associated Criteria for Buildings and Other Structures (7-16)

https://www.washingtonpost.com/news/energy-environment/wp/2017/09/28/storm-driven-power-failures-in-the-caribbean-spur-new-interest-in-renewable-energy/

https://www.noaa.gov/news-release/atlantic-hurricane-season-shows-no-signs-of-slowing

https://retc-ca.com/

https://pv-magazine-usa.com/2017/11/07/solar-survives-the-storms-in-puerto-rico/

IEC 61215-2:2021, Terrestrial photovoltaic (PV) modules – Design qualification and type approval

IEC 61730-1:2016 Photovoltaic (PV) module safety qualification

UL 61730 – PV Module Safety Standards Updates: Making the Transition

IEC TS 63209-1:2021 Photovoltaic modules – Extended-stress testing

IEC TS 62782:2016 Photovoltaic (PV) modules – Cyclic (dynamic) mechanical load testing