Over the past two years, the solar community has made significant progress in our understanding of the impact of extreme weather on large solar plants—the number-one source of PV insurance claims. At the same time, the methods of mitigating the potential damage from extreme weather induced events such as hail, wind, hurricanes, heavy snow-load, flooding, extreme temperatures, and power loss at site have also evolved considerably.
Nextracker has been at the forefront of this focus on understanding and mitigating the risks posed by extreme weather. Our pair of white papers on mitigating extreme weather risk (PART 1: Understanding How Differentiated Design and Control Strategies Unlock New Opportunities for Solar Development and PART 2: Surviving High-Wind Events and Dynamic-Wind Effects with Differentiated Solar Project Design and Control Strategies) as well as blogs, webinars and contributed articles have raised the awareness of the industry and helped call attention to the challenges and solutions. Nextracker’s ongoing collaboration with Renewable Energy Test Center (RETC) and others has resulted in fresh data and insights on the topic, some of which were presented in last month’s webinar moderated by Tim Sylvia of pv magazine USA.
Hail damage and mitigation strategies were the primary discussion points, along with new perspectives on the difference between hail and wind risks on their own and correlation between the two. Cherif Kedir presented data from RETC’s comprehensive Hail Durability Testing (HDT) program that has, so far, been used to assess the risk of hail damage to hundreds of solar module models with different constructions, materials, and sizes. I talked about new weather research information and corresponding damage risk profiles that result from it as well as the RETC HDT assessments. We have developed the ability to characterize the coincidence of wind and hail, and have specific tracker stow strategies and capabilities that de-risk solar plants in hail-prone areas like Texas and the U.S. Midwest. One thing is clear from our discussion: To perform risk analysis and implement a proper mitigation approach, one needs to assess both hail and wind risk, individually and together, and have both a passive and active risk reduction plan in place.
Here are a few highlights from the webinar.
RETC PV Module Durability Hail Testing Results
As Cherif explained, RETC has developed a test protocol, the RETC Hail Durability Test (HDT) sequence, which subjects different PV modules to a range of hail impacts with varying kinetic energies based on hail size, mass, and velocity at impact, with a variety of different impact angles. This test program assesses the PV module’s construction, which is a passive risk of being damaged by hail. The regimen goes well beyond the minimum required tests PV modules are conventionally subjected to receive IEC certification. The results were presented in three groups based on module construction.
The tests revealed a clear correlation between glass thickness and the average kinetic energy required to cause the glass to shatter: Modules with thinner glass broke at lower kinetic energy than modules with thicker glass construction. More interestingly, however, the results showed a noticeably wide range in the kinetic energy at which glass breaks. Cherif noted that this points to the glass heat-strengthening or tempering variability as well as other contributing factors, such as module construction bill of materials (BOM), and the manufacturing process quality.
HDT is much more than a simple ice ball versus module glass, pass-fail test. Once the hail tests are conducted and characterized, the sequence continues with a round of thermal cycling, followed by another round of characterization. This step simulates the long-term effects of a hail impact on a module by causing small invisible stress fractures in the cells to propagate into full cracks (see image 1). The PV modules are then subjected to hotspot testing, followed by one last characterization round before the final test results are compiled.
These additional tests show that in some cases, modules that seem to have held up well to the ice ball impacts may in fact experience latent stresses and weakening, leading to delayed cell microcracking or other potential failure mechanisms that could cause cell hotspots. In some cases, this can lead to severe localized heating and damage to the encapsulant, solder connections and the PV module substrate (see image 2).
These results suggest that the historic cost reduction efforts of the module manufacturing community, which have thinned the glass and reduced the strength of the module framing, may in some cases be close to hitting limits and is having unintended consequences and eroding the safety margins, especially in the larger format modules. As always, BOM quality and consistency is a critical factor in determining the durability and reliability of any module. In addition, the ruggedness and strength of the trackers and other mounting structures must be enhanced to protect the modules from extreme weather forming another layer of passive risk reduction.
Importance of Assessing Hail and Wind Risk
Despite all the attention it has been getting, it’s important to understand that hail is an infrequent, albeit costly and highly unpredictable, weather event. In terms of catastrophic risks, wind is a much more frequent and ubiquitous threat. And even though wind and large hail rarely occur at the same time, one thing we’ve learned is not to think of extreme weather threats in a vacuum, that optimizing risk reduction for one threat may not be the best choice. In the case of hail, the industry may have course-corrected too much. As noted earlier, both hail and wind risk need to be assessed, individually and together, and both a passive and active risk reduction plan must be in place as part of a comprehensive risk mitigation strategy.
This area of weather risk assessment is nascent but there are newly developed tools to assess the likelihood of a damaging storm over the life of the project. VDE America has developed a set of tools to help developers specifically assess the return interval of hail—which means how often, in years, that hail of a particular size is likely to occur—as well as the probability of hail and wind for a given region. Such tools are critical in devising the right mitigation strategies for a power project. This should start with assessing the module tolerance to projected hail impacts as well as using the right tracker and devising an appropriate stowing strategy for hailstorms and just as importantly for projected wind speeds expected for any given storm.
The return interval is a key attribute of hail risk we need to understand, as it estimates the probability of damaging hail (>2 in./50 mm diameter) occuring at a location. This measurement is not for a precise location, but more for a limited-size area, because there are too few hail reports to create statistics as we get smaller in geographic scale.
On very large scales like the state of Texas, damaging hailstorms are highly probable. On the other hand, PV systems, even as large as 100 MW, are still too small in area to create a site-specific hail return interval for based on historic data and other analysis tools are required that are still in early stages of development.
Smart Tracker Mitigation Analysis and Strategies
Our analysis derives its power by dividing up the data into regions and breaking it into hail size intervals, so we get a much better understanding of joint hail and wind risk. This analysis then can be used to help construct a kinetic energy distribution that combines the test data from RETC, with wind speed and ice ball diameter to anticipate when glass or cells may break for specific module types.
For example, in thunderstorms where wind speed is at or above 40 mph, we believe the best defensive position is to stow in a front-winded orientation or to the nearest hard-stop position without crossing the tracker through zero degrees. This active risk reduction feature is programmed into Nextracker systems that automatically maintain correct priority order of safety stows, and when used correctly, it can prevent unintended and risky zero angle crossings.
In lower windspeeds, where the user has been alerted to an approaching hailstorm, NX Navigator allows hail stow in either direction, but we would still recommend a front-winded stow since sudden changes in storm strength would be protected from wind damage. The difference in impact energy for median wind speeds in the existing hailstorm records is marginal between front- and back-winded positions.
Nextracker systems benefit from a fail-safe battery backup since any active risk mitigation strategy that fails to protect against a foreseeable event such as a utility power outage during a storm is no risk mitigation strategy at all. A utility-outage will immediately result in all trackers going into a protective stow configuration. Also noteworthy is that our trackers can transition quickly, when activated, moving 60 degrees in 90 seconds.
Decreasing impact angle is an effective way to avoid PV module damage. With the NX Navigator™ operation software, arrays can place PV modules into a stow position that can avoid damage, as this test shows.
Key Takeaways for Developers and Asset Owners
The industry has learned, in some cases the hard way, that utility solar plant owners in hail-prone areas must pay special attention to understanding their project sites’ probability for significant hailstorm risks. When selecting their modules, they should work with independent testing labs to determine the modules’ actual hail tolerance by BOM, since there is significant variability. Nextracker’s risk-mitigation approach employs the tracker as an active tool to reduce extreme weather risk, avoiding dynamic failures by not letting the system transition through zero degrees, making the transition quickly to stow positions, using battery back-up as an active fail-safe protection, and giving the site operator a choice of whether to take the tracker to front-stow position when the median wind speeds reach a certain level. The tracker materials and design take into account extreme weather and form a passive protection from damage. This combination of passive and active strategies featuring smart tracker stowing minimizes the damage when the weather gets nasty.
NX risk mitigation white papers:
Risk Mitigation white paper Part I
Risk Mitigation white paper Part II
Extreme weather risk mitigation blog – December 2020
Reduce the threat of hail damage blog – March 2021
Kent Whitfield, Vice President of Quality, Nextracker
Kent Whitfield has been involved in a range of PV activities from testing, analysis and certification through product manufacturing, system design and deployment for 30 years. He has previously been a R&D manager and principal engineer and for renewable energy technologies at Underwriters Laboratories and held Sr. Director roles in Engineering, Reliability, and Quality for Beamreach, SunEdison, Solaria and MiaSole. He has created two ISO/IEC accredited testing and certification laboratories and represents the USA for IEC standard development for PV products.