In the first installment of this two-part series on hail loss prevention, I presented the results of a real-world case study demonstrating the efficacy of the solar industry’s hail defenses. Specifically, three projects in Fort Bend County, Texas—Cutlass I, Cutlass II and Old 300—successfully used defensive hail stow procedures to weather a series of 1-in-500-year hail events, one of which produced hailstones up to 100 mm as measured by radar.
The hail defense success stories from Fort Bend County stand in stark contrast to the significant damage suffered at the nearby Fighting Jays project, as reported by news outlets. The effective implementation of operational hail stow protocols accounts for these very different outcomes. Here, in Part 2 of this series, I provide some best practices for operational hail defenses, offering specific, actionable insights proven to safeguard solar projects against hail damage.
Site risk and project resilience
The foundation of effective hail protection begins with accurate risk assessment and equipment selection. As a best practice, project stakeholders should require hail monitoring and tracker stow for project sites where the return interval for ≥45-mm hail (as determined by the 95th percentile largest hail diameter) is 100 years or less. This conservative return interval threshold accounts for forecast uncertainties and climate change effects, which are expected to increase hail frequency in the Northeast and hail size in other regions.
In addition to hail stow, PV module selection significantly impacts resilience. Hail durability test results published by RETC show the effect of module glass thickness and strength on hail resilience. Specifically, modules made with 3.2-mm fully tempered front glass are approximately twice as resilient to hail as modules made using 2.0-mm heat-strengthened front glass. Though 2.0-mm dual-glass bifacial modules are the most common module type in the United States, project stakeholders can improve asset resilience by fielding 3.2-mm front glass modules in hail-prone regions.
Hail alert and response
At VDE Americas, we recommend a two-tiered alert system to initiate hail stow. The first tier is a preemptive stow response that utilizes National Weather Service severe thunderstorm watches and warnings to trigger hail stow consistent with tracker manufacturer guidelines. All else being equal, hail stow defenses are optimized by moving modules to the highest tilt angle the tracker can attain, facing away from the prevailing wind direction.
The second tier uses specialized alerts from a qualified weather alert service provider to trigger hail stow to the closest extreme angle the tracker can attain without going through the flat position, which is where modules are most vulnerable to hail damage. As the primary alert threshold, we recommend triggering hail stow when a storm is detected within 30 miles that has a ≥30% probability of severe hail (≥0.75 inches in diameter). Secondarily, we recommend triggering hail stow whenever a storm with severe hail is detected within 5 miles, regardless of hail event probability. To qualify weather alert providers, we recommend a validation study that compares the accuracy and timing of the historical alerts to radar-based observations or on-site hail measurements.
To avoid communication failures and critical delays, alerts from weather alert service providers should interface with the project’s SCADA (Supervisor Control and Data Acquisition) system through API integration—not email—ideally at a one-minute interval. Note that remote operations centers should always be staffed and should have clear procedures for both automated and manual stow initiation. If automated systems don’t respond to a hail alert within one minute, operations center staff should immediately engage manual backup protocols.
Default protocols and testing
Site-specific hail stow protocols should include guidelines to minimize risk during construction, when hail monitoring and stow are not yet operational, and during overnight hours. Although hail is most likely to occur in the late afternoons or early evenings, the Fort Bend County case study demonstrates that severe hail can occur at any time. Specifically, the March 16, 2024, hailstorm that damaged Fighting Jays hit the project around 2:30 a.m. local time.
Wind direction during hail events is unpredictable and often differs from prevailing wind direction. Having said that, most hailstorms in the continental United States approach from westerly directions. Therefore, east-facing stow is recommended as the default orientation for preemptive stow, night stow, and construction stow positions, unless site-specific analysis indicates otherwise.
Because of the severe risk of hail damage to unstowed systems, we recommend testing the entire alerting and stow system monthly or whenever changes are made to system components. We also recommend testing as part of commissioning and performance testing, prior to substantial completion and final funding. Testing can take place during overcast conditions or at dawn or dusk to reduce loss of energy production and revenue.
Active storm management
When severe hail risk is present, rapid response becomes paramount. Given that hail has proven to be much more harmful than wind—and damage during the most severe wind events, such as tornadoes, is unavoidable—we recommend that hail stow override wind stow during active hail alerts.
Consult with the equipment provider to confirm the tracker’s ability to maintain hail stow position in high winds based on historical data for the project site. According to co-probability of wind gust speed and hail size data for Fort Bend County, Texas, for example, a tracker must be able to withstand 67-mph winds, after applying a safety factor, in hail stow position to resist 55- to 65-mm hail.
During a severe hail event, operators should monitor tracker positioning in real time and document any anomalies while staying out of harm’s way. They should immediately investigate and try to remotely remediate stow failures. Lastly, they should maintain hail stow for at least one hour after either the last weather alert or the all-clear notice.
Post-storm procedures
Comprehensive post-storm procedures prove critical to long-term protection through the maintenance, repair, and improvement of hail monitoring and stow systems. Sites should deploy at least one hail sensor per square kilometer, integrated with SCADA to log hail characteristics. Hail sensors enable operators to gauge the effectiveness of hail monitoring services.
After a hailstorm, teams should collect event-specific forensic evidence including hail samples (stored in a freezer), weather reports, witness statements, SCADA data, hail sensor data, high-frequency meteorological readings, and especially wind data. Damage assessment should utilize both visual inspection with supporting photos and advanced technologies like aerial imagery and infrared scanning. This collection of data is very useful for warranty and insurance claims.
Contracts, modeling and outlook
Different weather alert service providers and trackers have different capabilities and features. Therefore, each project should have its own hail monitoring and stow protocol. This protocol should be clearly documented in operations and maintenance contracts and operating manuals, with clear delineation of roles, responsibilities, and procedures.
It’s important to note that while operators can implement best practices for hail protection, they cannot assume unlimited liability for hail damage given the potential magnitude of losses, which can exceed tens of millions of dollars per event. Standard contract termination provisions and performance penalties should apply rather than making operators liable for the full cost of catastrophic hail damage.
Properly estimating hail risk in project financial pro formas and downside sensitivity models is a very effective approach to confirm the sufficiency of hail insurance coverage, operating expenses for deductible payments or repairs due to losses below deductibles, and exposure beyond insurance coverage. These financial models should incorporate average annual loss estimates, but importantly, they should also include downside stress tests for the rare scenarios where very extreme hail events cause damage to projects in hail stow and scenarios where hail stow systems fail.
Although we’re still working on the best way to do it, the effects of climate change and the El Niño Southern Oscillation (ENSO) cycle should be considered to bookend estimated losses over a project’s lifetime. We typically see more hail during La Niña years, when the jet stream brings more cold air to the southern part of the United States, where it mixes with warm, humid air from Mexico and the Gulf of Mexico to form more hailstorms.
Though we will never be able to claim that we have achieved zero hail risk for solar assets, hail risk mitigation measures continue advancing through improved materials, prediction capabilities and refined stow strategies. The success stories from Fort Bend County demonstrate that well-implemented protection strategies work to prevent hail damage and financial losses.
As our industry continues to prove its ability to mitigate hail risk through comprehensive protocols and rigorous implementation, we not only strengthen our physical infrastructure, but also strengthen our relationships with insurance providers and financiers, who are wisely starting to require the hail risk assessment and mitigation strategies discussed in this article.
Jon Previtali is a 20-plus-year veteran of the solar power industry who has worked in project development, operations, asset management, finance, and engineering. He is currently the vice president of digital services and product manager for hail risk assessment and mitigation services at VDE Americas.
Read the first article in the series, Texas storms show industry’s solar hail defenses work.
The views and opinions expressed in this article are the author’s own, and do not necessarily reflect those held by pv magazine.
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