From oversight to innovation: Hail stow protocols provide crucial solar project protection

Texas storms show industry’s solar hail defenses work

Hail risk to solar assets, largely unknown prior to 2018, emerged as projects were deployed in hail-prone regions, such as Texas. This two-installment series chronicles a journey in confronting and overcoming a significant oversight in hail protection, ultimately leading to innovative solutions that are reshaping project design and operation. The first part of this article highlights the benefit of hail monitoring and stow by telling the story of three projects in Texas that were exposed to the same system of hailstorms as Fighting Jays Solar, a large project that suffered major hail damage, but that effectively defended themselves using hail stow. It ends with another analysis suggesting that null events, where severe hail doesn’t damage solar projects, are much more common than we may think. It also offers a way to bring these examples to light.

The solar industry's blind spot

In fall 2019, while overseeing technical due diligence for tax equity investments of wind and solar projects at a major bank, I faced a startling revelation. Project sponsors were notifying us of the economic nonviability of hail insurance for solar farms and exercising their right to not renew policies. This was the fallout from the Midway solar farm hail loss earlier that year near Midland, Texas, where a catastrophic hailstorm damaged over 400,000 modules, resulting in a reported $70 million insurance claim.

Our industry had made a critical misstep by deploying vast fields of glass in hail-prone regions like Texas without truly understanding the risks or implementing adequate defenses. Our reviews of hail risk had merely confirmed that PV modules met the typical International Electrotechnical Commission (IEC) standard for 25 mm diameter hail resistance—a size that would prove woefully inadequate—and relied on hail insurance to cap the risk.

Our mistaken overreliance on insurance became apparent when we faced the reality of policy nonrenewal. We had examined historical insurance trends, which showed remarkably stable rates and consistent policy renewals, leading us to believe insurance would always be available and affordable. We were lulled into a false sense of security, presuming that insurance underwriters had properly characterized the hail risk. As we would learn, that was far from correct.

In fact, there was no good way of characterizing hail risk for solar projects at the time. Historical hail data, often based on claims, was scarce in remote project locations due to population bias. Moreover, vulnerability curves used for solar in leading catastrophe peril risk models were based on light metal buildings, not acres of glass. The Midway incident catalyzed the hardening of the hail insurance market, causing premiums to skyrocket, coverage to plummet, and in many cases insurance to become unavailable. This left operating projects at major risk of loss and made many projects yet to be completed unfinanceable without ironclad parent company guarantees to rebuild. That was something only the largest owners could offer that would exist as a liability on their books through at least the end of the financing period.

A quest for understanding and solutions

Driven by this dark realization and a legitimate sense of failure, I dove into researching hail risk and mitigation. I discovered ongoing work by Nextracker and the Renewable Energy Test Center (RETC) demonstrating how tilting modules significantly reduces hail damage—a process we now know as hail stow. For an imminent utility-scale Texas solar project, I included a requirement for hail monitoring and stow, and I added similar language to our group’s term sheet template for new deals.

That was just the beginning. I also needed a targeted and reliable hail risk assessment and mitigation strategy. With that in mind, I reached out to VDE Americas, our engineering consultant for solar technical due diligence. It responded by enlisting VDE’s Dr. Peter Bostock and Central Michigan University’s Professor John Allen, experts in physics and atmospheric sciences, respectively, to develop a new approach to characterizing hail risk using a combination of weather data, hail impact data on solar modules, and lots of math. This effort also led to the formation of a small team at VDE Americas dedicated to hail risk assessment for solar, which I now manage.

The Fort Bend County case study

Nearly five years later, we’re seeing the benefits of hail stow in the public domain. My team recently collaborated with Array Technologies on a study of severe hailstorms in Fort Bend County, Texas, during March 15–16, 2024—the same storm system that significantly damaged the Fighting Jays solar project. 

Array Technologies trackers are deployed at several utility solar projects in the immediate vicinity to Fighting Jays. Based on initial reports, the company understood that three of these sites—specifically, Cutlass I, Cutlass II, and Old 300—had experienced hail but sustained little to no damage. To better understand event severity and what those sites had done to prevent hail damage, Array Technologies engaged VDE Americas to conduct a forensic analysis. 

The first major storm struck on March 15, 2024, between 5:00 p.m. and 6:30 p.m. CDT, bringing powerful wind gusts up to 51 mph and hail sizes ranging from 30 mm to 75 mm across the project areas. The second, even more severe storm hit in the early hours of March 16 (2:30 a.m. to 3:30 a.m. CDT), with wind gusts up to 31 mph and massive hailstones exceeding 100 mm in some areas. 
The exposure varied by project. Old 300 and Fighting Jays experienced 75–100 mm hail on March 16, Cutlass I and Cutlass II encountered 40–50 mm and 50–75 mm hail, respectively, on March 15, and Cutlass I saw a small amount of 50–75 mm hail on March 16. The timing of the overnight storm was particularly unusual, as hailstorms rarely occur in the early morning hours. They usually occur in late afternoon or early evening.

Despite experiencing two >500-year hailstorms within hours, characterized by high winds and hail sizes up to 100 mm recorded by radar, we found that these projects implemented hail stow and reported minimal to no damage. All three projects integrate 2-mm glass-on-2-mm glass modules and use Array Technologies trackers, and thus were in 52° hail stow. We were not able to learn in which direction the trackers were stowed relative to wind at the time of hail. Cutlass I and II reported no damage, while Old 300 suffered minor damage due to wind-blown objects and a tracker motor issue that left about 40 modules unstowed, a proof point for hail stow. 

These results provide compelling evidence of hail stow’s effectiveness and the importance of overnight hail stow. However, it’s important to note that while hail stow can dramatically reduce risk, it’s not a guarantee against all damage. Stowing away from the wind is best, but we haven’t figured out how to predict wind direction when hail falls. It often changes from the prevailing direction of the storm. We weren’t able to discern in which direction Old 300, Cutlass I or II were stowed relative to wind direction, but we can confidently say, based on modeling and other hail loss events we’ve done forensics on, that implementing hail stow in either direction significantly reduces the risk of loss, typically far below the $15 million to $20 million insurance sublimits now available for large solar projects.

Industry chatter had suggested the Fighting Jays location was classified with low to moderate hail risk. This was likely based on a FEMA hail risk map that appears to be based mostly on loss claims and thus can be misleading due to population bias. Radar data solves this problem. VDE Americas has developed more accurate hail risk maps using data from the National Weather Service’s NEXRAD radar system, which we then calibrate using spotter data, records of hail size observed by trained hail spotters. By comparison, our ArcGIS-based hail return interval maps, which account for both spotter and Doppler radar data, clearly show the Fort Bend County area as a high risk for hail.

Validating hail stow effectiveness with more data

While the Fort Bend County study demonstrates the effectiveness of hail stow protocols on three projects, the solar industry needs many more examples for underwriters to consider hail stow a reliable solution. Owners only make insurance claims for losses above deductibles, meaning we have a long list of loss events, but virtually no null events.

To create a more balanced data set, we’ve initiated a Null Hail Event Study in collaboration with the Solar Energy Industries Association (SEIA) and the insurance providers CAC Specialty and FM Global. Our goal is to find at least 25 projects that experienced severe hail (>45 mm) but were in hail stow—either by design or by chance because hail fell when tracker rows were at high tilt angles. We plan to anonymize the data and make it publicly available through the National Renewable Energy Laboratory (NREL) DuraMAT DataHub.

In support of this effort, we’ve used our database to identify nearly 200 severe hailstorms within 10 km of 165 solar projects >50 MW. These projects use various technologies: 73% use crystalline PV modules, 27% are thin film, and 80% have single-axis trackers. While some projects reported hail damage, many have not, suggesting hail losses were mitigated. Please contact me if you’d like to contribute to the study.

As our industry continues to grow and face new challenges, the lessons we’ve learned about protecting against hail loss are proving invaluable. In Part 2 of this series, I’ll explore the best practices that have emerged from these experiences, offering specific, actionable insights for safeguarding solar projects against hail damage.

Operationalizing effective solar hail defenses

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 seen on 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 intelligence at VDE Americas.