SOLAR VS HAIL: PIVOTING AWAY FROM DANGER
AXIS Best Practice
To help the hail survivability of a solar PV project, AXIS recommends considering the following pillars for best practice:
Accurate forecasts
Understanding hail exposure and integrating forecasts that inform automated or manual hail stow decisions.
Appropriate technology
Selecting PV modules designed and tested to withstand hail impacts; as well as trackers able to stow to greater than 60 degrees.
Well-informed operations strategy
Implementing robust and foolproof procedures and protocols through construction to operation, allowing personnel to act fast.
Accurate Forecasts
Proper characterization and understanding of location-specific hail exposure risk is essential early in the project development phase, as well as consistent 24/7 awareness of when a hailstorm might strike, which is why integrating real-time weather forecasts is essential.
Understanding site exposure
Severe convective storm and hail-specific risk spatial probability maps can indicate the risk to a site location, even with broad resolution. These are often presented with a return interval, or exceedance probability, compared to a hailstone diameter—a number of these are shown in The Threat of Severe Convective Storms chapter.
VDE Americas comparative hail risk map of the US. Provided with permission of VDE Americas.
As solar PV hail damage and claims have increased from areas that were underserved by accurate maps predicting the long-term probability of hail strikes, providers have mobilized to develop bespoke maps and services that benefit developers as well as insurers. Some services can provide a more comprehensive understanding of the financial risk over time for the technology, incorporating hail stow strategy.
Two key metrics are probable maximum loss (PML) and annual average loss (AAL) estimates, which are determined in PML reports like those produced by VDE Americas [1]. These reports are based on probabilistic modelling and equipment-specific vulnerability and replacement costs. They present estimated losses that consider the full range of hail sizes expected to fall over a given project’s lifetime for the AAL metric, and the largest expected loss event over a 500-year period for the PML metric. The PML report also includes business interruption (BI) estimates of revenue lost to downtime.
Two site specific scenarios were modelled by VDE Americas and provided to AXIS in another type of analysis, the Pro-Forma Risk Exposure (PRE) report. The results of the modelling determine an estimated breakage percentage loss* for a 95% non-exceedance probability over 10 and 40 years of the solar project lifetime for the specific PV module and tracker technology. The first site modelled is in an established area for large and frequent hail exposure, Amarillo, Texas. The model assumes 3.2mm monofacial PV modules that exhibit on average twice the hail resiliency compared to their thinner 2mm bifacial module counterparts.
*Note: VDE Americas usually provide estimated cumulative losses in USD, which can be adjusted depending on site specific design, such as specific PV module technology.
PV module breakage with 95% non-exceedance probability (P95), based on VDE Americas data from a project example in Amarillo Texas, 100MWdc, 3.2mm monofacial PV modules with singe-axis tracker. This modelling scenario assumes a blended average of module breakage facing into and away from the wind.
It can be observed that a zero-degree, or horizontal, tilt angle exceeds 100% cumulative loss over the 40-year period, and therefore also exceeds the total cost of replacing all PV modules in the project more than twofold. This can occur due to multiple replacement cycles following repeated hail damage events and associated costs beyond PV module replacement (labor and disposal), representing a scenario with exceptional frequency or severity of hail events.
In the optimistic situation that a tracker system implements a perfect 75-degree stow on each occurrence, the modelling results determine that breakage probability becomes almost negligible. Therefore, a risk exposure assessment such as this could support project developer decisions on trackers, for example, to opt for a tracker system offering a 70 or 75-degree stow, due to the far reduced risk of damage over the project lifetime when compared to a 50 or 60-degree stow limitation.
Georgia, a lesser-known state for hail exposure, is also modelled as a project scenario using more vulnerable 2mm bifacial PV modules compared to the 3.2mm monofacial PV modules considered in the last scenario. This example demonstrates that a non-Texas location can still expect to exceed 100% losses over the project lifetime with a horizontal tilt angle, and as 57% PV breakage probability with a 50 degree stow angle.
PV module breakage with 95% non-exceedance probability (P95), based on VDE Americas data from a project example in Georgia, 250+ MWdc, 2mm bifacial PV modules with single-axis tracker. This modelling scenario assumes a blended average of module breakage facing into and away from the wind.
Confidence from real-time weather forecasts
During construction and operation, having onsite weather equipment and monitoring, combined with an accurate weather forecast provider with specialty in hail is essential. The system should be configured so that it provides real-time alerts, sometimes referred to “nowcasting”. The offering should provide sensible lead times on incoming severe storms and hail—ideally, the initial warning should come more than 30 minutes before the hailstorm reaches the closest edge of the site, with VDE Americas recommending five miles proximity as the hail stow trigger. Regular updates should then follow, allowing as much time as possible for the site to respond. It is important that the spatial resolution is reasonable, such as one square kilometer to ensure high accuracy and precision. The weather provider should also have a successful track record, demonstrating limited false negatives.
Importantly, the weather provider alerts should integrate seamlessly into the tracker control system or SCADA (Supervisory Control and Data Acquisition), which is key for enabling automatic stow protocols if utilized. In addition, these alerts must also be provided to the operators via the preferred communication method.
Smart Technology Selection
The resilience of the solar PV system against damaging hail is strongly influenced by early decisions around materials, mechanical design, and tracker performance.
Robust PV modules
One of the most critical passive protection measures is the selection of PV module type and glass. Developers should specify fully tempered glass, usually accompanying a front-glass thickness of 3.2mm or greater, offering significantly greater mechanical strength and fracture resistance than thinner or non-tempered alternatives. The globally established standard IEC 61215-1: 2021 (Terrestrial photovoltaic (PV) modules - Design qualification and type approval - Part 1: Test requirements) requires modules to pass a hail impact test (called MQT 17). This is widely regarded as inadequate for hail exposed regions where hailstones regularly exceed this diameter and can strike panels at greater wind speeds. For example, MQT 17 Hail Test subjects PV modules to 11 strikes in pre-defined areas of the PV module, which are fired at a perpendicular impact with 25 mm (one-inch) ice balls (approximately 7.5 g) at 23 m/s but does not consider:
- Irregular hail shapes
- Angle of attack/module tilt
- Wind which could increase impact speed
- Microcracking
Distributed impact energy
Concentrated impact energy
Whilst IEC 61215 (2021) does define incremental ice ball hail tests that can be completed up to 75mm to ~40 m/s, these are not mandated, and manufacturers rarely complete and/or publish results of these higher-grade tests.
These limitations contribute to many PV modules being vulnerable to horizontal and angled strikes in large number of cases.
Therefore, preference should be given to PV modules that have passed optional enhanced hail impacts and test protocols, especially those conducted by third-party independent testing, such as:
- The Renewable Energy Test Center (RETC) which offer their Thresher Test and Hail Durability Test using larger ice balls than those required by IEC 16125. RETC also publish a PV module index report on an annual basis, ranking PV modules on a number of metrics including this test.
- PV Evolution Labs (PVEL)’s “hail stress sequence” involving 50 mm hail impacts followed by thermal cycling to gauge longer-term effects. PVEL publishes a Module Reliability Scorecard on an annual basis.
- GroundWork’s PV Test lab, who have comparison data between field measurements and lab models.
Single-axis trackers protecting against hail and wind
From an active protection perspective, as established from the PV module breakage results in chapter three, tracker stow strategy is pivotal. Selection of a reputable tracker OEM with proven capability and success metrics is essential. Systems should be capable of reaching close to a near-vertical hail stow angle, like 75 degree, which significantly reduces the available surface area of the array exposed to the falling hail and kinetic energy transferred from hailstones to the module surface by increasing deflection and decreasing impact normality.
Ideally, this angle should be the same as the wind stow position and lock into place so it cannot experience oscillations and move out of position. This angle should be achieved via rapid rotation, ideally in under three minutes from operational position once a credible hail threat is identified. Tracker systems must be designed to maintain structural integrity during movement and while in stow, typically requiring validated performance at extreme wind speeds.
Consideration of the Wind
High wind sites, and windstorm-exposed regions, present specific challenges for the design and reliability of solar PV systems. As hailstorms are often associated with high wind speed events, the solar PV project systems must also be designed to take into consideration the concomitant wind loading and coupled hail-wind effects.
Determining the wind characteristics and risk
AXIS is aware of the challenge of constructing solar PV sites that often have no or very little local historical data that can be used for the project’s wind site assessment. The projects mainly rely on meteorological data of airports, sometimes located very far from the project site location, and mesoscale models to identify wind speed at the site. These approaches, although being common practice, lead to significant uncertainties and may not capture extreme local events like thunderstorm updrafts and downbursts, which we have seen through our claims can result in extensive damage.
While best-practice design incorporates wind tunnel tests investigating pressure loads on the modules and aerodynamic instabilities for various tracker angles, row spacing, and other configuration aspects, these assessments do not typically include potential extreme wind events, especially if they have not been flagged in the site wind assessment.
Site-specific wind speed must be assessed and characterized adequately, ideally from on-site wind speed measurements alongside relevant local historical data and in order to derive the project extreme wind speed, the design wind speed, in conjunction with building codes and standards’ requirements. No matter of the tracker system, the full solar PV system comprising PV modules, racking, trackers, and foundations should resist wind speeds up to the design wind speed.
In most cases, the failure mechanism is a cascading effect that primarily starts at the PV module level (exposed to the wind). Some of the most affected systems have been of 2P configuration. Examples of structural damages resulting from high wind speeds as reflected in AXIS claims include:
- Dislodged PV modules from the mounting structure, due to bolts being pulled out of purlins/beams (use of inappropriate fixings, like T-clamps)
- Teared PV modules around their fixation points
- Failed tracker drives and motors
- Bent or twisted torque tubes and racking
- Pulled out foundation piles
- Damaged or teared off DC cables
Stowing for wind and hail simultaneously
If hail is detected and this stowing mechanism prevails, the system must be designed so that it can withstand the design wind speed in the hail stow position, primarily so that it resists damage to accompanying wind and remain within hail stow for the duration of the storm.
Some solar trackers and sites adopt a zero-degree wind stow angle strategy—trackers in horizontal position—aimed at mitigating wind loads primarily by reducing the surface area of the modules, although there are two serious impediments to this:
- Impediment: Under certain wind conditions, a pure horizontal stowing angle means the system can enter in excitation mode and become dynamically unstable due to aerodynamic instabilities (torsional fluttering, galloping), increasing the risk of system structural failure
- Recommendation: It is therefore crucial for the project to investigate aeroelastic instabilities by performing wind tunnel tests and/or advanced CFD simulations, although these can be quite onerous
- Impediment: A horizontal wind stowing position means trackers are in the worst position in case of concurrent hailstorm, unless there is a complementary hail stowing strategy that would in that case prevail and place the trackers into a hail stowing position.
- Recommendation: The project should ensure that if a horizontal wind stowing strategy is adopted at a site, it should be proven that the site presents no risk of hailstorm, or that the tracker system can withstand design wind speed loads in hail stow position. The more robust design is to adopt a solar tracker system which has the same stow mode for hail, and for wind, which locks into place, can withstand the maximum wind speed without risk of oscillations, which beneficially prevents the risks from conflicting stow scenarios.
Keeping tracker systems online, 24/7
Tracker systems, which include critical components from motor drives to autonomous tracker controllers that integrate with real-time weather systems, must have an uninterruptible power supply (UPS) with a suitable auxiliary power source to ensure reliable execution of hail response even during periods when connection to the grid is lost.
Trackers and/or tracker rows should be controlled by a network control unit, which taps into a robust wireless mesh network that is secure with encrypted communications, while ensuring high-priority signals are not blocked or cause congestion, preventing any interference or slowing of stow commands. The communication systems should be suitable for outdoor use and feature redundant paths to ensure the stow signal reaches all controllers. Systems should also be designed such that a loss of communications triggers a failsafe state, aka hail stow. Additionally, low battery should also trigger an automatic stow mode.
Other research suggested that PV modules with a central support bar on the back (mid-span frame) suffer less glass breakage. However, there are still unknowns while in-situ data is not openly shared so it is important for developers to consider the latest research and releases, in what is a rapidly evolving field.
Well-informed Operational Strategy
AXIS considers a robust operational strategy that implements robust operational procedures and protocols, considering the many eventualities, is essential for PV solar sites with hail and severe convective storm exposure.
Design stage
A decision on whether to incorporate automated stow to the tracker system should be made with the tracker supplier. If real-time weather alerts can be programmed into the tracker system software allowing for autonomous decision-making, this can reduce errors and delays by the operator. Automatic stow in partnership with a procedure for manual action enables operators to make quick decisions based on manual alerts or even a phone call from a neighboring site that is experiencing changes in wind direction. As well as hardware considerations when selecting a trusted tracker supplier, as described in Smart Technology Selection, the user interface design is important, and should be clear and not create complications for those on site who may need to initiate an emergency stow mode.
It is important to work with the tracker supplier to understand their stow priorities when there are conflicting perils, ensuring that personnel cannot move the tracker system out of a critical stow position like hail or wind until the event has passed, and that the tracker system does not pass the horizontal and risk exposes the PV modules to hail in their most vulnerable angle.
Throughout the project stages, it is essential that emergency action plans are firmed up in writing, with a clear chain of command for zoned and site-level protection.
When ordering PV modules, a suitable number of spares should be considered and stored securely on site as a reserve.
During construction
Consideration should be taken to when the construction phase will take place, avoiding the region's "hail season" if possible. PV modules should be stored with consideration, and those in laydown should not be exposed to the sky and sheltered from high winds. Arrays with tracker systems should be placed into the maximum stow mode manually if not yet energized. Ideally, there should be a staged energization process so that zones will have already been commissioned while others are under construction, allowing some degree of control if a severe weather event strikes.
During operation
Trackers should be programmed to move automatically into maximum stow at night, facing the direction of the prevailing wind. Weather warnings should however still be received, as it may be best in the face of an approaching storm from the opposite direction that the trackers reposition.
AXIS has experienced several claims where a project stow effort has partially failed due to unforeseen tracker system damage, like damaged cable, and therefore it is strongly recommended to organize regular stow testing, especially in the approach to the main hail season, to maintain equipment readiness and identify issues before a real event occurs. Personnel should also receive regular training and drills.
It has been proven that the loss production from false positives and tracker testing is minimal, also since skies are overcast during storm events and irradiance low. Data provided to AXIS by a major asset owner determined energy loss of only 0.07% in 2023 and 0.06% in 2024 from hail stowing at Solar PV sites surpassing 500MW power capacity.
Successful and unsuccessful stowing is best identified via an accurate reporting tool, which accounts either for every tracker in the array or the row, dependent on tracker system.
When a hail alert is received, site staff should be notified using the preferred communications approach, such as radio or phone network, to reach a safe evacuation point or O&M building as soon as possible. Personnel must not be adjacent to modules during storm conditions when they may move into a stow position.
Systems and asset registers that store data and reports such as inspection logs should be maintained and updated, so that information is readily available post event to support assessment of damage and any insurance claims that may arise.
Questions to consider
- What is the stow priority for extreme weather conditions?
- Is manual override available on automatic decision making, could this be a risk to the solar PV assets?
- What site personnel will have access to the tracker stow control system?
- If opting for manual stow decision making: what is personnel response time?
- Can a manual stow command override another manual stow command?
Post event
Following an event when damage has occurred and repowering is required, a thorough review of the event should be considered, and whether the site should have been expected to survive given the hailstone size and impact. If the answer is “yes” and there was still extensive damage, a redesign should be considered to prevent a reoccurrence.
As an additional measure hail sensors can be employed on the site to unlock an immediate visibility of hail size and spread post event. Not only does this support the damage assessment and claim process, it provides a better understanding of successful scenarios, and whether the forecasts matched the ground truth.
Preparing for Hail Damage to Other Site Components
Inverters and Transformers:
Inverters, MV transformers and other electrical balance of plant (EBoP) equipment are usually housed in metal enclosures that offer decent hail protection, but large and wind driven hail can dent housings, louvers, or cooling/radiator fins, which have featured as part of solar claims. If a cooling fan or vent is damaged or disrupted, it risks the EBoP needing to shut down until repaired, or in the worst case, triggering a thermal event, something inverters can be prone to if overheating. Taking extra steps to protect this equipment is beneficial, and there are options via the use of metal mesh guards or high strength louvers for instance. Electrical protection devices add resilience—if hail damage to any component leads to an electrical short, string fuses in combiner boxes should blow to isolate the fault before upstream equipment is harmed.
Cabling
Exposed cabling, particularly DC string cables, risk damage or disconnection if not in a conduit, so should be designed to remain secure on impact.
Auxiliary Systems
Small weather sensors and CCTV cameras on site can be broken by hail if not shielded, as can UPS and controller boxes mounted on the trackers. In many cases, these ancillary damages are minor compared to PV module damage, but they can complicate recovery and repowering. Disrupting monitoring can also open up the site to the vulnerabilities like theft.
In general, a practical approach for mitigating impacts and minimizing downtime of these types of components is ensuring plenty of spare parts are readily available on site and having rapid repair plans are in place, similar to the PV modules.
