Enhanced hail durability testing
Module manufacturers and testing laboratories must subject product designs to higher impact energies to better understand severe hail effects in real-world applications. Unlike certification tests, enhanced stress tests are not pass/fail in nature. Rather, beyond-certification tests provide comparative data that differentiates product designs.
In May 2019, hail damaged roughly 400,000 modules at the Midway Solar farm in West Texas, resulting in a previously unimaginable $80 million of insured losses. In response, RETC designed the industry’s first enhanced testing program for severe hail.
“The HDT [Hail Durability Test] program expands and improves upon UL and IEC requirements,” explains Kedir, referring in the first instance to the safety body formerly known as Underwriters’ Laboratories. “First, the program subjects modules to higher kinetic impact energies to better reflect the risk posed by hail over a 25- or 30-year operating life. Second, it thoroughly investigates a range of possible outcomes – from cell cracking to glass breakage – which provides valuable data for probabilistic analyses. Third, the HDT program includes thermal cycle and hot-spot tests to reveal potential long-term module degradation modes.”
Stakeholders developing projects in hail-prone regions should encourage or even require module suppliers to provide these beyond-certification hail test data and make system design and product procurement decisions accordingly.
Front glass thickness and strengthening
In the absence of product-specific data, front glass thickness provides a general indication of hail resilience. Anonymized hail durability test data indicates that PV modules with thinner front glass are less resilient to large-diameter hail, as compared to modules with thicker front glass.
This is partly due to the fact that thinner glass has less cross-sectional area to absorb a shock without shattering. Additionally, thinner front glass materials cannot be fully tempered via traditional means and must be strengthened via alternative methods.
The impact energies of a hailstorm vary based on the array-tilt angle and the direction and speed of the wind. Although array-tilt angles are generally fixed in roof-mounted applications, tracker-mounted systems are not static. This ability to move out of harm’s way in advance of an approaching storm is relatively unique within the built environment.
Many large-scale PV systems integrate intelligently controlled single-axis trackers that can execute peril-specific defensive stow strategies. Leveraging these capabilities, plant operators can manually or automatically rotate a tracker-mounted PV array to an optimal tilt angle for hail risk mitigation. This defensive posture will reduce hail-impact energies and the exposed hail-field impact area, effectively decreasing the number of direct hail strikes and decreasing hail-impact energies.
Hail impact mitigation
Generally speaking, tracker-mounted PV modules are most vulnerable to hail in a horizontal position, as is the case at solar noon. Impact energies are greatest when hail strikes are perpendicular to the plane of the array. Moreover, the exposed impact area is largest when an array is stowed at zero degrees. Some trackers are also susceptible to wind-induced instability when the modules are flat.
If plant operators want to move an array through a horizontal position in advance of a storm, they must factor in the time required to safely complete this operation ahead of approaching winds. For best results, work with the tracker provider to reduce latency and improve stow-response speeds.
Exercising caution can ensure that the right steps are taken to mitigate hail risk while not increasing risk of wind-related damages. During severe convective storms, the direction of the wind is uncertain and may even be variable and swirling. Whether it is better to rotate into or out of the dominant wind direction will depend on hardware and storm-specific variables.
All else being equal, maximum hail mitigation is accomplished by rotating the array to the maximum tilt angle facing out of the wind, as this results in the smallest possible impact energies and exposed area. Unfortunately, facing out of the wind also results in a large sail effect, which may induce wind-stability issues.