SOLAR VS HAIL: PIVOTING AWAY FROM DANGER
Trackers and Stow Importance
Introducing Trackers
Single-axis tracker systems enable solar projects to optimize energy output by orientating the PV module perpendicular to the sun’s zenith angle, moving east to west throughout the day.
The PV modules are attached to a mounting frame that locks into the tracker system and usually operates across sections of a row, or the entire row via the torque tube. The range of motion facilitated by the tracker, commonly actuated by a slew-drive motor, provides additional benefits in protecting the PV module from a number of perils.
Some examples can be seen below:
Wind
The tracker rotates the PV module to its wind stow position, either at the maximum angle where it is locked in place, or to a horizontal, dependent on design.
Flood
The tracker rotates the PV module to a horizontal stow position, to prevent the PV module edges from becoming submerged.
Snow
The tracker rotates the PV module to its wind stow position, either at the maximum angle where it is locked in place, or to a horizontal, dependent on design.
When responding to a hail threat, the optimal decision is to rotate the PV modules to the nearest maximum tilt angle as quickly as possible, which presents a smaller cross-section to falling hail. By avoiding the flat (0-degree) position, which would take hailstones as direct perpendicular hits, the high-tilt stow causes most hail to glance off the glass at an angle, dramatically reducing impact, kinetic energy, and damage potential. Field and lab data have confirmed that such “defensive stow” can significantly limit broken glass and cell microcracking during hailstorms, and so is one of the most effective protective measures.
Breakage Probability Tests
Nextracker, in collaboration with RETC, has provided data, which estimate PV module breakage rates (tested to failure) from the kinetic energy on impact of different hailstone diameters and strike inclinations, indicative of tracker stow angles. Two PV module types were considered, which were 2.0mm (front and back glass) bifacial PV modules with heat-strengthened glass and 3.2mm monofacial PV modules with fully-tempered glass. To complete extensive modeling, over 150 PV module models were considered from several manufacturers, and a module breakage, or resiliency, curve was produced for the two main PV module types.
Key assumptions:
- A median curve was used for predictions, aggregating data taken from hail strikes to different PV module models of the same type*. An individual PV module model under a given manufacturer, may under or over perform compared to the curve, which would produce a different heat map given every PV module has its own unique characteristics. As the curve represents an impact towards the center of the PV module, this can also make a difference as different PV module models have different vulnerabilities at different points. For example, some PV modules are more vulnerable near to their corners by the frame.
- Hailstones considered were near perfectively spherical freezer ice balls produced under laboratory conditions, with a density of 910 kg/m3. In reality, during a hailstorm, hailstones would have differing densities due to trapped air. Differences like this in how they form naturally could affect their mechanical integrity and also weight, thus the kinetic energy on impact. Additionally, hailstones are very often irregular in shape, with points and bulbs, not properly representative of a sphere.
- Hailstone terminal velocity used the methodology determined by Juergen Grieser and Mark Hillin[1] and a constant drag coefficient was applied, which in real conditions is not a linear relationship and also varies with different hailstone shape and size.
Firstly, assuming the hailstones fall exactly vertically (no wind), the breakage rate heat maps for the two PV module types demonstrate substantial reduction in breakage rate for a 75-degree stow, especially where very large hailstones of three inches or greater. It is evidenced that the PV module breakage probability is lessened for the 3.2mm front glass monofacial type, for example demonstrating higher survivability, with 21% breakage probability for a 3.5 inch hail strike at 60-degree stow, when the bifacial module in the same scenario shows a near to 100% breakage probability.
*Logistic regression was used to take the discrete data from each PV module type and produce a continuous variable function.
Heat maps for wind-blown hail, assuming a 20mph (8.94m/s) wind directed at the front-glass, show an increase in probability of breakage for all tracker stow angles and hailstone sizes. Overall, this is a more accurate reflection of the conditions in the field.
