Severe Weather Resilience in Solar Photovoltaic System Design

The estimated initial costs of 13 storm hardening measures are listed in Table 1.

Table 1. Storm Hardening Measures for PV Systems and Their Added Cost

¹ If 2% of the fasteners in a system are torque checked.

² If 100% of the fasteners in a system are torque checked.

When exposed to wind, all objects vibrate, and depending on several characteristics of the array structures, arrays may experience violent resonance or severe frame member deflection, which could lead to catastrophic losses. Ensure that the racking design is engineered to withstand highly turbulent wind forces.

Due to the turbulence generated by wind flowing over parapets and around roof penthouses, solar PV roof systems should not be fully ballasted. Use mechanical attachments at strategic locations to prevent catastrophic loss.

Racking designs with vertical frame members need bracing to prevent lateral movement.

Recommended Actions for Lateral Bracing

Require that racking designs anticipate lateral movement and are properly braced. One simple indicator of a racking system's ability to resist lateral movement is the presence of cross-bracing in design drawings.

Rack frame elements comprised of light gauge structural channels have been shown to lack torsional strength and tend to flex in wind events. Torsional movement and significant flexing create a load path to the mounted modules and fasteners. This can damage the fasteners holding the modules in place or even fracture the modules in extreme cases.

Fastened joints are bolts, clips, and brackets designed to hold two or more parts together. Fastened joints are found throughout a solar PV system to mount solar modules to racking systems, hold racking frame elements together, and provide a means of mechanical attachment to foundations (for ground and carport arrays) or to a building structure (for roof arrays). Critical fastened joints are defined as the assembly of fastened components whose failure will result in a catastrophic outcome. For example, if fastened joints used to mount the module to the racking framework fail, modules could become dangerous airborne debris. Module mounting and racking assemblies typically have many critical fastened joints.

Many failures observed in the field have been the result of issues with fastened joints and components. There are broad common categories of failures seen with fastened joints: 

• Procurement of sub-standard components
• Poor installation practices; not using torque wrenches and thread lubricant
• Inadequate strength for the actual loading
• Vibration-induced loosening of threaded fasteners
• Joint relaxation – loosening of fasteners after initial installation
• Module mid-clamps fail, releasing an entire row of modules causing cascading failures.

Recommended Actions for Critical Fastened Joints

To reduce likelihood of fastened joint failures, use the following design guidelines:

• Through-bolt modules to the racking system to avoid use of top-down clamping systems when possible. Add hardware to compensate for short fastener lengths. 
   
• Never assemble critical structural fastened joints with self-tapping sheet metal screws or clamps.
   
• Threaded fasteners should include locking mechanisms rated to DIN 25201 part B standards (DIN 25201-4) to address vibrational loosening and resulting joint slip. Avoid using unrated hardware such as split washers, nylon insert nuts, double-nutting, star washers, and/or serrated flange nuts. Thread locking compounds, while effective, have unknown longevity when exposed to exterior elements and should be avoided as a sole locking means of a fastened joint. Movement or slip in a joint is the main force that induces loosening of fastened joints. Many fastened joints in a solar PV system are subjected to transverse slip, so it is recommended that vibration resistant fasteners be specified and installed on all critical fastened joints in a solar array.

Tracker manufacturers have developed controls strategies that drive rows of modules into a stow position if high wind is indicated. This stow position is typically near horizontal but with a slightly negative angle of attack with respect to the wind. This feature could be specified for tracking systems in areas with high wind risk.

Wind pressure can put substantial loads on the front and back of modules and lead to micro-cracking of the solar cells and even fracture of the module glass.

Recommended Actions for Front and Back Pressure Ratings

For modules placed in service at a site where the FEMA NRI tool shows relatively high risk of a strong wind event, specify modules with front and back pressure ratings. PV modules should be tested per ASTM E1830-15 prescribed test parameters for loading (snow and wind) of solar modules (front and back). This test also covers several other stress factors relevant to high winds such as the "twist test." 

Minimum Front and Back Pressure Ratings for Loading Strength of Modules

Poorly secured or routed wire can be damaged from weather events and cause electrical faults. Plastic wire ties (regardless of rating or material composition) can fail from exposure to heat, ultraviolet light, and moisture.  There is general concern that even exterior-grade wire ties rated to be ultraviolet-resistant are inadequate for longevity in field use. If plastic ties fail, wind can cause the loose wire to rub against sharp edges or abrasive objects, resulting in electrical faults.

Most electrical switchgear is shipped with default cabinet options which are not suitable for areas with marine misting, high winds, or wind-driven rains. The default cabinets can collapse when exposed to heavy wind, driven rain can bypass door gasketing, and a standard door latch can blow open in high wind.

Consider including one of these additional, extensive hail test standards to your solicitation and contract documents:

Tracker manufacturers have developed controls strategies that drive rows of modules into a stow position if a hailstorm is predicted. This stow position is typically the maximum vertical tilt the tracker system allows, which should decrease the angle of impact of most hail balls. Some manufacturers have lab-tested these stow strategies using hail cannons and have demonstrated good results. Consider requiring the use of tracker technologies if the FEMA NRI resource indicates that hail is rated as a "relatively high" risk or greater for your site even though this may add cost.

Weather events that produce standing water and/or wind can flood underground pull boxes and conduit. Water can then flow into electrical cabinets that house inverters, switchgear, controls, meters, and transformers. Array fields that are either uphill from or at about the same elevation as supporting electrical equipment are at risk of unintentional flooding through pull boxes and conduit.