Solar Photovoltaic Hardening for Resilience – Winter Weather

Modules are tested for how much load they can withstand and are typically certified according to the test standard IEC 61215. Load ratings are typically given in Pascals (Pa). A typical module rating will be 2400 Pa. 

For locations at risk of heavy snow accumulation, select modules certified to withstand at least 5000 Pa. Refer to module installation manuals to find static (snow) test load ratings for the specific mounting configuration to be used. Adding mounting points can also increase the load rating. Smaller modules can typically withstand higher loads. 

Because PV modules are normally installed in a tilted position, snow will slide down the panels and accumulate unevenly at the bottom edge of the panel at the frame (see Figures 4 and 5 under the section, "Framed Versus Frameless"), resulting in greater stress on the lower portion of the module. IEC standard 62938 for framed modules provides a method to determine the load-bearing capability of modules subjected to non-uniform snow loads.

Snow loads for a specific site are typically calculated using ASCE 7 that result in a ground snow load in pounds per square foot (psi). These ground snow loads are used in formulas which factor in site specifics to yield a roof snow load for rooftop systems. 

When calculating loads, do not assume a uniform loading across a PV system; factor in that the lower side of the modules typically gets loaded more than the higher side, causing increased stresses on the module. 

Modules typically have metal frames around the perimeter of the PV module, though some modules are frameless. 

Frameless modules can shed accumulated snow more quickly since the frame can prevent snow at the lower tip of the panel from sliding off after a heavy snowfall. Frameless modules, however, may have lower snow and wind load ratings than framed modules. Load rating should be the priority consideration over the ability to shed snow.

Figure 4 shows the success of the PV systems without the frame after a snowfall along with framed PV systems that have retained more snow.

Under heavy snow weight and ice buildup, frames have come detached from modules as well (Figure 5).

Under heavy loads, invisible cracks can form in solar cells. Modules with multi-busbar solar cells have a lower risk of propagating these cracks. The solar cell width between the busbars on multi-busbar PV panels has a smaller surface area than traditional busbar technology and so cracks have limited room to grow. If cell cracks grow, they can lead to decreased production and possibly fire safety risks.

Coatings and treatments for PV systems that would facilitate the automatic removal of snow and ice from the surface of the system are currently under development and are not mature at the time of writing. 

Higher tilt angles allow snow to shed from systems more quickly and reduce the risk of damage. The steeper the angle, the better the snow shed (up to 60 degrees). 

Significant gains are seen around 30–35-degree tilt angles, which is significantly steeper than a typical roof. Furthermore, the steeper the module tilt angle, the less snow weight is transferred to the module. 

Selecting a higher tilt angle, however, will result in higher wind loads on modules, and thus higher costs, so the system designer should find the ideal compromise between these drivers as well as solar production. 

PV tracker systems can adjust their angle with respect to the ground. If installing a tracker system in a heavy snow region, select one with the capability of a "snow stow" mode that will tilt the panels to a steep angle in a heavy snow event, thus aiding shedding of snow off the modules. This can be engaged manually or automatically. 

This functionality may not be available in heavy snow events, especially if communication with the system is lost. Ensure the system designer designs for the worst-case scenario where the tracker is not able to stow or shed any snow.

Overall, tracker systems typically mount modules closer to the center of the module, leaving the ends less supported. If installing a tracker system, ensure that module load calculations are performed using the actual attachment points on the module, rather than simply using load ratings given on a specifications sheet.

Snow that has fallen or been shed from a PV system may accumulate on the ground. In areas prone to heavy snowfall, elevate the bottom edge of systems higher above ground to accommodate, at least two feet above maximum normal snow depth. This will have cost implications and increase the wind load, so it is important to weigh these factors and risks. 

A greater number of attachment points on a module could increase its ability to withstand heavy snow loads. Modules typically use four attachment points, though more are possible. 

A National Park Service site in Mount Rainier National Park had a harsh winter that left modules crushed. They replaced the damaged modules (that were attached at four points) and racking with new modules that attached to the racking at eight attachment points. They also added extra module supports underneath the modules. These retrofits survived the following winter without damage. 

Read more about the successful deployment of a solar power system at Mount Rainier National Park.

As ground in cold climates freezes and thaws, foundations on structures can work their way up out of the ground. This can cause damage to or even toppling of a ground-mounted PV system. 

Consideration of frost heave is not always required on projects and is not mandated in ASCE 7 building codes that are typically used, so require design engineers to account for frost. 

Ensure frost depth is considered in the design, type, and depth of foundations; the pile depth should extend significantly below the frost depth.

Use a design engineering firm with experience and success installing PV in cold weather climates and require that they perform frost heave calculations in the system design. Soils vary from site to site and can vary across a site so design foundations for the specific conditions present. This may require a pile pull test. 

Orienting PV modules in landscape format can help accelerate shedding of snow or ice that is covering a PV panel. This orientation will also increase production as snow typically melts and first exposes the tops of the modules. The PV module produces power from three discrete sections: when viewed in landscape this would be the top third, middle third, and lower third. When each of these thirds of the module are clear of snow, they can produce power.

In areas where the wind generally blows from a single direction, PV system operators can take advantage of the venturi effect, where a current of fast-moving wind crests over the top of a sloping obstacle and then speeds away from the object. For PV systems, installing a curved "venturi" deflector at and pointing the top of the PV panel against the direction of the wind can help ensure that snowdrifts or water-bearing winds do not make contact with the surface of the panels, reducing the risk of snow or ice accumulation.

Vertical PV installations are fixed at a 90-degree angle relative to the horizon. Not only are they less likely to accumulate snow or ice compared to horizontally tilted systems but any snow or ice that ultimately adheres is more likely to shed off the panel. 

At high latitudes, their extreme orientation also means that they can capture the low angle of the winter sun more effectively. 

Ground-mounted vertical PV can be placed in areas that would otherwise be infeasible for ground-mounted horizontal PV since the vertical PV uses space more efficiently and may be placed among existing land uses, such as parking or farmland. 

Wind loading, though, will be higher on vertically mounted PV systems, which needs to be weighed against the benefits of a vertical PV system.

• Avoid systems with substantial cantilevers.
   
• Thicker and deeper frames around modules could increase the load capacity of the module and help prevent glass breakage.
   
• Thicker gauge racking will give a more robust structure more likely to withstand heavy snow and ice loads.
   
• More spacing between modules or between "tables" of modules can possibly give snow more space to shed, aid melting, and break up large masses of snow accumulation. 
   
• Consider combined snow and wind loads in the design as these hazards often occur together.

While always important to keep water out of electrical components, water ingress can pose additional risk in cold weather climates. Water that enters an electrical enclosure can freeze and expand, damaging system components (Figure 6). Enclosures should be NEMA 4 or higher rated.

As snow melts and freezes, it can form icicles on a PV system. Icicles can add weight and pull on light system components such as wiring, so use wire management solutions that will hold under added weight. Do not use plastic wire ties and ensure wire connections are under modules or otherwise protected from where icicles will form.

In situations where many feet of snow bury a PV system, avoid the potential for accidental damage (e.g., stepping on the glass surface of the PV panel or damaging it with snow removal equipment) by installing poles several feet in height along the corners and perimeter of the system and signage to alert crews of the presence of a PV system.

These poles can help snow removal workers identify the system's location and avoid damaging the system. This is especially important in systems adjacent to parking lots or roads where snowplows may inadvertently damage a PV system (Figure 7). 

Removal of accumulated snow or ice is not recommended. If attempted, it should be done with extreme caution and care.

Shovels can cause damage and attempting to remove snow off panels can do more harm than good. Stepping or standing on panels will cause damage, and crews are often not aware of these risks or may unintentionally put weight on panels.

If snow accumulation is excessive and a site chooses to remove snow, use softer tools, such as brooms and brushes to remove snow above the panels.