With team members from all over the country, and even the world, SEIPS can say that we’ve worked on PV systems in many types of weather and climate. Now that we are deep into wintertime here in the northern hemisphere, appropriately we thought we’d share some thoughts on the various impacts that snow has on our PV system design decisions. We will start by looking at ground-mount systems and move on to rooftops, which can be a little more complicated, in a future article.
Snow Load Data
Snow loads can vary wildly across even short geographical distances, so being as accurate as possible up-front is important. First and foremost, we start with finding the local requirements for snow loading from whom ever has jurisdiction in the project area. Sometimes this is provided in the building code itself and other times it is provided by a state-wide organization like the Structural Engineering Association of Colorado, or it is provided by the local building department. It is important to determine who really does have jurisdiction. Asking the local building department is typically the fastest way of finding an answer and many times it is on their website.
It is also important to have some familiarity with snow load magnitudes and what they mean. For example, a snow load of 60 pounds per square foot (psf) may seem like a lot to some people, but not to others, depending on where you live. But what is more important is knowing some basic thresholds for PV systems so you know when you need to start paying attention to addition variables during the design process. For example, the PV module selected may be rated by the manufacturer for a maximum snow load of 60 psf. This means that the module and its frame can withstand 60 psf of snow if it is mounted to the racking system properly. Other modules could be rated upwards of 120 psf.
The above example also applies to racking systems. Some manufacturers only provide engineering up to a maximum snow load and then they stop providing support during the design process. You may need to hire an engineer (like SEIPS) to add some structural engineering assistance in high snow load areas.
In addition to the actual load numbers, knowing the expected snow depth is also important to determine array heights and for keeping equipment safe.
In the following section we list some of the more important considerations when dealing with snow.
General Design and Installation Considerations
- Leading Edge Height (Ground to Module)
- Space between the modules and ground is needed to allow for snow to shed and accumulate.
- Additional clearance for snow accumulation between rows may also be needed.
- Refer to manufacturer specific design parameters regarding ground to module height.
- Snow Load.
- Verify modules are rated for the snow load.
- Make necessary adjustments in racking and clearances for high snow loads.
- Icicles can form from snow melt and can drip through racking gaps onto exposed wiring, therefore connectors should not be placed where ice can form.
- Ensure clearances and coverage structures are designed to prevent significant water intrusion.
- Design coverages structures to carry the expected snow loads.
- Consider box locations from edges and protect them accordingly.
- Example tracker manufacturer method for reducing the impact from snow:
- The manufacturer can program the tracker to “park” (the position of tracker at night) at the maximum tilt angle of 52º, rather than flat, to reduce snow accumulation in the evenings and night.
- The tracker moves at eight-minute intervals, therefore it will only be flat during the mid-day.
- An optional snow sensor can detect snowfall. If snowfall occurs a signal is sent to the tracker to move to the maximum tilt (52º) to shed the snow.
- Ground to module height design recommendations. For sites with >20psf of snow, the manufacturer recommends 18″ + 6″ per each additional 10psf over 20psf.
- The quantity and loading of the foundations and torque tubes are designed on a site by site basis for snow loading (see structural considerations below).
- Snow is accounted for in the monthly “soiling” percentages of our PVSyst Production model. We use the snow loss model originated by Bill Marion and developed by NREL as a computer model to generate estimates for monthly snow losses.
- In locations with very high snow loads, the array may only be able to be designed vertically, which impacts performance, but snow will not accumulate on the array.
- Consider bi-facial modules. If the front of an array is covered with snow, there may still be some production from the reflection of sunlight onto the back of a bifacial module.
Structural and Civil Considerations
- The basic criteria for structural engineering of PV arrays are based on applicable codes. Check with the local building department for the adopted code version.
- Consider drifting snow and unbalanced loading if the array is located adjacent to tree-lines or structures.
- The following snow loading criteria is used with adjustments based on the final design:
- Risk Category
- Frost depth
- Ground snow load, Pg
- Exposure Factor, Ce
- Thermal Factor, Ct
- Importance Factor, based on Risk Category
- Slope Factor, Cs
- Design Snow Load, Ps = Cs*0.7*Ce*Ct*I*Pg
- Civil design considerations include the following:
- Access roads to critical equipment must be designed to allow access in any condition. Re-graveling certain roads may be necessary at the end of construction to reduce mud and soft road conditions to ensure ease of access under any climatic condition.
- Design of foundations should be to a minimum frost depth to avoid soil heaving. Check with the building department for frost depth requirements.
- Grade should be sloped away from all foundations to rapidly remove rainfall from soaking into the surrounding soils.
Keep an eye open for a following SEIPS Tech Tip where we will cover additional snow impact considerations for rooftop PV systems!