The Evolution of Snow Management on Solar Arrays
In May of 2012, Alpine Snow Guards (Alpine) applied for, and was later awarded a patent for a snow retention device that attaches to solar panel frames. Over the past 9 years Alpine continues to innovate new forms of solar snow retention in order to satisfy the needs of our customers. Let us take a look at some of the ongoing complications, and how to resolve the problem of snow and ice sliding off rooftop mounted solar panels.
Consider that the most common roofing material that solar panels are installed on in “snow country” is asphalt or composition shingles.
(Asphalt shingle roof retaining snow)
Prior to the installation of a solar array, there was not a sliding snow problem to be solved. In fact, most of the inquiries we receive for solar snow retention come after the first winter; often following the first good snow fall. It’s usually after such an event occurs that the property owner realizes the danger of snow and ice siding off from the new array in one large mass.
(An unlucky home owner wishing he had a form of snow retention.)
The problem is obvious. The desire of the array owner is to generate as much energy from their array as possible. However, there is now a falling snow mass concern for the building owner that did not exist prior to the installation. Often the best placement of the arrays happens to be directly above points of egress, decks, hot tubs, driveways, sidewalks, etc., which makes the sliding snow off the roof even more dangerous.
Meeting the requirements:
Most new buildings constructed in the United States must meet local building codes. There are many codes; the one we are concerned with is the Ground Snow Load (GSL).
(Ground Snow Load map by geographic regions)
GSL is applied to roof structural calculations which is then reduced by a factor of 0.7 according to the American Society of Civil Engineers (ASCE) (7- 16, Chapter 7 “Snow Loads” and the roof pitch). To avoid confusion, these snow loads are estimated to withstand the weight of a 50-year storm.
Arriving at a roof design snow load value is not complicated. However, within the structural engineering community, there is a commonly accepted practice, that goes something like, “If it is desired to add a structural element to a building’s roof surface, the imposed load should not be more than an added 5% of the design load.”
Applying this 5% rule to the reality of adding solar panels to a roof surface creates a problem. For Example: GSL is 30 pounds per square foot (psf), the flat roof snow load is 70% of that, or 21psf. Therefore, the roof design snow load is 21 pounds per square foot. This is where it gets a little tricky. According to “Energy Sage”, the average solar array weighs 2-4 psf. In this example of a roof design snow load of 21 psf, by adding the low end of the estimated weight of a solar array (2 psf) the structure now has nearly 10% added to its roof design load. This is more than the engineering “Rule of Thumb”, that says no more than 5% added weight before structure comes into question.
So, you have to ask yourself, “How is it possible that engineers are allowing solar arrays to be installed on these roofs?”
Before we get there, it is important to stress that all of these calculations and building codes have built in safety factors. There should not be a case where the added weight of a solar array leaves the design unsafe. On the contrary, the engineering community is operating with industry accepted rules designed to facilitate safety. That being said, the answer to the question is the “slippery surface” load reduction. Certain roof surfaces that include metal, slate, glass, rubber, and plastic membranes with smooth surfaces, are considered slippery.
(Non-restricted solar panels shed snow)
This allows the engineer to reduce the roof snow load further based on roof pitch and other factors that vary (see ASCE 7-16) up to as much as 37%. Using the example above, this means by adding solar panels to the roof at 2-4 psf you actually reduced the loads that the roof was originally designed to support. In this example, 30 psf GSL x .7 = flat roof snow load of 21 psf. Taking the flat roof snow load * the slope factor of 0.68 (6/12 roof pitch) = 14.28psf sloped roof snow load for a slippery surface. By using the “Slippery Surface” factor the roof design load in this example (14.28 psf) combined with the highest array weight of 4 psf now brings the roof design load to 18.28 psf. Since the structure in this example was originally designed with a roof design snow load of 21 psf, this meets all of the design criteria; assuming no snow.
Note: This calculation was reached using the ASCE 7-16 guidelines. Drifting and other factors are not considered in this example.
What does all of this mean for the property owner?
It means that your solar array can be installed on your composition shingle roof without over loading the structure. However, it also means that the engineering of your array is taking into consideration that the snow and ice that would have accumulated on your composition shingle roof is now designed to evacuate the smooth surface quickly.
That’s correct! You, the building owner, upon agreeing to installing a solar array, have accepted that to be within standard engineering practice, the array will shed snow and ice.
This may be okay if there is nothing in the path of the avalanche. However, if you do not want your solar array to shed snow and ice all at once above areas of safety concern, you need a way to manage a slower release. To be perfectly clear, you cannot retain the snow and ice on the array indefinitely due to the structural issues as previously mentioned and a loss of power generation. From a functionality perspective, you would not want to restrict energy collection for a long period of time.
This is where Alpine SnowGuards comes in. Alpine has several solar snow management products that clamp to solar arrays that will help to slow the evacuation of snow and ice from a solar array. Alpine receives many calls where customers are looking for solutions to prevent all the snow and ice from quickly evacuating the panels. If you have followed the engineering and structural design practices above, you understand that this cannot be done without creating potential structural issues.
Alpine’s snow management products for solar arrays are designed to slow the evacuation of snow and ice from a solar array; they are not designed to prevent it. This is an important distinction between traditional roof-mounted snow guards and solar array-mounted snow guards.
Traditional roof-mounted snow guards come in two typical categories, pad-style systems and pipe-style systems. I have written about this in depth in;
To briefly summarize; pad-style guards are intended to add friction points to slow the release of snow. They are not intended to prevent the release of snow.
(ASG Fusion-Guard pad style system)
Pipe-style snow guards are thought of as a barricade, intended to prevent the release of snow. However, even pipe systems can still be overloaded as a snow mass reaches 2-3 times the height of the guards.
(PP145 two-pipe protecting a smoke stack and what roams below the eave)
To achieve the desired mitigation effect on solar arrays, Alpine designed a 2” tall rail-based snow retention systems for the leading edge of the arrays. This system will typically retain snowfall amounts of 2-3 times the 2” height (4”- 6”). Upslope from the array, we offer a 1” tall devices in the form of bars or pads. This height is governed by the need to prevent shading of the solar panel. Both are effective, but again, will only retain 2-3 times their height in snowfall.
(Solar Panel retaining +5″ of snow)
There may be times and conditions where snow accumulates to depths greater than 6” behind the Alpine Solar SnowMax and Solar SnowPad. If this happens, it is the responsibility of the building owner to clear the excess accumulation. This will ensure structural integrity of the solar array racking system and the building structure. This will also help to protect critical safety areas below the array and allow for more power generation.
As stated above, Alpine currently offers three Solar Snow management options; The SolarSnowMax-2” to be installed along the leading edge of the array.
The Solar SnowMax- 1” to be installed in subsequent tiers of panels to supplement the 2” edge mounted system.
The Solar SnowPad-3” used in subsequent tiers of panels to supplement the 2” bar.
(SSP-T3 Solar SnowPad)
Based on our experience over the past 9 years, and our customers feedback, what the industry really wants is the Solar SnowMax -2” along the leading edge and a supplemental Solar SnowPad in subsequent tiers.
(Solar panel with recommended solar retention)
There have been multiple requests for a longer Solar SnowPad. To accommodate this evolving demand, we are now introducing the “Solar SnowDog”. The SnowDog is 6” long (Double that of the Solar Snow Pad). This new innovation; the Solar SnowDog-6, combined with a SnowMax 2” bar (SSM-BAR 2”) along the leading edge of the array will standardize snow retention for solar arrays.
(The new Solar SnowDog)
Use of these devices will vary from customer to customer depending upon their experience, geographical region, and desired effect. In general, panels installed in portrait will use 2 pads per panel, while panels installed in landscape will use 3 pads per panel (both having the Solar SnowMax 2” at the leading edge). It is the decision of the installer, and the building owners desire as well as the structural limitations of the project.
This combination of systems will help to slow solar avalanches, but will still allow the arrays to clear. It will simplify snow management solutions on solar arrays and allow distributors to stock a standard solution.
Until next time,
Alpine Snow Guards