Tax incentives, profit of power buyback programs, and ever-rising electrical bills help justify the cost of solar panel installations for home and business owners. Cost-benefit analysis and the return on “solar investment” look attractive on paper over a 20-year term; however, the underlying risks of roof-mounted solar panels are typically not well known to owners and insurance companies.
Some of the risks involved with solar installations can be especially severe in areas of North America that experience very cold temperatures and cold and ice in the winter. Solar arrays placed on roofs that are not structurally sound may cause roof collapse or water intrusion. Solar panels are subject to great fluctuations in temperature, moisture exposure, and freeze-thaw cycles. Direct exposure to the outdoor environment accelerates wear and tear and increases the likeliness of component failure. Because there is currently no uniform standard in North America to provide guidance for the structural design, installation, and maintenance of solar panels, the risks vary case by case depending on the project characteristics including size, location, and installation approach.
For residential properties certain risks arising from rooftop solar panels such as damage caused by peril (e.g., fire, windstorm, and hail) may be covered under a standard home policy since they are typically considered as part of the property. However, for commercial buildings the business interruption loss resulting from solar panel failure may not be covered under a standard policy. Hence, commercial buildings are typically at a potentially greater risk of damage as a result of roof-mounted solar panels than residential buildings, although statistical studies in this area are limited.
Solar farms share similar environmental risks with roof-mounted solar panels, e.g., hail, freeze-thaw, and wind damage. However, they are exposed to additional losses such as frost heave, foundation failures, and significant movements due to variation in soil moisture or flooding. Solar farm development can pose a risk of wildfire to an extended area although there are certain ways to mitigate such risk. Given the amount of required land for a solar farm, aesthetics, proximity to residential areas, and the potential impact on wildlife risk can also be concerns during solar farm development.
Many engineering standards prescribe consideration of ice load for ice-sensitive structures. ASCE 7 defines ice-sensitive structures as “structures for which the effect of an atmospheric icing load (i.e., freezing rain) governs the design of a portion or the entire structure.” For structures such as steel power towers, guyed masts, ski lifts, etc., radial layers of ice can form around the structural elements which elevates their weight, internal stresses, and wind-exposed faces. As previously discussed, there is no generally acceptable structural standard for the design of solar panels. However, forensic experience and site inspections conducted after ice storms showed that solar racks can be categorized as ice-sensitive structures. This not only affects the design of solar racks (supporting frames) but can change the ultimate load applied on the supporting structures/ foundation system.
PV systems are typically designed for a lifespan of 20-25 years; however, in cold regions the effective life expectancy of ground-mounted systems may be shorter due to some aggressive environmental conditions. Frost heave may affect the power generation and even stability of solar racks. In sub-zero temperatures, water in the soil freezes, and the volume of the soil around the footings, e.g., micro piles, increases. This results in upward movement of the solar racks. Depending on the type of the soil, this cyclic frost-heave movement can occur at a rate ranging from 1/64” to 3/4” of an inch per day for an embedded foundation in certain weather conditions. The risk of frost-heave is higher in clay and silt and lower in sand and gravel, according to the US Army Corps of Engineers.
Frost heave result in structural deflection and changes in the angle of solar panels. Non-uniform deflections of the footing system may cause failure or deformation of connections, racking systems, disconnection of conductors, and/or grounding in the frozen soil. Proper design of PV systems for possible impact of frost heave can reduce the risk of damages. For instance, the footings or micro piles can be designed under the frost line per the building codes recommendation; however, determining the uplift force by frost-heave on piles is challenging although 15 psi is a typically recommended value. Nonetheless the piles can be pushed out of the ground even if driven under the frost line.
Mechanically attached PV systems on sloped residential roofs typically weigh 2-4 pounds per square foot. This is not a considerable amount of weight as an intact well-designed roof is expected to withstand this additional load with limited or no modification. However, alteration in structural systems or loads of an existing building may require code upgrades that may affect the original design of the building due to more stringent requirements. Gravity loads of ballasted PV systems on flat roofs are considerable and may impose an additional 5-30 pounds per square foot on the roof framing for which a detailed structural analysis is required.
The main challenge for both methods of installation is that the weight of solar panels is not uniformly distributed on the roof and may have critical local effects on certain structural elements. If detailed drawings for the location of arrays are not produced, the actual loads on the roof will differ from the design loads as repositioning of solar arrays on the roof in the construction stage is likely.
Water intrusion and ponding
Solar panels on a sloped roof are attached to the roof framing using mechanical fasteners. For existing buildings, this means that numerous holes must be drilled into the roofing system. The anchor holes are typically sealed to prevent water leakage into the building. However, the performance of the seals is a factor of installation accuracy and long-term behavior of the sealant material’s exposure to environmental elements. Vibration of the panels due to wind forces, as well as thermal expansion/contraction cycles, may result in seal breaks and water intrusion.
On flat roofs, solar racks may disturb the drainage path of water to the roof drains depending on their supporting configuration. Ballasted solar racks are equally spaced in parallel rows on the roof and may result in rainwater or snow melt accumulation between the modules. Water ponding can occur in these strip regions which may lead to accelerated delamination, cracking, and sagging of the roof and, ultimately, water intrusion to the building.
Assessing risks in advance
Installation of solar panels on flat or sloped roofs may alter the roof geometry and its capacity, especially when factoring in exposure to environmental loads. Snow load, ice load, wind load (plus wind on ice), additional dead load, water ponding, drainage obstruction, and water intrusion not only influence the structural design of buildings but may also affect their long-term functionality.
The risk of damage to buildings with roof-mounted solar panels is simply higher due to the presence of the panels. Insurers may unknowingly bear a considerable portion of this risk, and the owners may not be aware of the risk exposure. Lack of a uniform engineering standard, which includes all aspects of the design and installation—particularly for flat roofs—adds more complexity to the liability arising from the solar panels.
Movement of footing as a result of frost-heave may lead to permanent damage to the solar rack and power generation. Wind damage to solar farms is likely resulting from the complexity of the wind design and the effect of vortex shedding that may impose an excessive uplift load on the panels.
Simply put, the design of solar farms or roof-mounted solar panels is a multi-faceted problem that should be assessed by qualified engineers and insurers should know if the property includes roof-mounted solar panels to consider the potential risks.
Ben Daee and Sadegh Khosravi are engineers at J.S. Held, a global consulting firm providing specialized technical, scientific, financial, and advisory services.
The views and opinions expressed in this article are the author’s own, and do not necessarily reflect those held by pv magazine.
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Excellent article, some of these factors are not taken into consideration when installing solar panels, especially on roofs.
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