Potential Environmental Risks of Solar Project
When several rows of solar arrays are installed on a flat roof, snow may accumulate in a saw-tooth pattern as the spaces between the solar arrays can be filled with snow drifts as a result of obstructions. This pattern of snow accumulation on PV systems has been witnessed and reported in commercial roof damage subsequent to a heavy snowfall in certain geographical regions of North America. This pattern alters the load distribution and can increase the amount of snow load up to 25% in comparison to roofs without solar arrays. Moreover, non-uniform loading intensifies the internal stresses of certain roof elements which were not anticipated in the original design and is potentially enough to even cause damage to a well-designed roof absent of added solar panels.
GraceSolar - GS-smart Ground Mounting System in Hokaido,Japan
For residential sloped roofs where PV panels are flush-mounted, the alteration in the original amount of snow load is not typically critical since the roof geometry remains relatively unchanged. However, the panels convert the distributed snow load to several concentrated loads at their anchor points. Since the panels are typically installed on only one or two roof facets, the structural design of the roof framing needs to be rechecked for such load combinations. Some studies have shown that solar panels on sloped roofs may reduce the amount of snow load by 5% as result of their heating and thermal absorption characteristics. However, this may lead to a higher likelihood of formation and retention of ice on the roof, which, in turn, may cause other problems.
Snow accumulation and frost formation on ground-mounted solar panels can reduce the performance of the PV panels. Spaces between each row, and the minimum distance from the bottom of panels to the ground, can reduce the risk of damages to the PV system due to the accumulation of snow. In general, adverse risk of snow on ground mounted PVs is less than that on roof-mounted systems.
Wind loads on ground-mounted PV arrays are usually critical, particularly if the patterns of wind flow around the arrays (vortex shedding) create a matching vibration with the natural frequency of the structure.
For large and high-profile projects, wind tunnel testing is the only reliable method to determine the wind load on roof-mounted solar panels as other approaches may lead to over-designed or under-designed results.
For mid-size projects, Computational Fluid Dynamics (CFD) analysis is an appropriate approach wherein results can be compared with the available recommended values of engineering guidelines and deployed for the design.
GS-Light Intelligent Tracking System
For smaller projects, though, using simplified approaches could be an option as long as an experienced consultant evaluates the structural performance of the existing building and considers all loading scenarios which are likely to occur. Such approaches, although common, sometimes result in either oversimplification or conservatism in structural design which can be risky or costly.
Similar to snow load, wind load is typically not critical for flush-mounted PV systems on sloped residential roofs if the system’s total height is limited to a few inches. In this case, the air plenum underneath the panels tracks the pressure above the panel and does not create considerable wind uplift on the roof depending on the size and spacing of the panels.
There is a common misconception that panels situated at angles of 10-15 degrees cannot significantly change the wind load applied on a roof. Wind engineering and forensic studies have shown the opposite—roof collapses after the addition of solar farms are not uncommon.
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.
Resource：Solar Builder by Dr.Ben Daee