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Solar modules on flat roofs – why worry about wind?
A “low-rise building” refers to a structure of modest height and substantial girth: consumer retail stores, wholesale clubs, warehouses, and distribution centers are common examples. Low-rise buildings are characterized by large flat (or nearly flat) roofs. When a strong wind approaches a side of such a building, the structure forces the wind to flow up and over the top, as illustrated in Figure 1. The air does not, however, flow smoothly over the roof. Instead, it breaks away at the leading edge of the roof, leaving a zone of swirling air beneath it.
Figure 1: Flow separation and reattachment
This zone of swirling air is called a flow separation. If the building is large enough (and most low-rise buildings are), the wind above the building eventually comes back down and meets the roof. The point at which this happens is called the reattachment point, and although this point shifts around during a high wind event, on average it is located about twice the building height from the roof edge. Beyond the reattachment point, the wind once again flows parallel to the roof.
Figure 2 shows a wind zone map for the roof of a typical low-rise building. The light gray areas are dominated by flow separations: the wind can come from any direction, so there are four such zones. If we are concerned about loads on the roof itself, each of these zones is about half as wide as the building is high (a » 0.5h).
Links
Wind & Roof-Mounted Panels: A Common Scenario (Less Technical)
How to Calculate Wind Loads on Roof Mounted Solar Panels in the US
Wind Effects on Solar Panels Brochure
Renewable Energy Technical Papers
