Wind Actions on Flat-Roof-Mounted Photovoltaic Panels : a Comparison of Design Guidelines

Abstract: Wind actions on roof-mounted solar collectors dier from those on an open range-deployment, due to turbulence and wind stream deection induced by the underlying building. As of December 2010, no construction codes in eect describes the wind loads on such structures, with the exception of the preliminary standard NVN 7250 from the Netherlands, with the latest draft released in 2007. The roofs considered for this type of plants often consists of large-surface at-roofed industrial halls with tar-bitumen roof sheeting. Penetrating the roong for xtures of the solar collectors is often not allowable, so instead, wind-induced uplift forces are counter- acted by attaching ballast to the photovoltaic mounting systems. This practice causes the need for both safe and economical wind action guidelines, since uneconomical dimensioning causes the ballast quantities to exceed the residual load capacity of the roof, rendering the construction infeasible. A static model was developed for a typical photovoltaic mounting system. Results from three wind tunnel investigations on roof-mounted photovoltaics for dierent roof geometries were gathered, and the guidelines from the Dutch pre-standard NVN 7250 were adapted to the geometric boundary conditions of the respective studies. For all approaches to the wind loads, roofs are divided into dierent load areas, were modules situated in the dierent load areas are assigned dierent wind loading coecients. The static model was used to determine the ballast quantities needed for static equilibrium, using the results from the wind tunnel investigations, and the adaptions from the NVN 7250 respectively. The pre-standard was found to underestimate the ballast compared to one of the wind tunnel investigations, while signicantly overestimating the ballast compared to the two other results. Further, large dierences were found between the wind loading coecients from the wind tunnel investigations, derived for seemingly similar module layouts and mounting system geometries. This led to the conclusion that the aerodynamic properties of the mounting system itself plays a large role in determining the wind loads. The description of the mounting systems in the pre-standard, where the systems from the investigations all fall into the same category, seems insuciently ne-grained, since the wind actions in the end turned out both over- and underestimated. It is therefore not advisable to use the pre-standard NVN 7250 to dimension mouting systems and verifying static equilibrium.