b'CLEAN ENERGY WIND DESIGN VS. RESISTANCE OF PHOTOVOLTAIC ROOFASSEMBLIES (PVRA) BY SUDHAKAR MOLLETI AND MAURICIO CHAVEZCLEAN ENERGY AND GREEN INITIATIVESC ommercial rooftops provide extensive areas repre- American Society of Civil Engineers (ASCE) 7-16 and National senting the desired platform to install PhotovoltaicBuilding Code of Canada (NBC) 2015. (PV) systems. Roof assemblies, with or without theNBCC introduced the wind loads on roof-mounted solar PVsysteminstalled,mustfulfillvariousstandardpanels in the User GuideNBC 2015 Structural Commentaries andfunctionalrequirements.Thecombinationofroofing(Users GuideNBC 2015:Part 4 ofDivision B). Sections 56 and assembly and PV system is termed as Photovoltaic Roofing57 in the guide provide the procedure to determine the net Assembly (PVRA).As PVRA become more common, it isdesign pressure on solar panels. This procedure is limited to essential for the roofing and solar industries to work togetherlow-slope roofs (up to seven degrees) but is applicable to all toensurethatPVRAintegrityismaintainedandthePVbuilding heights. performance is optimized. For simplification, the process involved in determining the Thewinddynamicsonalow-sloperoof-mountedarraynet design pressure can be categorized into five simple steps depend on the winds characteristics, the buildings geometry,as follows:the PV arrays aerodynamic properties and the array layout on the roof. Wind flowing over the PV array exerts pressure orStep 1: Determine the roof zonessuction on the surface of the array. This pressure transmits forces onto the clamping fasteners, the racking system, andIn this step, the size of the roof corner, edge and field zones eventually to the roof assembly. The PV system must haveare determined. Figure 1 (referenced as Figure I-8 in the guide) enough resistance to resist the expected wind loads. Not onlyprovides the zone dimensions correlated to the roofs mean that, but the roof assembly should also withstand the trans- height. This figure applies to solar installations on any height mitted load without compromising its structural, thermal andof the building. Those familiar with roof cladding wind design waterproofing functionalities.will notice significantly larger edge width dimensions, which Before a PV system is installed on a building, a local buildingincreasedfive-foldfrom0.4hforroofcladdingto2hfor code official must verify the installations wind integrity. He willthe roof that will be installed with solar panels (where h is a need two parameters to validate the wind performance: thebuilding height). design load and the resistance. The PV systems resistance needs to meet or exceed the design loads for potential installation onStep 2: Determine reference velocity pressure, q, and the factors, , that particular building to demonstrate code compliance. ThisThese parameters are obtained from the Static Procedure in article explores these two parameters to provide some directionNBC Article 4.1.7.3.And again, those familiar with Part 4 of on the practical design of PVRA. the NBCC will notice that these parameters are the same in the roof cladding design.NBCC wind design of roof-mounted solar panelsFor many years, there was little guidance on determining theReference velocity pressure - NBC table C-2 wind loads on rooftop PV systems. With the efforts of SEAOC Exposure factor (NBC clause 4.1.7.3.(5)(b)) and numerous wind tunnel studies conducted by reputed labs,Topographic factor (NBC sentence 4.1.7.4.(1)) the design methodology for determining wind loads on PV systems has been established, and this has become part of the Importance factor (normal, NBC table 4.1.7.3) 18lROOFINGBC l FALL 2020'