Determining wind loads on roof-mounted solar arrays to calculate ballast weight required to secure the system in place
Since 2009, interest in renewable energy in the form of solar photovoltaic (PV) arrays has grown rapidly, bolstered by government incentive programs and Ontario’s retirement of coal fired generation. The North American industry grew rapidly, and it was recognized early that the approval process required home-grown science behind the designs. hb Solar was one of the industry’s early players and continues to provide well-designed systems for solar installations across North America.
Unused rooftops of industrial and commercial buildings are major sources of real estate for solar PV arrays. However, they also present a challenge regarding the weight of the ballast needed to hold arrays in place during strong windstorms. There are several factors in determining the weight needed, including wind climates of different regions, building aerodynamics, and specifics of the racking system’s aerodynamics.
Since market economics didn’t justify site-specific wind tunnel testing for most installations, wind tunnel tests of generic configurations were needed to provide the design loading information.
As part of this project, RWDI’s team of experts:
- Constructed scale models of a generic building and many rows of PV modules
- Installed pressure taps at hundreds of locations to simultaneously measure peak fluctuating surface pressures
- Conducted well-proven wind tunnel testing to assist in determining the weight of the ballast needed to hold solar arrays steady during strong windstorms
- Organized vast amounts of wind tunnel data into coefficients to replace those provided for common building forms in building codes
- Produced tables detailing worst-case values to allow design engineers to work within a familiar wind load calculation framework with input specific to racking geometry and module position within the array
The resulting table of values enabled designers to use building code-like calculations to determine wind loads and incorporate common site-specific parameters, such as wind speed, exposure of the site created by surrounding terrain and structures, and building importance. Tables providing guidance for variations such as parapet height, tilt angle, row spacing, and array skew to the building edges, were also made available.