Delivering design guidance for the safe installation of solar arrays
Demand for renewable energy from photovoltaic (PV) solar collectors has grown rapidly in North America over the past decade. hb Solar is a significant player in the North American market, providing solar arrays for a wide range of industrial, agricultural and commercial operations.
Unused rooftops of industrial and commercial buildings represent a major source of real estate for the application of PV solar. But solar arrays that are improperly installed or have insufficient ballast to hold them in place can risk being lifted and dragged by the wind, damaging valuable equipment and causing safety risks. Guidelines for safe installation of large solar arrays can vary according to local wind climate, the aerodynamics of the buildings on which the arrays are being installed, and the composition of the arrays themselves and their racking systems. Building codes tend to offer insufficient guidance on this relatively new design issue, and conducting wind-tunnel testing of each individual installation scenario would be impractical and prohibitively expensive. hb Solar turned to us for help establishing sound, evidence-based parameters for solar array installation that could be effectively adapted to a range of local wind climates and building scenarios.
Our team has conducted thousands of wind tunnel tests on a diverse range of structures, from skyscrapers and long-span bridges to stadia and air traffic control towers. We were able to draw on this rich experience as we set out to establish generic testing conditions for the solar arrays. Our goal was to conduct the wind tunnel tests in such a way that the insights we gathered could offer useful guidance in a wide range of installation scenarios.
We began by developing a scale model of a generic flat-roof building, typical of many big-box, commercial and industrial buildings. We then installed numerous scale models of hb Solar’s PV solar arrays on the model’s roof, adding pressure taps on both the upper and lower surfaces of the models so we could measure wind pressure at hundreds of points across the array simultaneously.
A turntable inside the wind tunnel lets us test a model’s performance in turbulent winds from any direction of approach. Typically, we rotate the model in ten-degree increments, testing the structure in winds coming from a total of 36 directions. In the case of the solar arrays, our wind tunnel tests provided detailed information on the wind loads required to lift and drag the solar arrays from the roof – and thus gave us insight into the ballast weights necessary to hold the arrays in place under extreme wind conditions.
One key insight from the testing process was that for flat-roof-mounted solar arrays, the turbulent wind flow structures created by the building itself were the most important factor in determining the peak loading conditions. In particular, vortices (or spinning flow) – generated when wind strikes the building at an angle and flows over a corner – created the largest peak loads and therefore the greatest threat to the arrays. We found these vortices interacted such that the wind forces on different solar modules varied depending on each module’s position within the array.
Building codes offer guidance on wind loads by providing tables of values that let designers and builders calculate necessary design measures in particular contexts.
To produce a roughly similar type of guidance for hb Solar, we synthesized the vast amounts of data we compiled through our wind-tunnel testing and generated simple-to-apply, code-like parameters for solar array installation.
The tool we produced included common site-specific input parameters such as:
- 50-year design wind speed;
- exposure of the site depending on surrounding terrain and structures; and
- properties of the building itself such as size, shape and orientation.
Our tool also accounted for variables specific to the solar arrays themselves, including:
- the specific racking system and its layout;
- parapet height;
- solar panel tilt angle;
- row spacing; and
- the skew of the arrays relative to building edges.
By synthesizing and repackaging our wind tunnel testing data this way, we enabled the design engineers to work within a familiar wind load calculation framework but with guidance that related specifically to the properties of the solar arrays, including the geometry of the racking system and the configuration of individual solar modules within the arrays.