Designing for Wind: What Current Codes Don’t Tell Us, But Should

Rapid growth in the solar industry has triggered a pressing need for consistent solar engineering and construction standards, particularly in connection with the structural design of rooftop PV systems. Over the last decade, we’ve seen an incredible variation in requirements and requests from project engineers and permitting officials across North America for more than 1,000 projects.

In some cases, the requirements have been surprisingly sparse, raising the concern that dangerously poor designs might find their way onto rooftops.  Conversely, we’ve also seen exceedingly onerous requirements, begging the question as how anyone might afford to get a system designed, permitted and installed in certain jurisdictions. Neither of these extremes is healthy for the industry.

Clearly, rooftop solar systems must be designed to handle the wind, snow and seismic loads appropriate for each building and site. The challenge is that, while there are established snow and seismic load standards that can be applied to PV systems in a fairly straightforward manner, there is very little guidance on wind loads. Engineers and permitting officials have therefore been left with the choice of applying the building code in ways not intended or accepting designs based on wind tunnel testing without a standard means to validate the testing approach or results. Neither method assures that appropriate wind design values are used.

To ensure the long-term safety, durability and affordability of rooftop solar installations, the time has come for the industry to stop improvising by applying design standards not intended for rooftop arrays, and to develop consistent criteria for evaluating structural adequacy based on testing and analysis specific to the PV industry.

To that end, we’ve conducted seven years of research at one of the world’s leading boundary layer wind tunnel facilities under the guidance of one of the leading experts in low-rise building aerodynamics. Through this extensive testing program, which ran 70 models and configurations through more than 1,000 tests, we have developed a fundamental understanding of how tilt angle, roof height, row spacing, building height, set-backs from the roof edge, and various deflector/shrouding strategies affect wind loads on solar arrays (see www.sunlink.com/researchanddevelopment). 

For example, our findings indicate that while wind loads on rooftop mounted solar modules are typically lower than what would be expected on the roof surface itself, they can be much higher than many of those in the solar industry might assume. This is particularly true when looking at an individual module or small collection of modules in an array. By nature, wind gusts that are smaller in size can be much more powerful than wind gusts that are spread over larger areas. It is therefore critical that mounting structures have sufficient structural capacity to take these high local pressures and spread the load over larger areas. In light of these results, current standards may not be sufficient in high wind zone areas.

We have also found that while adding properly designed wind deflectors or shrouding around modules can reduce the overall wind loads on an array, the effect is not substantial enough to mitigate the challenge of high localized pressures. And, because the effectiveness of shrouding is dependent upon keeping the shrouds very close to the roof surface, the structure of a shrouded array needs to be especially stiff and strong to control local displacement under locally high wind gusts. 

It is outcomes like these that must translate into standardized criteria and design requirements that guide rooftop solar installations. We look forward to sharing more of our findings in coming months to help accelerate the development of sensible standards for rooftop solar.

Christopher Tilley is CEO of SunLink Corp., based in San Rafael, Calif. Sunlink manufactures integrated PV balance of system solutions for the photovoltaic industry

Image: Olena Mykhaylova via Shutterstock

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