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A Mighty Wind (Sidebar)

Managing wind is a challenge, but it offers significant opportunities for cost reduction

By Kelly Vaughn

Among the many barriers to implementing large-scale, cost-competitive rooftop- and ground-mounted solar arrays, wind is a major issue—not wind-generated electricity, but the physical force of wind, which threatens to damage or destroy solar modules exposed to it.

At RMI’s recent Solar PV Balance of System Design Charrette, held this past June in San Jose, Calif., industry stakeholders explained that wind is the No. 1 driver of physical structure costs.

“The effect of wind on mounting systems is far from a trivial concern,” said Robin Shaffer, senior VP of sales and marketing at Sunlink. “Currently there are few organizations in the world that tackle the issue at an adequate level.”

Wind is complex, dynamic, and location-dependent, and it puts considerable stress on solar installations. Managing wind forces safely and in accordance with building codes requires rigorous engineering and testing of design, and it increases labor and materials costs for both rooftop and ground-mounted PV installations.

“Most commercial roofs can’t hold a lot of additional weight—so making solar systems both light and wind resistant can be challenging,” said Lena Hansen, a principal with RMI’s electricity practice.

Building codes require that components installed in or on buildings be tested or certified for obvious safety reasons. The testing establishes the amount of wind-generated force that solar modules on buildings can withstand to ensure the safety and integrity of the system.

To meet safety and integrity requirements, rooftop solar panels must be anchored for wind using either heavy ballasts or an affixed anchoring system. Ballasts dramatically increase the weight of the system, and may exceed rooftop load capacities. Anchoring systems generally require rooftop penetrations, which have their own challenges, including increased installation time and leakage risks. Wind poses an equally daunting challenge for ground- mounted systems. These systems may require pile-driven support beams, heavy concrete foundations, or ballasts to anchor the systems to the ground, requiring expensive equipment, materials, and labor.

Bio-Inspired Design and Charrette Ideas

Tim McGee, a charrette participant and biologist with the Biomimicry Guild, encouraged the team to look to nature for inspired design that could address these engineering challenges. “Life has faced the same design puzzles we have faced, but has had 3.8 billion years to work on them,” he said. “When we’re looking for new inventive ways to solve problems, nature has an abundance of solutions.”

Mimicking a bristlecone pine, which twists to reduce wind drag, or the spikes on a cactus, which create a protective barrier around the plant, could inspire the next innovative structural solution. Charrette participants discussed numerous strategies to accommodate wind forces, including:

Minimizing Exposure

Reducing a panel’s tilt angle reduces wind forces on the panel (which can act like a sail). Keeping panels closer to the ground further reduces wind exposure.

Spoiling & Deflection

Deflectors could be placed on panels where wind forces are the greatest, and vertical wind turbines, mesh wind curtains, and border fencing could be used to deflect wind before it reaches the panels.

Optimizing Site Layout

Reconfiguring the system so there are gaps between panels—allowing wind to pass easily through the arrays—could reduce system-wide wind forces, and designing the system to address local wind characteristics could reduce the costs of materials in certain regions.

Flexible Racking Structures & New Structural Concepts

“Change the physics that are hurting you into the ones helping you out,” said Sandy Munro, charrette attendee and CEO of Munro & Associates. Controlled failure mechanisms (that safely collapse panels when the wind reaches certain speeds) and flexible, compliant structures that automatically react to high wind speeds were viewed as possible opportunities for advanced systems to minimize wind damage. And, as RMI’s Amory Lovins points out, it is often cheaper to bear structural loads with tension (e.g., using cables) than with mass and stiffness. However, the practicality of these concepts is uncertain.

Exploiting Wind Loads

New horizontal wind turbines mounted just inside the edge of a conventional roof can convert wind to electricity while reducing wind forces on photovoltaic racks placed toward the center of the roofs.

A great deal of work remains to determine how much these ideas can contribute to cost-competitive and scalable PV systems. Ongoing analysis and testing will be required, and we certainly won’t see changes right away. Perhaps the single most important challenge will be keeping a whole-system perspective, precluding a solution that could improve the effectiveness of one portion of the solar value chain while compromising the effectiveness of others.

As RMI continues to promote the exchange of ideas, broaden perspectives, and push cutting-edge thinking, we hope to see considerable progress in what the industry believes is possible.

Kelly Vaughn is RMI’s public relations specialist.

--Published August 2010

 
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