The prairie has long been the provenance of wind energy. Long, flat ranges seemingly conjure strong and steady winds.

Urban winds are wily, given to irregularities as they encounter trees, alleys, and buildings. In Boston, engineers spent more than a year studying wind patterns atop the city's Museum of Science before installing five small-scale wind turbines there. Even then, they had trouble predicting the performance of the turbines. The technology was simply too new.

Nevertheless, more turbines are ascending to high places, whether the terrace of PepsiCo's Chicago headquarters or the summit of the city's Sears Tower. In New York, rooftop turbines preside over Brooklyn Navy Yard. In Portland, OR, a quartet of turbines rises 45 feet from the roof of Twelve West, a 22-story mixed-used development overlooking downtown. Chicago is home to more than 300 turbine installations.

More than half are for show, says Andre DeRosa, chairman of Chicago-based Balanced Wind, a supplier of small-scale rooftop turbines. As such, he says, performance is often less a consideration than image, prompting the more analytically minded to dismiss rooftop turbines as non-starters.

That simply isn't the case, says Brad Lystra, manager, economic development, with the Washington, D.C.-based American Wind Energy Association. The rooftop turbine can be a surprisingly good fit for buildings, capable of generating electricity precisely where it is needed for as little as a penny per kilowatt (kW), assuming wind speeds are sufficient – and sufficiently calculable – to generate a consistent flow of energy.

Seeking Direction

Suppliers have spun countless variations on the windmill, from the so-called helix to box-like assemblies, to make the most of the wind. Each has its own performance features. The Horizontal Axis Wind Turbine (HAWT) is a variant of the three-blade assemblies found on rural wind farms. The windward facing HAWT – so named for blades that spin on a horizontal axis – has proven peerless in capturing high, sustained winds if they arrive from a single direction. That isn't often the case in urban settings. Although the HAWT can orient itself to predominant wind, opposing winds are often of similar velocity, prompting the turbine to continually recalibrate.

"The wind," says DeRosa, "rarely hits it on the nose."

More effective in this situation is the vertical-axis wind turbine (VAWT), which captures wind from any direction due to the vertical orientation of its main rotor shaft. Because the generator and other primary components can be sited near the roof, the VAWT is also easier to maintain. However, less wind is available at lower altitudes, meaning less energy may be available to the turbine, according to Lystra. But by the same token, buildings typically redirect wind over their roofs, potentially doubling wind speed at the turbine, he says.

VAWTs generally operate at lower start-up wind speeds than HAWTs and can be installed on sites where taller structures are prohibited or require civic review.

Two Variants of the Vertical-Axis Turbine

The two predominant VAWTs – the Darreius and Savonius – differ significantly in design and performance.

Resembling an "S" in cross-section and comprised of opposing curved "scoops," the Savonius, named after Finnish engineer Sigurd J. Savonius, is a drag-type turbine and it extracts less wind energy than the Darrieus, a lift-type assembly patented by Georges Jean Marie Darrieus, a French aeronautical engineer. The Savonius is well suited to applications that place greater emphasis on cost and reliability than efficiency. It also performs well in turbulent settings.

The Darrieus, which resembles an eggbeater, is more efficient, but prone to torque ripple and cyclic stress on the tower, conditions that can hinder reliability. Because their starting torque is low, they also require an external power source or additional Savonius rotor to start turning.

Balanced Wind manufactures an "inverted" Darrieus whose blades pull away from the turbine structure rather than lift inward. "We did trials of hundreds of blade designs to identify the strongest performer, which happened to be a lift," DeRosa recalls. "We also discovered we got better performance from the inverted lift than traditional inward lift, and we refined our designs from there."

Best of Both?

Mercy Lakefront Housing, a 96-unit, single-resident housing development in Chicago, relies on turbines that incorporate aspects of both the Darrieus and Savonius designs. Mercy's turbines, supplied by Chicago-based Aerotecture, include both Savonius (S) and Darrieus (D) rotors, the latter to increase overall RPM at high wind speeds without compromising safety and efficiency. According to Aerotecture CEO Bil Becker, "the hybrid balance of S and D rotors provides good starting torque and higher RPMs while providing a cylindrical form that is easy to integrate with a variety of building sizes, shapes and design schemes."

Nevertheless, the Mercy installation isn't generating the amount of wind energy engineers anticipated, according to Peter Hays, associate principal architect with Chicago-based architect Murphy Jahn, the building's designer. The five-story structure is oriented to capture southwest winds, and it features a rounded roof edge to funnel winds from the facade to a network of horizontally mounted turbines at the roof's center.

Specialty designs tailored to accommodate a building's specific conditions come at a price, or even a hefty premium, because the rooftop turbine doesn't enjoy the same economies of manufacturing scale that streamlined turbines do, says Lystra.

"Cost is currently a significant barrier to entry," agrees DeRosa, who markets a 1-kW unit for $3,500, a 5-kW unit for $18,500 and a 10-kW unit for $35,000. Assuming that production ramps up, "our goal is to drop the price of the 10-kW unit by one-third, to $20,000, in the next five to seven years," he says.

Depending on the building's size, owners may require several turbines to meet their objectives – a potentially significant investment, even when projected energy savings are factored into the equation.

The good news is that owners currently can obtain a 30-percent, no-cap investment tax credit from the federal government to recoup a portion of their costs. Illinois and other states provide similar incentives.

Best to Test

Before investing, owners do well to ensure their building and its site are well suited to wind energy. Although wind meters can take up to six weeks to identify prevailing conditions, new software-driven methods can significantly reduce test times.

Owners should also evaluate candidate technologies for vibration and noise. "The larger the unit, the more vibration and noise it's going to generate," says Lystra.

The HAWT is particularly problematic, since it concentrates the majority of its weight atop a slender tower, conditions that promote vibration. VAWTs are comparatively quiet, though noise and vibration remain a challenge.

Some have likened the noise and vibration to that generated by rooftop mechanical units. However, turbine-generated vibration is more difficult to predict, and it is a factor that building owners should evaluate when investigating the potential of rooftop winds. B

John Gregerson is a contributing editor based in Chicago.

For more on renewable energy, read The Surges of Green Power.