Windy Cities: Rooftop Turbines Provide Economical Wind Power

Small rooftop turbines are scaling some big city buildings. Is your building a candidate for economical wind power?

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.

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 city 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.

 The Horizontal Axis Wind Turbine

Suppliers have spun countless variations on the windmill, from the so-called helix to box-like assemblies, to make the most of the wind. 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.

 Vertical-Axis Wind Turbine (VAWT)

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, making them the preferred choice for a rooftop turbine in an urban environment.

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