The Folly of Building-Integrated Wind
April 29, 2009
without a cap) provides a strong financial incentive. A year ago, Mayor Michael Bloomberg even suggested building-integrated wind as a greening strategy for New York City’s many tall buildings. What’s not to like about it?
It turns out that, despite some benefits, building-integrated wind doesn’t make much sense as a renewable-energy strategy. In this article, we’ll examine both the pros and cons of this technology, look at some examples of how it’s been tried, and explain why it’s usually a bad idea.
Context for Building-Integrated WindThe wind power industry has gone through a steady evolution since the 1970s, when interest in generating electricity from the wind was reawakened. Wind turbines from the early 1970s were generally small, a few kilowatts (kW) in rated output, and most were for residential applications. Aided by significant research support from the U.S. Department of Energy, the wind industry pursued the significant economies of scale with larger turbines, leading to machines with output in the tens of kW, then hundreds of kW, then in the megawatt (MW) scale.
Another major shift, starting in the 1980s, was to aggregate wind turbines into
wind farms. By situating multiple wind turbines close to each other on windy ridges, such as Altamont Pass and Tehachapi Pass in California, maintenance could be more efficient, and power could more easily be fed into the utility grid.
Some suggest that a third shift is underway today: putting wind turbines on top of buildings or integrating them into buildings in other ways.
In some cases, building geometry can enhance wind turbine performance. Several manufacturers of building-integrated wind turbines are taking advantage of the increased wind velocities at building parapets—where the wind rises up the façade of a large building and curls over the edge. Some architects are designing wind scoops right into the structures of buildings or situating building towers to funnel wind into turbines.
Most of our electricity is used in buildings, and generating the electricity on site reduces the need for transmission. This in turn reduces transmission losses as well as the materials needed for wiring and poles. In addition to this practical benefit, wind turbines spinning on a building provide a visible testament to a building owner’s commitment to the environment. While building-integrated photovoltaics (PV) can make a similar statement, the modules just sit there; we don’t see them generating electricity.
Finally, many consider wind turbines to be beautiful. The graceful AeroVironment wind turbines that top an office building at Logan International Airport are an aesthetic feature. Architects and building owners spend a lot of money on non-functional, decorative elements of buildings; why not install decorative elements that actually do something?
Facing Up to Reality
Unfortunately, building-integrated wind often doesn’t live up to its promise. The turbines must overcome several challenges to meet performance expectations and be cost effective.
laminar wind, in which all of the air flows in a single direction. But on top of even very tall buildings, wind flow is highly turbulent. Bob Thresher, director of the National Wind Technology Center at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, explains that as wind flow comes over the edge of a roof or around a corner, it separates into streams. “Separating the flow creates a lot of turbulence,” he told
According to Ron Stimmel, the small wind technology expert at the American Wind Energy Association (AWEA), this turbulent flow confuses a wind turbine, affecting its performance. “Even if it feels really windy [on top of a building], it’s probably more turbulent than steady wind,” he said. A common rule of thumb, according to Stimmel, is to elevate a wind turbine at least 30 feet (9 m) above anything within a 500-foot (150 m) radius, including the building itself.
What about the increased wind velocity at building parapets that manufacturers like AeroVironment use? Although AeroVironment’s turbines successfully harvest this band of higher-velocity wind, they do so only in a fairly narrow band, which limits the potential size and output of wind turbines. Because the turbines are small, the economics are not as attractive as with larger wind turbines.
Noise and vibrationNoise and vibration from wind turbines are among the greatest obstacles to integrating them into buildings. Based on the recent surge in building-integrated wind, one might think that engineers had beaten this problem. In truth, some wind turbines are a lot quieter than others—vertical-axis machines among them—but managing noise and vibration remains a huge challenge. Roger Frechette, P.E., of Skidmore, Owings & Merrill (SOM) in Chicago, who led the engineering team on the Pearl River Tower, opted for vertical-axis turbines to minimize noise and vibration but still put them in unoccupied “technical floors” to isolate them from occupants in the building.
Engineer Paul Torcellini, P.E., Ph.D., of NREL points out that the vibration from wind turbines is variable. He said that with HVAC fans on buildings, where the frequency of the fan is known, controlling the vibration and noise requires carefully engineered housings and mounting systems to isolate that vibration from the building—and it’s still a problem.
In one of the only extensive surveys of actual performance of building-integrated wind turbines (the Warwick Wind Trials Project, the only turbines able to generate close to their projected electricity output were mounted on high-rise apartment buildings. And these wind turbines remained
switched off throughout most of the test period because of complaints from the residents about noise.If you try to put a turbine on a tower on top of a building—to get away from the turbulent flow and into the most productive wind—the stresses on the building are magnified. Randy Swisher, the past executive director of AWEA, notes that wind turbines are subjected to a great deal of stress, and if installed on a building, “that stress can be transmitted to the building structure, creating substantial problems.”
EBN interviewed explained that turbulent flow creates stress on the drive gear in a turbine, creating vibrations. These vibrations can, in turn, create harmonic resonances within a building structure. Metal roof decks made from thin roll-formed steel sheet, common in commercial buildings, can act like drumheads and amplify these resonances. In fact, AeroVironment, the building-integrated wind energy company that has done more than any other to understand the aerodynamics of wind around buildings, suggests in its sales literature that their turbines are only appropriate for buildings constructed of concrete.
EBN found no evidence of injury or damage from building-integrated wind turbines, a building such as the Bahrain World Trade Center, with its 95-foot-diameter (29 m) rotors, might not be insurable in the risk-averse and litigious North American market.
Poor measured performance
Despite the growing number of building-integrated wind turbine installations around North America and the rest of the world, obtaining actual
measured performance data is like pulling teeth. Most manufacturers of these wind systems either claim not to have such data or are unwilling to share it. The reason for this reluctance may be that actual electricity production is much worse than expected.
power curves for their turbines that show projected electricity outputs at different wind speeds. There is also a
power output at a specific wind speed, though the wind speed used for this rated output differs among manufacturers. Referring to small-scale, rooftop wind turbines, Ron Stimmel of AWEA said that “it’s very, very difficult to get them to perform at anywhere near their rated capacities.” He told
EBN that he has yet to find one that achieves its expected performance.The municipal utility company Madison Gas and Electric, in Wisconsin, set out last year to find out for itself whether small-scale, building-integrated wind made sense. The company installed a vertical-axis wind turbine made by the Finnish company Windside, whose turbines are widely installed on rooftops in Europe. Madison Gas and Electric installed a turbine on a pole, with the top at 42 feet (13 m)—about the height it would be on a one-story commercial building—and has been tracking performance continually since November 2008.
According to senior engineer David Toso, P.E, the 12-foot-tall (3.7 m) by 3-foot-diameter (0.9 m) WS-4C turbine is rated at 10 kW AC power output, but he has never seen it produce more than 600 watts—6% of its rated output—even on a very windy day. The turbine cost $40,000 and was purchased from Bright Idea Energy Solutions in Evansville, Indiana (which is no longer distributing the Windside product, although the company does offer a similar, U.S.-made product). When
EBN checked the real-time cumulative electricity production from the wind turbine in early April 2009 (go toand click on “Our Environment”), we found that it had produced only 33 kWh total in four months—about a quarter kWh per day. “Either someone was too aggressive with their projections, or they missed a decimal point,” Toso told
EBN. “They’re not quite ready for prime time.”
Power generation systems are typically rated by a
capacity factor, which is the percent of electricity generated compared with the output if the system were operating at the rated capacity over that time period (although with wind turbines there is no standard for the wind speed on which the rated capacity is based). Freestanding wind turbines in good wind sites typically operate at a 10% to 30% capacity factor—the better the site, the higher the capacity factor. By this analysis, the Madison Gas and Electric wind turbine is operating at a capacity factor of just 0.11%. Fixed-pitch PV systems in selected cities, by comparison, have capacity factors ranging from 11% in Seattle to 18% in Tucson, according to data supplied by Steven Strong of Solar Design Associates.
The previously mentioned Warwick Wind Trials Project in the U.K. measured turbine performance of 26 building-mounted wind turbines from October 2007 through October 2008 and found an average capacity factor of 0.85%. All were very small (“microwind,” defined as less than 2 kW) turbines, including the Ampair 600 (600 W), Zephyr Air Dolphin (1,000 W), Eclectic D400 StealthGen (400 W), and Windsave WS1000 (1,000 W). For each installation, measured electricity production was compared with predicted production based on the manufacturers’ supplied power curves and both predicted and measured wind speeds. The study found that predicted performance exceeded actual performance by a factor of 15 to 17. With the worst-performing systems, the electricity required to run the electronics exceeded the electricity production, so the wind turbines were net consumers of electricity!
A 2008 report on 19 small wind turbines installed in Massachusetts, written by the Cadmus Group with support of the Massachusetts Technology Collaborative, found far lower performance than expected. While these were freestanding rather than building-integrated turbines, the measured capacity factor was just 4%, versus the projected 10%. In other words, the performance was 60% worse than predicted. Various reasons were given as to why this large discrepancy may exist, including inaccurate wind speed estimates, incorrect power curves, inverter inefficiencies, and greater losses due to site conditions (turbulence and wind shear) than expected.Even AeroVironment’s wind turbines, are not performing at the level the company had originally hoped for. Since 2006, when their parapet-optimized wind turbine was introduced (see
EBN), the company has adjusted downward its expectations of energy production, according to Paul Glenney, director of AeroVironment’s Energy Technology Center, though installations are matching their predicted power curves.
Perhaps the greatest impediment to building-integrated wind energy is the economics. While large free-standing wind turbines provide the least expensive renewable electricity today, small wind turbines are far less cost effective, and when small turbines are put on buildings, the costs go up while the production drops.
How does building-integrated wind compare with PV? AeroVironment installations have been running at $6,500–$9,000 per kW of installed capacity, which is fairly close to the cost of PV installations, which averaged $7,600 in 2007, according to a February 2009 report from Lawrence Berkeley National Laboratory. An AeroVironment wind system will deliver, according to Glenney, 750–1,500 kWh annually per kW of rated capacity (depending on the wind resource), while a fixed-pitch, commercial-scale PV system will deliver annually 1,100–1,200 kWh/kW of rated capacity in Boston and 1,400–1,560 kWh/kW in Tucson, according to data provided by Strong.
When you factor in the fact that the PV system is likely to deliver closer to its rated output on a building than the building-integrated wind system, while costing less to maintain, PV is just a better deal. According to Paul Gipe, a leading advocate of wind power for 30 years and author of numerous books on the topic, if you’re looking to put renewable energy on buildings, “there’s nothing better than photovoltaics.”
Quite a few manufacturers offer wind turbines for rooftop installation. The following is a small sampling of what’s available today.
AeroVironment has pursued horizontal-axis, rather than vertical-axis, wind turbines. Vertical-axis machines “are inherently less efficient by a wide margin,” according to Glenney. “Our patent for leveraging the accelerated wind flow includes vertical-axis wind turbines, but we’ve never pursued them simply because the lower efficiency significantly increases turbine size and, thus, material costs,” he said.
Aerotecture International helical rotor wind turbinesAerotecture founder Bill Becker, a professor at the University of Illinois, invented this unique wind turbine, described on the company website as a “helical rotor and airfoils housed within … a steel cage.” The lightweight, 10-foot-tall by 5-foot-diameter (3 x 1.5 m) 510V turbine is designed for vertical mounting and rated at 1 kW output—at 32 miles per hour (14 meters per second). While the 510V turbine is rated at 32 mph, the power curve for the unit shows less than 200 watts of output in 20 mph (9 m/s) wind. The cut-in windspeed (when the turbine begins generating electricity) is listed as 6.3 mph (2.8 m/s). The slightly modified 520H is made up of two 510V turbines that are installed horizontally; it is rated at 1.8 kW at 32 mph.
Eight 520H Aerotecture wind turbines were installed on a Mercy Housing Lakefront single-room occupancy building in Chicago in May 2007. Each of these was rated at 1.5 kW (somewhat lower than the currently listed rated output for the 520H)—for a total rated capacity of 12 kW. Unfortunately, there is no data available on the actual performance of these turbines. Aerotecture would not return
EBN’s calls, referring us to a public relations agency, which told us by e-mail that “the company is focused on internal development not media coverage at this point, so it’s frankly just not possible to get your query on the agenda.”
Larry McCarthy, the vice president for property management at Mercy Housing Lakefront, told
EBN that the turbines “are not all working at this time,” adding that a couple of the alternators are frozen up. A Chicago resident
EBN spoke with said he has “rarely seen more than one of the turbines rotating and often not even one.”
Quiet Revolution QR5 vertical-axis wind turbine
Currently available only in the U.K, Quiet Revolution’s QR5 is an elegant, eggbeater-style (Darrieus) wind turbine with blades and spokes made from carbon and fiberglass. The 16-foot-tall (5 m) by 10-foot-diameter (3.1 m) turbine is designed for mounting on a mast that is installed either stand-alone or on top of a building. The peak DC power output in 31 mph (14 m/s) wind is 6.2 kW, with the British Wind Energy Association (BWEA) rated power output at 24.6 mph (11 m/s) is 3 kW DC. Power generation can begin at 10 mph (4.5 m/s), and the turbine cuts out at 36 mph (16 m/s). Data from the company on noise production from the turbine shows about 50 dB(A) at 13 mph (6 m/s) and 58 dB(A) at 22 mph (10 m/s). The company’s website lists the price for the turbine and control electronics at 29,600 British Pounds (about $43,000), plus mast and installation.
To date, more than 65 Quiet Revolution turbines have been installed in the U.K., and expansion to other countries is anticipated in 2010 or 2011, according to Phillipa Rogers of the company.
I want to like building-integrated wind. There’s a wonderful synergy in the idea of combining form and function by generating electricity with turbines that reach into the sky on the buildings they will help to power. But in most cases, at least with today’s technology, it just doesn’t make sense.
There is a huge economy of scale with wind power. This has fueled the evolution of ever-larger wind turbines from a few kW of capacity in the 1970s to a few MW today. Small turbines, even stand-alone, pole-mounted turbines, are not very cost-effective. When we put those small turbines on top of buildings, the costs go up and the performance goes down.
Rooftop installations—even the best of them—are too small to be cost-effective, and the air flow too turbulent to be effectively harvested—whether vertical-axis or horizontal-axis. The truly integrated installations that are large enough to generate significant power will be too hard to permit or insure in North America to become a serious option, even if the vibration and noise concerns are successfully addressed.
Paul Gipe vociferously discourages building-integrated wind. Wind just isn’t a good fit, he argues. Cost-effective wind turbines are “too big for the structure of buildings.”
Wind energy has a very important role to play in our energy future, but it is with large, freestanding wind turbines, located on ridgelines, in Midwestern agricultural fields, or in offshore wind farms. The bottom line regarding cost is that while large stand-alone wind farms provide the least expensive renewable electricity today, small, building-integrated wind turbines provide electricity that is more expensive than that produced by PV, while the turbines are more costly to maintain and less dependable.
By all means, power your buildings with wind energy, but do it on a larger scale, remotely, where the turbines can operate in laminar-flow winds and where their vibrations and noise won’t affect buildings and building occupants.
For more information:
American Wind Energy Association
National Wind Technology Center
National Renewable Energy Laboratory
Warwick Wind Trials Project
Aerotecture International, Inc.
Quiet Revolution, Ltd.
Swift Wind Turbines
Tangarie Alternative Power