A look at small-scale urban wind power

Recent discussion about developing wind farms in Lake Michigan, possibly out from Milwaukee or Manitowoc, has blown wind power tantalizingly back into the public view.

But while ambitious projects such as these hold promise for providing renewable energy to the people of Wisconsin, they still rely upon the enterprise of large corporations.

Ordinary citizens might reasonably wonder: With those lake breezes, why not just put up a small backyard or rooftop wind turbine to offset at least some electricity usage?

It’s not illegal to do so-Wisconsin State Statute 66.0401 prevents municipal restrictions of solar or wind systems, essentially unless they threaten public health or safety (though interpretation of this statute varies).

But wind power in a “built environment” like a city-or even anywhere near the ground-is not feasible for several reasons, said Mick Sagrillo, wind energy specialist for Focus on Energy.

The first is turbulence. Sagrillo gave the anecdote of a twig tossed into a river. Near the shore, the twig will tumble and spin. Only if it gets out in the middle of the river will it flow purely downstream.

“Water’s a fluid. Wind is a fluid. They both follow exactly the same laws of physics: fluid dynamics,” Sagrillo said.

The nearshore zone of the river is like the near-ground region of the atmosphere-a “zone of friction” near the ground generates turbulence. “Turbulence robs you of quality fuel,” Sagrillo said.

Turbulence can also cause damage. Sagrillo described a friend in upstate New York who wanted a wind system he could afford, so he attached a small rotor to his roof. The turbine ended up ripping his roof off-which proved more expensive to repair than the turbine.

The second reason Sagrillo said small-scale wind power isn’t viable in a city is low average wind speed.

Because power is a product of the cube of the wind speed, Sagrillo said, even slightly faster winds can be highly more productive.

“If you can go from eight to 10 miles per hour, you get 100 percent increase in power available to the wind generator,” Sagrillo said.

Average wind speed is greater farther from the ground and that’s the reason for wind turbines mounted on tall towers in rural areas. “That’s where the resource is,” Sagrillo said.

Also, larger turbines prove more cost-effective than small ones. The larger the rotor, the more power can be generated. Larger, higher turbines do cost more but provide a greater return.

In contrast, a small two- or four-foot turbine mounted on roof-even assuming a windy environment-would only provide enough electricity to power a taillight bulb or exit sign, Sagrillo said, not an economical return for a technology that is still going to cost thousands.

“Do you want to generate electricity or do you want to make a statement?” Sagrillo said. If the latter, then he cautioned: “make sure they’re willing to feed that statement for a long time.”

The U.S. Department of Energy’s “Small Wind Electric Systems: A U.S. Consumer’s Guide” recommends a minimum rotor height of 30 feet above any obstacle, like trees or roofs, within 300 feet. Property owners will thus need space on which to erect 60-foot or taller towers, not common inside cities, even leaving aside turbulence and zoning issues.

Even potentially prime spots above surrounding land require a significant investment to tap the wind resource, if there is one. Sagrillo offered an example of the Urban Ecology Center north of downtown Milwaukee, which he said was interested in a wind turbine. Sagrillo and Focus on Energy took a look at the site and recommended a “minimum tower height of 120 feet, just to get up into airflow.”

Just because winds feel strong doesn’t mean a particular site is good for wind power.

Wind is essentially a rural technology, Sagrillo said-unless someone comes up with a cost-effective way to mount turbines on towers on top of skyscrapers, or on tall building parapets facing the wind.

Last year, Kettle Foods installed 18 1-kilowatt AeroVironmnent Architectural Wind turbines on the roof of its new facility northwest of Beloit, Wis.

Kettle Foods claims the swiveling turbines are on track to meet an estimated annual electricity generation of 28,000 kilowatt-hours, or enough to make 56,000 bags of potato chips. The company could not provide average site wind speed, however.

Sagrillo said he’s been trying to learn about their performance but has been told their wind data is proprietary information. Lacking data, he suspects the Kettle Foods system has underperformed expectations.

Average wind speed predictions, key to predicting output, have proved erroneous before.

Sagrillo pointed to the recent Warwick Wind Turbine Trials in the United Kingdom, which sought to measure the effectiveness of already-installed rooftop wind turbines. Trial anemometers showed that actual wind speeds were significantly less than predicted, resulting in poor performance at many sites.

“In a built environment, wind, it has a terrible track record. People take them down because they’re disgusted with the performance.”

More sophisticated wind mapping promises to identify optimal sites for wind turbines throughout the state, but it won’t change the physics that make small-scale urban wind power economically unappealing, Sagrillo said.

When urbanites ask Sagrillo if they should consider putting up a wind turbine, he usually offers this advice: “Buy photovoltaic. Buy solar. That is technology designed for an urban environment. Look at the rooftops.”

Report on Lake Wind Farms

An Oct. 10 report by the Public Service Commission of Wisconsin concluded that “Off-shore wind projects are technically feasible in the near-shore areas of the Great Lakes with present day technology. There are significant technical challenges with the development of wind projects in deeper water locations where the best project sites may be located, based on wind resources and other considerations.”

The report also concluded that Wisconsin’s existing transmission system could support up to an additional 600 megawatts, before substantial upgrades would be needed.

The study group recommended three more years of wind speed data collection. The PSC is looking for public comment on its report by Nov. 10 (psc.wi.gov).

Sagrillo said Focus on Energy funded a turbine on the Racine Reef and observed an 18 mph average wind speed-just 60 feet above the lake.

By way of comparison, Sagrillo, in Kewaunee County, gets an average 15 mph at his 200-foot tower. The small difference equates to a 72 percent increase in energy.

“You would have a very substantial wind resource out there,” Sagrillo said. “They’re talking about gigawatts of energy that are available out there.”

If you live in a rural area and want to consider a wind turbine, Focus on Energy can provide a renewable energy site assessment (focusonenergy.com).