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Small wind turbines attached to individual homes work fine unless you have lousy wind.

That’s the upshot of a multi-site study called the Warwick Microwind Trial project, a year-long survey on the performance of roof-mounted turbines done in the U.K. done by Encraft, a low-carbon technology consulting firm.

The researchers picked 26 sites in the U.K where microturbines tied to the power grid were attached to people’s homes to offset their electricity use. Many of the consumers clearly purchased the turbines to lower their fossil fuel energy use as some already had solar panels and ground-source heat pumps.

One of the building-mounted roof turbines tested in a year-long study of urban wind turbines in the U.K.

But the Encraft study, which came out last month, found that many of the turbines didn’t meet manufacturers claims for power generation. Some turbines needed to go offline at times because of technical problems or complaints over noise.

“The gap between average performance (or expectation) and reality is much larger than people could reasonably expect,” Encraft managing director Matthew Rhodes said in a summary.

The survey doesn’t conclude that small wind turbines, in general, are uneconomical or unsuitable for the U.K. Instead, the data points to the need for accurate wind measurements before installing microturbines, particularly in cities.

From the study: “Overall the trial has painted a picture of an industry and technology that is still at development stage and is likely to make a tangible contribution to energy and carbon saving only on the most exposed sites and tallest buildings. The combination of this reality, aggressive and over-optimistic marketing by some suppliers, and the enthusiasm and credulity of the market (and regulators) has potentially led to an unfortunate outcome where the wind industry as a whole is in danger of suffering from a setback in credibility.”

In response to the report, the British Wind Energy Association noted that many of the urban sites chosen in the study have wind lower than 5 meters per second (or 11 miles per hour), considered the lower limit of commercial viability. It also said that the British government’s online wind assessment tool overstated the available wind in cities.

“The overwhelming majority of small wind system installations are a success–when they are sited properly they save money and energy. The Warwick trials do not show that small wind is not viable. We know that it is, and the experience of thousands of UK users bears this out,” said Alex Murley, BWEA Small Systems Manager, in a statement.

Poring over wind maps
In the U.S., urban wind turbines account for only one percent of installations but there is growing interest, according to the American Wind Energy Association (AWEA). In a report (click for PDF) on how municipalities should permit small wind turbines, the AWEA said understanding one location’s particulars and how high a turbine is placed–which in general, means more wind–is crucial in urban locations.

“Siting becomes especially important for turbines in urban settings. Wind patterns behave very differently around buildings and in densely-built areas, so a turbine must be sited very precisely inorder to gain access to wind of sufficient quality,” according to a report by AWEA.

Small wind turbines overall are a fast-growing segment of the wind industry. Incentives to install small wind turbines improved last year, boosting the federal tax rebate to up to $4,000. Politicians, including New York City Mayor Michael Bloomberg, have called for placing turbines atop skyscrapers and public buildings.

To get a read on the suitability of a location, start-up 3Tier offers a free, Web-based wind and solar assessment tool, which gives people an idea at a high level how good wind is. There are state-level wind programs, and the Energy Information Association and National Renewable Energy Laboratories both publish national maps.

In isolated areas, a pitched roof causes wind to speed up. But the presence of nearby buildings in an urban setting can cause wind to slow down when it hits a roof, according to research from the U.K.’s Centre for Renewable Energy Systems Technology.

(Credit:S. Watson, Loughborough University)

Southwest Wind Power, which makes a small wind turbine that typically is mounted on a pole rather than a roof, offers guidance on minimum requirements, which include 10 mile-per-hour average wind and 20 feet clearance above obstructions.

Another manufacturer, Cascade Engineering, publishes specifications that assume a very good wind resource. The specifications for its roof-mountable small turbine indicate that it can turn out 1.5 kilowatts and the 2000 kilowatt-hours per year–or about one third or one quarter of a U.S. home’s electricity. To meet that that threshold, however, a site needs 31 mile-per-hour wind, or 14 meters per second.

Some customers, meanwhile, don’t do rigorous economic analysis beforehand. The Museum of Science in Boston, for instance, is now working on an educational project to put eight small wind turbines from different manufacturers on its roof

Harvard University, which plans to put some small turbines on an office complex and on a parking garage, toldThe New York Times that the installations are experiments and “outward symbols of our commitment to renewable energy and sustainability here on campus.”

Aerovironment is targeting commercial customers for its small-wind turbines that are optimized for gusty wind coming off buildings.

Small wind manufacturers say that the payback on small wind turbines–rooftop or pole-mounted–varies greatly based on how a machine is sited, which points to the importance of a good installer.

With good wind and high electricity rates, a homeowner could recoup the upfront investment of a small wind turbine in five or six years. But low electricity rates and marginal wind could mean 15 or 20 years, Andy Kruse, the CEO of Southwest Windpower, said last month.

Kruse is lobbying for government-sponsored work to make wind and solar maps that offer more specific information. Similar as air source heat pump,wind turbines will become popular with energy saving.

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