BOSTON--Despite the growing enthusiasm for home wind turbines, an analysis of microwind turbines in Massachusetts found that they fell short of performance expectations.
The Massachusetts Renewable Energy Trust commissioned a study last year to review electricity output from 21 small wind turbines in the state and the results were surprising: the data showed that the estimated production was about three times higher than the turbines' actual production.
The analysis is not the final word on small wind generators, but is significant because few states have done similar reviews, say the study's authors.
The Massachusetts Renewable Energy Trust was "taken aback" at the discrepancy in expected versus actual performance and made changes to its "small wind" rebate program earlier this month to address the issue, said James Christo, a program director from the quasi-public state agency. Christo spoke on a panel on small wind--defined as less than 10-kilowatt capacity machines--at the Northeast Sustainable Energy Building Energy Conference here last week.
"We're certainly disappointed by the performance we've seen to date but we believe there is potential for microwind," Christo said. "People just need to take a more critical look and be cautious."
With the growing interest in clean energy, dozens of different small wind turbine types have emerged from sources as varied as designer Philippe Starck to Aerovironment, a company with roots in military aircraft.
The problem is not that the technology doesn't work. Aerovironment's roof-mounted turbines installed at Boston's Logan airport and other spots, for example, have performed well. The challenge is finding--and choosing--sites with sufficiently strong wind, particularly in cities.
"One of the challenges as an installer is that everyone has a windy site," said Mark Durrenberger, the president of New England Breeze who also spoke at last week's "small wind" panel. "But what you feel on the ground has nothing to do with what you have 100 feet up."
Higher is better
Most small wind turbines are scaled-down versions of giant utility-scale turbines, which look like a fan with three blades. Southwest Windpower's Skystream, for example, is rated at 1.8 kilowatts with at least 10 mile-per-hour winds, which should offset a large portion of a home's electricity consumption.
But there are many other types, including vertical axis wind turbines, such as Mariah Power's Windspire, where a structure spins on a pole to generate electricity.
Having already installed a few turbines in Massachusetts, Durrenberger offered a few simple rules of thumb on small wind turbines. He doesn't recommend roof-mounted models because of the vibration they cause in a home. "It will be like having a sub-woofer in your basement," he said.
When it comes to optimizing for capturing wind energy, higher is usually better. His company will not install a turbine unless it's 30 to 40 feet above any other obstructions, such as buildings and trees.
Good sites are places with smooth terrain like a field with minimal obstructions, according to the Cadmus Group, a consulting firm that performed the analysis for the Massachusetts Renewable Energy Trust. (Click for a PDF with slides of the study). Massachusetts as a whole has only an average wind resource.
The Massachusetts state analysis tried to pinpoint the reason for the underperforming turbines and found that installers often worked without sufficiently good information.
Area wind maps for the region tended to overestimate on average by 10 percent how good the wind was for certain locations, according to Shawn Shaw, an analyst at the Cadmus Group who worked on the study.
Another problem is the rated capacity--how much electricity a turbine can produce--that manufacturers publish aren't always reliable for extrapolating expected performance, Shaw found. Industry associations are trying to come up with standard ways of reporting capacity which will help, he added.
"You want to be internally honest about your (wind resource) assessments," Shaw said. "The economics are going to probably be the best driver in Massachusetts."
A state like Massachusetts has a good wind resource near the coast, but its hilly and woody terrain means that finding a good site requires some investigation.
Installers and customers should be aware, for example, that nearby obstructions can have a significant impact. A 100-foot wind tower placed next to a 50-foot tree is effectively the same as a putting turbine on top of a 50-foot tower, which means it will get a lot less wind, Shaw said.
The results from the Massachusetts study echoes a similar survey done in the U.K. over the past two years, called the Warwick Trials.
That study focused specifically on urban microwind turbines, some of which were roof-mounted. Overall, it found that the performance of these systems fell below expectations as well and that a number suffered technical glitches.
"The truth of the matter is that (urban wind) hasn't been studied very much, at least in the U.S.," said Shaw. "There's a tremendous amount of uncertainty."
To test urban wind turbines, Christo said the Massachusetts Renewable Energy Trust is sponsoring a "science experiment" to put up five turbines from different manufacturers at the Museum of Science, a project expected to go up this spring.
NIMBY and urban wind
Massachusetts on average is far from having the best wind resources in the U.S.--it ranks right in the middle of pack compared to other states.
Following this review, the state revamped its wind power program with the hopes of getting only the best projects developed, said Christo. To get a rebate, installers are now required to do a more stringent wind study and use a specially designed software tool to assess the wind resources in a projected site.
At the federal level, the tax credit for small wind turbines was increased this year, giving investors a 30 percent credit on the installation cost.
Performance issues aside, Durrenberger said that inconsistent zoning and not-in-my-back-yard (NIMBY) sentiment are also serious barriers to erecting a turbine.
Some cities, such as San Francisco and Seattle, have passed bylaws to specifically enable microwind generators. But the rules vary from town to town or are ambiguous.
In an unscientific survey of the 351 towns in Massachusetts, Durrenberger found that 60 percent had no rules for or against wind turbines. Twenty percent specifically allow them and 20 percent have rules, such as height restrictions, that either disallow them or make it difficult to get permitting.
"I promise you, if you want to put one of these things up, you will hear from your neighbors...so contact them before the building inspector does," he said. But "despite NIMBY and the folks in Nantucket (opposing the offshore wind project Cape Wind), there is still a lot of support for wind so you could change your town's bylaws."