West Tisbury is too quiet for turbine
A wind turbine project proposed by the Up-Island Regional School District (UIRSD) recently came to a noisy end.
For more than two years, the UIRSD, which serves the towns of Chilmark, Aquinnah, and West Tisbury, has been considering the feasibility of installing a 250-kilowatt (kW) wind turbine at the West Tisbury School to help offset the school's electricity demand and rising energy costs.
Superintendent of schools James Weiss said the school district received a grant for a feasibility study and hired Boreal Engineering.
The completed feasibility study showed there was sufficient wind. However, Boreal suggested the school also do an acoustical noise study, which the company had not been empowered to do. The UIRSD school committee agreed.
"We hired an acoustical engineering firm" Mr. Weiss told The Times, "and they came back with a message that West Tisbury is a very quiet place. So quiet, in fact, that putting a 250-kilowatt turbine up there would go above the acceptable change in sound."
Based on that information, the UIRSD school committee decided at a meeting on December 21 to postpone any further action on the wind turbine project.
The results of the acoustic study that was conducted by Tech Environmental showed that the community-scale wind turbine would increase the sound level for West Tisbury School's property boundaries and nearby residences beyond the acceptable level allowed by the Massachusetts Department of Environmental Protection's (DEP) Noise Policy.
Can you hear me now?
According to the noise study, since the site for the proposed wind turbine is north of the school, existing sound levels on the school grounds were measured at three locations west, north, and south of the proposed site. The study was conducted from November 24-30, 2009, when hub-height winds measured 2 to 43 miles per hour (mph).
As the study explains, the loudness of a sound depends on the sound pressure level, which is a ratio of the measured sound pressure from a source, divided by a reference pressure, the quietest sound people can hear. This is the unit of sound pressure familiar known as the decibel (dB).
To put this in perspective, the quietest sound most people can hear is 0 dB, and the loudest, 120 dB, the study said. Most sounds heard in daily life range from 30 dB to 100 dB.
Sound exposure in a community is commonly expressed in terms of the A-weighted sound level (dBA), the study explains, which approximates the frequency response of the human ear.
For example, the sound in a quiet bedroom at night measures 30 dBA, while the inside of a New York subway car measures 100 dBA.
In general, the background, or ambient, sound level in a given area is the level exceeded 90 percent of the time. The equivalent, or average, sound level is the steady-state sound level measured over a period of time.
At the three locations at West Tisbury School where sound was measured, "The audible sounds were wind in the trees, children in the playground, and intermittent traffic noise," according to the study. Measured sound levels were higher, of course, at the location next to the school building because of the presence of people.
According to the study, data showed that existing background sound levels at the three monitoring stations during times when winds are high enough to support wind turbine operation are in the range of 22 to 50 dBA. Average sound levels in the area are 26 to 63 dBA. Existing sound levels increase with wind speed.
In conclusion, the study said, "The maximum sound level increases [for the proposed wind turbine] are in the range of 24 to 31 dBA and far above the maximum 10-dBA increase allowed in the DEP Noise Policy."
Tilting at windmills
The UIRSD's proposal actually made it through several stages before noise stopped its blades from ever spinning.
The West Tisbury School wind turbine proposal involved collaboration between the UIRSD, which owns the property, and a town committee, the Up-Island Regional School (UIRS) Energy Committee. Original plans called for building a 235-foot wind turbine at the school.
Noise emerged as a concern from the project's beginning. In a poll of about 150 towns with wind turbine projects conducted by the UIRS Energy Committee, chairman Shelton Bank said noise was cited as the most common complaint.
A wind site survey conducted by the Renewable Energy Research Laboratory in January 2007 identified West Tisbury School as a favorable location. Interestingly, that study also referenced noise as a potential problem.
"Noise will be an important siting constraint for a wind turbine at West Tisbury School," the study read. "Consideration of the neighbors will be an important factor in micro-siting and sizing a wind turbine on this parcel."
As a "rule of thumb" for minimizing possible noise, the study explained, a wind turbine should be sited three times the blade-tip height from residences. Based on the turbine initially proposed at West Tisbury School, that meant it should be located about 1,000 feet from residences.
"There are no potential wind turbine locations on the West Tisbury School parcel where the distance to the nearest house is greater than 820 feet," the study said. "From a noise perspective, the 'three-times-blade-tip' distance guideline suggests that a large-scale turbine could be a nuisance to neighboring residences."
Because the West Tisbury School location is only 2.2 miles from the airport, the UIRSD and energy committee requested an aeronautical study from the FAA to determine whether there would be height restrictions for a wind turbine.
On October 23, 2008, the FAA issued a notice of presumed hazard, recommending a reduction in the wind turbine structure's height to 202 feet above ground level.
The UIRSD received the feasibility study grant after adjusting plans for the new height restriction. Last fall, however, height became an issue again in the Martha's Vineyard Commission's (MVC) consideration of a proposed land zone as a district of critical planning concern (DCPC) in an Island Wind District.
The area nominated as a DCPC included airspace 150 feet above all lands and inland waters within Dukes County, except the Elizabeth Islands, the Indian Common Lands, and the settlement lands.
However, in response to concerns raised by Mr. Weiss and Edgartown officials about proposed wind turbine projects at West Tisbury School and the Edgartown Wastewater Facility, the MVC agreed to amend the DCPC designation to exclude all school property and the town of Edgartown.
Otherwise, the projects would have been delayed by a one-year building moratorium imposed once the DCPC was designated.