December 15, 2006

Unique challenges face wind power developers, buyers

Dr. Robert Peltier, PE

Sound advice: Make wind turbines seen but not heard

By Teresa Drew and Roger Treagus, Golder Associates Ltd.

For many developers of wind power projects, the "noise issue" is one that just won't go away. It comes up in public forums, in meetings with local government bodies, and in kitchen-table discussions with local landowners.

Given the relatively low noise level of today's wind turbines, why are they considered noise polluters a priori? Early models were louder for a variety of reasons, including a higher turning speed and less-efficient designs. Better sound shielding of mechanical noise sources, improved aerodynamics, and other technical advances have substantially reduced, but not eliminated, the noise produced by today's turbines of equivalent size.

The human factor

One plausible reason for wind turbines' bad aural reputation is their big visual impact. Because sight dominates human sensation, any source of noise that can be seen seems louder. Wind turbines are large, out in the open, often on high ground, and they move—all of which attract attention. If people don't like the look of a wind farm, they may transfer this dislike to the sound it makes.

There are many steps that developers can take to reduce the actual and perceived noise impact of a wind power project. The most important is to consider noise early in the planning process. Most developers focus first on wind speed and reliability, as they should. However, noise should not take a back seat to other environmental consequences of a project, such as those on bird and bat populations, rare plant and animal species, and sensitive habitat and ecosystems.

To account for noise, the site selection process needs to be broader than is often the case with other developments, right from the start. Hiring environmental and noise professionals to vet potential locations for certain deal-killers can spare a developer from wasting money and time on dead-end candidates.

Which way is upwind?

When evaluating a potential site for noisiness, fluctuations in prevailing wind patterns should be taken into account. Because sound carries farther downwind, it's important to know both the dominant direction and how often the wind deviates from it. The minimum distance, or setback, from a "receptor" such as a farm house or subdivision (including those planned for the future) should be increased if it is frequently downwind of the wind farm.

As an example, consider a recent wind farm project in which Golder Associates was involved. In our preconstruction projections, we indicated that, to limit the noise impact on a cluster of nearby homes, our client would need to shut down some of the turbines closest to it. But it would only have to do so when the wind blew in a particular direction (historically, only one or two days a year on average) and only when it blew above a certain speed. These parameters were integrated into the farm's operating plan, and it now works at full efficiency almost all of the time.

When planning to limit noise pollution, realize that topography and ground conditions can significantly affect noise propagation. Studies have shown that lining a ridge with turbines produces "shadow zones" on its slopes: noise levels are actually lower near the turbines than at a distance in the valleys below. For sites near a body of water, remember that reflections off the water's surface can amplify noise.

Past, present, and future

Finally, the planning of a school, hospital, or subdivision (or the recent rezoning of a property for such a land use) may create limitations on wind farm development. It's a good idea for developers to build good relations with local authorities and regulators to learn what land use changes are possible. Noise criteria normally do not contain a land use component, making it even more important for developers to be certain that encroachment on their site is unlikely.

Sound level projections are more acceptable if they are based on real-world results. Developers need to conduct thorough preconstruction projections of a project's noise impacts and then verify them later. Through practice, such diligence improves the accuracy of projections for future projects. It also comes in handy during discussions with municipal leaders and others who could possibly be affected by a future wind farm. If individuals are shown that previous projections of noise levels were accurate, they"ll be more willing to trust the projections being made for a wind farm under consideration.

—Teresa Drew and Roger Treagus are sound engineers in the Calgary office of Golder Associates Ltd., a ground engineering and environmental services specialist. They can be reached at 403-299-5600 or via teresa_drew@golder.com and roger_treagus@golder.com.

Wind now on the radar—literally

Another wind farm siting problem was the focus of a recent U.S. Department of Defense (DoD) report of obvious interest to any wind power developer. The report, titled "The Effect of Windmill Farms on Military Readiness," concludes that wind farms located in the line of sight of an air defense radar system could degrade the system's effectiveness. The impact, of course, would depend on the number and location of the farm's turbines.

The report, which was released this past September, was prepared in response to a congressional call for the secretary of defense to submit a report on the effects of wind farms on military readiness. Pending completion of the report, in March 2006 the DoD and the Department of Homeland Security (DHS) adopted an interim policy (pending publication of the final report in the Congressional Record) that warned that they would contest any wind farms "within radar line of site of the National Air Defense and Homeland Security radars." The Federal Aviation Administration (FAA) subsequently began issuing "Notices of Presumed Hazard" to wind projects located within 60 nautical miles of long-range radar installations.

At least 12 projects in Illinois, Wisconsin, North Dakota, and South Dakota received such notices, placing a de facto moratorium on construction. According to a statement issued by Senator Dick Durbin (D-IL) on June 2, the "overwhelming percentage of land in this country is classified as within the 'radar line of sight' and possibly obstructive."

The DoD's final report evaluated several mitigation measures and defined mitigation as follows: "either an approach that completely prevents any negative impact from occurring," or an approach that sufficiently reduces any negative impacts so that "there is no significant influence on the capability of air defense or missile warning radar." The report also noted that the only proven mitigation measure is "line of sight mitigation"—in other words, not placing wind turbines within a radar's field of view. Line of sight mitigation may be achieved by employing the "bald earth" approach (increasing the distance between the radar system and the turbines), terrain masking (hiding the turbines behind a mountain), or terrain relief (when the radar is relatively higher than the turbines).

The impact of the DoD/DHS policy and the FAA actions on wind farm developers nationwide is immediate, significant, and potentially costly. Any new project now must apply to the FAA for a hazard determination prior to starting construction on a new plant. The developer must submit a "Notice of Proposed Construction or Alteration" to the Air Traffic Division Manager of the FAA Regional Office with jurisdiction over the site location. The report notes that specific DoD bases and installations are assigned management responsibilities for specific sections of airspace. Wind developers should continue to work with local base and installation managers to eliminate potential impacts on existing facilities.