SOLAR TODAY - March 2009

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Zoning for Small Wind Turbines
A reasonable setback for a wind turbine tower
is equivalent to the height of the structure itself.
By MICK SAGRILLO

Mick Sagrillo, a small-wind consultant, owns Sagrillo Power & Light and is wind energy specialist for Focus on Energy, Wisconsin’s renewable energy program. Research for this article was funded in part through Wisconsin’s Focus on Energy Program. Contact him at msagrillo@ wizunwired.net.

SOUTHWEST WINDPOWER

Wind tower setbacks don’t have to be onerous, but consider the possibility that a power line could fall against the tower.

The past two columns have examined zoning considerations for small wind turbines. In the first, we explained why wind turbine towers need to be so tall, while the second laid out the best practice rules for determining a wind turbine’s minimum tower height. In this column, we’ll look at the issue of setbacks for towers.

Let us assume that zoning regulations on small wind turbines exist to protect public health and safety. We will not address the third leg of many zoning ordinances, general welfare, a catch-all category for noise, visual impact and other annoyances. Health and safety are objective categories that can be backed up with facts and experience. General welfare is far more subjective, often based on emotion, whim, what’s in vogue, “community standards” or pressure from a disgruntled neighbor. As a result, regulating on subjective criteria is categorized as “arbitrary and capricious” in legal terms.

On the other hand, health and safety threats are more or less easily documented and verified. I say more or less, because some groups opposed to wind farms have imagined a variety of health threats. Over the 30-year history of wind in the United States, we have a complete lack of evidence that wind turbines pose any kind of health threat. So we will address only the safety concerns of small wind turbines, with the goal of meeting the concerns of the applicant, the neighbors and the local zoning authority.

Misunderstandings

People have erroneously referred to the need to protect the “fall zone” around a wind turbine. The “fall zone” thus became a circle of land around the base of the tower where the tower might land should a catastrophic failure occur. The term “fall zone” conjures up a vivid image of the consequences of what might happen in that area. The only problem with the term is that few wind turbine towers have ever fallen over.

There are several reasons why few towers have fallen completely. The first is the engineering involved. Conventional engineering calls for a safety factor of twice the failure mode. Conventional engineering for towers, however, often exceeds this margin of safety, frequently going as high as five times the failure mode. Towers, based on the number of failures seen in the field relative to other structures, are pretty safe structures. This includes their foundations.

In addition, when we analyze the failure modes for towers of all sorts (communication, utility, observation and wind turbine towers), we find that towers, when they

do fail, tend to buckle, not fall over. It is actually quite a rare tower failure where the tower falls completely over, pivoting at its base.

Reasonable Setbacks

Zoning regulations frequently include setbacks (from the property line) for various types of construction, so it is common to delineate a setback for a wind turbine.

A distance that is reasonable to protect safety is a setback from the tower base equal to the “total structure height” of the tower with one blade raised in the upright position. This is the tallest that a wind turbine and tower could ever be. For example, a wind turbine with a 10-foot blade on a 120-foot (36-meter) tower would dictate a setback of 130 feet ( 39 meters).

This setback can be required from adjacent property lines or from neighboring inhabited structures, but also from overhead utility lines and roads. In theory, a setback can be shortened with the permission of the affected party, usually an adjacent landowner or the local department of transportation.

It is my experience that, for liability reasons, utilities will not grant permission to do anything. Utilities typically have far shorter setback requirements. Just look at the proximity of trees to utility lines, and the power outages that occur when limbs and whole trees fall over. It is still good practice to adopt a total structure setback from overhead utility lines. I know of no wind installers who are interested in getting close to a utility line while climbing a tower. For that matter, you don’t want the utility line falling against the tower, either.

Finally, the tower setback does not apply to the homeowner’s residence or other structures on the property.

Are Greater Setbacks Needed?

While some neighbors may call for greater setbacks for any number of subjective reasons, there is really no rational justification based on safety for a setback greater than the total structure height.

On the other hand, there are many instances of shorter setbacks for wind turbine towers. I know of one tower only four feet from the property line. This distance accommodates the foundation that extends four feet from the base of the tower. The adjoining neighbor made no objection to the installation.