Calculating Wind Turbine Setbacks With Science Instead of Politics

Wind turbine blade failure happens – setbacks are important for public safety

If there is one hot button issue in communities where wind turbines are being discussed, it’s setbacks, how far must wind turbines be kept from property lines, roads and occupied buildings. Wind developers want a very short distance so they can install the maximum number of turbines while residents and property owners want them kept as far away as possible, so the effects of the turbines are minimized. Wind developers come prepared with the usual rules of thumb or so called “industry standard” setbacks and seem shocked and amazed when anyone questions their numbers. The one thing they don’t provide is any basis for those “industry standards.” It’s almost as though they made them up out of thin air. There has to be a better way, and there is.

Engineering analysis or a finger in the wind

Recently, we discovered an excellent research article written by three aerospace engineers: A method for defining wind turbine setback standards, published in the journal Wind Energy. In the introduction they explained why they did the research:

These setback standards are intended to protect people and property from rotor blade fragments released from failed wind turbine blades. However, required setbacks are often based on rules of thumb involving some combination of turbine height and blade radius and typically have little or no rigorous physical foundation. There is currently a strong demand for re-evaluation of turbine setback distances in view of both increased turbine reliability and the desire to install more large turbines on small parcels of land. Specifically, it would be desirable to provide a technique that allows regulators and wind farm developers to determine setback requirements given a specific turbine model, the site parameters and an acceptable level of risk.

We couldn’t have said it any better ourselves.

Three turbines, surprising results

The engineers selected three different sized turbines for their study, a 660 KW, a 1.5 MW and a 3.0 MW. Blade radius for each respectively was: 77 feet, 115 feet and 148 feet with hub heights of 164 feet, 262 feet and 262 feet.

The full research article explains exactly how to calculate throw distances based on the operational specifications of specific wind turbine models. In all cases in the article, even with the smallest 660 KW turbine, the throw distance was far greater than the 1000 foot 1.75 times turbine height setback from a property line called for in the proposed wind ordinance for North East Township.

Though it may be reassuring to use a very simple and easily understood formula for setbacks, as the township officials have done in the ordinance, this is one instance where what they have called for is very simple and wrong.

Public safety demands objective analysis, not political compromise

In the last public meeting in North East Township, one supervisor, when asked where the 1000 foot number came from said it was a compromise. Unfortunately, compromising public safety is a very poor choice, especially when there is an objective method of determining setbacks, as illustrated in this article.

In light of this new information, and especially because the wind turbine manufacturers themselves call for a 500 meter (1640 foot) evacuation radius around a malfunctioning turbine, the township supervisors and the planning board are very strongly advised to rewrite the setback requirements in this ordinance to protect the interests of all residents of North East Township. Public safety demands it.

It would be extremely unfortunate if township officials were to insist on using any other factors besides public safety to influence their decisions and if they do so, the residents, taxpayers and voters will be asking for a full explanation of how their decision was made.