Permitting setbacks for wind turbines in California

The California Wind Energy Collaborative was tasked to look at barriers to new wind energy development in the state. Planning commissions in the state have developed setback standards to reduce the risk of damage or injury from fragments resulting from wind turbine rotor failures. These standards are usually based on overall turbine height. With the trend toward larger capacity, taller towers and longer blades, modern wind turbines can be "squeezed out" of parcels thus reducing the economic viability of new wind developments.

Current setback standards and their development are reviewed. The rotor failure probability is discussed and public domain statistics are reviewed. The available documentation shows rotor failure probability in the 1-in-1000 per turbine per year range. The analysis of the rotor fragment throw event is discussed in simplified terms. The range of the throw is highly dependent on the release velocity, which is a function of the turbine tip speed. The tip speed of wind turbines does not tend to increase with turbine size, thus offering possible relief to setback standards. Six analyses of rotor fragment risks were reviewed. The analyses do not particularly provide guidance for setbacks. Recommendations are made to use models from previous analyses for developing setbacks with an acceptable hazard probability.

Executive Summary


California counties have adopted setbacks for wind turbines primarily to account for the risk of fragments from the rotor. These setbacks are usually based on overall turbine height, which includes the tower height and the radius of the blade. With evolution in the industry to larger turbines, these setbacks increase in total distance and become a hindrance to wind energy development. The authors present a hypothetical example where the total energy production of a windplant is reduced with the application of larger, modern turbines.


The purpose of this report is to summarize wind turbine setbacks in California and to describe any connection between rotor failure and windplant setback requirements.

Project Objectives

The objectives of this study of wind turbine setbacks were to:

    *  Document and compare current wind turbine setbacks in California
    *  Report on how the setbacks were developed
    *  Report on the probability of rotor failure
    * Study existing analyses of the rotor fragment hazard and determine if setback criteria can be developed with existing information.

Project Outcomes

The outcomes of the project were:

    * The authors gathered information regarding turbine setbacks by interviewing county planning personnel, studying the county ordinances, and conducting a literature search of the subject. Wind turbine setbacks were documented for California counties with existing and future wind energy development, including Alameda, Contra Costa, Kern, Merced, Riverside, and Solano counties. Comparisons were made between the various ordinances.
    * From this data the authors developed a picture of how the turbine setbacks were established. The majority of the ordinances were developed by ad hoc groups of local interests and the fledgling wind energy industry.

    * The authors conducted a literature survey regarding the probability of rotor failure. Several sources of information were obtained. These include failure reports of turbines in Alameda County, failure data from Denmark and Germany reported in the WindStats periodical, and a Dutch report on European rotor failures. The probability of rotor failure varied from 1-in-100 to 1-in-1000 turbines per year.

    * The authors present a simplified analysis of the rotor fragment hazard to compare to more complex analyses. The analyses of six researchers were found in a literature survey of varying complexity. Results were compared to determine if setback criteria could be developed.


Wind turbine setbacks vary by county. The counties typically base the setback on the maximum of a fixed distance or a multiple of the overall turbine height. A common setback is three times the overall turbine height from a property line. There is no evidence that setbacks were based on formal analysis of the rotor fragment hazard.

The most comprehensive study of wind turbine rotor failures places the risk of failure at approximately 1-in-1000 turbines per year. The maximum range of a rotor fragment is highly dependent on the release velocity that is related to the blade tip speed. Tip speed tends to remain constant with turbine size; therefore, the maximum range will tend to remain constant with turbine size. In the analysis of rotor fragment trajectories, the most comprehensive models yielded results that showed the shortcomings of simpler methods. Overall, the literature shows the possibility of setbacks for larger turbines may be based on a fixed distance and not the overall height.


The authors recommend that a comprehensive model of the rotor fragment hazard be developed based on the results of the literature review. This tool would then be used with a variety of turbine sizes with the objective to develop risk-based setback standards.

Benefits to California

The information provided in this report can be used by California planning agencies as a background for evaluating wind turbine setbacks. Researchers can also use the information as background for developing models of the rotor fragment hazard.

Permitting Setback Requirements

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NOV 1 2006
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