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Documents > Projection de débris > Permitting setbacks requirements for wind turbines in California

Permitting setbacks requirements
for wind turbines in California

Prepared by:
Scott Larwood and C. P. van Dam
California Wind Energy Collaborative
Davis, California

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Prepared for:
California Energy Commission
Public Interest Energy Research (PIER) Program

November 2006


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.


Courtesy of Lisa Linowes, president of Industrial Wind Action Group

About the report she said: "I've had several conversations with the author, Scott Larwood, and he acknowledged the study did NOT consider:

  1. ice fragments that can fly away from the turbine;
  2. noise, shadow flicker or other issues created by a properly working wind turbine;
  3. smaller fragments that may have blown much further distances after the failure occurred."


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