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Ice throw from wind turbines: Assessment and risk management 

Author:  | Norway, Safety


What is in-cloud icing?

  • If temperatures are below 0°C and the structure is located inside a cloud (above cloud base height) we get in-cloud icing.
  • The ice accretion rates increases with the relative windspeed and the moisture content of the cloud.
  • Because the blade of a wind turbine moves fast there is an elevated hazard associated with ice throw and fall from turbines located in icing conditions.

How far can the ice be thrown?

  • Maximum throw distance (screening) : 1.5 × (Diam. + Ht.). ~ 350 m.*
  • Ice debris have so far not been found at this distance.
  • Ice pieces have been found at 68 % of the maximum throw distance.
    • 1.4 × tip height (Cattin). 1000 ice pieces with 3 % above tip height
    • 1 × tip height (Lunden, 2017). 500 ice pieces total.

*Strict German/Austrian regulations

  • Seifert screening formula of danger zone: (Hubheiht + rotordiameter) × 1.5
  • In Germany/Austria it is required to have ice detection systems if there are roads or buildings within this distance.
  • Restriction on production: turbine must stop when there is icing.
  • If detection systems are reliable and sensitive, then the potential hazard is most likely associated with ice fall and not throw of smaller ice pieces.

How dangerous is the ice?

  • An impact kinetic energy of more than 40 J is considered fatal.
  • 40 J correpsonds to a 0.2 kg ice piece with density 500 g/dm3 falling from an elevation of 30–50 m.
  • Because of the turbine height all ice pieces larger than approximately 0.2 kg are potentially fatal.

How large a risk can we accept?

Localized individual risk metric: the probability that an average unprotected person, permanently present at a specified location, is killed in a period of one year due to an accident at a hazardous installation

Acceptable risk:

  • Ski tracks, hiking areas < 10−4
  • People walking along public road, industrial sites, scattered houses < 10−5
  • Houses, cafés, shops, etc. < 10−6
  • Schools, kindergartens, shopping malls, hospitals, etc. < 10−7

See also: R.E. Bredesen, H. Farid, M. Pedersen, D. Haaheim, S. Rissanen, G. Gruben and A. Sandve, “IceRisk: Assessment of risks associated with ice throw from wind turbine blades” (PO.339). https://windeurope.org/summit2016/conference/allposters/PO339.pdf, in WindEurope Summit, Hamburg, 2016.

Rolv Erlend Bredesen, Kjeller Vindteknikk
IEA Wind Task 19, Winterwind 2017
February 15, 2017

Download original document: “Ice throw from wind turbines: Assessment and risk management

This material is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

The copyright of this material resides with the author(s). As part of its noncommercial educational effort to present the environmental, social, scientific, and economic issues of large-scale wind power development to a global audience seeking such information, National Wind Watch endeavors to observe “fair use” as provided for in section 107 of U.S. Copyright Law and similar “fair dealing” provisions of the copyright laws of other nations. Queries e-mail.

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