Sunday, September 11, 2005

Some data on blade failure and throw

At the 2004 California Wind Energy Collaborative Forum (click the title of this post for proceedings, both PowerPoint PDFs and audio MP3s), Scott Larwood of the California Wind Energy Collaborative, University of California, Davis, presented "Permitting setbacks for wind turbines and the blade throw hazard."

His research concludes that a reasonable expectation for blade failure is 1 per 100 turbines per year. Thus setbacks in consideration of blade or fragment throw are indeed important to establish.

His calculations (or his reporting of a Danish study) establishes, first of all, how far a blade or fragment could be thrown at tip speeds at and above the normal operation maximum, expressed as multiples of the total turbine height, using data for 1.5–2.0-MW turbines. At the normal maximum, a blade could be thrown to a distance almost 1.5 times the turbine height and a hazardous fragment over 3.5 times. At twice the normal tip speed, a complete blade could be thrown over 2.5 times the turbine height and a hazardous fragment almost 6.5 times. The maximum fragment distance is 6.5 times the turbine height.

Second, Larwood calculates blade and fragment thrown as a function of turbine height, finding that as height increases, the absolute distance they might be thrown increases, but as a multiple of turbine height it decreases. For example, a 50-m (164-ft) turbine (height to blade tip) could throw a whole blade about 120 m (2.4 × ht) and a fragment over 250 m (5 × ht); a 100-m (328-ft) turbine could throw a blade about 125 m (1.25 × ht) and a fragment about 375 m (3.75 × ht).

Larwood does not recommend specific setbacks, presumably because they involve other considerations as well, such as noise, high voltage, and visual intrusion.

On another note, he cites the distance the turbines should be from each other for minimal wind interference: three rotor diameters when aligned perpendicular to the wind and 10 rotor diameters when parallel to the wind. Thus, the GE 1.5-MW turbine, with a 70.5-m rotor span, requires 37-123 acres per tower. Each Vestas V90 1.8-MW turbine, with a 90-m rotor, requires 60-200 acres.

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