Editor's note: This short document prepared by personnel at the Wind Application Engineering Group of GE Energy provides some insight into the risks of ice throw. The content of this document confirms that ice fragments can be thrown large distances from the turbines.

Selected Extract:

 

The Risk

The Accumulation of ice is highly dependent on local weather conditions and the turbine's operational state. Any ice that is accumulated may be shed from the turbine due to both gravity and the mechanical forces of the rotating blades. An increase in ambient temperature, wind, or solar radiation may cause sheets or fragments of ice to loosen and fall, making the area directly under the rotor subject to the greatest risks. In addition, rotating turbine blades may propel ice fragments some distance from the turbine -- up to several hundred meters if conditions are right. Falling ice may cause damage to structures and vehicles, and injury to site personnel and the general public, unless adequate measures are put in place for protection.

Risk Mitigation

The risk of ice throw must be taken into account during both project planning and wind farm operation. GE suggests that the following actions, which are based on recognized industry practices, be considered when siting turbines to mitigate risk for ice-prone project locations:

Turbine Siting: Locating turbines a safe distance from any occupied structure, road, or public use area. Some consultant groups have the capability to provide risk assessment based on site-specific conditions that will lead to suggestions for turbine locations. In the absence of such an assessment, other guidelines may be used. Wind Energy Production in Cold Climate provides the following formula for calculating a safe distance:

While this guideline is recommended by the certifying agency Germanischer Lloyd as well as the Deutsches Windenergie-Institut (DEWI), it should be noted that the actual distance is dependant upon turbine dimensions, rotational speed and many other potential factors. Please refer to the References or more resources.

Physical and Visual Warnings: Placing fences and warning signs as appropriate for the protection of site personnel and the public.

Turbine Deactivation: Remotely switching off the turbine when site personnel detect ice accumulation. Additionally there are several scenarios which could lead to an automatic shutdown of the turbine:

- Detection of ice by a nacelle-mounted ice sensor which is available for some models (with current sensor technology, ice detection is not highly reliable)

- Detection of rotor imbalance caused by blade ice formation by a shaft vibration sensor; note, however, that it is possible for ice to build in a symmetric manner on all blades and not trigger the sensor

- Anemometer icing that leads to a measured wind speed below cut-in.

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