They produce clean energy, sure, but they can also hurl a mean snowball.

DECEMBER 9, 2008 | KATE GALBRAITH | New York Times

How do wind turbines fare in winter weather?
Not so well, according to one little town in England. The Wisbech Standard reports a harrowing tale in which “lumps of ice three or four feet long flew through the air” and smashed into a carpet showroom and a parking lot.

They apparently came off the spinning blades of a 410-foot-tall wind turbine.

No one was hurt, but residents of Whittlesey, in the southeastern part of England, would not rest until the turbine was shut down. One local businessman described the ice shards as “javelins” coming off the blades.

The wind industry concedes that, as with all tall things (buildings, for example, or trees), ice and snow can build up and, eventually, fall down, creating a hazard to people and structures below.

But the industry denies that “ice-throwing” — another concern surrounding wind power — is a problem. “Ice can end up at places other than exactly at the base of the turbine, but it’s a myth that a turbine will (and can) operate at high speed with ice on it and fling ice for miles,” said Ron Stimmel of the American Wind Energy Association, in an e-mail message.

Just as an airplane will not be able to fly with too much ice on its wings, Mr. Stimmel said, wind turbines are designed to stop or shut off automatically, he said, when they sense the extra weight of ice.

Its recommendations include placing fences and warning signs around turbines, and locating them a safe distance from buildings or roads. They also recommend deactivating turbines when ice begins to form.

Swiss report  last year, titled “Wind Turbine Ice Throw Studies in the Swiss Alps,” focused on a turbine near a ski area. That report found ice throw to be a “significant safety risk.” The most dangerous place for ice was underneath the turbine, but about 5 percent of fragments landed more than 80 meters — or 260 feet — from the turbine.

A chart from the study shows where and how far ice and snow were flung, relative to the position of the turbine:

Distribution of ice throw relative to the wind turbine.  
(Source: Wind Turbine Ice Throw Studies in the Swiss Alps)

As a general recommendation, it can be stated that wind farm developers should be very careful at ice endangered sites in the planning phase and take ice throw into account as a safety issue. Each incident or accident caused by ice throw is an unnecessary event and will decrease the public acceptance of wind energy.

via Ice-Tossing Turbines: Myth or Hazard? – The New York Times