Here is a reprint of an email about ice throw at Green Mountain Power's Searsburg wind energy facility in Searsburg Vermont. The email was written by John Zimmerman to an American Wind Energy Association listserv in 2000. Mr. Zimmerman managed the development of the Searsburg facility
E-mail from Randy Swisher (AWEA)
Date: Thursday, February 26, 2004 6:37 PM
Fwd: Re: Ice shedding from turbines and public
Here is a comment from John Zimmerman. He states that wind turbines don't belong at ski areas, but I think it is really just a question of what is the appropriate setback. John describes some of the ice they've seen at the Searsburg site and it sounds pretty intimidating but manageable with proper setbacks.
I think that is the last of the information I have on the topic. It isn't a lot, but I hope you find it helpful. Please let me know if you need anything else, and feel free to consult our web site at www.awea.org as well.
Thanks again for your interest. I hope next time I'm riding my bike in the Northeast Kingdom I will actually have a chance to see a few wind turbines!
From: "John Zimmerman"
To: Vindpet~groups.com> AWEA / John
Date: 20 Jan 200010:51:43 -0500 Zimmerman (Vera)
Organization: VERA Icing
Subject: [a-w] Re: Ice shedding from turbines and public safety
Jim and Peter,
I've watched over the wind turbines GMP [Green Mountain Power - Searsburg]has had installed in Vermont over the last 10 years and have several thoughts that be useful to this discussion.
Here in Vermont, and elsewhere in the northeastern US, the winds blow at the strongest at the mountain tops, where it is also the most icy. A common first question to wind developers in this region is 'why don't you put the wind turbines at the ski areas (where there already is human development)'? The answer is because of the danger to public safety due to ice throws. Ski areas are not a good place for wind turbines.
Back in the mid 1980s one of the windy areas that was being considered for wind development was near to ski trails. Boeing and/or Hamilton Standard did some work to determine how far we must stay away from the ski trails to be safe from ice being thrown trom their turbines (the MOD 5b was the boeing machine at the time). Without going back to dig up those papers, and if I remember correctly, the distance was between .25 and .5 miles away, downwind. It's a function of blade tip speed, so applicable to present day turbines too.
While the Boeing study was academic, the danger from ice being release from rotor blades overhead is real -- and a hard hat is not going to provide you with much comfort. I have stood near the turbines GMP had on Mt. Equinox in the early 1990s and more recently the Zond 500 KW turbines in Searsburg Vt during and after icing events. When there is heavy rime ice build up on the blades and the machines are running you instinctually want to stay away.
They roar loudly and sound scarey. Probably you would feel safe within the .5 mile danger zone however.
One time we found a piece near the base of the turbines that was pretty impressive. Three adults jumping on it couldn't break. It looked to be 5 or 6 inches thick, 3 feet wide and about 5 feet long. Probably weighed several hundred pounds. We couldn't lift it. There were a couple of other pieces nearby but we wondered where the rest of the pieces went.
In the winter, icing is a real danger and GMP therefore restricts public access to the site(s). Maintenance workers have developed a protocol for working on turbines during icing conditions, though I am not familiar with the details. I'll 'dig into it' if you want.