Ice throw from wind turbines
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To the Editor:
Reprint of an email about ice throw at Searsburg, written by John Zimmerman to an American Wind Energy Association listserv in 2000. Mr. Zimmerman managed the development of the Searsburg facility:
"I've watched over the wind turbines GMP 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 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 from 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.
[Email from John Zimmerman, VERA, to listsaver-egroups-awea-windnet@egroups.com, Thu, 20 Jan 2000 10:51:43 -0500.]