Land hurricane tests wind turbines
Posted at 8:54 AM on October 29, 2010
by Mark Steil
This week's 'land hurricane' was a first time event for Minnesota's wind power industry. Wind turbines have been part of the state's landscape for about twenty years, but the storm was a real test.
Dan Juhl has been involved with the business from the start, he currently owns or manages about 50 wind turbines on southwest Minnesota's Buffalo Ridge. Juhl says he's never before seen a storm where the winds have been "so sustained, at such a high level, for so long."
National Wind Assessments of Grand Forks have data to back up Juhl's claim. Debbie Jacklitch-Kuiken is project manager for the company. It has wind measuring towers throughout the Midwest.
Jacklitch-Kuiken says a tower near Atwater, Minn., recorded a gust of 77 miles per hour Tuesday afternoon at 4:20. The gust was measured 325 feet above the ground. At the same time, a gust of 76 mph was measured at the tower's 263 foot level.
Dan Juhl says the winds were the strongest he's ever seen outside of brief gusts in a thunderstorm. He says most of his turbines automatically shut down during the storm as a self protection measure. Juhl says the turbines automatically shut down when sustained winds reach 55 mph. The biggest machines are more than 200 feet tall.
Juhl says at 55 mph the blades of the wind turbine "pitch so their leading edges are facing into the wind instead of the flat part of the blade". He says that means the blades catch only a little wind, slowing then stopping the rotation of the blades.
It's like holding your hand out the window of a car. Face the palm of your hand into the wind and you can feel the push of air. Place the edge of your hand against the wind and the pressure practically disappears.
Juhl says "the forces are just staggering" when wind speeds reach this week's level.