Demystifying science | The slowness of wind turbines

Every week, our journalist answers scientific questions from readers.

Why do wind turbines spin so slowly?
– Alain Cornellier

Because the tips of their blades rotate much faster than their centers, and excessive speed must be avoided which could create turbulence and damage the structure or mechanisms of the wind turbine.

“What limits the speed of wind turbines is the speed of sound,” explains Frederick Gosselin, professor of mechanical engineering at Polytechnique Montréal. The tip of the blade travels very fast and the length of the blade multiplies the speed. It’s like an airplane wing: when you get close to the speed of sound, there are all kinds of effects of compressible fluids in the air that affect the behavior of the wing. […] In addition, around the blade, the wind is accelerated, even if it does not appear when looking at it from the ground. »

The speed of sound, or Mach 1 in honor of a 19th century Austrian physicist^e century, varies depending on temperature and altitude. On the cow’s floor and at 15°C, it is 340 m/s, or 1235 km/h. At -25°C, the speed of sound is 10% slower. Ernst Mach was the first to describe the turbulence surrounding an object approaching the speed of sound, the famous “sound barrier” that planes break with a “bang”.

Mr. Gosselin studies the “instability phenomena of slender structures”, such as airplane wings.

“The largest wind turbine blades in the world are manufactured in Gaspésie,” notes Mr. Gosselin. At a radius of 110 m (107 m of blade plus the hub), if the wind turbine rotates at 7 revolutions per minute, that’s a blade tip speed of more than 80 m/s or 290 km/h! At 7 revolutions per minute it seems slow, but it’s so big it goes fast. »

This blade size is used in offshore wind turbines (offshore). Wind turbine blades on land are typically half that size.

Another Polytechnique professor, Ion Paraschivoiu, is the author of the manual Wind Turbine Design, used in universities since its publication in 2002. “I had the chance to work on the Cap-Chat vertical wind turbine in the mid-1980s,” says Mr. Paraschivoiu. It was shaped like an egg beater, with a radius of 32 meters. It was spinning at only 15 revolutions per minute, which is not fast seen from a distance. But the tips of the blades were going fast enough that we had to do stability calculations. »

The Cap-Chat vertical wind turbine was only operated for four years, due to a gust which damaged its mechanism.