wind turbine effects

[GuavaHalf]

There are a growing number of windfarms springing up all over the country. As they extract energy from the wind presumably they must slow the wind down. If there were enough wind farms would the effect be noticable. ie would the average wind speed measureably decrease?

[Longman]

Any object that is exposed to a movement of air will change the wind apeed around it to a greater or lesser degree so in that sense a wind turbine is really no different than, say, a building. Wind turbines don't really 'extract' energy from the wind they mearly convert energy (kinetic presumably) into a mechanical/electrical form. There is a slight possiblility that would would notice a drop in wind speed directly behind a turbine but there would have to be pretty much 100% coverage of turbines for there to be any nticable effect at ground level!

[Longman]

sorry very bad spelling of 'merely' there!

[New Forester]

There's an awful lot of wind, and the turbines can only reach a little of it near the ground -- where it's already slowed a lot by frictional contact with ground, trees, waves etc. That's why they put them on hilltops or at sea, or make them very tall. You'd have to hang them through a large part of the depth of the atmosphere to slow it significantly, and as Longman says, have an awful lot of them. They do have an effect downwind -- you can see roughly how far, by how far apart they put them. I seem to remember that an efficent turbine drops the speed by about 30% immediately downwind. When sailing, you can feel the effect of another sail (say 10 m tall) passing upwind of you from a hundred metres or so away. Probably similar with turbines. NF's pedant wind-up for today: people who call wind turbine generators "windmills". Windmills are mills, for grinding things. Wind turbines are things that turn in the wind. Generators generate. OK, rant over... (You both got it right, anyway).