[andlier]

Even if we covered the earth in windmills, we'd still only affect the lowest 100-200m of the atmosphere. And windmills can only extract 50% of the winds energy. I'm thinking a forest full of trees might even reduce the windspeed more than a field of mills.

[vlehto]

I'm pretty sure that 50% number is for single mill. For a farm there should be no such limit.

[danmaz74]

Placing a windmill behind another one would be stupid. So, when you have a farm, you still capture a maximum of 50% of the energy that flows through the farm - only, you cover a bigger area and thus you get <= 50% of a bigger mass of moving air.

[Someone]

In (current) practice, yes, but not necessarily.

If windmills are dirt cheap, energy and land scarce/expensive, or transport losses large, a windmill in the wind shade of another windmill could still make economic sense, just as it can make economic sense to install solar cells on roofs in places far from the equator that often have cloud cover.

[Namrog84]

I don't know how practical they are but I've seen that inflatable flying windmill wind turbine thing that is meant to be 500-1000ft or higher? So doesn't that make the 100-200m a bit low on what altitude we can extract from?

[Qwertious]

The inflatable flying windmill thing isn't particularly efficient, the future high-atmosphere wind energy source is much more likely to be kite-based wind:

https://www.youtube.com/watch?v=vMTchVXedkk

[menacingly]

And I assume that the two systems (upper and lower atmosphere) aren't so isolated from one another that they don't interact

[mcv]

50% sounds way higher than I'd expect. Windmills don't exactly block the wind. Most of it just blows through it unhindered.

[Z1nfandel]

In order for the windmill to turn, some energy must be taken from the wind and turned into that rotation. This hinders the wind.

If you were going to try and go turn that windmill, would you just freely spin it with 0 effort? No you'd have to push, it would hinder you.

[toasterlovin]

I think the parent is saying is that, if you consider the circle traced by the outer tips of the windmill blades, the windmill blades themselves take up a very small fraction of the surface area of that circle. Thus, the windmill is fundamentally only capable of capturing a small fraction of the energy that flows through that circle.