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I have noticed that windmills are generally built in empty fields with no trees around, and I've been wondering why...

A windmill is generally way taller than trees and I can imagine that trees don't actually affect the flow (see picture).

enter image description here

But is this the reason why there is nothing around or is there something else that makes them waste so much space around? And is the velocity profile from the first figure a realistic one?

If I search for the theory of external convection, the velocity profile looks like this for flow over a plateau: enter image description here In the second case, wouldn't the velocity be higher if there is an obstacle? Or is it because the obstacle is a porous medium that damps the flow? And lastly, is the velocity profile at earth level looking like that? Is there really an origin to it, or is it fully turbulent all the way?

Physther
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3 Answers3

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Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue.

One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book "Wind Energy - The Facts". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence.

Open land also makes physical access to the site easier. That's particularly important during construction, when very very long vehicles carry each blade (40-60 metres long) in one piece.

enter image description here

410 gone
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There is a non-engineering reason as well: the people most open to placing turbines on their land are farmers who are interested in the extra income.

Turbines are not (yet) placed near built-up areas, which generally leaves large farms, undeveloped lots and nature reserves. Getting a building permit for a commercial enterprise in a nature reserve is close to impossible, and undeveloped lots may be zoned for housing, which would also prohibit the erection of wind turbines.

Simon Richter
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Look at how and why the "Betz limit" works.

As energy is lost in your first diagram to the trees, the airflow through and around the trees expands and slows down.

That slowdown reduces the energy available to the turbine.

(note. I am not the resident wind energy expert!)

user_1818839
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