Wind Power and Soil

I recently had the somewhat questionable pleasure of driving across most of the Great Plains.  It has been over a decade since I last did a long distance drive across the Plains, and a new feature is starting to pop up: Wind turbines.  Sometimes in ones and twos, sometimes by the tens or hundreds.  I may be biased, but I find modern wind turbines to be among the most beautiful built structures in the world.  They have a slow, graceful motion that belies their Brobdingnagian scale.  They were particularly beautiful on a foggy evening driving as I drove through a wind farm near dusk, when I could see only the bottom half of the giant blades as they swept gracefully down out of the mist in a slow motion appearing and disappearing act.

The other feature of the generally broad and open landscape were lines of trees, sometimes bordering the interstate, and sometimes bordering fields.  I recall from a US history class in high school that these wind breaks were planted in response to the 1930′s dust bowl.  A little web research led me to the Shelterbelt Project, which seems to be what I recalled (somewhat inaccurately) from high school:

Established by President Franklin D. Roosevelt under executive order on
July 21, 1934, the Shelterbelt Project provided for a tree barrier one hundred miles wide extending twelve hundred miles north to south from the Canadian border through the Texas panhandle. It was designed to
reduce wind velocity, which had occasioned severe soil erosion across the Midwest and dust storms to the eastern seaboard.

In some ways, the Shelterbelt project can be seen as an early experiment in geoengineering. I sincerely hope that any future projects are so successful and benign.

Wind turbines, too, reduce wind velocity.  After all, a wind turbine’s function is to take wind energy, and convert it to electricity.  This led me to wonder just how many turbines would it take on the Great Plains to significantly lower the average wind speed in the region?

According to FTExploring, a wind turbine can extract about 35% of the wind energy passing through the swept area of its blades.  A typical 2.5 MW wind turbine from General Electric (GE) has a rotor diameter of 100m.  To get a ball-park figure, imagine two rows of GE 2.5MW turbines were installed from north to south along the Shelterbelt project (1200 miles) with rotor blade tips inches apart.  If the two lines were offset, wind blowing from east to west or west to east would have to pass through one or two rotors, losing 35% to 58% of its energy along the way, and exiting the back of the turbines 15% to 25% slower.

Such a double row of turbines would require about 386,000 turbines, or about 1 million MW of wind.  So, according to this back-of-the-envelope calculation, 1 million MW of wind installed in the Great Plains (even if not installed in a north-south line) should be enough to noticeably decrease the overall wind speeds in the region, and not only reduce soil loss from wind, but also reduce the cost effectiveness of installing more turbines.  Assuming a 35% capacity factor, this equates to about 3,066 million MWh.  In 2008, the US produced 4,119 million MWh of electricity, so 1 million MW of wind represents about a 75% of electricity production from wind in the Great Plains.  Even if there were sufficient transmission to distribute the power across the country, and geographic diversity greatly moderated the the overall variability of wind, such high penetrations would be impossible without prohibitive investments in electricity storage. 

With current storage technology, a greatly enhanced national grid, and a full roll out of smart grid technology used to better match demand to supply, I would guess that the upper limit for wind penetration would still be only 50% (and considerably lower if any of these things fail to materialize, especially the diversifying benefits of a robust national grid.)   This upper limit (and the fact that only a fraction of wind power is likely to be generated on the Great Plains) means that we’re probably unlikely to need to cut down any trees on the Great Plains in the hope of increasing the wind output of out turbines.

The United States had a cumulative 35,000 MW of wind installed by the end of 2009 (about 3.6% national penetration using the numbers above) so we’re still a long way from slowing the wind significantly on the Great Plains, or anywhere else.

I wonder if farmers who lease some of their land to wind farms notice any local slowing of the wind?  Is that a positive externality worth accounting for?

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10 Comments

  1. David Levy said

    Tom,
    I agree about the graceful poetry of large wind turbines, but a bit doubtful that the trees or turbines could do much to slow the overall wind velocity. They might have some effect on the next field or two, but remember that they are only absorbing energy from the first 100 meters of atmosphere – you have to think about the total wind energy passing through a much larger vertical area. Its true that close to the ground the air is more dense, but wind speeds are higher at higher altitudes. I’m not a fluids dynamics expert, but suspect that the wind at higher levels soon mixes back with ground level air. Think about a small obstruction at the edge of a river, say 1% of its width – creates a few eddy currents for a while…

    • Tom said

      I agree with you that how much slowing would occur is open to question… in other words, if we do slow the wind in the first 100 meters, how soon will energy from higher wind layers speed it back up. I need to ask an expert.

      On the other hand, the Shelterbelt project worked- it slowed bind speeds at ground level and greatly reduced dust storms.

    • Tom said

      One other thought on that… I think I got the germ of this idea from a talk by James Hansen. I recall him saying (with no detail) that most estimates of wind power potential for the Great Plains were too high, because once wind farms reached a certain density, they would lower the overall amount of wind energy available.

      Unfortunately, I don’t have a reference other than my memory.

  2. daniel ziskin said

    Wind near the surface is turbulent and sporadic. Wind at higher altitudes is streamline and far more regular. At this latitude it blows steadily from west to east with a few north-south wiggles. It would be nearly impossible to make a dent in the wind flows at heights above 50m. So I agree with a previous post…even if the wind at the surface was blocked by an impenetrable wall running north-south across the great plains, energy and momentum from higher altitude winds would replenish the surface winds. Consider, for example what happens when there is a huge mountain range (the Rockies) blocking all the surface wind from the western slope. We get even more wind as air masses rush down the mountains.

    • Tom said

      Thanks for the quick response, Daniel. (Daniel is my atmospheric scientist acquaintance.) I probably should defer to your judgment here, but your reasoning does not seem to follow for me.

      Regarding the laminar flow above 50m, the GE turbines I used in the example have a 100m diameter, and since hub heights are well above 50m so that the turbines can take advantage of the laminar flow you refer to, the turbines will be drawing wind energy from this laminar layer. So we are not talking about laminar wind at 50-100m re-accelerating wind below 50m, we’re talking about wind at 150m+ re-accelerating wind below 150m.

      Surely the reason that the first 50m is relatively slow and turbulent, is trees and other obstructions on the ground? When the obstructions start rising to 150m in large numbers, as with a wind turbines, would this not slow the flow at that level as they become more numerous?

      Regarding the Rockies, I don’t see the analogy. Indeed, the Rockies block wind and also create wind… but how would wind turbines create wind?

      The calculation I would have liked to do goes like this. The total power of wind on the Great plains up to 150m is P (I could not do the calculation because I don’t know P.) If P is say, 10 million MW, then 1 million MW of wind turbines would reduce the energy of the wind by 10%, and wind speed would be reduced by about 3.5% (since speed is the cube root of power.)

      So P depends on how high up we should go before we decide that the energy in the wind should not be included in this calculation. We know that there is an upper limit because the jet stream does not have a significant effect on surface wind speeds.

      Thoughts?

      • Daniel Ziskin said

        It’s true that I’m an atmospheric scientist by training – but this is out of my professional expertise. I’m arguing by physical intuition so I welcome the challenge.

        First – let me remind you that wind is created by at least four factors.

        1)The uneven heating of the Earth’s surface (both diurnally and latitudinally) creating air pressure differences between here and there.

        2)The rotation of the Earth dragging the atmosphere with it by friction.

        3)Orthographic effects – air is a fluid and will flow down slope.

        4)Stirring and mixing by motion.

        Of these forces, I would suggest that the fourth is significantly the smallest. If the wind on a large scale were affected by surface motion then we would feel the effect of highways and airports at a distance. So by analogy if we cannot create much wind by speeding vehicles, we probably cannot stop much wind with turbines soaking up some of its energy.

        I agree that the turbines are higher than the surface and into the laminar regime. And I believe that in that case – force #4 becomes even less significant.

        Here’s another approach…Kinetic Energy is defined as 1/2 m v^2. So for a m^3 of air moving at 5 m/s we get:

        K m^-3 ~ 1/2 * (1 m^3 * 1 kg/m^3) * (5 m/2)^2 ~ 12.5 J m-3

        The area of a 100m diameter turbine ~ 7.8E3 m^2

        So in 1 second we get:
        P = (7850 m^2) * (12.5 J m-3) * (5 m/s) ~ 0.5 MW

        I’m not sure whether this calculation is relevant except to show that there is plenty of KE in moving air.

      • Tom said

        Daniel,
        You seem to be challenging the assumption that a wind turbine extracts about 1/3 of the energy that passes through its swept area, a result I got (but did not check) from FT exploring (See the article for the link.)

        Anyway, I’m now approaching this with more basic physics… conservation of energy. Please take a look at that article… after those calculations I ended up agreeing with you, but there is still an open question about your 1) above…

        BTW, your #2 does not make a lot of sense… our frame of reference here is the Earth’s surface, in which frame of reference there is no rotation of the earth’s surface. Are you referring to (nonfictional) Coriolis and gyroscopic effects?

  3. [...] under Wind power, wind ·Tagged atmospheric science, Great Plains, wind I received serious skepticism to my idea that wind turbines could significantly slow the wind speed on the Great Plain…. One of the criticisms came from an atmospheric scientist I asked to weigh in on the matter. The [...]

  4. I’m not a atmospheric scientist either, so with respect to the science I’ll just read and learn. But your description of a foggy drive past wind turbines caught my attention.

    Just in early January I had occasion to drive on a foggy day from Lubbock to Dallas along a route that takes me straight through the Roscoe/Sweetwater/Abilene area where much of Texas’s wind energy is. On clear days, for nearly a 100 mile stretch there is always a wind turbine in sight. On the day of my drive I probably went about 30 of those miles before seeing as much as a turbine pole, and another 10 or 15 before I saw a turbine blade spin.

    It was an almost eerie feeling, like I was driving among invisible giants.

  5. [...] This is no surprise to anyone. Trees and other objects on the ground slow the wind, and as you get higher, you enter the region of … [...]

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