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Wind Power and Energy

  1. Jan 10, 2006 #1

    G01

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    I was wondering if anyone would be able to answer this question for me:

    Where the the energy collected from wind power come from? Now I know that it comes from wind (obviously) but where does the wind get its energy?

    I was thinking the energy came from the earth's rotation, and that havesting wind energy would steal a very small amount of energy from the rotation of the earth. Am I on the right track here?
     
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  3. Jan 10, 2006 #2

    LURCH

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    Sort of. The energy mostly comes from the Sun, which heats air thereby causing it to expand and contract, rise and fall. This heating however would have much less effect if the rotation of the planet were not causing the surface to heat up and cool down as it faces toward and then away from the Sun. It is, of course, the changes in temperature that generate most of the movement of the air.
     
  4. Jan 10, 2006 #3

    rcgldr

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    Isn't there also an effect from the air trying to circulate around a rotating sphere?
     
  5. Jan 10, 2006 #4
    The main reason for wind to blow is the temprature difference caused by either sun -directly- or the poles. Sometimes rotation of the earth also has a part to play - jet stream, you might have head of, is due to the rotation of the earth.
    But no harvesting wind energy will not "steal" away anything from earths rotation. for example, sitting next to a room heater or campfire won't affect the heat energy it radiates.
     
  6. Jan 10, 2006 #5

    Mk

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    But could it affect wind currents?
     
  7. Jan 11, 2006 #6
    It depends, around the blades of a wind turbine there will be some swirling, rotating and kind of random currents in the wind. But at a distance everything will be fine again.
     
  8. Jan 11, 2006 #7
    This site has different meteorologists explaining the phenomenon of wind:

    http://www.newton.dep.anl.gov/askasci/wea00/wea00187.htm

    It's common to say that hot air rises and cold air rushes in to take it's place. This implies the hot air rises by itself creating a vaccuum that sucks the cold air in. Actually, though, what's happening is the opposite: cooler air is denser and therefore weighs more per unit volume. It exerts more pressure in all directions and squeezes the less dense, warmer air up to higher altitudes by the force of buoyancy. The movement of the cooler, denser, higher pressure air into the lower pressure zones is wind. It's more comprehensive to understand wind as caused both by the sun and by gravity.

    As described at that site, the rotation of the earth adds an extra twist to the general phenomenon of high pressure air moving toward the equator displacing low pressure air toward the poles.
     
    Last edited: Jan 11, 2006
  9. Jan 11, 2006 #8

    DaveC426913

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    In principle, anything that makes the surface of the Earth "rougher" will have an effect on weather by friction. Wind turbines are particularly rough. While we won't notice any effects in the short term, it would (again, in principle) have an effect in the very long term. Turbines will "steal" wind from the air - wind that will not go into erosion and other natural processes - and will instead divert that energy and ultimately convert it into heat released from "heat islands" (cities).
     
  10. Jan 11, 2006 #9
    A windmill removes energy from the wind for sure. That has to have "an effect". But is the effect it has really of importance? Probably not at all. Consider the effect on the wind of planting a tree instead of erecting a windmill. The drag created by the tree will remove energy from the wind, but no one would think to be concerned about it.
     
  11. Jan 11, 2006 #10

    DaveC426913

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    Well, mostly true, yes.

    One tree will steal "one tree unit" of energy from the wind and distribute it locally. Many trees (i.e. forest) are an inefficient way of scaling this up, since most trees in a forest are too closely packed to have much effect on the wind.

    But wind turbines are deliberately positioned to catch as much wind as possible (via their spacing).

    In fact, let's get our head out of the details. The whole *point* of a wind turbine farm is to extract as much energy from the wind as possible. The more efficient a wind farm is at stealing wind, concentrating and converting it, the better. The "ideal" turbine farm would extract all the energy, leaving still air.
     
  12. Jan 11, 2006 #11

    rcgldr

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    Well there would have to be some flow of the air, else air would just accumulate at the farm, creating a stagnate zone at the farm.

    There's also a very relatively small torque force applied to the earth by a windmill.
     
  13. Jan 11, 2006 #12
    In terms of the OP's worry that harvesting energy from the wind might have a deleterious effect somehow, though, it's extremely doubtful. I mentioned a tree because it disrupts wind without anyone blinking an eye. Maybe a better example would be skyscrapers and other tall buildings which, taken all together, must represent an amazing amount of man made drag on the wind, but which no one has ever thought to worry about, since there's no indication this has a bad effect.
     
  14. Jan 11, 2006 #13

    Bystander

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    By all means, "do the math." I'm a little curious to see whether other people get the same order of magnitude as I.
     
  15. Jan 11, 2006 #14

    G01

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    Not really worried about the deletrious effect as much as I am curious about it.
     
  16. Jan 11, 2006 #15

    DaveC426913

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    Actually, on Daily Planet there was an article about exactly that.

    Niagara Falls kicks up some mist, but over the years it has been getting worse. Scientists built a giant scale model of entire area and placed the appropriate buildings on it. Sure enough, as existing buildings were added to the model, the wind exhibited a marked backdraft, carrying lots of mist over the area in exactly the opposite direction of the (otherwise) prevailing winds.

    Then they took the sim one step further - into the future. They placed buildings on the sim that have been proposed (they are much larger than the existing buildings - huge hotels as the casinos grow). Interestingly, there was very little additional effect. It appears we have already done the damage.
     
  17. Jan 11, 2006 #16
    I stand corrected then. I guess people have started to worry about it. Since this instance shows a definite change due to a concentration of buildings I suppose people are going to start doing simulations in all kinds of cases to see what effect cities have had on the winds.
     
  18. Jan 11, 2006 #17
    Oh, I misread your intention, then. I got the impression you were concerned it could have an effect on the earth's rotation.
     
  19. Jan 11, 2006 #18
    Not me. I wouldn't know where to begin.
     
  20. Jan 12, 2006 #19

    Bystander

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    Huh? Sure you would --- how 'bout
    ,

    or, http://www.physicalgeography.net/fundamentals/7p.html .

    I'm not asking you for a quantitative global model, just an order of magnitude estimate/calculation for a circulation cell that interests you.
     
  21. Jan 12, 2006 #20
    Of course buildings have a significant impact n the winds, as anyone who's walked the concrete canyons of Manhattan knows. But it's a local effect. There's already some frictional coupling between the wind and the ground. Putting in a building only redistributes the effect. A tall, flat building transfers the force of the wind to the foundation, giving a push to the earth in that direction, and leaving slower moving wind in it's wake. Absent the building, the higher speed wind pushes on the earth more gently but over a larger area, eventually yielding the same net result globally. Given measurements of wind or the Earth's rotation made in Nebraska, the presence of New York is likely undetectable.

    This disregards the secondary effects like increased evaporation of water from high wind areas or the transport of soil, which could in the complex world of weather shift the attractors of the overall weather patterns into different configurations. But I suspect the effect is marginal, even over millenia.
     
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