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Would a wind turbine on a moving car generate energy efficiently?

  1. Aug 14, 2010 #1
    I was talking on another forum about an idea someone proposed. They said if you put a miniature wind turbine as a Hood Ornament on a car, as the car drives, it would produce extra usable energy.

    I said this would not work as if the wind is not blowing at the correct angle and velocity then the wind turbine on the car would actually slow it down to a minute degree. Meaning, the friction between the air and the wind turbine would slow down the car(to a very small degree) and the car would have to speed up to compensate for the loss of kinetic energy. The energy used to speed up the car to make up for the loss would be greater than the energy produced by the turbine meaning the energy consumption is being increased even more.

    Who is correct and can you explain why? I've already tried to explain that the energy has to come from somewhere to move the wheel, and if it isn't coming from the non-moving air then it must be coming from the car.
  2. jcsd
  3. Aug 14, 2010 #2


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    You have correctly explained it yourself, in useful terminology. Although elaboration is possible, it certainly isn't necessary. You're right; they're wrong. (Feel free to quote the Laws of Thermodynamics if you feel so obliged.)
  4. Aug 14, 2010 #3
    Yes but I need someone to explain it in clearer terms. I have tried to explain it to the person (same as I did above) and they didn't understand me. I'm going to be linking them to this topic so if anyone else has a clearer explanation, feel free.
  5. Aug 14, 2010 #4
  6. Aug 14, 2010 #5


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    You could build a model.
    Take the motor-generator I built this winter for example.


    On the left is the generator. Driving the generator via a fan belt is the motor.
    When the fan belt was removed, the motor consumed 200 watts.
    When the fan belt was installed, the motor consumed 300 watts.
    This was just drag from the idling generator and belt.
    If the generator was then hooked to a 40 watt load, the power consumed by the motor would probably go up another 100 watts.
    The same thing is happening in your wind driven generators.
    For every watt they generate, your vehicles primary engine is going to consume more than that watt. (Unless of course you turn off the car while driving or going down a hill and turn off the car, which would be unwise as power steering and power brakes would fail and you would probably die trying this experiment)

    I was trying to explain the concept of regenerative braking to someone once, and they could not get it into their head that a generator under electrical load exerts a torque. Unless that torque is countered, the generator will quickly stop rotating.

    The torque required to keep your friends little hood ornament fans rotating under load is called wind drag. That wind drag has to be countered by your vehicles primary engine.
  7. Aug 14, 2010 #6
    Any energy you got from the generator could more efficiently be subtracted from the wind drag of the vehicle by increasing its aerodynamics. I.e. I think removing the hood ornament altogether would generate convert more energy into forward motion than it would deliver as electricity. If you want more electricity, why not just increase the output of your alternator and take the energy directly from the engine instead of first converting it into forward momentum and then getting it back from the wind drag?
  8. Aug 15, 2010 #7


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    Wind turbines have a rather phenomenal amount of drag, and the amount of drag produced by the turbine will always outweigh the amount of energy you can get out of it.
  9. Aug 15, 2010 #8
    Just quote the http://en.wikipedia.org/wiki/Conservation_of_energy" [Broken]. When the car is standing still the turbine isn't moving. When the car is moving the turbine will move. So the energy for the turbine's movement comes from the car's movement. And because of the law of conservation of energy either the car will slow down a bit or will use more gas. If we would live in a frictionless and dragless world and you had a 100% efficient generator attached to the turbine you would still only get back the same amount of energy the car lost (in speed or gas). Now we don't live in such a world, so a portion of that energy will for example be lost to heat due to friction with the air and friction inside the turbine. So the turbine could never generate extra energy.
    Last edited by a moderator: May 4, 2017
  10. Aug 15, 2010 #9
    One the most fundamental laws of physics states that this won't work. You can't make energy, you can just convert it from one form to another...and every time you do so, you WILL lose some. What your friend is proposing is a perpetual motion machine: something that creates more energy than it consumes. It doesn't work and it will never work. If he doesn't understand this then it's going to be very difficult to explain.

    The main reason a car uses fuel in the first place is because of air resistance. If it wasn't there, you could just accelerate to speed and practically coast most of the way. A wind turbine on a car will increase air resistance, requiring more fuel. What's more, it will ALWAYS increase the air resistance by such an amount, that the car will require to burn more fuel than the amount of energy the turbine puts back in. Always. No matter what elaborate scheme you come up with...you can use turbines, magnets, slopes, gears, levers, thermocouples, you can split atoms and enlist all the forces of nature, and you will ALWAYS lose.
  11. Aug 15, 2010 #10


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    There is one practical application—a turbine mounted on a stationary vehicle in the presence of a significant breeze. You could, for instance, stick one on the roof of a parked RV to power your TV or computer. In that case, there would be no parasitic losses to the vehicle.
  12. Aug 15, 2010 #11
    The total wind power incident on a stationary horizontal axis wind turbine (HAWT) is P = ½ρAv3 (Newton-meters/sec), where ρ is the air density, A is the frontal area of the HAWT, and v is the wind velocity. The maximum theoretical efficiency of converting this wind power to turbine power is 16/27 = 59% (called the Betz limit). because the air needs sufficient residual energy (velocity) to move away from behind the turbine after passing through the blades. See


    Actual HAWT maximum efficiencies are ~40 to 45%. So putting a HAWT on the hood of a car will produce more drag force (F = ½ρCdAv2, Cd is drag coefficient) than it produces in electrical power. See

    http://en.wikipedia.org/wiki/Drag_(physics [Broken])

    Bob S
    Last edited by a moderator: May 4, 2017
  13. Aug 15, 2010 #12
    What if the wind turbines were placed in such a way as to reduce drag by redirecting the flow of air, such as at the back of a vehicle. If the turbines shifted the air flow so that it reduced the low pressure behind the vehicle, couldn't the vehicle theoretically move forward more easily while the turbine motion also generated electricity?
  14. Aug 15, 2010 #13


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    What you are describing is a fan, not a turbine. A turbine impedes airflow.
  15. Aug 15, 2010 #14
    This is a very interesting point. The power loss due to air drag by a vehicle is P = ½ρACvv3, where Cv is the coefficient of drag, and A is the frontal area of the vehicle. The object of streamlining a vehicle is to reduce Cv. If a wind turbine could be placed in a location such that the reduction in the vehicle air drag power loss was >= the output power of the turbine, then you could theoretically have turbine "efficiency" over 100%. Any comments?

    Bob S
  16. Aug 15, 2010 #15


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    Are the moderators ahead of the send action ?
    Just had a post disappear as I started to hit send. Anyhow I'll just say, warmer air flowing through a turbine of some sort, converting energy and pushing cold air into the draft area behind the vehicle will reduce drag to a point that the turbine should power the vehicle and have a surplus energy.

    I'm sure I could have said it better, but that does not happen often.

  17. Aug 15, 2010 #16
    The situation we're talking about involves 0 air movement. Meaning all of the energy MUST be coming from the cars kinetic energy as it moves forward. Even if drag and friction on the turbine were minimized, the turbine still could never produce 100% of the energy that it takes to spin it to generate a select amount of energy. If it somehow manages to generate more than 100% of the energy which it took to spin it, then you just made a perpetual motion machine.
  18. Aug 15, 2010 #17


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    Well it happens all the time in a jet engine, but a car driving down the highway is not a jet engine. A turbine by itself doesn't do any pushing and even in a jet engine is removing energy from the exhaust that would otherwise be propelling the plane.
  19. Aug 15, 2010 #18
    Perpetual motion machine is an exaggerated claim. If the vacuum forming behind the vehicle is increasing the drag, then it is already taking unnecessary energy from the engine. All this turbine would be doing is getting that energy back from the drag and reducing the drag at the same time. Probably it would be more efficient to just reduce the drag without the turbine, though, I'm guessing.
  20. Aug 15, 2010 #19


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    There is more than one thing going on in respect to the car and the hood ornament windmill.
    Wind blowing straight at a stationary car at the speed of 50mph will produce the same exact drag friction as if the car is moving at 50mph. Drag is calculated using the square factor.
    Now consider a windmill, it is stationary and if a wind of 50mph blows straight at it, the energy of the wind is calculated using a cube factor.

    In my mind, energy in the air motion affects, at the expense of drag on the car shell, can be two different values. Proper ducting and turbine design can extract this energy to a high level of efficiency.

    My understanding is that a volume of warm air flowing at some velocity and performing work on a turbine of flow through design, will discharge that same volume at a colder temperature.
    Will energy required to move a volume of air, not be proportional and related to it's temperature ? if so then that amount of work (heat) has to be transferred into some action. I would propose on an electric car a Regen system.

  21. Aug 17, 2010 #20
    I'm not sure what the power formula for a turbine spinning in air is. But, I'm going to go ahead and guess that it's similar to the formula for how much power a car needs to push that turbine through the air. In fact, the formulas are probably damn near identical.

    There's probably an air pressure, an area, and a V^3 in there.

    That said, I just don't see how any air pressure/ air deflection/ strategic placement trick would result in the power extracted by the turbine being greater than the power used to get it to spin.
  22. Aug 17, 2010 #21
    Try this simple intuitive thought experiment:

    1) build any vehicle with any configuration of wind-driven electric generators.
    2) disengage the generators so they they rotate freely without the friction of generating current.
    3) measure how much drag is reduced by disengaging the generators.
    4) compare X (the reduction in drag) to Y (the amount of energy generated when the generators are engaged).
    5) Is Y greater than or equal to X?
  23. Aug 17, 2010 #22
    From Bob S
    A spoiler on the rear of an automobile with some wind turbines in it may be more efficient (lower overall drag coefficient) than no spoiler at all. But then, the spoiler without the turbines would be even better.

    Bob S
  24. Aug 17, 2010 #23
    Especially if the spoiler had neon lighting and green flames painted on it. Sorry, couldn't resist:)
  25. Aug 17, 2010 #24

    Andrew Mason

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    A windmill on a car could be useful if you had a cross wind and wanted to take advantage of it to supplement the energy delivered by the motor. Conceivably it could be designed so as to almost eliminate drag. You could turn it to forward to supplement braking and recover some of the car's kinetic energy in a battery, say. A novel twist on dynamic braking. But you cannot use the virtual wind created by the car itself, for the reasons already given.

  26. Aug 18, 2010 #25
    Using a windmill to convert the car's kinetic energy when braking seems kinda not optimal. Using a flywheel (if you want to use it to accelerate again) or some kind of spring for that is much better.
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