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Wind Turbine with Generator & Gearbox on the Ground?

  1. Aug 11, 2014 #1
    I have often wondered why conventional large wind turbines have evolved with their generators and gearboxes in the nacelle at the top of the tower where they are hard to reach and maintain. Despite this fact, I wish to believe that there may be benefits to a conventional horizontal shaft wind turbine that utilized some form of line shaft down to a ground based gearbox and generator in a small enclosure next to the base of the tower.

    I understand that the weight of the generator and gearbox only adds a fraction of the forces to the tower that the large blades do - but still feel the whole "giant crane" requirement is a big negative to the existing standard design.

    What ideas do others have for a large wind turbine with ground based generator and gearbox if applicable? I am mostly interested in ideas on horizontal shaft machines because I think they have more potential than vertical shaft units. If a large turbine pump can have a line shaft between the ground mounted vertical shaft motor and multistage vertical shaft pump several hundred feet below the ground, why can't a tall wind turbine utilize a line shaft arrangement (somewhat) efficiently? Isn't there a way around the tower bending and flexing?
     
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  3. Aug 11, 2014 #2

    AlephZero

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    One basic difference from your turbine pump example is that the turbine and/or pump doesn't have to rotate to face into the wind. You would need some device to allow the turbine to rotate as necessary, but also to stop it just rotating around the vertical shaft instead of turning the shaft and driving the generator.

    Think about the tail rotor on a helicopter, which has the same basic function (i.e. preventing and controlling the rotation of the helicopter body), and imagine scaling something to perform the same function up to wind-turbine size. Putting the generator on top of the tower solves the problem the easy way - by not creating it in the first place.
     
    Last edited: Aug 11, 2014
  4. Aug 13, 2014 #3
    I agree, but believe the same design philosophy as existing machines could be used. A large nacelle on top of the tower with four synchronized yaw gear motors on a diameter of 10 to 15 feet ought to compensate against the counter torque of the vertical drive shaft. I thought the biggest issue may be power phase compensation due to the yaw movement back and forth into the wind. However, perhaps the electronics can compensate for this assuming a 3 phase AC generator. Of course if a large slow speed generator is used with multiple poles, the gear box can maybe be simplified. I would also be concerned about line shaft torsion and possible torsional stress induced by system feedback / oscillations.
     
  5. Aug 16, 2014 #4

    Baluncore

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    Since power = torque * RPM, the best way to reduce the torque is to increase the rate of rotation.
    The right angle gearbox in the nacelle that drives the vertical shaft should have a ratio that increases the speed.
    The vertical shaft can have a smaller cross section as it handles less torque at higher RPM.
    A long vertical shaft will need multiple mounted bearings to maintain alignment and prevent twirling.

    It is easier to use an axial gearbox with the generator, all mounted in the nacelle.
    The shaft is then replaced with slip-rings and fixed power cables.

    The water pump turbines are usually fitted with a reduction gearbox and crank.
    The connecting rod rises and falls sinusoidally, directly driving a piston pump in the well below.
     
  6. Aug 16, 2014 #5

    AlephZero

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    Gearing up the speed of the shaft would make no difference to the torque that is trying to yaw the turbine away from the wind direction. To change that, you would have to change the RPM of the turbine itself.

    EDIT: This is wrong - see the next two posts.
     
    Last edited: Aug 16, 2014
  7. Aug 16, 2014 #6

    Baluncore

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    I don't understand how that could be. Can you explain why power is not torque * RPM.
     
  8. Aug 16, 2014 #7

    AlephZero

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    Oops, my mistake. Of course the torque at the two ends of the gearbox is different, in proprortion to the RPM ratio. I had a brain-fart about which way was horizontal and vertical.
     
  9. Aug 16, 2014 #8

    Baluncore

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    Thank you AlephZero, it is such a relief to know I've not had another “senior moment”.
     
  10. Feb 5, 2016 #9
    Hi,
    I am a retired Electrician with an automotive background and a backyard builder.I want to throw out some ideas and I have no idea if they would work.
    First, to eliminate the issue of how large a wing needed to keep the blades facing the wind, why not plan it with the blades facing away from the wind?
    Next, I envision an automobile rear end differential on top of the pole or tower. The blades would be mounted on one shaft (either the drive shaft input or one of the axle shaft outputs). By chopping off one of the axle shafts and freezing it in place, all the rotational energy would be transmitted around the corner and down. Then a vertical shaft would transmit the rotational power down to the ground where you might be able to use the reverse to drive the ground based generator...The gearing would have to be experimented with but by flipping the design around there would be many RPM output possibilities. If a high speed fan rotation is expected make the drive shaft the fan input shaft. If a low fan speed if expected make an axle shaft the fan input. There would be more power losses in the mechanics but the build cost might be very low. and the tower would only have the blades and part of an auto rear end, instead of a heavy generator.
    And second hand auto parts are readily available. If the vertical drive shaft was made of actual drive shafts, imagine those middle bearing mount that used to be in the old split drive shafts of some trucks. You could mount as many of them as needed to the pole/tower to mount several stock drive shafts.
    Any thoughts?
     
  11. Feb 5, 2016 #10

    Baluncore

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    The blades needs to “fly” through clean air. The air downstream of the tower structure is disturbed as there is an eddy below the tower. As blades regularly pass through that disturbance they suffer a change in lift and so develop an oscillation that can destroy them if it builds up in amplitude near resonant speeds.

    The size of the tail is important. It is eliminated in power generation turbines so they can be turned side on in wind storms. That prevents destruction at high uncontrolled RPMs. Having the blades downstream, acting as a tail will present the same problem unless the turbine azimuth is controlled by a motor.

    When not mounted flat, auto differentials, have lubrication problems. The internal oil bath is then flooded against an oil seal rather than bathing the outer crown wheel and transferring that oil to the pinion.
    I have observed several local car and truck “rear end differential” based experimental wind turbines built. They have all failed due to lubrication, lack of directional control, or because it is easier to transmit power down the tower in other ways.

    The vertical shaft drive concept is a brain virus, once someone gets it into their head it is very hard to dislodge. The vertical axis Savonius rotor is the most inefficient turbine available. The same people who consider auto differentials usually go through a period of Savonius rotor worship also.
     
  12. Feb 6, 2016 #11

    CWatters

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    None of the turbines I've seen in the UK have a tail. They all use motors to keep it pointing into wind.

    A tall crane is needed to lift the blades so putting the generator at the bottom doesn't avoid the need for a tall crane.

    All this sounds like a solution to a non existent problem.
     
  13. Feb 17, 2016 #12
    Just build a flying wind turbine for a fun experiment... That's what I thought. Fun all-right. but it gets a bit more serious the longer you stare at it.
     
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