Salient White Elephant

June 27, 2009

Cable Untwisting System for Small Wind Turbine

Cable Untwisting System for Small Wind Turbine

The diagram shows the turbine yawed to the position where its power cables are completely untwisted. In this case, the cable that untwists the yaw system attaches to its topmost pulley wheel in such a way that it is not wrapped around that topmost pulley wheel at all. (The topmost pulley wheel is the one with the axis of rotation that is coincident with the turbine’s yaw axis). Now as the turbine yaws, it doesn’t matter which direction the turbine yaws in. Whichever way the turbine yaws, it wraps the untwist cable around the topmost pulley wheel, and in so doing it draws the twist sensing component that is attached to the untwist cable (colored purple) up to a higher elevation. When this part of the sensor passes close by the topmost twist sensor component (colored red), the control system knows that the power cables are twisted up. To untwist the turbine, the controller simply turns on the small electric motor at the base of the tower until the cable mounted twist sensor component passes by the lowest red colored twist sensor component, then turns the motor back off. If the system fails for some reason, the result is that the small untwist motor will burn up or blow a fuse – a minor repair indeed. The controller might wait until the wind isn’t blowing before untwisting the power cables.

Sensorless Variation

In this variation, the controller merely untwists the turbine every time the wind speed drops to zero (rotor blades not turning). Some kind of slip clutch mechanism might be provided to keep the untwist motor from burning up if it runs too long. Alternatively, the motor could be turned off whenever the power it draws jumps up by a large value (indicating the turbine has been completely untwisted). Or a simple mechanical switch could be tripped whenever the turbine is completely untwisted.

Manual Variation

A manual version of this device might also work well. In this case, the controller might issue some kind of mechanical or telecommunications signal to let someone know that the turbine needs untwisting.

April 10, 2009

Precision Wind Turbine Yaw Fin

The upwind variation has two airfoils that are each pitched near the stall angle. When the wind shifts directions, one of the airfoils stall, while the other generates the yaw moment at nearly maximum lift:

Aerial View Precision Wind Turbine Yaw Fin (Upwind Variation)

I suppose the upwind variation will produce stable yawing behavior only if the airfoils have gentle stall characteristics, and only if they can be pitched very near to the stall angle.

Now for the downwind variation. If the wind shifts by (say) 2 degrees, then the angle of attack for one airfoil increases by 2 degrees while the angle of attack for the other airfoil decreases by 2 degrees. The yaw moment thus generated is as if the wind had actually shifted by 4 degrees.

Both of these variations allow for the use of asymmetrical airfoils, which produce more lift than symmetrical airfoils.

Aerial View Precision Wind Turbine Yaw Fin (Downwind Variation)

March 17, 2009

Darrieus with Inverting Asymmetric Airfoil

One disadvantage of the traditional Darrieus Turbine is that it requires symmetrical airfoils of zero pitch. This is unfortunate, since a pitched asymmetric airfoil has much better aerodynamic characteristics. Because a given side of the Darrieus airfoil must serve as the high pressure side for half a rotor turn, and then as the low pressure side for the other half turn, Darrieus machines have not been able to take advantage of the superior performance of the pitched asymmetric airfoil.

This post describes a technique for inverting the airfoils twice per rev. This permits the Darrieus to employ pitched asymmetric airfoils:

Blade Inverting Darrieus Has Asymmetrical Airfoils

Darrieus with Constant Non-Zero Pitch Inverting Asymmetric Airfoil (Side View)

I believe in three bladed Darrieus machines, but it’s usually easier to use two bladed machines in diagrams and explanations, and that is what I have done here. As the two-bladed Darrieus approaches the rotational angle at which it produces no power, the airfoils may rotate freely. This doesn’t matter because they aren’t torquing the rotor anyway. As the rotor enters the other half of its power producing arc, the net wind velocity vector shifts so that it is no longer parallel to the airfoil’s velocity vector. Now the airfoil is mechanically stable, but unless its high pressure side is upwind and its low pressure side is downwind, it is aerodynamically unstable. If unstable, it will flip over so that the polarity of the airfoil is appropriate for the given power producing arc.

The embodiment depicted in the above diagrams is only one of many variations. Basically, it boils down to this:

  • Each airfoil is able to rotate about about a horizontal axis tangent to its circular path of motion. (If the airfoil is pitched, then its chord will not be parallel to its axis of rotation. In this case, the chord will sweep out the surface of a cone if the airfoil is rotated 360 degrees. However, the axis of rotation is still tangent to its circular path of motion.)
  • The mass of each airfoil is balanced with respect to its axis of rotation. (“Axis of rotation” here means the airfoil axis – not the rotor axis.) Because the airfoil is balanced, centrifugal force will not cause it to rotate.
  • The airfoil is aerodynamically stable when the high pressure side of the airfoil is upwind and the low pressure side is downwind. The airfoil is unstable when oriented with the opposite polarity. In this case, aerodynamic forces will rotate the airfoil, causing it to flip to the other side.

Actuated Variation

A similar approach would have the controller actuate the airfoils. In this case, the controller has a wind vane just like a horizontal axis machine, and uses this information to determine when to flip the airfoils.

Unbalanced Variation

Note that if the circular path of the VAWT blade has a large diameter, centrifugal forces are greatly diminished. In this case it may not be necessary to counterbalance the inverting blade.

March 12, 2009

Precision Wind Turbine Yaw Sensor

The following diagrams show the relationship between the tower shadow and the rotational angle of the rotor:



Tower shadow is detected by monitoring the instantaneous power produced by the turbine. Tower shadow produces a very short negative spike (drop) in the turbine’s power output. If this negative spike is sharp (very short in duration), and if it occurs when one blade passes the 6 o’clock position, then the turbine is correctly yawed. If the spike occurs “too early” or “too late”, then the turbine is not correctly aligned with wind direction. The lower diagram above shows that when the yaw angle is not correct, the tower shadow for the blade root occurs at a different time than the tower shadow of the blade tip. In this case the duration of the tower shadow increases. Furthermore, since the blade tip is generating torque when the root is in the shadow, and since the root is generating torque when the tip is in the shadow, the magnitude of the negative spike in power should decrease. So the controller knows that the turbine is misaligned whenever:

  • the duration of the tower shadow is increased,
  • the magnitude of the negative spike in power is reduced, and
  • the tower shadow occurs earlier or later than 6 o’clock.

In designing the yaw angle control system, the traditional vane sensor might provide an estimated value for yaw angle while the precision yaw angle sensor provides for fine tuning this value.

February 24, 2009

Spoiling Wind Turbine Yaw Control

Simply fit one turbine blade with the light-weight, reliable, inexpensive, and fast-acting spoiler of your choice. Now if the spoiler is deployed at 2 o’clock and retracted at 4, the turbine will yaw in one direction. If the spoiler is deployed at 8 o’clock and retracted at 10, the turbine will yaw in the opposite direction. That’s all there is to it!

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