Salient White Elephant

March 2, 2009

Highly Scalable Horizontal Axis Wind Turbine

Downwind View, Highly Scalable Wind Turbine, 3 Bladed Rotors

Upwind View, Highly Scalable Wind Turbine, 3 Bladed Rotors

The Highly Scalable Wind Turbine is remarkable in that guy wires assist in supporting all of the large tower loads that are carried by the machine. These loads include:

  • overturning moment on central (vertical) tower tube (due to thrust of operating rotors or drag during storm winds),
  • moment that tends to bend the ends of the upper (nearly horizontal) tube in the downwind direction (due to thrust of operating rotors or drag during storm winds), and
  • moment that tends to bend the ends of the upper tube down towards the ground (due weight of nacelles and blades).

Note that the configuration of the Highly Scalable Wind Turbine allows guy wires to attach to the tubes they support at any desired location along the tube’s longitudinal dimension, including at the very end of the tube. This feature facilitates the design of a very large turbine that does not require a correspondingly massive tower. It also permits the use of very high towers on sites that have favorable wind shear.

Looking Into The Wind, Highly Scalable Wind Turbine

Downwind View, Highly Scalable Wind Turbine

Aerial View, Highly Scalable Wind Turbine

The top tower tube is just long enough to allow the rotors to clear the guy wires that support the central tower tube. Tubes or lattice structures may be substituted for guy wires if greater control over tower resonance is required, or if an upwind design is selected.

Note that the nacelles do not rotate with respect to the top tower tube. Instead, the nacelles and the top tube rotate as a unit about the central vertical tube in order to regulate yaw angle. The top tube is bent in order to lower the center of gravity. This way, the upper structure will naturally want to correct any tendency for one rotor to dip lower while the other rises up. In fact, it may even be desirable to allow limited motion in this direction so that the structure doesn’t have to bear the associated load. (Of course, using a bent tube in the upper structure rather than a straight tube also results in a tendency for the rotors to rock “out of the page” in the upwind view above.)

Although the machine in the diagrams has two-bladed downwind rotors, it is readily apparent that this configuration would be equally effective with upwind or downwind rotors, regardless of how many blades they have. (Some guy wires will be replaced with tubes or lattice structures if an upwind design is selected.)

Teetering to Shed Loads

The Highly Scalable Wind Turbine has a remarkable ability to yield in response to sudden changes in loads. If a wind gust hits one of the rotors but not the other, the machine will compensate by yawing. If two bladed, downwind, teetering rotors are used, then each rotor can teeter in response to variations in the wind that flows through its rotor disk. In selecting turbine configuration options, one must remember that providing more degrees of freedom will render the machine more difficult to understand and model, and may also lead to instabilities.

It may be useful to explore the option of letting the upper structure teeter about both of the axes that are orthogonal to the yaw axis. In this case, the upper structure “balances” on top of the vertical tube.

Tilt Down Option

The highly scalable turbine has yet another extraordinary feature – it is naturally counter-balanced! This means that a tilt-down version may be designed without adding expensive, heavy components. Both nacelles and rotors may be serviced at ground level, or at least near to ground level.

Highly Scalable Wind Turbine, Tilt Down Option

Here’s a folding tilt-down version:

Highly Scalable Wind Turbine, Folding Tilt Down Option

I guess this is getting pretty outrageous, but imagine the controller folds the turbine not only to facilitate service and repair, but also in response to storm winds! The procedure might go something like this:

  1. Both rotors are feathered,
  2. the brakes lock both rotors at appropriate angles,
  3. the machine is yawed to an appropriate angle, and finally
  4. the upper structure is folded as depicted above.

Might the controller do all this without human intervention? Seems pretty far out, but if this can be achieved, what a fantastic high-wind drag profile!

Another high wind shutdown option would simply have one rotor produce (say) 50% power while the other rotor produces only (say) 20% power. In this case, the rotor operating at 50% capacity would be dragged to the most downwind location, and the machine would maintain this somewhat low drag profile throughout the storm. This action is reminiscent of the way some small HAWTs turn their rotors to the side in high winds. It offers the advantage of quickly and continuously yawing in response to changes in the direction of the storm wind.

Advantages of the Highly Scalable Design

I am hypothesizing that the scalable design presented here is superior for several reasons. The tower of the traditional horizontal axis turbine must support the total aerodynamic drag on the rotor and nacelle during storm wind conditions. Since this force is applied with a lever arm equal to the height of the nacelle, the tower must be large, heavy, difficult to transport, difficult to erect, and it must be expensive. The vertical tube of the scalable machine is strengthened by guy wires, allowing it to be lighter, more manageable, and less costly. The penalty is the addition of another tube (the upper structure tube) that must support the weight of the rotors and nacelles. But this force is applied with a lever arm only slightly longer than the length of the rotor blades, and there are guy wires above the tube to help carry this load. Making the worst case assumption that the scalable turbine is not folded or tilted down, and that its upper structure tube is at a right angle to the direction of the wind, then the scalable turbine’s upper structure tube must support the same storm wind aerodynamic drag as the one supported by a traditional turbine. But again, the lever arm is only slightly longer than the blade length, and there are guy wires on the upwind side of this tube to help. There are also guy wires on the upwind side of the vertical tube to help carry this load, and these guy wires may attach to any place on the vertical tube, even at the very top of the tube!

In the early 1990s, Carter Wind Turbines, Inc. developed a downwind, tilt-down, two bladed 300 kW turbine. This machine was considerably lighter and more flexible than the more common upwind, three bladed turbine. The lightweight, small diameter tower was possible because it had guy wires. It is important to realize that this tower was strong enough to:

  • withstand storm wind aerodynamic drag loads applied with a lever arm equal in length to the height of the nacelle, and
  • support the weight of the nacelle and rotor (applied with the same lever arm) when the tower was in a nearly horizontal position. (This would be the case when tilting the machine down.)

tilting-down-the-carter-300

If the provision of guy wires can make the lighter, less costly Carter design feasible, then it can make the lighter, less costly scalable design feasible as well. This is so because the scalable turbine must support exactly the same loads. In fact, the lever arm for the scalable turbine load is likely to be shorter than than the lever arm for the load on a traditional machine. Furthermore, it is very important to realize that the Carter machine achieved all this with guy wires that could reach no higher than the height of the tip of a blade in the 6 o’clock position. The scalable design enjoys the luxury of guy wires that can attach to its tubes at any desired longitudinal position.

Now that utility scale turbines have reached a size of two or three megawatts, transportation of turbine components and construction of the turbine have become very significant cost and technology issues. A turbine’s maximum size may be influenced by some combination of the following factors:

  • cost of transportation,
  • feasibility of transporting the turbine’s largest components to the wind farm site,
  • cost of construction,
  • maximum weight that can be lifted by a crane,
  • maximum height that can be reached by a crane,
  • cost of the crane.

The industry is attempting to design larger and larger wind turbines because increasing scale potentially lowers the cost of energy. However, the benefits of scale may be eroded by any or all of these factors. The highly scalable turbine proposes to solve this problem by reducing the size and weight of tower components, and by facilitating the design of a tilt-down machine. The tilt-down design allows tower top components to be assembled either on the ground, or at least at a much reduced elevation. Furthermore, since the scalable turbine has two rotors rather than one, it provides twice the power of the traditional horizontal axis machine for a given blade length. Of course, it also has two rotors, two gearboxes, and two generators. But it has only one yaw drive, transformer, and foundation. And finally, it makes more efficient use of tower resources. You can see this by noting that the scalable turbine extracts twice the benefit from every extra meter that is added to the height of the vertical tube.

Yaw Drive

If the pitch of the rotor blades is variable, then the machine is easily yawed as follows:

  • If a rotor is feathered, the machine will yaw in the direction that will move the feathered rotor in the upwind direction.
  • If a rotor is stalled, the machine will yaw in the direction that will move the stalled rotor in the upwind direction.

If the pitch of the rotor blades is fixed, then equipping each blade with a spoiler will allow the machine to be yawed in a manner similar to that just described. That is, if the spoilers on the blades of one of the rotors are deployed, then the machine will yaw so as to move the rotor with active spoilers in the upwind direction.

Another yaw drive option would employ a tail fin. The highly scalable turbine configuration certainly has plenty of room for a yaw fin. The yaw fin may be mounted on a supporting tube that extends in the downwind direction from the vertex of the bent tube in the upper structure.

Yet another possibility would use a yaw rotor that is similar to a helicopter tail rotor. Like the tail fin, the yaw rotor would be suspended in a position downwind from the vertex of the bent tube. It would rotate about a horizontal axis that is parallel to the plane of the upwind view in the first diagram. The yaw rotor has symmetrical airfoils with zero pitch (i.e., the airfoils are symmetrical about the plane of rotation). Such a rotor will generate a strong correcting yaw moment whenever the yaw angle deviates even slightly from its correct value. If the yaw rotor airfoils have variable pitch, then the turbine can be yawed even when there’s no wind. This may be useful for untwisting the power cables, and for adjusting the yaw angle to a value that is appropriate for tilting the machine down.

Untwisting Power Cables

One final yaw drive strategy that might be effective for untwisting power cables would use one of the rotors as a propeller. Suppose there’s no wind. One of the generators drives its rotor like a propeller, yawing the upper structure until the power cables have been untwisted. If the machine does not have any other active yaw drive system, then this same trick may be used to adjust the yaw angle to a value that is appropriate for folding or tilting the machine down.

Synchronized Drive Variation

Since the rotational axes of both rotors are parallel, I wonder if they could by synchronized with a sprocket and chain drive (kind of like the drive on a bicycle). In this case, both rotors could drive a single generator that is situated somewhere near the top of the vertical tube. This has the added advantage of reducing the amount of weight that the upper structure tube must support, and it would probably also stabilize the upper structure somewhat, reducing any tendency to tilt from side to side. In fact, maybe the generator could be lowered somewhat in order to further lower the center of gravity of the upper structure.

Semi-Direct Drive Option

The following semi-direct drive option is possible because the nacelle is fixed with respect to its supporting tube. This design eliminates the gearbox.

Upwind View, Highly Scalable Wind Turbine, Semi-Direct Drive Option

Or the streamlined ring can be pinched between two tires:

Upwind View Highly Scalable Wind Turbine Semi-Direct Drive Option (2 Generators)

 

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