Salient White Elephant

May 10, 2009

CounterRotating Direct Drive Wind Turbine

A number of posts to this blog describe turbines (both HAWTs and VAWTs) having blades that are supported at the high-speed blade tips rather than at the low speed parts of the blades. This is usually accomplished by having a blade tip engage some sort of slot that is cut into a blade guiding track, so that the action is somewhat reminiscent of the way a rail guides the wheel of a train. One of the biggest problems with this approach is how to come up with a simple, reliable way of converting the kinetic energy of the blades to electricity. The conversion apparatus should not be unweildy or cumbersome, and should not require too much hardware. (For example, in some of my earlier posts I have suggested distributing generator windings all along a very lenghty blade guiding track. This is clearly undesirable because it would make the tracks very heavy and very expensive.)

I think I may have stumbled on a good way to deal with this problem just a minute ago while writing the post entitled: Skyscraper with H-Rotors. I didn’t do a very good job of describing the counter-rotating drive idea in that post, so I’ll attempt to do a better job of it here. (Although the technique described here may be applied to many of the HAWT and VAWT turbines proposed on the Salient White Elephant, you might want to read Skyscraper with H-Rotors first, since I’ll draw the diagrams and everything assuming that we’re applying the counter-rotating direct drive idea to that particular turbine.)

Counter-Rotating Direct Drive Wind TurbineThis turbine produces power in pulses. Each time two blades that are traveling in opposite directions pass each other, their generator components (permanent magnets and coils) pass close to each other as well. So a pulse of power is produced when two blades pass each other. Obviously, it would be better for a turbine to produce power at a smooth constant rate. This is desirable for many reasons. For one thing, producing power in pulses applies a cyclic fatigueing load on the mechanical components, and this is obviously bad news. For another, the electricity is easier to process and manipulate if it is produced at a smooth regular rate. But I am hypothesizing that the design proposed here may be a good one because it allows blades to be supported at both blade tips, even as both tips travel at high velocity! This is a tremendous advantage. But the main advantage of this design is that although it allows blades to travel long distances guided only by slots that are cut into blade guides, it does not require for these long distances to have generator components (magnets and/or coils) distributed along these long portions of the blade guides. Instead, the generator components are compact, and are attached to the ends of the airfoils. You can think of all of the airfoils that rotate (say) clockwise as comprising the generator “stator”, while all of those rotating counter-clockwise comprise the generator rotor. Of course, another disadvantage of this approach is that slip rings would be required to get the power away from the blades and into the electrical system. But there’s another advantage as well – the fact that the generator’s rotor and “stator” rotate with equal and opposite rpm’s effectively doubles the relative speed with which the coils and magnets pass each other.

So before closing, let me address one of the biggest disadvantages of the idea proposed here – that power is produced in pulses. First of all, the fact that generator rotor and “stator” components are counter-rotating means that more pulses per second are produced than you might otherwise expect. (The more pulses the better. If we had enough pulses then they’d all bunch together and we’d have continuous power. As a matter of fact, three phase power is produced in pulses as well, yet these pulses combine to produce power that is perfectly constant. Might we find a way to exploit this three phase effect to make the power output from this machine constant? Don’t know, and too tired to think about it right now, so maybe I’ll revisit this later. But anyway it may not matter. I’m not concerned about the electrical pulsing – we can easily deal with that using power electronics. I’m more concerned about the pulsating mechanical loads, because these will fatique mechanical components and cause them to fail. On the other hand, the good news is that this pulsating load is confinded with a small space that is enclosed by the slots that guide the blades. This is good, because the more confined it is, the more options we have for dealing with the cyclic load. One option being, for example, just beefing up the support structure in that area. This is possible because, again, this area is aerodynamically shielded from the outside wind because it lives inside the slot.) Anyway, as I said, the because the blades are counter-rotating, they pass each other at a relatively high frequency. So maybe we can just design the machine to have many small blades (i.e. many blades, each having a short chord). Now when all these blades counter-rotate, we may end up with so many pulses that the output power looks like DC with a ripple on top. (Remember that adjacent blades don’t necessarily need to be separated by a constant angle. For example, just because there are (say) 6 blades that rotate (say) clockwise doesn’t mean that each adjacent blade must be separated by an angle of 360/6 = 60 degrees.)

Ring Generator Option

If the pulsating loads turn out to be a showstopper, then we can always fall back on the ol’ ring generator approach. In this case, we have the advantage that the rings are counter-rotating, thus doubling the velocity between magnets and coils.

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April 28, 2009

High Speed Centrifugally Stable VAWT

(Note – there are some errors in this post that I haven’t had time to fix yet, but I’m sure that if you know mechanical engineering you can easily correct the errors yourself. I think this idea might have potential once the errors are corrected. Note also that the torque tube will probably remain fixed with respect to the stationary tower rather than rotating around it. Also note that the struts each need to be connected by a vertical lattice (near the stationary tower) to keep them separated… that is, to prevent the load that tends to bend the ends of the struts towards each other from being transferred to the rest of the structure, thereby defeating the fundamental purpose of the idea.)

(Okay, here’s a pic with some errors corrected, but with no explanation:

High Mechanical Efficiency Centrifugally Stable Darrieus Turbine

)

High Speed Centrifugally Stable VAWT, Side View

High Speed Centrifugally Stable VAWT, Aerial View

This is a 3 bladed turbine, but I have drawn only two blades in order to make the illustration easier to understand. And I realize there are a lot of “legitimate” mechanical designs to realize this concept, likely using gears instead of tires and so forth. But I’m not a mechanical engineer, and so I just want to draw something that will give the real designers an idea they can play with.

Because the tower does not rotate, the rotor can be very tall, very slender, and it can spin at high rpm without becoming centrifugally unstable. But can’t the stationary tower can bend just as much as the rotating tower? And if the stationary tower bends, won’t this cause the rotating part of the structure to become centrifugally unstable just as if the tower were rotating? No. To see this, consider what happens when the middle of a rotating tower bows in response to the lifting forces transmitted to the tower from the airfoil by the middle strut. In this case, the middle of the rotating tower bows in the downwind direction, but its rotational axis does not change. Therefore the mass of the rotating tower has been displaced from the rotational axis, and centrifugal force now acts to cause even more bowing, and the rotor has become unstable. But when the middle of the stationary tower bows in the downwind direction, the rotational axis of the middle struts and airfoils moves along with it. And so although the rotor’s axis of rotation is no longer straight, it is at least centrifugally stable.

Another advantage of this design is that the guy wires are not connected to the tower through bearings. This should provide a big reduction in mechanical losses, since the bearings at the top of a traditionally guyed Darrieus bear a very heavy load – the rotor’s overturning moment. Of course, the overturning moment must be supported somewhere by some bearings. This design has bearings inside the rings that the struts attach to. So is there any advantage in this compared to the traditionally guyed Darrieus? I’m not a mechanical engineer, so I don’t know. Maybe there’s no advantage at all, but I’m wondering if the approach here isn’t better because it is easier to influence the bearings at design time. For one thing, you can spread the load over as many bearings as you want, while the traditional design requires two sets of bearings – one at the top of the tower and one at the bottom. For another thing, the guy wires in the traditional design are not only trying to torque the bearings about a horizontal axis, they are also doing this cyclically, from very low torque to very high torque several times a second. Surely this can’t be good. Of course, the present design also places a cyclic load on the bearings – there’s no way to avoid that. But at least it’s a “typical” load in that it doesn’t try to twist the bearings to a new axis. So maybe this is a better approach. It seems to me that mechanical losses will be decreased by eliminating the torquing thing, but again, I don’t really have the background to know if this claim is accurate.

April 24, 2009

HAWT with Torqueless Shaft

HAWT with Torqueless Shaft

April 23, 2009

Wind Farm Hydro Drive

Filed under: Wind Turbine Drive — Tags: , — Salient White Elephant @ 10:47 pm

The gearbox and generator are eliminated with this hydro drive. Instead of electric cables carrying power, water pipes carry power from each turbine to a central collection point. The water pressure at the central collection point is converted to electricity with a water turbine. An air tank buffers pressure changes.

Wind Farm Hydro Drive

Wind Farm Hydro Drive Driving Generator

I guess this idea needs a means of regulating pressure on a per turbine basis. That way if the wind speed is lower for one turbine than for all the others, the low wind speed turbine will still be able to actuate the hydro-drive. On the other hand, I guess this could be solved by providing each turbine with a variable speed transmission. Actually, this would be pretty easy to do if a crankshaft is used to drive the water pump piston. All you need to do is vary the radius of the off-axis part of the crankshaft.

Reciprocating Electric Generator Variation

This variation eliminates the water turbine at the central station where water pressure is converted to electricity. The water turbine is replaced with a reciprocating piston exactly like the kind that the turbines use to raise water pressure in the first place. So you just reverse the process, and the reciprocating piston drives the electric generator.

April 11, 2009

Reciprocating System for Transferring Wind Turbine Power Down the Tower

Reciprocating System for Transferring Wind Turbine Power Down The Tower

April 3, 2009

Closed Loop Direct Drive for Highly Scalable Horizontal Axis Wind Turbine

Closed Loop Direct Drive for Highly Scalable Horizontal Axis Wind Turbine

This drive system has sensors in the turbine blade that measure how much each blade is bent by lift and other forces. The shrouds that house direct drive generator windings are moved in the upwind or downwind direction in order to properly position them for the approaching turbine blade. What kinds of sensors might provide the proper information to the closed loop control system that adjusts the position of the generator windings? I don’t know. Strain gauges embedded into the blade spars that communicate with the controller via a radio link? A cylindrical cavity inside the blades that runs in the longitudinal direction and that has a laser at one end and a mirror at the other?

Another idea would give the turbine blade a permanent magnet that can protrude (longitudinally) from the blade tip. With this design, the shroud that houses the generator windings move to a location that is completely outside of the rotor disk (i.e. the swept area). As a blade approaches the windings, its magnet is pushed out of the blade tip. Once it passes the generator windings, the magnet is retracted so that it doesn’t create drag during the rest of the blade’s trip around the rotor disk.

Well it’s a couple of days after I wrote this, and re-reading it, it seems like a pretty stupid idea. I guess I just keep thinking about how once the turbines starting getting really big (1 megawatt or so), then the really sexy power electronics came onto the scene. Making this blog has proven to me that wind turbine innovation is likely to come from integrating other already well-known technologies, like power electronics. They didn’t invent power electronics for wind turbines, but once the machines got big enough to justify the cost of integrating these well understood devices and circuits, the addition of them provided much improved performance, and probably cut the cost of energy as well. So I’m just thinking that closed loop control technology might be one of the next things. Like for example, suppose it’s difficult to manufacture a giant ring generator and still maintain the tight tolerances required for good performance (given temperature changes, etc). Then maybe strain gauges of other sensors could be added, and a closed loop control system could adjust the radius of the rotor or stator at various angular positions… kind of like the way you turn the little nut-like things on a bicycle wheel to make it perfectly round and so that it won’t wobble side to side. So instead of trying to make the perfect ring generator, you just let the control system continually adjust the various tolerances based on sensor readings taken every minute or so. One thing’s for sure… pretty much every wind turbine configuration that can be thought of has already been thought of over all these centuries. So I wish I was in a position to look at how other technologies might be integrated in order to lower the cost of wind electricity… but I’m just not in a position like that. (Another thought that comes to mind is self-health checks… like they have in avionics systems… they have raised the detection of equipment failure or underperformance or abnormal performance to a high art form.)

Magnets In Rotating Shroud

Magnets In Rotating Shroud
This design is applied to the Highly Scalable Horizontal Axis Wind Turbine. I would put the rotating shroud on the other side of the nacelle, away from the blades, but that would require the mechanical power to go through the shaft, and I was trying to avoid that. I wonder if you could just have a rotating shroud with no magnets in it, and wedge its trailing edge between two tires and directly drive an off the shelf generator?

Rotating Shroud Drives Tires

March 23, 2009

Direct Drive System for Horizontal Axis Wind Turbine

Direct Drive System for Horizontal Axis Wind Turbine, Mechanism That Catches Blade Tip

Direct Drive System for Horizontal Axis Wind Turbine

The diagrams are not very well done, and certainly not sufficiently detailed. But basically what happens is that a blade catching mechanism follows the blade that it catches through the arc shaped guide. After the blade moves past the end of the guide, the blade catching mechanism begins its return trip back to the beginning of the arc shaped guide. The blade catching mechanism will arrive at the beginning of the arc just in time to catch its blade as it comes back around. All three of the blade catching mechanisms behave this way, and they are synchronized in order to catch each blade with little mechanical shock on impact (i.e. a blade catching mechanism is moving at the same speed as the blade at the moment it makes contact with the blade).

The mechanical system that incorporates the three blade catching mechanisms also drives the (off-the-shelf) generator.

March 19, 2009

Flow Multiplying Anti-Vortex Drive

This machine has a blade tip that simultaneously performs three functions. It is a kind of elongated, arc-shaped vortex spoiler, and it is pitched relative to the oncoming wind so that it also accelerates flow through the rotor disk. It’s third and final function is to streamline and support the permanent magnets of a direct drive ring generator:

Flow Multiplying Anti-Vortex Drive

Flow Multiplying Anti-Vortex Drive

How can the (generator) rotor pass through the stator without hitting it and still maintain the close tolerances that are required? I don’t know, but perhaps some sort of guide system can be designed that routes it through the stator. The following diagram gives the general idea. It shows a wheel approaching a guide. The wheel is not necessarily aligned with the guide, but the funnel-like shape of the guide causes it to make any necessary adjustments in position.

router

One disadvantage of the Flow Multiplying Anti-Vortex Drive is that the structure housing the stator coils must yaw with the nacelle and rotor. However, if the drive idea is used together with the Highly Scalable HAWT idea, then the stator coils may remain fixed, and do not need to yaw with the rotor blades.

March 18, 2009

Flow Accelerating Ring Generator for Horizontal Axis Wind Turbine

Flow accelerators don’t seem appropriate for utility scale horizontal axis machines. It’s probably much easier to just make the blades longer, especially given that blades will probably generate far less drag in storm winds. But what if the real purpose of the flow accelerator is to hide a ring generator? In this case, the size of the flow accelerator is the minimum required to shroud and contain the generator. Any increase in power output due to accelerated flow is “icing on the cake”, since otherwise the accelerator is designed to have minimum storm wind drag profile.

Flow Accelerating Ring Generator for Horizontal Axis Wind Turbine
Though not shown in the diagrams, the end of the airfoil goes through a slot in the accelerator, so that the blade tip is actually inside the accelerator. Alternatively, some kind of metal extension can extend from the tip of the airfoil through the slot in the accelerator. The part of the blade that is inside the accelerator is attached to an arc shaped housing for the permanent magnets of the ring generator.

Given modern wind machines are approaching a scale that leaves even the engineers pointing and goggling, it makes little sense to keep taking the torque off the wrong end of the airfoil.

There are two major challenges to this design. The first is how to accommodate for bending and vibration of the end of the rotor blade. The second is how to maintain close mechanical tolerances in such a large ring generator. I have not given thought to the specifics of how these problems might be solved, but I do have a philosophical approach I’d like to share with you. When a pilot turns the steering wheel of his state of the art modern airplane, the ailerons are electronically actuated. As expected, electric motors and microelectronics realize the actuating system. This system has a very cool trick for moving the ailerons to just the right angle. The trick does not involve impossibly accurate and brittle (in the sense of non-robust) design and manufacturing processes. The electric motors are not precision Swiss watches. Instead, a feedback loop is employed. The challenge in designing such a system mostly involves control theory – a well understood science. I am suggesting that the successful implementation of a “mega-ring generator” may possibly employ the same approach.

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