Salient White Elephant

May 19, 2009

I Dream of Genie Wind Dam

I Dream of Genie Wind Dam Construction - 1

I Dream of Genie Wind Dam Construction - 2

jeannie4

I Dream of Genie Wind Dam Construction - 3

Genie

I Dream of Genie Wind Dam Construction - 4

Now take a bunch of the structures depicted in the last step and connect them together with arcs to form a giant circular wind dam:

I Dream of Genie Wind Dam

Genie

Drive System

Notice that since a given “Totem Pole” of HAWT rotors are stacked one on top of another, they can share a common drive chain. This common drive chain ultimately drives a sprocket that drives the generator shaft. The generator is located in a small compartment on the ground, beneath its “Totem pole” of turbine rotors.

Does This Turbine Require a Yawing System?

I can think of two variations of the I Dream of Genie Wind Dam as regards the yaw drive. The first version would always have all of the rotors turning, regardless of which direction the wind is blowing in. This would be easier to explain if I could draw a nicer 3D model, but I’m not much of an artist, so bear with me here. All rotors can rotate simultaneously because some of the high speed wind moving around the outside of the dam seeks the “smallest radial path”, meaning it will eventually force its way through a rotor to the inside of the wind dam. The wind outside the dam that does not manage to find its way into this smaller radial space will instead shoot off the bowed out end of one of the wall sections (much like the wind shoots off the trailing edge of an airfoil). This will create a low pressure region in the smaller radial locations that “suddenly appear” next to this high speed air at the moment it shoots off that bowed out edge of the wall. This low pressure sucks low speed high pressure air from the inside of the dam through the rotor that is near the high speed air that is shooting off the bowed out end of the wall.

In the variation just described, the rotors do indeed need to assume one of two yaw angles, each separated by 180 degrees. (Alternatively, the blades may have variable pitch so that they can accommodate the flow of wind in either direction through the rotor.)

The other variation would have doors that can block the “hole” that is occupied by the rotor. In this variation, all the rotors that pass wind from the outside of the dam to the inside of the dam (given wind direction) have their doors opened, while the doors of all the other rotors are closed, rendering those rotors inoperable. Or you could do it the other way around, allowing air to be sucked out of the inside of the wind dam, but not to be forced from the outside of the wind dam to the inside. In this case a yaw system is not required. This is so because the door will always be closed when wind has a tendency to flow through the accompanying rotor in the “wrong direction”.

Third World Variation

Think of how easy it would be to build a low cost variation of this machine for the developing world! Imagine that instead of building a circular dam, we’ll build one that is polygonal, with an approximately circular shape. Maybe the circle has 10 or 12 sides. So we put concrete columns or telephone poles up in the shape of the polygon. We have one set of telephone poles for the “smaller radius” polygon, and another set for the “larger radius” polygon. Now we string cable between the poles. The following diagram shows just one side of the polygon, and uses blue to represent the cables connecting the “larger radius” polygon, and green to represent the cables connecting the “smaller radius” polygon:

I Dream of Genie Wind Dam, Third World Variation, 1

Now take some of that beautiful multi-colored fabric like they have in India and wrap it back and forth around the cables to imitate the I Dream of Genie Channels:

I Dream of Genie Wind Dam, Third World Variation, 2

When wind flows around sharp edges, it tends to create turbulence. To avoid this tendency, we might like to use something with a larger diameter than cables. How about we use cheap PVC pipe? Then we can string the cables through the inside of the PVC pipe. This way the cables can provide a great deal of strength and stiffness to the structure, yet the cables will have no adverse affect on the aerodynamics.

For More Information on Wind Dams

Genie
For further information related to this idea, see the earlier Salient White Elephant post: Circular Wind Dam.

May 12, 2009

Practical Artificial Pressure Differential Wind Turbine

Explanation of Artificial Pressure Differential Turbine 1

Explanation of Artificial Pressure Differential Turbine 2

In this way we have brought the low pressure on the downwind side of the parachute to the top of the tower.

Now, in your mind’s eye, eliminate the low pressure tube, and make the parachute whole again. Now attach a high pressure tube to the vertex of the parachute:

Explanation of Artificial Pressure Differential Turbine 3

Now we have techniques for bringing the high pressure of the upwind side of the parachute back to the tower, and for bringing the low pressure of the downwind side of the parachute back to the tower. Next, we build both the high pressure and low pressure extending tubes into the parachute at the same time. The low pressure tube has the smaller diameter, and it connects to the hole in the vertex of the parachute which has the same diameter. The diameter of the high pressure tube is larger than the diameter of the low pressure tube, and it encircles the low pressure tube so that a cross-section of the two tubes makes them look like concentric circles.

The next step is to transmit the high and low pressures to the bottom of the tower using the same technique. The tower is actually two towers – an inner smaller diameter low pressure tube with an outer larger diameter high pressure tube surrounding it so that a cross-section of the two makes them look like concentric circles. The high pressure and low pressure regions are connected at the bottom of the tower and, as expected, a HAWT rotor (with a vertical axis) is positioned between the two. The rotor, gearbox, and generator are at all at ground level.

The only thing I haven’t explained is how the low pressure and high pressure tubes make a right angle turn at the top of the tower. I was planning to draw some pictures of this, but I don’t think it’s really necessary. There are probably a million ways to do this. I will just note here that the parachute part automatically seeks the downwind position, and so it doesn’t require a yaw system. The right angle joint can be yawed, or it can simply be a cylindrical piece with vents positioned radially about its center. The vents have doors in them that can open and close to simulate yawing, though few moving parts would actually be required. (To illustrate, imagine an aerial view of this machine. Suppose the high and low pressure tubes approach the tower near the 9 o’clock position. Then the vents in the cylindrical piece at the top of the tower that are at positions 8 o’clock, 8:30, 9:00, 9:30, and 10 o’clock would all be open, while all of the other vents would be closed tight and aerodynamically sealed.)

The tower supports little weight, and it can be fitted with vents to let storm winds pass through unimpeded. This means the tower can be very light in weight, very inexpensive (relative to a typical HAWT tower), and it can have a large diameter if necessary. The parachute and the fabric part of the low pressure and high pressure tubes may have similar vents so that they also create little drag during storm winds. Maybe the parachute and fabric vents could even be somehow rolled up and stored inside the tower during storm winds.

Finally, note that the tower could be incredibly high. This is true because it supports little weight, has little overturning moment in storm winds, and can accomodate multi-level guy wires that can attach to the tower at any elevation, including at the very top of the tower!

Artificial Pressure Differential Turbine

Yawing Variation

Now we might imagine a long horizontal tube extending from the top of the tower. The supporting cords that tether the parachute are attached to the end of the horizontal tube that is far from the top of the tower. This way, the “vertex” (downwindmost end) of the parachute is right at the top of the tower, and the low and high pressure regions may easily be connected to the vertical (tower) low and high pressure concentric tubes. The horizontal tube yaws to align with the wind.

There are many other variations like this. Maybe we just build something that looks like a giant radio telescope dish, and attach its vertex to the top of the tower. This might not be so ridiculous if the parabolic dish has slats that automatically open when the pressure differential between the upwind and downwind sides of the slats exceeds a safe value.

Jet Stream Ram Air Wind Turbine

In earlier posts I have mentioned that a turbine capable of harvesting the energy of jet streams would probably be better for newspaper headlines than for an economical approach to wind electricity, since it would probably be cheaper and more effective to build several smaller low altitude turbines than a single monster that could tap into the jet streams. But it got me to realize that there are no jet stream turbines on the Salient White Elephant. This is Salient, to be sure, but is it White Elephant? Certainly not! And already I can hear not a little hubbub from the Canadian Parliament behind me patting their tables and gushing heah heah! So let’s just round things out with a couple of jet stream turbines before tensions run too high and one of the hairs on the head of the Right Honourable Stephen Harper springs noticeably out of place, shall we?

Jet Stream Ram Air Wind Turbine

For some reason, I’m usually biased toward using suction rather than high pressure in my flow accelerator ideas. But one advantage of using ram air pressure in the machine proposed here is that it would keep the long fabric tube inflated. This is very significant of course, since one of the biggest challenges in designing an airborne turbine is keeping weight to a minimum. Using high pressure might eliminate any rib-like supporting structure that would otherwise be required for the tube. I guess you’d have to stabilize the fabric tube by attaching it to the tethering cables at various intervals, but who knows… maybe somebody can design a way around this requirement.

Triple Tethered Variation

Jet Stream Ram Air Wind Turbine, Triple Tethered Variation

Multiple Blimps Variation

There are many variations of the ideas proposed here, but let me discuss one in particular. This idea emphasizes a technique I’d like to use to bring these pie-in-the-sky airborne turbines a little closer to feasible. Imagine eight blimps. Each is tethered by at least three cables to keep the blimps from moving around too much. An aerial view would reveal that the blimps are situated at the vertices of a gigantic octagon. It is important to note that the “diameter” of the octagon is far from insignificant. I can’t give you a number… maybe two or three football fields? Each blimp has a parachute and a high pressure tube, just as described above. All of the high pressure tubes converge at the center of the octagon, where they connect to a single larger high pressure tube that takes the jet stream wind down to the ground.

What’s so great about this variation? Well… let me first list what I believe may be the salient objectives of airborne turbine design:

  • If possible, no moving parts in the air.
  • If possible, no fiberglass, electrical cable, gearboxes, drive shafts, or electrical generators in the air. (Ever notice how the components of a wind turbine that have to do with mechanical and electrical power are about the most dense (heaviest) things known to engineering kind?!)
  • MINIMIZE WEIGHT, MINIMIZE WEIGHT, MINIMIZE WEIGHT!!!!!!!!!!!

So the idea here is that instead of having eight different tubes, we attempt to minimize weight by having a single large tube carry wind from the jet stream to the ground. This is desirable because the really long distance is from the jet stream to the ground. Once at the center of the jet stream octagon, it isn’t much further to the blimps. So could we use this trick to reduce the overall weight of the machine?

Well, whether this trick will work or not… I think you see my point. What is needed is a kind of linear programming style optimization that minimizes weight of fabric per kilowatt of capacity.

Can We Really Reach the Jet Stream?

No. The jet streams are like 30 to 40 thousand feet off the ground. (The cruising altitude of jet airplanes!) So we can’t reach the jet stream with the design proposed in this post. But we can certainly reach a higher altitude than today’s state of the art wind turbines! If you want to see a more practical configuration that uses the principles described in this post, check out the Practical Artificial Pressure Differential Wind Turbine.

May 10, 2009

SkyScraper with H-Rotors

This is a very simple idea. Imagine a skyscraper that looks round in an aerial view. Now just put a bunch of disks around the outside walls of the building… (say) one disk every 6 stories or something like that. Now near the outer perimeters of these disks, you cut a slot as described in many of the other posts on this blog. (See for example 20 Megawatt VAWT.) The ends of the airfoils engage these slots. The blades go around the building with each level rotating with the same polarity as its upstairs and downstairs neighbors, or else with adjacent levels rotating with opposite polarity to double the speed difference between the generator rotor and the generator stator:

SkyScraper with H-Rotors

Mechanical energy may be converted into electrical energy in any of several ways. Once again, all of these conversion methods have been described multiple times on this blog, so I won’t repeat them here. I’ll just give a short list of the possibilities:

  • Generator windings are embedded into the slots. A permanent magent is attached to the ends of the blades and the motion of the blades forces these magnets past the windings.
  • Cables are attached to the middle of each rotor blade. These cables are attached to a smaller diameter cable. The diameter of the smaller diameter cable is just large enough to around the building without hitting it. The smaller cable drives the generator. All cables should be designed to carry only the power producing components of the airfoil loads. The larger (non-power producing) loads are carried by the disks that support the blades tips.
  • The slots that engage the blade tips are provided with a great many small wheels inside. Airfoil motion imparts spin to these wheels. All wheels are mechanically linked so that they all rotate at the same rpm. The rotational motion of the wheels (or of their mechanical linkage) drives a generator.

One question is whether blades that are rotating and producing power would be a visual distraction to the people inside the building. I honestly don’t know what the answer to this question would be. Because the building is high, and because the building itself acts as a flow accelerator, you can imagine that the blades might be traveling at a very high velocity indeed. I have no idea what the velocity would be, but let’s say they’re going 200+ miles per hour. Could you even see a blade moving at that speed? Anyway, if this turns out to be a problem, maybe one should explore the opposite approach – a great many blades having a slow, gentle movement.

Direct Drive Counter-Rotating Cat’s Eye Variation

In this variation, each level (each set of adjacent supporting disk-like structures) is provided with two sets of blades. Half rotate clockwise, and the other half rotate counterclockwise. (These blades may or may not be designed to imitate the behavior of the Cat’s Eye Darrieus Rotor.) The ends of all of the blades rotating with one polarity are attached to permanent magnets, while the ends of all the blades rotating with the other polarity are attached to windings. As two blades that rotate with opposite polarity pass by each other, their magnets and coils pass close to each other in a manner similar to what would be witnessed if you could look inside of an electric generator. The magnets and coils are attached to the parts of the ends of the airfoils that are hidden inside the slot, and thus create minimum drag and turbulence. The mechanical tolerances that must be maintained to minimize the air gap that the flux must traverse are acheivable also because these components live within the rigid and tightly controlled confines of the blade guiding slot.

Geodesic Dome Turbine

Start with a Geodesic Dome:

Geodesic DomeNow Cut a hole in the top, and cover the hole with a shroud that can yaw in order to keep its opening pointing upwind. Also add vents near the lower part of the dome can than be either opened or closed:

Geodesic Dome TurbineThe variation depiced above has air flowing into the hole at the top of the dome and out of the vents below. I’m not sure this is the best arrangement. The alternative would be to have air flowing into the lower vents and out of the hole in the top. In this variation, the shroud over the top hole in the diagram would be yawed (rotated) 180 degrees, and (I’m guessing) the left two vents in the diagram would be open, while the other vents would be closed. I guess one of these ideas is probably aerodynamically superior to the other, but I don’t know which is which. It’s worth noting that the real low pressure should be at the top of the dome, since this is where the wind has been accelerated the most. Seems like it might make sense then to let the wind flow in to the lower vents (where pressure is naturally higher), and out through the hole in the top. I don’t know much about the theory of fluid flow, so I’ll leave the rest to those of you who have the academic background to model and solve a problem like this.

Before ending this post, however, I’d like to point out an aspect of this idea that is particularly intriquing. Since you can have lots and lots of vents, but only one hole in the top of the dome, it stands to reason that it should be easy to provide the dome with many square feet worth of vents, given the area of the hole at the top of the dome. This means that the velocity of wind flowing through the vents may be caused to vary by only a small amount relative to the velocity of the wind outside the dome. For this reason, it would seem that the vast part of the lower part of the dome could be made to be quite comfortable for people, and this means the dome can have alternative uses. For example, the dome could house a giant botanical garden for the public to enjoy. If you really wanted to control the environment for the people inside, two concentric geodesic domes could be placed one on top of another, creating a thin (say) 40 foot wide gap between the two ceilings. The gap could be used for wind flow and the harvesting of its energy, while the part inside the lower (smallest) dome could be used for just about anything – commercial office space, a manufacturing plant, basketball court, … you name it!

Retrofit Option

Depending on how attractive this structure could be made to be, and on its cost effectiveness, we might imagine putting one of these things on top of an existing structure. Rooftop wind turbines are generally frowned upon, but I think this is mostly because of the harshly turbulent conditions within which a rooftop turbine must normally operate. The dome solves this problem in several ways. First, it doesn’t have any sharp turbulence producing edges. Second, its rotors are small and therefore less sensitive to turbulence, and they can be located in a short cylindrical shroud that is equipped with the same kinds of turbulence attenuating apparatus as is found in wind tunnels. And finally, if a small rotor does eventually develop a crack due to turbulence-induced fatique, simply replace it. It’s small, and so the cost of replacing it is no big deal.

I saw a medium sized, three story motel the other day that looked like it could easily accomodate a Geodesic Dome Turbine. If the dome had enough vents to open during a wind storm, it would seem likely that the motel could accomodate the turbine in spite of the fact that the building designers had not accounted for the extra load.

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 25, 2009

Horizontal Savonius Circular Wind Dam

Horizontal Savonius Circular Wind Dam

Sustainable Skyscraper

A very tall building reserves some floors for producing energy. I guess you’d have to produce a heck of a lot of energy to generate as much revenue as you could get by leasing the space instead. I haven’t crunched any numbers or anything, but since the wind might be very strong at these altitudes, I’m guessing maybe it would work. Remember that the turbines would produce power 24 hours a day, and seven days a week. The office space would only be used for a fraction of that time:

Sustainable Skyscraper, Side View

Sustainable Skyscraper, Aerial View

Even more energy can be concentrated at the turbine if some of the wind from the non-turbine floors could also be collected. This might not be as difficult as it sounds:

Aerial View of Floor Used For Office Space Showing Flow Concentrating Windows, Sustainable Skyscraper

High pressure develops on the upwind side of the building in the cavity formed by the flow concentrating windows. Much of the air will simply escape around the outside edges of these upwind windows, but a lot of it will escape by flowing in the vertical directions (both up and down). Once this air escapes by moving either up or down, it will find itself at the entrance of the flow concentrating panels on one of the power producing floors. It thus augments the flow through the pie slice shaped flow concentrator. The same thing happens (but with opposite polarity) on the downwind side of the building. Note that these outside windows may span several floors of office space. In this case, we don’t have several levels of windows. Instead, just one tall window spans however many office floors are between the power producing floors.

It also might not be that difficult to think of ways to make sure no accidents happen. For example, the windows that can swing open are certainly designed so that there’s no way the wind could ever be strong enough to tear them from the building. But just to make sure, a cable could attach to the middle of the top and bottom of each window. The other end of the cable would be right above or below on one of the power generating floors, and it could attach either to the floor or the ceiling as the case may be. An alarm is activated if ever the window detaches from the building. Now there are 4 mechanisms that simultaneously guarantee the safety of the public:

  1. the windows are designed to be strong enough to withstand any weather conditions,
  2. the bottom of the window is tethered to the building in case the design proves to be flawed and the window tears away anyway,
  3. the top of the window is also tethered,
  4. an alarm notifies the authorities if ever a window becomes detached from the building (leaving it hanging from the 2 tethering cables). If the alarm is ever triggered, the streets below may be quickly evacuated.

Narrow Flow Concentrating Channels Variation

Here’s a variation that looks like it might make better use of real estate. (A mathematical analysis should be developed to verify whether this is indeed the case.) It is difficult for me to draw this variation, so let me first present a crude drawing that has some structures omitted, then I’ll explain how it works:

Aerial View, Sustainable Skyscraper with Narrow Flow Concentrating Channels

The panel in the center of the building yaws so as to separate the upwind flow concentrating channels from the downwind channels. The height of this panel is equal to the height of the building. High pressure air in the upwind channels is forced up to the roof of the building. This air is now drawn down from the roof of the building through the low pressure downwind channels. The turbine rotor and generator are on the roof. One side of the turbine rotor faces the high pressure air and the other side faces the low pressure air. In this way, a single turbine rotor and generator converts the wind to electricity. Alternatively, several (2 or 3) rotors are positioned (say) 10 stories apart, so that each turbine converts 10 stories worth of wind energy to electricity.

April 23, 2009

Blimp VAWT

Since no people will ride in the blimp, I guess it could be filled with hydrogen.

Blimp VAWT

I’m always drawing 2 bladed VAWTs when I really mean 3. A three bladed VAWT produces relatively constant power, and more importantly in this case, thrust. Since wind speed will normally increase with altitude, blade radius increases with altitude so as to maintain a more constant tip speed ratio. For simplicity, I drew only 2 guy wires. In reality, at least three would be required.

The turbine’s rotational axis can tilt, and the gearbox and generator tilt along with the rotor axis. This allows the wind turbine to seek its optimum orientation given wind speed and other parameters. Alternatively, perhaps the lower end of the wind turbine (the generator) is on wheels and is able to move if the turbine wants to move. Since the structure will be very heavy, the control system would likely control the movement of the generator this way and that along the ground as wind conditions change. But if we let the lower end of the turbine move, then we may as well save some weight by using only one guy wire or lattice to moor the blimp. In this case, the turbine will “yaw” by roughly tracing out a circle about the guy wire anchor point as the wind direction sweeps through a 360 degree arc.

The teardrop streamlined shape of the blimp is critical. If any old shape is used for the floatation device, then the wind will tend to carry it downstream. I don’t know what the optimum aerodynamic profile would be, but you would want to minimize drag. For this reason, perhaps a very elongated shape (kind of like a cigar) might be used.

Kites and Airfoils

Additional lift may be generated if a kite is positioned on top of the blimp. Imagine, for example, those modern parachutes that look like a big airfoil made out of something like nylon. If one of these is mounted on a vertical pole that extends upward from the blimp, then it will “take flight” when the wind speed reaches a sufficient value. In this case it will be available to add further support for the turbine whenever the turbine is running and producing power. Some kind of inflatable airfoil might do the same trick. If a kite or modern parachute is used, it will be suspended by its supporting structure in a shape and orientation that “prepares it” for taking flight when the wind picks up.

Other Variations

I think there must be many ways to develop better configurations if more than one blimp is employed. For example, two blimps could be moored to a giant circular railroad track. The anchor points are “yawed” so that a line connecting them forms a right angle with wind direction. The blimps are connected by a cable, and multiple VAWT turbines are suspended from the cable that connects the two blimps.

Weight is obviously one of the primary issues with airborne turbines. Perhaps some sort of inflatable airfoils can be used. The airfoils may even themselves be filled with hydrogen. Or a design similar to those light-weight modern parachutes might be used for the blades.

April 21, 2009

Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Ideal Path for Wind Turbine Blade

Doesn’t it seem like the ideal path for a wind turbine blade would have the entire blade moving at “tip speed”, spending the majority of its time traveling in a direction that is orthogonal to the direction of the wind?

Description of the Machine

Aerial View Semi-Direct Linear Turbine With Yawing Oblong Track Built Into Cornfield

(Note that the word “power cable” in the diagram below does not refer to an electrical cable, but rather to a cable that carries the mechanical power to the pulley wheels at the generators.)

Side View of Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Looking at Leading Edge of Airfoil Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Cross-Wind View of Tower Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Bowed Airfoil Rides Padded Rims of Pulley Wheels Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Aerial View of Apparatus That Routes Airfoil and Power Cable Round Tower Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Drag on Moving Cable

How much aerodynamic drag does a moving cable add to the system compared to the drag of a strut or the low speed part of a troposkein Darrieus blade? Consider an H Rotor supported by a strut. The instantaneous velocity of a given radial location on the strut is at right angles to its longitudinal dimension. For this reason, it is able to leave a semi-evacuated space behind it as it moves. This space is filled with turbulent air. But the velocity of a given cylindrical cross-section of a cable is along its longitudinal dimension. Because there is always more cable following any cylindrical cross section, it is impossible for the cable to leave a semi-evacuated space in its wake. The only semi-evacuated, turbulated space it can leave is on its downwind side, and the force thus produced cannot drag down the system because it is orthogonal to the velocity of the cable. The only aerodynamic drag that is possible is that drag that is created by the cable’s ability to “stick to the air around it”. I’m not an aerodynamicist, but I’m guessing this might turn out to be pretty small. Also, the moving cable in the machine presented here has small diameter because it moves at “tip speed”, and because it is only required to carry the torque at tip speed. Because tip speed is high, torque will be low, and the cable will have small diameter.

Circular Variation

One obvious disadvantage of the oblong path is the need to yaw the track. We can eliminate this problem by providing the turbine with a polygonal path that approximates a circle:

Aerial View Showing Polygonal Path That Is Approximately Circular

Airfoils Could be Slowed to Go Around Tower

To avoid inflicting excessive centrifugal force on the blades as they go around the tower, maybe the control system could sense the point at which blades are nearing towers, and slow them down somewhat as they make their way around the tower. Alternatively, mechanical support could be provided on the opposite side of the airfoil to prevent excessive bowing due to centrifugal force:

Wheels Support Airfoil as it Rounds Tower

Low Drag Airfoil Variation

The airfoils need to have gaps near the regions where the load carrying cables penetrate them. This is necessary to accomodate the mechanical supports for the apparatus that steers the airfoils around the tower. But if the airfoils carry a small generator, like the generator that rubs against a bicycle tire in order to power a light for the bicycle, then these gaps could be filled with streamlined shapes during the long path between the two towers. The streamlined shapes would be pulled out of the way as the airfoil makes its way around a tower. The actuating mechanism is powered by electricity that is generated from the small generator just described that is located inside the blades.

Alternatively, maybe the streamlining shapes could be pushed out of the way by some mechanical means as the airfoil goes around a tower.

March 29, 2009

Eye of the Cat

From an aerodynamic perspective, the ideal vertical axis wind turbine rotor is the “H Rotor”:

H Rotor

Unfortunately, from a mechanical point of view, the H Rotor is terrible. This is so because as its blades bow out, seeking the troposkein shape, a tremendous load is applied to the horizontal beams that support the blades. This load is in exactly the direction that the beams are least able to support (tending to pull the ends of the beams together).

Mother Nature Doesn't Always Care What We Want

Now you know why the only H Rotors you see on YouTube are on small turbines rather than on utility scale machines. The small H Rotor is essentially a brute force solution to the problem. That is, if you’re only interested in small machines, then you can just make the stiffness of the blades and blade supports large enough to resist the tendency to assume a troposkein shape. Don’t get me wrong… I think the H Rotor is a good idea for small machines that will run the meter backwards at somebody’s home. But I just want to point out the irony in this approach, for if you go in the opposite direction, the problem goes away! To see this, simply imagine an H Rotor with a diameter of 10 miles. Now the velocity of the rotor blades is essentially linear, and there is no centrifugal force to bow the blades! (A little thought will reveal that the centrifugal force disappears because the airfoil velocity is always the same, regardless of the rotor diameter. This is true because the blade velocity is determined by wind speed and by the desired tip speed ratio. So if the wind speed is 20 mph and the desired tip speed ratio is 6, then the airfoil will travel at 120 mph regardless of the rotor diameter.) Of course, the aerodynamic forces also cause the blades to bend. In any case, it would be nice if there were an alternative technique, and there is! It is the mysterious and enchanting Eye of the Cat, and it eliminates the tendency of the blades to bow, whether this tendency is caused by aerodynamic forces or by centrifugal forces!

Eye of the Cat Darrieus

The aerodynamics of this rotor are close to ideal because we can select any blade shape that has the symmetrical force-balancing properties depicted above. (Actually the centrifugal force on the outer blade is greater than the centrifugal force on the inner blade, but they are approximately equal. If necessary, weight can be added to the inner blade to balance the opposing forces.)

I’m not sure what to think of the Cat’s Eye Turbine. It seems to me that it will work as described, but it does have some strange properties. For one thing, it is unstable. To see this, imagine the blades are supported by cables instead of beams. We have carefully added weight to the inner blades so that the vertical forces tending to increase the distance between the ends of the cables exactly balance the vertical forces that draw them together. However, any perturbation that slightly increases blade curvature also increases the centrifugal force on the outer blades, and decreases the centrifugal force on the inner blades. In this case, if the blades are sufficiently flexible, they will subsequently increase in curvature until the outer blade and the supporting cables form an approximately troposkein shape, while the inner blade mirrors the shape of the outer blade in the opposite direction. Furthermore, any perturbation that slightly straightens the blades will also increase the centrifugal force on the inner blades, and decrease the centrifugal force on the outer blades. In this case, both blades will eventually become perfectly straight, and then both blades will bow all the way out together until the whole structure, cable and blades, assumes a troposkein shape.

But on the positive side, consider how even the aerodynamic forces seem to resolve into approximately equal and opposite vertical forces on the ends of the supporting beams! Meow!!

It guess the feasibility of this machine boils down to whether the beams that support the blades can have stiffness sufficient for keeping the rotor from becoming unstable, and also whether the designers can put the rotor’s natural frequencies into an appropriate range.

Rotating Guy Wire Variation

Cat's Eye Darrieus, Rotating Guy Wire Variation

Traditionally Guyed Variation

Cat's Eye Darrieus, Traditionally Guyed Variation

Non-Vertical Axis Applications

The Eye of the Cat can also be applied to the Direct Drive Linear Turbine With Yawing Oblong Track, Radially Displaced HAWT Rotor, the HAWT With No Shaft.

March 26, 2009

Unmanned Aerial Vehicle (UAV) Turbine

Unmanned Aerial Vehicle (UAV) Turbine

One of the most significant features of the UAV Turbine is that its tethering system connects the airborne device to either two tracks, or else to one track and a ground anchor situated at the axis of rotation. Other airborne wind machines tether the airborne device with only a single cable. In this case, the airborne device is poorly positioned to make the transition from its upwind arc to its downwind arc, and vice versa. Of course, tethering with two cables isn’t very appealing. But one of these cables can be eliminated if an oblong track is used. For more on this, see Direct Drive Linear Turbine With Yawing Oblong Track.

Using small, light-weight tubes that are shaped like a symmetrical airfoil to tether the airborne device allows them to make a small contribution to output power. Otherwise the only contribution they make is drag.

The UAV Turbine can use kites instead of airplanes. Or it can use floating devices, such as the airfoils described in the Helium VAWT post.

March 25, 2009

Displaceable Guy Wires

A number of Salient White Elephant blog posts describe guy wire, tube, or lattice supported towers in which the supporting structures can be moved to make way for a yawing rotor or for other yawing components. Here’s an idea for realizing this functionality:

Displaceable Guy WiresTower With Three Guy-Wire-Like Displaceable Support Tubes

March 24, 2009

HAWT With No Shaft

HAWT With No Shaft

HAWT With No Shaft, Side View

HAWT With No Shaft

What if the blade is longer than the diameter of its supporting frame? In this case, the blade tends to act like a spring, pushing against the frame so that it deforms slightly into an oval shape. When the blade spins, centrifugal force makes the blade push even harder against the frame. But the aerodynamic force tends to bow the blade, giving it tend to pull on the frame and shorten its diameter. Perhaps these two opposing tendencies could approximately balance each other, thereby reducing loads on the frame. Bowing the blade has another interesting effect – it tends to make the blade tips orthogonal to local flow:

Tips of Bowed Blade More Orthogonal to Local Flow

The HAWT With No Shaft is supported by 5 guy wires. When the rotor yaws, it makes contact with only one guy wire at a time. When it comes to a point at which is about to make contact with a guy wire, the guy wire is moved out of the way. For ideas on how to move guy wires out of the way see my earlier blog post Scalable Tower for Very Large Wind Turbine. Here’s a combination of the HAWT With No Shaft and the Highly Scalable Horizontal Axis Wind Turbine:

HAWT With No Shaft, Highly Scalable Wind Turbine Variation

Radially Displaced HAWT Rotor

Radially Displaced HAWT Rotor, Side View

Radially Displaced HAWT Rotor, Downwind View

Only the airfoils rotate in this machine. The flow accelerators are supported in the manner of a wheel with spokes. Each end of each airfoil is attached to a cable that moves like a rotating ring inside of its flow accelerating shroud. The cables drive the generators. (Generators not shown in diagram.) Alternatively, the flow accelerating shrouds can house direct drive generators in the manner described in an earlier post entitled Flow Accelerating Ring Generator for Horizontal Axis Wind Turbine.

It might be worth considering a combination of this idea and the one described in Highly Scalable Horizontal Axis Wind Turbine.

March 13, 2009

Fat Tower Darrieus

A Fat Tower Darrieus is simply a Darrieus whose tower has a large enough diameter for the guy wires to go inside of it:

Fat Tower Darrieus

Counter-Weighted HAWT

Sometimes the simplest ideas are the most effective. What’s wrong with a turbine that has guy wires that attach to the top of the tower and a counter-weighted nacelle?

Counter-Weighted HAWT

One advantage of a three bladed rotor is that it is much quieter than a two bladed rotor. This is true because a three bladed rotor rotates more slowly than a two bladed rotor. But maybe a four bladed rotor could turn more slowly still, and maybe it would be even quieter. We can achieve this with counter-balanced counter-rotating two bladed rotors:

Counter-Weighted Counter-Rotating HAWT

One advantage of this last machine is that the downwind rotor can extract some of the rotational energy that the upwind rotor supplied to the streamtube. It may also be possible for one wind rotor to turn the generator rotor, while the other turns the generator “stator” in the opposite direction. This doubles the effective generator rpm.

Imperial Crown Darrieus

Imperial Crown Darrieus, No Wind

Imperial Crown Darrieus, Producing Power

Imperial Crown Darrieus, Lower Blade Tips on Rail Cars

Imperial Crown Darrieus, Lower Blade Tip Runs On Track

March 12, 2009

Extremely Large Horizontal Axis Wind Turbine With Three Rotors and Three Towers

This post describes an idea for an extremely large Horizontal Axis Wind Turbine. The turbine has three towers and three rotors. This idea is similar to the Radially Displaced Darrieus Rotor. (You might want to read that post first.)

This turbine has the same three lattice towers as the Radially Displaced Darrieus Rotor, and it also has a ring on top that can rotate about the center of the structure. In addition to this structure, put a long horizontal tube on top of the ring, and mount three horizontal axis rotors on the tube:

Horizontal Axis Turbine With Three Rotors and Three Towers

Horizontal Axis Wind Turbine With Three Rotors and Three Towers

Now if the three nacelles and their rotors are instead suspended from the horizontal tube on three short vertical tubes, then the force of gravity keeps the rotor disks vertical. If desired, the short vertical tube from which a nacelle and its rotor is suspended may rotate somewhat about the supporting horizontal tube above. This provides a teetering action that helps make to mitigate the fluctuating loads associated with wind gusts.

Radially Displaced Darrieus Rotor

This post describes an idea for an extremely large Darrieus Wind Turbine.

Imagine a lattice tower supported by two guy wires:

Lattice Tower with Two Guy Wires

Now add two more towers, each supported by two guy wires:

Three Lattice Towers Each Supported by Two Guy Wires

Now connect the tops of the towers with I-beams:

Lattice Towers and Connecting I-Beams

Now put a ring on top that can rotate with respect to the towers:

Lattice Towers With Ring On Top

Connect the ring to a vertical “torque tube” that is centered about the axis of rotation:

Lattice Towers, Ring, and Torque Tube

Now put a rotating ring near the bottom and add a couple of Darrieus blades:

Radially Displaced Darrieus Wind Turbine

The following diagram gives the general idea for synchronizing the rotating rings:

Synchronizing Rings and Torque Tube

Here’s another way to synchronize the rings:

Synchronizing Rings

Actually, I wonder if the gearbox would even be necessary in the above diagram. If the radius of the rings is large enough, then perhaps this could be a direct drive machine.

The diagrams above depict a two bladed Darrieus. A better design would have three blades in order to provide a relatively smooth power output.

Exceeding the Betz Limit

One of the disadvantages of a Darrieus turbine is that it typically extracts less energy from the wind than a horizontal axis turbine with the same swept area. However, consider a radially displaced Darrieus rotor with an extremely large radial displacement (i.e., the diameter of the rotating rings is very large). In this case the de-energized air will be re-energized by the time it reaches the downwind blade. Horizontal axis wind turbines are typically spaced two or three rotor diameters apart in order to allow the wind to be re-energized as it travels from one turbine to the next. If we make the radial displacement of the radially displaced Darrieus rotor blades equal to this distance, then this should be sufficient for allowing the wind to be re-energized as it travels from the upwind blade to the downwind blade. In this case, the Betz limit says that we should be able to extract a maximum of 2 x 60% = 120% of the wind’s kinetic energy. For this reason, a radially displaced Darrieus rotor should realize a much higher efficiency than a horizontal axis turbine with the same swept area.

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