h a l f b a k e r yIncidentally, why isn't "spacecraft" another word for "interior design"?
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In 2005 I posted an Idea, "Venetian Turbine" (linked), which was actually derived from this one. That is, I thought of the Sail Turbine first. Then I thought about how to intercept even more wind with it, and came up with the Venetian Turbine notion. Also, the VT is in some ways mechanically simpler
than the ST, as is indicated in my Dec 10/11 2007 annos of that Idea.
According to my personal records, I originally wrote down the Sail Turbine notion in 1980. I'll try to post one of the sketches I made at that time, and link it. Some years later I thought about patenting it, and discovered that a substantially similar gadget had been patented in 1984 (#4,424,002, linked), after being filed in 1981. There's no telling how long the other fellow had it in mind before filing it, so maybe I was actually first, maybe not.
Well, anyway, the Idea never became widely known, so why not post it here at the HalfBakery? (Not to mention that US patents typically expire after 17 years or so, which means anyone is free to make/sell this type of windmill now.)
The biggest difference between the two versions of the Idea is that mine uses sails and the other apparently uses rigid vanes, to intercept the wind. I chose sails because of the need to make the thing survive high wind speeds (sails can be furled! --see link).
OK, enough background; now consider this ASCII top-view sketch:
__---
_/_o_\
___|
Pretend the underscore characters represent the wind blowing from left to right. The small "o" in the center is the main axle of revolution. The dash at top, the vertical bar at bottom, and the two slashy characters at left and right represent 4 sails and their orientations, with respect to the wind. (There could be more sails, of course; the patent shows 6 vanes and my 1980 sketch has 8 sails.)
So, each sail rotates something like the Earth rotates, as it revolves/orbits around the central axle/sun. (Except, of course, the Earth's rotation axis is tilted, and here all the axles are parallel/vertical.) Also, it takes two full revolutions of the overall windmill for any single sail to do one whole rotation (vastly slower than Earth's 365+ rotations per orbit!).
Basically, the sail orientations change exactly as they would for a sailboat that the skipper decides to sail in a perfect circle.
Thus in that ASCII sketch the lower sail is receiving the wind full-on (and is pushed in the same direction as the wind); the tilted sails at left and right are oriented to be pushed across the wind, and the sail at top is oriented to not intercept any wind, as the overall (counterclockwise, here) revolution of this windmill moves that sail against the wind.
Obviously some mechanisms are needed to properly link the overall revolution of this windmill with the necessary orientations of the sails, at each point during their revolution. The patented device uses gears and sprockets and chains; my 1980 notion used a sort of giant cam at ground-level.
Next, note that this particular type of windmill, despite being a vertical-axis style, needs to be "pointed" with respect to the wind direction. That is, it works best if the full-on sail orientation is actually receiving wind full-on. So some sort of extra mechanisms are required to do that. I merely made my giant cam rotatable, and connected it to a big wind-direction-vane.
Finally, it seems to me that if a sailboat skipper ever wanted to try a windmill-powered boat, THIS is the style that would be preferred!
Venetian Turbine
Venetian_20Turbine
As mentioned in the main text. [Vernon, Jan 04 2012]
US Patent 4,424,002
http://www.google.com/patents/US4424002
As mentioned in the main text. [Vernon, Jan 04 2012, last modified Mar 28 2015]
On furling windmill sails
Furling_20Sails_20Automatically
As mentioned in the main text. [Vernon, Jan 04 2012]
"EOL Process" VAWT
http://www.youtube....watch?v=qg7hxaPAjnw
[FlyingToaster, Jan 04 2012]
Sailboat Whirligig picture.
http://www.eldreds....s/13jun06/03723.jpg
Artsy [baconbrain, Jan 04 2012]
Sailboat Whirligig video
http://www.youtube....ure=player_embedded
Fartsy [baconbrain, Jan 04 2012]
1980 Sketch (overhead view)
http://www.nemitz.n...non/SailTurbine.jpg
As mentioned in the main text. The piece of paper I scanned is showing its age (more than 30 years). [Vernon, Jan 05 2012]
2nd 1980 Sketch
http://www.nemitz.n...on/SailTurbine2.jpg
This has a cam-track schematic. There is a crossing point that may need a mechanism like a railroad track-switcher, to ensure the cam followers take the correct track from the crossover point. Not difficult to implement, to work automatically, so not shown. [Vernon, Jan 06 2012]
3rd 1980 Sketch
http://www.nemitz.n...on/SailTurbine3.jpg
Side view. Having a rail under the cam may help make it stronger. The sail orientations are probably at about 45 degrees here (neither full-on or edge-on). This drawing was made long LONG before I thought about an automatic furling mechanism for it, so the wind-vane and the guy-wires here would be in the way of the free-weights described in that other linked Idea, and the crossbraces through the sail area don't help with furling, either. However, if might be better, with respect to allowing a sail to "billow", to furl these sails from the edge instead of from the bottom. [Vernon, Jan 06 2012]
D-Dalus
http://www.google.c...jdOP7KWxz1hNXSujpZQ
In my linked "Venetian Turbine" Idea, I mentioned that it could work in reverse to blow air. So can this Idea, although the sails need to be stiff vanes in that case. And this link is the proof. [Vernon, Feb 02 2012]
Flapping blades
http://www.youtube....=PLEE6502E1E4645358
a whole list of videos showing this kind of turbine. [pashute, Jul 25 2013]
[link]
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Hey that's my idea... (well it is, though like yourself I don't pretend I'm the only one to have thought of it). |
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As a twist that I haven't seen yet though, If you can make do without a center shaft that protrudes between the sails, then you can get the same coverage with only 2 sails/vanes instead of four: make the sails (almost) twice as wide as the distance between sail axes. |
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[edit: not quite, you can't cover all the distance between, but you can get up to sqrt2/2] |
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o . . .-----X--------- o ---------------
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'X' is the major axis, 'o's are the sails axes.
(pay no attention to the . . . s) |
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In this diagram the --- sail is sweeping through the | sail; the X is rotating counterclockwise and the o's are rotating at half the angular-speed of the X, clockwise. |
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Wind coming up from the bottom of the page. |
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[FlyingToaster], the reason you haven't seen that twist is because it doesn't work. The vanes will collide with each other. Note they both have to rotate in the same direction (COUNTERclockwise is appropriate for your sketch, if the wind approaches it from the bottom) to catch the wind at appropriate angles during their overall revolution about the central "X". So, for the vane on the right, its left half will be moving downward/rightward (to have angle suited to push vane across wind toward the left), and for the vane on the left, its bottom half will be moving rightward/upward (to have angle suited to push vane across wind toward the right)... For vanes like these to be so close and not collide, they have to rotate in opposite directions like meshing gears --but then they can't catch the wind correctly. (And I'm pretty sure I noticed the collision problem back in 1980, when thinking about how wide my sails could be...) |
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I thought of it in the last 5 years, not 25. |
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I do have the motions right regarding the diagram and wind though. The minor axes have to rotate counter to the major axis because you want the sails to only turn 180 to every complete revolution of the major axis. |
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possibly wrong concerning the width of the sails, but they can definitely be much wider if the center post is removed from impinging the sail area. |
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Somewhere, possibly on Seattle's Lake Union houseboat neighborhood, I saw a wind toy kind of like this. It was much simpler, though. |
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It was a bicycle wheel mounted horizontally (vertical axis). Around the rim were three or four sailboat models, with real cloth sails. The sails had a boom, and the boom had a string to allow it limited motion side to side, much like a real boat. |
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As the boats sailed around, attached to the wheel rim, the sails worked automatically, tacking and jibing. |
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See Sailboat Whirligig links. |
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My imagining was gears: one around the major axis, not turning, and one around each vane axis twice the size of the middle gear to impart the backwards motion. The sum of the motion is a positive half revolution of the sails proper, for every full revolution of the armature that connects them. |
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[bigsleep], your first link causes my browser to crash (so, NO). The second link shows something I have seen, years ago. And the third is new to me. |
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[FlyingToaster], if the overall revolution is counterclockwise, while the individual vane rotation is clockwise, then the vane at the right will tilt from its current horizontal state to having an angle like this: \ --and with wind coming from the bottom of the page, that's the wrong angle for the wind to blow it toward the upper-left (as part of overall counterclockwise revolution). When I get my 1980 sketch linked you will see more clearly (because has 8 sails) that the directions of rotation and revolution must always be the same. And you WILL get a collision between adjacent vanes if they are too wide --which always includes "wide-enough to want to remove the central vertical post"! |
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[baconbrain], while that sounds good, I doubt that that system could be efficient at a large scale ("workable" is fine for toys, but not acceptable for generating significant power.) |
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mmmm nope, I might not be right about full width, but they can definitely be wide enough to cross over the major axis. I've gotta check my own files, in this case mental ones that probably need some dusting off, so it could be awhile. |
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But I'm pretty sure I've got it right [V], the sails are tacking as they come into the wind from a | position: it's still a \, but it's so slight a \ that the overall thrust vector still pushes the armature in a counterclockwise motion. |
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Check the link, you'll see the tacking. |
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[bs] the link to EOLProcess shows what Vernon means. The vanes move around their own axes as the axes move around the main axis. |
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[V] you're right'ish about the vane width: a bit of slapstick geometry shows that they can be only sqrt2/2 the width in order to avoid crashing into each other, *but* they still can cross over the center post. |
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[bs] you mean the Indian one with the bicycle wheel and stuff ? |
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[FlyingToaster], I'm quite right about the rotation issue. Note in my prior anno I specified the vane at the RIGHT, not the vane oriented |. And, for that vane on the left, if it rotates clockwise, then its orientation will become like this: /, which is also the wrong way for wind pressure, from the bottom of the page, to encourage the entire assembly to revolve counterclockwise. |
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I'll agree that you are partly right about the vane width ONLY if the turbine is restricted to having only two vanes (and perhaps three vanes). But since this is not as energy-efficient as having 4 or more, nobody will have a good reason to build it that way (without a center post so as to accommodate wider vanes). |
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[V] In my diagram the entire mess is rotating counterclockwise around point X, as noted. Each vane/sail is rotating around points o clockwise at half the angular speed as X. The SUM of the rotations results in the sail rotating counterclockwise as well, but at half the angular displacement it would have if it was fixed to the main armature. |
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So while the sails *are* turning clockwise in relation to their axis, the end result is that their turning counterclockwise. |
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Each time a sail comes to any point in X rotation it's orientation is 180 degrees (not 0 or 360) from what it was the previous time it passed through that point. It's half flipped over, not wholly flipped over. |
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[FlyingToaster], keep describing it the same way; it still WON'T WORK. Let me be more specific about exactly why. Start with that vane on the left, currently oriented |, which if it rotates clockwise will become oriented /. Wind coming from the bottom will be deflected off of it such that two force-vectors will appear, one, pushing directly downwind, and one pushing it farther toward the left. |
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Both forces can be ignored for the moment, because the vane on the right, full-on against the wind, will receive a greater force, all of it pushing downwind. Some counterclockwise revolution of the whole turbine will occur because of that, thereby forcing the vane on the left to move upwind, while the vane on the right moves downwind. |
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However, if that vane also rotates clockwise, then its orientation will change from --- to \. Wind bouncing off of it will be associated with two force-vectors, one of which pushes toward the right, while the other pushes downwind. After 90 degrees of turbine-revolution, this vane can't go downwind any farther, so only the sideways vector is relevant --which is TO THE RIGHT (clockwise), not to-the-left-counterclockwise. |
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Note now that BOTH vanes will be tilted exactly 45 degrees from their starting orientations. The vane that started on the left and was forced to move upwind now might be able to go downwind. The sideways vector is still to-the-left, so again the net effect is to try to rotate the whole turbine clockwise, not counterclockwise. |
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Therefore your turbine stalls after revolving 90 degrees, and starts to backtrack. Then it stalls again, because backtracking (revolving clockwise) causes the downwind vane to begin to rotate (counterclockwise) back toward the --- orientation, where soon it will receive more direct wind pressure than the other vane (which also starts rotating counterclockwise back to the | orientation). |
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So, as I previously wrote, it is essential for the vane-rotations to be counterclockwise if the turbine revolution is counterclockwise (and for everything to go clockwise if you want it to run that way). |
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A counterclockwise rotation of the | vane at the left will cause it to rotate to become \; as before, it is forced agains the wind by the --- vane, which rotates counterclockwise to become /. After 90 degrees of revolution, the sideways vectors are exactly what we want, for the turbine to continue rotating counterclockwise. |
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[baconbrain], thanks for the image. I can see why only an oscillating motion of those ship-sails is workable; it is because they pivot at a vertical sail-edge, and not on an axis through their middles, as portrayed in the patent, and by [FlyingToaster]'s ASCII sketch (and my own 1980 sketch). |
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The problem with oscillating motions is "efficiency". You have to make the mass stop moving in one direction and start moving the opposite direction (wasting effort). Meanwhile, pure rotation always goes the same direction. So, as a toy, the oscillations are fine, but for generating power, it is impractical. |
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[Vernon] Again: the vane axes are rotating clockwise, BUT the main axis is rotating counterclockwise twice as fast angularly. So the | doesn't become a /, it becomes a \. Look at your own notes; I'm sure they say the same thing. |
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Except for the point where the | vane is *exactly* side on to the wind, it's still contributing to pushing the armature around. |
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Look at the | vane for instance: for the next quarter revolution of the main axis, it will be physically moving into the wind but it will still be pushing the contraption around by tacking. The key is that the angle of the wind and the angle of the vane have to match up to push the thing, not backwards but in a circle. |
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[bigsleep], I tend to agree with your point about a part of the sail or vane that extend beyond the turbine's central rotation axis. (A reason to disagree might emerge from considering the extra "lever length" available to the sail/vane, from the OTHER side of its axle --it may more-than-balance the negative effect you mention.) However, neither [FlyingToaster] nor myself were technically "wrong" about it, because neither of us commented on that particular aspect of the turbine. We were basically discussing collisions between vanes/sails only. |
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With a sail width of (sqrt2 x the distance between sail axes) the sails kiss when they're one behind the other, both at 45deg angles (in different directions). Of course if you give the vanes depth, making them a stretched ellipse or a skinny rhomboid, you could close the gap completely but I'm not sure there'd be any advantage. Too bad symmetrical airfoils don't do anything. |
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As I mentioned in my last anno, I'm not sure if the inner tip of the sail crosses the line during the backswing but, reciprocating mass aside, I don't think it matters, as long as the wind angle and sail (face) angle are mirrored across the tangent line (at the sail axis, on the circle scribed by the sail axis as it travels around the main axis)... or words that sound something like that. |
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[FlyingToaster], ok, I apologize for ignoring the relativism involved, which indeed means that the vanes would be slowly rotating opposite to the overall turbine revolution. My descriptions were based on absolute orientations, and are correct as long as only absolute orientations are considered. That is, by ignoring the contribution of the overall turbine revolution, the individual vanes do seem to be rotating in the same direction. Thank you. |
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However, now I should comment about this that you wrote: "My imagining was gears: one around the major axis, not turning, and one around each vane axis twice the size of the middle gear to impart the backwards motion." |
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If you have a stationary "sun" gear, then any planetary gear that revolves around it counterclockwise will also be rotating counterclockwise. The relativism here is between the overall revolution of the planet and the stationary sun --the sun gear can be perceived as rotating clockwise, while the planet doesn't revolve-- but the planet will still rotate (because the sun is rotating), and that planetary rotation will be counterclockwise. |
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You will need an intermediary direction-reversing gear, between the sun and the planet. As for myself, I'll stick with the giant cam idea, mechanically simpler! --and it occurs to me that because I designed my Sail Turbine with a cam to force sail-rotation, that made the relativism of the situation less obvious to me. |
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//gears// ah right, some bitrot between the ears there; I meant to change the anno and forgot to. Darned if I can remember how I had it arranged: I'm positive I didn't have 5 gears involved. |
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Fanbelts possibly. fixed gear in the middle, fan belts going out to double-sized gears fixed to the sail axes. Or one continuous fanbelt encompassing all 3 gears, with guide pulleys to keep it attached to the center gear. |
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At one point I entertained the idea of having the sails position themselves independently; each sail axis would have its own weather-vane-tail thing sticking out downwind, and little gears to position the angle of the vane. But I think it would have to contend with reciprocal motion of some kind. |
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Or it could be done with a little electric motor on each sail axis and a pole with a windvane and anemometer on top off to the side: controlling the pitch of the vanes electronically. That way it could be feathered in case of very high winds. |
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(From a quick look, the patent guy has one pulley, and a hula-hoop that attaches to the tip of each vane to move them all at the same time) |
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//relativism// yeah, I knew what the miscommunication was but couldn't untangle it. |
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Have you considered mounting the contraption sideways?, ie: have two complete turbine assemblies, one sticking out of each side of a pole horizontally,rotating in the pitch axis, instead of one vertical assembly rotating in the yaw axis. That way the middle gears or cams would be really fixed, the tail-feathers would be attached to the pole instead. And if you had them rotating in opposite directions you'd double the speed of a generator placed in between. |
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I'd like to see the cam arrangement if you've got it uploaded somewhere. |
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Folks, the 1980 sketch I've been talking about has been posted. I hadn't expected to need to post any others, but I do have a couple more, also made back then, which could be scanned. |
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I think you should have included the advantage the turbine type has over other designs, mainly that the design is just basic'ish geometry: no background in aerodynamics is needed, no expensive, complex propeller to purchase/fabricate, no unfathomable twisty origami curves, just a bunch of flat plates. You could make a trip to the Home Depot, stop off at an auto supply store on the way home and have it built by the end of the weekend (more or less). Arguably simpler than a Texas windmill. |
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[FlyingToaster], besides being furl-able, sails offer the advantage of using curved vanes, without the disadvantage of needing to turn a solid vane's curvature around for the turbine to keep working right (note place on my 1980 sketch where it says that the "sail flutters from outside to inside"). |
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Also, regarding "mounting it sideways", well, the Horizontal Axis version of this Idea is the "Venetian Turbine" (linked). |
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Mathematically, with the exception of the one point where the vane is *exactly* edge-on to the wind (the | sail in my diagram), the vane is *always* contributing positively, even as it's physically proceeding into the wind. (In reality, even an edge-on vane will still have a little bit of cross-section facing the wind from its supporting structure, so a couple degrees in either direction). |
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I've thought of using fabric stretched across a frame, but that's for weight reasons. |
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I don't understand why you prefer a billowing sail. |
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[FlyingToaster], like I wrote in my last anno, a billowing sail can emulate a curved vane. Why are curved vanes used in steam turbines and gas turbines? Because they are more efficient. In this case the efficiency derives from a curved sail enhancing the sideways force-vector, in terms of contributing to the overall revolution of the turbine. (For the full-on wind-receiving orientation, no significant advantage.) |
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hmm... I imagine there's quite a dead-zone coming into the wind... unless you're furling/unfurling the sails on the fly that is. |
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//no significant advantage// less than none actually since any billow would be decreasing the effective surface area. |
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At this point I'm imagining your contraption's sails as being stretched out between two poles, and the cam arrangement variously moves the poles closer together and farther apart ? |
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Folks, I think I know a simpler automatic way to protect the sails from high winds. On each sail-rotation axis, between the part that follows the cam and the lower sail-support strut, put a torsion spring. This would allow the sails to twist in a high wind independently of the orientation that would normally be forced by the positions of the cam followers. They would essentially automatically "feather" themselves. |
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Naturally, we want those torsion springs to be strong enough to prevent such twistings at ordinary windspeeds. |
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But your sails are symmetrical: they won't twist. |
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Oops. OK, so the sails have to be mounted somewhat nonsymmetrically. Not a big problem. |
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But since they rotate half a turn every turn, they will be asymetrical in the wrong direction half of the time. Also, how would that spring based feathering system work as the sail passed the point where it is normally flat into the wind (9 o'clock in the 1980 sketch)? As it approached that point it would be feathered like /. After that point it would be feathered like \. During normal winds it passes thorugh the __ position, but during high winds it would need to make that transiation by passing through the | position to avoid damage. |
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Regarding the pictures of the cams you added today, I don't think your cam configuration will work reliably as drawn. There is nothing to ensure that as the sail passes the 9 o'clock position that the sail will rotate counter-clockwise (relative to the support struts) rather than clockwise. I suspect that it would varry based on wind variation/gusts. You need at least 3 guides, but that would require 6 tracks since each guide would then alternate between two tracks. If you have 4 guides and 4 tracks, it might be optimal. |
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That's not too difficult: a disk with two grooves in it, one bridges the inner track to the outer, the other, set 90deg off, does the outer to inner; a front or back roller goes through and runs over a lever that mechanically rotates it 90deg to let the next one through... or flips back and forth... or is gear driven, etc. |
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Cams/tracks allow for fine-tuning the sail, non-circular gears could achieve the same thing, but the offset hula-hoop used by the patent guy couldn't pull it off. |
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[scad mientist], you have a good point about the torsions-spring idea, and perhaps it won't work, after all. I'd have to think about it more, to decide. Regarding the cam track, though, the picture says "schematic". Not every detail needed to make it work reliably was included; it is a general notion that I'm confident CAN be made to work reliably. |
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For example, consider the first sketch, and notice that if there are 8 sails, then when one of them is at the 9:00 hour-hand position, there is another at the 7:30 hour-hand position which is blocking some of the wind it could receive. This means that the 9:00 sail is receiving more wind-force at the side of the sail that we want to move faster, in terms of following the cam-tracks. |
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And other things could be done. Note that what you are describing is basically a "top dead center" problem, similar to "which way should a piston cause the crankshaft to rotate on the down-stroke". A flywheel solves that problem, and likely could work here, too. The sail-rotation is quite steady when the wind is steady, after all (it's simply fairly slow, but flywheels don't really care about speed; only momentum matters, and mass can substitute for velocity in that situation). |
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Not a huge problem, therefore. |
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I haven't had time to keep up with this one, but I've a few things that have been bothering me. |
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I worked up a similar idea for a while, then decided to give it up. There were too many basic faults for this former sailor. |
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A sailboat going across the wind ("reaching") goes much faster than a sailboat going directly downwind ("running"). A circular sail-wheel is not going to be getting as much power out of the downwind segment. You could slow the wheel down, but then you'd need more gearing up to get efficient electrical generation. But still, the parts where the sails are acting as wings is going to work a lot better than the spots where the sails act as parachutes--look at the size of spinnakers if you don't believe me. |
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So it isn't really a circular and steady arrangement. The power is going to oscillate, and your rig is going to be flexing like mad. |
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Speaking of oscillating, when I worked this up, I wound up using rotating cylinders for the sails--Magnus effect, Flettner rotors. I had the cylinders spring-loaded and flywheeled a bit, so they'd be lifting one way and then the other as they wound up and down twice each revolution of the turbine (with a bit of a boost, of course, and the dead spots on the up-wind and down-wind arcs). |
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I gave all that up for a purely oscillating wing pump setup. Oscillation isn't as wasteful as was mentioned above, provided what stops the moving mass is the generation of power. (Yeah, pure oscillating is as in-efficient as a flag.) It takes a bit of design, but that's the charm of it. |
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If you do want to use sails in a situation like this, especially as shown in the linked sketches, you shouldn't use soft cloth. You want a battened sail like a sailboard sail, with stiffened leading/trailing edges. A sheet of thin plywood would work, too, especially when making a bi-directional sail. |
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(This writing attempt is still too abrupt. I'll try to clarify later.) |
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I was thinking the same, [baconbrain]; in other words, this is a hybrid lift/drag turbine, like a Savonius, rather than a pure lift turbine, like a Darrius. |
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[Vernon] and [FlyingToaster] are assuming that the apparent wind (experienced by the sails) is in the same direction as the true wind; that approximation only holds for very slow, low-power turbines. Part of the reason sailing boats are faster on a reach than on a run is that the apparent wind speed is greater, so they are able to harvest more power. The same holds for wind turbines. That is why Darrius turbines look like this: |
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Those skinny, tall (high aspect ratio) wings are moving much faster than the wind, and therefore harvesting much more power per unit area than slow, broad sails would - in addition to using efficient lift, rather than inefficient drag. |
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There are VAWTs with variable blade angle; but the angle only changes slightly (less than 5 degrees each way, at a guess and the phase is governed by a wind vane (your design does not work properly if the wind changes direction, as far as I can tell)). That reflects the fact that blade speed has a greater bearing (pun intended) on the angle of attack than does wind direction. |
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[spidermother], since a Sail Turbine is intended to be lower-speed (and higher-torque), compared to other windmills, some of the argument about catching relatively-fast wind becomes less relevant. |
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Also, look at the 2nd and 3rd sketches, and you will see that I planned on a way of keeping it oriented correctly, if the wind direction changes. Note that it is quite possible, simply by changing the attachment point of the wind-vane to the cam, to change the overall orientation of a Sail Turbine, with respect to the wind direction. If you really can harvest more energy by not having a particular orientation square to the wind, then it is easy to accomplish. |
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In general, the greatest power is harvested when the apparent wind (the wind that the sail feels) is always at the angle of greatest lift-to-drag ratio (or possibly slightly steeper). As a rule of thumb, this is about 5 degrees for rigid aerofoils, maybe a bit more for fabric sails. |
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An empirical method I've read for finding the best angle for fabric sails is to let the sail out until it begins to luff (flutter in the wind), then haul it in until it just stops luffing. That's close to the most efficient angle for that particular sail. |
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There's nothing drastically wrong with slow, fabric-sail turbines; their low efficiency is offset by lower cost, partly because they aren't exposed to such extreme forces and wind speeds, and you don't need close tolerances. They've pumped a lot of water and ground a lot of grain over the years. If you only had natural-fibre cloth and wooden spars to work with, your design would be better than nothing. |
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So what factor is supposedly making a low speed unit a low power unit ? |
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The thing with the tacking sailboat going faster upwind'ish than downwind is a non sequitur: all that means is that you could increase the size of the downwind-travelling sailboat, utilizing the same sails, without a corresponding loss of speed. |
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That's the problem with propeller turbines: they're efficient at one speed range only: too slow and too much air is getting through without hitting a blade. |
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A low speed unit is like driving in too high a gear, with the clutch slipping. I don't know if that helps. |
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To be clear, a low speed unit is a low power-per-unit-area unit (in terms of both sail area and swept area). Obviously, increasing the sail area increases the power. |
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My understanding is that all lift-type wind turbines (which includes the 'propeller' (HAWT) type) are efficient at a particular ratio of blade speed to wind speed; that is, if the wind is twice as fast they need to turn twice as fast to maintain an efficient angle of attack. There are formulas to calculate the optimum ratio for a particular number of blades and rotor diameter. So for a given wind speed, a given turbine works best within a fairly narrow speed range. |
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Also, in HAWTs, drag is entirely counterproductive (as it can only possibly slow the blades, not speed them up), so good blade design is important; whereas in the type of turbine discussed here, drag can be used to produce power, so there is more leeway (again, pun intended) in the design tolerances. But drag is always far less effective than lift - both because the apparent wind speed is less, and because much energy is lost as turbulence. Lift, correctly applied, slows and deflects the wind. The deflection is wasteful (lift-induced drag), but it is unavoidable, and minor. This suggests one further trade-off - for low speed designs, lift-induced drag is high, but for high speeds, form drag is high. The optimum is somewhere in between - but for good aerofoils, it's PDQ. |
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//non sequitur// Don't you mean red herring? I think it is an apt comparison. Since hull speed is associated with power, isn't it clear that the greater speed, and smaller sail area, of a boat on a reach indicates greater power per unit area of sail? And I think you have things back-to-front; in the case of a run, in between the extremes of a stationary boat and one moving at the same speed as the wind (zero power in each case), there is a precise speed at which a given sail generates maximum power. In contrast, the sail on a reach generates more and more power as speed increases, until air resistance dominates. Maximum power is when speed * net force is maximised; and that's considerably faster than the wind, and considerably more power per unit area than a spinnaker can produce, whatever the load. |
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// An empirical method I've read for finding the best angle for fabric sails is to let the sail out until it begins to luff (flutter in the wind), then haul it in until it just stops luffing. That's close to the most efficient angle for that particular sail. // |
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Quite true. FWIW, some sails have little yarn streamers a few places. Those start to flutter a little sooner. |
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// The thing with the tacking sailboat going faster upwind'ish than downwind // |
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A reaching sailboat goes directly crosswind much faster than the same boat could go straight downwind. Sailing speed records are set at right angles to the wind. |
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//is a non sequitur: all that means is that you could increase the size of the downwind-travelling sailboat, utilizing the same sails, without a corresponding loss of speed. // |
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No, sailboats carry honking huge sails called spinnakers, and none of them go downwind faster than the wind. |
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A boat going due crosswind can move faster than the wind's MPH. |
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but I'm talking about well... plate area in these types and disc area in the propeller types. Obviously in terms of blade area a propeller type is most efficient, but the disc it swings through ? |
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Again, the orientation is not important. Darrius (vertical axis) and 'propellor' (horizontal axis) turbines with good aerofoils are about equally efficient, both in terms of swept area and blade area. |
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Both are more efficient that either Dutch style sail windmills* and the type exemplified by this idea, in terms of both swept area and blade (sail) area. |
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But efficiency isn't everything; "bang for buck" is what really matters. As long as it generates more useful work than was required to build it, you've gotten somewhere. |
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Oh, and the large difference in efficiency is only because it's possible to achieve a large lift-to-drag ratio in a relatively non-viscous fluid such as air. This design would be fine for a custard turbine. |
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* These generate power through lift, but are inefficient because they have a low lift-to-drag ratio, and turn slowly for their blade number and length |
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^ except the idea isn't about flapping blades. |
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Vernon's idea has sails changing position by
rotation on their own axis, so that the one in the
wind is in position to receive full power, the two
at 90 degrees are set to have the wind force push
them in the direction they are going, and the
directly upwind sail is set so as to not interfere. |
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His mistake of course is that a sail flat against the
wind "directly downwind" will slow the speed of
the turbine, because it can go only at the speed
of the wind due to its stalled flow, while the sails
when at a sharp or acute angle in the direction of
the wind can use the "apparent wind" in an
"attached flow" to utilize the extra lift, and "sail
faster than the wind" in the wind's direction. Both
sails against the wind can only produce drag... |
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Anyway, Vernon's idea, WITH the mistake, is the
exact
configuration in the second video ("pivoted panel
wind turbine") except instead of sails they have
"shutters". |
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