Talk:Darrieus wind turbine

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The cant also makes the turbine self-starting from any position

Can you provide a reference to this, or explain it to me? I can't see how this helps. Since at zero rotational speed there is no "airspeed" over the blades, there can't be any lift, so it won't start moving. Any airspeed due to the wind alone will surely be cancelled by the drag on the rest of the structure - it's not enough to get it started. I may have missed something howevr; I'd be very interested to know about it if I have because I'm interested in building some turbines like this. Graham 05:08, 29 January 2006 (UTC)

Contents 1 self-starting 2 Reversing the motor for rotor startup 3 Lower tip speed 4 Figures 3 & 4 5 Figure in explanation is partially wrong

[edit] self-starting

To GrahamUK: A 2-bladed giromill or eggbeater probably won't self-start if the blades are aligned with the wind, but may do so otherwise. Having more blades increases the chances of a suitably placed blade, but it may just move to a less favourable position, before another blade comes into a good position. I think thicker blades start better, but run slower. The camber line should follow the turbine's curvature, to reduce parasitic drag.

The cant also makes the turbine self-starting from any position

I have removed this, after reading Turby's website. I think a helical turbine would self-start at any position if the flow rate were large enough; otherwise it can be motor-started at a flow rate at which it can then sustain the rotation. The Turby wind turbine claims "Starting is achieved by the generator in motor operation" [1]

Can you provide a reference to this, or explain it to me?

The helical Gorlov turbines claim "Self-starting in water current flows as low as two ft/s" [2].

I can't see how this helps. Since at zero rotational speed there is no "airspeed" over the blades, there can't be any lift, so it won't start moving. Any airspeed due to the wind alone will surely be cancelled by the drag on the rest of the structure - it's not enough to get it started. I may have missed something howevr; I'd be very interested to know about it if I have because I'm interested in building some turbines like this.

There is airflow over the blades from the wind. Admittedly some of it is in unfortunate directions, and will cause negative torque. However, I think the net torque will be positive at any wind speed (but it has to overcome friction and generator cogging). Turby claim "cut-in wind speed 4 m/s". This may mean the self-starting speed, or the self-sustaining speed.

Relative to the blade (or cross-section at one point if helical), the airflow is the vector sum of oncoming air plus rotating wind, giving a varying angle-of-attack and magnitude as the blade rotates. At standstill, there is simply the wind vector, which effectively comes from all angles because of the canted blades.

Importantly, owing to a fluid's flexibility and, for a gas, compressibility, we cannot add the forces due to these two causes, or resolve the relative flow into tangential, radial and axial components, although we can do this with the resultant force.

At every point around the turbine, the line-of-action of the resultant force (after resolving onto the radial plane if there is any axial component) can be projected inwards, past the turbine axis at a certain perpendicular distance, giving a positive or negative torque to the shaft. The net torque is the sum of these.

The aeronautical terms lift and drag are, strictly speaking, forces across and along the approaching relative airflow respectively, so they are not useful here. We really want to know the tangential force pulling the blade around, and the radial and axial forces acting against the bearings. Sometimes the tangential force is called thrust, but for a HAWT this means the axial force on the bearings (as in a propeller).

Some people talk about using just one single blade, or blades within blades, or twin blades side-by-side, or a cylindrical "distributor" within the turbine, or an external duct to funnel the air.

See these websites for info: [3] [4] [5]

Melilot 02:13, 20 February 2006 (UTC) Melilot 16:06, 24 February 2006 (UTC)

The way I think about the self-starting issue (though I could certainly be wrong) is as follows: The blades are airfoils, and if there is a wind across them, they will produce lift. However, if the rotor is stationary and the blade is pointed directly into the wind, the lift it generates (just like a plane wing) is directly away from the rotational axis, and doesn't do any good as far as rotating the assembly. When the rotor is turning, however, the actual wind combines with the APPARENT wind due to the rotation of the rotor, which results in a lift vector which points in a different direction...it points somewhat forward, which results in a net force continuing to drive the rotor. I don't think having more or fewer blades helps, it's the lack of the apparent wind that keeps the rotor from self-starting. However strong the wind is, the lift force generated by the blades will be directly away from the rotational axis and doesn't have any forward component. Middlenamefrank 00:59, 22 July 2007 (UTC)

[edit] Reversing the motor for rotor startup

Hello, just about the thinking of ""Starting is achieved by the generator in motor operation" [1] ... Can you provide a reference to this, or explain it to me?"

...Electric Motors are working on a very similar way as generators. A basic electo-motor has permanent magnets, so any (let it be a rotation-) part of the equipment which has a coil (current through that) can generate also magnetic field to pull itselves to the permanent magnet. This is the way the motors try to turn the axis. In addition; if you move the coil (this is a long wire rendered in an efficient way, looking like a life belt) near to the permanent magnet(s), there you will recognise some (potential), electic current, or electric power on the coil. So in this case your Motor works as a Generator.

I tryed to make it simple, as I can... SO, You can use a simple Electric Motor for the purpose of Generator, and in this case you can gain electric power from the motors (electrical) connectors. Regards, Csaba (mailto:CsabaBalog@hotmail.com)

Very simply, electric motors and electric generators are very similar devices, just run in reverse of each other. The motor provides motive power when electrically energized; the generator provides electricity when driven. Some designs work very poorly in the opposite mode, but some are designed to work at least acceptably well in both modes. Pumped-storage hydroelectricity discusses a reversible pump/generator application that shows decent efficiency on an enormous scale. Middlenamefrank 01:09, 22 July 2007 (UTC)

[edit] Lower tip speed

I removed text which says that darreus turbine uses lower tip speed than normal HAWT, both use same wing-tip-ratio so there is no difference in bird safety or noise. I have heard just some darreus or gyromills rotating and both where very noisy, I think that noise came from guy-wires.

I think that there should be reference that there is no any commercial Darreus any more.

I don't think that's correct, I do believe the HAWT has a higher tip speed than the Darreus. The blades on a propeller-type turbine are arranged radially out from the axis, so the tips of the blades move much faster than points closer to the center, and considerably faster than the average blade speed. The Darreus' maximum blade speed is at the center for an 'egg beater' type, or uniform for the straight-blade type. Also, you should remember to sign your posts (four tildes does the trick) so we know who we're talking to. Middlenamefrank 00:44, 22 July 2007 (UTC)

[edit] Figures 3 & 4

Where are they? Too Old 18:26, 25 April 2006 (UTC)

They were removed because they were not tagged properly or had copyright issues. I've removed the text that references them for now - it's not too much of a loss for a general article, though if someone who knows more about this type of design wants to add it back (and sort out the image issues) then I wouldn't stand in their way. Graham 05:00, 26 April 2006 (UTC)

[edit] Figure in explanation is partially wrong

I really think this figure is wrong. Indeed, the tangential velocityfor the blade in the back is in the inverse direction, so the aerodynamical force is also in a wrong direction. With the opposite tangential direction lift changes, but drag does not, thus, the total aerodynamical force only changes its vertical component which has to be pointing to the front of the apparatus. Carlos 00:22, 4 March 2008 (UTC)