Self-generating Darrieus windmill ?

[Externet]

A Darrieus windmill is meant to spin a generator at its base; where its windings 'cut' magnetic lines to generate electricity. Cutting stronger or more lines of force can produce more power, and the wind force should be capable of keeping it up to a speed in order to produce a decent generation.

http://upload.wikimedia.org/wikipedia/commons/3/3c/Darrieus-windmill.jpg

 

But, if the blades were actually made a winding preserving its aerodynamics, and no generator at its base but slip rings; the magnetic lines being 'cut' would be the earth field. Weak but massive, the area being 'cut' would be comparatively huge.

Finer wire wound would yield more voltage because of number of turns; thicker wire would yield less voltage but at a higher current. The power should be about the same.

 

Do you think the huge spinning wound blades would be capable of extracting from wind a similar amount of power without having a generator at its base ?

----> How to calculate power based on the earth magnetic field 'constant', the huge area enclosed by the blades and the rotation speed ?

 

Edited-added:

Say 0.5 Gauss; 10m^2; 100rpm; 2 blades=1winding.

Edited February 12, 2012 by Externet

[InigoMontoya]

I've no idea as to whether more or less power would be generated, but I strongly suspect that such construction would be very expensive and as such kW per dollar would drop.

[Xittenn]

This wouldn't be terribly difficult to put some numbers to. I will revisit the thread, if anyone else hasn't, when I'm a little more motivated!

[insane_alien]

Thing is, the earths magnetic field is very weak. Those things turn an alternator at the bottom which has its own magnets that are MUCH more intense. I don't think you're going to get better than it by adding more generating equipment to the blades.

 

If you're making the blades heavier then you'll reduce the ease with which the air can turn it. I think this would be counter productive.

[Xittenn]

Is there a reason why most of these seem to be twin bladed? I ask because electricity as a utility tends to be delivered as three phase and converting it after would be a point of issue.

[Joatmon]

Is there a reason why most of these seem to be twin bladed? I ask because electricity as a utility tends to be delivered as three phase and converting it after would be a point of issue.

I think you will find that these machines produce AC of varying voltage and frequency because of varying wind speed. this leads to the necessary steps AC (very variable) - DC - AC (very stable and locked in phase with the mains). The link seems to cover this:- http://ethesis.nitrk...yofDFIG_pdf.pdf

Because conversion to DC is part of how it works (IMO) it wouldn't really matter whether the generator was single or three phase.

However, I can see some advantage in starting with three phases and using each basically as a separate single phase supply. Each phase could then have its own inverter to provide for one phase of the mains. For "inverter" see link:- http://en.wikipedia....er_(electrical)

Edited March 5, 2012 by Joatmon

[Xittenn]

Excellent observation old chap! : )

[Joatmon]

Excellent observation old chap! : )

Bless you, my child.

 

 

 

[Enthalpy]

Every recent wind turbine has a converter to avoid the need of synchronization with the power grid.

Some Darrieus have three blades and this would be MUCH better in the geomagnetic field since the produced power and overall torque would be constant over one turn.

Though, the torque on each blade would still cog! Two maxima per turn. Acceptable since torque by teh wind also cogs at a Darrieus.

 

In this three-phase case, an AC-AC converter typicaly has one resonance converter per input phase (or phase pair if delta operated) so the current it draws is a sine in phase with the voltage: this gives smooth operation. The DC sum of the three outputs is constant (three pulsed sin^2 at 0°, 120°, 240°), and so is the torque at the shaft. Then, DC is converted to the power grid three-phase.

 

A much simpler six-phase rectifier would be less good, as it lets the DC current flow through each of the three generator outputs for 1/3 of a period in one direction, idle for 2/6, and for 1/3 in the other direction. Not bad, but the pulsed current creates some mechanical cogging, the output voltage also fluctuates a bit, and you still need a DC-DC converter to stabilize the voltage.

 

The puzzling idea of geomagnetic operation can be compared with a separate generator.

• Earth's horizontal induction can be 0.4G=40µT for instance, so one blade sweeping a half-cross-area of, say, (2/pi)*h60m*R20m cuts a flux of 0.03Wb only. As little as a separate generator with 1T mean gap induction and just 0.03m² cumulated pole area.
• Then this flux varies a slowly as the Darrieus rotates. Take 100m/s blade speed, it's only 0.8Hz, giving a peak induced voltage of just 0.15V per coil turn. A separate generator could have 48 poles to create comfortable 19Hz and 24 times more voltage per coil turn.
• The copper cross-section you would reasonably put in the Darrieus isn't bigger than what a separate generator accepts, and the longer windings mean more copper losses and hardware expenses.
• That's why electric machine design tries very hard to maximize the flux within the minimum size. The Gramme design was a fabulous improvement, it hasn't changed since then...

So: the coils in the blades are not quite as good, and they concentrate more engineering constraints on fewer parts.

 

For wind turbines, you might give a look at my "electrostatic alternator" (Google together with "Marc Schaefer", follow the links, begin at the drawings). At these slow generators, the design with an insulating liquid is smaller and more efficient than an induction machine; it could be overall better.