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wind generator design question

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Posted by brian mitchell on March 29, 2006, 3:34 am


from looking around at all the information on wind turbines I get the
sense that there's a certain scientific macho at work. Everything has to
be super-efficient, with rotor-blades as thin as knitting needles,
perfect pitch and razored lift, etc.

As I (unscientifically) understand it, this is because the power
generation is all predicated on wind *velocity*. This may be fine if you
live where Atlantic gales come screeching up the loch and you need heavy
stones on the roof to keep it in place, but not many of us do. I
actually live in Wales in the UK, which can be quite windy, but I am in
sight of a trio of large wind turbines and they never seem to be turning
--though they look impressive.

My question is whether, for people who don't live in ideal (regular,
strong, non-turbulent) wind conditions, it wouldn't be better to think
in terms of wind *pressure*? With this, the rotor-blade would be more a
sail than a wing and one would be wanting to maximise area rather than
lift. Being slow-turning, the alternator part would have to be geared.
Or, as I've been pondering, be arranged around the rim of the rotor so
as to utilise the tip speed.

Does anyone know if a) this makes any sense, b) anyone else is thinking
in this general direction? Are there plans available? I know there is
the Savonius rotor, which is a pressure device, but would seem to me to
suffer from only being driven for about a third of its rotation, the
rest being windage. But I could be very wrong, I know. Does anyone have
experience of these rotors?

Looking forward to hearing some thoughts on this...

brian mitchell

Posted by spinning on March 29, 2006, 7:10 pm

The energy available from the wind is in the form of kinetic energy.
"The kinetic energy of a given [air] mass varies with the square of its
velocity. Because the mass flow increases linearly with the wind speed,
the wind energy available to a wind turbine increases as the cube of
the wind speed."  http://en.wikipedia.org/wiki/Wind_energy

The wind *velocity* causes *pressure* on the rotors, so these two
parameters go hand-in-hand.  You need one to have the other right?

Also, to generate power, you need both pressure and rotor movement.
Remember, power equals torque times rotational speed.

Also, in a wind gust, if the rotor blades are thin, they will spin-up
more quickly.

Thin turbine blades will have a small area, but once they're moving,
their "swept area" is large.

Posted by brian mitchell on March 29, 2006, 9:05 pm

"spinning" wrote:

Yes. To be very specific, with an aerofoil blade, such as is used in
most wind generators, the wind passing over the leading edge creates a
pressure differential, known as 'lift', between the two surfaces of the
blade. Increased velocity = increased lift. But this doesn't address my
main point, which is that you need quite high velocities to get useful
speeds of rotation. This may be the most *efficient* way of utilising
the kinetic energy of the wind, but not necessarily the most effective
way in all conditions.

Thanks. Torque is really what's at issue. In light winds, a rotor which
uses the kinetic energy to simply *displace* its blades by converting a
horizontal force into a rotational one, like a windmill or a fan in
reverse, may develop more torque than one dependent on lift alone.
Sailing a boat downwind in light airs you throw up as much canvas as you
can. Since electricity generation increases with rotational speed, it
might be a better trade-off to use the torque of a displacement-type
rotor with a geared-up generator than a lift-type rotor with blades
several metres long. Especially if you want to mount the whole thing on
the roof of a suburban semi!

I'm sure both these are true, but it seems to be a fact that aerodynamic
wind turbines give disappointing results --electricity-wise-- in areas
where the windstream is broken, turbulent, confused, etc., and in light
winds anywhere. Rather than saying :- these conditions are no good for a
wind turbine so don't bother- perhaps it would be better to ask: how can
we take best advantage of these conditions?

brian mitchell

Posted by BobG on March 29, 2006, 9:59 pm

My idea is to use technology to tweak the efficiency.... a small
gearhead motor can tweak the blade pitch to get rotation in small or
large winds... feather the blades in high wind... and pwm field control
on the alternator to control torque load on the rotor so light wind can
be used for just a few watts. This would open up many other areas of
light wind to windpower.

Posted by brian mitchell on March 30, 2006, 7:11 pm

"BobG" wrote:

Yes. The most adaptive machine possible seems the best idea.

How would that be controlled?

I don't know what pwm field control is...? But I agree that the
electrical side should also be adaptive. Hugh Piggott's well-known diy
generator can have its 6 coils wired in two configurations, star for low
speeds and delta for higher speeds. He speaks of having a switch to
change modes and it could be centrifugally operated inside the

And I, in my Mad Victorian Inventor mode, wonder if it would be feasible
to have a centrifugally operated auto-transformer, such that with
increasing speeds the permanent magnet generator fed into a load of
increasing impedance and the output would be a nearly constant voltage.
Perhaps there's a wholly electronic way of achieving the same end, I
don't know.

brian mitchell

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