Giant Wind Turbine

I notice wind turbines are all very big in size, and often have 3 blades.
These features make them more efficient than otherwise, I suppose.
Anybody can explain in simple technical terms why they have to be so ?
John
Reply to
John C
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That they are very big in size is easily understandable. What you try to do is retrieve energy present in the kinetic energy of the air. The larger the volume of air you take energy from, the more energy you can get. Increasing the volume is realized by increasing the diameter of the rotor. The alternative of using many small rotors leads to an increase of mechanical losses in the machinery, plus more area you block with the structure needed to keep the rotors in the air.
With regard to the number of blades the basic idea is that the smaller the number of blades the better to minimize the effect of drag on the blades. Since you need a balanced rotor this would lead to two blades. Many smaller wind turbines have therefore been built with only two blades. Unfortunately the wind field is not completely uniform, close to the ground the velocity is less while also the pylon leads to disturbances. With two blades this leads to high dynamic load components when one blade passes the lower part of the arc. With three blades the load per blade is less, and so the dynamic load components will be less as well. With more blades the dynamic loads would even be less, but so would be the efficiency. It appears that three blades give a reasonable compromise.
Timo
"John C" schreef in bericht news:4264ae7e$ snipped-for-privacy@news.starhub.net.sg...
Reply to
Timo de Beer
"Timo de Beer" wrote
Many thanks for the clear and concise information. Do I understand that these two or three bladed turbines have to turn pretty fast? And is this the reason that makes them so different in looks from the traditional windmills with many blades, as the latter turn slow.
On a related matter, if I were to transform a ceiling fan into a home-made wind turbine, what do I have to do to change the ac motor (which I suppose is an induction motor) into a generator?
John
Reply to
John C
First I want to state that I am not a turbine designer, so I may be wrong, but this is as far as I understand the situation: - it is clear that a turbine that stands still is not capable of getting energy from the complete area spanned by the rotordiameter, since the wind would blow unhindered through most of the circle. This would suggest a kind of minimum revs required (which I believe you suggest). Not being an aerodynamicist I am not sure exactly how large this speed would be, but since modern blade profiles are fairly effective, I would expect it to be rather low. In fact large wind turbines are not going very fast - a more important factor for keeping the revs low is the effective angle of attack of the wind: the wind speed experienced by the blade profile is the vector sum of the wind speed and the forward speed of the blade. If the blade moves very fast the angle of attack becomes very low near the blade tips, so that it becomes difficult to get the required lift from the wind while dragforces increase - having the revs too low gives some problems in the gearboxes. In order to get number of revs sufficiently high to drive the generator you need quite high gearratios, specially if the turbine runs slow. Gearboxes appear to be the least reliable part of many windmills. Of course with the advent of reliable semiconductor power electronics the need for having a fixed and high generator speed to match the power grid frequency is importantly reduced nowadays, adding to the flexibility of the windmill designer
The actual speed of windmills is a kind of compromise of the above. Of course for small windmills the angle of attack issue is less pronounced, so that they can run at higher revs, reducing the need for expensive gearboxes.
Timo
"John C" schreef in bericht news:426fa891$ snipped-for-privacy@news.starhub.net.sg... Do I understand that these two or three bladed turbines have to turn pretty fast? And is this the reason that makes them so different in looks from the traditional windmills with many blades, as the latter turn slow.
Reply to
Timo de Beer
How complicated can this get?
Since kinetic energy, as opposed to pressure drop, is all you have to work with, wouldn't a 100% impulse turbine make the most sense?
Just install some stator blades to redirect the wind in a tangential -- or possibly radial -- direction into "buckets" just like in a gas turbine engine.
It must be a pretty deceptive branch of fluids if the Dutch have been doing on it for centuries and STILL haven't figured it out.
Where is Bernoulli when you really need him?
Bret Cahill
Reply to
Bret Cahill
Just apply the Euler turbo equation -- the mechanical energy balance -- to get the tip speeds, power outputs and best design.
Bret Cahill
Reply to
Bret Cahill
Getting something to run in the wind is not that difficult indeed, you can buy these things for a few cents on the beach. Doing it with efficiency for a non uniform wind field over a large range of wind speeds is getting more difficult. And then there is the issue of the reliability of the mechanical components.
Indeed in Holland we used four bladed low speed wind mills for some centuries. In the city of Schiedam where I work there are still three ancient wheat mills that run regularly. Unfortunately their efficiency is not that high, in contrast to the cost of keeping them running.
Timo
"Bret Cahill" schreef in bericht news: snipped-for-privacy@z14g2000cwz.googlegroups.com...
Reply to
Timo de Beer
I understand some wind turbines are going up near Flagstaff near the Grand Canyon.
No one lives there -- 6 people / mile^2 -- so they ought to have all the juice they need.
Bret Cahill
Reply to
Bret Cahill
< Doing it with efficiency for a non uniform wind field over a large
< range of wind speeds is getting more difficult.
Just use adjustable stator and rotor blades like everyone else in turbo.
It's surprising no one from GE, Pratt or Rolls is doing anything with wind turbines. It's the exact same science. They are always whining about inefficiency of aircraft engines over a range of speeds.
Have you noticed commercial flights only have two speeds? Wide open up and coast down.
Well that's the reason. They are trying to save fuel.
Bret Cahill
Reply to
Bret Cahill
One blade works fine, too. However, that is the fewest that provides useful amounts of energy.
Unfortunately
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Al

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