Wind turbine questions

[hobz]

I am under the impression that a wind turbine generates lift because the rotor blades move through the air. At the blade tips, the speed by which the rotor blades move through the air, is much greater than the wind speed.

 

However, I fail to see how the incoming wind adds to the movement of the rotor blades?

 

Also; can wind turbines start by them selves, or do they need a certain jolt of energy (sort of like a starter on a gasoline engine) to begin converting wind power into lift?

[jules]

the rotor blades generate energy because of the wind pressure on the angled blades causes a rotational force to the turbine, they may go around faster than the wind speed but obviously wont go faster than the wind is pushing them as this would mean that they are using an engine to power them wasting energy thus making them redundant (meaning they don't create lift, the shape is for efficiency of generating energy, i think). there are many different types of turbine the smaller variety's generally don't need a motor to start as for the big daddy's im not sure.

have you tried wiki for an explanation or howstuffworks.com generally pretty simple but should give you what you need:-)

[CaptainPanic]

I am looking at a large wind turbine from my office window. The blades rotate once every 3 seconds (approx.). The blades about about 50 meters long. The tip of the blade therefore travels:

 

distance = 2*pi*r = 2*3.14*50 = 314 meters in 3 seconds. That equals over 100 m/s...

 

I can tell you that the wind here is definitely not 100 m/s (I really wouldn't be sitting here).

 

So, yes, blade tips definitely go faster than the wind.

 

The concept is quite easy I think.

The wind hits the blade. The blade has a shape which deflects the wind to the side. Since action=reaction the blade itself will move the other way... so it starts to spin.

I am not sure that lift has much to do with it. Why would a blade need to generate lift, and in what direction would that be? Forward? It's a stationary machine. Although aerodynamics is definitely the right field to study the wind turbines, not all aerodynamics has to do with lift...

 

But there is no reason why a blade tip would not be able to go faster than the wind speed. Make a force balance, and try to find the force that will slow down a blade. In a well designed wind turbine, that force is the gearbox and generator.

 

If you let a windturbine spin freely, and thus let it go out of control, the blade tips might even break the sound barrier.

 

[J.C.MacSwell]

Lift is by definition at right angles to the overall flow (not the orientation of the blade or wing, though they are often close to the same)

 

The direction of the flow can be relative to the turbine, or relative to the blade, in which case it would change direction from hub to tip, so there is probably a convention of taking it as relative to the turbine as a whole unless otherwise stated.

 

Drag is in the direction of the flow, at right angles to lift.

 

You can get a turbine to turn with drag alone, usually with the axis of rotation at right angles to the flow, almost always vertical, but they are generally less efficient.

 

Note: if you squeeze a pea (pip) it will take off much faster than you squeezed it.

Edited March 25, 2009 by J.C.MacSwell

[hobz]

Thanks for replies.

I have figured this out. The blades are angled in such a way, that the relative wind to the blade is moving faster on the upper side, than the lower side (upper side faces wind direction). By Bernoullis principle, this causes the pressure on the bottom side to exeed the pressure of the upper side, causing a lift force on the bottom side. A component of the lift is in the direction of rotation, thereby adding force to the rotor.

[J.C.MacSwell]

Thanks for replies.

I have figured this out. The blades are angled in such a way, that the relative wind to the blade is moving faster on the upper side, than the lower side (upper side faces wind direction). By Bernoullis principle, this causes the pressure on the bottom side to exeed the pressure of the upper side, causing a lift force on the bottom side. A component of the lift is in the direction of rotation, thereby adding force to the rotor.

 

 

Everything else makes sense if you got that part wrong, the bolded part, somehow.

 

 

(The wind direction is the direction the wind is coming from, just in case that might be the error)

[hobz]

I don't see your problem?

Usually the blade are angled so that the turbine faces the wind. Thus the wind direction will face the upper side of the blades.

[J.C.MacSwell]

I don't see your problem?

Usually the blade are angled so that the turbine faces the wind. Thus the wind direction will face the upper side of the blades.

 

I assumed upper side meant the side facing away from the wind since that would correspond to the upper side of a wing with a similar angle of attack. Not sure why you would call it "upper" otherwise.

 

But if that is the case, your explanation in post 5 is incorrect.