Why are wind turbine blades so skinny?

[spikespiegal]

I was looking at the box fan in my living room, and noting that the blades are much fatter and wider than wind turbine blades. Common sense would seem to indicate that you'd want propellar blades to take as much lateral surface area as possible to transfer as much kinetic energy from that linear area as possible.

So, why are wind turbine blades skinny? Airplane props are the same way, although in that case their high angular velocity in relation to the wind speed moving through them likely makes up for it. Wind turbine blades though don't move nearly as fast....so why are they long and skinny as opposed to fatter and wider like a house fan?

 

[jagec]

glider wings are really skinny too. It's an efficiency thing.

 

[tcsenter]

Wind turbine blades mostly make up for smaller cross-sectional area by being much longer than, say, ventilation or cooling fans. The biggest reasons blades are not wider is because you would kill your axle and bearings in a short amount of time, and would massively grow the size of the structure required to withstand the torque.

http://en.wikipedia.org/wiki/Wind_turbine_design

 

[CycloWizard]

What tscenter said. I'll add that the point of the blades is to convert an axial force into torque, which is essentially achieved by converting the axial force into a radial force. The pitch and curvature of the impeller are the essential blade design parameters. Chemical engineers learned long ago by studying agitation (that is, converting torque to axial/radial velocities for mixing purposes) requires careful design of the impeller. The "high-efficiency" impellers that were developed early on were simply flat plates with the corners bent at a specific angle. The flow patterns over such impellers are extremely complex and very difficult to predict even with modern computational fluid dynamics because the solution is always transient. In other words, the problem isn't as simple as it first appears.

 

[silverpig]

The diameter of these things can be 300 feet...

 

[spikespiegal]

In other words, the problem isn't as simple as it first appears.

Yup, I never took torque and other parameters into account. A turbine with long, skinny arms is going to have an easier job generating the required torque than a turbine with the same surface area, yet fatter and shorter arms. Correct?

 

[CycloWizard]

Originally posted by: spikespiegal
Yup, I never took torque and other parameters into account. A turbine with long, skinny arms is going to have an easier job generating the required torque than a turbine with the same surface area, yet fatter and shorter arms. Correct?

Correct. The restriction on how long the arms can be is dictated mostly by structural concerns, as tcsenter stated previously.

 

[dkozloski]

It's all about aspect ratio. A propeller is a rotating wing. A high aspect ratio(L:W) wing gives the best lift distribution and lowest spanwise flow. The limtations on how far it can be carried are structural.

 

[BEL6772]

Another aspect of the design is the need to have a variable pitch on the blades. Varying the pitch allows the blades to maintain a constant rotational velocity over a range of wind speeds.

The root of the blade needs to be skinny enough to rotate around without extending past the hub.

Most high performance prop planes use the same principle. They vary the pitch of the props to keep the engine at optimal RPMs.

 

[flyted]

Actually ALL high performance aircraft use variable pitch for maximum thrust. You box fan would benefit from that also if it wasn't for the cost/benefit issue. The best angle of attack for the blade varies with, in the case of wind turbines, the wind speed. With aircraft the angle varies by either being set by the pilot for a certain rpm or varies with the loads imposed on it by climbing or descending.

 

[tcsenter]

Originally posted by: spikespiegal
Yup, I never took torque and other parameters into account. A turbine with long, skinny arms is going to have an easier job generating the required torque than a turbine with the same surface area, yet fatter and shorter arms. Correct?

Well, more like a longer narrow blade will generate as much radial torque as possible, with as little axial torque as possible, to the hub (axle/bearings). You want to catch enough wind to get the turbine moving, without catching so much wind that the excess torque must go somewhere else, like to your hub.

This is more difficult than it sounds because wind is an impossibly complicated mistress. Wind is made up of smaller currents which can have different velocities, randomly emerge from or merge into other currents, and at different angles of attack. A small wind turbine isn't nearly as design-heavy because it will be exposed to a much smaller spectrum of these currents. However, larger turbines can have substantially different wind velocities and angles of attack hitting different blades. A stronger wind current hitting one blade relative to everywhere else could generate a significant amount of axial torque or actually impede rotation.

These differences must be dealt with by designing the turbine to yield an averaging effect over a certain degree or arc of rotation. The blade is designed to 'sample' only part of the wind to which it is being exposed at any given time. If you have a significant differential, only a portion of that differential makes it into the system.

 

[Vee]

Originally posted by: jagec
glider wings are really skinny too. It's an efficiency thing.

QFT

Originally posted by: dkozloski
It's all about aspect ratio. A propeller is a rotating wing. A high aspect ratio(L:W) wing gives the best lift distribution and lowest spanwise flow. The limtations on how far it can be carried are structural.

QFT

Regarding the OP's question: It's all about aerodynamics. And nothing about torques, bearings or structures.

Lowest span wise flow! Forget all that radial vs axial torque nonsense.

The mission is to suck the energy out of the wind. As much as possible. For that reason you want the wind to travel from leading edge to trailing edge as much as possible.

Remember, you get to choose the speed of the blade.
So you don't need a large wing area. You don't need a big torque. Energy = torque * revs. Make more revs, sweep more air instead. Cut more and thinner slices of wind. And, actually, you get considerable axial force from this but, unlike what many of you speculated, this is irrelevant.
(besides, you can't escape this axial force anyway, whatever you do. If you make a more inefficient turbine you will need a larger for the same energy. More drag, more axial force.)

Thinner slices, higher blade speed means less angle = higher glide ratio in glider plane terms.
A glider wants to achieve as high glide ratio as possible. This is achieved by maximum lift/drag ratio. And this again is not achieved with a large wing area. Lowest sink rate is achieved with large area. And highest lift. But that is not what is wanted. lift/drag ratio is wanted. This represents the optimum energy conversion.
And this is achieved at higher airflow speeds, higher sink rates, higher wing loading, smaller wing.

With the exception of having increasing speed over the span towards the tips, propellers and wind turbines represent an equivalent problem of optimization as glider wings. It's the same.

So you get a moderate area instead of large because that is actually the aerodynamic optimum.
And you get that area distributed in a long narrow wing because you must minimize spanwise flow. Spanwise flow results in vortex and turbulence at the tip, pure energy loss.


I have no intentions to provide you with a complete course in aerodynamics, so I'll limit myself to telling you that the reason your box fan has wider blades has to do with scaling and a thing called 'Reynolds number'. It essentially describes the conditions of the airflow and that is dependant on the speed and chord length of your air foil. Basically, your fan is too small and rotates too slow. A wider chord compensates because it makes your fan 'less small'. ...to the airflow.

 

[CycloWizard]

Axial force is hardly irrelevant. Thrust number depends very much on Reynolds number and impeller design, and is a major design consideration for any structure containing an impeller. If I have a very long support and a very large axial force, it will fail.