on wind turbines, which is more efficient, a 2-blade or a 3-blade? 1

[etrobal]

in wind turbines:
1. which is more efficient, a 2-blade or a 3-blade?
2. which is more efficient, a blade with a root to tip twist or one with none? at what angle would be more efficient, if ever?
3. when you specify power regulation as either pitch or stall, which would be more efficient?
4. can we classify blade tip air brake and yaw brake as power regulation and should therefore be consistent with the bidder's power regulation offer?

appreciate your advises and should wind assessment data be needed for a clear reply, then am sorry i could not provide that. i have come to trust this forum more than our lowest bidder consultants. thank you again indeed.

 

[GregLocock]

You ask an unnecessarily complex question since you have not defined what you mean by efficient.

For a wind turbine the following are possible definitions

average kWh per year produced per $ invested

proportion of the available windpower that is turned into electricity

RoI

etc etc

Until you know that, the technical details are irrelevant.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

As I understand it, the 3-blade design has become conventional because of gyroscopic stability considerations.

And if you figure out the relationship between the wind velocity vector and the tip velocity vector at the root and at the tip, you will find that if you want a comparable angle of attack into the wind, a root-to-tip twist is absolutely necessary and is found on every good design. Take a look at an airplane propeller from the 1930's, this is not new.

I don't know enough about the topic to answer the third and fourth questions.

And the above post is correct; in most applications like this "efficiency of dollars spent" is more appropriate than the thermodynamic definition.

 

[btrueblood]

"Take a look at an airplane propeller from the 1930's, this is not new."

Or Orville & Wilbur's original Wright Flyer propellors, from ~1902.

Or ship propellors from the late 1800's.

 

[KENAT]

Greg makes a very good point.

Also, a similar question was asked not so long ago, hunt around for it.

I see lots of 2 blade wind turbines every week. They are mostly used for water pumps I believe, but they are still 2 blades.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[verymadmac]

I dabbled in wind turbines as a uni project, but here my thoughts, assuming we are talking about one bigger than you can put in your back yard
2 blades are more efficient in terms of structural mass for area swept but do have different failure modes but these guys seem to do them alright

http://www.windflow.co.nz

The only time you are would have no blade twist is if its a vertical axis wind turbine, the range of angle of flow is something like 60 degrees from root to tip

Stall regulation verses pitch, my only comment would be that stall regulation is likely to be better at preventing wind gust overloads, but dependent on the aerofoil used could lose a lot of power from the reduction in angle of attack required to reattach the flow (the proper term escapes my spell this morning).

Got no insight in to 4

Be aware that there are large differences between island (enven large islands) & continental sites, partiiculay issues caused by wind gusts.

 

[FredRosse]

Neglecting cost and a host of mechanical issues, such as balance, a single blade wind turbine definately has the capability for higher efficiency than a two blade, a two blade has better potential for efficiency than a three blade, etc.

The US DOE built a rather large and expensive single blade wind turbine during the 1970s - 1980s.

In the real world, additional considerations come into play, as mentioned by others within this post, to evolve the designs into the most practical configuration. This is currently a three blade configuration, with two blade turbines at a close second place.

Consider the "American" type windmill, the only practical machine for farm duty of several decades past. It had very many blades, and a generally low efficiency. However it had good starting torque, and hence was the most practical device for running reciprocating well pumps in rural America.

 

[Ralph2]

Why would "more" not be better? The wind is acting on each blade which, as seperate entity, is transmitting torque. It is also forcing the other blades to turn but I would think it would take less energy than what the extra blade provides?

 

[btrueblood]

1. Interference between blades - i.e. the wake from one blade messing up the flow for adjacent blades. Fewer blades minimizes such interference. Analogies to monoplanes, biplanes, and triplanes - induced drag and wing interference drag effects.

2. Minimum surface area (minimum skin friction drag) for the same swept area, i.e. fewer blades = less skin friction.

 

[rb1957]

i'd follow greg's lead ... you can't talk about efficiency untill you define the cost function, which will have a host (or not) of varibles so then you can optimise your solution.

if you don't know then look at where the different designs (vertical axis, horizontal axis, 2-blade, 3-blade) are being implemented at figure out what the applications were trying to accomplish (minimising which cost varible) and then ...
1) were they effective in meeting the design goal ?
2) how does this apply to your project ?

 

[FredRosse]

A good example of what rb1957 is talking about are the vertical axis machines used to charge the lighting system on floating marine buoys. This application requires a simple and rugged wind turbine. The vertical axis machines meet this requirement, although their efficiency (at converting wind energy into shaft energy) is rather poor. The these vertical axis machines are self starting, without having to point the turbine in any direction. The trade off here is the very high cost of maintenance and repairs, so a more fragile, higher efficiency device is not generally used.

 

[racookpe1978]

FredRosse:

The other related advantage (or limit, depending on how you view things) about a vertical turbine on floating sea buoy is the relatively small vertical height required: A vertical windmill is less efficient than a high-tower-mounted "classic" two or three bladed propeller-type unit, but it can receive winds from any direction without twisting or wrapping the buoy's anchor chain around. Also, no tall tower is required = The buoy doesn't tip over and is more stable. Coastal winds tend to be steadier than inland breezes, and of course there are no buildings or mountains nearby, so overall, there is more of a chance of getting "some" wind than you'd get at a roadside sign or hilly location with lots of trees.

The top of the turbine blades in a vertical unit are close to the ground, obviously the mid-point of the blades are at half that height, and winds close to the ground are less = less "apparent" efficiency than in a tower facility, but the design is better under actual installed conditions.

 

[hokie66]

Is there a difference in noise? Where these wind farms have been situated close to housing, they are driving people crazy. I don't think a person could ever get used to the eerie sound they make.

 

[racookpe1978]

Sort of.

But the noise problems are very bad - or "very good" - depending on how well you like sleeping. And how much you hate/dislike/like/want/love the "ideals" of wind power.

Imagine a one-blade prop going "whoooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh..........whooooosh" for the next five years (Until it breaks down.)

Then compare that low frequency, slow ranging noise to a three-blade prop's faster but slightly higher frequency
"whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...whooosh...