Science Questions

Why do wind turbines only have three blades?

Sun, 22nd Jun 2008

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Question

Eddie Cunningham asked:

Why do wind turbines only have three blades?

Answer

Not all wind turbines do have three blades.  I’ve seen some in Spain which have four and some older ones only have two.  Some old-fashioned windmills have up to six or eight.  Three seems to be the optimum for wind turbines.  There’s a few reasons behind that.  One of them is that if you have too many blades on a wind turbine each blade as it moves through the air leaves a vortex behind it.  It’s very like if you look at a plane taking off you can see swirling air behind to two wigs of the plane.  Wind turbine blades are very much like a plane’s wing.  You get a swirl left behind a moving blade.  If those interfere with one another that can cause big problems.

That’s similar to a boat propeller only having three blades.  If you had more blades you’d think you’d move more water but actually it would become self-demolishing in terms of the benefit.  That’s the optimum choice between weight and materials and efficiency.

If you have only two blades because the wind’s moving faster higher up than lower down then it tends to, if the blade’s pointing upwards, it puts a big twisting force on the bearings. It can damage the bearings with the horizontal force.

You can actually steer a boat that way.  With big boat propellers, really big propellers the water is denser at the bottom of the propeller than it is at the top.  The propeller as it turns round is creating more of a push to the side at the bottom of its run than at the top.  If you know which way your propeller rotates by giving the engine a big burst of power you can enable it to do what’s called prop walking.  It gives you an extra kick in one direction which can make it really easy.  That’s how these really clever people get their boats into really small spaces just because they’ve learned they can use that trick just to get some extra movement in one particular direction.

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Eddie Cunningham asked the Naked Scientists: Why do wind turbines only have three blades? What do you think? Eddie Cunningham, Sat, 21st Jun 2008

I imagine the number of blades is the result of an optimisation process. Parameters to optimise will include:
- maximising total energy captured
- minimising total mass (ie material cost/weight, bearing-wear etc)
- minimising downstream turbulence (efficiency and, importantly, noise pollution)
- minimising bird-strike
- (possible) minimising radar/TV cross-section (to reduce interference)
- limiting 'cross section' in gale conditions when the blades are deliberately stalled.

I'm guessing that a 4- or 5-bladed design would capture slightly more wind energy, but degrade most of the other considerations.
techmind, Sat, 21st Jun 2008

Aerodynamics is one of the least intuitive of all bits of Engineering.  Just 'looking at' a design can easily give you the wrong impression about its suitability. There are so many parameters involved and 'they' have certainly looked at all possibilities. lyner, Sun, 22nd Jun 2008

Not an expert on wind turbines but generally, fewer blades are cheaper to make and maintain.  On a wind turbine however, to sink the same power, reducing the count to two blades would require each of the blades to be quite a lot longer, and to generate the same power, each blade attachment would have to be proportionally stronger, pushing the costs back up.  In addition, because the blades would be longer, the tower would have to be higher, further increasing the costs.  On the other hand, reducing the blade count from four to three wouldn't incur the same degree of penalty, and as the turbine has to be at a minimum height anyway, three blades seem to be optimimum, all things considered.
LeeE, Mon, 23rd Jun 2008

Do the all have only three blades? Windmills one sees on farms have more. Alan McDougall, Sun, 29th Jun 2008

Maybe the farmers did not understand aerodynamics. Atomic-S, Mon, 14th Jul 2008

Small turbines (like you get on boats) have multiple blades. (Damned noisy, tho) lyner, Tue, 15th Jul 2008

If you're thinking of the wind-pump windmills one sometimes sees on farms or in western movies, where they are more like a fan than a propeller, I think it's largely due to the blades not being aerodynamically shaped - they're just planks of wood with no aerofoil cross-section, so you need to use lots of them to make up for the inefficiency.  Small turbines that you'd be likely to find on small boats can operate at much higher rotation speeds before they run the risk of shedding blades and so don't need a pitch-change mechanism, much reducing the cost per blade - when the wind gets too strong you just collapse or lower it - so you can afford to use lots of blades and reduce the diameter at the same time. LeeE, Tue, 15th Jul 2008

Why do wind turbines have three narrow blades, but ceiling fans have five wide blades?
Dale E. Berg, a member of the technical staff in the wind energy technology department at Sandia National Laboratories, replies:

The differences between wind turbine and ceiling fan blades arise from the contrasting design criteria: the wind turbine is intended to capture high-velocity wind to generate electricity efficiently; the ceiling fan needs to move air at low velocity with inexpensive components.

To keep drivetrain costs low, a wind turbine must capture the energy in fast-moving air and rotate at relatively high speed—within limits, so as to avoid excessive noise generation. (Slow rotation would increase the torque and require heavier and more expensive drivetrain components.) Such high-efficiency energy conversion dictates the use of lift-type turbine blades, similar to airplane wings, of twisted and tapered airfoil shapes. The blade design creates a pressure difference in wind—high pressure on one side and low pressure on the other—that causes the blades to turn. A combination of structural and economic considerations drives the use of three slender blades on most wind turbines—using one or two blades means more complex structural dynamics, and more blades means greater expense for the blades and the blade attachments to the turbine.

The ceiling fan, on the other hand, is built to keep the occupants of a room comfortable by moving air gently. Its engineers work to minimize noise while the fan rotates at low speed (for safety reasons) and to keep the construction costs, and therefore the purchase price, low. Energy efficiency is not a primary concern, because operation is inexpensive—a typical ceiling fan running 24 hours a day consumes about 60 kilowatt-hours a month, for an average electricity cost of about six dollars. For this reason, most ceiling fans incorporate blades that are comparatively inefficient drag devices; rotating the pitched blades pushes air vertically out of the way. Wide, flat blades are inexpensive to build and work well as drag devices. More blades are better, up to a point, and the usual layout of four or five blades is the result of balancing trade-offs between efficiency and expense.

A 2001 article in Mechanical Engineering chronicled the quest of a man named Danny Parker to create a more efficient ceiling fan. Parker’s initial blade prototype looked a lot like a wind turbine blade, but the end result (because of man­ufacturing, safety and operating concerns) was a hybrid between a standard ceiling fan blade and a wind turbine blade.

http://www.sciam.com/article.cfm?id=wind-turbine-fan-blades paul.fr, Wed, 28th Jan 2009

How about using screw formed devices?

Would that work?
(Maybe in concord with some sort of windfunnel creating a stronger airflow.)
yor_on, Wed, 28th Jan 2009

this isn't a question or an answer, but an opinion, adult to adult, ok? this is a question many people will search for and they will find this page, so i think it's important they should read an opinion that shares their skepticism, and offers an alternative view to the witewash presented here. wind farm blades are exactly like a PROPELLOR, not a turbine. it's the elephant in the room and that's why this page is here. these are covert weather weapons / defences. they are so bad at catching the wind, the operators have been caught feeding them power FROM the national grid, just so they go round and people think they're doing something. it's also tragic that clean energy projects have been compromised in the process. lazyhorse, Tue, 28th Sep 2010

Maybe in the olden days the farmers had three blades and now it's a tradition so thy don't want to change it? Ash, Thu, 22nd Sep 2011

lazyhorse is onto something. It's been discovered that the plan is to implement enough of these and run them off the electrical grid. The government would then be able to speed them up and slow them down at will. This would actually speed up and slow down the rotation of the earth and they could get rid of the daylight savings time which causes such a problem twice a year when everyone needs to convert. Not too many people know about this plan. dr_wind, Sun, 9th Oct 2011

If Three vanes reduce the length of the vane required to do the same job as two would , then why not six vanes or more ? Allowance being made for the speed for lift etc. in an aeroplane ? Would smaller, but more vanes, not be better and cause less vane fatigue, and engine percepted noise, and also reduce lift-off speed in mono mini 'planes ? Aidan Colgan, Mon, 30th Apr 2012

The section on prop walk attributes this phenomenon to the difference in water density between the top and bottom of the prop. I doubt, this, though. Water is a very incompressible fluid, so the difference in density between the top and bottom of even a very large prop would be negligible. I would say that other factors have to account for this phenomenon. Tom Einertson, Mon, 4th Jun 2012

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