Why do windmills have three thin blades?

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Offline thedoc

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Why do windmills have three thin blades?
« on: 09/09/2016 09:53:01 »
asked the Naked Scientists:
   Hi Chris,

I am listening to the Naked Scientist podcast every week. And I also listened to you on the Nature podcast some times ago. You do a great job !
I have a naive question for the podcast question section.>

Why do modern windmills have three thin blades?
This seems counter-intuitive. We would expect either two (as in plane propellers) or three thick ones (as in boats) or four or more to offer more resistance to wind.

I understand that this results from optimization, but I don't see why even slightly thicker blades wouldn't do the job a little better.

Best wishes,
What do you think?
« Last Edit: 09/09/2016 09:53:01 by _system »


Offline alancalverd

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Re: Why do windmills have three thin blades?
« Reply #1 on: 09/09/2016 11:14:04 »
Optimisation of a windmill is surprisingly complicated as it is a balance between meteorology, fluid dynamics and economics.

Start with economics. Design and testing is an expensive business, and blades break or wear out from time to time, so you want the minimum number of different designs to make stock control and maintenance as easy as possible.

Now consider how you are going to make money from the turbine: if it is an isolated farm unit, you need it to work over the widest possible range of wind speeds, capital cost is crucial, but efficiency isn't particularly important, so a small multiblade fan can usually be accommodated on a low mast close to the farm sheds. If you are feeding into the grid, capital cost is less significant - you can get paid for not producing electricity when demand is low - but you want maximum efficiency at low windspeeds because the coldest days are the least windy and the most profitable, so a long, thin wing (think glider rather than aeroplane) on a tall tower is best. But if the wing is too long, the tip will approach sonic speeds so there's an upper wind limit beyond which you need to feather the wing to reduce its efficiency and prevent tip damage.

The problem with multiple blades is that, at low windspeeds, each blade is moving into the turbulence of its predecessor and therefore not extracting the maximum available power from the wind. Early wind generators and some model airplanes (used for low speed "maximum climb" competitions) use a single blade with a counterweight. A fixed two-blade airplane propellor works OK for light aircraft (up to 120 knots) and low-speed wind generators, but faster turboprop planes use 3,4 or 5 short, fat blades with variable pitch to optimally match rotation to airspeed - and of course jet aircraft use dozens of short blades in a 500 kt bypass fan. The practical limit for a large wind turbine is probably a 3-blade variable-pitch wing with an efficient operating range of 0 - 40 knots.     
« Last Edit: 09/09/2016 11:16:29 by alancalverd »
helping to stem the tide of ignorance


Offline evan_au

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Re: Why do windmills have three thin blades?
« Reply #2 on: 09/09/2016 12:41:44 »
I understand that a 4-bladed windmill can extract slightly more energy from the wind than the 3-bladed variety.
But the cost goes up by almost a third, which is more than the extra efficiency.
So it comes down to economics.