Above: Image © Leaflet, Wikimedia Commons

D.J.- Grade 11 - London, Ontario

This question follows from a previous question on "where does wind come from?"

As you'll remember, wind is air in motion. It is created by temperature differences on the earth's surface. When warm air rises, cool air comes in to take its place. This process is known as convection and is responsible for the movement of air which we know as wind. Air also exhibits a mass — the product of density times volume. So when air is in motion (wind), it contains energy in that motion (kinetic energy).

Did you know? Port Martin, Antarctica is the windiest place in the world where the average annual wind speed is 64 km/h (40 mph) and gale force 8 winds are experienced for over a hundred days per year.

Wind turbines were developed to capture the kinetic energy of moving air by slowing it down and transforming its energy into electrical energy. Two important factors that contribute to the operation of a wind turbine are the design and configuration of its blades.

Wind Turbine Blades

If you've ever had the opportunity to watch a wind turbine start, you would have noticed that the blades start rotating very slowly and then begin accelerating faster and faster. This is because of the aerodynamic design of the blades.

The blades of a wind turbine are shaped similar to an airplane wing, with one side (rear) much more curved than the other (front). With a wing, air flows fastest over the top which reduces the pressure and causes the lift needed for the aircraft to fly. Turbine blades also rely on pressure differentials due to changes in air speed in order to operate.

When the wind begins blowing and passing over the blade, air behind the blade starts travelling at a higher velocity than air in front of the blade. In fact, the greatest velocity is at the rounded front edge which creates a pocket of low-pressure air. This literally pulls the blade forward and we get the start of rotation.

Once the blades are rotating, they create their own headwinds (like what we feel on our face when cycling). The velocity of this additional wind helps to lower the pressure on the back side of the blade and contributes to even more lift. This causes the blade to rotate faster and produce additional headwind. The net effect is that the blades of a turbine spin more rapidly until they reach their maximum velocity.

But why 3 blades?

The more blades there are on a wind turbine, the higher will be the torque (the force that creates rotation) and the slower the rotational speed (because of the increased drag caused by wind flow resistance). But turbines used for generating electricity need to operate at high speeds, and actually don't need much torque. So, the fewer the number of blades, the better suited the system is for producing power.

Did you know? About 90% of the installed wind turbines today have three rotor blades.

Theoretically, a one-bladed turbine is the most aerodynamically efficient configuration. However, it is not very practical because of stability problems. Turbines with two blades offer the next best design, but are affected by a wobbling phenomenon similar to gyroscopic precession.

Since a wind turbine must always face into the wind, the blades will have to change their direction vertically when there is a shift in wind direction. This is referred to as yawing. In the case of a two-bladed system, when the blades are vertical (i.e., in line with the tower and the axis of rotation) there is very little resistance to the yawing motion.

But when the two blades are in the horizontal position, the blades span a greater distance from the axis of rotation and so experience maximum resistance to yawing (notice how a spinning figure skater slows down when they bring their arms away from their body. As a result, the yawing motion starts and stops twice per revolution, and this leads to stress on the turbine due to blade chattering.

On the other hand, a turbine with three blades has very little vibration or chatter. This is because when one blade is in the horizontal position, its resistance to the yaw force is counter-balanced by the two other blades. So, a three-bladed turbine represents the best combination of high rotational speed and minimum stress.

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Stan Megraw

Stan is a writer/researcher, a PhD graduate of McGill University and was a member of the CurioCity team for several years. As a kid he dreamed of playing hockey in the NHL then becoming an astronaut with NASA. Instead, he ended up as an environmental research scientist. In his spare time Stan enjoys working on DIY projects, cooking and exploring his Irish roots.

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Avatar  Sean

I was reading something recently about lift and newer research shows that Bernoulli pressure differentials don't actually add much whereas the major contribution is the deflection of air mass. Would expect it to be the same here. The pitch of the blades cause them to rotate not the lower pressure pockets.

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Avatar  MD

Yes, some people like to put the cart in front of the horse, it gets the manure to market sooner. Bernoulli's mass-flow equations only apply under constrained flow conditions (static boundary conditions) they are easy to teach but lead to flaws in physical understandings (like equal transit time fiction, which is often still taught at university and Flying school levels).. Note the wind over the top of a lift generating wing arrives at the trailing edge (low pressure) a long-time before the wind under the wing (high pressure) nothing equal about the transit times. Velocity often follows pressure, ie flow accelerates due to reduction in pressure, but on the other hand if you physically increase the flow rate (yes there must be a higher pressure somewhere) total pressure will reduce (due to dynamic pressure differentials.. It's all a complex interaction, which is why there are no simple formulas to calculate fluid dynamics systems (only circular equations which need a wind tunnel to measure Coefficients of lift and drag), only the Navier-Stokes etc family of equations which must be solved iteratively and continuously for every particle in the fluid to understand what is happening. It Flow and pressure are is like Voltage and Current in Electricity theory , a lot of "experts" don't understand it and therefore can't teach it very clearly.I have even had University Lecturers shout at a class "It's all Bernoulli", obviously that fitted the curriculum, and for thosewho progressed to study Aerodynamics at Post Graduate Level, hopefully they cone to realise that Bernoulli only plays a part in real life, Newton also gets a look in (under Inertial Reference Frame Assumptions). All Aerodynamic Wind turbines generate power by taking that power out of the airstream (by redirecting airflow), the lower the Tip Losses and the lower the Drag, reduction of Tower Shadow and Yawing Precession issues (not truly gyroscopic as there is no Rotating Disc more issues of dynamic stall and reversal of Angle of attack)) leads to more efficient and long lived devices. IF Blade length is unconstrained ie higher towers and longer blades have always been possible (for a cost) the lower blade count wins. Airplanes only turned to more blades on propellers because the engines couldn't be made to be any further from the ground or Fuselage, without increasing the overall weight and decreasing the efficiency and stability of the aircraft system. Once we get to maximum blade size, likely in the 100+ m range we will start to see more blades on wind turbines (Will nearly always be odd numbers for resonance reasons).

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Avatar  Idk

Useful

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Avatar  Zainab

Would it be okay to have more than 3 blades and they would be more heavier?

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Avatar  Arnold Schwarzenegger

This page is great. I love to read all about this and i will be showing my manager all of this and i will show this to my friends!!

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Avatar  George Boomhower

Would four blades make the whole wind turbine easier to possitively balance it's forces into the massive base and then into the supporting earth; thereby reducing the amount of very low vibration into the surrounding "neighborhood"? A large part of the upset, other than visual, involves a sense of unease possibly caused by low frequency vibration. George Boomhower. goldenyrs@myfairpoint.net

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Avatar  Cristina Grace Dole

Practically, yes!!!

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Avatar  Cristina Grace Dole

I made some research.

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Avatar   Arnold Schwarzenegger

no you did not the person that wrote thi did a lot of reasearch.

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Avatar  Zainab

How do you know the person did alot of researching?

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