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.