How Wind Turbines Really Work: The Hidden Secrets

Here’s a sanitized version of the provided YouTube transcript:

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Sponsored by Brilliant. Why are there three blades? Why are they so high? Why are they so slow? And how does it even generate electricity? Grab a pad and paper to make notes and sip from your engineering mindset mug. Let’s find out!

This basic wind turbine can power a small LED, while this larger one can power a small home. However, these mega turbines can power entire towns. A wind turbine simply converts the kinetic energy of the wind into mechanical energy, which is then converted into electrical energy. We can feel the energy of the wind on our hand; we know that it can turn a windmill. We can turn that into motion, or we can attach a generator to it, and it will produce electricity.

Try it yourself: take a simple DC motor, spin the shaft, and you’ll notice it produces a voltage. Just attach a blade to it, and it will spin in the wind and generate electricity. The speed of the wind increases the higher we go, and it’s also less turbulent. The larger the blades, the more wind energy we can capture. Large blades need to be higher off the ground, but the speed of the wind is the largest influencer in power generation.

Large turbines are difficult to transport, so we often find the largest turbines out at sea, where space isn’t a problem. Although it is cheaper and easier to install them on land, they do mark the landscape, cast long flickering shadows, and can create some noise. Wind turbines need a deep, strong foundation. We can extend down into the seabed, but some waters are so deep it’s easier to float the wind turbines on a platform.

You might notice that smaller wind turbines have a tail fin at the back, but large ones don’t. I’ll explain why later in the video. The wind turbine needs to face the wind, and since the wind changes direction, we could use a vertical wind turbine, which will work in any wind direction. There are many designs, but they are usually less efficient in comparison and don’t scale up very well.

The wind turbine can be upwind or downwind. Upwind is more efficient because the wind hits the blades before the tower. However, the blades need to be stronger so that they don’t bend in the wind and hit the tower. How do you feel about wind turbines? Would you live next to one? Tell me in the comment section down below.

When we look at a large wind turbine, we notice the tubular tower rising up out of the ground. It reduces in diameter as it reaches the top, rising high into the sky to reach the strongest wind. Inside the tower, we have an access ladder for engineers, power cables, and often find a transformer at the base. At the top of the tower, we find a large bearing and a ring gear. Attached to this bearing is the bedplate, which is the main support.

A set of motors are bolted onto the bedplate and their gears lock with the large bearing gear. This controls the direction of the turbine. A small encoder counts how far the turbine has rotated. I’ll explain this part later on in the video. We also find a set of hydraulic brakes and a large disc brake, which will hold the turbine in position. At the back of the bedplate, we find the electrical generator, along with an electrical and controls panel.

The generator connects to a gearbox via the high-speed shaft. Attached to the shaft is a disc brake, which is hydraulically controlled. This will be powered from the hydraulic control set. The gearbox then connects to the main low-speed shaft, which is supported by the main bearing. This connects to the hub at the very front of the turbine, where the blades will bolt onto the hub via some geared bearings. The metal hub is covered with a nose cone to protect it and improve aerodynamics.

Inside the hub, we typically find three motors. These are attached to the hub and their gears lock with the geared bearings, allowing the blades to be tilted. The bedplate and all the main components are covered with a fiberglass housing, forming the nacelle. This case protects the components from the wind, sun, rain, etc. On top of the nacelle, we find a wind vane, which determines the wind direction, and there’s also an anemometer to measure the wind speed.

The wind vane will determine the wind direction, and the controller releases the brakes, allowing the motors to turn the nacelle to align it with the wind. Once aligned, the brakes are reapplied. The wind flows over the blades, forcing them to rotate. This rotates the hub, which rotates the shaft. The shaft rotates slowly but with high torque. The bearing supports this and allows low-friction rotation.

The shaft will rotate the gears in the transmission, and then the output shaft connects to the electrical generator’s rotor. The rotor turns and induces a voltage, generating electricity. The output of the generator flows through a cable and down the tower to the transformer, where it will be sent to the electrical grid and distributed to towns and houses. Other power generators, such as solar or nuclear, will also feed into the grid.

Wind and solar are a great combination of renewable energy because it’s either sunny or windy. The blades are typically made from reinforced fiberglass, making them strong and lightweight, allowing them to be longer to capture more wind energy. Metal or wooden blades are expensive, heavy, and more likely to fail. Heavy blades are hard to turn and stop.

The blades have an aerofoil shape, with the shape changing along the length of the blade and often twisting to improve aerodynamic efficiency. Smaller wind turbines usually have fixed-angle blades, but large turbines can change the angle of the blade. The front of the aerofoil is known as the leading edge, while the rear is called the trailing edge. The line between these two points is the chord line.

When the blade tilts, the difference between the chord line and the relative wind direction is known as the angle of attack. The blade obstructs the wind’s path, forcing it to go under and over the aerofoil. Air is a fluid, and when an object passes through it, we get friction across the object’s surface and resistance from its shape. We call these forces drag, which acts parallel to the wind, slowing the blade down.

The aerofoil is designed to minimize drag forces and maximize lift. The air has a longer distance to travel over the top due to the curved profile, meaning the speed increases along the top and slows down along the bottom, resulting in two streams arriving at different times. As air speed increases, pressure decreases, creating a lower pressure region over the top and a higher pressure region along the bottom. The higher pressure side pushes the blade into the lower pressure region, creating lift.

Air colliding with the underside of the blade also provides force. The air on the top and bottom is deflected downwards, creating an equal and opposite upward force, helping push the blade into the lower pressure region and adding to the lifting force. The tip of the blade has a higher velocity through the incoming windstream than the hub, so the lift and drag will be different. The shape of the blade is twisted to account for this and improve the angle of attack.

We tilt the entire blade to alter the amount of lift produced. As the angle of attack increases, more lift is generated, but at a certain point, the streams will separate and become turbulent, reducing lift and increasing drag, which slows the rotation down. We can see with this model wind turbine that if the blades are perpendicular to the wind, maximum drag occurs with no lift, and the blades do not turn, generating no voltage.

If the blades are parallel to the wind, very little lift is generated, the rotation is slow, and only a small voltage is produced. It’s also very easy to stop this rotation. However, if we tilt the blades to an optimal angle, we generate a large amount of lift, the hub spins very fast, and we generate a few volts. The blade’s design will have an optimal angle of attack, which we can find on the design chart.

We can see with this DC generator that the faster the shaft rotates, the more voltage is generated. However, if it spins too fast for too long, it becomes very hot and will eventually fail. Our generator might be rated for, say, 2 megawatts, so we have to tilt the blades to control how fast they rotate, which controls how much power we generate and helps us stay under the maximum rating of the generator.

The wind turbine won’t start until a minimum wind speed is reached, known as the cut-in speed. As the wind speed increases, the power output also increases. At a certain wind speed, the wind turbine will tilt its blades to stop generating power, and the brakes will be applied to protect the turbine. This is known as the cut-out speed. The anemometer measures the wind speed, and the controller changes the angle of the blades.

So, how many blades do we need? Each blade generates lift, causing rotation, but they also generate drag, which slows the blades down. Using this model wind turbine, we can change the number of blades to find out. With one blade, it’s very slow, unstable, and doesn’t produce much voltage. It also doesn’t self-start, so this is not a good design.

With two blades, it self-starts, is much more stable, and can produce a higher voltage. With three blades, it produces only a slightly higher voltage but is much harder to stop because it’s catching more wind energy. With four blades, it again produces a slightly higher voltage, but with five blades, the voltage starts to drop slightly, and at six blades, it produces an even lower voltage but is very hard to stop.

The three, four, and five-blade versions produce the most energy. The three-blade version is very stable and costs the least to build, making it the obvious choice. Two blades are also common in medium-sized turbines because they are cheap and fairly stable. Micro wind turbines might have many blades because they are installed lower, experiencing slower and weaker wind speeds.

Large wind turbines rotate quite slowly, but the blades are very long, so the tip of the blade travels much faster than the hub. At a certain point, the blade tip will travel so fast that it will break the sound barrier, creating a sonic boom and causing the blades to rip apart. Even at low speeds, there are large centrifugal forces acting on the blades.

Additionally, the generator needs to rotate at a certain speed to produce 50 or 60 Hz electricity for our homes. The gearbox increases the speed so the rotor doesn’t need to rotate very fast to achieve this. Small wind turbines have a large tail fin, allowing them to align their blades into the wind. Without this, they will turn away from the wind, making the wind energy hit the nacelle and tower first, which is less efficient.

Vertical wind turbines do not need a tail system; they work in any wind direction. However, large wind turbines don’t use a tail fin because they would need to be so large to work, adding a lot of moving weight. They also swing in turbulent winds, which creates uncontrolled force on the structure, bearings, and blades. Engineers opted to use a wind vane to indicate wind direction, and a computer controls motors that rotate the nacelle around the large gear on the tower to keep it facing the wind for optimal performance.

Some brakes hold the turbine in position once the nacelle is aligned with the wind. A small encoder tracks the rotation of the nacelle in large turbines because the power cables need to connect from the generator down into the tower. If it rotates too far, it will twist the cables and eventually snap them. The nacelle will turn in the wind, but shortly after, the wind direction will change, causing the nacelle to realign and undo the twist. The cables are suspended to reduce twisting, and the computer controls how far the nacelle can turn to avoid twisting them.

Small turbines just use slip rings to avoid that, but large turbines produce much more power, making it cheaper, safer, and easier to use a cable and track the rotations. Small wind turbines are typically direct drive, often using permanent magnet generators. Large wind turbines turn much slower, so we use gears to increase the speed of the rotor to produce sufficient power and output frequency at the generator.

Typically, we find a three-stage gearbox consisting of a planetary gear set and two spur stages. The input shaft is low speed but high torque. The gearbox converts this into high speed and low torque. The rotational speed is controlled by the pitch of the blades, so the input speed might be just 18 RPM, and the output speed is 1,800 RPM. We need to achieve this speed to control the output of the generator.

We also have a hydraulic disc brake at the back of the gearbox because the shaft is low torque, making it easier to stop. The blades are first used to stop the rotation, and the brakes will then hold it in place, for example, during maintenance. The doubly-fed induction generator is the most common generator for large wind turbines. Smaller domestic wind turbines might use a three-phase brushless motor or a brush DC generator.

When we pass DC current through a coil of wire, it produces an electromagnetic field. When we pass AC current through the coil, it produces a magnetic field that changes polarity. The rate of change depends on the frequency of the AC current applied to the coil. A basic generator has a magnet at the center of the rotor and a coil of wire on the stator. When the rotor rotates, the magnetic field interacts with the electrons in the wire, pushing them forwards and pulling them backwards, creating an alternating current with a sine wave.

The electrical outlets in our homes provide either 50 or 60 Hz, meaning the sine wave repeats 50 or 60 times per second. To achieve that, the magnet would need to rotate thousands of times a second. However, if we add another magnet and coil, we can reduce the distance and time taken for the North and South Pole to pass a coil, reducing the rotational speed to just 1,800 RPM. The gearbox increases the speed by around 100 times, so we only need 18 RPM of the hub for the blades to achieve that.

We can see with this simple two-pole AC generator that the frequency produced depends on the rotational speed of the rotor shaft. In the wind turbine, the rotor connects to the blades. The faster the wind, the faster the shaft rotates, although we have some control over the shaft speed by rotating the blades to change the amount of lift or drag produced. If the wind speed is too fast, the turbine shuts down, which is our cut-out speed.

A generator rotates easily and produces voltage, but when we connect a load to it, it’s much harder to rotate, adding mechanical load to the blades and slowing them down. Therefore, the wind turbine won’t start until a minimum wind speed occurs, known as the cut-in speed. The doubly-fed induction generator consists of a rotor attached to the high-speed output shaft from the gearbox. The rotor has three sets of coils connected to slip rings at the end, while the stator surrounds the rotor and also has three sets of coils inside.

When the blades turn, the shaft turns, and the rotor rotates while the stator remains stationary. The rotor is connected to a three-phase electrical supply via the slip rings. Each coil produces an alternating magnetic field at slightly different times based on the AC frequency applied to the coil. These are positioned around the rotor to combine and create an equivalent rotating electromagnetic field. The direction of rotation depends on the timing of the phases.

A controller determines the frequency and direction of the rotating electromagnetic field. As the electromagnetic field rotates, it induces voltage into the coils of the stator, generating AC current that we then export to the grid. If we apply a 60 Hz supply to the rotor, we generate and export 60 Hz back to the grid. Because the wind controls the rotor speed, it might rotate at 1,800 RPM, slower, or faster.

The magnetic field is rotating and attached to the shaft, so these will combine. If the rotor speed drops to 1,600 RPM, that’s equivalent to 53.33 Hz, so we would need a 6.67 Hz frequency on the rotor coils to make up the difference and achieve 60 Hz. The speeds will combine to produce an equivalent 60 Hz rotating magnetic field, generating 60 Hz at the stator.

If the shaft speed increases to 2,000 RPM, that’s equivalent to 66.67 Hz, which is too fast, so we need to subtract 6.67 Hz by rotating the electromagnetic field in the opposite direction to the rotor. If the rotor is exactly at the required 1,800 RPM, we require zero Hz, which is DC electricity. We apply a constant current to the rotor, and the magnetic field rotates only with the shaft at the same speed, resulting in 60 Hz on the rotor and 60 Hz at the stator.

The controller constantly adjusts the frequency of the rotor current to ensure a 60 Hz output is maintained. The engineering design of a wind turbine is complex, but with Brilliant, our sponsor, you can learn core engineering principles. They offer many amazing courses and even a 30-day free trial, allowing you to learn about electricity, magnetism, planetary gears, mathematics, Python programming, and data analysis—all key skills to become a brilliant engineer.

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