Wind energy is the fastest-growing renewable energy sector, even beating out solar. We have been harnessing the power of wind throughout much of human history. That means wind turbines are becoming an increasingly common part of our landscape. Though unlike solar, wind turbines are almost exclusively large-scale projects built by industrial and commercial producers. So the average person is less likely to be familiar with how a wind turbine works. Keep reading to learn about what wind turbines are and how they produce electricity.
How do wind turbines generate electricity?
The most common way to generate electricity is by spinning a turbine which connects to a generator. This is commonly applied to fossil fuels, where a fuel is burned, creating pressurized steam or gas that spins a turbine and generates electricity. Wind turbines work on the same premise, but they use the wind. The blades of a wind turbine capture kinetic energy from the wind, which causes them to spin. This spinning motion then generates electricity.
Step 1: Wind strikes the turbine
Before you can harness wind energy, you need the wind to blow. The wind strikes the blades, spinning the rotor. The blades are shaped like an airfoil, similar to the wings of a plane, causing them to generate lift. This is the force that generates electricity. Since the blades are always in motion, the wind strikes the blades at a relative angle. The blades are designed with this in mind, incorporating twists and angles to take advantage of the angular momentum and capture as much energy as possible.
Step 2: The turbine spins
Once the turbine is spinning, the wind energy is transferred into the gearbox. The wind turbine itself rotates too slowly to generate electricity, so gears are needed to amplify the torque. The gearbox consists of a low-speed shaft and a high-speed shaft. The blades are connected to the low-speed shaft, which connects to the high-speed shaft. The high-speed shaft connects to the generator.
Step 3: Electricity!
The gearbox is like a funnel, forcing a large amount of energy into a small rotor, which spins an electromagnet inside the generator. Electricity is generated as the mechanical energy from the wind is converted into electrical energy.
Step 4: Power Transmission
The electricity flows through cables in the tower to a transformer at the base of the wind turbine. The generators in most wind turbines produce AC power, so there’s no need to convert from DC to AC like at a traditional power plant. The transformer amplifies the voltage for large-scale distribution.
Wind Direction & Speed
The velocity and direction of the wind have a major effect on the power generated by the wind turbines. Stronger winds generate more electricity, although there is a limit. Turbines shut off when wind speeds go above 55 mph to prevent damage, and there are instances of turbines being completely destroyed by high winds during storms. Similarly, wind speeds that are too low carry little capacity to produce energy. Most wind turbines shut off when wind speeds drop below 8 mph.
Turbines work best when the wind is perpendicular to the plane of the rotors, or when the rotors and direction of the wind form a 180-degree angle. The efficiency of the turbine drops when the angle begins to deviate. Nature isn’t perfect, and wind direction can change abruptly without much notice, so most wind turbines are equipped with a yaw system and motor which orients the turbine towards the direction of the wind.
Upstream vs Downstream
Upstream is where the wind is coming from, or the area in front of the turbine. Downstream is where the wind is going, or behind the wind turbine. Since wind turbines are extracting energy from the wind, the wind speeds downstream are always going to be lower than upstream wind speeds. The wind speed flowing through the plane, or working section of the turbine is the average of the upstream and downstream velocities. The efficiency of the turbine is highest when the wind speed downstream is one-third of the upstream wind speed. The efficiency of wind turbines is capped by the Betz’s Limit, which states that no wind turbine can extract more than 59.3% of the available wind energy.
What are the parts of a wind turbine?
From the outside, the design of a wind turbine seems simple enough. It’s just a giant fan, right? Wrong. Getting the best efficiency means that a ton of engineering, physics, and fluid dynamics are put into the design of a wind turbine. They’re a complex system of parts that work to extract the most energy possible.
Blades & Rotor
The main feature of a wind turbine is the blades. Most have three, though a few designs call for just two blades. The blades are shaped like an airfoil, like the wings of a plane. This aerodynamic design creates more lift than drag, causing the blades to spin.
Since the blades spin, they experience the wind in a relative manner. Though the wind is perpendicular to the blades, the top end of the airfoil experiences the most wind. So designers tilt the blades in the relative direction of the wind to maximize efficiency. The relative wind speed and direction changes slightly as you go from the base of the blade to the tip. The most efficient blades have a slight twist to capitalize on this effect.
The blades are attached to a cone-shaped hub. Together, the blades and hub form the rotor, which rotates in response to the wind.
By themselves, wind turbines rotate too slowly to produce energy on their own. In order to spin the generator fast enough to generate electricity, a gearbox is necessary to speed up the rotation. The gearbox consists of a high-speed shaft and a low-speed shaft. The rotor is connected to the low-speed shaft, which connects to the high-speed shaft, and in turn, connects to the generator. The gearbox ensures that enough torque, or rotational energy, reaches the generator to generate electricity.
The generator is where the electricity is produced. The torque produced by the rotor is amplified in the gearbox and is then converted into electrical energy. Like most electrical generators, the generator in a wind turbine spins a rotor connected to an electromagnet, which produces electricity.
To prevent damage during high-speed winds, wind turbines are equipped with speed brakes. If the wind speed surpasses 55 mph, the breaks turn on, stopping the rotation of the blades. This protects the blades, gearbox, and generator from potential damage.
All of the above parts are housed inside the nacelle. The nacelle is the box behind the blades and rotor. It protects the gearbox, breaks, and generator from exposure to the elements.
Anemometer & Wind Vane
The anemometer is a device that measures wind speed. The anemometer sends a signal to the speed brakes when the wind speed is too high or low. The wind vane measures the direction of the wind and sends a signal to an electronic device that controls the yaw system. The anemometer and wind vane both sit atop the nacelle, usually towards the back.
The yaw system keeps the wind turbine oriented in the right direction. Like we said earlier, maximum efficiency is achieved when the turbine rotors are perpendicular to the direction of the wind. The yaw system receives input from the wind vane about the direction of the wind, and orients the turbine accordingly. It is composed of a system of motors and brakes, and uses either electric gears or hydraulic bearings to rotate. The yaw system is located where the nacelle meets the tower.
Wind tends to be stronger at higher altitudes, so wind turbines are placed atop large towers to take advantage of the higher wind speeds. The height of the tower is important when assessing potential power generation. Raising or lowering the tower height can have a large effect on the projected power capacity. Whereas Indiana had a projected power capacity of 30,000 MW, that figure rose to 40,000 MW when the tower height was raised from 50 m to 70 m. Average tower height is around 65 meters. To avoid buckling, doubling the tower height requires doubling the diameter of the tower and increasing the amount of material by a factor of four.
The majority of towers are made of steel and make up anywhere between 30% to 65% of the total weight of the turbine. Researchers and engineers are looking into higher-grade steel alloys that weigh less but still provide stability. Steel has its disadvantages, as it is not strong enough to support the construction of towers taller than 90 meters. Research is being done on steel mixed with prestressed concrete, which may be able to support the construction of super-tall wind turbines.
The tower houses the cables which run electricity to the power converter at its base. In most turbines, the electricity generated is already an alternating current, which can be run through a transformer to raise or lower the voltage, then fed directly to the power grid.
Types of Wind Turbines
The most common type of wind turbine is the horizontal axis wind turbine, called HAWT for short. The HAWT design produces the vast majority of wind power today. It’s simply the most efficient and well-researched design available. The blades on a HAWT generate lift, causing the turbines to spin and generate electricity. Unlike other designs, HAWTs must face the wind to be effective, and are equipped with a yaw system to make sure that a constant 180-degree angle is achieved. Most HAWTs have three blades, but two-blade designs are common as well.
VAWTs, or vertical access wind turbines, are less common. As the name suggests, VAWTs have vertically oriented turbines which spin around a central shaft. A major advantage of VAWTs is that they don’t have to be pointed at the wind, which is great for areas that have variable wind direction. While a few VAWTs are large-scale projects, most tend to be used on a small scale, like powering a single building or small structure.
Saponius VAWTs and Darrieus VAWTs are the two most common designs. Saponius VAWTs are among the simplest ways to generate wind power, made using a few airfoils, or scoops, fused into a central rotating shaft. They’re self-starting, but suffer from low efficiency because they produce energy using drag instead of lift. Darrieus turbines use two large curved airfoils attached to the top and base of a central shaft. Darrieus VAWTs are quite efficient since they generate electricity using lift, but they suffer from structural weaknesses. They also need an external power source to get started.
A group of wind turbines generating power in one location is called a wind farm. A wind farm can vary in size from a handful of wind turbines to several hundred. The number of wind farms is rising across the globe, and as wind turbines become increasingly efficient, fewer wind turbines are needed per unit of power. Wind farms can be onshore or offshore. The largest onshore wind farm is the Gansu Wind Farm in China. The farm has a current capacity of 8 GW, and engineers plan to expand the farm to 20 GW. The largest offshore farm is Hornsea 1 in the UK, with a capacity of 1.2 GW. When the entire Hornsea project is complete, it will have a total capacity of 6 GW.
How much electricity can a wind turbine produce?
Large wind turbines can provide anywhere from a few hundred kilowatts to several megawatts of power, but since wind speeds often vary, so does the power output of a wind turbine. If a wind turbine is rated at 1.5 MW, then it is expected to generate significantly less power in practice. Generally speaking, offshore wind turbines are larger than onshore turbines, and so can generate more energy. The largest offshore wind turbines have a power output as high as 8 MW.
Newer wind turbines are deliberately designed to not operate at full capacity. A small portion of the energy is stored either in the generator or electrical grid for times of need, such as a failure in the electrical system. This extra energy is also used during times of low wind speeds to sustain a constant electrical supply.
Where should wind turbines be placed?
Any region with strong winds is a desirable location for a wind farm. Generally speaking, the strongest winds are found directly offshore and in mountain passes. Mountain passes tend to be the ideal location, as they have strong, steady winds that come from a single direction. Access to electrical systems, physical geography, and local electricity prices play a major factor as well. Some of the windiest places in the world are near the poles, but the extreme cold and lack of densely populated areas means that there’s little reason to build a wind farm.
Wind turbines capture energy from the wind. This energy is then amplified using a gearbox, which turns the rotor in a generator, converting the mechanical energy into electrical energy. Wind turbines have an efficiency of 40% to 50%, but their efficiency is capped by the Betz’s Limit, which states that no wind turbine can extract more than 59.3% of the wind’s energy.
Horizontal access wind turbines, or HAWTs, or the most common wind turbine. HAWTs are more efficient and stable. Most HAWTs have two or three blades, and they use drag to generate life. HAWTs must face the wind to be effective. Vertical access wind turbines, or VAWTs, are less common. VAWTs are usually used on a smaller scale to power single buildings or small structures.
The Gansu Wind Farm in China is the largest in the world, with a current capacity of 8 GW, though future plans call for a total capacity of 20 GW. The largest offshore wind farm in the world sits just off the coast of England. Hornsea 1 has a capacity of 1.2 GW, but once the four-phase project is complete, the Hornsea project will have a capacity of 6 GW.
Strong winds are best for generating wind power. The wind should ideally be between 8 and 50 mph, and come from a single direction. It should also be a steady current of wind, without turbulence or sudden gusts.
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