What are wind turbines?
Wind turbines represent a renewable energy form that can be installed both on-shore and offshore. They work by harnessing the kinetic energy of the wind to rotate a turbine, which in turn generates electricity via an electrical generator.
Wind turbines come in a variety of sizes, from small ones that can be installed for domestic usage (kW size), to large ones that can power an entire town (MW size). They are also becoming increasingly more affordable as technology advances.
Wind Turbines
There are many advantages to using wind turbines, including reducing humanity's reliance on fossil fuels (coal, oil etc.) and lowering greenhouse gas emissions. There are however also some disadvantages to consider, such as noise and visual impact. Generally, wind turbines are considered a clean and efficient way to generate renewable energy.
What is wind and how is it caused?
Wind is simply the movement of air within our atmosphere. This movement is driven by pressure differences that occur due to various factors. When air is heated by the sun, it rises, creating a low-pressure area. Conversely, when air cools, it descends, resulting in a high-pressure area. These pressure differences cause the air to move from high-pressure areas to low-pressure areas, as the pressure tries to equalise.
The rate at which air moves depends upon the magnitude of the pressure difference. A larger pressure difference leads to stronger winds, whilst a smaller pressure difference results in gentler breezes. If there is no pressure difference, there would be no wind, but this does not occur in real-world situations. The continuous process of pressure equalisation leads to a constant movement of air, and it is this that shapes the weather patterns and climates that we experience on a daily basis.
To better understand how wind occurs, we can refer to the process of natural convection. Heated air rises as it becomes less dense and cooler air displaces it due to its greater density. This displacement pushes the less dense air (hotter air) upwards, creating a cycle of ascending/rising air, and descending/cooling air. Natural convection occurs on a much larger scale in the Earth's atmosphere, but the basic principle remains the same.
Natural Convection
What are the main types of wind turbine?
Wind turbines can be classified in various ways, but one of the easiest ways to classify a wind turbine is by its axis of rotation. Horizontal axis wind turbines (HAWT) are used for large commercial applications and are the ones that most people have seen. Vertical axis wind turbines (VAWT) are less common, less efficient, and not used for large commercial applications.
What are the main parts of a wind turbine?
A brief overview of each wind turbine component is given below for familiarisation purposes; a detailed description of each part is then given.
Wind Turbine Parts
The main parts of a wind turbine are:
- Foundation – supports the weight of the turbine and all its parts. The foundation is typically constructed of concrete or steel.
- Tower – provides structural support and elevates the nacelle.
- Nacelle – houses all the power generation machinery.
- Rotor – comprising of the blades and a hub.
- Blades – designed with an aerofoil shape to capture the wind's kinetic energy and convert it to mechanical power.
- Hub – a common piece to which all blades connect.
- Low-speed shaft – transfers the mechanical energy (rotational motion) from the rotor to a gearbox.
- Gearbox – takes the rotational speed of the low–speed shaft and increases it.
- High-speed shaft – the high–speed output of the gearbox connects to the high–speed shaft.
- Generator – converts the mechanical energy from the high–speed shaft into electrical energy.
- Brake system – stops rotor rotation in an emergency, or when the wind speed is too low, or when maintenance of the turbine is occurring.
- Yaw control system – a hydraulic system that aligns the turbine rotor with the wind direction for optimal energy capture.
- Pitch control system – a hydraulic system used to change the pitch of the rotor blades. Varying the pitch (angle of attack), varies the power output of the turbine.
- Electronics – regulates the generator voltage and frequency output.
- Transformer – usually installed at the base of the tower or mounted within the nacelle. The transformer increases the voltage output of the generator for efficient power transmission.
A detailed look at each of a wind turbine's main parts will now be given.
Tower
The tower elevates the nacelle, thus maximising its exposure to the higher wind speeds that are present at greater heights. The greater the wind speed, the greater the amount of energy available for conversion to electrical power.
Nacelle
The nacelle is an enclosure situated atop the tower. It houses all the power generation machinery, including the gearbox, generator, and control systems. The nacelle protects the power generation machinery from the environment, thus reducing the effects of wear-and-tear, corrosion, and erosion.
Rotor
The rotor consists of two main parts: the blades and the hub. The rotor captures wind energy and converts it to mechanical power. As the wind flows over the blades, it induces a rotational force (torque) that is transmitted to the hub, and consequently to the rest of the turbine's rotating assets (low-speed shaft, gearbox, high-speed shaft, and generator).
Blades
The blades convert the kinetic energy of the wind to mechanical energy; this is achieved because each of the blades has an aerofoil shape. The aerofoil design causes lift to be generated as air flows over the blade's surface. Lift is the linear force that acts upon the blade, but it is torque -a rotary force- that causes the blades to rotate.
Hub
The hub serves as a central connection point for all the turbine blades. The hub transfers the rotational motion generated by the blades' to the low-speed shaft, and then onto the turbine's other rotating assets.
Low-speed Shaft
The low-speed shaft transmits the rotational motion of the rotor to the gearbox.
Gearbox
The gearbox takes the low rotational speed of the low-speed shaft and increases it before passing it to the high-speed shaft.
High-speed Shaft
The high-speed shaft transfers the rotational energy from the gearbox to the generator.
Generator
The generator is responsible for the final energy conversion. It converts the mechanical energy from the high-speed shaft into electrical energy.
Brake System
The brake system enables safe operational control of the wind turbine. It can halt the rotor rotation during emergencies, low wind speeds, or for maintenance activities.
Yaw Control System
The yaw control system is used to adjust the orientation of the nacelle and rotor with reference to the wind direction. This system optimises energy capture by ensuring that the blades face the wind at the most efficient angle. Yaw control systems are usually electrically or hydraulically actuated.
Pitch Control System
The pitch control system modifies the angle of attack of the rotor blades with reference to the wind. Adjusting a blade's pitch adjusts the total power output of the turbine. Due to the nature of wind and its varying conditions, pitch control is essential for efficient operation of a wind turbine.
Electronics
Electronics manage and regulate various aspects of the turbine's functioning. They control generator voltage and frequency output, ensuring that the turbine can be connected (synchronised) with the grid in a safe and efficient manner.
Transformer
Usually located either at the base of the tower or within the nacelle, the transformer serves to increase the voltage output of the generator. This step-up (increase) in voltage causes a reduction in current, which means that transmission power losses can be reduced significantly.
Cables and Substation
Cables convey electrical power from the wind turbine to a substation. Multiple wind turbines will connect to a single substation. The substation connects the wind farm to the electrical grid.
How do wind turbines work?
Commercial wind turbines are tall structures, usually with three blades, that rotate around a central axis. The central axis is formed of a low speed shaft between a rotor hub and gearbox, and a high speed shaft between the gearbox and electrical generator (the two shafts are not literally installed along a central axis, but are slightly offset).
The turbine blades are rotated by the wind's kinetic energy (energy from movement). Each blade has an aerofoil (airfoil) shape to maximise lift and minimise drag. Lift is a force that acts upon the blades as the wind passes over them, but it is torque (rotary force) that causes the blades to rotate around the central axis.
Aerofoil Design
The blades connect to a common rotor hub, which connects to a low speed shaft, then a gearbox, then a high speed shaft, and finally a generator. Kinetic energy from the wind is converted to mechanical power by the turbine's blades; this power is then transferred to a generator where it is converted to electrical power. All of these components are housed within a nacelle, which is installed at the top of a tower.
Wind turbines often use a cylindrical monopole tower design, although lattice type constructions are also possible. Monopole towers are typically manufactured from steel, but may also be constructed from concrete.
How big can wind turbines be?
The largest commercial wind turbine is General Electric's Haliade-X. The Haliade-X is 260 m (853 ft) tall, has a rotor diameter of 220 m (721 ft), and can generate almost 13 MW of electrical power (enough for about 10,000 homes).
Why are wind turbines so tall?
Wind speeds are greater at higher elevations. The amount of energy a wind turbine can extract from the wind is proportional to the wind's speed. Higher wind speeds contain more potential energy, thus wind turbines are built tall to reach this potential energy and harvest it.
What are the advantages of wind turbines?
Wind turbines are increasingly being used as a source of renewable energy. There are many benefits to using wind turbines over traditional fossil fuel sources, including their lack of emissions (once installed and commissioned), low maintenance costs, and ability to generate electricity even at low-wind speeds. Wind turbines can also be used to supplement other power sources, such as solar (photovoltaic, often abbreviated as 'PV'), hydro, and non-renewable power plants (coal, gas, oil etc.). In addition, wind turbines can be built on a variety of land types, including farmland, desert, urban areas, and even at sea (offshore).
What are the disadvantages of wind turbines?
Wind turbines are an increasingly popular source of renewable energy, but they also have some drawbacks. One of the most significant problems is that wind turbines can be visually unappealing to some people, particularly in rural areas. They also represent a source of noise pollution, as a turbine's blades may generate noise due to the movement of the blades through the air, and also sometimes due to vibration. Wind turbines can pose a threat to wildlife, as birds and bats may be killed if they fly into the blades. Finally, wind turbines can be a source of light pollution, as the rotating blades create flickering shadows.
Overall, wind turbines offer a relatively clean source of energy, but they also come with minor disadvantages.
Where can a wind turbine be installed?
Wind turbines can be found in a variety of locations, from open fields to mountaintops. Often, they are grouped together in 'wind farms' that can generate significant amounts of power (hundreds of Mega Watts). Some of the largest wind farms are located in the United States, China, UK, and Germany. As the use of wind energy continues to grow, it is likely that wind turbines will become even more common.
Wind Farm
Resources for learning more about wind turbines:
The U.S. Department of Energy's Wind Program has a wealth of information on its website.