Anatomy of a Wind Turbine
Current utility-scale wind turbines installed in the US range from a 1-MW generator with a 57-meter rotor diameter on a 65-meter tower to a 3-MW generator with a 114-meter rotor diameter on an 120-meter tower. The average capacity of all wind turbines installed in the U.S. in 2009 was 1.75 MW, up from 1.67 MW in 2008. Many manufacturers are developing larger turbine models for both land-based and offshore applications.
In 2009 alone, the U.S. wind industry installed over 5,700 turbines. To install that number of turbines, the U.S. industry required 17,000 blades and the same number of tower sections, approximately 3.2 million bolts, 36,000 miles of rebar, and 1.7 million cubic yards of concrete (enough for more than 7,630 miles of 4-foot wide sidewalk). There are over 8,000 components in each turbine assembly.
The rotor for a typical utility-scale wind turbine includes three high-tech blades, a hub, and a spinner. The blades are one of the most critical aspects for a wind turbine and are considered a strategic component by wind turbine OEMs. Most manufacturers create multiple blade types for a single wind turbine in order to enhance performance in different wind conditions. The blades range in size from about 34 to 55 meters and are made of laminated materials – such as composites, balsa wood, carbon fiber, and fiberglass – that have high strength-to-weight ratios. These materials are molded into airfoils to generate lift which causes the rotor to turn. The blades also often include material to protect against lightning strikes. They are bolted onto the hub, with a pitch mechanism interposed to allow the blade to rotate about its axis to take advantage of varying wind speeds. The longest blade made by LM Glasfiber, the world’s largest wind turbine blade supplier, is 61.5 meters long and is made for the 5-MW REPower turbine.
The hub – usually made of a ductile cast iron – is one of a wind turbine’s heaviest components, weighing 8 to 10 tons for a 2-MW turbine. The hub is designed to be rigid yet able to absorb a high level of vibration.
The hub is covered by a nose cone. The nose cone is designed primarily with aesthetics in mind but can protect the hub slightly from the environment. The nose cone is manufactured with composites similar to those used for the blades.
The nacelle of a wind turbine is the box-like component that sits atop the tower and is connected to the rotor. The nacelle contains the majority of the approximately 8,000 components of the wind turbine such as the gearbox, generator, main frame, etc. The nacelle housing is made of fiberglass and protects the internal components from the environment. The nacelle cover is fastened to the main frame, which also supports all the other components inside the nacelle. The main frames are large metal structures that must be able to withstand large fatigue loads.
The heart of the wind turbine is its electricity generating system. Inside the nacelle of a typical wind turbine, the rotor drives a large shaft into a gearbox, which steps up the revolutions per minute to a speed suitable for the electrical generator. A wind turbine gearbox must be robust enough to handle the frequent changes in torque caused by changes in the wind speed. The gearbox requires a lubrication system to minimize wear. Wind turbines being sold in the U.S. have either induction or permanent-magnet generators, depending on the model being sold. Induction generators are more common and require a gearbox as described.
Some wind turbines avoid the gearbox completely and use a direct drive system. A direct drive system connects the rotor directly to a permanent-magnet generator. These turbines avoid the mechanical problems associated with a gearbox, but require extremely heavy and expensive generators that can produce electricity capable of supplying the grid.
All turbines have a yaw drive system to keep the rotor facing into the wind and to unwind the cables that travel down to the base of the tower. The yaw drive system usually consists of an electric or hydraulic motor mounted on the nacelle which drives a pinion mounted on a vertical shaft through a reducing gearbox. The yaw drive system also has a brake in order to be able to stop a turbine from turning and stabilize it during normal operation.
To control the functioning of the wind turbine, it is fitted with a number of sensors to read the speed and direction of the wind, the levels of electrical power generation, the rotor speed, the blades’ pitch angle, vibration levels, the temperature of the lubricants and other variables. A computer processes the inputs to carry out the normal operation of the turbine, with a safety system which can override the controller in an emergency. The control system protects the turbine from operating in dangerous conditions and ensures that the power generated has the proper frequency, voltage, and current levels to be supplied to the grid.
The nacelle and generator are mounted on top of a high tower to allow the blades to take advantage of the best winds. The power available to a wind turbine is proportional to the cube of the wind speed. Therefore, a 10% increase in wind speed would result in a 33% increase in available power. Towers are typically made of three or four tubular steel sections coated with paints and sealants and joined by flanges and bolts. Today’s wind turbine tower is usually about 80-100 meters tall. Most towers come with load lifting systems with load-bearing capacity of more than 400 pounds.
Transportation of turbine components often involves road, rail, and water. Given the increasing size, weight, and length of components, innovative transportation, manufacturing and logistics solutions are necessary. With respect to trucking, only a fraction of the industry is capable of managing the heavy-load long-haul requirements of the wind turbine industry. One turbine can require up to eight hauls (one nacelle, three blades and four tower sections). A truck carrying a tower section must be able to support a load with a propensity to roll that is over 30 meters long and weighs over 150,000 pounds. Due to tunnel and overpass restrictions, rail transport can be even more dimensionally limited than over-the-road transportation, though innovations in rail transit have helped to increase the use of rail.
Once the nacelle, blades, and tower sections are delivered to the wind farm site, construction of the wind farm can begin. The tower is normally fitted with a base flange, which can be attached to the foundation by screwed rods cast into concrete or bolted to an embedded tower stub. For the foundation, a variety of slab, multi-pile and mono-pile solutions have been used for tubular towers, determined by the condition of the ground where the turbine is being installed.
In addition to the erection of each turbine, there is additional construction work that must be conducted to connect each turbine to the power grid, such as access road construction, laying electrical cable, and installation of an electrical substation.
Image courtesy of GE Energy
More Wind Turbine Graphics
View more graphics of turbines, nacelles, generators and more in this presentation by Kinetik Partners. Wind Turbine Graphics