WIND TURBINE ANATOMY
The standard utility scale wind turbine for both onshore and offshore applications has, as noted already, a three-bladed rotor attached to a drive train and generator with the whole assembly mounted at the top of a tall tower inside a protective housing called a nacelle. The nacelle must be able to rotate, so it is attached to the tower through a yaw bearing that allows the complete structure to turn as the wind direction changes, with the rotor always facing into the wind and the nacelle behind it. In an upwind design, the rotation must be powered by a yaw motor although this is not necessary for a downwind rotor.
The drive train within the nacelle will often include a gearbox that increases the rotational speed of the drive shaft to be able to drive a generator that is synchronized to the local grid frequency, usually 50 Hz or 60 Hz. Other turbines use a variable-speed system with a power electronic converter to ensure the out- put is always at grid voltage even as the generator speed varies. There are, how- ever, an increasing number of wind turbines that dispense with the gearbox—a component that has often proved unreliable in the past—and use a direct-drive system instead. Direct-drive generators tend to be more expensive but the benefits in terms of higher reliability can outweigh this.
The tower-top structure may have a helicopter pad and will usually be accessible via a ladder or lift within the tower. There may also be a crane fitted to the top for maintenance purposes. Power from the generator will be carried down the tower in cables to a transformer fitted at the bottom of the tower that converts the output to the local distribution grid voltage. If the unit is part of a large wind farm this power may then be carried to a wind farm substation where the voltage is raised further to be fed into the transmission system.
The reliability of modern wind turbines is much higher than it was in the 1980s and 1990s, and most turbine manufacturers aim for lifetimes of 20 years of more. However, anecdotal evidence suggests that maintenance costs rise as machines get older.
The rotor is the part of the wind turbine that interacts with the wind and its design will determine the efficiency of the generation unit. The three-blade rotor used on the majority of modern wind turbines represents a balance between cost and efficiency. More blades can, in principle, extract more energy but make the rotor more expensive. Fewer blades are cheaper but lead to balancing problems.
The actual amount of energy that a wind turbine rotor can extract depends on its rotational speed. If the rotor rotates too slowly some wind passes between the blades without energy extraction, whereas if it rotates too fast the turbulence created by one blade will affect energy extraction of the next blade. The optimum rotational speed is usually defined by a parameter called the tip speed ratio
(TSR), which is the ratio of the speed of the blade tips through the air to the wind speed. For a three-blade rotor the optimum TSR is between 6 and 7. It will be clear from this that the optimum rotational speed varies with wind speed, irrespective of turbine size.
For onshore wind turbines the maximum practical unit size is around 3 MW. Beyond this it becomes excessively difficult transporting the massive components to the often remote sites where wind farms are located. Such machines can have rotors up to 120 m in diameter and individual blades up to 60 m. The latter are of a similar size to those used for larger offshore machines up to 5 MW.
Offshore, such 5 MW units are already in use and 10 MW units are being designed. Rotors for these units could have blade lengths up to 75 m, while 15 MW units, now on the drawing board, will require even longer blades. For a given site, onshore or offshore, the selected rotor diameter will also depend on wind speed. A larger rotor will harvest more energy at a low– wind speed site since energy capture will depend on the area swept out by the rotor. In contrast, a smaller rotor can be more economical for a high–wind speed site.
Early turbine blades were often made from wood but most modern wind tur- bine blades are built from fiberglass-reinforced polymers. Carbon fiber is also being introduced into longer blades to help increase stiffness and strength and this trend is likely to continue. Wind turbine blades are aerodynamically shaped to extract the maximum energy from the wind. The blades must also incorporate features that aid the control of rotor speed. Speed control serves two functions. The first is to enable the optimum rotor speed to be maintained at different wind speeds in variable-speed designs. The second is to ensure that the rotor does not run too fast in high winds. Although turbines would ideally operate in all conditions, if these become too severe, the turbine will normally be shut down completely.
Various methods of speed control are possible. Passive speed control involves designing blades that aerodynamically stall when the wind speed becomes too high, shedding wind. Stalling is a simple technique but it does not help to vary rotor speed with wind speed. The alternative, used by many modern designs, is active pitch control. This involves fitting each blade with a motor at the point where it joins the hub so that it can be rotated about its long axis to change the blade pitch as wind speed varies. Since the optimum rotational speed depends on wind speed, this also allows wind turbines with variable-speed generators to control the speed continuously for optimum efficiency.
Most large wind turbines have a maximum rotational speed of 20 rpm though smaller units may rotate faster. The speed is limited for two reasons. The first is to ensure centrifugal forces do not become too great within the blade. The second is to limit airborne noise, which is a function of blade tip speed. The faster the blade tips move through the air, the more noise they generate. Since blades on large rotors will have a higher tip speed than those on a smaller rotor turning at the same speed, maximum rotational speed on large rotors is normally relatively low.
Although most utility-scale turbine blades adopt a broadly similar shape and structure, there are a number of advanced blade designs under development. These include rotors that have the ability to alter the pitch of each blade independently. This capability may be used to alter blade pitch at different points in the rotational cycle to compensate for the changing wind speed at different heights. Other blades are able to twist under heavy loads, such as during very high gusts, to shed load, a passive system that can help reduce stress fatigue. Other advanced blades are being developed with a number of adjustable sections, each of which is independently controlled by a microprocessor. These complex blade designs also aim to reduce the fatigue loading on blades as well as controlling rotational speed.
Yet another type of new rotor design has the ability to change the length of each blade to create a variable diameter rotor. With a rotor of this type the diameter can be maximized for low wind speed and then reduced as wind speed increases, both controlling rotational speed and reducing the fatigue stress on the rotor blades.
The rotor of a horizontal-axis wind turbine must always be oriented so that the plane of rotation is perpendicular to the direction of the wind. This can be accomplished either by having the rotor face the wind with the nacelle behind (an upwind design) or with the nacelle facing the wind and the rotor behind (a downwind design). A downwind design is mechanically simplest because it is possible to use vanes on the nacelle that ensure the orientation is maintained passively simply by the effect of the wind.
Many early wind turbines took advantage of the simplicity of the downwind design but problems with this were soon recognized. The main difficulty arises because of the shadow effect of the tower as each rotor blade passes behind it. This leads to a momentary drop in wind pressure, generating additional fatigue stress in each blade. Noise problems can also arise from the same source. In consequence, modern designs adopted the upwind orientation.
Precise upwind orientation is important to avoid uneven stress on the rotor that can lead to other forms of fatigue. Maintaining an accurate upwind orientation requires that the turbine be equipped with a yawing motor to turn the nacelle. Modern turbines usually use a stepwise system of yawing to keep pace with any changes in wind direction.
The yawing motor also serves a further function. If the nacelle turned continuously in one direction to face the wind the cables from the top of the tower to the bottom would soon become twisted. The yaw motor enables this situation to be avoided by alternating the direction of the yaw as necessary.
Drive Trains and Generator
The drive train of a wind turbine begins with the shaft to which the rotor is attached (Figure 11.4). This transmits the mechanical energy generated by the rotor in the form of a rotational force or torque. In most early wind turbines and in many modern units the shaft is connected to a gear box that increases speed of rotation from perhaps 20 rpm to 1000 rpm or 1500 rpm (50 Hz) or 1200 rpm or 1800 rpm (60 Hz), suitable to drive a synchronized generator. A drive shaft from the gearbox is then linked to the generator.
The drive train has to endure more than simply the rotational torque produced by the rotor. The force of the wind on the rotor blades can be extremely uneven and this will generate lateral or bending forces too, which are transmit- ted into the gearbox and generator. While shock-absorbing components can help reduce the effect of such lateral forces, the effect on the gearbox can often be severe and this can lead to early failure.
Various attempts have been made to improve gearbox reliability but perhaps the optimum, if most expensive, solution is to remove the gearbox all together and drive the generator directly from the rotor. Direct-drive generators are becoming increasingly popular in large wind turbines and work is being carried out to develop a superconducting direct-drive generator for large offshore wind turbines of 10 MW or more.
The generators used in early wind turbines were asynchronous generators (often motors operated in reverse) that relied on the grid to control their rotational
frequency. This generally weakened the grid and large wind farms with this type of turbine usually required some form of reactive compensation to improve grid stability. Modern wind turbines are normally required to be able to maintain their synchronization with the grid independent of the grid itself, so a more sophisticated design is necessary. This has the added advantage of allowing them to help maintain grid stability rather than reducing it.
Designing an effective generator system for a wind turbine can be difficult because of the variable wind speed conditions. A conventional synchronous generator can only rotate at one speed if it is to supply power at the grid fre- quency so that the wind turbine must be maintained at a single rotational speed. To overcome this some wind turbine designs include two generators, one for low-speed operation and one for higher-speed operation.
While this is the cheapest variable-speed solution, maintaining fixed rotational speeds creates additional stresses on the rotor and drive train, something that manufacturers are seeking to avoid to improve reliability and lifetimes. The best way to avoid many of the problems associated with variable wind speed is to use a variable-speed generator. The disadvantage of this is that a generator operating at varying speeds will produce an output of variable frequency. Variable-speed operation can, therefore, only be achieved by using some form of power electronic frequency conversion system to maintain grid frequency independent of the frequency from the generator. These electronic systems convert the output from the generator to direct current and then back to alternating current at the grid frequency.
Two types of variable-speed generators have been used in recent years. The first is called a partial conversion system and uses a doubly fed generator to provide limited speed variation. The second is a full conversion generator that is more expensive but also more flexible.
Variable-speed operation reduces the stress on the rotor because the wind turbine can always operate at the optimum speed for the wind conditions. In addition, it means that energy can be harvested over a wider range of wind conditions than is possible with fixed-speed generators. A further advantage is that variable-speed generators with full AC–DC–AC converters can provide grid frequency support facilities, as noted before. This can make them easier to integrate into modern grids.
The tower of a wind turbine has to be tall enough to lift the rotor and blades so that the blade tips are both clear of the ground and clear of the layer of turbulent air found close to the ground or sea. This will often require a higher tower onshore than offshore for a similar sized rotor because the turbulent air layer is usually thicker onshore. In some cases the rotor may be lifted higher still to gain access to the higher wind speeds found at greater distance from the ground or sea.
Towers for early wind turbines were often made from a lattice steel structure but modern towers are of tubular construction, generally of steel or concrete. Most today are made from tubular steel sections that can be bolted together at the site. Towers are conical in shape, with the base having a larger diameter than the top. Aesthetically the optimum arrangement is considered to be when the tower height is the same as the rotor diameter.
Tower height is also important as the length of the tower is responsible for one of the key structural resonances of a wind turbine. It is critical that this should not be excited by the rotational frequency of the rotor as it could lead to tower failure. This is not normally a problem with onshore wind turbines because the towers are too short, but it can be with offshore turbines mounted on monopole towers with a substantial length below sea level.
Steel towers for large wind turbines are becoming extremely heavy as turbine sizes rise offshore and alternative structures are being sought. One possibility is to construct towers from prefabricated concrete sections. However, concrete does not normally offer the same structural strength as steel. As well as its load-bearing capability, which must be sufficient to support the tower top nacelle and rotor, tower strength is an important issue because the tower is subject to significant bending forces as well as torsional forces generated by the effect of uneven gusting on the rotor. Both must be resisted without significant fatigue stress.