Wind turbines are generally classified into two categories: horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). VAWTs incorporate a vertically oriented rotor shaft, with drivetrain components located at their base. Early VAWT designs include Savonius, Darrieus, and Giromill, or H-bar designs (Fig. 18.3). Although VAWTs have shown to have advantages over HAWTs in that they are omnidirectional and their drivetrain components easily maintained, their installation heights are limited, and their blades are prone to cyclic fatigue. Because VAWTs are relatively uncommon among modern turbine designs, they will not be discussed in this manuscript.
HAWTs incorporate drivetrains that are oriented horizontally, in a direction parallel to that of the wind. HAWTs are far more common for utility-scale applications (e.g., greater than 100 kW in capacity) than VAWTs in part due to their capability of being installed at higher altitudes, and consequently,
their potential to be exposed to greater wind speeds. The drivetrain components, generator, and associated systems are installed in a nacelle enclosure at the top of a tower, with the nacelle itself angled (yawed) to keep the rotor blades in the direction of the wind. Early wind turbines were designed to position the blades downstream of the support tower and be yawed passively by the wind itself (known as downwind HAWTs, shown in Fig. 18.4). However, the blades in downstream HAWTs are exposed to the turbulent wake that is caused by the tower, which was shown to cause fatigue failures due to cyclic loading. For this reason, almost all downwind HAWT designs have been replaced in favor of turbines that position the blades upwind of the tower (i.e., upwind HAWTs), as enabled by a yaw system that is actively controlled using a wind sensor and control mechanism. Because upwind HAWT designs are used almost exclusively in modern turbines, they will be the focus of this manuscript.
The power output from a wind turbine is given by the following:
where p is the density of air, CP is the power coefficient, A is the rotor swept area (i.e., the area of the imaginary circle formed by the blade tips), and U is the wind speed. The power coefficient CP, which denotes the fraction of wind power that can converted into usable mechanical work, is primarily a function of the tip speed ratio (commonly denoted as k), which is defined as the ratio of the rotor tip speed to free wind speed. The maximum theoretical power coefficient is denoted as the Betz limit, which is specified to be 0.593. In practice, lower maximum power coefficients in the range of 0.47 and below are commonly seen in utility-scale turbines, with optimal tip speed ratios between 6 and 8.
The power coefficient of a wind turbine is also dependent on the blade pitch angle, that is, the angle of attack of the blades with respect to the direction of the wind. Most modern utility-scale turbine designs use pitch angle to control the rotation of the rotor, and in doing so, fall under three main classes: (1) passive stall-controlled, (2) active stall-controlled, and (3) pitch-controlled. Passive stall control indicates that the rotor blades are designed to stall at large wind speeds, and thus do not incorporate a pitching mechanism at the blade roots.
In regard to the solidity of the rotor swept area (i.e., the total blade area divided by the swept area), it is well accepted that utility-scale turbines have three rotor blades, which corresponds to a solidity of approximately 0.0345. Having a high solidity (i.e., more than three rotor blades) results in a relatively narrow range of tip speed ratios k at which CP is optimum, in addition to increased production costs due to the large number of blades that must be manufactured, shipped, and installed. Meanwhile, turbines with relatively low solidity (i.e., one or two rotor blades) have been shown to experience excessive cyclic loading within their drivetrain components, and have also been shown to have less aesthetic appeal than 3-bladed turbines. For this reason, almost all utility-scale turbines have three rotor blades. The rotor speed of the wind turbine must be limited for a number of reasons:
• The tip speed ratio has a narrow range (generally, between 6 and 8) for optimal performance, and it is prudent to maintain the rotor speed within the range for efficiency purposes.
• Extreme rotor tip speeds have been shown to cause excessive noise, because the noise emissions from rotor tips vary by the fifth power of blade speed . For this reason, wind turbine designers are forced to have firmer restrictions on turbine rotational speed when the wind speed, and corresponding ambient noise levels, are relatively low. This limitation is less stringent in the case of offshore wind turbines.
• The rotor and hub must be kept within centrifugal force limits. Since centrifugal force increases with the square of rotation speed, excessive rotation speed can result in catastrophic damage to the rotor and/or bearings.
Based on Eq. 18.1, it can be easily seen that the power rating of a wind turbine is largely dependent on wind speed and rotor swept area. A wind turbine manufacturer can therefore design for increased turbine power capacity by either designing the turbine with longer rotors, or by installing the wind turbine at a location with higher wind speeds. Many wind turbine developers are thus working to install larger turbines offshore, as wind speeds tend to be larger offshore, while visual appearance is less of an issue for large, offshore wind turbines. It must be noted, however, that tip speed limitations require an increase in rotor size to be accompanied by a decrease in rotation speed.
The generator within the wind turbine receives rotational energy from the drivetrain and converts it into electrical energy. In utility-scale turbines, utility requirements call for wind turbines to produce three-phase alternating current (AC) at a fixed frequency of 60 Hz in the United States or 50 Hz in Europe for transfer to the electrical grid. Two types of generators are common in modern turbines, namely, synchronous generators and asynchronous (otherwise known as induction) generators. Both types of generators operate by spinning a rotor within a stator, with a narrow gap, known as the air gap, separating the two. Because power is generated based upon the movement of an electromagnetic field past the windings within the stator, the frequency of the power that is generated is a function of the rotor speed.
An important parameter pertaining to generator operation is known as the synchronous speed ns, given as follows:
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