13-04-2013, 02:48 PM
A Wind Turbine
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INTRODUCTION
A wind turbine -is a device that converts kinetic energy from the wind, also called wind energy,
into mechanical energy; a process known as wind power. If the mechanical energy is used to
produce electricity, the device may be called wind turbine or wind power plant
Wind is a form of solar energy and is a result of the uneven heating of the atmosphere by the sun,
the irregularities of the earth's surface, and the rotation of the earth. Wind flow patterns and speeds
vary greatly across the United States and are modified by bodies of water, vegetation, and
differences in terrain. Humans use this wind flow, or motion energy, for many purposes: sailing,
flying a kite, and even generating electricity.
The terms wind energy or wind power describe the process by which the wind is used to generate
mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into
mechanical power. This mechanical power can be used for specific tasks (such asgrinding grain or
pumping water) or a generator can convert this mechanical power into electricity.
Rotor
The rotor is the heart of a wind turbine and consists of multiple rotor blades attached to a hub. It is
the turbine component responsible for collecting the energy present in the wind and transforming
this energy into mechanical motion. As the overall diameter of the rotor design increases, the
amount of energy that the rotor can extract from the wind increases as well. Therefore, turbines are
often designed around a certain diameter rotor and the predicted energy that can be drawn from the
wind.
The predominant aerodynamic principles that rotor designs are based upon are Drag Design and Lift
Design. Drag design rotors operate on the idea of the wind “pushing” the blades out of the way,
thereby setting the rotor into motion. Drag design rotors have slower rotational speeds but hightorque
capabilities, making them ideal for pumping applications. With Lift design rotors, the blades
are designed to function like the wing of an airplane. Each blade is designed as an airfoil, creating
lift as the wind moves past the blades. The airfoil operates on the basis of Bernoulli’s Principle
where the shape of the blade causes a pressure differential between its upper and lower surfaces.
This disparity in pressure causes an upward force that lifts the airfoil. In this case, this lift causes the
rotor to rotate, once again transforming the energy in the wind into mechanical motion.
Rotor blades
Rotor blades are a crucial and basic part of a wind turbine. The design of the individual blades also
affects the overall design of the rotor. Various strains are placed on them, and they must withstand
very big loads. Rotor blades take the energy out of the wind; they “capture” the wind and convert its
kinetic energy into the rotation of the hub. The profile is similar to that of airplane wings. Rotor
blades utilize the same “lift” principle: below the wing, the stream of air produces overpressure;
above the wing, the stream of air produces vacuum. These forces make the rotor rotate. Today, most
rotors have three blades, a horizontal axis, and a diameter of between 40 and 90 meters. In addition
to the currently popular three-blade rotor, two-blade rotors are also used to be common in addition
to rotors with many blades, such as the traditional wind mills with 20 to 30 metal blades that pump
water. Over time, it was found that three-blade rotor is the most efficient for power generation by
large wind turbines.
Hub
The hub is the centre of the rotor to which the rotor blades are attached. Cast iron or cast steel is
most often used. The hub directs the energy from the rotor blades on to the generator. If the wind
turbines have a gearbox, the hub is connected to the slowly rotating gearbox shaft, converting the
energy from the wind into rotation energy. If the turbine has a direct drive, the hub passes the
energy directly on to the generator. The rotor blade can be attached to the hub in various ways:
either in a fixed position, with an articulation, or as a pendulum. The latter is a special version of the
two-blade rotor, which swings as a pendulum anchored to the hub. Most manufacturers nowadays
use a fixed hub.
Nacelle with drive train and other equipment
The nacelle contains all the machinery of the wind turbine, i.e. the drive train including the
mechanical transmission (rotor shaft, bearings and the gearbox) and the electrical generator, and
other equipment such as the power electronic interface, the yaw drive, the mechanical brake, and
the control system, among others. Because it requires rotating in order to track the wind direction, it
is connected to the tower via bearings. The build-up of the nacelle shows how the manufacturer has
decided to place the drive train and other components above this machine bearing.
Drive train
Mechanical transmission
The gearbox is the major component of the mechanical transmission. Due to their huge diameters,
the rotors of large scale wind turbines tend to have very slow rotational speeds (generally 18–50
rpm). In most cases, these speeds are insufficient to operate their generators at maximum efficiency
(for most generators, somewhere in the range of 1200–1800 rpm). The solution is to include a
gearbox transmission between the rotor output shaft and the generator input shaft so that the rotor
speed can be geared up to the appropriate rpm required by the generator for maximum power
generation. In the case of multi-pole synchronous generators coupled to the electric grid via a full
scale power converter, which decouples entirely the generator system from the utility grid, since it
can operate at low speeds the gearbox can be omitted. Consequently, a gearless construction
represents an efficient and robust solution that is beneficial, especially for offshore applications,
where low maintenance requirements are essential. In the case of wind turbines with smaller rotor
diameters, the gearbox transmission between the rotor and generator can be also omitted. A decrease
in rotor diameter results in a smaller arc-length that the rotor must travel per revolution, eventually
causing a comparatively larger rotational speed than that of a larger rotor for a given wind speed. If
these larger rotational speeds are appropriate for the type of generator being used, the rotor can be
connected straightforwardly to the generator resulting in a direct-driven system in the same way as
in the system linked with the power converter. These smaller direct-driven wind turbine systems are
predominately used in stand-alone (not grid-connected) DC applications (battery charging, etc).
Electrical generator
The generator is the component of the wind turbine responsible for converting the mechanical
motion of the rotor into electrical energy. The blades transfer the kinetic energy from the wind into
rotational energy in the transmission system, and the generator is the next step in the supply of
energy from the wind turbine to the electrical grid.
There are many different types and sizes of electric generators for a wide range of applications.
Depending on the size of the rotor and the amount of mechanical energy removed from the wind, a
generator may be chosen to produce either AC or DC voltage over a variety of power outputs.
There are two major types of electrical generators for converting mechanical energy. The first is the
synchronous generator. The synchronous generator operates on the principle that as a magnet is
rotated in the presence of a coil of wire, the changing magnetic field in space induces a current, and
therefore a voltage in the coil of wire. In this case, the magnet is attached to the input shaft of the
generator and is surrounded by several coils of wire, individually referred to as a pole. As the shaft
rotates, so does the permanent magnet which creates a changing magnetic field in the presence of
the poles which surround it. This induces a current in each of these poles and electrical energy is
produced. Synchronous generators are typically quite simple and can be used in a wide variety of
applications.