Basic Wind Power Operating Principles
The basic wind energy conversion device is the wind turbine. Although various designs and configurations exist，they are generally grouped into two types; vertical axis wind turbines, in which the axis of rotation is perpendicular to the wind stream and the ground; and horizontal axis turbines, in which the axis of rotation is parallel to the wind stream and the ground.
Figure 1 shows illustrations of these two types of turbines and typical subsystems for an electricity generation application. The subsystems include: a blade or rotor, which converts the energy in the wind to rotational shaft energy; a drive train, usually including a gearbox and a generator; a tower that supports the rotor and drive train; and other equipment including controls, electrical cables, ground support equipment，and interconnection equipment.
Figure 2 illustrates the basic operating principles of a horizontal-axis wind turbine. The wind passes over both surfaces of the airfoil-shaped blade. The wind passes more rapidly over the longer side of the airfoil, creating a lower pressure area above the airfoil. The pressure differential between top and bottom surfaces results in a force, known as lift, which causes the airfoil to rise. Since the blades are constrained to move in a plane with the hub at its center, the lift force causes rotation about the hub. In addition to lift force, a drag force perpendicular to the lift force impedes rotor rotation. A prime objective in wind turbine design is for the blade to have a high lift to drag ratio.
The output of the wind turbine varies with the wind's speed through the blades. This relationship is usually expressed graphically in a power curve (Figure 3). The "rated power" is the wind speed at which the "rated power" is achieved and generally corresponds to the point at which the conversion efficiency is near its maximum. In many systems, the power output above the rated wind speed is mechanically maintained at a constant level, allowing better system control. Note that at lower wind speeds, the power output drops off sharply.
Wind resource evaluation is a critical element in projecting turbine performance at any given site. The energy available in a wind stream is proportional to the cube of its speed, which means that doubling the wind speed increases the available energy by a factor of eight. Furthermore, the wind resource itself is seldom a steady, homogeneous flow. It varies with time of day, season, height above ground, and type of terrain. In addition proper siting in windy locations, away from large obstructions, enhances a wind turbine's performance. In general, annual average wind speeds greater than 4 meters per second (12 miles per hour) are required. Wind resources of this power are available in most parts of the world.
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