Among the many possible technical designs for power generation, turbines with a horizontal axis and three blades oriented towards the wind (so-called upwind turbines) are the most successful ones. Depending on the field of application wind turbines with a vertical axis or with more or less than three blades have been developed for niche applications.
Lift turbines
The most common type of wind turbine consists of a tower, a nacelle (located on top of the tower) and a rotor with the blades. The tower ensures sufficient height where stronger and steady winds prevail. The nacelle contains the gears and a generator or in some cases only a generator, which transforms the rotation of the rotor into electricity.
Maximuim efficiency of a wind turbine
On grounds of the physical laws of fluid mechanics not all of the energy contained in the wind can be extracted. If all energy would be extracted from the wind, the air behind the rotor would be at standstill. Accordingly, air would “accumulate”, while the pressure behind the rotor increased. In turn, the speed of the wind in front of the rotor would decrease. When calculating this effect, the maximum of energy possible to be extracted amounts to roughly 59 percent.
From wind speed to the rotation of the rotor
The blades of a wind turbine have a particular shape with a high lift coefficient. Therefore the buoyant or lift force is quite strong. When the rotor turns around, the blades experience a flow from two different sides, from the direction of the wind, but also through their own rotational movement.
Thus the flow onto the rotor blade is composed of two components: From the incident wind from the front and from the “headwind” due to the rotation. The speed of the rotation is larger than the speed of the incident wind many times over. Therefore the resulting inflow is diagonally from the front. Due to the specific profile of the blade a high lift force and a low drag force results.
The angle of attack (the angle with which the resulting wind hits the blade) has to be correct at all times in order to create the highest possible lift. Therefore the rotor blades have to be adjustable. Especially during the start of the operation of a wind turbine, the angle of attack has to be continuously adjusted to the increasing rotational speed of the rotor.
The rotational movement of the rotor is due to the lift force. Via a shaft the rotation is transmitted to a generator, which in turn generates electricity.
