Operation principle of wind turbines
Wind turbines or aerogenerators transform the kinetic energy of the wind into electrical energy with no use of fuel and passing through the phase of conversion into mechanical rotation energy carried out by the blades. Turbines can be divided into "lift" machines and "drag" machines according to which force is generated by the wind and exploited as "motive force". To understand the operation principle of a wind turbine, reference is to be made to the most widespread turbines, that is the "lift" ones. In the "lift" turbines, with respect to the "drag" type, the wind flows on both blade surfaces, which have different profiles, thus creating at the upper surface a depression area with respect to the pressure on the lower surface.
This pressure difference creates on the surface of the wind blade a force called aerodynamic lift (Figure 1.7), as it occurs for aircraft wings.
Lift force on the wings of an airplane can lift it from the ground and support it in flight, whereas in a wind turbine, since the blades are bound to the ground, it determines the rotation about the hub axis.
At the same time a drag force is generated, which is opposed to the motion and is perpendicular to the lift force. In the turbines correctly designed, the ratio lift-drag is high in the field of normal operation. An aerogenerator requires a minimum wind velocity (cutin speed) of 3-5 m/s and delivers the nameplate capacity at a wind velocity of 12-14 m/s. At high speeds, usually exceeding 25 m/s (cut-off speed) the turbine is blocked by the braking system for safety reasons. The block can be carried out by means of real mechanical brakes which slow down the rotor or, for variable pitch blades, "hiding" the blades from the wind, by putting them in the so-called "flag" position.
The main advantages of the wind plants can be summarized as:
- distributed generation
- effective conversion of the wind energy into electrical energy (59% theoretical efficiency)
- lack of emission of polluting substances
- saving of fossil fuels
- reduced service (there are no costs for the fuel supply) and maintenance costs
- easy dismantlement of the wind turbines at end of life (20/25 years)
- the generation capability of the wind turbine ranges from few hundreds of Watts to some MWatts, thus meeting the requirements of both single dwellinghouses, as well as of industrial applications or of injection into the network (through wind power stations).