2.2 Renewable Energies

Humans have been utilising the energy of the wind for over 2000 years. For the majority of this period, the focus was on converting wind energy into mechanical work, for example for irrigation purposes, milling grain or driving sailing ships. However, by the beginning of the 20th century at the latest, the use of wind energy was largely replaced by systems powered by steam, combustion engines or electric motors. In the last 40 years, however, the use of wind energy has experienced a renaissance in the course of the generation of renewable electricity. 

When talking about the use of wind energy, the wind capacity P_Wind is the key parameter:

                                                        P_Wind [W] = 0.5*ρ*A*v³

ρ is the density of air and a constant factor of 1.225 kg/m3. A describes the area through which the wind flows. The wind velocity v has a particularly large influence on the capacity of the wind. The capacity of the wind increases with the third power of the velocity, which means that a doubling of the velocity leads to an eightfold increase in the carried wind capacity. The average wind speed of a site therefore plays a key role in the search for suitable areas for the construction of wind energy plants.

Modern wind turbines slow down the wind and extract energy from the flowing air mass with the help of rotor blades. Wind turbines convert the kinetic energy of the wind first into mechanical energy (the rotation of rotor system) and then into electrical energy. However, it is physically impossible to extract all the energy from the wind: if we would extract all kinetic energy, the wind speed would become zero and then the path would be blocked by the non-moving air while the moving air would just go around the wind turbine so that no energy can be extracted any longer. Thus, a wind turbine can theoretically convert a maximum of 59.3 % of the wind power into mechanical power at the rotor. This ratio of extractable power to maximum wind capacity is described by the so-called Betz's coefficient.

The basic components of all wind turbines used today to generate electrical energy are:

Common, commercially available wind turbines almost invariably have three rotor blades. The rotor blades converge at the rotor hub. There, the rotational movement of the blades is transferred to the power train. The power train connects the rotor system (Rotor blades + rotor hub) with the generator, in which the mechanical energy is converted into electrical energy. Cables ensure the electricity transport to the base of the tower, where the wind turbine is connected to the grid. Another important component of modern wind turbines is the yaw drive, which enables the rotor to be aligned against the wind.