Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 294

7 Wind Power Generation
7.1 Principles
Wind energy converters (WEC) harness the kinetic energy contained in flowing
air masses. In the following, the fundamental physical principles of this type of
energy conversion are explained. However, explanations do not include wind en-
ergy utilisation by sailboats, for instance.
Energy extraction from wind, by wind energy converters, is always related to a
certain time difference as wind and operational conditions are usually subject to
constant changes. This is why in most cases the instantaneous energy value
(power) is determined in order to calculate its useful energy contribution (work)
by summation over time (i.e. integration). Kinetic wind power P Wi is thus deter-
mined by air density ρ Wi , wind passage area S , and wind velocity v Wi, 1 . By means
of wind energy converter the wind power station extracts part of the wind power
by reducing the wind speed.
Most modern wind energy converters are equipped with rotors to extract wind
power, and consist of one or several rotor blades. The extracted wind power gen-
erates rotation and is thereby converted into mechanical power P Rot at the rotor
shaft. Mechanical power is taken up at the shaft in the form of a moment at a cer-
tain rotation and is transferred to a machine (such as a generator or a pump). The
entire wind power station thus consists of a wind energy converter (rotor), a me-
chanical gear and a generator.
It is physically impossible to technically exploit the entire wind energy, as in
this case air flow would come to a standstill. In this case, air would fail to enter
the swept rotor area, and wind power would no longer be available.
There are two different physical principles to extract power from wind. The
less efficient airfoil drag method is based on the wind drag force incident on a
wind-blown surface. The second principle - also referred to as aerodynamic or
airfoil lift principle, which is based on flow deviation inside the rotor - is pre-
dominantly applied for wind energy conversion. When compared to the drag prin-
ciple, double or triple the power output is achieved for a given cross-section area.
Chapter 7.2 thus focuses entirely on plants based on the airfoil lift principle. Nev-
ertheless, both principles are outlined throughout the following sections to explain
the main differences following a discussion on the maximum achievable wind