Wind Turbines - Wind Energy Systems for Electric Power Generation - page 10

Chapter 2
Wind Turbines

2.1 General

The history of windmills goes back more than 2000 years. They have been used
predominantly for grinding cereals and for pumping water. Important examples of
more recent times are the Dutch Windmills which appeared in different variants
and were erected in large numbers in the 17th and 18th century in Europe. Another
memorable development of the 19th century was the Western Mill, found in rural
areas especially in the USA up to the present day. Modern constructions of wind
energy converters were developed in the 1920s, but it was not before the 1980s that
they found professional interest as a prominent application of renewable energies.
From the standpoint of fluid engineering we have to distinguish wind energy
converters with drag force rotor and with buoyant force rotor. While drag force
rotors utilize directly the wind pressure and attain only low efficiencies in the order
of 0 , 1 ... 0 , 2, buoyant force rotors develop considerably higher values, inherently
limited at approximately 0,59. A suitable theory was published not before the start
of the 19th century (Joukowski 1907).
Modern windturbines are mostly constructed as fast running machines with hori-
zontal shaft, upwind arrangement and preferably 3 rotor blades. The machine ratings
have steadily increased so that the average installed power per unit is currently above
1.700 kW. For offshore-wind-parks ratings of up to 6.000 kW are in pilot stage.
A selection of relevant topic literature on wind turbines is in [Hau06, Gas02,
Gas07].

2.2 Basics of Wind Energy Conversion

2.2.1 Power Conversion and Power Coefficient

From the expression for kinetic energy in flowing air follows the power contained
in the wind passing an area A with the wind velocity v 1 :
P w = 2 Av 1 3 (2.1)