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B. Wind Turbine Characteristics
A wind turbine turns the kinetic energy of the wind into
mechanical energy. Wind passes over the blades exerting a
turning force. The rotating blades turn a shaft inside the
nacelle, which goes into a gearbox. The gearbox increases
the rotation speed for the generator, which uses magnetic
fields to convert the rotational energy into electrical energy
It is very well known fact that the wind power P is
proportional with the cube of the wind velocity v:
1 3
P = ρAv
(1)
2
However, due to various losses the maximal usable wind
power is equal to 59% of the theoretical value, which is
known as a Betz coefficient. In the reality wind turbine
efficiency is approximately 40%.
There are several aspects considered at turbine design
[4]:
orientation of axes (vertical or horizontal),
number of blades and
types of rotor power regulation (pitch or stall).
All modern turbines have horizontal axes, so this aspect
will not be addressed in much more details.
Most modern wind turbines are three-bladed designs with
the rotor position maintained upwind. Two-bladed and onebladed
wind turbine designs have the advantage of saving
the cost of one or two rotor blade and lower turbine weight.
However, they require higher rotational speed to yield the
same energy output. This is a disadvantage both in regard to
noise and visual intrusion.
There are two principal means of limiting rotor power in
high operational wind speeds - stall regulation and pitch
regulation. Stall regulated machines require speed
regulation. As wind speed increases, providing the rotor
speed is held constant, flow angles over the blade sections
become steeper. The blades become increasingly stalled and
this limits power to acceptable levels without any additional
active control. For this to work, the speed of the rotor must
be held essentially constant, which is achieved through the
connection of the electric generator to the grid. Briefly, a
stall regulated wind turbine will run at approximately
constant speed in high wind, not producing excessive power
and yet achieving this without any change to rotor
geometry. Pitch regulation involves turning the blades about
their long axis (pitching the blades) to regulate the power
extracted by the rotor. In contrast to stall regulation, pitch
regulation requires changes to rotor geometry. This involves
an active control system to sense blade position, measure
output power and instruct appropriate changes of blade
pitch. The objective of pitch regulation is similar to stall
regulation, namely to regulate output power in high
operational wind speeds. For pitch and stall, costs are quite
similar for each design type, but pitch regulation offers
potentially better output power quality and pitch regulation
with independent operation of each pitch actuator allows the
rotor to be regarded as two independent braking systems for
certification purposes.
The latest developments of wind turbine technology are
directed towards turbines with higher rated power especially
for off-shore applications, larger rotor diameter,
development of new materials for lower blades mass,
increased “grid friendliness” (variable speed drive) and offshore
applications.
C. Electricity Production from Wind Energy
Annual amount of produced electricity should be
calculated according to expected wind characteristics. Total
electricity produced from wind turbine ET in specific time
period T depends on the wind velocities v and is determined
by the following equation:
∞
E = T∫
Pv
0
T p(
v)
dv
(2),
where Pv denotes wind turbine power at wind velocity v and
p(v) is Weibull wind velocity probability distribution
function.
Apart from windiness of the site, the amount of
electricity produced from a wind turbine depends also on
wind turbine availability, which denotes the capability to
operate when the wind is blowing and is typically 98% or
above and on the way the turbines are arranged (the ideal
position for a wind turbine generator is a smooth hill top,
with a flat clear fetch, at least in the prevailing wind
direction).
The energy production is calculated based on the power
curve of a selected wind turbine and on an average wind
speed at hub height for the proposed site. The typical power
curve is shown in Fig. 2. The calculation should also include
losses of energy transformation as well as wind potential
assessment uncertainty factor (usually 10%).
kW
900
800
700
600
500
400
300
200
100
0
0 5 10 15 20 25 m/s
Fig. 2. Typical wind turbine power curve
D. Electric Generators Concepts
Apart from stochastic nature of energy carrier, what
distinguishes wind power plants from conventional once is a
power source, i.e. electricity generator. There are three types
of wind turbine-generators couplings:
constant-speed turbine coupled with squirrel-cage
induction generator,
variable speed-wind turbine coupled doubly fed
(wound rotor) induction generator and
variable speed-wind turbine coupled with direct
drive synchronous generator.
A squirrel cage induction generator is an asynchronous
machine, composed of a squirrel cage rotor and a stator with
three distributed windings directly coupled to the grid. The
wind turbine rotor is coupled to the generator through a
gearbox. Substantially this is a constant speed wind turbine
because the power converted from the wind is limited by
designing the turbine rotor in such a way that its efficiency
2