Environmental Engineering Reference
In-Depth Information
If the blade angular speed w is too small, most of the wind may pass undisturbed
though the blade swept area making little useful work on the blades. On the con-
trary, if w is too large, the fast rotating blades may block the wind fl ow reducing
the power extraction. Therefore, there exists an optimal angular speed at which the
maximum power extraction is achieved. For a wind turbine with n blades, the opti-
mal angular speed can be approximately determined as [49]:
2 u
nL
p
w
(25)
opt
where L is the length of the strongly disturbed air stream upwind and downwind
of the rotor.
Substituting eqn (25) into (24), the optimal tip speed ratio becomes
2 lr
nL
p
+
(26)
l
opt
Empirically, the ratio ( l + r )/ L is equal to about 2. Thus, for three-blade wind tur-
bines (i.e. n = 3), l opt
4 p /3.
If the aerofoil blade is designed with care, the optimal tip speed ratio may be
about 25-30% higher than the calculated optimal values above. Therefore, a wind
turbine with three blades would have an optimal tip speed ratio [49]:
4
p
(27)
l
=
(1.25 ~ 1.30)
5.24 ~ 5.45
opt
3
5.3.6 Wind turbine capacity factor
Due to the intermittent nature of wind, wind turbines do not make power all the
time. Thus, a capacity factor of a wind turbine is used to provide a measure of the
wind turbine's actual power output in a given period (e.g. a year) divided by its
power output if the turbine has operated the entire time. A reasonable capacity fac-
tor would be 0.25-0.30 and a very good capacity factor would be around 0.40 [50].
In fact, wind turbine capacity factor is very sensitive to the average wind speed.
5.4 Wind turbine controls
Wind turbine control systems continue to play important roles for ensuring wind
turbine reliable and safe operation and to optimize wind energy capture. The main
control systems in a modern wind turbine include pitch control, stall control (pas-
sive and active), yaw control, and others.
Under high wind speed conditions, the power output from a wind turbine may
exceed its rated value. Thus, power control is required to control the power output
within allowable fl uctuations for avoiding turbine damage and stabilizing the
power output. There are two primary control strategies in the power control: pitch
control and stall control. The wind turbine power control system is used to control
the power output within allowable fl uctuations.
 
Search WWH ::




Custom Search