Stalling
works by increasing the angle at which the relative wind strikes the
blades (angle of attack), and it reduces the
induced drag (drag associated with lift).
Stalling is simple because it can be made to happen passively (it
increases automatically when the winds speed up), but it increases the
cross-section of the blade face-on to the wind, and thus the ordinary
drag. A fully stalled turbine blade, when stopped, has the flat side of
the blade facing directly into the wind.
A
fixed-speed HAWT inherently increases its angle of attack at
higher wind speed as the blades speed up. A natural strategy, then, is
to allow the blade to stall when the wind speed increases. This
technique was successfully used on many early HAWTs. However, on some
of these blade sets, it was observed that the degree of blade pitch
tended to increase audible noise levels.
Blade Pitch
control
Furling
works by decreasing the angle of attack, which reduces the
induced drag from the lift of the rotor, as well as the cross-section.
One major problem in designing wind turbines is getting the blades to
stall or furl quickly enough should a gust of wind cause sudden
acceleration. A fully furled turbine blade, when stopped, has the edge
of the blade facing into the wind.
Standard
modern turbines all pitch the blades in high winds. Since
pitching requires acting against the torque on the blade, it requires
some form of pitch angle control. Many turbines use hydraulic systems.
These systems are usually spring loaded, so that if hydraulic power
fails, the blades automatically furl. Other turbines use an electric
servomotor for every rotor blade. They have a small battery-reserve in
case of an electric-grid breakdown. Small wind turbines (under 50 kW)
with variable-pitching
generally use systems operated by centrifugal force, either by
flyweights or geometric design, and employ no electric or hydraulic
controls.
Other controls
Horizontal
Nacelle Yawing
Modern
large wind turbines typically use wind vanes located on the back of the
nacelle to keep the turbine facing into the wind.
By minimizing the yaw angle (the misalignment between wind and turbine
pointing direction), the power output is maximized and non-symmetrical
loads minimized. However, since the wind direction varies quickly the
turbine will not strictly follow the direction and will have a small
yaw angle on average. The power output losses fall approximately with cos3of
the yaw angle.
Vertical Nacelle Pitching
By use of a horizontal vane and a hinge to the rear of the center of
gravity, the wind turbine speed is limited by increasing the vertical
yaw angle of the nacelle.
Electrical
braking
You can slow down a small wind turbine by dumping energy from the
generator into a resistor
bank, converting the kinetic energy of the turbine rotation into heat.
This method is useful if the kinetic load on the generator is suddenly
reduced or is too small to keep the turbine speed within its allowed
limit.
Cyclically
braking causes the blades to slow down. Slowing increases
the stalling effect thus reducing the efficiency of the blades. This
way,
the turbine's rotation is kept at a safe speed in higher winds
conditions while maintaining (nominal) power output. You don't usually
see this method used on large grid-connected wind turbines.
Mechanical
braking
A
mechanical drum brake or disk brake
is used to hold the turbine at rest for maintenance. Don't try to apply
the mechanical brakes until after blade furling and electromagnetic
braking
have reduced the turbine speed, as the mechanical brakes would wear
quickly if used to stop the turbine from full speed. There can also be
a stick brake.
For HAWTs, tower heights
approximately two to three times
the blade length have been found to balance material costs of the tower
against better utilization of the more expensive active components.
Distance to Batteries, Inverter and Grid
Connection
Power
output from your wind turbine can be in the form of AC or DC depending
upon you wind turbine design. The least expensive way to transport this
power over distance is to increase the voltage and therefore decrease
the current. The higher the current the greater the power loss in
transmission.
The best windmill sites are usually some
distance from the location of your house or business. The wind turbine
output (usually variable frequency AC) needs to be changed from low
voltage to higher voltage at the windmill in order to move it
economically to the battery and inverter location. The
smaller the current the smaller and cheaper the transmission wire.There
the AC is converted into DC of the proper voltage to charge the battery
and run the inverter.
For a 220 volt system, 1.59kw needs wiring to
handle 7.23amps. A 110 volt system would need to handle 14.46 amps. Make sure you know the the max capacity of your wind
turbine before you plan the wiring.
