It is very critical to understand the concept of wind before we think of
generating electricity from it. There are many basics like wind blade designs,
speed of wind, tower height, site location etc. which should be given due
importance before going into windmill construction. In this article we will help
you understand these things and take you further toward the completion of
windmill designing.
Height:
Turbines are usually 100-120 m high. Increase in the diameter of the
blades will increase the height with it. With more height, the wind power
increases and power yielded is more. But the costs increases too and hence a
balance is sought when deciding on the height of the tower.
Power:
Power in
watts yielded by the wind turbine alternator equals the angular velocity (radians
per second = 2 pi RPM/60) multiplied by the torque which is in Newton-meters. In
case of a permanent magnet alternator, voltage and current are proportional to
the RPM. Hence the power is proportional to the square of the RPM.
A very important thing to note here is that wind
speed is not a one to one function of the speed. In fact energy increases by
the cube of the wind speed. If we double the wind speed, we get eight times the
energy. Hence looking at the maps become all the more important. Even a small
difference in wind speed within a given area can have a big impact on the
amount of energy a wind turbine can generate. It is also one of the reasons why
a taller wind tower can make so much of a difference. You can get a sense
of the energy produced by the turbine by the power curve graphs given by the
manufacturers along with the turbine.
Speed
§ Start-up Speed – The speed at
which the blades begin to rotate.
§ Cut-in Speed – The minimum
speed at which the turbines will begin producing electricity.
This factor will depend
on the wind permanent magnet alternator.
§ Rated Speed - The rated speed is the
minimum wind speed at which the wind turbine will generate its designated rated
power. For example, a "10 kilowatt" wind turbine may not generate 10
kilowatts until wind speeds reach 25 mph. Rated speed for most machines is in
the range of 25 to 35 mph.
§ Cut-out Speed - At very high
wind speeds, typically between 45 and 80 mph, most wind turbines cease power
generation and shut down. The wind speed at which shut down occurs is called
the cut-out speed, or sometimes the furling speed. Having a cut-out speed is a
safety feature which protects the wind turbine from damage. Shut down may occur
in one of several ways.
Temperature
Utility-scale
wind turbine generators have minimum temperature operating limits which apply
in areas that experience temperatures below –20 °C. Ice accumulation
should always be avoided.
Axis
Vertical axis windturbines (VAWTs) have become popular
in recent times. Manufacturers claim that these designs are quiet, efficient,
and economical. The rotor shaft in a VAWT is placed perpendicular to the
ground. They can be used to generate electricity without getting bothered about
placing them in the direction of the wind. These are Omni Directional. Hence
they require less parts which makes them less costly and gives ease of
maintenance.
They produce a
good amount of torque. Its blades have a larger surface area. Hence they can be
coupled directly to pumps etc. Also they
are designed to spin at much lower speeds, hence they can function efficiently
at higher wind speeds. This decreases noise and vibrations and provides
stability.
Horizontal
axis wind turbines (HAWTs) are the
most common type of wind designs used today. In fact most grid connected
commercial wind turbines are today designed with propeller-type rotors mounted
on a horizontal axis on top of a vertical tower. These turbines need to be
aligned with the direction of the wind, hence allowing the wind to flow
parallel to the axis of rotation.
Upwind
rotors need a yaw mechanism to keep the axis aligned with wind direction. Downwind rotors are placed on the lee side of
the tower. A great disadvantage in this design is the fluctuations in the wind
power due to the rotor passing through the wind shade of the tower which gives
rise to more fatigue loads.
Generator
torque
With wind
permanent magnet alternator, we can set the cut in speeds as well as the cut
out speeds. Hence we can generate electricity at a variable speed. Let us take
an example. Suppose the turbine speed is 200 RPM at a torque of 40 NM. With
permanent magnet generators, the torque generated will be around 90% of 40 NM
at same RPM. Other machines are able to provide just around 70% of this
torque.
Blade
design
The ratio
between the speed of the blade and wind speed is called tip speed ratio. High
efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7. Modern
wind turbines are designed to rotate at varying speeds. Low inertia is achieved
by the use of aluminum and composite materials in the blade design. Hence the
blades start rotating quickly.
The speed and
torque at which a wind turbine rotates must be controlled for several reasons:
§
For keeping the generator
within the speed and torque limits.
§
To optimize the aerodynamic
efficiency of the rotor in light winds.
§
To keep the rotor and hub
within their centrifugal force limits. The centrifugal force from the spinning
rotors increases as the square of the rotation speed, which makes this
structure sensitive to over speed.
§
To keep the rotor and tower
within their strength limits. Because the power of the wind increases as the
cube of the wind speed, turbines have to be built to survive much higher wind
loads (such as gusts of wind) than those from which they can practically
generate power.
§
To facilitate maintenance. Since
it is dangerous to have people working on a wind turbine while it is active, it
is sometimes necessary to bring a turbine to a full stop.
§
To reduce noise. As a rule
of thumb, the noise from a wind turbine increases with the fifth power of the
relative wind speed.
When you gather all these basic information regarding the
designing of the wind mill, send us the requirements and the conditions and the
skilled and experienced R&D heads at PM generators Ltd. will design an
alternator which will match the mechanical power output as per the alternator
requirement. PM Generators Ltd. will work on the provided specifications and
develop a customized design which will suit your requirements to the fullest.
