Wind Technology.
By Emil Bedi, CANCEEand Hakan Falk, "Energy Saving Now".
Wind turbines are moved by the wind
and convert this kinetic energy directly into electricity by spinning a
generator. Usually they use blades like the wing of an plane to turn a central
hub which is connected through a series of gears (transmission) to an electrical
generator. The generator is similar in construction to the generators used in
traditional fossil fuel power plants. The variety of machines that has been
devised or proposed to harness wind energy is considerable and includes many
unusual devices. Nevertheless modern wind turbines come in two basic
configurations:
Horizontal axis turbines (HAT)
Horizontal axis turbines (HAT) are the most common type seen sitting
on top of towers with two or three blades. The orientation of the drive shaft,
the part of the turbine connecting the blades to the generator, is what decides
the axis of a machine. Horizontal axis turbines have a horizontal drive shaft.
The blades may be facing into the wind, upwind turbine, or the wind may hit the
supporting tower first, downwind turbine. Horizontal axis wind turbines
generally have either one, two or three blades or else a large number of blades.
Wind turbines with large numbers of blades have what appears to be virtually a
solid disc covered by solid blades and are described as high-solidity devices.
These include the multi-blades wind turbines used for water pumping. In
contrast, the swept area of wind turbines with few blades is largely void and
only a very small fraction appears to be ‘solid’. These are referred to as
low-solidity devices.
Extracting energy from the wind as
efficiently as possible means that the blades have to interact with as much as
possible of the wind passing through the swept area of rotor. The blades of a
high-solidity, multi-blade wind turbine interact with all the wind at a very low
tip speed ratio, whereas the blades of a low-solidity turbine have to travel
much faster to virtually fill up the swept area, in order to interact with all
the wind passing through. Theoretically, the more blades a wind turbine rotor
has, the more efficient it is. However, large numbers of blades interfere with
each other, so high-solidity wind turbines tend to be less efficient overall
than low-solidity turbines.
The pumps that are used with water
pumping wind turbines require a high starting torque to function. Multi-bladed
turbines are therefore generally used for water pumping because of their low tip
speed ratios and resulting high torque characteristics.
Vertical axis turbines (VAT)
Vertical axis turbines
(VAT) have vertical drive shafts. The blades are long, curved and attached to
the tower at the top and bottom. There is not so many manufacturers of such
turbines in the world. Flowind is the most noted manufacturer of them. Vertical
axis wind turbines have an axis of rotation that is vertical, and so, unlike
their horizontal counterparts, they can harness winds from any direction without
the need to reposition the rotor when the wind direction changes. The modern VAT
evolved from the ideas of the French engineer G. Darrieus.
Despite the different appearances of HAT and VAT, the basic mechanics of
the two systems are very similar. Wind passing over the blades is converted into
mechanical power, which is fed through a transmission to an electrical
generator. The transmission is used to keep the generator operating efficiently
throughout a range of different wind speeds. The electricity generated can
either be used directly, fed into a transmission grid or stored for later
use.
Wind turbines can be built with two different forms of
operation: pitch- or stall-regulation. Both systems have advantages and
disadvantages. With pitch regulation, the blades can be pitched, which means
better utilisation of the wind and more energy from the wind turbine; on the
other hand, the turbine has to be equipped with blade bearings, a blade-pitch
regulation system, etc- parts which experience shows can give rise to operating
problems. With stall regulation the blades are fixed and there is no pitch-
adjusting system. A stall-regulated wind turbine is so to speak self-regulating
and thus simpler, and it requires less maintenance and service; on other hand,
one cannot utilise the wind quite as well as with pitch regulation.
Wind System Components
Modern wind turbine
usually consists of following components:
 Blades,
Rotor,
Transmission,
Generator,
Controls.
Blades are the part of a turbine
that capture the wind. Advanced designs have led to higher energy capture. Two
or three blades most often make up a rotor. Blades are made from fiber glass,
polyester, or epoxy resins. Some have wood cores. These materials have the
needed combination of strength and flexibility (and they don’t interfere with
television signals!). Blade diameters for commercial size turbines range from 25
to 50 meters and can weigh over 2000 pounds each.
Blades
 Brakes
 Gearbox
GeneratorThe rotor is all the blades and the centre hub which
the blades are attached to. The hub is attached to the drive shaft (or it is
attached directly to a large gear in some systems). Upwind machines have their
rotor in front of the tower (wind hits the rotor before the tower). Downwind
machines are just the reverse arrangement.
Transmission and gears are
important in order to transfer the rotating power through the spinning drive
shaft to a generator.
The output from the transmission is then
connected to an electric generator that produces electricity from motion.
Several control systems are all co-ordinated and monitored by a computer
and can be accessed from a remote location. Pitch controls twist the blades to
improve performance at different wind speeds. Yaw controls point the whole
turbine into the wind.
Electronic controls keep the same voltage
flowing from the generator as it changes speed. This variable speed generator is
an important part of making wind turbines cost effective.
 
WIND TURBINES
A wind
turbine is a deceptively difficult product to develop and many of the early
units were not very reliable. A PV module is inherently reliable because it has
no moving parts and, in general, one PV module is as reliable as the next. A
wind turbine, on the other hand, must have moving parts and the reliability of a
specific machine is determined by the level of skill used in its engineering and
design.
Modern wind turbines come in a wide range of sizes, from
small 100 watt units designed to provide power for single homes or cottages, to
huge turbines with blade diameters over 50 m, generating over 1 MW of
electricity. The vast majority of wind turbines produced at the present time are
horizontal axis turbines with three blades, 15 - 40 m in diameter, producing 50
- 600 kW of electricity. These turbines are often grouped together to form “wind
farms” which provide power to an electrical grid. Modern large wind turbines
generally produce electricity at 690 volts. A transformer located next to the
turbine, or inside the turbine tower, converts the electricity to high voltage
(usually 10-30 kilovolts).
Modern wind turbines costs around 800
USD/W what is sharp decline from 2500 USD/W for a turbine built in 1981.
MEGAWATT WIND TURBINES
Through the short history of the modern wind turbine, electric utilities
have made it clear that they have held a preference for large scale wind
turbines over smaller ones, which is why wind turbine builders through the years
have made numerous attempts develop such machines - machines that would meet the
technical, aesthetic and economic demands that a customer would require.
Considerable effort was put into developing such wind turbines in the early
1980s. There was the U.S. Department of Energy's MOD 1-5 program, which ranged
up to 3.2 MW, Denmark's Nibe A and B, 630 kW turbine and the 2 MW Tjaereborg
machine, Sweden's Näsudden, 3 MW, and Germany's Growian, 3 MW. Most of these
were dismal failures, though some did show the potential of MW technology.
A number of R&D facilities in Europe decided to take advantage of
these incentives and most received either partial to full financial support to
develop prototype wind turbines. The first of these was completed and installed
at the end of 1995. Today several have been installed and have been up and
running for a years. One company, Nordex, has even been marketing one of these
machines for more than a 3 years. Leading wind turbine manufacturers continue to
up-scale their 500 kW machines. It appears the marketing strategy of most of
these companies is to maintain a market hold with their proven turbines in the
500-800 kW class (39-50 meter) while expecting that commercial MW machines will
be in greater demand in the near future.
For the most part, manufacturers seem to be sticking close to the basic
design of their smaller machines in the design of their MW plant. One exception
is Tacke Windtechnik of Germany. Tacke introduced a pitch regulated, variable
speed turbine which was not previously part of its stable of machines. Four
largest wind turbines on the market are Enercon, Nordtank, Tacke and Vestas,
each rated at 1.5 MW.
Installation of MW machines under all circumstances presents new
challenges for meeting planning and siting requirements. In areas that have
already been filled to near capacity with smaller turbines, it is going to be
difficult find locations for MW turbines where they can be incorporated
harmoniously with existing turbines. Studies have been conducted in Denmark
which focus on the special siting considerations necessary for installing MW
turbines in the "technical" landscape. Results of these studies indicate there
is available space in areas such as harbours and industrial areas for about 200
units, or about 200-300 MW. Power production of such machines can be enormous.
It has been showed that 1 MW turbine can annually produce more than 5 million
kWh at average wind speed higher than 9 m/s. Turbine with 1,3 MW rated power can
produce more than 7 million kWh per year under such conditions.
POWER PRODUCTION
Important figure describing wind turbine is its rated power. This tells
you how much e.g. kilowatt-hours (kWh) the wind turbine will produce when
running at its maximum performance. 500 kW turbine will produce 500 kilowatt
hours (kWh) of energy per hour of operation at its maximum with wind speed say
15 metres per second (m/s). According to the experience large single turbines
can generate a considerable amount of electricity. Usually 600 kW machine will
generate about 500 000 kWh per year with an average wind speed of 4,5 m/s. With
an average wind speed of 9 metres per second it will generate up to 2 000 000
kWh per year. The amount of energy produced can not be simply calculated by
multiplying of capacity (here 600 kW) and average annual wind speed. Here we
have to deal with the capacity factor what is another way of expressing the
efficiency of power production by a turbine during the year in particular
location. Capacity factor is actual annual energy output divided by the
theoretical maximum output, if the machine were running at its rated (maximum)
power during all of the 8766 hours of the year. For example if a 600 kW turbine
produces 2 million kWh in a year, its capacity factor is = 2000000 : ( 365,25 *
24 * 600 ) = 2 000 000 : 5 259 600 = 0,38 = 38 %. Capacity factors may
theoretically vary form 0 to 100 per cent, but in practice they will usually
range from 20 to 70 %, and mostly be around 25-30 %.
A very important factor which influences the performance of the wind
turbine is the location. In general, wind speeds increase with elevation. This
is why most wind turbines are placed at the top of a tower. Because the higher
you are above the top of the neighbouring obstacles, the less wind shade. The
wind shade, however, may extend to up to five times the height of the obstacle
at a certain distance. If the obstacle is taller than half the turbine height,
the results are more uncertain, because the detailed geometry of the obstacle
will affect the result. Limitations in the strength of affordable materials has
limited most towers to heights of approximately 30 m. On wind farms, turbines
are most often spaced at intervals of 5 – 15 times the blade diameter. This is
necessary to avoid turbulence from one turbine affecting the wind flow at
others.
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