|
WIND
ENERGY |
Wind energy is a form of solar energy produced by
uneven heating of the Earth's surface. The sun radiates 100,000,000,000,000 kilowatt
hours of energy to the earth per hour. In other words, the earth receives
10 to the 17th power of watts of power. About 1 to 2 per cent of the energy
coming from the sun is converted into wind
energy. That is about 50 to 100 times more than the energy converted into
biomass by all plants on earth.
For several thousand
years now, man has known how to extract energy from the wind by means of ships,
sails or wind wheels, because the kinetic energy of wind is available more or
less all over the world. Wind energy is environmentally attractive for many
reasons. It produces no health-damaging air pollution, forest-destroying acid
rain, climate-destabilizing carbon emissions, or dangerous radioactive waste.
Wind, as the primary energy source, costs nothing and can be used decentrally.
There is no need for an extensive infrastructure such as that required for a
power supply network or for the supply of oil or natural gas.
HISTORY
Wind has been used by humankind as a natural
source of energy for tens of thousands of years. The use of wind energy dates
back to the dawn of civilization when sailing vessels were powered by the wind.
The first simple sailboats were set afloat in Egypt about 5,000 years ago.
Around the year 700 AD, in what is Afghanistan today, the first wind machines
rotating around a vertical axis were employed to grind grain. The famous
fixed-tower windmills with sails provided irrigation for many parts of the
Mediterranean island of Crete. Wind-driven gristmills were one of the greatest
technical challenges of the Middle Ages. In the 14th
century, the Dutch improved on the design that had spread throughout the Middle
East and continued to use it for its primary purpose of grinding grain.
A wind powered water
pump was introduced in the United States in 1854. It was the familiar fan type
with many vanes around a wheel and a tail to keep it pointed into the wind. By
1940, over 6 million of these windmills were being used in the United States
mainly for pumping water and generating electricity. The “Wild West” was
won at least in part with the help of these wind pumps that were used to supply
water for the massive herds of cattle.
However, the 20th
century soon brought an end to the widespread use of wind energy, which gave way
to the “modern” energy resources, oil and electricity. It was not until after
the oil crisis that wind energy options met with renewed interest. As a result
of the drastic rises in oil prices at the beginning of the 1970s, energy
planners have once again been turning their attention increasingly to the
utilization of wind energy. State-sponsored research and development grants in
many countries have provided a fresh stimulus to the development of technology
for the utilization of wind energy. Efforts have been concentrated on
developing wind energy converters for generating electricity, because in the
industrialized countries the application of wind pumps is of minor importance.
USA
The oil embargo of 1973 was the driving force behind wind turbine
development programs in the United States. Westinghouse Electric developed
first generation of 200 kW wind turbines, known as
MOD-OAs. The largest of this series the 3,2 MW MOD-5B
is operating in Oahu, Hawaii. The Public Utilities Regulatory Policies
Act (PURPA) of 1978 and a 25% tax credit for investors in turbines jump started
commercial development of the United States wind industry and resulted in 6870
turbines being installed in California between 1981 and 1984. The tax credits
expired on Dec. 31, 1985. None of the small wind turbine companies, however,
were owned by large companies committed to long term market development, so
when the federal tax credits expired and oil prices dropped to USD 10 a barrel,
most of the small wind turbine industry once again disappeared. The companies
that survived this “market adjustment” and are producing small wind turbines
today are those whose machines were the most reliable and whose reputations
were the best.
Nevertheless since the
year 1998 the interest in wind energy is back again and installed capacity
reached 25.170 MW in 2008 and USA became the top world producer of wind energy.
Wind power represents around 1% of total U.S. consumption but varies in
different regions. In 2008 Texas became the leading wind power state in the
U.S. and due to the favorable condition there it is expected to rise further in
the future.
DENMARK
Denmark's wind energy industry is a major commercial success story. From standing start in the 1980 to a turnover of 1 billion USD in
1998. Danish wind turbines dominate the global market. From a few
hundred workers in 1981 the industry now employs 15.000 people. Its turnover is
twice as large as the value of Denmark's North Sea gas production. Output , mainly for export around the world, has increased
to 1216 MW of capacity in 1998. Now over half of the wind turbine capacity
installed globally is of Danish origin.
The Danish government
introduced support for renewable energy technology in 1979, covering 30% of
capital cost. State aid encouraged the development of a highly successful wind
turbine industry (it has also been used to promote the use of straw, biogas and
solar projects).Danish wind turbine manufacturers were advised on ways of
improving the performance and reducing costs of their machines by experts
based at the National Wind Turbine Test Centre at Riso. The grants for wind
turbines were reduced to 15% in 1986 and finally phased out all together in
1989 as the industry became established. They have since been replaced by tax credits
– the owners of wind turbines obtain a proportion of the income from the sale
of electricity tax free.
Huge wind power
development In Denmark was mainly based on activity of local people organized
in co-operatives. Here is one example from Bryrup Wind turbine Co-operative
(Jutland), 110 km from the West-coast and 50 km from the Eastern coastline.
This co-operative has 70 partners owning three wind turbines installed between 1986
and ‘89. The effects is as follows: one 95 kW
producing 184 000 kWh a year and two 150 kW each producing 275,000 kWh. Thus
average total production amounts to 734.000 kWh annually.
Total price for all three turbines including foundation and connection to
the public grid amounted to 2,5 million DKr (1 USD equals
6.2 DKr). This investment is split up in 734 “shares!’,
each related to a production (and a consumption) of 1000 kWh, at a cost of
3,400 DKr. This equals half a month salary after tax for an unskilled Danish worker.
Each partner can buy “shares” in proportion to his annual consumption of
electricity plus 30%. If for instance annual consumption is 10 000 kWh you may
add 3 000 kWh and thus be able to acquire maximum 13 “shares”. This restriction
is applied because the profit for co-operative partners is tax- free, and the
Danish legislators did not wanted this profit to be unreasonable. The partners
have bought an amount of “shares” at numbers between 1 and 28. At the
democratic general assemblies each partner has one vote despite numbers of “shares”.
The reason for putting shares in quotation marks is related to the fact that these
“shares” can not be traded like normal shares. By coming sales, buyers must apply to the rules referring to electricity
consumption. The economy of this co-operative is good. They distribute every
year - after putting aside a reasonable amount for maintenance and renewals -
510 DKr per “share”, which gives a tax-free Interest rate of 15% what is more
than banks can offer for your money. Today installation of wind turbines is a
bit more costly. A share will amount to 4000 DKr, thus reducing interest rate
to 12,75%. The Danish governmental support for wind
power has caused that every tenth Danish family is member of a wind turbine
co-operative or single owner of a wind turbine.
GERMANY
In contrast to the situation in Denmark or California, where a large number
of wind generators were installed early on, the revival
in Germany was relatively late in coming. In 1989, the German Federal
Government initiated a promotion programme which called for the installation of
wind generators with a total capacity of 250 MW over the next seven years.
German legislation supports the installation of wind turbines through the
feed-in tariff system. Investors receive fix rates for their wind electricity
for several years. The feed-in tariffs were set so that they are interesting
for the investors (large and small). Wind-generated electricity is supplied to
the public power mains by any operator of the grid who is obliged to buy the
wind electricity from producers. This programme has led to a rapid increase in
the number of installations and today Germany is ranked second in the world in
installed wind power capacity (23.903 MW in 2008).
CURRENT DEVELOPMENT
Wind power has retained its status as the fastest growing energy source in
the world. Globally the wind installed capacity reached 121.188 MW in
2008. Capacity in EU has hit 65.255 MW in end of this year. USA, Spain,
Germany, China and India are leading markets. Leader of the new installation in
2008 were USA with almost 9000 MW. China and India are expected to be the
largest wind energy market in the future. The global rate of growth fell from
26 % in the year 2003 down to 21 %. This is the result of slowing down of
traditional markets in Denmark, USA and, to a lesser extend, Germany.
Hundreds more megawatts
of energy capacity are scheduled to be built in years to come, encouraged by a
tariff systems in Europe and tax breaks in other parts of the world. World wind
production capacity more than quadrupled between 2000 and 2006, doubling about
every three years. Wind energy now accounts almost 20 percent of national
electricity production in Denmark. European and the U.S. success for wind
energy development is just the beginning. It is estimated that this source, if
the appropriate policies are put in place.
Country |
Installed
capacity in MW |
Installed
capacity in MW |
16.819 |
25.170 |
|
22.247 |
23.903 |
|
15.145 |
16.740 |
|
5.912 |
12.210 |
|
7.850 |
9.587 |
|
2.726 |
3.537 |
|
2.477 |
3.426 |
|
EU TOTAL |
56.614 |
65.255 |
WORLD TOTAL |
93.927 |
121.188 |
During 2003 and 2008 the
average growth rate in new wind power installations has been 27,6 percent a year and the forecasts until 2013 expect growth
around 15-16 percent per year.
The cost
of wind power continued to decline through advancements in design, siting
practices and the cost of capital from around 14 US cents per kWh in 1986 to
below 5 cents per kWh in 1999. Wind power is now cost-competitive in many
electric power applications and that is why it is experiencing rapidly growing
deployment.
Over the past two years
wind energy capacity has been expanding at an annual rate of more than 30%. In
contrast, the nuclear industry is growing at a rate of less than 1% whilst coal
has not grown at all in the 1990’s. Europe is the centre of this young and
high-tech industry. 90% of the world's manufacturers of medium and large wind
turbines are European. The average size of turbine increased
by 150 kW to 900 kW.
POTENTIAL
Wind power potential is much greater than current world energy consumption.
There are several studies which found[update]
that this potential (land and off-shore) is around 72 TW (72 million MW) per
year. This is five times more than the world's total energy production from all
fuel sources. This potential covers only areas with mean annual wind
speeds less than 6.9 m/s at 80 m.
According to the study Wind
Force 12 – a blueprint to achieve 12% of the world's electricity from wind
power by 2020 - there are no technical, economic or resource limitations to
achieve this goal. By 2020 the industry is capable of installing 1,260,000 MW
of wind power throughout the world. According to the study the cost of
generating electricity with wind turbines is expected to drop to 2.5 US
cents/kWh by 2020, compared to the current 4.0 US cents/kWh.
|
Wind
Force 12, by 2020 the wind industry can deliver: 12% of global electricity
demand, assuming that global demand doubles by 2020. installed
capacity of 1,261,000 MW, generating 3,093 terawatt hours (TWh), equivalent
to the current electricity use of all Europe. Cumulative CO2 savings of 11,768
million tones. Creation of 1.475 million jobs. |
JOBS
Renewable energy has become an important employer. There are over 110.000
jobs in the manufacture, installation and maintenance of renewable energy
technologies in the European Union. Wind energy accounts for around 20%
of this. Most of the 700 companies involved are small and medium sized
enterprises. As the industry grows, so more jobs are created. At the end of
1999 more than 20.000 Europeans were estimated to be employed in wind energy,
and this figure is projected to grow to 40.000 by the year 2005 and to more
than 1,4 mil. in 2020.
Markets
|
Wind power
systems are being built all over the world. They are ideally suited to the
needs of developing countries, which urgently need new capacity. They can be
brought on line relatively cheaply and quickly in comparison with large power
stations, which need major electrical infrastructure and grid systems to
transmit their power. Developed countries are also a key growth area as they
turn to wind power for environmental and economic reasons. Wind energy can be
integrated into existing electrical systems, reducing the amount of power
which needs to be generated by burning fossil fuels. |
ENERGY IN THE WIND
Wind resources are best along coastlines and on hills, but usable wind
resources can be found in most other areas as well. As a power source wind
energy is less predictable than solar energy, but it is also typically
available for more hours in a given day. Wind resources are influenced by the
ground surface and obstacles at altitudes up to 100 meters. The wind energy is
thus much more site specific than solar energy. In
hilly terrain, for example, two places are likely to have the exact same solar
resource. But it is quite possible that wind
resource can be different at both places because of site condition and
different exposure to the prevailing wind direction. In this regard, wind
turbines planning must be considered more carefully than solar technology. Wind
energy follows seasonal patterns that provide the best performance in the
winter months and the lowest performance in the summer months. This is just the
opposite of solar energy. For a Denmark conditions a PV plant has a production
per month varying between 18% in January and 100% in July. The wind power plant
produces 55% in July and 100% in January. For this reason small wind and solar
systems work well together in hybrid systems. These hybrid systems provide a
more consistent year-round output than either wind-only or PV-only systems.
It is important to know that the amount of wind power generated is
proportional to the density of air, area swept by the rotor blades of the wind
turbine, and to the cube of the wind speed.
AIR DENSITY
Blades of the wind generator rotate because air mass is moving them. The
more air can move the blades, the faster the blades will rotate, and the more
electricity the wind generator will produce. From the physics comes out that
the kinetic energy of a moving body (e.g. air) is proportional to its mass (or
weight) so the energy in the wind depends on the density of the air. Density
refers to the amount of molecules in unit volume of air. At normal atmospheric
pressure and at 15° Celsius air weighs some 1,225 kg per cubic meter, but the
density decreases slightly with increasing humidity. Air is more
dense in winter than in the summer. Therefore, a wind generator will
produce more power in winter than in summer at the same wind speed. At high
altitudes, (in mountains) the air pressure is lower, and the air is less dense.
It is obvious that the density of air is variable that we can't do anything
about.
ROTOR AREA
The rotor of the wind turbine “captures” the power in the mass of the air
that are passing through. It is clear that the larger
area covered by a rotor means, the more electricity it can produce. The rotor
area determines how much energy a wind turbine is able to use from the wind.
Since the rotor area increases with the square of the rotor diameter, a turbine
which is twice as large will receive four times as much
energy. But increasing rotor area is not as simple as putting bigger
blades on a wind generator. At first glance, this appears to be a very easy way
to increase the amount of energy that a wind generator can capture. But by
increasing the swept area we have also increased all of the stresses on the
wind system at any given wind speed. In order to compensate for this change and
let the wind system survive, it is important to
make all of the mechanical components stronger. Obviously this approach is
going to get very expensive.
WIND SPEED
The wind speed is most important factor influencing the amount of energy a
wind turbine can convert to electricity. Increasing wind velocity increases the
amount of air mass passing the rotor, so increasing wind speed will also have
an effect on the power output of the wind system. The energy content of the
wind varies with the cube (the third power) of the average wind speed. Thus, if
wind speed doubles, the kinetic power gained by the rotor increases eight
times. From the following table you can estimate the power of the wind for
standard conditions (dry air, density 1.225 kg/m3, at sea level pressure). The
formula for the power in Watts per m2 = 0.5 *
1.225 * v3, where v is the wind speed in m/s (according to Danish Wind Turbine
Manufacturers Association).
m/s |
W/m2 |
1 |
1 |
3 |
17 |
5 |
77 |
9 |
477 |
11 |
815 |
15 |
2067 |
18 |
3572 |
21 |
5672 |
23 |
7452 |
Nature provide us with a different wind conditions and wind speed is continuously
changing. Wind turbines are specially build to make use of wind which range in
speed between 3 to 30 m/s. Higher wind speed can damage the turbine so large
turbines are equipped with the brakes. Smaller turbines can make use of wind
speeds lower than 3 m/s.
Wind speed scale:
Wind speed m/s |
Type of wind |
0-1,8 |
Calm |
1,8-5,8 |
Light |
5,8-8,5 |
Moderate |
8,5-11 |
Fresh |
11-17 |
Strong |
17-25 |
Gale |
25-43 |
Strong gale |
more than 43 |
Hurricane |
ROUGHNESS CLASS OF THE TERRAIN
Earth surface with its vegetation and buildings is the main factor reducing
the wind speed. This is sometimes described as roughness of the terrain. As you
move away from the earth's surface, roughness decreases and the laminar flow of
air increases. Expressed another way, increased height
means greater wind speeds. High above ground level, at a height of about 1 kilometer,
the wind is hardly influenced by the surface of the earth at all. In the lower
layers of the atmosphere, however, wind speeds are affected by the friction
against the surface of the earth. For the wind power utilization it means the
higher the roughness of the earth's surface, the more the wind will be slowed
down. Wind speed is slowed down considerably by forests and large cities, while
plains like water surfaces or airports will only slow the wind down a little.
Buildings, forests and other obstacles are not only reducing the wind speed but
they often create turbulence in their neighborhood. The lowest influence on the wind speed have the water surfaces. When
people in the wind industry evaluate wind conditions in a landscape they
describe it by roughness class. Higher roughness class means more obstacles in
terrain and larger wind speed reduction. Sea surface is described as roughness
class 0.
Roughness Class and
Landscape Type:
0 = Water surface
0.5 = Completely open terrain with a smooth surface, e.g. runways in
airports, mowed grass, etc.
1 = Open agricultural area without fences and hedgerows and very scattered
buildings. Only softly rounded hills
1.5 = Agricultural land with some houses and 8 meter tall sheltering
hedgerows with a distance of approx. 1250 meters
2 = Agricultural land with some houses and 8 meter tall sheltering hedgerows
with a distance of approx. 500 meters
2.5 = Agricultural land with many houses, shrubs and plants, or 8 meter
tall sheltering hedgerows with a distance of approx. 250 meters
3 = Villages, small towns, agricultural land with many or tall sheltering
hedgerows, forests and very rough and uneven terrain
3.5 = Larger cities with tall buildings
4 = Very large cities with tall buildings and skyscrapers
In the industry also the
term wind shear is used. It describes the fact that the wind profile is twisted
towards a lower speed as we move closer to ground level. Wind shear may also be
important when designing wind turbines. Here large rotor diameter and only a
few meter higher tower could mean that the wind is blowing with higher speed
when the tip of the blade is in its uppermost position, and wit much lower
speed when the tip is in the bottom position.
TECHNOLOGY
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) are the most common type seen siting 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) 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 utilization 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 utilize
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
Generator
The 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 600-800
kW class (39-50 meter) while expecting that commercial MW machines will be in
greater demand in the near future. The
world's largest turbines are recently (2008) produced by German companies Enercon
(E-126 with capacity of 6 MW) and REpower (REpower 5M with capacity of 5 MW). The
overall height these turbines is around 198 m and
has a diameter of 126 meters.
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 harbors 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 meters 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 meters 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 neighboring 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.
WIND POWER COST
Wind power has no fuel
cost but high proportion of capital cost. The estimated average cost
of wind energy per unit incorporates the cost of investment and construction of
the turbine, transmission facilities, estimated annual production, and other
components averaged over the projected life of the equipment, which may be more
than 20 years for typical wind power generators. Energy cost depends on these
assumptions and they differ substantially. According to several reports average
wind production cost for onshore wind power plants is around 5-6 US cents per
kWh (2005). Cost per kWh produced (0,056 USD/kWh) was comparable to the cost of
new coal (0,053 USD/kWh) and natural gas (0,052 USD/kWh) generating capacity in
the USA in 2006.
APPLICATION OF WIND TURBINES
LARGE WIND TURBINES - WINDFARMS
The development of wind turbines started with small units for small
applications, but as the turbines grew in size, they became less and less
attractive as a source of electricity for individual or household consumption.
Consequently, almost all of the electricity generated by such plants today is
fed into the grid. The output of a wind turbine of typical size is already so
high that it exceeds the capacity of the local electricity mains. This is
precisely the case in areas along the coast with a good wind regime but often
lacking electricity facilities, making it necessary to install new and
higher-capacity mains facilities, with the related additional costs. Because
the additional expense is not an economically viable venture in the case of
individual units, there has been an increasing tendency to install several
plants (at least five in most cases) in consolidated areas known as wind farms.
The output of several turbines is combined and sold under contract to the
utility company.
Starting in the early 1980’s, larger wind turbines were developed for “wind
farms” that were being constructed in windy passes in California. In a wind
farm a number of large wind turbines, now typically rated between 400-600 kW
each, are installed on the same piece of property.
In the USA the wind farms are usually owned by private companies, not by the
utilities. Although there were some problems with poorly designed wind turbines
and overzealous salesmen at first, wind farms have emerged as the most cost
effective way to produce electrical power from wind energy. There are now over
16,000 large wind turbines operating in the California and they produce enough
electricity to supply a city the size of San Francisco. Large wind turbine
prices are coming down steadily and even conservative utility industry planners
project massive growth in wind farm development in the coming decade, most of
it occurring outside California. One recent study actually called North Dakota
the “Saudi Arabia of wind energy”.
Offshore Wind Turbines
The success story of onshore wind energy created an interest for the
exploitation of wind energy at offshore sites since suitable locations on land
are becoming scarce or do not have good enough wind conditions. On sea the wind
blows harder and a large amount of space in shallow waters not too far from
shore is available especially in most states of Northern Europe. Both aspects
are essential for a future large scale development. Firstly, a ten
percents increase in the mean wind speed can result potentially in 30% more
energy yield. Secondly, it is generally believed that the continental
shelf with water depth up to some 30 m and distance from shore of up to about
30 km offer considerable economic advantages. In the future technological
progress, e.g. floating offshore wind farms or HVDC (High Voltage Direct
Current) power transmission, may also enable exploitation of deeper water
locations as typical for the Mediterranean and many sites outside Europe as
well as more remote offshore sites. In a recent study carried out in the scope
of the European non nuclear energy research programme JOULE the potential of
offshore wind energy in the European Union has been estimated to be nearly two
times the total consumption.
In the
1990s first promising steps were taken to develop the required technology and
to gain experience. The general feasibility of offshore wind energy was
demonstrated and together with the demand for environmentally green technology
it was seen as a considerable and renewable contribution to the energy supply
in Europe. Utilization of wind energy offshore has even less environmental
constraints than on land due to large available space and relaxed noise
limitations. Generally the prospects are assessed quite positively and
investment in offshore wind energy today is a preparation for a big market
tomorrow. Offshore wind energy is an extremely promising application of wind power,
particularly in countries with high population density, and thus difficulties
in finding suitable sites on land. Construction costs are much higher at sea,
but energy production is also much higher. The Danish electricity companies
have announced major plans for installation of up to 4000 MW of wind energy
offshore in the years after the year 2000. The 4 000 MW of wind power is
expected to produce some 13,5 TWh of electricity,
equivalent to 40 % of Danish electricity consumption. Four possible areas
(ranging from 135 to 500 km2, water depths from 5 - 15 m) are designated
suitable to erect turbines at sea, with only few conflicting interests (e.g.
environment, landscape, fishing, defense, communication, transport and national
monuments). Production prices of about USD 0,05/kWh
(20 years loan, 5% discount rate) are estimated.
Offshore wind farm in the Netherlands.
In spring 1998, five offshore wind farms were realized in Denmark, The Netherlands and Sweden, respectively. These farms are demonstration
projects, characterized by medium sized wind turbines of the 500 kW class,
moderate farm capacity up to 5 MW, low water depths (less than 10 m) and small
distance from shore (between 40 m and 6 km). The energy prices of the pilot
plants are considerably higher than onshore wind farms at good coastal sites.
Some e.g. the Danish ‘Plan of action for large scale offshore
wind farms’, show that the cost of energy for large plants is
competitive with onshore wind farms at average sites. Moreover the price of
wind energy is close to or in the range of other energy sources.
The world's first
offshore wind farm is located North of the island of Lolland in the Southern
part of Denmark Vindeby. The Vindeby wind farm in the Baltic Sea off the
coast of Denmark was built in 1991 by the
utility company SEAS. The wind farm consists of eleven 450 kW wind turbines,
and is located between 1,5 and 3 kilometers North of the coast of the island of
Lolland near the village of Vindeby. The turbines were modified to allow room
for high voltage transformers inside the turbine towers, and entrance doors are
located at a higher level than normally. Two anemometer masts were placed at
the site to study wind conditions, and turbulence, in particular. The park has
been performing flawlessly. Electricity production is about 20 per cent higher
than on comparable land sites, although production is somewhat diminished by
the wind shade from the island of Lolland to the South.
Vindeby offshore wind farm in
Denmark.
The world's second offshore wind farm is located between the Jutland
peninsula and the small island of Tunø in Denmark. The Tunø Knob offshore wind
farm in the Kattegat Sea off the Coast of Denmark was built in 1995 by the
utility company Midtkraft. The wind farm is situated in an area where the sea
depth varies from 3-5 m. The Tunø Knob area is of considerable environmental
interest, both as a resting area for birds and as a beautiful part of the
coastline and landscape. Furthermore, a careful archaeological investigation of
the site has been carried out as part of the off-shore wind farm planning
process. The Wind farm consists of ten 500 kW wind turbines. Each turbine is a
horizontal axis pitch regulated machine, orientated up-wind with a tubular
tower, and a 3-bladed rotor of 39 m diameter. The turbines are mounted on
specially-developed, reinforced concrete caisson foundations. The turbines are
connected to the national grid via a 6 km submarine cable to the mainland of
Jutland. Each turbine is controlled remotely. The production manager can
monitor the performance and operation of the wind turbine from an operation
centre in Hasle. The control system is continuously collecting all relevant
data. The data are transmitted via a radio system from the individual
data-collecting unit of each wind turbine to computers at Hasle. On-site
maintenance is estimated to be needed only twice a year, when engineers will
sail to the wind turbines and carry out the regular scheduled maintenance
programme.
The
turbines were modified for the marine environment, each turbine being equipped
with an electrical crane to be able to replace major parts such as generators
without the need for a floating crane. In addition, the gearboxes were modified
to allow a 10 % higher rotational speed than on the onshore version of the
turbine. This will give an additional electricity production of some 5 %. This
modification could be carried out because noise emissions are not a concern
with a wind park located 3 kilometers offshore from the island of Tunø, and 6 kilometers
off the coast of the mainland Jutland peninsula. The park has been performing
extremely well, and production results have been substantially higher than
expected. In November 1995, its production was 1,3 GWh
almost 40% more than originally estimated. The total production price/kWh is
expected to be DKr 0,49 with an annual total
production of 15 GWh. The entire costs of the off-shore farm are estimated to
be about DKr 78 million.
The on-shore noise from the wind turbines has been calculated, at the nearest
island of Tunø, to be less than someone whispering [15 dB(A)].
On the mainland it is inaudible.
SMALL WIND TURBINES
Small wind energy systems can be used in
connection with an electricity transmission and distribution system (called
grid-connected systems), or in stand-alone applications that are not connected to
the utility grid. A grid-connected wind turbine can reduce consumption of
utility-supplied electricity for lighting, appliances, and electric heat. When
the wind system produces more electricity than the household requires, the
excess can be sold to the utility. With the inter-connections available today,
switching takes place automatically.
Stand-alone wind energy
systems can be appropriate for homes, farms, or even entire communities (a
co-housing project, for example) that are far from the nearest utility lines.
Either type of system can be practical if the following conditions exist.
Small wind generator sets for
household electricity supply or water pumping represent the most interesting
wind-energy applications in remote areas. Such generators can be very promising
for the Third world countries as well where millions of rural households will
be without grid connections for many years to come and will thus continue to
depend on candles and kerosene lamps for lighting as well as batteries to operate
radios or other appliances.
Wind turbines for domestic or rural applications range in size from a few
watts to thousands of watts and can be applied economically for a variety of
power demands.
In areas with adequate wind regimes (more than five meters per second annual
average), simple wind generators with an output range of 100 to 500 W can be
used to charge batteries and thus supply enough power to meet basic electricity
needs. The families assign a very high priority to electricity and the range of
services made possible by it (lighting, operation of radios and TVs). But
relatively high investment costs of a complete wind-power system, which range
from several hundred to a thousand US dollars or more, can be an obstacle for
many households in developing countries.
In the past reliability of small wind turbines was a problem. Small turbines
designed in the late 1970’s had a well deserved reputation for not being very
reliable. Today's products, however, are technically advanced over these
earlier units and they are substantially more reliable. Small turbines are now
available that can operate 5 years or more, even at harsh sites, without need
for maintenance or inspections. The reliability and cost of operation of these
units is equal to that of photovoltaic systems.
WIND vs. DIESEL OR GRID EXTENSION
Small wind mills are sometimes better than diesel generators or extension of grid because
they offer a number of other socio-economic benefits. Wind systems are smaller,
modular and have a shorter lead-time than grid extension. In many countries for
grid extension distances as short as one kilometer a wind system can be a lower
cost alternative for small loads. While they cost more initially than diesels
they are much better from the users point of view.
Some donor agencies, for example in developing countries, typically supply
diesels at no cost, but leave operational costs (fuel, maintenance and
replacement) to the local people. This requires scarce hard currency and
usually results in limited utilization and a shortened life of the diesel
because of inadequate maintenance. Many countries must also import their fossil
fuels, further magnifying the burden imposed by diesels. In such case small
wind mills seems to be the better alternative.
The economies of scale in small wind turbines makes
them particularly competitive in cost for sizes above 250 watts. For daily
loads as small as one kilowatt-hour per day a wind turbine will be less
expensive than diesels, grid extension, or photovoltaics for virtually any wind
resource above 4 m/s. This wind resource is available in most of the developing
world. For larger daily load requirements the economics of wind power get
progressively better. For a 10 kW wind turbine a wind resource of only 3-3.2
m/s will usually make wind the least cost option. There are not many areas of
the world that have average wind speeds below 3 m/s .
In Asia, for example, 50
000 wind generators are currently in operation in Inner Mongolia. The success
story in Mongolia was made possible by favorable climatic conditions, on the
one hand, and a consistent development and marketing policy, on the other. A minimum
monthly velocity above 5 m/s throughout the year in many parts of the vast
grasslands provides for a continuous supply of electricity to the semi-nomads
living in the region. Operating electric lights, a radio and a TV is one of the
few modern technical conveniences available to the people living in these
remote areas. On the other hand, several private companies competing with one
another have developed cheap and affordable designs. The wind generators are
sold locally. The local government subsidizes the price of the equipment with
up to 50 % of the production costs.
COSTS
Small wind turbines can be an attractive alternative, or addition, to those
people needing more than 100-200 watts of power for their home, business, or
remote facility. Unlike PV’s, which stay at basically the same cost per watt
independent of array size, wind turbines get less expensive with increasing
system size. At the 50 watt size level, for example, a small wind turbine would
cost about USD 8/W compared to approximately USD 5/ for a PV module. This is
why, all things being equal, PV is less expensive for very small loads. As the
system size gets larger, however, this “rule-of-thumb” reverses itself. At 300
watts the wind turbine costs are down to USD 2,5/W,
while the PV costs are still at USD 5/W. For a 1500 W wind system the cost is
down to USD 2/W and at 10 000 watts the cost of a wind generator (excluding
electronics) is down to USD 1,50/W. The cost of regulators and controls is
essentially the same for PV and wind. Somewhat surprisingly, the cost of towers
for the wind turbines is about the same as the cost of equivalent PV racks and
trackers. The cost of wiring is usually higher for PV systems.
SMALL WIND TURBINE COMPONENTS
The wind systems for remote or rural application is essentially the same as
used with a PV system. Most wind turbines are designed for battery charging and
they come with a regulator to prevent overcharge. The regulator is specifically
designed to work with that particular turbine. PV regulators are generally not
suitable for use with a small wind turbine because they are not designed to
handle the voltage and current variations found with turbines.
Small wind turbines usually consists of : blades,
alternator, regulation and control electronics.
Blades
are usually made of carbon fiber reinforced composite that twists as the
turbine reaches its rated output. This twisting effect changes the shape of the
blade, causing it to go into stall mode. This limits the revolving of the
alternator, preventing damage in high winds.
Some small turbines do not have
brakes and during period of strong winds they can change their orientation.
Alternator
is optimized to match as close as possible the energy available in the wind. It
is constructed with permanent magnets and is usually brushless for best
performance and maintenance-free operation.
Regulation and control electronics performs several functions to assure
maximum output and safety for the user. The control electronics maintains a
load on the alternator at all times to make sure that the turbine never over
speeds, regardless of the condition of the battery. In case of battery
charging, the sophisticated regulator periodically checks the line, correcting
for voltage loss and monitoring charge rate. Once the battery has reached its
optimum charge level the regulator shuts the current off, preventing the
battery from being overcharged while maintaining a load on the alternator at
all times to prevent over speeding.
APPLICATION OF
SMALL WIND TURBINES
|
When
considering renewable energy sources and their use in some remote areas wind
energy is today once again a possible alternative to the diesel engine as an
economical means of converting energy. The principal ways in which wind
energy can be exploited in rural areas are as follows: |
Water pumping
Wind
energy has always been used extensively for pumping water, since there are no
major problems involved in storing sufficient quantities of water without loss.
Current estimates calculate that 100 000 wind pumps are installed around the
world. Most of them are located in rural, non-electrified areas. They are used primarily
by farmers for drinking water supply and livestock-watering. Wind pump
technology is still of major interest for applications in the developing
countries because of the importance of water supplies in rural areas, and the
relative simplicity and transparency of the technology.
In view of the varying amount of wind energy available and the fact that,
for economic reasons, the amount of storage capacity is limited, it can only be
assumed in extremely rare cases that a single wind pump installation will be
capable of ensuring a 100 percent reliable power supply. Hence, as a rule,
these renewable energy sources can only be used as part of a combination of
different systems appropriate to the case in question.
This means that for pumping water, be it for a drinking water supply,
irrigation, or drainage, a suitable combination of different pumping systems
with an optimized storage capacity should be installed. For small pump
capacities up to approx. 10 m3/day, systems such as hand and foot pumps,
capstans and, with certain limitations, solar pumps may be considered in
addition to wind pumps where the water requirement is greater, motor pumps
(diesel or electric) become competitive.
The question as to which combination of possible systems is the right one,
i.e., the one which is most economical and best adapted to local conditions,
depends on a variety of physical, socio-economic and sociocultural conditions
which can differ considerably from one region to another. All of these
conditions, which are not dealt with in more detail here for reasons of space,
are of vital importance in the planning of rural water supply systems. Failures
of projects for the introduction of wind pumps can, without exception, be
traced back to the non-observance of one or more of these conditions or
prerequisites.
Thus, for example, a combination of wind and hand pumps can be the right
solution for providing a drinking water supply for a settlement, always
provided that there is a sufficient amount of wind available. In the case of a small-scale
irrigation system with wind pumps, a small, transportable diesel pump which can
be used by several farmers is more suitable as a back-up system.
Other factors which have proved to be essential for dissemination on a
larger scale are the existence and financial and technical capability of
potential operators as well as the availability of marketing and service
facilities in the area.
Today there are several water-pumping windmills on the market. They are
designed to pump water in wind speeds as low as 2 m/s to 4 m/s from depths
reaching 1000 meters. Typical water pumping windmill with a 3-m rotor can draw
up to 2000 liters per hour from a depth of 10 meters at a wind speed of 3 m/s.
Windmill with a 7-m rotor, can draw up to 8000 liters per hour under the same
conditions. These systems can be used for irrigation, land reclamation or
drinking water in remote areas. Windmills are designed for easy installation
and require minimal maintenance.
IRRIGATION
The use of wind pumps for irrigation purposes seems to be problematic, since
the water requirement and the availability of wind energy were generally
subject to wide variations over the year. A good and above all constant wind
regime is required to make them a viable option. Generally speaking, an annual
average wind speed of four meters per second is a prerequisite for economic
operation.
Typical project involving wind pump for irrigation was realized in Eastern
Indonesia. This area has a short rainy season and traditional practice is for
farmers to raise one rice crop per year. Two thirds of the time,
during the dry season, the rice paddies are used only for grazing
cattle. But many areas have substantial ground water resources which can be
used for irrigation. In one project they dig wells, installed pumps, and
trained the local farmers to use irrigation to raise higher value crops
year-round. In most cases small 5 horsepower kerosene pumps are used for
irrigation. These pumps are inexpensive and the fuel costs are partially subsidized
by the government. But they also only last a few years and they operate at poor
efficiency, so their life-cycle costs are quite high. Small wind systems cost
more initially, but they have lower life-cycle costs. Project in Oesao, where
the water table is only 2-5 meters below ground level, was based on use of the
wind turbine which drives a surface mounted centrifugal pump. Pump is operated
at variable voltage and frequency and its speed varies with the rotor speed of
the wind turbine. The peak flow rate is ~3 liters/second. The system requires
no fuel and no regular maintenance. A kerosene pump is, however, used for
back-up. The Oesao system was installed in 1992 as a pilot project to show that
wind power could be effective for water pumping in Eastern Indonesia. Since that
time fifteen additional systems have been installed and more systems are
planned.
TELECOMMUNICATION
Wind power is an excellent source of power for telecommunications sites
because the height and exposure that make for a good antenna site also make for
a good wind energy site. But wind turbines for this application must be
particularly rugged because of the harsh conditions often encountered on
mountains.
BATTERY CHARGING
Utilization of small wind turbines for lighting, TV or refrigeration is
quite simple through battery charging. Storing wind produced electricity in
battery gives a homeowner a possibility to use this power whenever it is
needed. Many small wind turbines directly produce 14 or 28 V
. Some smaller wind turbines and other larger types produce higher
voltages. 12 V o 24 V output from the battery can be used directly for DC
appliances or inverted to 240 VAV current. For standard
domestic appliances. It is usually best to directly charge the battery
from the wind as this will not load the wind turbine at low speed causing
stalling of the rotor.
HEAT STORAGE
If there is a need for hot water it is
better to use direct wind generated electricity via an immersion heater to
standard hot water tank and store the hot water. Battery storage is always more
expensive than heat storage. The simplest system for water heating uses a
thermostat to protect the water from boiling. The immersion heater should match
the wind turbine rating. If a 1 kW turbine is used the immersion heater should
also be rated at 1 kW (most domestic immersion heaters are 3 kW).
Wind - Solar Hybrid Systems
Solar and wind energy are complementing each other well under average
seasonal conditions. In winter, when there is much wind, room heating is needed
while in summer with much sun domestic hot water is needed. The combination of
solar-wind is very interesting in the so-called off-grid electricity systems.
These are self-supplying plants which are not coupled to the public electricity
grid. A photovoltaic plant has a relatively high production in summer and a
relatively small production in winter. This means that an off-grid system will
either result in a heavy over-production in summer or should be equipped with a
seasonal storage. Both solutions will be very expensive. A wind power supply
can have serious problems in summer when periods with no wind may occur. The
combination of solar-wind is therefore evident.
The important question, what the proportion between the solar and wind plant
should be, have to be answered by the planner of the facility. It is obvious
that the answer depends on energy needs during the year and a site conditions.
ENVIRONMENTAL IMPACTS OF WIND
POWER
In many part of the world, there is such a dearth of electricity generation
that the public welcomes wind turbines with open arms. Where there are
alternative choices, however, environmental impact is of major significance for
development. Note that impacts may be judged as either beneficial or harmful.
The impacts of wind turbines and the factors influencing these are:
ACOUSTICS
Noise is mostly generated from blade tips (high frequencies), from blades
passing towers and perturbing the wind (low frequencies) and from machinery,
especially gearboxes. Since noise is essentially a sign of inefficiency and
because of complaints, manufacturers have reduced noise-generation intensities
greatly over the last five years. The critical noise intensity is usually
considered to be 40 dBA, or less, as judged necessary for sleeping. This level
of acceptance is usually attained at distances of about 250 m or less. However,
attitudes to noise are strongly psychological; the owner of a machine probably
welcomes the noise as a sign of prosperity; whilst neighbors may be irritated
by intrusion into “their space”.
LAND AREA AND USE
Turbines should be separated by at least five to ten tower heights; this
allows the wind strength to reform and the air turbulence created by one rotor
not to harm another turbine downwind. Consequently, only about 1 % of land area
is taken out of use by the towers and the access tracks. The
taller and larger the turbines, the greater the separation. Megawatt
machines should be spaced between half and one kilometer apart. Neither
buildings nor commercial forestry can be established between, so the land is
thereafter safeguarded against such development and can be used for
agriculture, leisure or natural ecology.
VISUAL IMPACT
Wind turbines are always visible from places in clear line of sight. The larger the machines, the greater the distance between them.
The need for a long fetch of undisturbed wind, and the economic bias to large
machines, means that machines will potentially be visible from distances of
tens of kilometers. However, at such distances, the majority of the public will
have their view obscured by hills, trees, buildings etc. The most likely people
to notice the machines on land are walkers and pilots. For the former, beauty
is in the eye of the beholder, and for the latter there is danger for
exceptionally low flying. For offshore machines, visual impact is largely, as
yet, unassessed.
BIRD STRIKE
Birds often collide with high voltage overhead lines, masts, poles, and
windows of buildings. They are also killed by cars in the traffic. Birds are
seldom bothered by wind turbines. Radar studies from Tjaereborg in the western
part of Denmark, where a 2 megawatt wind turbine with 60 meter rotor diameter
is installed, show that birds - by day or night - tend to change their flight
route some 100-200 meters before the turbine and pass above the turbine at a
safe distance. In Denmark there are several examples of birds (falcons) nesting
in cages mounted on wind turbine towers. The only known site with major bird
collision problems is located in the Altamont Pass in California. A "wind
wall" of turbines on lattice towers is literally closing off the pass.
There, a few bird kills from collisions have been reported. A study from the
Danish Ministry of the Environment says that power lines, including power lines
leading to wind farms, are a much greater danger to birds than the wind
turbines themselves. Some birds get accustomed to wind turbines very quickly, others take a somewhat longer time. The
possibilities of erecting wind farms next to bird sanctuaries therefore depend
on the species in question. Migratory routes of birds will usually be taken
into account when siting wind farms. Offshore wind turbines have no significant
effect on water birds. That is the overall conclusion of a three year offshore
bird life study made at the Danish offshore wind farm Tunø Knob.
There have been many
independent studies of birds killed by rotating blades. This undoubtedly
happens, but perhaps to a similar or lower frequency than strikes by a car,
against the windows of a building or : against grid
transmission cables. Every death is regretted. The counter argument, again
attested by experts, is that land around wind turbines may provide excellent
breeding conditions. The exception to this argument is the possibility of
strikes by large migratory birds flying in the dark and by raptors intent on
their prey.
ELECTROMAGNETIC INTERFERENCE
TV, FM and radar waves are perturbed in line of sight by electrically
conducting materials. Therefore, the metallic parts of rotating blades can
produce dynamic interference in signals. It is easy, but not necessarily cheap;
to install TV and FM repeater stations to provide another direction of signal
for receivers. Radar interference is, as yet, a largely undocumented effect, of
most concern to the military. However, wind turbines are a fact of life that
has to be accepted by the military on an international scale. There are many sites
of wind turbines close to airfields, and no significant difficulties occur.
GUIDELINES FOR WIND POWER APPLICATIONS
Wind turbines have to compete with many other energy sources. It is
therefore important that they be cost effective. They need to meet any load
requirements and produce energy at a minimum cost .
When you have decided that it is time to consider buying and installing a wind
turbine you have to examine first two things: how much energy you require, and
what is the average wind speed at the height of the wind turbine. Sometimes, it
sure seems windy in your area, at least part of the time any way. But how can
you tell if a wind turbine generator will really be optimized in term of power
output versus wind speed. The common response is that you must monitor the wind
speed at your site for at least one year and compare the results with
historical data that had been recorded for some years. Or, contract a professional
who will do a ‘feasibility study’ to estimate the yearly average wind speed and
the estimated annual energy that would be captured by the wind turbine.
Usually, which way to choose depends on the amount of investment you are
willing to pay for having the wind turbine. For small
applications when the amount of investment is relatively small, it is
unrealistic to pay more than the cost of the wind turbine for obtaining the
yearly average wind speed.
Wind systems are at the
mercy of their site survey. Without an extended site survey or real wind data
for a specific location, it is really impossible to specify a wind turbine for
the system. While PV and micro hydro systems are often effectively designed by
their users, wind systems should seek help from someone who really knows wind
power. Here are some guidelines for siting and sizing small wind turbines.
SITING A TURBINE
A common way of siting wind turbines is to
place them on hills or ridges overlooking the surrounding landscape. In
particular, it is always an advantage to have as wide a view as possible in the
prevailing wind direction in the area. On hills, one may also experience that
wind speeds are higher than in the surrounding area. You may notice that the
wind can bend some time before it reaches the hill, because the high pressure
area actually extends quite some distance out in front of the hill. Also,
you may notice that the wind becomes very irregular, once it passes through the
wind turbine rotor. As before, if the hill is steep or has an uneven
surface, one may get significant amounts of turbulence, which may negate the
advantage of higher wind speeds.
DISTANCE BETWEEN OBSTACLE AND TURBINE
The distance between the obstacle and the turbine is very important for the
shelter effect. In general, the shelter effect will decrease as you move away
from the obstacle, just like a smoke plume becomes diluted as you move away
from a smokestack. In terrain with very low roughness (e.g. water surfaces) the
effect of obstacles (e.g. an island) may be measurable up to 20 km away from
the obstacle. If the turbine is closer to the obstacle than five times the
obstacle height, the results will be more uncertain, because they will depend
on the exact geometry of the obstacle.
ROUGHNESS
The roughness of the terrain between the obstacle and the wind turbine has
an important influence on how much the shelter effect is felt. Terrain with low
roughness will allow the wind passing outside the obstacle to mix more easily
in the wake behind the obstacle, so
that it makes the wind shade relatively less important. A good rule of thumb
is that we deal with individual obstacles which are closer than about 1000 meters
from the wind turbine in the prevailing wind directions. The rest we deal with
as changes in roughness classes.
OBSTACLE HEIGHT
The taller the obstacle, the larger the wind shade. If the turbine is closer
to the obstacle than five times the obstacle height, or if the obstacle is
taller than half the hub height, the results will be more uncertain, because
they will depend on the exact geometry of the obstacle. In that case the
programme will put a warning in the text box below the results.
WAKE EFFECT FROM WIND TURBINE
Since a wind turbine generates electricity
from the energy in the wind, the wind leaving the turbine must have a lower
energy content than the wind arriving in front of the turbine. This follows
directly from the fact that energy can neither be created nor consumed. A
wind turbine will always cast a wind shade in the downwind direction. In fact,
there will be a wake behind the turbine, i.e. a long trail of wind which is
quite turbulent and slowed down, when compared to the wind arriving in front of
the turbine. Wind turbines in parks are usually spaced at least three rotor
diameters from one another in order to avoid too much turbulence around the
turbines downstream. In the prevailing wind direction turbines are usually
spaced even farther apart.
TURBULENCE
Turbulence decreases the possibility of using the energy in the wind
effectively for a wind turbine. It also imposes more tear and wear on the wind turbine, as explained in the section on
fatigue loads. Towers for wind turbines are usually made tall enough to avoid
turbulence from the wind close to ground level.
AVERAGE WIND SPEED
To correctly site and size a wind turbine, it is helpful to have the
information about average wind speed for the location. The annual average wind
speed is used to describe the general windiness of a place. Shorter-term
averages (monthly, hourly) are used in more precise analyses where the time
relation between wind energy availability and energy demand is particularly
important. The time variation of wind speed at a given site is described by the
relative probability of the wind speed at any moment being greater or less than
the average wind speed. A typical distribution of wind speed (called the
Rayleigh Distribution, special case of Weibull Distribution) usually means that
there is little probability of absolutely no wind; the most frequent wind speed
is about 75% of the average wind speed; and wind speeds above twice the average
wind speed do occur, but not often.
Wind Speed Measurement
Don't consider wind power without a thorough
measurement of the wind speed at your specific location. In most cases, four
months should be the minimum recording interval and one year is preferred. If
you are going to spend a lot of money on a wind system, this extra eight months
could mean the difference between a good investment and a bad one.
The measurement of wind speeds is usually done
using a cup anemometer. The cup anemometer has a vertical axis and three cups
which capture the wind. The number of revolutions per minute is registered
electronically. Normally, the anemometer is fitted with a wind vane to detect
the wind direction. Other anemometer types include ultrasonic or laser
anemometers which detect the phase shifting of sound or coherent light
reflected from the air molecules. Hot wire anemometers detect the wind speed
through minute temperature differences between wires placed in the wind and in
the wind shade (the lee side). The advantage of the
non-mechanical anemometers may be that they are less sensitive to icing. In
practice, however, cup anemometers tend to be used everywhere, and special
models with electrically heated shafts and cups may be used in arctic areas.
Determining the exact average annual wind speed is not an easy task and it is
an expensive process. After all it might be unnecessary. For small wind
turbines applications what we need to do is get some idea of the average annual
wind speed for the area, and that can be available by observing few physical
phenomena around the site. Start by your feeling, while they are hardly
scientific, then try to check the airport and weather
station data for your area. Use these data as a raw baseline, which you have to
tune to make them represent your area.
Meteorologists already
collect wind data for weather forecasts and aviation, and that information is
often used to assess the general wind conditions for wind energy in an area.
Precision measurement of wind speeds, and thus wind energy is not nearly as
important for weather forecasting as it is for wind energy planning, however.
Wind speeds are heavily influenced by the surface roughness of the surrounding
area, of nearby obstacles (such as trees, lighthouses or other buildings), and
by the contours of the local terrain. Unless you make calculations which
compensate for the local conditions under which the meteorology measurements
were made, it is difficult to estimate wind conditions at a nearby site. In
most cases using meteorology data directly will underestimate the true wind
energy potential in an area.
It is because weather stations monitor wind speeds at or slightly above
street level, where people live. They don't monitor wind speeds at 20 - 30
meters, where the wind turbine is usually located. Similarly, airports data has
limited value. Because airplanes traditionally had problems taking off and
landing in windy locations, airports were sited in rather sheltered locations.
Virtually all airports are sheltered. After having the raw data from
nearby airport or weather station, you need to extrapolate these numbers to
your location using a concept know as shear ‘factor’. Based on these numbers
and the topographical difference or similarity between your site and theirs
(weather station and airport), you can theoretically estimate your average wind
speed at any proposed height.
Very simple anemometer
can be build by yourself. Here is the way how to
construct it. Materials needed : five paper Dixie
cups, two straight plastic soda straws, a pin scissors, paper punch, small
stapler, sharp pencil with an eraser.
Procedure: Take four of the Dixie cups. Using the paper punch, punch one
hole in each, about a half inch below the rim. Take the fifth cup. Punch four
equally spaced holes about a quarter inch below the rim. Then punch a hole in
the centre of the bottom of the cup. Take one of the four cups and push a soda
straw through the hole. Fold the end of the straw, and staple it to the side of
the cup across from the hole. Repeat this procedure for another one-hole cup
and the second straw. Now slide one cup and straw assembly through two opposite
holes in the cup with four holes. Push another one-hole cup onto the end of the
straw just pushed through the four-hole cup. Bend the straw and staple it to
the one-hole cup, making certain that the cup faces in the opposite direction
from the first cup. Repeat this procedure using the other cup and straw
assembly and the remaining one-hole cup. Align the four cups so that their open
ends face in the same direction (clockwise or counter clockwise) around the
centre cup. Push the straight pin through the two straws where they intersect.
Push the eraser end of the pencil through the bottom hole in the centre cup.
Push the pin into the end of the pencil eraser as far as it will go. Your
anemometer is ready to use. Your anemometer is useful because it rotates at the
same speed as the wind. This instrument is quite helpful in accurately
determining wind speeds because it gives a direct measure of the speed of the
wind. To find the wind speed, determine the number of revolutions per minute.
Next calculate the circumference of the circle (in feet) made by the rotating
paper cups. Multiply the revolutions per minute by the circumference of the
circle (in feet per revolution), and you will have the velocity of the wind in
feet per minute. The anemometer is an example of a vertical-axis wind
collector. It need not be pointed into the wind to spin.
FLAGGING
Another useful tool to help determine the potential
of a wind site is to observe the area's vegetation. Trees, especially conifers
or evergreens, are often influenced by winds. Strong winds can permanently
deform the trees. This deformity in trees is known as flagging. Flagging is
usually more pronounced for single, isolated trees with some height. On the upwind
side of the tree, the branches are noticeably stunted. On the downwind side, they're
long and horizontal. The flagging was caused by persistent winds from, more or
less, one direction. Look around especially for single trees, or trees on the
outskirts of a grove. Unless they have grown considerably above the common tree
line, trees in a forest will not show flagging because the collective body of
trees tends to reduce the wind speed over the area. While the presence of
flagging positively indicates a wind resource, you should not conclude that the
absence of flagging in your area precludes any suitable average wind speeds.
Other factors that you are not aware of may be affecting the interaction of the
wind with the trees.
For very rough estimate of the
average wind speed Griggs-Putman Index of Deformity can be used.
VARIATION OF WIND SPEED
While average wind speed is meaningful, there are other wind parameters that
are just as meaningful. Other wind parameters worth knowing are maximum wind
speed, number of days (hours) between winds of greater than 5m/s. Number of
consecutive days (hours) where the wind is in excess of 5 m/s, and the times of
year where the either wind or not wind periods occur. The wind speed is always
fluctuating, and thus the energy content of the wind is always changing.
Exactly how large the variation is depends both on the weather and on local
surface conditions and obstacles. Energy output from a wind turbine will vary
as the wind varies, although the most rapid variations will to some extent be
compensated for by the inertia of the wind turbine rotor.
All important data is
not available from garden variety recording anemometers. A recording anemometer
that will take all the data mentioned above will cost much. Such anemometers
are more computer than wind sensor and cost between USD 2,000 and USD 4,000.
SIZING A SMALL TURBINE
This is a job for someone with experience
with all types of wind turbines. Not only must the wind turbine be well made,
but it also must fit the wind conditions at your particular site and must
produce the power that the system requires. Modern turbines usually produce
some specie of low voltage and only the very large units make 60 cycle, 120/240
VAC directly.
When choosing a turbine the rated power for a wind turbine is not a good
basis for comparing one product to the next. This is because manufacturers are
free to pick the wind speed at which they rate their turbines. If the rated
wind speeds are not the same then comparing the two products is very
misleading. Usually manufacturers will give information on the annual energy
output at various annual average wind speeds. These figures allow you to compare
products fairly, but they don't tell you just what your actual performance will
be.
TOWER
The power in the wind is a function of
(among other things) the cube of the wind speed. Therefore, the easiest way to
increase the power available to a wind generator is to increase the wind speed.
We can increase wind speed by either installing a taller tower or by moving to
a windier location. Note that as a percentage, wind speed increases much faster
over terrain cluttered with trees and buildings than over flat open ground.
With the exception of the middle of a lake or desert, wind speed increases significantly
with height. For example, power available at 30 meters can be up to 100% higher
than power available at 10 meters. Said another way, two wind generators on two
10 meters towers will produce as much power as one wind generator on a 30 meter
tower. And the system with the 30 meters tower will be cheaper to install than
the “twin” systems at 10 meters. The rule of thumb for siting is that the wind
generator must be at least 10 meters above any obstacle within 100 meters.
Consider 15 meters to be a realistic minimum and after that, go as high as you
can. Smaller turbines typically go on shorter towers than larger turbines. A
250 watt turbine is often, for example, installed on a 15-20 meter tower, while
a 10 kW turbine will usually need a tower of 20-30 meter. A wind turbine must
have a solid tower to perform efficiently. Turbulence, which is highest close
to the ground and diminishes with height, reduces the
performance of the turbine.
For small wind mills the least expensive tower type is the guyed-lattice
tower, such as those commonly used for ham radio antennas. Smaller guyed towers
are sometimes constructed with tubular sections or pipe. Self-supporting
towers, either lattice or tubular in construction, take up less room and are
more attractive but they are also more expensive. Telephone poles can be used
for smaller wind turbines. Towers, particularly guyed towers, can be hinged at
their base and suitably equipped to allow them to be tilted up or down using a
winch or vehicle. This allows all work to be done at ground level. Some towers
and turbines can be easily erected by the purchaser, while others are best left
to trained professionals. Anti-fall devices, consisting of a wire with a
latching runner, are available and are highly recommended for any tower that
will be climbed. Aluminium towers should be avoided because they are prone to
developing cracks. Towers are usually offered by wind turbine manufacturers and
purchasing one from them is the best way to ensure proper compatibility. Be
sure that the tower is strong and well installed. Sloppy tower installation can
bring the whole system crashing down. Guyed towers are more secure and less
expensive than unguided towers.
Choosing a wind controller
In almost every case, the manufacturer of
the wind machine also makes a regulator for that specific model. So, the user doesn't
have to select a regulator because it is bundled in with the wind machine.
These controls are shunt types that divert the turbine's output to maintain
control of the system's voltage. Diversion regulator schemes are really the
only type used, because unloading the wind machine will cause over speeding and
damage to the turbine.
Sizing the Wind system's battery
The size of a wind system battery storage is
determined by the longest period of windless weather. This can be very
difficult to determine in advance. For this reason wind systems usually have
more days of battery storage than do PV systems. Shoot for a minimum of seven
days of storage and extend this to fourteen days if you can afford it. Wind
power comes in gusts and spurts, having a large battery makes
more effective use of nature's least consistent power source.