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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-destabilising 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 civilisation
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 and the largest in the world, 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.
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 15000 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 1986and 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 organised 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.
CURRENT DEVELOPMENT
Windpower has retained its status as the fastest growing energy
source in the world. Installed wind energy capacity in Europe has reached
20,447 MW in autumn 2002, accounting for 74% of the global total. Germany
has commissioned 1,896 MW in the first nine months of 2002, with Spain
in second place with 742 MW. Hundreds more megawatts of energy capacity
are scheduled to be built in France next year, encouraged by a new tariff
system. 84 per cent of European wind energy capacity is installed in Germany,
Spain and Denmark. Wind energy now accounts for 4 per cent of national
electricity consumption in Germany, and 18 per cent in Denmark. European
success for wind energy development is just the beginning; within eight
years, the total amount of wind power installed globally can more than
ten times that achieved in Europe today, if the appropriate policies are
put in place.
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Autumn 2002 |
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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
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. Wind Force
12 outlines that by 2010 the industry is
capable of installing 230,000MW of wind energy worldwide, 100,000MW
in Europe. By 2010 the global wind power market could be worth a cumulative
€133 billion. The 20,000MW represents a total cumulative investment
of around €20 billion.
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.
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Wind Force 12, by 2020 the wind industry can deliver:
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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
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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 metres. 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 metre, 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).
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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 kilometre, 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 utilisation 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 neighbourhood. 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 metre tall sheltering
hedgerows with a distance of approx. 1250 metres
2 = Agricultural land with some houses and 8 metre tall sheltering
hedgerows with a distance of approx. 500 metres
2.5 = Agricultural land with many houses, shrubs and plants,
or 8 metre tall sheltering hedgerows with a distance of approx. 250 metres
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 describe 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 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)
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.
Blades,
Rotor,
Transmission,
Generator,
Controls.
Brakes
Gearbox
Generator
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.
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 windfarms. 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 “windfarms” that were being constructed in windy passes in California.
In a windfarm 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 windfarms 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, windfarms 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 windfarm 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.
Offshore
windfam in the Netherlands.
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 kilometres 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.
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
kilometre 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 favourable 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.
 |
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:
for pumping
water and producing compressed air;
for generating
electricity (and thus also for developing technologies which depend on
electricity);
for powering
mechanical devices. |
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.
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 realised
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
subsidised 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 litres/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.
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 neighbours 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 kilometre 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 kilometres. 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.
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 optimised 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 microhydro 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 metres from the wind turbine in the prevailing wind directions.
The rest we deal with as changes in roughness classes.
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.
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 overspeeding 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.
TOP
12% of global electricity
demand, assuming that global demand doubles by 2020.