Power Quality and Utilisation Guide

Section 8 – Distributed Generation

Wind Power∗
Ton van de Wekken, Fred Wien, KEMA Consulting

Autumn 2006

Introduction

What is Wind Power?

General
Wind turbines make a major contribution to the production of renewable energy. When
the oil crisis occurred in the 1970s in Europe, the development and commercial production
of wind turbines for generating electricity was strongly stimulated. Developments in
harnessing wind power continually improved and during the last decennia a considerable
scaling up has taken place in the wind power industry. Turbines have become larger,
efficiencies and availabilities have improved and wind farms have become bigger.
The consumption of electricity keeps growing on a worldwide basis. Most European
governments have set targets to reduce the emission of carbon dioxide in order to stop
the Earth from warming up further. The widely accepted opinion is that these targets
∗ c
°European Copper Institute and KEMA Consulting. Reproduction is authorised provided the ma-
terial is unabridged and the source is acknowledged.

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can only be met on the one hand by energy-saving incentives and on the other hand by
the large scale application of renewable energy.
The use of wind turbines is a serious alternative for achieving these aims. Several Eu-
ropean countries have impressive plans for the coming years for installing large amounts
of wind power generation. Some governments support these actions by providing tax or
investment incentives. The northwest of Europe with its coastal windy waters and fine-
meshed but strong electric grid give promising opportunities for investing companies and
wind farm developers.
Basic principle
Wind turbines extract the energy from the wind by transferring the thrusting force of the
air passing through the turbine rotor into the rotor blades. The rotor blades are aerofoils
that act similarly to an aircraft wing; this is the so-called principle of lift. This can be
seen in the cross-section of a rotor wing in figure 1.

wind speed resulting wind speed

rotational
rotor direction

rotational speed of the rotor

wind direction

Figure 1: Cross-section of a rotor wing showing speeds and directions

As an effect of the resulting air flow, the windward side of the aerodynamic profile is over-
pressured while the leeward side is under-pressured. This differential pressure creates
a thrust force. This lifting force is perpendicular to the direction of the resulting force
(resulting wind speed) reacted by the flowing wind towards the turbine wing and the local
rotational speed of the wing. As a result, the lifting force is converted into a mechanical
torque. The torque makes the shaft, as part of the turbine rotor, turn. The power in the
shaft can be used in different ways. For hundreds of years it was used for the grinding
of wheat or pumping water but the large machines of today, with integrated generators,
convert the shaft power into electricity.

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Power and efficiency rates

Mass in motion carries a certain amount of energy. This kinetic energy varies in proportion
to the product of the mass and the square of the velocity. In units of time, this energy is
similar to the power.
Kinetic energy per second is:

P = 1/2 · m · V 2 (1)
P power (Nm/s or Watt)
m air mass per second (kg/s)
V wind velocity (m/s)
This physical law is also applicable to air in motion. The mass of air flowing through
the rotor has to be imagined as a cylindrical disc. The volume of the disc is equal to the
surface area of the rotor and the length of the disc is equal to the wind speed.
Air mass through the turbine rotor per second is:

m=ρ·A·V (2)
ρ density of air (kg/m3)
A rotor surface area (m2)
V wind velocity (m/s)
This leads to an important characteristic; the amount of energy is similar to the speed of
wind raised to the third power.
Both formulas combined make the theoretically power:

P = Cp · 1/2 · ρ · A · V 3 (3)
Cp mechanical power coefficient (at slow shaft)
As an example:
At a wind speed of 6 m/s the energy content is 132 W/m2 (Watt per square meter). When
the wind is blowing at a speed of 12 m/s, the energy content is 1053 W/m2 . Summarized:
twice as much wind speed gives eight times as much power.

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Not all the energy present in the wind can be converted into usable energy at the rotor
shaft. Using physical calculations it can be proven that the theoretical maximum efficiency
of wind power is limited at about 59
The net electrical power output of a turbine can be determined when mechanical and
electrical performance rates are also taken into account.
Net electrical power output:

Pelec = Ce · 1/2 · ρ · A · V 3 (4)

Ce electrical efficiency rate (%)
Today, large modern turbines are able to achieve a total net efficiency Ce of 42 to 46%
with respect to the energy of the undisturbed wind in a circular tube with a cross-sectional
area equal to the gross rotor area.
Basic comparison with conventional electricity production and benefits of wind
power
There are several reasons for the success of wind power over recent years. When compared
to conventional electricity production, wind turbines produce clean energy. In operation
there is no carbon dioxide emission or other air, water or soil pollution.
Other advantages of ”Wind power fuel” are that it is free, it is inexhaustible and it is
abundant. Wind turbines have a modular concept (reference to Section 8.4.2 ”Standards”
of this application note), are quick to install and are also independent of the importation
of fuels and fluctuations in fuel prices caused by oil politics. Looking at the reliability
of a modern wind turbine, the operating availability (the proportion of time in which a
windmill is available to operate) is about 98%. No other electricity generating technology
has a higher availability rate.
A disadvantage of wind power is the unpredictability of wind. Storm fronts in particular
can cause a sudden increase in the wind power. Furthermore, periods of low wind give
little wind power.
The expansion of a grid with power produced by wind turbines is not as obvious as
it seems. A certain percentage of the total generated power still needs to be provided
by (central) ”stable” conventional plants. This percentage depends on the composition
and stability of the grid. If the instability of a grid is foreseen, it can be prevented
by using intelligent control systems that interface between different kind of production
units, consumers and the intermediate grid. In many EU countries, grid companies or
(independent) associations are carrying out research into this.

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Application of wind power

Description of typical situations in which wind power can be/is suitable

The amount of electrical power produced by a turbine depends on the size and type of
the turbine and where the turbine is located. A characteristic that represents a typical
power output in relation to the wind speed is given in figure 2. At low wind speeds, no
electrical power is generated. From 2 Beaufort (approx. 3 m/s) and above the turbine is
rotating, and at about a wind force 6 Beaufort (12 - 13 m/s) the turbine is supplying its
maximum power.
At wind speeds over 25 m/s former generations wind turbines were designed to shut
down in a controlled way to avoid overloading or damaging the turbine’s installation or
construction. Modern turbines however are equipped with a pitch control that changes
the angle of the rotor wing in extreme weather. The result is that the power supply can
be guaranteed even in bad weather conditions. When very heavy storms occur it still is
necessary to stop the turbine.

power in the wind maximum theoretical usable power

specific
power

100%
real power curves
75%

50%

stall controlled
25% pitch controlled

0
0 5 10 15 20 25 30
Undisturbed wind speed (m/s)

Figure 2: Typical turbine characteristic; power output in relation to wind speed

An average turbine in an ideal location can deliver an electrical power output on an annual
basis of about 850 kWh per square meter of rotor surface area. Another simple rule for
estimating the annual energy yield of a wind turbine is that on an average wind site the
output is about 2000 full load hours and at high wind sites approximately 3000 hours.

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For example, on average a 1.5 MW wind turbine produces 3.106 MWh, consisting of 2000
hours at 1.5 MW.
Costs of wind energy
Without taking into account tax benefits or other incentives on production of wind energy,
the costs of wind energy are summarized in Table 1.

Wind Energy cost breakdown 2000 full load hours 2500 full load hours
[EUR/MWh] [EUR/MWh]
Investment
(12 year annuity at 4%) 40 to 50 30 to 40
Operation and maintenance
including major overhauls 12 12
Other operational expenses 8 8
Total 60 to 70 50 to 60

Table 1: Summary of wind energy cost breakdown

Included in other operational expenses are: daily management, insurance, land lease,
compensation for visual or noise nuisance and taxes.
Currently, the costs of wind energy are slightly higher than the feed-in tariff for electricity
produced from conventional fossil fuels or by nuclear power plants.
However, most European countries have incentives to stimulate the production of renew-
able energy, including wind energy. Although each country or state applies its own rules,
common features are:
• Tax benefits on investments in new renewable energy (wind) assets;
• Grants on installing new renewable energy (wind) assets;
• Lower interest rates from green funds for financing renewable energy (wind) assets;
• Incentives on the production (kWh) of renewable energy (wind).
As a consequence of one or more incentives, investments in wind energy can be profitable.
In the past, tax benefits up to 50% of the investment costs were not uncommon. In the
case of a feed-in tariff, including incentives, of 80 to 100 e/MWh, the cost recovery period
is between 6 (> 2700 full load hours) and 10 (> 1900 full load hours) years.
Site selection wind energy
Next to issues such as sufficient space for an envisaged wind farm site, accessibility by
heavy equipment such as cranes, limited nuisance to the neighborhood and the presence

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of a medium voltage substation with sufficient capacity, the most important issue is the
presence of sufficient wind.
As a first guide, investors and developers may use the European Wind Atlas [2] to estimate
the long-term wind speed. A second source is wind data from meteorological stations
located up to 30 to 40 km from the site. In case of wind farms installed in the surrounding,
say less than 5 to 10 km, these production data may also serve as a good starting point.
More in detail the site wind speed and subsequently the envisaged wind farm output can
be predicted with the WAsP software tool [3]. WAsP needs the long-term wind speed
distribution of at least three surrounding meteorological stations. The accuracy of the
results increases when the met stations are located close to the potential site. Subsequently
the envisaged site and its surroundings, more precisely the surface roughness, are modeled
as accurately as possible. The output is the long-term wind climate at site.
In case of doubt, certainly for complex terrain like hilly and mountainous areas, additional
wind measurements are required. The measurement period has to be at least a year and
if possible extended to two years.
Project risks
The main project risk is that the long-term wind climate at the potential site is lower than
anticipated during the feasibility phase. As a result of the cubic law relation between the
wind speed and the power, a relatively small decrease in the long-term wind speed may
have a large effect on the energy output. A significant lower energy yield, i.e. more than
10 to 15%, may result in cost recovery times of more than 12 to 15 years instead of less
than 10 as estimated. The result is a loss-making project.
It is therefore advisable to use a somewhat lower mean wind speed in the financial and
economic calculations. Instead of using a wind speed with a 50% probability of being
exceeded, use a lower value with an 80 or 90% probability of being exceeded. By doing
this, in 8 to 9 out of 10 years one is assured of a wind speed, and therefore an energy
output, higher than estimated.
The following items have to be considered when building a wind turbine:
• There must be sufficient space and plenty of wind. Nearby the applicable area many
deflections can occur. These deflections can, for example, be caused by hill slopes
or obstacles
• The territory must have a permit to operate a wind farm. In practice, this means
that mostly territories that are marked as industrial areas are to be considered.
Otherwise it is necessary to change the local zoning scheme
• The territory must be accessible. During the erection of the wind turbine it is
necessary for large hoisting cranes to be able to reach the construction site

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• It has to be possible to connect the wind turbine to the electrical grid. The voltage
level can be from 10 to 30 kV when connected to the local distribution grid. In the
case of a wind farm, where the generated power is much larger, it is necessary to
connect at the voltage level of a transmission grid. For more details, refer to Section
8.3.5 ”Integration and interconnection”.
Wind turbine power control
The previous chapter demonstrated that the power increases with wind speed according to
a cubic law. Most wind turbines reach maximum power, also called the rated or nominal
power, at wind speeds between 12 and 14 m/s. At higher wind speeds, the power has
to be kept constant in order not to overload the wind turbine structure or the electrical
connection.
Wind turbine technology applies the following methods to control the power above the
rated wind speed (see also figure 2):
1. Stall controlled rotors
The rotor is kept at a constant speed and the mostly asynchronous generator is
connected to the 50 or 60 Hz public grid without the use of a converter or other
power electronics. Power control is based on the aerodynamic principle that if
the flow angle-of-attack reaches a certain limit (stall point), the lifting force and
subsequently the rotor torque stabilizes or even decreases in magnitude. The main
advantage of this concept is its simplicity; no mechanical or electronic systems are
required to limit the power because this is a completely passive system. In early
days of modern wind technology, the 80s and 90s of the last century, stalling was the
most widely used power control system. The Danish wind turbine manufacturers
in particular gained extensive experience in this control principle. Currently stall
control is not often applied any longer. The main reason is that when stall control
is applied to a wind turbine greater than 1 to 1.5 MW, this may lead to resonance
problems in the rotor blades and drive train. Another disadvantage is the relatively
poor power quality obtained from stall wind turbines.
2. Variable speed rotors
Although this concept was already known and also applied on limited scale in the
80s and 90s, this control mechanism has been developed further and used widely
since the end of the 90s. The rotor speed is variable and increases in proportion
to the wind speed. At the rotor speed producing the nominal power; the power
is kept constant by pitching the blades towards the wind. By pitching the blades
into the wind direction, the angle-of-attack is lowered and the lifting force and
rotor torque are reduced. The synchronous generator is connected to the grid using
a converter or other power electronics that can deal with alternating frequencies.
The advantage of this control mechanism is that it can be applied to MW wind

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turbines without introducing undesirable mechanical resonances. Applying blade

pitch together with up-to-date control techniques may lower design loads and may
serve as a good starting point for further up-scaling activities. Last but not least,
modern, IGBT or IGCT-based, converter technology improves the quality of the
generated power.
3. Intermediate power control solutions
In the past two decades, several power control methods have been introduced that
are based in one way or another on the above-mentioned control mechanism. A
power control mechanism applied in the 90s by a limited number of manufacturers
is the so-called ”active stall” control. This method combines stall control including
constant rotor speed with blade pitch to optimize the stall characteristics. An-
other variation is the combination of stall/constant speed and power electronics to
optimize the power quality.
More details are given in reference [1].
Wind power applications & opportunities in different sectors
The owner or operator of a wind turbine mostly sells the electricity produced to the utility
company. Various systems of price rates have been determined in the EU to structure the
trade in electrical power. Owners or operators can be:
• Initiatives from private persons or companies. They finance wind projects with own
money or loan capital. Many taxation regulations are applicable for companies
• Cooperatives; private persons create a legal structure to set up a wind turbine or
wind farm together. Shareholders participate in profits according to the efficiency
of the turbine
• Utility companies; as turbines grew in capacity, more utilities became interested in
the wind industry. The companies are particularly interested in large wind farms
and will probably participate in the development of new offshore wind farms.
Current status of wind power
The manufacture of commercial wind turbines started in the 1980s, with Danish technol-
ogy leading the way. From units of 40-60 kW with rotor diameters of around 10 m, wind
turbine generators have increased in capacities over 5 MW and rotor diameters of 120 m
and above.
Continual improvements are being made in the ability of wind turbines to capture as
much energy as possible from the wind. One result is that employment in the European
wind energy sector has been growing rapidly. In Denmark, for example, the number of
people employed both directly and indirectly in wind turbine manufacture increased from
about 2,900 in 1991 to 21,000 by 2002.

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Estimations based up on the EWEA scenario ”Wind Force 12”, employment in Europe
could reach almost 200,000 by 2020, with double that number for global employment.
Other facts of wind power worldwide and in Europe are:
• By the end of 2005, worldwide 60,000 MW wind power capacity had been installed
• Over the last few years, the worldwide yearly increase has been approximately 25%,
worldwide in 2004 7,500 MW and in 2005 11,600 MW wind power capacity has been
installed
• The majority, 60 to 704
• It is estimated that 15,000 MW of wind power will be installed worldwide in 2006
• Outside Europe, most wind power is installed in USA and the last years China and
India are rapidly increasing wind power capacity
• Wind energy has grown most consistently in Europe, with capacity multiplied by
27 times over the decade between 1992 and 2002
• The leading nations in wind energy are Germany, Spain, Denmark and the Nether-
lands, who account for 84% of the total European wind capacity. Emerging markets
include Austria, Italy, Portugal, Sweden and the UK. All ten Member States that
joined the EU in May 2004 have also adopted targets for the level of renewable
energy they are expected to achieve
• In Germany last year, the wind industry turnover was e 4.2 billion

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WIND POWER INSTALLED IN EUROPE

BY END OF 2005 (CUMULATIVE)
EU - 40,504 MW
ACCESSION COUNTRIES - 28 MW
EFTA COUNTRIES - 279 MW Norway
Finland
82
267
Sweden
Iceland
500
0
Estonia
Faroe Islands
4 30
Latvia
Rep. Of Ireland Denmark 26
495.5 3.122 Lithuania
UK
1,353 Netherlands 7
1,219 Poland
Germany
Belgium
73
18,428
167
Luxembourg CzechRepublic Ukraine
35 26 82
Slovakia
France
757 Austria
5
Switzerland
819
Hungary
11.6 Romania
Slovenia
17
1.4
Portugal 0 Croatia
1,022 6
Spain Italy Bulgaria
10,027 1,717 1

Turkey
20
Greece
Source: EWEA(www.ewea.org) 573

Malta
Cyprus
0
0

Figure 3: Map of Europe with installed amount of wind power per EU-member in MW

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Wind turbine manufacturers

Worldwide, the market for wind turbine manufacturing and installation is dominated by
a limited number of manufacturers. The leading ten manufacturers have a worldwide
market share of 95% in wind turbine production and installation, see figure 4.

Market share worldwide 2005

Ecotecnia (ES) Mitsubishi (JP)
2.1% 2.0%
Nordex (GE) Others
Repower (GE)
2.6% 5.0%
3.1% Vestas (DK)
27.9%
Siemens (DK)
5.5%
Suzlon (IN)
6.1%

Gamesa (ES)
12.9% GE Wind (US)
Enercon (GE) 17.7%
13.2%

Figure 4: Worldwide market share of wind turbine manufacturers (production and instal-
lation).

Market leader is Vestas Wind Systems (VWS) with a market share of almost 30%. Since
approximately 2000 there is growing tendency for large multi-national operating compa-
nies in power generation equipment and plants to expand their activities towards wind
energy. Examples are Mitsubishi, GE and Siemens with the takeover of Bonus from
Denmark.
Trends
In perception of economical and technological aspects, three major trends can be recog-
nized in recent years concerning grid connected wind turbines:
• Turbines have grown larger and taller
The average capacity of turbines installed in Germany and Denmark increased from
approximately 200 kW in 1990 to almost 1.5 MW during 2002. Turbines in the 1.5
to 2.5 MW range have more than doubled their share of the global market from
16.9% in 2001 to 35.3
• Investment costs have decreased
The average cost per kW of installed wind power capacity currently varies from

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900 e/kW to 1 200 e/kW. The turbine itself comprises about 80% of this total
cost. Foundations, electrical installation and grid connection mainly account for
the remainder of the cost. Other costs are land, road construction, consultancy and
financing costs.
• Turbine efficiency has increased
A mixture of taller turbines, improved components and better siting has resulted in
an overall efficiency increase of 2 - 3
In addition to the previously mentioned trends, there is also the fact that offshore wind
farms have become larger in number and size. In the beginning, offshore turbines were
”sea-adjusted” versions of land-based technology, with extra protection against sea salt
incursion. Present generations include more substantial changes; such as higher peripheral
rotor speeds and built-in handling equipment for maintenance work. The turbines must be
firmly positioned on the sea bed based on a precise design. Many kilometers of cables have
to be laid both between individual turbines and back to shore to feed the generated power
into the grid. To ensure a high reliability of wind turbines, it is of great importance to
perform effective maintenance on turbines. This requires service vessels that can transport
maintenance crew in extreme weather to the turbine platforms.
By the end of 2003, a total of almost 600 MW of offshore wind farms had been constructed
around Europe in the coastal waters of Denmark, Sweden, the Netherlands and the UK.

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Wind Turbine Technology

The technology of modern wind turbines has developed rapidly during the last two decen-
nia. The basic principle of a wind turbine has remained almost unchanged and consists
of two conversion processes. These processes are performed by its main components:
• The rotor that extracts kinetic energy from the wind and converts it into generator
torque
• The generator that converts this torque into electricity and feeds it into the grid.
Although this sounds rather straightforward, a wind turbine is a complex system in which
knowledge from the areas of aerodynamics and mechanical, electrical, and control engi-
neering is applied.
Rotor and wings
A modern wind turbine consists of two, preferably three wings or blades. The blades are
made of polyester strengthened with glass fiber or carbon fibers. On a commercial basis,
wings are available from 1 meter up to 100 meter and more. The wings are mounted on
a steel construction, called the hub. As mentioned, some wings are adjustable by pitch
control.
Nacelle
The nacelle can be considered to be the machine room of the turbine. This housing is
constructed in such a way that it can revolve on its (steel) tower to face the rotor into the
right wind direction. This facing into the wind is controlled by a fully automatic system
and is set by a pennant on the turbine housing. The machine room is accessible from the
tower and contains all the main components such as the main shaft with bearing, gearbox,
generator, brakes and revolving system. The main shaft transfers the rotor torque to the
gearbox.

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pennant + wind meter

gearbox
main
bearing generator

control cabinet

revolving motor

Figure 5: Cross-section of a wind turbine nacelle

Gearbox
A gearbox increases the number of revolutions from the shaft to the desired number of
revolutions of the generator. A turbine with a capacity of 1.0 MW and a rotor diameter
of 52 m turns at about 20 revolutions per minute and the generator at 1 500 rpm. The
necessary transmission rate will be: 1 500 divided by 20 is 75.
Generator
Currently, there are three main wind turbine types available. The main differences be-
tween these concepts concern the generator and the way in which the aerodynamic ef-
ficiency of the rotor is limited during wind speeds above the nominal value in order to
prevent overloading. As for the generator, nearly all wind turbines currently installed use
either one of the following systems (see figure 6):
• Squirrel-cage induction generator
• Double fed (wound rotor) induction generator
• Direct-drive synchronous generator.
An asynchronous squirrel cage generator is a wind turbine with the first generating sys-
tem. Because of the great difference between the rotation speed of the turbine and the
generator, a gearbox is used to couple them. The stator windings are connected to the

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grid. This concept is called a constant speed wind turbine, although the squirrel cage
induction generator allows small variations in rotor speed (approximately 1%). A squirrel
cage generator does consume reactive power from the grid. This is not a desirable situa-
tion, especially in a weak grid. For this reason, the generator’s need for reactive power is
compensated for with capacitors.
The other two generating systems allow for about a factor of 2 between the minimum and
maximum rotor speed. These different speed levels are intercepted by the decoupling of
grid frequency and rotor frequency. For this, decoupling power electronics is used.

rotor rotor doubly fed rotor direct drive

squirrel cage (wound rotor) synchronous
induction generator grid induction generator grid generator grid

gearbox gearbox converter

converter

compensating
capacitors

Figure 6: From left to right, commonly applied generating systems in wind turbines:
squirrel cage induction generator, double-fed (wound rotor) induction generator, direct-
drive synchronous generator

The first variable speed concept is based on the double-fed induction generator. Through
the power electronics, a current is injected in the rotor windings of the generator. The
stator windings of the generator are directly connected to the grid. The frequency of the
current injected into the rotor windings is variable so electrical and mechanical frequencies
are decoupled. By doing so, operation with variable speeds is possible. A gearbox adapts
the two different speeds of rotor and generator.
A direct drive synchronous generator is used in the second variable speed concept. The
additive ”direct drive” refers to the fact that these turbines don’t have a gearbox. Gen-
erator and grid are fully decoupled by power electronics. In this configuration, variable
speed operation is also possible. For this concept, some manufacturers use special low
revolution generators. Generators with low speeds are recognizable by their relatively
large diameters that are positioned close to the turbine rotor.
As can be concluded from this description, there is a fundamental difference between
conventional thermal or nuclear power generation on the one hand and wind power on
the other, namely: in wind turbines, generating systems different from the synchronous
generator used in conventional power plants are used.
Braking system

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Wind turbines are equipped with a safety system of a high degree. An aerodynamic
braking system is part of this. In case of emergencies, or for parking the turbine for
maintenance, a (disk) brake is usually fitted.
Control system
Wind turbines are high-tech machines. After commissioning, a turbine is controlled fully
automatically by an internal computer system. Information on the status of the tur-
bine can be retrieved remotely by owner or manufacturer by telecom transmission (e.g.
modem).

2.2 Future developments

At the moment, wind turbines with proven technology are available in the range of 1.5 to
3.0 MW. Especially in Western Europe private wind farm developers and utilities focus
on wind turbines in the power range of 2.0 to 3.0 MW. All leading manufacturers have
one of more wind turbines in the MW+ segment.
In some countries, e.g. Southern Europe, Asia and Latin America, with less developed
transport infrastructure or dominated by mountainous areas, wind turbines having more
limited dimensions are more appropriate. This is the reason why wind turbines in the
power range between 0.8 and 1.3 MW are most in demand worldwide.
Wind turbine prototypes are available in the power range 5 to 6 MW, these turbines are
characterized by a shaft height of 120 meter or higher and rotor diameters of more than
110 meter. These wind turbines will become commercially available from 2006. Besides
the still high costs of these 5+ MW turbines per installed MW, the main problem is
the weight and outer dimensions of the components. Components are of sizes that are
hard to transport over the Western European road infrastructure. Some manufacturers
solve this problem by offering these turbines only for offshore sites or sites accessible by
waterways. Other manufacturers manage this logistic problem at least partly by building
and installing the towers from prefab concrete parts or in-situ concrete tower structure
instead of tubular steel segments.
In wind turbine technology, the following developments are under preparation on the
drawing boards or are anticipated by experts:
• The market share of variable speed rotor technology, including modern power elec-
tronics, will increase further
• Also in the segment of MW+ wind turbines, the gearbox is one of the weakest links,
requiring frequent maintenance, and refurbishment or replacement is expensive. A
select number of manufacturers offer gearless wind turbines. These wind turbines

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are characterized by large, with diameters up to 5 meter, multi-pole synchronous

generators. Hybrid designs are also obtainable, e.g. a one-stage step-up gearbox
followed by a less massive multi-pole generator. For the coming 5 to 10 years it is
anticipated that these different concepts will be developed side by side
• The development of MW+ wind turbines will focus on saving weight and limiting
dimensions in order to simplify road transport and limit the required capacity of
building cranes on site. Ways to achieve this are the optimization of the control
strategy leading to less loaded and so less massive components. Another method is
to integrate more functionality in a component or system leading to less or more
compact parts.
• Currently offshore wind turbines are equal to or derived from onshore turbines. In
the near future the development of offshore wind turbines will deviate from onshore
wind turbines. Offshore wind will focus on reliability, remote control and high
installed power per wind turbine (up to or more than 10 MW). Onshore wind will
concentrate on low and acceptable nuisance (i.e. noise) for the neighborhood, high
efficiency, easy and low cost transport to the site, installation by fast and on short
notice available building cranes, and limited installed power (up to 6 to 8 MW).

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Costs and benefits

Costs of wind power
Cost prices of wind power depend to a considerable degree on the location of the turbine.
The wind speed and costs for the connection to the grid can vary per location. For
commercial use (budget and depreciation over ten years), cost prices vary from 5 ce/kWh
on good windy areas up to 8 ce/kWh inland. In comparison: the cost price of electricity
generated by fossil fuels is approximately 4 ce/kWh. Payment for the delivered energy
consists of avoided fuel costs, partly ecotax (grants for green energy) and a part that is
determined by the market for renewable power. This cost calculation is based on the
following assumptions:
• A new medium-sized wind turbine of 850 - 2 500 kW capacity
• O&M costs averaging 1.2 ce/kWh over a lifetime of 20 years. The aggregated
operational expenses (land rent, taxes, insurance, daily operation, O&M) is approx-
imately 2 ce/kWh.
During the last twenty years, the investment costs per kW installed wind power has
dropped. These average costs for a wind turbine project range from e 900 up to e 1 200
per kW installed power. Since the start of the present development in the end of 1970s,
generating costs have dropped by about 80%. Expectations tell us that this trend will
continue with a reduction of a few percent each year.
The other principal cost element in generating electricity from wind power is operation
and maintenance (O&M). Obviously, there are no fuel costs. O&M costs include regu-
lar maintenance, repairs, insurance, spare parts and administration. Because not many
machines are more than 20 years old, data is not always available, or comparable. For a
new machine, O&M costs might have an average share over the lifetime of the turbine of
about 20-25% of the total amortized cost per kWh produced. Manufacturers are aiming
to reduce these costs significantly through the development of new turbine designs requir-
ing fewer regular service visits and therefore reduced downtime. The trend towards larger
wind turbines also reduces O&M costs per kWh produced.

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Except for investment and O&M costs, costs for the following items also have to be taken
into account:
• Project development
• Preparation of the building site
• Bedplate of the turbine
• Connection to the grid
• Real estate taxes.
Benefits of wind power
The owner of a wind turbine sells his generated electricity to a utility company. The
value of wind power, as seen by the utility, is to be determined by the costs as caused
by the replacement of coal or gas. The average price of this grey power has gone up to
approximately 2 to 3 ce/kWh. If the owner were only to be compensated for this part,
a wind turbine could not be utilized sufficiently when the present costs are taken into
account.
Utility companies also pay for a guaranteed supply of power. ”Back up” power is not
needed if the power supplied as a high level of availability. Statistics have shown that
wind power can, in slow wind speeds, represent approximately 25% of the guaranteed
power.
Future costs:
Can wind power be competitive with conventional existing power plants at this moment?
In this comparison, wind power does not have an advantage because of existing plants
that are partly written off. Decisive is, how electricity made by wind will act in ten
years in relation to electricity produced by then newly built conventional plants that are
powered by fossil fuels; of which all the exhausted gasses need to be cleaned and probably
all emitted CO2 has to be stored. Because sources of fuels are running out, there’s a
realistic chance that the prices of fossil fuels will remain high. On the other hand, the
costs of wind power are expected to continue dropping.
If wind power works out in a positive way for the next ten years, it has a good chance of
becoming a serious competitor to conventional energy sources.
Taxes and incentives
In most European countries, wind power has at this moment no chance of surviving
without an incentive contribution from EU governments.
A substantial reason for providing an incentive is that wind power, as (almost) clean
energy source has almost no external costs. These costs have been recently determined

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as part of a study made by the EU. The European Union states that ”external costs
are incurred when the social or economic activities of one group influence another group,
and when this influence is not entirely being compensated or taken into account”. For
example, a conventional power plant produces SO2 . This gas causes a shortness of breath
in people who are asthmatic and damage to building materials. However, the owner of
the plant does not pay for the extra health care or the repair of buildings. The owner
shifts these costs to others. The EU could introduce an ecotax for this damage. As a
consequence of this, the energy per kWh would be 2 up to 7 cemore expensive.
The other way around is to encourage clean energy sources by incentives, so social and
environmental costs are avoided. These subsidiaries are allowed, if not encouraged by the
EU. In some European countries for example, wind power is rewarded with subsidy rates
of approximately 8 or 9 ce/kWh depending whether it is located on land or in the sea.

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Policy and regulation

Relevant regulations, EU policies and directives concerning wind power
There are major disadvantages in being dependent on oil imports and fluctuating oil
prices. Sources of fossil fuels are running out. The environment is also becoming a major
issue in terms of CO2 emissions or how to store nuclear waste.
In the background, many industrialized countries are exerting a major effort to develop
renewable energy sources; especially solar, biomass, hydro and wind power. Shell expects
that one third of all energy demand worldwide will be provided by renewable sources in
the year 2050. The ambitions to apply wind power are high. Some countries within the
European Union have set individual targets to have (for example) 9% of all electricity be
generated by renewable power by the year 2010. Half of this percentage might be wind.
These ambitions are very modest compared with the targets that have been set by the
European Union. Most countries of the EU now already have a higher share of renew-
able power generation by the availability of hydropower and a higher use of biomass and
wind power. In the former EU (15 countries) and by the year 2010, 22% of all generated
electricity must be produced by renewable energy sources. The Union is constantly en-
couraging all former and new European members to achieve this goal. (Concerning the
subject of ”Policy and incentives”, refer to Section 8.4.1 of this application note).
Local effects of wind power:
When wind power is used, this can cause local inconvenience. Detailed planning can limit
the impact of wind power consequences to an acceptable level.
Birds
Birds can run into the rotor blades of a turbine or get caught by the turbulence behind the
rotor. Inquiry has shown that the risks for collisions are relatively small. The estimated
number of ”collision-victims”, at an installed power of 1000 MW is approximately 21,000.
On an annual basis this seems to be a high number, but this is a small amount when
compared to the number of birds that get killed by traffic (2 million) or that die because
of power lines (1 million).
Most wind turbine casualties are caused at night, during twilight or in bad weather situ-
ations. Birds know their forage and resting grounds very well; they avoid wind turbines.
When installing turbines, the breeding and foraging areas of the birds have to be consid-
ered well.
Fish
Wind turbine (farms) at sea also have positive effects. Over-fishing is a well-known
problem; the stocks of many species of fish are threatened. The sailing of ships and

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therefore fishing is prohibited in and around wind farms. Sea biologists expect that these
areas will develop into a breeding ground of several species of fish, and overall this will
have a positive effect on fish stocks. Recent research near wind farms confirms this effect.
Noise
Wind turbines produce noise. The rotor makes a zooming sound and the generator and
gearbox can also be heard. Carefully designed rotor blades, a limited revolution speed,
and effective sound insulation of the gearbox and generator limit the noise emission. By
maintaining a sufficient distance from residential or other sound-sensitive areas, noise
pollution can also be avoided.
Shadow
Sunshine creates moving shadows when the rotor of a turbine rotates. In winter, when the
sun is low, the shadow can be annoying when it falls into a window. Giving wind turbines
an appropriate orientation towards houses is sufficient to prevent this problem. If on a
yearly basis a small number of hours give inconvenience, the turbine can be stopped at
these moments without any particular loss of energy.
Blending into the landscape
Wind turbines are striking structures in the landscape. They can be made to blend in by,
for example, by arranging them in lines along a dike or waterway. In doing so, the lines of
the landscape are taken into account. Research has shown that positioning wind turbines
in clusters is more accepted when it is clear to neighboring people that in this situation
a great yield can be generated. Whether the lining-up of several turbines is liked or not
is and always will be a matter of taste. More important is the relationship between the
altitude of the shaft and the diameter of the rotor. Another significant item is the size of
the rotor. Rotors that have larger diameters rotate slower and because of this they are
quieter.

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Summary
For thousands of years, wind power has been used for various purposes. Mainly since the
oil crisis a substantial development has taken place and impressive wind farm projects
have been realized.
Wind technology is still developing. Turbines are becoming more efficient, power rates
are increasing and intelligent power electronics are being introduced. In the meantime,
impressive wind farms are rising out of the sea.
Not only environmental issues make wind turbines more often seen on the horizon. The
continuously lowering of the investment and maintenance costs of wind turbines make
this technology interesting for investors and wind farm developers.

References
[1] Thomas Ackermann (ed.), Wind Power in Power Systems, John Wiley & Sons, Ltd,
2005, ISBN 0-470-85508-8
[2] Troen, I. and E.L. Petersem, European Wind Atlas, RisøNational Laboratory,
Roskilde, Denmark, ISBN 87-550-1482-8
[3] WAsP (Wind Atlas Analysis and Application Program), software version 8,
RisøNational Laboratory, Roskilde, Denmark
[4] Jos Beurskens, Gijs van Kuik, Alles in de wind, answers and questions concerning
wind power, October 2004
[5] Wind power technology, operation, commercial developments, projects, grid distri-
bution, EWEA, December 2004
[6] Wind power economics, wind energy costs, investment factors, EWEA, December
2004
[7] The current status of the wind industry”, industry overview, market data, employ-
ment, policy, EWEA, December 2004
[8] Windenergie winstgevend, Ministry of the Flemish Community, department of re-
newable sources and energy, 1998

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