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CHAPTER 7

RENEWABLE ENERGY –
WIND ENERGY
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After reading this chapter, you should be able to…

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Describe the history of wind power

Define and differentiate between vertical axis and
horizontal axis wind turbines

Describe a modern wind turbine and how it works

Describe wind farms and the logistics involved in
constructing them
 P2P: Would you want a wind farm in your
backyard?

Explain how Europe is pioneering the development of
offshore wind power

Discuss the potential for wind power to meet global
energy needs
 P2P: How many wind turbines would we need to
supply global energy demand?

Explain the environmental impact of wind power

Wind is air in motion. Differences in atmospheric pressure from one

location to another cause air to move. The kinetic energy of moving air, or
wind, is an indirect form of solar energy and is considered renewable.
Wind turbines harness the kinetic energy of wind and convert the
mechanical energy of a rotating blade into electrical energy from a
generator. The objective of this chapter is to discuss the use of wind as a
source of energy for generating useful power. We begin our discussion of
wind energy by reviewing the history of wind power.
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Renewable Energy – Wind Energy 159

7.1 HISTORY OF WIND POWER

Wind has been used as an energy source for thousands of years.
Historical applications include sails for ship propulsion and windmills for
grinding grain and pumping water. Wind is still used today as a source of
power for sailing vessels and parasailing.
The earliest known applications of wind as an energy source come
from Persia [Manwell, et al., 2002]. Around 900 A.D., wind was used to
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drive early vertical axis windmills. Modern wind turbines are classified as
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either horizontal axis turbines or vertical axis turbines. A vertical axis

turbine has blades that rotate around a vertical axis and its visual
appearance has been likened to an eggbeater. A horizontal axis turbine
has blades that rotate around a horizontal axis (see Figure 7-1).
Horizontal axis turbines are the most common turbines in use today. Early
vertical axis windmills had a simple design and were particularly
susceptible to damage in high winds.

Figure 7-1. Comparison of Vertical Axis and

Horizontal Axis Windmills

The first known use of horizontal axis windmills, a precursor to the

technology seen predominantly today, appeared in Europe in the Middle
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160 Energy in the 21st Century

Ages. Horizontal axis windmills could be used for a variety of simple tasks
such as pumping water, grinding grain, and sawing wood. Early European
windmills typically had four blades, as shown by the Dutch windmill in
Figure 7-2.
Wind power was a significant contributor to European energy for
centuries prior to the Industrial Revolution. As time passed, use of wind
as an energy source lost favor because it was difficult to distribute and it
was not always available when it was needed. Eventually coal entered
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the energy mix because coal had several advantages: it could be used
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when and where it was most needed, and helped reduce dependence on
wood as a combustible fuel, which was deforesting some areas. To a
lesser extent, water also overtook wind as a source of power because
water could be moved through waterways such as canals and stored for
more timely use in containers such as ponds.

Figure 7-2. Dutch Windmill (Fanchi, 2003)

Blades on early European windmills were usually designed to rotate

together in the wind. This design made the mill susceptible to damage
because of the large number of moving parts. European windmill
technology eventually matured to the point that only a small part of the
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Renewable Energy – Wind Energy 161

mill, the rotor, turned to move the blades and adjust to wind direction
without affecting the adjoining structure. Controls also were implemented
to allow the blades to rotate on their own and reduce the amount of
operational supervision. Scientific testing of windmills led to a more
sophisticated knowledge of the forces at work in a wind energy system.
Many of the advances made in Europe have been integrated into modern
wind turbine designs.
As use of windmills declined in Europe, a new form of windmill came
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into widespread use in the Western United States (Figure 7-3). This type
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of multi-bladed windmill, or ‘fan mill’, was used primarily for pumping

water in the more arid regions of the United States. The water was
needed for irrigation, supplying livestock with water, and providing water
for steam-driven locomotives. A simple but effective regulating system
was developed for these windmills that allowed them to function for long
periods unattended, and set the stage for automatic control systems that
are integral parts of modern turbines.

Figure 7-3. Windmill in the Western United States (Fanchi, 2009)

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162 Energy in the 21st Century

The first wind turbines for electricity generation appeared in the late
th
19 century in the United States. Following the development of the
electric generator, it was only logical that someone would try to turn
generator shafts using wind power. Early wind turbines were built to
provide electricity for residential areas on a very small scale, usually one
turbine for one home. The first larger-scale wind turbine was built by
Marcellus Jacobs in the early 1920’s. These turbines had three rotor
blades with airfoil shapes and resemble wind turbines in common use
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today (Figure 7-4). The Jacobs turbine could provide up to 3 kW power

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and could be integrated into an electrical grid to provide power for

distribution to many consumers. A decision by the United States Rural
Electrification Administration to expand the central electric grid in the
1930’s may have delayed the adoption of an expanded role for wind
energy in the United States energy mix. With that decision, small scale
wind power lost much of its appeal since fewer areas were isolated from
the electrical grid.

Figure 7-4. A Modern Wind Turbine Blade in

Lamar, Colorado (Fanchi, 2009)

A large number of small-scale wind turbines with electrical production

ranging from 20 kW to roughly 60 kW were developed in Denmark. Then,
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Renewable Energy – Wind Energy 163

shortly after World War II, Johannes Juul erected the 200 kW Gedser
turbine in southeastern Denmark. This turbine had several major
advances built into it, including an aerodynamic design that enabled
greater control of power output by varying the angle of the blade in
response to changes in wind speed, and a generator that could be
connected directly to the electrical grid. Also around this time, in the
1950’s, German Ulrich Hütter made advances in the application of
aerodynamic principles to wind turbine design. Many of Hütter’s ideas led
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to concepts in use today. More discussion is provided by Kühn [2008] and

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the American Wind Energy Association [AWEA, 2009].

The use of wind energy for electrical energy began in the late 19th
century when windmills were being used on a very small scale, such as
for individual houses or farms. Over the course of the 20th century, wind
energy received increasing attention as a potential power source, but the
technology did not exist for large scale use. By the middle of the century,
wind energy was still limited to small scale use despite attempts to
develop the technology. Even in 1980, the total power from wind turbines
for electricity generation was less than 1000 MW, which had a small
impact on the energy mix. Over the next 20 years, wind technology
advanced significantly. Modern turbine technology appeared between
2000 and 2005. Today, a single wind turbine can produce up to 6 MW
electrical power, and researchers are seeking to increase power output
up to 10 MW using new technology, such as superconducting generators
[Matthews, 2009].

7.2 WIND TURBINE

A schematic of a typical modern wind turbine is shown in Figure 7-5.
Moving air rotates blades attached to a generator shaft in the machine
cabin. The machine cabin is known as the nacelle. It contains the
electrical generator which converts the rotational energy of the rotating
blades to electrical energy. Electricity is transmitted through a line in the
post that connects each wind turbine to the electric grid. Therefore, the
generator produces electricity that is routed directly to the electric grid.
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164 Energy in the 21st Century

A typical horizontal axis turbine consists of a rotor with three blades

attached to a machine cabin set atop a post that is mounted on a
foundation block. The machine cabin contains a generator attached to the
wind turbine. The rotor blades can rotate in the vertical plane and the
machine cabin can rotate in the horizontal plane.
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Figure 7-5. Schematic of a Wind Turbine

Most modern horizontal axis turbines have three rotor blades instead
of two. Rotor blades attached to a generator shaft, or rotor, make up a
rigid body with a particular moment of inertia. Rotational properties of a
rigid body, such as angular momentum and torque, depend on moment of
inertia. The moment of inertia of the wind turbine depends on the number
of rotor blades and their orientation. Turbines with two rotor blades have a
higher moment of inertia when the blades are vertical than when they are
horizontal. The difference in moment of inertia between the horizontal and
vertical configuration of two blades introduces a mechanical imbalance
that can increase wear on the system. By contrast, the use of three
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Renewable Energy – Wind Energy 165

equally spaced blades adds the cost of another blade but allows a
symmetric placement of blades that makes it easier to balance the blades
as they rotate. The improved stability of turbines with three rotor blades
increases wind turbine reliability and reduces maintenance costs.
Figure 7-5 shows a yaw mechanism attached to the post just below
the machine cabin. The purpose of the yaw orientation system is to align
the rotor shaft with wind direction. Figure 7-6 illustrates the difference
between yaw, pitch and roll in aeronautical terms. Yaw refers to rotation
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of the nose in the horizontal plane around the vertical axis. Rotation of the
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airplane around the B to B’ line is called pitch, and rotation of the wings
around the A to A’ line is called roll.

Figure 7-6. Aeronautical Orientation System

Modern wind turbines are several hundred feet tall. For example, a
wind turbine that generates 1.6 megawatts of electrical power is
approximately 113 meters (370 feet) tall from its base to the tip of the
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166 Energy in the 21st Century

rotor blade. The storage tanks in Figure 7-7 illustrate the scale of a
modern wind turbine and demonstrate that wind turbines can be erected
on existing industrial properties. For example, wind turbines have been
erected near fossil fuel fired power plants, along highways, on ranchland,
and in shallow waters offshore.
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Figure 7-7. Modern Wind Turbine near

Rotterdam, Holland (Fanchi, 2003)

If the speed of rotation of the tip of the rotor blade is fast enough, it
can be lethal to flying animals such as birds or bats that enter the fan
area of the rotor blade. In addition to direct contact between wildlife and
rotating rotors, air pressure disturbances between the front and back of
the wind turbine can harm fragile wildlife. Environmental hazards can be
minimized by selecting locations for wind turbines that avoid migration
patterns and nesting areas.
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Renewable Energy – Wind Energy 167

7.2.1 Turbine Power Output

Wind power can be maximized when wind direction is perpendicular to
the plane of rotation of the rotor blades. A change in wind direction can
put stress on wind turbines.
Electrical power output from a wind turbine is affected by the reliability
of the wind turbine. Wind speed is seldom constant; it can vary from still
to tornado or hurricane speed. Wind turbines have three regions of power
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output, illustrated in Figure 7-8, that depend on wind speed. Region I

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occurs when the wind speed is low and little or no power is produced.
Region II occurs with intermediate wind speed allowing the turbine to
begin producing meaningful power output. Region III occurs when the
wind speed is high and the turbine reaches maximum power. Eventually,
if the wind speed is great enough, the turbine will reach a cutout point at
which time it shuts down entirely. This is generally reserved for events,
such as hurricanes or tornadoes, when the wind speed is dangerous to
the equipment.

Figure 7-8. Wind Turbine Power Output Regions

The peak power of a wind turbine is its maximum power output at

optimum wind speed. Since wind turbines do not usually operate at peak
power, their performance is measured using capacity factor, which is the
average output power of a wind turbine divided by its peak power.
One factor that affects the efficiency of a wind turbine is the efficiency
of converting mechanical energy of the rotor blade into electrical energy.
In 1928, Albert Betz showed that the maximum percentage of wind power
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168 Energy in the 21st Century

that can be extracted is approximately 59.3% of the power in the wind.

The rate of rotation of the rotor blade depends on wind speed and size of
the blades. If the wind speed is too great, the rotor blade can turn too fast
and damage the system.

7.3 WIND FARMS

A wind farm or wind park is a collection of wind turbines. The areal extent
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of the wind farm depends on the radius R of the rotor blades and the
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effective radius Reff of the wind turbine (Figure 7-9). A wind turbine must
have enough space around the post to allow the fan of the rotor blade to
face in any direction. The minimum spacing between the posts of two
equivalent wind turbines must be 2Reff to avoid collisions between rotor
blades. If we consider the aerodynamics of wind flow, which is the factor
that controls turbine spacing, the turbine spacing in a wind farm should be
at least 5 to 10 times rotor diameter, which is given by 2R [Sørensen,
2000, page 435].

Figure 7-9. Wind Turbine Spacing

The distance between wind turbine posts is designed to minimize

wind turbulence between wind turbines and enable the restoration of the
wind stream to its original undisturbed state after it passes by one turbine
on its way to the next turbine. Wind turbine spacing, or the distance
between turbines, is an important factor in determining the surface area,
or footprint, needed by a wind farm. Figure 7-10 shows the spacing of
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Renewable Energy – Wind Energy 169

wind turbines in a Texas wind farm near Sweetwater, Texas. The train on
the left side of the figure shows the scale of the wind turbines.
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Figure 7-10. Wind Turbine Spacing on a

Texas Wind Farm (Fanchi, 2009)

7.3.1 Offshore Wind Farms

Current technology favors onshore wind farms as the most economical
form of wind farm. There are drawbacks to onshore wind farms that can
be mitigated by construction of offshore wind farms. For example, the
sight of a 200-300 foot tall wind turbine does not appeal to everyone, and
wind turbines do make some noise. Offshore wind turbines can be placed
beyond visible and audible range of many populated areas.
Several countries around the world do not have good conditions for
onshore wind farms but do have good conditions along nearby shoreline.
Some conditions that would hinder the development of onshore wind
farms include unreliable onshore wind patterns or a population density so
large that access to land is too costly. Wind currents tend to be stronger
and smoother offshore than on land. Onshore topographic features like
mountains, valleys, and skylines can disrupt the smooth flow of air and
introduce turbulence. Offshore wind farms can be built to provide power
to densely populated coastal areas where the cost to access land is
prohibitive.
European countries with coastlines began constructing offshore wind
turbines in the early 1990’s, led by Denmark and the United Kingdom.
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170 Energy in the 21st Century

The wind resource map in Figure 7-11 shows that the Gulf Coast, Atlantic
coast, Pacific coast, and Great Lakes of the United States are areas
where wind conditions are better offshore than onshore. Offshore wind
conditions give these areas access to wind power.
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Figure 7-11. Wind Patterns in the United States

[NREL Wind, 2012]

A concern that is particularly significant to the United States is the

cost of transporting large, heavy components to isolated parts of the
country. Transportation costs for development of offshore wind farms can
be lower because marine shipping and handling equipment is better
suited for the heavy components needed for wind farms.
Offshore wind farms can have many benefits compared to onshore
wind farms, but there are a number of drawbacks. One issue is the
difficulty of creating an infrastructure system to get the electricity from the
turbine to shore without significant loss of power. This concern also exists
with onshore wind farms located a significant distance from the end user.
In that case, the cost of transmitting power over long distances is
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Renewable Energy – Wind Energy 171

comparable for both onshore and offshore power generation by wind

farms.
Another drawback of offshore wind farms is increased maintenance
costs. Costs are greater for offshore facilities than facilities on land for
several reasons. Salt in seawater has a corrosive effect that increases
water damage to equipment. The location of turbines offshore increases
the difficulty of accessing them. Offshore turbines are subjected to more
severe weather conditions, including strong wave activity, which
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increases the risk of serious damage to equipment.

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Countries with sea coasts adjacent to shallow sea shelves include

Denmark, the Netherlands and the United Kingdom. Shallow water can
extend more than 12 miles from shore. These countries have a
geographic advantage for placement of offshore wind farms because the
cost of building an offshore wind farm in shallow water is less than the
cost of building an offshore wind farm in deep water. Two major
difficulties of constructing an offshore wind farm in deep water are
installation and control of wind turbines to ensure that are adequately
anchored to the sea floor.

Point to Ponder: Would you want a wind farm in your backyard?

The first offshore wind farm to be built in the United States is
proposed for the Nantucket Sound area of Massachusetts. The
wind farm was approved by the U.S. Department of the Interior
in April 2010. The farm, called Cape Wind by farm sponsors, will
consist of 130 wind turbines constructed over five miles from the
nearest shoreline in a region called Horseshoe Shoal. According
to CapeWind.org, the Shoal “has strong, consistent winds; is
located in protected shallow water; has close proximity to
landfall and electrical interconnections; and is out of way of
shipping lanes and commercial boating traffic.” The farm is
designed to produce 420 MW of energy. Each turbine would be
spaced 1800 to 2700 feet apart, which is enough space for easy
navigation of the surrounding waters by fishing craft and shallow
water boats.
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172 Energy in the 21st Century

So what is the problem? The largest concern exhibited by

the local population is that the aesthetic impact will drastically
decrease the value of beachfront homes. A related concern is
that the drop in home values would lead to a drop in tax revenue
gathered from property taxes. CapeWind.org answered these
concerns by pointing to a similar wind farm constructed at
Tunoe Knob, Denmark in 1995. The concerns of Tunoe Knob
residents were similar to those held by Massachusetts
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residents. Tunoe Knob home values remained steady years

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after construction of the wind farm, and the presence of the wind
farm is no longer a major topic of discussion.
The Cape Wind project is expected to create jobs in the area
and have a positive impact on the local economy. Opponents of
the project argue that it could seriously damage the natural
beauty of Nantucket Sound, a popular tourist destination, which
in turn could reduce tourism, tourism-related jobs, and harm the
local economy.
Projects such as Cape Wind can provide clean, renewable
energy, create energy-related jobs, and increase energy
independence. They can also have social and environmental
consequences that need to be considered.

7.3.2 Advances in Offshore Wind Technology

Offshore wind turbines provide access to steady sea winds and do not
require onshore surface area in countries with high population densities.
Figure 7-12 shows the progression of wind turbines from onshore to
deeper offshore waters. Shallow water offshore wind turbines are
designed for up to 30 m of water depth. Some offshore wind turbines are
designed to float and maneuver in accordance with wind and wave
patterns, while others are designed to be moored to the sea floor. Seabed
mounted wind turbines are being developed for transitional depths
ranging from 30 m to 60 m. Floating offshore wind turbines can be used in
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Renewable Energy – Wind Energy 173

waters that are too deep for conventional turbines, and are being
developed for water depths greater than 60 m. A floating turbine can be
designed to align with wind direction.
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Figure 7-12. Offshore Wind Turbines [after DoE Wind, 2012]

Wind turbines must be sturdy enough to withstand bad weather

conditions, and there must be a way to transmit power to the electrical
grid. Many European nations have long shorelines and are pioneering the
development of offshore wind turbine technology. Their efforts are
discussed below.

7.4 CASE STUDY: EUROPEAN WIND POWER

Over the past decade, advances in the commercial and technological
viability of wind have increased support for wind as an alternative to fossil
fuels. Technological advances in wind turbine technology, electrical
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174 Energy in the 21st Century

transmission technology and power grid organization have improved the

efficiency and cost effectiveness of wind dramatically. With these
improvements, nations lacking fossil fuel resources have begun viewing
wind as a possible primary energy source for the future. While the United
States is taking large strides in this direction, Europe is leading the
development of wind power, especially offshore wind power.
Many members of the European Union have a high UN Human
Development Index score. We showed in Chapter 1 that quality of life is
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correlated to energy use. European countries have relatively small

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conventional fossil energy reserves. Some European countries have

access to unconventional fossil energy resources. Overall, Europe is
concerned about the environmental impact of fossil energy consumption
and has chosen to develop clean, renewable sources of energy. In an
attempt to secure a local energy resource with a fixed fuel cost, these
countries have turned to wind and solar energy. Wind and solar energy
are abundant natural resources that are renewable and free.
Denmark was among the first nations in the world to begin producing
large amounts of wind power and to include wind as a major contributor to
their national power supply. By the end of 2011, 26% of Denmark’s power
supply was provided by wind power, which is the largest percentage of
any European country. Spain (15.9%), Portugal (15.6%), Ireland (12%)
and Germany (10.6%) were the only other countries over ten percent.
[EWEA Stats, 2012, Figure 3.6]. Overall, member countries of the
European Union were using wind to provide 6.3% of total electricity
demand by the end of 2011.
Many European countries have limited land area to develop onshore
wind farms, but have extensive coastlines. The North Sea has very strong
winds and a shallow continental shelf that makes it a desirable location
for installing offshore wind farms. Global installed offshore wind capacity
at the end of 2010 is shown in Table 7-1 [EWEA Pure Power, 2011].
All but 36 MW of the European offshore wind capacity has been
installed since the beginning of 2001. This demonstrates the rapid growth
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Renewable Energy – Wind Energy 175

in wind power, particularly offshore wind power, in this region [EWEA

Pure Power, 2011, Figure 2.10]. Similarly, over that same period of time,
total EU wind power capacity grew from approximately 13 GW to 94 GW
[EWEA Stats, 2012]. While it should be noted that the EU has expanded
from 15 to 27 members in that time frame, 95% of the total EU wind
capacity comes from 15 members that have been present since 2000 or
earlier.
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Table 7-1
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Installed Offshore Wind Capacity

MW of Offshore Wind Power
Country
(End 2010)
United Kingdom 1,341
Denmark 854
Netherlands 247
Belgium 195
Sweden 164
Germany 92
Finland 26
Ireland 25
Total EU 2,944
Norway 2.3
China 102
Total World 3,048

The European Union is one of many international organizations taking

steps to formulate a plan for future power production and use. The 2009
EU Renewable Energy Directive was created as a means to force EU
Member States to begin actively addressing their energy future. The
Directive required each EU member to estimate its energy consumption
for both renewable and nonrenewable energy sources each year from
2010 to 2020. This consumption must include the three factors that use
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176 Energy in the 21st Century

the most energy: heating/cooling, electricity, and transportation. The

Directive requires every country to produce at least 12% of their total
power from wind by 2020, although the relatively low cost of wind in
today’s energy market may enable countries to exceed this number.
Many of the wind power projections for EU members are ambitious.
One example is France, which generates most of its electricity from
nuclear energy and produced 5,660 MW of power from wind at the end of
2010. France is expected to increase its wind capacity to at least 23,000
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MW by 2020. This capacity requires adding 1,734 MW each year. France

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installed only 830 MW new capacity in 2011 [EWEA Stats, 2012]. The
23,000 MW total includes 4,000 MW of new capacity in offshore wind.
France must receive assistance from its neighbors to design the
technology for offshore wind farms and create the infrastructure
necessary to deliver this power to consumers.
As a whole, Europe has a high-end target of 265 GW of wind capacity
by 2020, which is approximately three times its wind capacity in 2010.
While this may seem ambitious, the reality is that the expectations are not
unreasonable. In order to achieve the desired continental goal by the end
of the decade, the number of turbines needed to be installed is actually
smaller than the number already installed. This is due to improvements in
turbine technology since the beginning of the last decade. Even if there is
no further improvement in technology, which seems unlikely given the
rate of recent progress, only 60,344 new 1.8 MW turbines will be needed
for the continent to reach its goal. That is 18% fewer turbines than the
71,620 already installed at the end of 2010.
A great deal of infrastructure development must occur to achieve the
goals of the EU plan. As many European nations face financial crises, the
ability to finance grid improvements is becoming more difficult. Another
concern is availability of wind turbines, cables, and other equipment
needed to install significantly more wind capacity. Today, manufacturing
capacity is able to meet demand and provide equipment at a reasonable
price, although there are concerns about looming supply bottlenecks
[EWEA Offshore, 2012]. The European plan calls for a significant
increase in wind power capacity in a short period of time, and this
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Renewable Energy – Wind Energy 177

increase could affect the price and availability of equipment and trained
personnel, both in Europe and globally.

7.5 CAN WIND MEET ALL OF OUR ENERGY NEEDS?

Wind appears to have many of the advantages that would make it an
appealing solution to our energy problems. When properly designed and
located, wind turbines are environmentally benign. Some people may
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object to large fields filled with wind turbines, but fields of wind turbines
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can be built on property to serve a dual purpose. For example, wind farms
can be built on West Texas ranch land or along the roadways in
Rotterdam, Holland. In both cases the turbines rotate well above activities
below.

Point to Ponder: How many wind turbines would we need

to supply global energy demand?
We can estimate the number of wind turbines that would be
needed to supply global energy demand by making a few key
assumptions. For the sake of argument, let us assume that the
world population in year 2100 will be eight billion people and the
amount of energy needed to provide each person an acceptable
quality of life will be 200,000 megajoules per year. Both of these
assumptions can be challenged, but they define a specific
scenario so that we can estimate the number of wind turbines
needed for the scenario.
Suppose we use wind turbines that can provide 4 MW
each. We would need about 12.7 million wind turbines to supply
global energy demand in 2100. If these wind turbines are
gathered in wind farms that can provide 1000 MW per wind
farm, we would need approximately 50,700 wind farms [Fanchi,
2004, Exercises 15-7 and 12-8]. If we assume the turbine radius
is 108 feet and assume the area occupied by each wind farm is
approximately square, with a turbine separation of about ten
times turbine radius, we estimate that each wind farm will
occupy about nine square miles. All of the farms would occupy
an area of about 465,000 square miles, or about 16% of the
area in the contiguous United States. The area is smaller than
the state of Alaska, but larger than the state of Texas.
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178 Energy in the 21st Century

We can build enough wind farms to provide the energy we need

based on area and power capacity, as illustrated in the Point to Ponder
above. The ability to build wind farms offshore further reduces concerns
about access to land area. Still, other issues must be considered. Wind is
an intermittent source of power. We need to provide energy when the
wind does not blow. We need to distribute the energy where it is needed
and when it is needed. We need to provide energy in a form that best fits
the need. We need to be willing to accept the environmental impact of
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wind farms, including their appearance and impact on wildlife.

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Some unresolved issues are technical, others are social. An example

of a technical issue is the question of how to provide energy on demand,
even when the wind is not blowing. There are several options to consider.
For example, we could use wind energy to charge batteries, develop
large scale energy storage capacity, or use wind energy to produce
hydrogen for use in fuel cells. One advantage of hydrogen production is
that we could use hydrogen in the transportation sector. Hydrogen is
considered in Chapter 10.
Much work remains to be done to identify the optimum strategy for
providing renewable energy. It is important to note that wind power in the
United States cost approximately US$0.04 to US$0.06 per kilowatt hour
in 2009, which was comparable to the cost of electricity from natural gas.
The cost-competitiveness of wind and the demand for cleaner energy by
consumers is encouraging the growth of wind energy around the world.
Figure 7-13 shows the exponential growth of wind production in the
United States. This figure can be misleading because it gives the
impression that wind is a major contributor to the United States energy
mix. To understand the actual role of wind in the United States, we
compare the production of energy from wind and coal in Figure 7-14.
Although the contribution of wind energy to energy portfolios is growing,
especially in Europe and the United States, the relative contribution of
energy production from wind is much less than the contribution of energy
production from coal.
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Renewable Energy – Wind Energy 179

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Figure 7-13. Energy Production from Wind in the United States

[US EIA Energy Production, 2011]

Figure 7-14. Comparison of Energy Production from Wind and

Coal in the United States [US EIA Energy Production, 2011]
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180 Energy in the 21st Century

7.6 ENVIRONMENTAL IMPACT

Wind energy is a renewable energy that is considered a clean energy
because it has a minimal impact on the environment compared to other
forms of energy. Wind turbines provide electrical energy without emitting
greenhouse gases. On the other hand, we have already observed that the
harvesting of wind energy by wind turbines can have environmental
consequences.
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Rotating wind turbine blades can kill wildlife and interfere with
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migration patterns. Judicious placement of wind turbines away from

migration patterns and nesting areas can reduce the risk to wildlife. Wind
farms can have a significant visual impact that may be distasteful to some
people. Wind turbines produce some noise when they operate. In the
past, wind turbines with metal blades could interfere with television and
radio signals. Today, turbine blades are made out of composite materials
that do not interfere with electromagnetic transmissions.
An aesthetic concern that is coming to the forefront as more wind
turbines are placed in populated areas is a concept known as “flicker.”
Flicker is the effect a wind turbine has on the area of land in its shadow.
Turbine blades that rotate in daylight cast a shadow that causes a
flickering light effect. This flicker can be irritating or distracting to people if
their homes or businesses are in the shadow of the turbine.

7.7 ACTIVITIES
True-False
Specify if each of the following statements is True (T) or False (F).
1. Modern wind turbines provide energy whenever the wind is blowing.
2. Solar energy and wind energy are intermittent sources of energy.
3. A sailboat can use wind energy and water currents to move.
4. Wind turbines must be spaced to minimize turbulence.
5. Two drawbacks of wind energy are the visual impact and footprint of
wind farms.
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Renewable Energy – Wind Energy 181

6. A horizontal axis turbine has blades that rotate around a horizontal

axis.
7. A wind farm is a collection of wind turbines.
8. Wind turbines have no adverse effect on the environment.
9. Use of wind as an energy source is a recent development (within the
last 200 years).
10. The United States was the first country to construct an offshore wind
farm.
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Questions
1. A renewable energy project will generate 400 MW of wind power
using 100 wind towers and will cost approximately US$ 600 million.
a. On average, how much wind power will be provided by each wind
tower?
b. What is the average cost per MW?
2. What is the minimum distance between two wind turbine posts on a
wind farm?
3. Does a wind turbine have any negative environmental consequences?
4. When and where were horizontal axis windmills first utilized?
5. Where were fan mills developed and what were their purpose?
6. What does the yaw mechanism on a wind turbine do?
7. Why do most modern wind turbines use three blades?
8. List three benefits of offshore wind farms relative to onshore wind
farms.
9. A 1.5 MW wind turbine provides enough power for 350 homes. What
is the average power used by each home, in kW?
10. What is “flicker”?