WIND ENERGY
David A. Johnson
Department of Mechanical and Mechatronics Engineering
University of Waterloo
Waterloo, Ontario
CANADA
2007 - updated continuously
Contents
0.1
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 INTRODUCTION
1.1 Introduction . . . . . . . . . . . . . . .
1.2 HISTORY . . . . . . . . . . . . . . . .
1.2.1 The English Windmill . . . . .
1.2.2 Dutch Windmills . . . . . . . .
1.2.3 Other Countries . . . . . . . . .
1.3 Wind Power in the 20th Century . . .
1.4 Machine Types . . . . . . . . . . . . .
1.5 Oil Crises 1970’s - USA and worldwide
1.6 State Of The Art . . . . . . . . . . . .
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2 THE WIND AS AN ENERGY SOURCE
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 The Extraction of Energy . . . . . . . . . . . . . . . . . . . .
2.3 The Structure of the Wind . . . . . . . . . . . . . . . . . . . .
2.4 Boundary Layers . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 Wind Variation with Time . . . . . . . . . . . . . . . . . . . .
2.7 Presentation of Data . . . . . . . . . . . . . . . . . . . . . . .
2.8 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9 Power Estimation From Wind Data Using Statistical Techniques
2.10 Wind Data Analysis Using Statistical Techniques . . . . . . .
ii
1
1
5
5
14
15
16
18
20
20
28
28
28
32
34
39
40
45
46
50
56
3 FUNDAMENTALS OF WIND MACHINES
59
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.2 Performance Parameters . . . . . . . . . . . . . . . . . . . . . 59
i
CONTENTS
ii
4 AERODYNAMICS OF WIND
4.1 Introduction . . . . . . . . . .
4.2 Aerodynamic Fundamentals .
4.2.1 Separation and Stall .
4.3 Horizontal Axis Turbines . . .
4.3.1 Tip Losses . . . . . . .
4.3.2 Optimization . . . . .
TURBINES
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71
71
71
76
82
89
92
5 Design of Wind turbines
5.1 Introduction . . . . . . .
5.2 Forces on Wind turbines
5.2.1 Simple Models . .
5.3 Control and Safety . . .
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102
102
102
104
107
0.1
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Preface
These notes are compiled based on those previously collated by Professor G.
Bragg in the very early days of wind development and I am grateful for his
contribution. They are by no means comprehensive as a one term course does
not allow much in-depth analysis of any one particular aspect of this field.
As such they represent an ongoing project as the subject matter changes and
matures.
Chapter 1
INTRODUCTION
1.1
Introduction
Wind as an energy source has always been significant in early human history. Through the use of sails for motive power, wind has been in use since
prehistory as shown in Figure 1.1. There is some indication that mills with
a vertical axis may have originated in India, Tibet, Afghanistan or Persia
(year 950?) as shown in Figure 1.2. Later examples are found in China. The
origin of wind mills is somewhat obscure and is an area of active historical research called Molinology http://www.molinology.org/. Windmills have been
used for grain grinding and water pumping in Europe since the late middle
ages. Only beginning in the early 20th century has the use of sails for power
decreased. Some historical development is covered in most texts including
Spera[9] and Hau[10]. The term windmill will be used as a term to describe
conversion of wind energy to mechanical energy (e.g. grain grinding or water
pumping) and wind turbine will be used to describe the conversion of wind
energy into electrical energy.
However, wind power for water pumping has had a continuous history of
usage up to the present. Until the late 19th century the large Dutch water
pumping or grain grinding windmill epitomized the best practice. From the
19th century until the present, the American farm windmill was developed
for water pumping or in later versions for electrical generation.
In the 1920’s and 1930’s several European countries built larger electricity
generating windmills. These were only variably successful and tended to
suffer from a lack of dependability. At that time there was considerable
1
Why we harness the wind.
From the ancient Egyptians to today’s modern wind farms, the
wind has always been a natural ally in propelling our societies
forward. Today, instead of grinding grain and pumping water, we
can harness the wind to generate electricity.
In Canada, there is more than enough wind potential to make
CHAPTER 1. INTRODUCTION
2
2
a big contribution to our energy needs. Wind energy is an
affordable and viable source of electricity, powering 315,000
Canadian homes in 2006. Using our untapped wind resources
might one day see us provide for 20% of our electricity needs
– enough to power 17 million homes.
Chapter 1: Windmills and Windwheels
The first reliable information about the existence of windmills from historical sources
originates from the year 644 A.D. [2]. It tells of windmills from the Persian-Afghan border
region of Seistan. A later description, including a sketch, dates back to the year 945 and
depicts a windmill with a vertical axis of rotation. It was obviously used for milling grain.
Similar, extremely primitive windmills have survived in Afghanistan up to the present time
6000 BC: wind powers
1600s: windmills pump
1888: Charles Brush develops
Early 1900s: Windmills drive
1941: Putnam’s 1.25 MW
2006: 3MW turbines in
(Fig.
the 1.1).
first sailboats along
water from Holland’s
first large wind generator
pumps & generators across
turbine demonstrates
production and 5 MW
Egypt’s Nile River
reclaimed wetlands
12 kW DC
rural North America
need for lighter materials
Some centuries later,
the first newsproducing
arrived
in Europe that
the Chinese were
also using prototypes are tested
wind wheels for draining rice fields. Whether the Chinese knew windmills even before the
Persians and whether the European mills might have been only an offshoot of the Chinese
Figurewith
1.1:
Wind
through
timethat[1]
invention, can no longer be determined
certainty
today.use
It is remarkable,
however,
the Chinese windwheels, too, were simple
structures
made
of
bamboo
sticks
and
fabric
sails resource.
The history of wind.
Canada’s bountiful
and that they had a vertical axis of rotation (Fig. 1.2).
No matter how far back we go in time,
So how much wind do we have at our
The“According
windmilltowith
a horizontal axis
of rotation,
which
is the
windmill,
the World
mankind
has relied upon
the wind.
The traditional
disposal?
In Canada,was
we have more than
probably
invented
Europe
of theused
vertical-axis
of the
ancient Egyptians
wind to sail thewindwheels
Nile,
we could
ever Orient.
use. Wind is abundant and
Energy
Council,induring
the independently
and
the ancient
created
free. Our
vast landscape, our three windy
The first
information
has its
in Persians
the year
1180theinfirst
the Duchy
of Normandy.
“ origin
lastverifiable
decade, global
wind
windmills
to grind
grain
and pump
water.
coastlines,
the plains
and mountains all
According
to
this
source,
a
so-called
post
or
trestle
mill”
is
supposed
to
have
stood
there.
energy capacity has doubled
contribute to this endless resource.
The Dutch
used windmills
to reclaim
their a post
Similarevery
information
also
points
to
the
province
of
Brabant,
where
windmill
was
3 years – about a
Wind: a power unlike any other.
30% increase annually.”
land from the sea by draining wetlands.
Windmills were first used to generate
electricity in North America in the 1800s and
continued to do so up until the 1930s when
the extension of the electric power grid to
rural areas brought the decline of demand
for electricity generated on-site. As we enter
the 21st century, the continued evolution of
wind turbine technology means wind energy
is poised to power us into the future.
Figure 1.1. Vertical-axis windmill for milling grain, Afghanistan
Today, we are just beginning to tap into
Canada’s potential wind resource, which
currently powers the equivalent of 315,000
Canadian homes. Tomorrow we hope to
do even more. Like Denmark, Canada has
more than enough wind resources to meet
20% of our electricity demands – enough to
power 17 million homes! As long as the wind
continues to blow, there is a great future in
wind energy.
(Deutsches Museum)
Figure 1.2: Vertical axis windmill for milling grain, Afghanistan (Deutsches
Museum) [4]
CHAPTER 1. INTRODUCTION
3
interest in windmills for electrical generation, with some interest in water
pumping, and occasionally space heating particularly in locations without
an alternative (cheaper) form of energy. Modern three bladed wind turbines
which have resulted from this interest frequently look little different from
some of the earlier versions.
As in all forms of energy which are non-nuclear the basic source of the
energy is the sun. It has been estimated that 12 % of the total solar radiation
falling upon the earth’s outer atmosphere is converted directly to wind energy.
However, this total quantity could not possibly be usefully adapted as an
energy source. For all practical purposes we are forced to consider the wind
energy in a region within a few hundred meters of the earth’s surface. It
is theoretically possible to show that the earth’s energy requirements could
be met with this restriction with the use of only a small proportion of the
earth’s surface. (See Chapter 2).
The total energy requirement of the modern world, however, has never
been met to any significant extent by wind. In spite of this there is considerable interest in the use of wind power. Modern machines differ from
previous machines in that they are built with considerable prior knowledge
of how they will behave. This is due to the applicable technology which is
now being used, much which was developed for use in other fields such as
helicopter blade design and modern tower construction.
The modern era of wind energy equipment design is generally considered
to have begun in the early 1970’s as a result of increasing oil issues and
environmental awareness. However, prior to this developments in Denmark
beginning in the 1900’s and culminating in the Gedser turbine (1955-56)
(Figure 1.9). A unique and important machine was designed in the United
States in the period 1934-1941. This is the Smith-Putnam turbine built in
the United States in Vermont. This machine built on a 610 m mountain was
rated at 1250 kilowatts (kW), Figure 1.3. The diameter with two stainless
steel blades was 53 m in and the hub was at a height of 37 m. This unique
machine, many years before its time, provided wind energy to the main power
grid of Vermont from October 1941 to March 1945 when a main blade failed
and in 1945 the machine was not rebuilt.
CHAPTER 1. INTRODUCTION
Figure 1.3: Smith Putnam wind turbine 1.25 MW, 53 m rotor 1940-1945
4
CHAPTER 1. INTRODUCTION
1.2
5
HISTORY
The generation and use of energy has always been in the forefront of human
technological abilities. The earliest as well as the latest windmills are no
exception to this and as a result have always represented some of the most
advanced abilities of mankind. The earliest windmills were used for water
pumping, irrigation and milling. The highest level of windmill technology
prior to the industrial revolution was in Europe. However, there are many
examples of windmills in the Middle East, in China and in North Africa.
There is mention in an Arab chronicle of windmills in use in Persia in the
seventh century. This is one of the earliest documentations of a windmill. In
Persia in the tenth century vertical axis windmills were used as water mills.
There is some evidence that the vertical axis machine concept went to China
with Ghengis Khan.
In Western Europe the earliest recorded record of a windmill is in France.
This refers to a windmill in a deed in the year 1180 and refers to a windmill
in Normandy. In England the earliest record is in a rental note dated 1185
for a windmill in Yorkshire. There are other records from 1191 of a windmill
in Sussex and as a result the origin of the European windmill is generally
thought to be from the twelfth century. The earliest record in Holland is
from the year 1274 and records a windmill in Haarlem. Windmills in southern
Europe became common only in the fourteenth century and fifteenth century.
During this period, i.e. the fifteenth century, windmills also appeared in
Spain and Western Asia. As is usual, clear information on the origin of
windmills is very difficult to come by.
The basic concept of the windmill having been disseminated basically in
the period from the year 1000 to the year 1500 underwent rather separate
development in various parts of the world. As a result it is rather easier to
discuss the basic types based upon considering each country separately.
1.2.1
The English Windmill
In England the windmill underwent several different types of development
ending about 1875. The earliest type of windmill is the type known as the
postmill, shown in Figure 1.4 without the sails (blades). A postmill in an
early version has the body which carries the sails and all the machinery
mounted on an upright main post which was usually of oak which can turn
through a full circle in order to face the wind. The post does not rotate
CHAPTER 1. INTRODUCTION
6
but on it is mounted a pin pole which rotates and is attached to the main
rotating body. The post is supported at the bottom by quarter bars which
transmit the load to the foundation. The main housing contains the wind
shaft which is the main drive shaft of the windmill and on which are mounted
the blades. The main spar of the blades is called a whip and a gear mounted
on the wind shaft drives the upright to which is connected the mill wheels or
the pump shaft. In order to counterbalance the weight of the sails the body
of the postmill is overweighted towards the rear. At the rear of the postmill
is the tail pole which is connected permanently to the rotating housing and
to a wheel resting on the ground. Moving this tail pole by hand allows the
rotation of the windmill into the wind. Stairways are usually connected to the
tail pole and rotate with it. In earlier versions of the postmill the wind shaft
was horizontal. This resulted in wear and eventual unbalance of the windmill.
In later versions the wind shaft was angled upwards in order to aid in this
balancing of the mill. Additional hardware included brakes both on the tail
pole and on the main shaft, subsidiary shafting and gearing to hoist sack and
drive secondary mill wheels. Early versions of the postmill in England were
usually boarded other than thatched on the housing. The earlier versions of
the postmill were almost entirely of wood. In later versions cast iron gearing
and bearings were used. The foundation was usually of stone in order to
prevent rotting of the timbers. The typically English postmill was fourbladed with a whip near the leading edge. Most early postmills were canvas
bladed but later versions were shuttered. In later versions of the postmill a
fan tail was added. This was a small windmill at right angles to the large mill
which would be used to obtain power for the rotation of the windmill into the
wind. With the windmill pointing into the wind the fan tail being at right
angles to the wind would not rotate. If, however, the wind direction changed
the fan tail would rotate and by suitable gearing could be arranged to drive
the wheels which would rotate the postmill into the direction of the wind.
Fan tails were mounted either high on the postmill above the main housing
or low near the stairways in the wake of the main housing. The ability of a
fan tail to bring a windmill into the wind depends upon its position. A fan
tail mounted high above the main body of the windmill can easily keep the
windmill within ten degrees of the wind direction. However, those mounted
close in the wake of the main housing are able to keep the windmill only
within approximately ±20o of the wind direction.
The other common type of windmill found in England is the small smock
mill shown in Figure 1.5. The name originates from the similarity to the
CHAPTER 1. INTRODUCTION
7
Figure 1.4: English Post Mill Garboldisham post mill [5]
English country smock. This is a multi-sided wooden mill usually mounted
on a brick base and topped by a cap which rotates and carries the sails and
wind shaft. If there is a fan tail this is also mounted on the rotating cap.
A balcony is frequently added in order to give access to the lowest rotation
point of the sails. These types of mill originated at least as early as 1650.
Since the amount of rotating equipment is much smaller than the postmill,
smock mills can be considerably larger. The tower mill is similar to the smock
mill except that it is constructed of brick up to the point where the cap lies.
Typically the tower mill is larger than the smock mill.
Drainage mills are of the same three types mentioned above and differ
from grinding mills only in that the internal equipment is rather simpler. The
basic internal working of a drainage mill has a vertical shaft running down
to the pumping area where a crown wheel turns a large pit wheel mounted
on the scoop shaft. The scoop or paddle wheel is typically of a slightly larger
diameter than the pit wheel and is usually of the order of 3 m in diameter. As
a result the typical lift of a windmill is approximately 1.8 m. Larger heights
of rise must be accomplished by staging windmills.
CHAPTER 1. INTRODUCTION
8
Figure 1.5: Lacey Green Windmill is pictured today near Princes Risbourgh,
Buckinghamshire. It is the oldest working smock windmill in England, dating
back to 1650 [6]
The Workings of Windmills
We can begin by considering a typical post mill used for grinding grain. The
usual machine was four bladed although many five and eight bladed machines
existed and a small number of multi-bladed machines with thirty or forty
blades were built. Cloth sails were most common and were laid out on a
wooden framework. The leading edge of the windmill for about one quarter
of the chord was typically solid. The whips which are the main load-bearing
member of the blade were, in a good machine, mounted at approximately
one quarter chord. This according to modern technology is the centre of the
bending moment on typical airfoil sections and hence probably represents an
optimum achieved by experience. The whips are bolted to the stocks which
are shafts running through the centre of the assembly at the end of the wind
shaft which is called a canister. The sails were arranged to pass near the
ground or near a balcony where they could be reefed. Four typical reefs were
used shown in Figure 1.6. These are full sail, dagger point, sword point, and
first reef. On a machine whose blades reached to the ground this could be
accomplished from the ground. Early sails were set up with a constant pitch
of approximately 20o . However later machines were aware of the necessity
CHAPTER 1. INTRODUCTION
9
Figure 1.6: Sails and reefing
for increased twist near the root and less twist near the tips. As a result, this
twist was added in later machines. In older English terminology, this twist is
called weather so that there is considerable weather near the roots and less
weather near the tip of the blades.
Wind mill development seems to have been somewhat trial and error up to
this point but a reliable and functioning wind mill had economic advantage
to the owner so it was a subject of scientific interest. The first organized
experimentation using scientific principles appears to be Smeaton[8] who
found the external wind to be too variable for studies and designed a rig to be
rotated at constant velocity by pulling a string. He studied the relationship
between weather and performance. He found small angles at sail tips and
larger angles toward the root gave better results. He reported that increasing
the surface area of the sails had a detrimental effect on performance. He also
determined that the load is proportional to velocity2 (approx)and Power is
proportional to velocity3 (approx). The effect of weathering was considered
CHAPTER 1. INTRODUCTION
10
Figure 1.7: Smeaton experimental apparatus for windmill testing [8]
experimentally by John Smeaton [8] in 1759 and as a result, weathering was
typically about 20o at the root and about 5o at the tip in later designs. The
cloth of the sails was supported by back stays and enabled the pitch of the
sail to be controlled fairly accurately. A diagram of his test apparatus is
shown in Figure 1.7.
In 1722, a Scot named Andrew Meikle, invented the spring sail. This
consists of a series of hinged shutters whose longitudinal axis is along the
chord of the blade. These shutters were arranged to be open or closed by
a central control mechanism. The effect was that of a Venetian blind. The
shutter bars were rotated to make the blade effectively solid by a bar which
ran towards the centre of the wind shaft. Linkages running through the shaft
CHAPTER 1. INTRODUCTION
11
could open or close the shutters. Later versions of the spring sail arranged
for this to be done automatically by the fan tail. In another version elliptical
springs near the centre of the wind shaft allowed centrifugal forces to control
the opening or closing of the slats. At high rpm, which would occur under
high winds, the slats flew open. At lower rpm the springs held the slats
closed and allowed power to be generated.
The blades were attached to the wind shaft, which as indicated before
was typically inclined to the horizontal by five to fifteen degrees in order to
help the balance of the windmill. Immediately inside the housing was the
brake wheel which is the main driving gear. The brake wheel had a brake
band around it with a series of levers to brake the windmill. This lever was
usually operated through a winch. Most windmills cannot be stopped in high
winds with the brake alone. The friction would cause heating and danger of
fire. The more common way to stop a windmill was to rotate it out of the
wind and then to apply the brakes.
The brake wheel as the main driving gear with its axis inclined to the horizontal drove a gear on a vertical shaft. This smaller gear called the wallower
and its vertical shaft was sometimes attached directly to the mill stones or
could through a further reduction drive the mill stones. In a post mill the
entire equipment described at this point plus the housing plus associated
stairs, subsidiary hoists and hoppers would rotate with the windmill. The
mill stones could be driven from a shaft which came downwards from gearing
above (overdrift) or could be driven from a shaft which came up through the
stationary bottom stone to the top rotating stone (underdrift). Typical gear
ratios were of the order of three or four to one from the brake wheel to the
wallower and approximately two to one from the driving upright shaft to the
mill wheels. Hence the mill stones typically grinding at five or six times the
rate of the windmill.
The earliest gear wheels were of course made of wood and typically had
pegs or wooden teeth inserted in manufactured circular discs. Smaller wheels
often consisted of two discs with wooden bars acting as teeth between them.
These were called lantern pinions. Later machinery of course was made of
cast iron. Wooden teeth and cast iron was a quiet combination which was
frequently used.
A common method of stopping the windmill which had got to too high a
speed was to choke the mill stones with grain, attempt to move the windmill
out of the wind by hand and then apply the brake. The slat or patent
sails allowed dumping of the wind and hence allowed braking to be directly
CHAPTER 1. INTRODUCTION
12
applied.
Postmills frequently had more than one set of grind stones in operation.
This was easily achieved by having the vertical shaft driven by the wallower,
drive gearing to two sets of grind stones. Subsidiary gearing can drive sack
hoists, flour dressers, grind stones and other machinery.
In water pumping mills used for drainage and other purposes, there is
considerably less machinery. Usually the brake wheel and wallower at the
top and a bevelled crown wheel at the bottom of the upright shaft.
The main body of a post mill which is known occasionally as the buck is
usually boarded up or occasionally thatched. That part of the buck forward
of the main pivot or pintel is called the breast or head and the rear part of
the main assembly is the tail. The crown tree rests on the pintel and rotates.
The crown tree is usually a heavy beam high up in the buck to which is
attached the entire buck assembly. The entire assembly must be extremely
strongly made, since the grind stones which are rotating with the windmill
must be kept level at all times. Some but not all post mills have a collar or
bearing lower on the pintel beam to take some of the sideways load. In post
mills which do not have a collar, much of the load will be taken on the tail
pole which is also used to orient the windmill into the wind.
Mill stones are approximately 1.2 m or slightly larger in diameter and 20
to 35 cm thick. Each stone can weight up to one and one-half metric tons.
The lower or bedstone is set level with its upper face slightly above the floor.
It is contained inside a vat or tun which is a wooden box slightly larger in
diameter than the stones. The stone has a central hole through which the
main drive shaft comes for an underdrift machine. Overdriven machines have
the main driveshaft coming down. The top rotating or runner stone also has
a hole in the centre for driving. The runner stone must be carefully balanced
and run central to the shaft. Balancing is usually achieved by running molten
lead into various cavities on the upper surface or through balancing weights
near the rim of the stone. The runner stone is raised or lowered through an
arrangement of screws and levers under the floor in the lower stage of the mill.
The distance between the two wheels must be carefully controlled. Under no
circumstances can the two wheels touch. However, the mills must be close
together and the distance between them allowed to vary depending upon the
quantity of grain being milled. Larger distances are used for larger quantities
of grain. The distance between the mills is controlled by a teetering screw
and a governor. This governor is usually a flyball governor and raises or
lowers the runner stone fractionally depending on the rotational speed. As
CHAPTER 1. INTRODUCTION
13
the mill slows down the governor drops and the runner stone is lifted to its
widest setting. The stones must be regularly disassembled and dressed. This
consists of renewing the necessary flatness and channels in the stone surfaces
which are worn away. Grind stones can grind between three hundred and four
hundred tons of grain between dressings. The dressing process can take up
to one hundred hours and was done either by the millwright or occasionally
by traveling tradesmen who specialized in this particular aspect of grinding.
Above the runner stone is a grain hopper. The flow of grain from the
hopper is controlled by a sliding door which controls the flow into a feed
shoe. The feed shoe is automatically vibrated by being held against the
drive shaft which in this section is square. As a result the hopper vibrates
four times for every rotation of the drive shaft and serves as a vibratory
feeder. A warning bell is also attached to the feed hopper such that the
bell will ring when the level of grain in the hopper decreases below a certain
value and warn the miller. Stones running free of grain can touch and this
can destroy the surface of the stones.
The ground grain moves between the millstone moving outward due to
the action of the slots or furrows on the millstones and falls outward from the
edges into the wooden vat around the stones. The interrelation of rpm, stone
distance, grain quality and grain type ensure that good grain milling was a
highly developed skill. In addition the grain is heated during the milling
process and must not be allowed to become hot. The stones must be run
to capacity at all times and neither be choked nor run empty. In England
during World War I, regulations required that white flour be produced to a
certain proportion of the total milling by windmills. This was not possible
for most millers and resulted in many windmills producing only animal feed.
A windmill can produce an excellent quality of whole wheat flour; however, it
crushes the bran making it harder to separate from the white by comparison
to the modern, power driven roller mill. In addition, combine harvesting is
not as clean as traditional hand cleaned grain and a windmill is not equipped
for a high quality cleaning process. These problems were at the root of the
decline of the flour milling windmill in England. However, in many parts of
Europe windmills today, produce excellent quality flour which can be sold
through speciality food stores.
CHAPTER 1. INTRODUCTION
14
Smock and Tower Mills
The interior mechanisms of smock and tower mills are not significantly different from those mentioned above. However, their larger size allows considerably more interior equipment, and the smaller quantity of rotating material
allows a larger size of blading. The brick bases of smock and tower mills are
waterproof as opposed to the wood which is necessary in the post mill. The
essential difference, however, is in the cap. The cap contains the wind shaft
and brake wheel which drives the wallower. The wallower is centrally located
on these mills so that the brake wheel may rotate about it. In addition, a
fan wheel if it is used is also mounted on the cap.
The cap has several requirements. It must be strong enough to contain
the forces on the sails, wind shaft and fan tail. It must move smoothly on
its base or curb and it must be weatherproof. A well designed cap will also
give minimal wind resistance. The basic construction of a cap consists of four
main beams arranged as a square which sit on a circular geared curb mounted
permanently on the tower. The wind shaft and its two main bearings sit on
this square array as does the structure required for the fan tail. The square
beams are heavily reinforced and arranged to provide maximum bearing on
the curb. Occasionally, rollers are fitted between the beams and the curb.
The curb will have cogs on it which engage with a worm or pinion wheel
which is driven by the fan tail or a hand winding mechanism. The worm
arrangement may be inside or outside the curb. The centering of the cap
on the curb is accomplished by centering wheels, usually four which are also
attached to the cap frame and run inside the curb. Brackets or flanges are
also arranged to keep the cap from being blown off the curb. The entire
assembly is then housed over with wood or sheet metal.
1.2.2
Dutch Windmills
Holland has the largest number of existing older windmills in Europe. Approximately 9000 existed in the 19th century and almost 1000 remain in
existence today. These comprise both milling and pumping mills. Pumping
mills are capable of pumping approximately 1.5 m. Since many dykes required pumping of 4.5 m or more, it was necessary to stage the windmills in
gangs of two or three to achieve the total lift. Since large quantities of windmills were required to empty the large areas behind the dykes, large sets of
windmills were often built called molengang. At Kindrdyjk in Holland there
CHAPTER 1. INTRODUCTION
15
are sixteen mills together in a molengang.
The form of post mill used to pump water in Holland is called a wip mill.
To transfer the drive of a post mill to a scoop wheel, it was necessary to
transfer the drive through a shaft which runs through a hollow post. Usually
the base of the wip mill is thatched to enclose the crown wheel at the bottom
of the vertical shaft. At a later stage the polder mill was developed for
water pumping. This is an octagonal smock mill with thatched sides and
a moveable cap. The base is usually brick or wood and contains the scoop
wheel. Thatching was desirable by comparison to brick since it was frequently
necessary to construct the windmill on soft soil.
The wip and polder mills are water pumping mills. As in England, many
corn grinding mills were also in existence. Some windmills were used for other
types of grinding. Chalk, spices and cement mills as well as oil presses are
frequently found in Holland. The Dutch mills often were used for lumbering.
The main vertical shaft drove a horizontal crankshaft. The crankshaft was
connected through a connecting rod to a saw frame. The saw frame had
a series of vertical saw blades under tension, arranged to cut on the down
stroke. The windmill also drives a carrier which forces the log against the
saw blades.
Removal of the Dutch windmills to be replaced by pump and modern
milling equipment continued until the 1930’s. However, today the Dutch
have undertaken extensive repair and restoration work and millers who work
mills are subsidized.
1.2.3
Other Countries
France, Portugal and Spain all have large numbers of windmills. Many of
these are derelict as in other countries. Two examples of many can be seen
in Fig 1.8. As in other countries, the post mill was the earliest form and
tower mills the later form.
In Canada and the United States where running water was fairly freely
available many water mills were in existence. As a result, windmills were
rather less popular and were concentrated on the East Coast. Since significant construction occurred after the development of the smock mill in America, the existing windmills in America are wooden smock mills. Although it
is known that post mills were built none are presently in existence. United
States windmills are copies of types existing in Europe. Many of the existing
American windmills are similar to French and Mediterranean designs.
CHAPTER 1. INTRODUCTION
16
It is interesting to compare the effectiveness of the earlier windmills with
present day efficiency and design techniques. A Dutch four arm windmill
has an efficiency of around 16 or 17%. This may be compared with the
efficiency to be expected from a modern machine of the same relative rotating
speed which could be as high as 50%. In spite of this relative inefficiency,
several important advances were made by trial and error. In particular, the
positioning of the main spar at the quarter chord, that is 1/4 of the way
between the leading edge and the trailing edge of the blade was achieved. It
is known today that this is the best location to prevent aero elastic instability
such as flutter. Drees [2] has studied the development of these early machines.
He also notes that on one machine the blade twist is quite similar to that
which should be achieved by modern design methods. The best match occurs
at approximately 70% of the way out of the blade.
As will be shown in Chapter 3, this is the approximate position of maximum torque generation. The poorer match occurs near the root and tip
where less power is generated and a poor match is more acceptable. In addition, good modern airfoil designs have a leading edge camber. That is the
front 1/4 of the blade is drooped. This ”droop-snoot” was found empirically
by the Dutch around the end of the 17th century.
1.3
Wind Power in the 20th Century
By 1900, the European water pumping and grain grinding windmill was in
a period of decay and many of these machines were being removed. The
exception perhaps is the development from 1891 to 1918 by Poul LaCour
in Denmark of electricity generating turbines (20-35 kW) based on Danish
smock mills. These machines generated DC and some were used to generate
hydrogen which was then stored and then burned for lighting of a school. Juul
in Denmark constructed a 200 kW aerodynamic stall-controlled induction
machine in the early 1950’s, a significant advance at the time (see Fig 1.9).
This could be considered the first modern wind turbine as it has 3 blades, and
is stall controlled. It featured electromechanical yawing, an asynchronous
generator, and emergency aerodynamic tip brakes which were released by
centrifugal force in case of over speed. The turbine, which for many years
was the world’s largest, was incredibly durable and ran for 11 years without
maintenance. The Gedser wind turbine was refurbished in 1975 at the request
of NASA which wanted measurement results from the turbine for the new
CHAPTER 1. INTRODUCTION
17
U.S. wind energy programme. The machine ran for a few years with test
measurements after which it was dismantled. The nacelle and rotor of the
turbine are now on display the Electricity Museum at Bjerringbro, Denmark.
The American farm windmill which had achieved its basic design in the
1800’s was entering a period of greatly increased popularity in the agricultural
parts of the United States.
The American multi-blade farm windmill achieves only approximately
30% efficiency. However, this is very good for the low rpm type of machine.
As will be pointed out in Chapter 4, lower rpm machines are limited to lower
efficiencies than the higher rpm devices. Many versions of this machine are
still in active production. In addition, many tens of thousands of these
devices are still in use throughout Australia, Africa and North America.
In the 1920’s and 1930’s there was increased interest in the use of wind
for both sail power and electrical generation. It was long known that a rotating cylinder in a crosswind would generate a thrust at right angles to the
wind. This is called the Magnus effect. In 1925, Flettner developed a ship
with rotating vertical cylinders to provide the main thrust. This ship actually crossed the Atlantic Ocean. Another version of this device proposed by
Madaras was constructed in the United States in 1933. In this case, a 27 m
high 8.5 m diameter cylinder driven by an electric motor was mounted on
a circular train track. It was proposed that the wind would drive the cars
around the circular track. Energy would be extracted through electric generators in the car axles and transmitted by the rails to a switchboard. Only
small components of this complete system were actually built and tested.
The program was eventually discontinued for economic reasons. Rotating
cylinder devices tend to be inherently less efficient than other types of wind
machine per unit of swept area.
Electrical generating machines were built in France where a 20 m diameter
machine was built in 1929 and Russia which constructed a 30 m diameter,
100 kilowatt machine in 1931.
By far the most impressive machine (from a scale perspective) built prior
to the present period (1941-1945) was the Smith-Putnam wind turbine built
on Grandpa’s Knob as shown in Fig. 1.3. As mentioned before, this machine
had two blades at 53 m diameter on a 37 m tower. The two blades were
downwind of the main tower. This allowed high winds to allow the blades
to cone. Coning is the downwind deflection of the blade due to high wind
speeds. The machine was designed to produce 1000 kilowatts of power from
a 13 m/s wind. The pitch of the blades was controlled hydraulically and
CHAPTER 1. INTRODUCTION
18
this system was also used to feather the blade to a low lift position in order
to prevent over speeding. A fly-ball governor was used as control. The
orientation of the blades into the wind was accomplished electrically. The
windmill drove a synchronous generator. In 1945 a cost study of this machine
suggested that approximately twenty of these units could supply power at a
cost approximately 50% higher than the existing cost of power to the Vermont
Power Commission. This was based upon a design which was not altered
through the lessons being learnt by the machine. The loss of this machine
due to fatiguing of the blade root was a blow to the development of wind
powered generators of large size.
1.4
Machine Types
Many different basic types of wind turbines exist. A sampling of some of
the more common ones may be seen in Figures 1.10, 1.11. The wide range
of possibilities may be divided into essentially horizontal axis and vertical
axis machines. Horizontal axis machines orient their main axis of rotation
directly into the wind. For this reason, they must have a device for achieving
this orientation except in very unusual circumstances where the wind may
be dependent upon not to vary significantly in direction.
Vertical axis machines have the corresponding advantage that many of
them do not need to be oriented into the wind and hence have the possibility
of being mechanically simpler. Vertical axis machines, except for the Chinese
version, are relatively more modern in origin. This is particularly true of the
Savonius, and Darrieus designs.
These basic designs may be combined. For example, the combination of a
Savonius and Darrieus rotor on the same shaft gives high starting torque due
to the Savonius rotator and high efficiency at high rpm due to the Darrieus
rotor.
The basic wind generator may be combined with various other devices to
give desirable properties. A possible addition is a diffuser or concentrator.
The concentrator concentrates wind from a larger area than the basic windmill through the windmill. A diffuser effectively slows the exit flow from the
windmill and as a result decreases the static pressure at the immediate exit
from the windmill. This decrease in static pressure may be used to increase
the power through the windmill compared to a windmill of equivalent diameter without a diffuser. The Enfield-Andrea windmill has a small windmill
CHAPTER 1. INTRODUCTION
19
in the shaft of a tower. A larger windmill rotating on a horizontal shaft produces a very low pressure at the tips of these large blades. A duct runs from
one side of the small windmill up through the main shaft and out through
the large rotating blades on the horizontal axis due to this low tip pressure.
Power is generated by the small windmill inside the shaft. Other forms of
concentrators include the venturi and confined vortex designs. These are
shown schematically in Figure 1.10 and 1.11
A fundamental parameter used in describing all wind turbines is the tip
speed ratio, λ. This is the ratio of the speed of the tip of the blade to the
wind speed.
ΩR
(1.1)
λ=
U∞
In a given wind a high tip speed ratio device will rotate faster than a
low tip speed ratio device. Typically, high speed tip ratio machines are more
efficient than low tip speed ratio devices. In addition, low tip speed ratio
machines typically have high solidity. The solidity is the proportion of swept
area which is covered by blading as seen from the wind direction. Because low
tip speed ratio machines must produce their work with lower tip velocities,
they must deflect the wind more. This larger deflection implies larger blade
surfaces. The Savonius rotor for example is a low tip speed ratio device and
the Darrieus device is a high tip speed ratio device. An extreme case of a low
solidity device is the single bladed horizontal axis turbine which as a result is
a very high tip speed ratio machine and has therefore the potential for very
high efficiencies. The single bladed rotor is of course counter-balanced.
The majority of the machines which can be conceptualized have not been
built. Of those which have been built, few show immediate potential for
economic viability. At the present time, commercial or government funded
machines which are producing electrical power are limited to the horizontal
axis multiple bladed devices and historically the Darrieus machine.
Machines which have received considerable development effort include the
above machines plus the gyromill, the Savonius-Darrieus combination, the
confined vortex and the unconfined vortex configurations. Of course, many
amateur machines of a wide range of designs have been built (see internet).
In all of these machines, the energy comes from the windmill in the form
of shaft power. This shaft power may be used as a direct mechanical drive,
to drive a mechanical pump, or to power an electrical generator. Some uses
of the shaft power also include direct heat generation to various frictional
devices. Once developed to either mechanical, electrical or heat energy, the
CHAPTER 1. INTRODUCTION
20
energy may be stored by any one of the complete range of energy storage
devices and used in any possible way.
Along with the wide variety of possible geometries for windmills, recent
years have spawned a large number of manufacturers in all parts of the world.
The majority of this activity is directed toward multi-bladed horizontal axis
machines. This is partly because this machine has received by far the largest
amount of developmental work. In the early stages, almost all the development work on large electrical generation machines was government funded.
In contrast, the smaller machines in sizes less than 15 meters in diameter are
increasingly being designed by private corporations or individuals.
1.5
Oil Crises 1970’s - USA and worldwide
The oil Crises in the mid-1970’s saw a renewed interest in wind energy in
the United States. There was a significant period of development in the US
government with the MOD series of wind turbines as seen in Figure 1.12. In
this figure the Mod-0A is a 200 kilowatt design for a 31 m/s wind. Three
machines of this type have been built. Mod-1 is a 2000 kilowatt windmill of
which one version was built. The tower height is 43 m and rotor diameter
is 61 m. There are two blades. The goal in 1978 was production of power
for an approximate cost of 2 cents per kilowatt hour. The large systems
which have presently been built are all prototype models. As a result, extrapolations must be made concerning the cost of multiple versions of these
machines. California led the way in the development of wind energy due
to incentives like investment tax credits and good winds in certain regions.
Many of these machines were still prototypes and performance was variable.
After tax credits were withdrawn the industry collapsed. From this period
onward the main proponents of wind energy technology were in Europe with
turbines from Denmark and Germany dominating the industry.
1.6
State Of The Art
At the present time there is significant worldwide interest in the development
of renewable energy. Of these technologies wind energy seems to be the most
viable at this time and has been growing significantly worldwide. There has
been a significant deployment of wind turbines in many countries partially
CHAPTER 1. INTRODUCTION
21
due to government power production incentives such as long term guaranteed
production rates and standard offer contracts. In conjunction with these developments wind turbine technology has improved due to advancements in
many engineering areas including: electrical controls, computers, modeling,
composite materials, aerodynamics, sensors, etc. Wind turbines are becoming larger and applications for offshore turbines will continue to drive industrial development. In a field which is developing at the rate of wind power,
it is obviously not possible to be aware of the latest developments without
access to the latest research literature.
CHAPTER 1. INTRODUCTION
Figure 1.8: Several inoperable windmills in area around Duras, France
22
CHAPTER 1. INTRODUCTION
Figure 1.9: Gedser wind turbine 200 kW 1957
23
CHAPTER 1. INTRODUCTION
Figure 1.10: Horizontal Machine Types [3]
24
CHAPTER 1. INTRODUCTION
Figure 1.11: Vertical Machine Types [3]
25
CHAPTER 1. INTRODUCTION
Figure 1.12: DOE/NASA wind turbine development
26
Bibliography
[1] Canwea Introduction to Wind Power.
[2] Drees, J.M. (1977) Blade Twist, Droop Snoot and Forward Spars, Wind
Technology Journal, Vol. 1, 1, pp. 10 - 16.
[3] Eldridge, F.R. (1980) Wind Machines, VanNostrand Reinhold, New York.
[4] Hau, E., (2006) ”Wind Turbines: Fundamentals, Technologies, Application, Economics”, Springer
[5] http://www.norfolkmills.co.uk/Windmills/garboldisham-postmill.html
[6] http://www.dailymail.co.uk/news/article-2398266/How-worlds-oldestsmock-dating-1650-collapse-volunteers-restored-it.html
[7] Beedell, S. (1975) Windmills, David and Charles, London
[8] J. Smeaton ”An Experimental Enquiry concerning the Natural Powers
of Water and Wind to Turn Mills, and Other Machines, Depending on a
Circular Motion” Philosophical Transactions (1683-1775), Vol. 51. (1759 1760), pp. 100-174.
[9] Spera, D.A. (1994) Wind Turbine Technology: Fundamental Concepts of
Wind Turbine Engineering, ASME, USA
[10] Hau, E. (1994) Wind Turbines Fundamentals, Technologies, Application, Economics, Springer, NY
[11] Wailes, R. (1967) The English Windmill, Augustus, M. Kelley Publishers, New York. N.Y.
27