i A STUDY OF WIND POWERED TURBINE GENERATION BY EZUGWU, NICHOLAS U. REG. NO: PGD/EEE/2003235010 DEPARTMENT OF ELECTRICAL/ELECTRONICS ENGINEERING SCHOOL OF POSTGRADUATE STUDIES FACULTY OF ENGINEERING AND TECHNOLOGY NNAMDI AZIKIWE UNIVERSITY, AKWA 2007
51
Embed
A STUDY OF WIND POWERED TURBINE GENERATION BY … · 2012. 2. 13. · i a study of wind powered turbine generation by ezugwu, nicholas u. reg. no: pgd/eee/2003235010 department of
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
i
A STUDY OF WIND POWERED TURBINE GENERATION
BY
EZUGWU, NICHOLAS U.
REG. NO: PGD/EEE/2003235010
DEPARTMENT OF ELECTRICAL/ELECTRONICS ENGINEERING SCHOOL OF POSTGRADUATE STUDIES
FACULTY OF ENGINEERING AND TECHNOLOGY
NNAMDI AZIKIWE UNIVERSITY, AKWA
2007
ii
APPROVAL PAGE
This project report entitled “A Study of Wind Powered Turbine Generation”
by Ezugwu Nicholas U. meet the regulations governing the award of the
Degree of Post Graduate Diploma (PGD) in Electrical /Electronic Engineering
of Nnamdi Azikwe University, Akwa and is approval or its contribution to
I am dedicating this project to my parents Mr. & Mrs. Steve U. Ezugwu, my
lovely wife Mrs. Ezugwu Nnenna and my only beloved sister Ezugwu, R.N.
iv
ACKOWLEDGEMENT
I wish to use this medium to thank the Almighty God for taking me this far. For
it is only by his mercy that am able to make this programme to the end. It is
now time to give him thanks and praise for he has lifted me up at last.
I cannot forget the encouraging, fatherly and inspirational words of my
project supervisor Engr. O.A. Ezechukwu who inspite of his tight schedules,
still made out time to meet me and discuss issue that led to the successful end of
this project.
My regards to my brothers Dr. Ezugwu C.O. and Ezugwu F.A. who due
to their financially commitment saw me through. May the good Lord guide and
protect them for me. May God grant them and their beloved families all their
heart desires that are not evil.
May the peace and love of God be with every soul that contributed in any
way to the success of this project all the days of their lives Amen.
v
ABSTRACT
The (PHCH) Power Holding Company of Nigeria Plc, effort to provide us with
a regular power supply has not yet been achieved. And it is because of the
search for another means of power generation that necessitated the development
of wind turbine.
The wind energy is abundant in several parts. Among the numerous
natural energy resources that Nigeria was blessed with, wind is among the once
in great quantity. Wind can be used in conjunction with turbines to produce
electricity in isolation to supply numerous consumers. The PHCN can as well
use it to serve its customer be it commercial or residential load. It is because of
this aggressive search that in this paper I try to discuss various methods of
converting wind energy to electrical energy. The places it is obtainable and how
it can be best used to achieve maximum result in Nigeria.
vi
TABLE OF CONTENTS
Title page i
Approval page ii
Dedication iii
Acknowledgement iv
Abstract v
Table of contents vi
CHAPTER ONE
1.0 Introduction 1
1.1 Background of study 2
1.2 Energy from wind 4
CHAPTER TWO
2.0 Consideration for wind powered generator 6
2.1 Basic principal of wind power generation 9
2.2 Performance 12
CHAPTER THREE LITERATURE REVIEW
3.0 Types of wind machine 13
3.1 Horizontal axis wind turbine 13
3.2 Vertical axis wind turbine 15
3.3 Power in the wind 17
3.4 Matching wind turbine to load 19
vii
CHAPTER FOUR DESIGN SPECIFICATION FOR WIND TURBINE
4.0 Strength Calculation for Structural Analysis & Safety 21
4.1 System Design 25
4.2 Integration and Control 30
CHAPTER FIVE
5.0 Viability in Nigeria 32
5.1 Analysis of Wind Resource 34
5.2 Identification of Wind Data Source 36
5.3 Advantages 39
CHAPTER SIX
6.0 Conclusion 41
Reference 43
1
CHAPTER ONE
1.0 INTRODUCTION
Since ancient times, people have harnessed the winds and energy. Over 5,000 years
ago, the ancient Egyptians used wind to sail ships on the Nile River. Later, people built
wind mills to grind wheat and other grains. The earliest known wind mills were in
Persia. These early wind mills looked like large paddle wheels. Centuries later, the
basic design of the wind mills was improved. It was given a propeller type blade. The
evolution has been slow but continuous, with two major phases. The first in which
wind energy was directly use for mechanical work, as in windmills, sawmills, water
lifting etc. and a second phase in which mechanical energy was changed into electrical
energy.
Since their first appearance, it have undergone many changes and improvements
in their major components. For instance, the major structure varied from wood frames
to more stable masonry works and subsequently to steel frames.
The interest here is on manufacturing system of which my goal will be looking at wind
as a means to effective power generation and system integration given a complete
program activities addressing electric power market rules.
This wind in question has played a long and important role in the history of human
civilization and is the world fastest growing energy technology, when
2
flowing wind is captured and is turned into electricity and this process is called
HYBRID WIND SYSTEM OR WIND GENERATED POWER.
Wind powered generation. Turbine and generator coverts the energy into
electricity which can be used in isolation or fed into the electrical grid to be used in
homes, farms, business and by industries.
1.1 BACKGROUND OF STUDY
As the search for energy is now probing the realm of the wind and the sun,
human beings are sure to get a lasting solution to electricity problem. In using energy
to produce mechanical work, humans, since the beginning of civilization, have focused
their attention on those forces found explicit in
nature. Animal, hydraulic, and wind energy did not require transformation into other
energy types.
Instead, the use of such energy type necessitated the invention of mechanisms
whose complexity has constantly increased with the progress of technology. In
particular, since ancient times, the kinetic energy of the wind has been used to propel
ships and machines, such as windmill that proceeded the industrial age. The wind
power has been used for the production of electricity itself, but in small scale.
However, because of world wide energy crisis, wind energy once again is being
studied for large-scale application, along with other
3
sources neglected. In the utilization of the wind or other natural energy forms, the
unlimited potentials of the source must be recognized in contraposition to the limited
supply of fossil fuel presently used.
Solar energy, which generates the wind is considered unlimited. The relationship
between the dynamic force in the wind and solar radiation is direct. Another major
characteristic of wind energy is the absence of any form of pollution, either in its direct
utilization for the production of mechanical work or in its transformation into electrical
energy. Yet, despite such positive factors, wind energy was neglected until a few years
ago. An estimate 1% to
3% of energy from the sun that hit the earth is converted into wind energy.
Most of this energy can be found at high altitude where continuous wind speeds over
160km/h (100mph) occur. This wind energy is converted through friction into diffuse
heat throughout the earth’s surface and the atmosphere.
The wind power estimated apply to areas free to local obstruction to wind terrain
features that are well exposed to wind such as open plains, table/and hilltops, Exposed
coastal areas are also rice potentials for wind application.
4
1.2 ENERGY FROM-WIND
Wind is simple air in motion. It is caused by uneven heating of the earth’s surface by
the sun. Since the earth’s surface is made of very different types of land and water, it
absorbs the sun’s heat at different rates. During the day, the air above the land heats up
more quickly than the air over water. The warm air over the land expands and rises,
and the heavier, cooler air rushes in to take its place. Creating winds at night, the winds
are reversed because the air cools more rapidly over land than over water.
In the same way, the large atmospheric winds that circle the earth are created because
the land near the earth’s equators is heated more by the sun than the land near the north
and South Pole.
Today, wind energy is mainly used to generate electricity. Wind is called a
renewable energy sources because the wind will blow as long as the sun shines.
Valleys often experience calm conditions at night even when adjacent hilltops are
windy, cool, heavy air drains from hillside and collects in the valleys. The resulting
layer of air is removed from the general flow above it to produce the calm condition in
the low lands.
5
Fig 1
Because of this, a wind turbine located on a hill may be able to produce power all
night, while one located at a lower operation stands idle. This phenomenon is more
likely to occur on high terrain features that reach at least several hundreds feet above
the surrounding land. High terrain features can accelerate the flow of wind. An
approaching air mass is often squeezed into a thinner layer so it speeds up as it crosses
the summit. Over a ridge, maximum acceleration occurs when the wind blows
perpendicular to the ridge side. Isolated hills and mountains may accelerate the wind
less than ridges because more of the air tends to flow around the side. The downward
side of high terrain features should be avoided because of the presence of high wind
turbulence.
Land areas adjacent to large bodies of water may be good wind site for two
reasons. First, a water surface is smoother than land surface so air flowing over water
encounters little frictions. The best shoreline site is one where the wind direction is “on
shore”. Second when regional wind are light as on a sunny day local wind known as
sea or lake breeze can develop because the land and water surface heat up at different
rate.
6
CHAPTER TWO
2.0 CONSIDERATION FOR WIND POWERED GENERATOR
In the 1970s, oil shortage pushed the development of alternative energy source. In
1990s, the push came from a renewed concern for the environment in response to
scientific studies indicating potential changes to the global climate if the use of fossils
fuels continuous to increase. Wind is a clean fuel; wind farms produce no air or water
pollution because no fuel is burnt.
Providing new cost effective advanced and innovative technologies valuation and
performance metrics that will enhance environmental performance and greater
efficiencies. The basic design concept for a wind powered outfit can be seen as in fig.
2.1 inside the wind turbine.
Inside the wind turbine
Fig.2.1
7
This section defines the various part and instrument found inside the wind turbine.
Anemometer: This is an instrument which is used to measure the wind speed by
rotating in the wind and generates a signal proportional to the wind speed and transmit
wind speed data to the controller. Most of which are designed with cups mounted on
shaft arms connected to a rotating vertical shaft.
Controller: The controller starts up the machine at wind speed at about 8 to 16 miles
per hour (mph) and shuts off the machine at about 65mph. Turbines.
Cannot operate at wind speed above the rate of 65mph because their generators could
overheat.
Generators: This is usually an off the shell induction generator that produces 60 cycle
A.C electricity.
Gear Box: Gear box connect low-speed shaft to the high-speed shaft from about 30 to
60 rotations per minute (rpm) to 1200 to 1500 rpm, the rotational speed required by
most generators to produce electricity. This gear box is a costly and heavy part of the
wind turbine.
Low-Speed Shaft: This is the connecting pipe between the rotor and the gear box.
Rotor: This is made up of the blade and the hub. It is a portion of the wind turbine that
collects energy from the wind and rotates about an axis (horizontally or vertically) at a
rate determined by the wind speed and shape of
the blade. The blade is attached to the hub, which in turn is attached to the main shaft.
8
Brake: A disc brake which can be applied mechanically, electrically or hydraulically
to stop the rotor in emergencies is used.
Yaw Drive: This is an equipment that is used to keep the rotor facing into the wind as
the wind direction changes. It is only found in the upwind turbine.
Yaw Motor: This powers the yaw driver.
High Speed Shaft: This drives the generator.
Wind Vane: This is an instrument that measures or decodes the wind direction and
communicate with the yaw drive to channel the turbine rotor properly with respect to
the wind.
Nacelle: This is the housing or casing which sits on-top of the tower and includes the
gear box, low and high speed shafts, generator, controller and brakes. The rotor is
being attached to it
Tower: This is being made of steel lattice and is very high because wind speed
increase with height and taller towers enable turbines to capture more energy and
generate more electricity.
9
2.1 BASIC PRINCIPLES OF WIND POWER GENERATION
Wind power is a measure of the energy available in the wind. It is a function of
the cube (third power) of the wind speed, if the wind speed is doubled, power in the
wind is increased by a factor of eight (i.e. 23) this relationship means that the small
difference in wind speed leads to a large difference in power.
Wind speed is therefore defined as the rate at which airflow past a point above
the earth’s surface.
The output of a wind turbine varies with the wind speed through the motor. The
“rated wind speed” is the speed at which the “rated power” is
achieved. This corresponds to the point at which the conversion efficiency is near its
maximum. In most system, the power output above the “rated wind speed” is
mechanically or electrically maintained at a constant level allowing more stale system
control.
The power output drops sharply at wind speed. This is better explained by the
cubic power law, which states that the power available in the wind increases eight
times from every doubling of the wind speed and decreases eight times for every
halving of the wind speed.
In a particular wind site, the power output expected at the average wind speed
can be determined by the power curve.
10
10 20 30 40 50
Wind speed in MPH
POWER CURVE
FIG. 2.2
Just like the weather, the wind can be unpredictable. It varies from place to place and
from moment to moment. It is invisible and as such can only be measured with a
special instrument. While wind turbines are mostly commonly classified by their
“rated power” at certain “rated wind speed”, energy output is also greatly influenced
by some subtle features of a wind turbine’s design.
These include:
1) Cut-in-Speed: This is the minimum wind speed at which the blade will turn and
generate useable power. This wind speed is typically between 7mph and 10mph.
2) Blade Air Foil Shape: This determines the power produced at moderate speed.
When air flows past the blade a wind speed and pressure differential
created between the upper and lower surface is greater and thus acts to “lift” the blade.
11
3) Tip Speed Ration (TSR): The tip-speed is the ratio of the rotation speed of the
blade to the wind speed. The larger the ratio, the faster the rotation of the wind turbine
rotor at a given wind speed.
4) Rated Speed: The rated speed is the minimum wind speed at which the turbine
will generate its designated rated power. Note that between the cut-in and rated speed,
the power output from a wind turbine increase. The rated speed for most machines is in
the range of 25mph to 30mph.
5) Cut-Out Speed: This is the wind at which the turbine may be shut down to protect
the rotor and drive train machinery from damage. A wind speed sensor is usually used
to activate the automatic brake. And normal wind operation usually resumes when the
wind drops back to a safe level.
6) Operating characters: This includes such thing as low speed on-off cycling, shut
down behavior and overall reliability. These things together determine the turbine
availability to produce power when the wind speeds are in its operating range.
7) Efficiency: The efficiency of the drive train components such as the generator,
the gear box and other can be defined as the ratio of the output to the input. It should
also be noted that the drive converts energy from one form to
8) Another. And in the process, some of the input energy/power is being consumed
to make the device operate.
12
For this reason, the efficiency of the operation is defined as energy output in given a
time.
Efficiency = Energy output in a given time = Wo Energy input in the same time Win
= Power output = Po Power input Pin
9) Frequency of Operation: This is the number of cycles completed in a second.
Actually this varies from time to time. But it is necessary to stabilize it, because
as soon as the frequency is made constant, it goes a long way to influence the
power/energy output of the wind turbine.
2.2 PERFORMACE
The wind does not start as long as the “cut-in speed” has not been reached. And when
it starts it has to reach the “rated speed” i.e. the lowest speed it has to reach before it
will be able to generate its “rated power”. The turbine also shut down automatically as
soon as it gets to the “cut-off speed” to avoid damage on the turbine.
Despite the use of frequency control, the wind turbine would be permitted to
rotate at a variable speed as the wind changed. How ever since the frequency
control has to be maintained. The wind turbine rotational speed can be controlled in
two distinct positions.
1) When the wind speed is between the cut-in speed and the rated speed, the speed
can be controlled by varying the load on the generator
2) When the wind speed is between the rated and cut-off speed, it can be controlled
by changing the pitch on the turbine.
13
CHAPTER THREE
LITERATURE REVIEW
3.0 TYPE OF WIND MACHINE
There are two types of wind machine (turbines) used today. This classification is
based on the direction of the rotating shaft (AXIS). Horizontal- axis wind machine
Vertical – axis wind machine. The size of wind machine varies widely. Small turbines
used to power a single home or business. This may have a capacity of 100 kilowatts.
Some large commercial sized turbines may be up to 5 million watts or 5 watts. Large
turbines are those that provide power to the electrical grid
3.1 HORIZONTAL – AXIS WIND TURBINE (HAWT)
Most wind machines being used today are the horizontal – axis type. Horizontal – axis
wind have blades like airplane propellers. A typical horizontal wind machine stands as
tall as 20 – story building and has three blades that span 20 feet across. The largest
wind machines in the world have blades longer than a football field. Wind machines
stand tall and wide to capture more wind.
The axis of rotation is parallel to the wind flow. Some very large turbine use a motor
driven mechanism that turns the machine in response to wind direction
14
sensor mounted on the tower. A prime objective in wind turbine design is for the blade
to have a relatively high lift-to-drag ratio. This ratio can be varied along the length of
the blade to optimize the turbine’s energy output at various wind speeds.
Horizontal axis
Fig. 3.1
ADVANTAGES OF HAWT
1) Blades are to the side of the turbine center of gravity, helping stability.
2) Ability to wing warp, which gives the turbine blades the best angle of
attack.
3) Ability to pitch the rotor blades in a storm, to minimize damage.
4) Tall towers allow access to stronger wind in site with wind shear. It allows
placement on uneven land or in shore locations.
15
5) Most of them are self-starting.
6) Can be cheaper.
DISADVANTAGES OF HAWT
1) IT has difficulty operating in near ground, turbulent winds because their
yaw and blade bearing need smoother wind flow.
2) The tall towers and long blades (up to 180feat long) are difficult to
transport.
3) Difficult to install, it needs very tall and expensive cranes and skilled
operators.
4) Their height can create local opposition based on impact to view sheds.
5) Downwind variants suffer from fatigue and structural failure caused by
turbulence.
3.2 VERTICAL AXIS WIND TURBINE (VAWT)
Vertical-axis wind machines have blades that go from top to bottom and the most
common type is (Darrien wind turbine). It looks like a giant two bladed egg beaters.
The type of vertical wind machines make up only a very small percent of the machine
used today.
Other vertical axis turbine designs include the savanis, which uses scoops to catch the
wind. A vertical axis machine need not be oriented with respect to
16
wind direction because the shaft are vertical, the transmission and generator can be
mounted at the ground level allowing easier servicing and lighter weight.
Vertical axis (Darrieus Wind Turbine)
Fig. 3.2
ADVANTAGES OF VAWT
1) Easier to maintain because most of their moving part are located near the
ground.
2) As the rotor blades are vertical, a yaw drive is not needed, reducing the
need for the bearing and its cost.
3) Low height useful where laws do not permit structures to be placed high.
4) Usually have a lower tip-speed ratio so less likely to break in high winds.
5) Does not need a free standing tower so is much less expensive and
stronger in high winds that are close to the ground.
17
6) In place like hilltops, ridgelines etc. we can get higher than and more powerful
winds near the ground than up high because of the speed effect of the winds moving up
a slope or funneling into a pass combining with the winds moving directly into the site.
In these places, VAWTS placed close to the ground can produce more power than
HAWTS placed higher up.
DISADVANTAGES OF VAWT
1) Most VAWTS need to be installed on relatively flat piece of land and some sites
could be too steep for them but are still usable by HAWTS.
2) A VAWT that uses guyed wires to hold it in place put stress on the bottom
bearing as all the weight of the rotor is on the bearing. Guyed wires attached to the top
bearing increase downward thrust in wind gusts solving this problem requires a
superstructure to hold top bearing in place to eliminate the downward thrusts of great
events in guyed wired models.
3) Most VAWTS are low starting torque.
3.3 POWER IN THE WIND
The power in the wind can be extracted by allowing it to blow past a moving
wing that exerts torque on a rotor. The amount of power transferred is directly
proportional to the density of the air, the area swept out by the rotor, and the cube of
the wind speed.
The power available in the wind is giving by P = 1/2 Pπ R2V3 .The mass flow of air
that travels through the swept area of a wind turbine varies with the wind speed and air
density. The kinetic energy of a given mass varies with the square of its velocity.
Because the mass flow increases linearly with the wind speed, the wind available to a
wind turbine increase as the cube of the wind speed.
18
As the wind turbines extracts the energy from the air flow, the air is slowed down,
which causes it to spread out and diverts it around the wind turbine to some extent.
Windiness varies and an average value for a location does not alone indicate the
amount of energy a wind turbine could produce there. To assess the climatology of
wind speed at a particular location, a probability distribution function is often fit to the
observe data. Different locations will have different wind distribution.
Because so much power is generated by higher wind speed, much of the average
power available to a windmill canes in short bursts. Half of the energy generated
arrived in just 15% of the operating time. The consequent is that wind energy does not
have a consistent output as fuel-fired power plants.
Additional output can only be made to compensate for load increase by utilizing
advanced wind storing technologies (e.g. giant compressed air storage tank facilities).
Since wind speed is not constant, a wind generator annual energy production is
never as much as its nameplate rating multiplied by the total hours in a year. The ratio
of actual productivity in a year to the theoretical maximum is called the capacity
factor. A well-sited wind generator will have a capacity factor of about 35%. This
compares to typical capacity factors of 90% for nuclear plants, 70% for coal plants,
and 30% for oil plants.
When comparing the size of wind turbine plants to fuel power plants, it is
important to note that 1000km of wind turbine potential power would be expected to
produce as much energy in a year as approximately 500km of coal-fired generation.
Though the short term output of a wind-plant is not
completely predictable. The annual output of energy tends to vary only a few percent
points between years.
The drag type rotor, such as Savonius rotor and American Multi blade, have lower tip
to speed ratio and power coefficient compared to the lift type propeller HAWT. The
19
curve from the propeller type rotor (RAWT) indicates that the rotor is able to maintain
high efficiency over a long range of rotor rpm while the sharp curve from Darreus
rotor experience drop drastically when the rotor rpm movies away from the narrow
optimum range, the Darreus rotor with low power coefficient at low tsr range indicates
a weak self-starting ability thus when the Darrius type VAWT is coupled with a low
efficiency Savonius rotor its initial stating torque is developed and also on the
alternative an induction machine could be coupled with the perils rotor to improve its
starting characteristics.
3.4 MATCHING WIND TURBINE TO LOAD
The rotor efficiency alone will not determine the efficiency of wind energy system as
the coupling of the rotor load like generators will further reduce the overall efficiency.
The power output curve for a system consisting of a rotor and generator depends not
only on the individual efficiency of both components but also how well they were
matched. The plot/curve below shows a torque of rotor at different speed.
Fig. 3.3 Relationship Between load and rotor tsr
Now considering the 6 m/s rotor torque curve and the medium load curve. When the
stationary rotor is subjected to a 6 m/s wind speed, the rotor rpm will accelerate from
zero to the rpm of the intersection between two curves and
stabilize there. The rotor will produce torque following the 6m/s curve until the rotor
torque balances with load torque at the intersection. So by plotting the, rotor torque
20
curve for various speeds, the intersection of these curves will form the actual operating
rpm of the wind turbine at various wind speed. However
the rotor/load torque at different wind speed is not the actual power output, the actual
output will be lower due to losses in the generator mechanical components and
conversion losses.
Fig 3.3 shows another 2 load which is a light and heavy load A light load will cause
rotor to operate at high rpm (high tsr) which will not maximize the rotor efficiency
while the heavy load causes the rotor to rotate slowly (low tsr) which also do not
maximize the rotor efficiency. By selecting a load nearer to the rotor maximum
efficiency point like the medium curve more power can be obtained.
21
CHAPTER FOUR
DESIGN SPECIFICATION FOR WIND TURBINE
4.0 STRENGTH CALCULATION FOR STRUCTURAL ANALYSIS &
SAFETY
It is usually the common practice in Engineering field to have prior knowledge of the
properties of the materials that will be used in a design so that the design could be
carried out with the awareness of the materials properties. With such prior knowledge
adequate provision could be made to compensate the inadequacies of the material
available for use in design.
It is in the light of the above that the strength calculation of these materials
must be done before and during the fabrication process. The most important test will
be the strength of wood/aluminum airfoils under centrifugal load as flying broken
pieces of airfoils are not welcomed especially in wind tunnel. The airfoils model can
be fabricated using either solid woods or aluminum sheets with spar and rib.
Having a wind turbine test model as basic of my estimates and calculation
these strength calculation/estimates can be sub-divided into.
different section as;
i) Airfoil bending stress
ii) Bolt tension at Airfoil joint and washed pressure
iii) Support arm vibration and deflection
iv) Shaft bending stress and vibration
v) Bearing size selection
vi) Pipe bending stress and vibration
22
i) Airfoil Bending Stress:
Hear two main loads on the airfoils are the centrifugal force and the
aerodynamic drop force. The rough comparison of these forces shows that the
aerodynamic forces are small compared to the centrifugal force for the mode material,
solidity and operating speed. The strength property of the wood airfoils will be
supported at the centre so the centrifugal forces will cause highest stress at the centre
where the airfoils join the support arm. To test the airfoil strength both sides of the
airfoil will be loaded with dumbell weights and it will be lifted off the ground by
holding the center of the airfoils. Note that stress concentration is definitely lower
when balancing using hand instead of the actual support.
Aerodynamic force comparism to centrifugal force on the air force is thus F aero = ⅔ Pair U2 Rcl X Cnorm = ½[U2 + (wR) 2]cL Cnorm F centrifugal ½ Pwood (AL) w 2R ½ Pwood (1/2 tcl) W2R – Equation 1 The uniformity distributed centrifugal force compared to the airfoil weight = F centrifugal = m (wR)2 W R - Equation 2 mg ii) Bolt Tension at airfoil joint and washer pressure
Here each airfoil is to be supported by 4 bolts with pairs of nuts which clamp the
wood airfoil. This bolts and nuts joint will further be tightened by using spring and
plate washer. The plate washer will hopefully distribute the stress to a wider area of the
wood.
Calculation on the tension in the bolts caused mainly by an assumed airfoil
imbalance and partly by centrifugal forces showed sufficient strength.
23
The wood will probably be dented in after some time of fast rotation and repeating
assembling/disassembling.
Bolt tension due to centrifugal Force
= F Atensil --Equation 3 Bolt tension and compression due to imbalance = 2Fx L H/2 --Equation 4 Atensil Total Bolt tension =Equ 3 + Equ 4 --Equation 5 Average Compression on contact Surface = F A --Equation 6 iii) Supporting Arm Vibration And Deflection
The rotor vibration might build up if the rotor rpm is operated constantly at fixed rpm
that coincide with the structure nature frequency. The rough estimate frequency is
made by assuming the support arm as a spring while the other handing components
are rigid mass and the supporting components as fixed support. This will not provide
the real structure natural frequency but just as a comparison of how stiff this part of
structure is, compared to the other parts. The pair of flat plates support arm will have
the possibility to act like a single beam which will be very stiff.
24
Cantilevered beam Vibration Formula
δ = PL3 3EL --Equation 7 K = P δ --Equation 8 F = K M --Equation 9 iv) Shaft Bending Stress and Vibrating
The vibration of the shaft at hub position was analyzed as an overhanging had
with simple support beam with the rotor as a lumped mass load. This shaft in question
is supported by a pair of bearing.
Deflection of hub δ = Pa2 (1+a) 3E1 --Equation 10 Stiffness K = p δ --Equation 11 v) Pipe Bending Stress, Deflection and Vibration
The pipe supports the shaft so it will also experience the centrifugal force from
the rotor imbalance. The pipe is very stiff with very high frequency and so in the
vibrating calculation / estimation, the frame was assumed to be a fixed support while
the pipe as a cantilever supporting the rotor mass.
25
vi) Bearing Size Selection
The bearing size should be as small as possible partly to reduce loss but most
importantly to reduce torque loss at start up.
4.1 SYSTEM DESIGN
Here there are two basic arrangements in respect to the system design namely:
i) Constant speed constant frequency (CSCF) system
ii) Variable speed constant frequency (VSCF) system
i) Constant speed constant frequency (CSCF) system: Is where the rotor speed is
held constant by continually adjusting blade pitch and or generator electrical
characteristics.
ii) Variable speed constant frequency (VSCF) system: Is where the rotor speed is
allowed to vary in proportion with the power resulting in maximum conversion factor
for most of the operating zone.
In the variable speed constant frequency alternators, the rotor is meant to rotate
freely at the wind speed. The actual speed of rotation is determined by these speed/load
characteristics of both wind rotor and the generator. The rotor speed determines the
efficiency of conversion from wind energy to mechanical energy. At a particular ratio
the conversion factor K1 is maximum and the efficiency of conversion from wind
energy to mechanical energy is optimum.
26
Since a constant frequency is needed from a variable speed machine an
additional circuit is necessary. A solid state electronic circuit is required to rectify the
instantaneous power output d.c power and another circuit comparing chopper is used to
invert the d.c power back again to a.c at a given constant
frequency (say 50Hz). These additional circuiting makes the VSCF system more
complex and of course more cost effective.
Improvement in the design of VSCF machine has led to the development of
special type called the “Double Output Induction Generator” (DOIG). In this, the solid
state handling only the slip power and this seems an interesting comparism between
the VSCF and CSCF Systems.
The solid state electronics is used for rectification and the line commutator
inverter inverts the slip power back to alternating current. Another version of the
VSCF system has power output at modulated frequency higher than the system
frequency fs (say 50Hz) and processes it down to the system frequency, fs by the
electronic circuitry. The system uses an 8.C exited alternator having a modulated
output. So the output is an amplitude –modulated wave. The speed of the rotor of the
machine nr-Lnw
Where nw = wind speed
L = loss factor
27
Thus the output is a modulated signal consisting of (fr + fs) and (fr – fs)
components.
Fig 4.1i shows the output of a varying speed generator couple to a rectifier. The
varying frequency of the generated power is rectified and inverted again in order to
achieve a constant frequency supply. The field of the generator is excited with d.c
current.
The same process used in fig 4.1i but the field coil of the generation is excited
with alternating current at the system frequency, fs. The effect of the a.c excitation of
the field is a modulated power output with the excitation frequency as shown. Thus if
the system (excitation) frequency is 50Hz, the generator output frequency will always
be fr = + 50H2 and after the conversion to a.c, a filter network is used to removed the
rotor frequency components.
In fig 4.1iii some principle applied in fig 4, 1ii is used but the commutator above
does the operation of the rectifier, inverter and filter. Thus the a.c commutator
generator seems to be simplest methods to obtain constant frequency output from a