Wind Turbines

Transcription

Wind Turbines

PEMP
RMD 2501
Wind Turbines
Session delivered by:
Prof Q.H.
Prof.
Q H Nagpurwala
14
© M.S. Ramaiah School of Advanced Studies
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Session Objectives
PEMP
RMD 2501
This session is intended to discuss the following:
g
• Types of wind turbine
• Working principle of wind turbines
• Efficiency of wind turbines
• Characteristics of wind turbines
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Introduction
PEMP
RMD 2501
E
Everyone
f l wind
feels
i d
but what is wind?
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Wind is the roughly horizontal movement of air (as opposed to an air current)
caused by uneven heating of the Earth's surface.
It occurs at all scales, from local breezes generated by heating of land surfaces
and lasting tens of minutes to global winds resulting from solar retard heating of
the Earth.
The two major influences on the atmospheric circulation are the differential
heating between the equator and the poles and the rotation of the planet (Coriolis
effect).
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Status
PEMP
RMD 2501
• Jun 02, 2008 GE Energy's Installed Fleet of 1.5-Megawatt
Wind
i d Turbines
bi
Surpasses 115 Million
illi Hours off Operation
i
Worldwide
• 8500 units installed of which 5200 exist in USA
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Classification of Wind
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RMD 2501
Winds can be classified either by their scale, the kinds of forces which cause
them (according to the atmospheric equations of motion), or the geographic
regions in which they exist.
exist
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Prevailing winds — the general circulation of the atmosphere
Seasonal winds – winds that only exist during specific seasons
Synoptic-scale winds; winds associated with large-scale events such as
warm and cold fronts and are part of what makes up everyday weather
Mesoscale winds; winds that frequently advances ahead of more intense
thunderstorms and may be sufficiently energetic to generate local
weather of its own
Microscale winds; winds that take place over very short durations of
time - seconds to minutes - and spatially over only tens to hundreds of
metres. Winds that produces convective events such as dust devils and
are essentially
ti ll unpredictable
di t bl
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Wind Resource
•
PEMP
RMD 2501
Surface features have a major impact on local wind, and can increase or
decrease in wind power and speed and cause turbulence
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Fl terrain
Flat
i with
i h obstacles
b
l causes turbulence
b l
and
d decrease
d
i wind
in
i d power
and speed for a significant distance from object
•
Surface Roughness Friction between the Earth and the wind cause the
wind speed to be lower closer to the surface.
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Wind Turbine
PEMP
RMD 2501
Why do we need wind turbine?
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Wind energy is abundant, renewable, widely distributed, clean and mitigates
the greenhouse effect if used to replace fossil-fuel-derived electricity.
Conversion of wind power/energy into more useful forms is done by wind
turbines.
Wind
i d turbines
bi
are usually
ll used
d to generate power but
b in
i certain
i applications
li i
are used as prime movers to pump water (wind mills).
Wind power is used in large scale wind farms for national electrical grids as
well as in small individual turbines for providing electricity to rural
residences or grid-isolated locations.
In 2005,
2005 worldwide capacity of wind
wind-powered
powered generators was 58,982
58 982 megawatts;
although it currently produces less than 1% of world-wide electricity use, it
accounts for 23% of electricity use in Denmark, 4.3% in Germany and
approximately
pp
y 8% in Spain.
p
Globally,
y, wind p
power g
generation more than
quadrupled between 1999 and 2005
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Wind Turbine
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RMD 2501
A wind turbine is a machine for converting the kinetic energy in wind into
mechanical energy. If the mechanical energy is then converted to electricity, the
machine is called a wind generator
Wind turbines are mounted on a tower to capture the most energy
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Wind Mills
PEMP
RMD 2501
A windmill is a machine for
converting the kinetic energy in wind
into mechanical energy to be used
directly by machinery, such as a pump
or grinding stones.
These four- (or more) bladed squat
structures, usually with wooden
shutters or fabric sails, were pointed
into the wind manually or via a tailfan.
Windmills were historically used to
grind grain or pump water from lowlying land.
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Working of Wind Turbine
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The blades act like wings of an airplane –
capturing the energy in the wind.
The blades cut through the air with an angle
off attack
k to the
h wind
i d causing
i
a pressure
differential.
The resulting pressure differentials cause a
force called lift,
lift which propels the blade
forward.
This lift is created because of the airfoil
shape
p of the turbines blades.
In order to propel the turbine, the net torque
caused by lift forces must be greater than
the net torque caused by drag forces.
The blades turn a generator that converts
blade rotation into electricity
The tail keeps the blades facing the wind
In high winds, the blades turn sideways to
limit speed
Powerhead
Tail Fin
Nacelle
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RMD 2501
Alternator
Blades
S i
Spinner
Tail Boom
Tower
Mount
Tower
Horizontal axis wind turbine
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Classification of Wind Turbine
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RMD 2501
Wind turbines are classified based on the following,
• Horizontal axis
• Co
Co-axial,
axial, multi
multi-rotor
rotor horizontal axis turbines
• Counter-rotating horizontal axis turbines
• Vertical axis
• Darrieus
D i
wind
i d turbine
t bi
• Giromill wind turbine or cycloturbines
• Saronium wind turbine
• Terra Moya Aqua wind turbine
• Location
• Onshore
O
• Off shore
• Deep water
• Aerial – Airborne wind
ind tturbine
rbine – Not in practice yet
et
• Ducted rotor
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Horizontal Axis Wind Turbine
PEMP
RMD 2501
Horizontal Axis – blades are placed on horizontal shaft.
• Co-axial, multi-rotor horizontal axis wind turbines –
• patented by Douglas Selsam.
• two or more rotors may be mounted to the same drive shaft turning the
same generator.
• wake vorticity is recovered as the top of a wake hits the bottom of the
next rotor.
• Counter-rotating horizontal axis turbines
• two rotor are mounted on the same shaft.
shaft
• used to increase the rotation speed of the electrical generator.
• allows the generator to function at a wider wind speed range than a
single turbine generator for a given tower.
single-turbine
tower
• to reduce sympathetic vibrations, the two turbines turn at speeds of
different ratios.
• taps more of the wind's energy at a wider range of wind speeds.
speeds
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Ducted Wind Turbine
PEMP
RMD 2501
Ducted wind turbines –
• the ducted rotor consists of a turbine inside a duct which flares outwards
att the
th back.
b k
• they are also referred as Diffuser-Augmented Wind Turbines (i.e.
DAWT).
• the
h main
i advantage
d
off the
h ducted
d
d rotor is
i that
h it
i can operate in
i a wide
id
range of winds and generate a higher power per unit of rotor area.
• another advantage is that the generator operates at a high rotation rate, so
it doesn't require a bulky gearbox, so the mechanical portion can be
smaller and lighter.
• a disadvantage is that the duct is usually quite heavy, which puts an
added load on the tower.
• this is still a research project
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Vertical Axis Wind Turbine
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PEMP
RMD 2501
Darrieus vertical axis wind turbines –
• These are eggbeater turbines.
• They have good efficiency, but produce large torque ripple and cyclic
stress on the tower, which contributes to poor reliability
• The torque ripple is reduced by using 3 or more blades
• they generally require some external power source, or an additional
Savonius rotor, to start turning, because the starting torque is very low.
Giromill vertical axis wind turbines
• this is a lift type of Darrieus turbine.
• Cycloturbine has variable pitch, to reduce the torque pulsation and selfstart.
• The advantages
g of variable p
pitch are high
g starting
g torque,
q , a wide and
relatively flat torque curve, a lower blade speed ratio, a higher
coefficient of performance, more efficient operation in turbulent winds,
and a lower blade speed
p
ratio which lowers blade bending
g stresses.
• Straight, V, or curved blades may be used.
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PEMP
RMD 2501
Saronium vertical axis wind turbines –
• these are the familiar two - or more scoop drag-type devices used in
anemometers and in the Flettner vents commonly seen on bus and van
roofs and some high-reliability low-efficiency power turbines.
• they always self-start when three scoops are provided.
• they can sometimes have long helical scoops,
scoops to give smooth torque.
torque
• the Banesh rotor and the Rahai rotor improve efficiency by shaping the
blades to produce significant lift as well as drag.
T
Terra
M
Moya
A
Aqua
vertical
ti l axis
i wind
i d turbines
t bi
–
• developed by Terra Moya Aqua using a combination of fixed and
rotating vanes.
• the
th efficiency
ffi i
i similar
is
i il to
t other
th wind
i d turbine
t bi designs,
d i
b t with
but
ith less
l
vertical height and visual impact.
• This design also reduces bird injury because birds avoid the fixed vanes.
hi design
d i has
h not yet entered
d commercial
i l production.
d i
• This
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Offshore Wind Turbine
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RMD 2501
Offshore
Off
h
wind
i d turbines
bi
–
• because water has less surface roughness than land, the average wind
speed is usually higher over open water.
• In stormy areas with extended shallow continental shelves turbines are
practical to install, and give good service
• The offshore environment is, however, more expensive. Offshore towers
are generally taller than onshore towers once one includes the submerged
height, and offshore foundations are generally more difficult to build and
more expensive as well.
• power transmission from offshore turbines is generally through undersea
cable
• the offshore environment is also corrosive and abrasive. Repairs and
maintenance are much more difficult, and much more costly than on
onshore turbines.
• offshore wind turbines are outfitted with extensive corrosion p
protection
measures like coatings and cathodic protection.
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Wind Turbine
PEMP
RMD 2501
Types of horizontal axis wind turbine
Horizontal axis
Vertical axis
Types based
on location
Vertical axis
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Types of Wind Turbine
PEMP
RMD 2501
Giromill wind turbine
Co-axial, multi-rotor horizontal axis wind turbines
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Some Applications of Windmills
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RMD 2501
1 kW, 9-ft rotor, 30-ft tower
Water pumping for cattle
Texas
10 kW, 23-ft rotor, 100-ft tower
Farm
windmill used for cereal
milling
Double windmill Texas
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The middle-18th-century
Th
iddl 18th
t
windmill of Nesebar, Bulgaria
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Working of Vertical Wind Turbine
PEMP
RMD 2501
As the aerofoil moves around,
the angle of attack changes to the
opposite sign, but the generated
lift is still positive and in the
direction of rotation, because the
wings are symmetrical.
The rotor spins at a rate unrelated
to the wind speed, (usually faster)
The surplus energy arising from
the lift is extracted and converted
into useful power by a generator.
When the rotor is stationary, no net rotational force arises, even if the wind
speed rises quite high - the rotor must already be spinning to generate lift.
lift Thus
the design is not self starting.
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PEMP
RMD 2501
Horizontal and Vertical Wind Turbines
Horizontal Axis Wind turbine
• typically have two or three
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blades.
blades
three-bladed wind turbines are
operated "upwind," with the
blades facing into the wind.
wind
two-bladed are operated
downwind.
horizontal arrangement has wider
operating speed range and are
self-starting.
horizontal wind turbines have
high tip speeds of up to 6x wind
speed, high efficiency, and low
torque ripple which contributes to
good reliability
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developed by Darrieus G J M, a French
aeronautical engineer,
g
, in 1931.
the turbine consists of a number of
aerofoils vertically mounted on a
rotating shaft.
these do not depend on direction of
wind
the generator can be placed at the
ground
d ffor easy servicing
i i
the main supporting tower is lighter as
much of the force on the tower is
transmitted to the bottom.
bottom
inefficient due to the physical stresses
and limitations imposed by a practical
design but theoretically more efficient.
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Wind Turbines in Various Settings
PEMP
RMD 2501
Utility-scale Wind Turbines
Small-scale Wind Turbines
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Parts of a Wind Turbine
1.
2.
3
3.
4.
5.
6
6.
7.
8.
9
9.
10.
11.
12
12.
13.
14.
15
15.
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PEMP
RMD 2501
Blades
Rotor
Pitch
Brake
Low speed shaft
Gear box
Generator
Controller
Anemometer
Wind Vane
Nacelle
Hi h speedd Shaft
High
Sh ft
Yaw drive
Yaw motor
T
Tower
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Parts of a Wind Turbine
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PEMP
RMD 2501
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Specification for Wind Turbine
Horizontal
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Rotor Diameter
: 100.4 m
Speed
: 18.5 rpm
Axle Height
: 100 m
Optimal power
: 3MW
Wind velocity (design)
: 11.8m/s
Vertical
PEMP
RMD 2501
Small scale
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PEMP
RMD 2501
Size of Wind Turbine
50 kW
400 W
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900 W
10 kW
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Large Scale vs Small Scale
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RMD 2501
• Utility-scale turbines
– Main purpose: provide wholesale electricity to grid
– Typically larger than 750 kilowatts in capacity
• Rotor diameters: typically greater than 80 feet
• Tower heights: typically more than 200 feet
• Small-scale turbines
– Main purpose: offset retail electricity for consumers
– Typically smaller than 10 kilowatts in capacity
• Rotor diameters: typically less than 20 feet
• Tower heights:
g
typically
yp
y less than 100 feet
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PEMP
RMD 2501
Rotor Diameter vs Raptor Wing Span
Raptor Wing Spans:
Utility--scale
Golden eagle: 5-7 ft
R d Wi
Red
Wing Hawk:
H k 2-4
2 4 ft
A. Kestrel: < 1ft
Sm
mall-scale
Golden eagle wing span
(for comparison)
10 m
10 k
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PEMP
RMD 2501
Power in the Wind
= 1/2 x air density x swept rotor area x (wind speed)3
A
V3
ρ
Density
D
it = P/(RT)
P - pressure (Pa)
R - specific gas constant (287 J/kgK)
T - air temperature (K)
kg/m3
A
Area
= π r2
m2
Instantaneous Speed
(not mean speed)
m/s
It can be seen that wind power density depends on the third power of wind
velocity
Modifying ‘Power in the wind' formula, P = ½.Cp ρ A V3
where,, P is p
power ((in watts)) available from the machine,, Cp
p is the coefficient of
performance of the wind machine.
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Power Developed by Wind Turbine
PEMP
RMD 2501
P
Power
extracted
t t d from
f
wind
i d turbine
t bi is
i
2
U2
U2
1
3
PWT = ρAU1 (1 + )(1 − 2 )
U1
2
U1
Ideal or Maximum power is when U2=0
Pideal
id l
1
3
= ρAU1
2
Power Efficiency
Pactual
Cp =
Pideal
Ideal turbine has a Cp of 0.593. This is called Betz Limit
Betz Limit cannot be achieved for the following reasons
• Aerodynamic drag
• Finite number of blades
• Rotation of wake
ake behind rotor
• Tip losses
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Performance Characteristics
PEMP
RMD 2501
Tip Speed Ratio - ratio of the blade velocity, U at the tip to the air velocity at inlet
ΩE
λ=
U1
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Velocity Triangle for Wind Turbine
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PEMP
RMD 2501
flow causes lift and drag force
dR is the resultant force.
the component dF causes the rotation
component in axial direction has to be arrested
by suitable bearings
Power delivered by the blade,
2 1
1
V
P = dF *U = ρC LUAV 2 (1 + 2 ) 2
2
U
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Aerodynamic Construction of Blade
PEMP
RMD 2501
Lift andd drag
Lif
d
on a stationary
i
airfoil.
Lift and drag on a translating airfoil
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Force F1 iin th
F
the direction
di ti off translation
t
l ti is
i
available to do useful work, F1 causes
torque which drives some load
connected to it.
it F2 used in the design of
aerofoil supports and structural integrity
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Aerodynamic Construction of Blade
PEMP
RMD 2501
Definition of pitch angle β and angle of attack γ.
g depend
p
on angle
g of attack
Both lift and drag
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Blade – Airfoil
PEMP
RMD 2501
Airfoil for BWC XL
XL.50
50
Commonly used airfoil shapes for vertical axis wind turbines are the
symmetrical NACA-0012, NACA-0015, and NACA-0018.
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Blade Manufacturing Process
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RMD 2501
Pultrusion Technology
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Factors governing Installation

G id availability
Grid
il bilit
Accessibility

PEMP
RMD 2501
for commissioning
Strong terrain / soil for proper foundation / civil work

Favourable environmental condition to prevent
corrosion & not prone to cyclone.
Area required per Wind Turbine = 5Acres (approx )
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Session Summary
PEMP
RMD 2501
In this session the students would have learnt about
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Different types of wind turbines
Working of horizontal and vertical axis wind turbines
Parts of wind turbine
Power developed by wind turbine
Characteristic of wind turbine
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PEMP
RMD 2501
Thank you
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