Introduction to
DESIGN OF WIND POWER GENERATING STATIONS
presented to
ME 195-3 Senior Design Projects Class
Department of Mechanical and Aerospace Engineering
San Jose State University
by
Tai-Ran Hsu, Professor
on
October 28, 2009
Overview of Wind Power Station
A Promising Fast Growing Clean Power Source
121.2 GW = 1.5%
worldwide electricity
Total solar PV power
generation = 6 GW
in 2008
Source: Wikipedia 2009
Countries
Source: Wikipedia 2009
ar
k
rt u
ga
l
Po
nm
UK
De
ce
ly
Fr
an
I ta
dia
In
ina
Ch
ai n
Sp
an
rm
Ge
US
y
30000
25000
20000
15000
10000
5000
0
A
P ow er G eneration
(M W )
The Top Ten Wind Power Producing Countries
in the World 2008
Wind Industry Growth Trends
•
•
•
•
Larger multi-MW turbines
Demand for new innovative technologies
Led by Europeans
Offshore & low wind regime focus in U.S.
Altamont
Region
10 m,
26 ft
0.15 MW
Large Wind Turbines
•
•
•
•
•
450’ base to blade
Each blade 112’
Span greater than 747
163+ tons total
Foundation 20+ feet
deep
• Rated at 1.5 – 5
megawatt
• Supply at least 350
homes
Wind 2030
A goal set by
US Department of Energy in July 2008:
“20% of US electricity generation by wind energy
by Year 2030”
Total US electricity generation in 2005 was 4017 GW
Major Components in Wind Power Plants
WIND
Wind Turbogenerator
Wind
Turbine
Gear Box
Electric
Generator
Horizontal axis wind turbine
Power
Electronics
Vertical axis wind turbine
Power
Storage
Batteries
Capacitor banks
Grid power system
Pumped water
Flywheel
Thermal
Superconducting magnetic
Design of Wind Power Station
Major Tasks in Design and Construction of
Wind Power Generating Stations
A. Site selection
B. Local wind resource survey
C. Selection of wind turbogenerators or wind farm
with multiple wind turbines
D. Power transmission and storage
E. Public safety and liability
F. Environmental impacts wildlife protections
G. Construction of power generating stations
A. Site Selection
Possible sites:
Flat Plain
Hill tops
Rooftops of (high rise)
buildings and structures
Offshore
In North
Sea
Site Visits:
The purpose of site visits is to look for the following facts:
● Available open space for wind power generating station
● Consistently bent trees and vegetation as a sure sign of strong winds.
● Accessibility for construction, monitoring and maintenances, and power transmission
● Check for potential site constraints:
● Competing land uses
● Permission for the wind plant or its transmission lines,
● Probable local land owners’ resistance to selling the necessary land and easements.
● Availability of possible location for a wind monitoring station.
B. Wind Resource Survey
- A major task in wind power generating station design
● Wind contains energy that can be converted to electricity using wind turbines
● The amount of electricity that wind turbines produce depends upon the amount
of energy in the wind passing through the area swept by the wind turbine blades
in a unit of time.
● Wind resource is expressed in terms of the wind power density and
wind speed in the locality
● Wind Power Density is a useful way to evaluate the wind resource available at a
potential site.
● The wind power density, measured in watts per square meter, indicates
how much energy is available at the site for conversion by a wind turbine
● Viable wind speed for power generation:
● Minimum threshold speed: 4 m/s
● Viable speed: 11 m/s
Average World Wind Energy Resources
(wind velocity at m/s)
● Wind resource in
various parts of
USA is
available from US
Geological survey
Wind speed in
SF Bay Area (m/s):
5.0 -5.5
5.5 -6.0
6.0 -6.5
6.5 -7.0
Wind velocity – Why it is important?
Wind power generation:
W ∝ AV 3
Wind Power vs. Wind Speed:
Power/Area (W/m^2)
2500
2000
1500
1000
500
0
0
5
10
Wind Speed (m/s)
High power output is possible with:
● High tower for higher wind speed
● Long blades for large swept area
15
20
Classes of Wind Power Density at 10 m and 50 m - for evaluation
10 m (33 ft)
Preferred for large scale
wind power stations
Wind
Power
Class
50 m (164 ft)
Wind
Power
Density
(W/m2)
Speed(b)
m/s (mph)
Wind
Power
Density
(W/m2)
Speed(b)
m/s (mph)
1
<100
<4.4 (9.8)
<200
<5.6 (12.5)
2
100 - 150
4.4 (9.8)/5.1
(11.5)
200 - 300
5.6 (12.5)/6.4
(14.3)
3
150 - 200
5.1 (11.5)/5.6
(12.5)
300 - 400
6.4 (14.3)/7.0
(15.7)
4
200 - 250
5.6 (12.5)/6.0
(13.4)
400 - 500
7.0 (15.7)/7.5
(16.8)
5
250 - 300
6.0 (13.4)/6.4
(14.3)
500 - 600
7.5 (16.8)/8.0
(17.9)
6
300 - 400
6.4 (14.3)/7.0
(15.7)
600 - 800
8.0 (17.9)/8.8
(19.7)
7
>400
>7.0 (15.7)
>800
>8.8 (19.7)
(a) Vertical extrapolation of wind speed based on the 1/7 power law
(b) Mean wind speed is based on the Rayleigh speed distribution of equivalent wind power density. Wind speed is
for standard sea-level conditions. To maintain the same power density, speed increases 3%/1000 m (5%/5000 ft)
of elevation.
(from the Battelle Wind Energy Resource Atlas)
Available Wind Energy Density and Wind Speed
Distribution of wind speed (red)
and energy (blue) for all of 2002
at the Lee Ranch facility in Colorado
(Ref: Wikipedia 2009)
Power/Area (W/m^2)
2500
2000
1500
1000
500
0
0
5
10
Wind Speed (m/s)
15
20
Wind speed increases with the height (altitude)
– Reason for high tower for wind turbine
Formula for extrapolation:
⎛ z ⎞
v( z ) = v( zo )⎜⎜ ⎟⎟
⎝ zo ⎠
n
v
vo
n=
z
ln
zo
ln
where
v(z) = Extrapolated wind velocity at elevation z
v(zo) = measured wind velocity at elevation zo
n = wind shear factor
Extrapolated wind velocity measured at IBM-ARC site
By SJSU student team in 2009
ground cover
smooth surface ocean, sand
low grass or fallow ground
high grass or low row crops
tall row crops or low woods
high woods with many trees suburbs, small
towns
n
0.1
0.16
0.18
0.2
0.3
Wind speed measurements:
● Conduct wind resource survey on specific site using tower with anemometers
: for measuring wind speed:
Wind vane
Cup anemometers
Data logger
Data logger by solar power
Thermal sensor
● Wind profile measured by Sodar transmitters using Doppler effects associate
with the shift of the frequencies of the acoustical waves of the transmitted and
received at various altitude in the atmosphere.
Sodar units manufactured by Atmospheric Systems
Corporation (ASC) can detect wind profile from 15
to 250m in elevation using acoustic waves at
4-6 kHz frequencies.
Intermittent Nature of Wind Power
Wind power varies randomly in:
(a) time of the days, (b) months of the year, (c) by the years
January 6, 2005 California Wind Generation
TOTAL
Load, MW
400
34000
350
32000
30000
250
200
26000
150
24000
100
22000
50
23:00:00
22:00:00
21:00:00
20:00:00
19:00:00
18:00:00
17:00:00
16:00:00
Hours in the Day
15:00:00
14:00:00
13:00:00
12:00:00
11:00:00
10:00:00
9:00:00
8:00:00
7:00:00
6:00:00
5:00:00
4:00:00
3:00:00
2:00:00
20000
1:00:00
0
0:00:00
MW
28000
Watts
300
Wind speed, m/s
Average Wind Speed, m/s
Hours of the Day
Required wind energy resource data for wind power generating station design:
Wind Energy on a Selected Site
Month of the Year
C. General Design Parameters for Wind Power Generating Station Design
Principal selection criteria of wind turbogenerators:
● The available wind energy on the site
● Site visit findings
● Other considerations:
Input Variables
Output-side
Average annual wind speed
Optimal rotor diameter
Total available wind energy on the site
Optimal generator capacity
Capital investment
Optimal RPM of rotor
Suitable wind turbine types
Optimal blade angle at each wind speed
Fixed or variable speed wind turbine
Torque on gear box at each wind speed
Blade coefficients of lift and drag at each
wind speed
Power produced at each wind speed
Gear box efficiency
Maximum total annual energy production
Generator efficiency
Power electronics efficiency
(Ref: “Wind Turbine Design Optimization,” Michael Schmidt, Strategic Energy Institute, Georgia Institute of Technology,
www.energy.gatech.edu)
Available Wind power on the Site:
Annual Wind Energy on a Selected Site
Wind power by the turbine:
1
P = ρ AV 3 (cb ) kW
2
where
ρ = mass density, kg/m3
A = rotor swept area, m2
V = wind speed, m/s
Cb = Betz limit < 0.59
cb
(
1 + Vr )(1 − Vr2 )
=
2
with Vr = Vout/Vin
Rotor selection:
% of Available wind
energy captured
100%
Variable speed rotor
Fixed speed rotor
9 →10 m/s
Wind speed
Selection of Wind Turbogenerator
Horizontal Axis Wind Turbine
Vertical Axis
Wind Turbine
Horizontal Wind Turbines
Advantages:
1) Variable blade pitch, which gives the turbine blades the optimum angle of attack.
Allowing the angle of attack to be remotely adjusted gives greater control,
so the turbine collects the maximum amount of wind energy for the time of day and season.
2) The tall tower base allows access to stronger wind in sites with wind shear.
In some wind shear sites, every ten meters up, the wind speed can increase by 20%
and the power output by 34%.
Disadvantages:
1) HAWTs have difficulty operating in near ground because of turbulent winds.
2) The tall towers and blades up to 90 meters long are difficult to transport.
Transportation can now cost 20% of equipment costs.
3) Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
4) Massive tower construction is required to support the heavy blades, gearbox, and generator.
5) Tall HAWTs may affect airport radar.
6) Their height makes them obtrusively visible across large areas, disrupting the appearance
of the landscape and sometimes creating local opposition.
7) Downwind variants suffer from fatigue and structural failure caused by turbulence.
8) HAWTs require an additional yaw control mechanism to turn the blades toward the wind.
Vertical Axis Wind Turbines
Advantages:
1) Does not need to be pointed into the wind to be effective
2) The generator and gear box can be placed near ground –
no need to be supported by a tower, and for easy maintenance
3) Does not need a yaw mechanism to turn the rotor against the wind
Disadvantages:
1) Difficult to be mounted on a tower. So it is almost all installed on the ground
- low wind speed in low attitude with low efficiency
2) Air flow near ground level with high turbulence
- cause excessive vibration, noise and bearing wear – a serious maintenance problem
3) May need guy wires to hold the turbine “vertical” – guy wires are not practical solutions
4) Major load on thrust bearings – need frequent replacement – not an easy job
Unique advantages:
1) More suited for roof-top installation
2) Optimum height of turbine ≈ 50% of building height
Design of Horizontal Axis Wind Turbines
Rotor
& Blades
Basic Structure:
● The rotor typically has
three blades.
Blade diameter can be as large
as 40 m
● The nacelle yaws or rotates to keep
the turbine faced into the wind
● The nacelle also houses the gear
box and generator
Hub
Nacelle
Tower
Controls, Transformer and
Power Electronics
Interior of a Nacelle
Design of Horizontal Axis Wind Turbines
● Odd number of rotor blades
(= 3, the optimum number from aerodynamics principle)
● A large rotor captures more energy but cost more
● Blades have slight twist optimized to capture the max.
amount of wind power
The power captured by a horizontal axis wind turbine is:
where
The coefficient cp is:
cp =
⎛ Vout
⎜⎜1 +
Vin
⎝
1
W = ρ AV 3 cp
2
⎞ ⎡ ⎛ Vout
⎟⎟ ⎢1 − ⎜⎜
⎠ ⎢⎣ ⎝ Vin
2
ρ = mass density of air
A = rotor swept area
V = wind velocity
cp = coefficient relating to efficiency
⎞
⎟⎟
⎠
2
⎤
⎥
⎥⎦ 16
=
= 0.592 = Betz limit
27
meaning the efficiency of HAWT
cannot exceed 59%
Other design considerations:
1) Synchronous or asynchronous electric generator
2) Fixed speed or variable speed
3) A large rotor-to-generator ratio captures more energy at low wind speeds
4) A small rotor-to-generator ratio captures more energy at high wind speed
5) So, this ratio must be optimized for site specific wind speed distribution
6) The variable speed captures more energy at almost all wind speeds.
- cost more in hardware and power electronics control system
Mechanical Engineering Design of
Wind Power Generating Station
● Performance Design
● Structural Design
Performance Design
Design Objectives:
● Design for maximum LIFT and minimum drag for the airfoil
cross-section of the turbine blades using aerodynamics
principle
● Design the yaw mechanism that provides fast response to
change of wind direction using mecahtronics principle
Lift & Drag Forces
• The Lift Force is
perpendicular to the
direction of motion. We
want to make this force
BIG.
• The Drag Force is
parallel to the direction
of motion. We want to
make this force small.
α = low
α = medium
<10 degrees
α = High
Stall!!
Airfoil Shape
Just like the wings of an airplane,
wind turbine blades use the airfoil
shape to create lift and maximize
efficiency.
Tip-Speed Ratio
Tip-speed ratio is the ratio of the
speed of the rotating blade tip
to the speed of the free stream
wind.
There is an optimum angle of
attack which creates the
highest lift to drag ratio.
Because angle of attack is
dependant on wind speed,
there is an optimum tip-speed
ratio
Where,
ΩR
TSR =
V
Ω = rotational speed in radians /sec
R = Rotor Radius
V = Wind “Free Stream” Velocity
ΩR
R
Performance Over Range of Tip Speed Ratios
• Power Coefficient Varies with Tip Speed Ratio
• Characterized by Cp vs Tip Speed Ratio Curve
0.4
Cp
0.3
0.2
0.1
0.0
0
2
4
6
8
Tip Speed Ratio
10
12
Wind Turbine Structural Design
Loading
● STATIC LOADING – Constant in time, e.g. weight
● CYCLIC LOADING – Structural vibration induced
● STOAHASTIC LOADING – Load varying with time
e.g. aerodynamic induced loading with varying wind velocity
● DYNAMIC LOADING – Inertia forces induced by
varying rotor speed, and Coriolis forces.
Common Structural Failure Modes
● Over-stress – Stress concentration near abrupt geometry
change areas
● Vibration-induced fatigue failure
● Failure due to resonant vibration
Loading on Horizontal Axis Wind Turbines
A. Loading on Blades
● Aerodynamic load:
● Intermittent with varying magnitudes along the
blade length → stochastic loads
● Lift forces for bending
● Drag forces for torsion
BLADES
● Centrifugal forces from rotation at high speed
● Gravitation load in large blades
ROTOR
MAIN SHAFT
B. Loading on Rotor
● Weight of blades → bending
● Aerodynamic forces on blades → bending
● Coriolis force → axial thrust
● Centrifugal forces on blades → bending
● Electromagnetic forces by the generator → torsion vibration
● Yaw forces → bending
C. Loading on Main Shaft
● Weight of blades → shear
● Electromagnetic force of generator → torsion vibration
TOWER
D. Loading on Tower
Uneven centrifugal forces
Aerodynamic forces
Aerodynamic forces
Uneven centrifugal forces
Rotor weights
Intermittent shearing
Cyclic tension/compression
Intermittent bending
Loading on Vertical Axis Wind Turbines
Stochastic aerodynamic loading
→ cyclic bending & torsion
Weights → buckling
Friction → wear
DESIGN ANALYSIS
Componenets
Geometry
& Dimensions
CAD
Solid models
Aerodynamic analysis
Flow patterns
Fluid-induced forces
Lift/drag coefficients
Other Input
Loads
CFD Analysis
Fluid-induced forces
Stress Analysis
using FEM
Material handbook
Lab test data
Material
Characterization
(e.g., fatigue failure models)
Phenominological Models
Fatigue
Safe/Fail?
Over-stress
Resonant vibration
Fatigue Failure of Wind Turbine Blades by Cyclic Stresses:
σ max − σ min
σ max + σ min
Stress range: σ r = σ max − σ min
=
σ
Stress
amplitude
:
a
=
σ
Mean stress: m
2
2
Fluctuating stress
Sinusoidal fluctuating stress
Non-sinusoidal fluctuating stress
Non-fluctuating sinusoidal stress
Repeated stress
Completely reversed sinusoidal stress
Typical Fatigue (S-N) Curves for Ferrous and Non-Ferrous Metals
(Laboratory Test Data for Specific Materials)
Note: Calculated stress can be: σm, or σa, or σr
D. Power Transmission and Storage
Two Major Cost Factors of Wind Power:
● Power transmission – involves hundreds miles of transmission from power generating
stations to the consumers. Transmission often require over
rugged terrains or over waters.
● Power storage – wind power is intermiitent in nature, There is rarely matching between
the time of power generations and that of the needs for power
January 6, 2005 California Wind Generation
TOTAL
Load, MW
400
34000
350
Wind Power
32000
300
30000
250
MW
28000
200
26000
150
24000
100
22000
50
23:00:00
22:00:00
21:00:00
20:00:00
19:00:00
18:00:00
17:00:00
16:00:00
15:00:00
14:00:00
13:00:00
12:00:00
11:00:00
10:00:00
9:00:00
8:00:00
7:00:00
6:00:00
5:00:00
4:00:00
3:00:00
2:00:00
1:00:00
20000
0:00:00
0
Mid-day
Peak needs by business
& industry
Power storage systems are essential parts of wind power generation
A Pumped-Storage Plant
● Generated wind power is used to pump water to a higher elevation for energy storage
● The high elevation water is released to drive hydraulic turbogenerator to generate
electricity to consumers when power is needed
A Viable Energy Storage System
- Net metering with local utility power generator
Excess energy fed to the grid for credit
To and from
utility, e.g. PG&E
Customer’s
Distribution
Panel
Synchronous
Inverter
Power generator
and user
Utility
Meter
IBM-ARC
Campus
Additional power requirements satisfied by the utility
● Most utility generators impose limit on how much power may be swapped with
the generators – a major design consideration
E. Environmental Impact Study
Environmental impacts by wind power generation are minor in comparison to
other means of power generations.
Major concerns are:
● Noise and vibration
● Avian/Bat mortality
Avian fatality < 1 in 10,000
● Visual impacts
● shadow flicker
Construction of Wind Power Stations
Turbine blade convoy passing
through Edenfield in the UK
Construction
sites
Principal References
● Rand, Joseph “Wind Turbine Blade Design,” joe@kidwind.org
● Ragheb, M. “Dynamics and Structural Loading in Wind Turbines”
● “Wind turbine Design,” Wikipedia,
http://en.wikipedia.org/wki/wind_turbine_design
● Schmidt, Michael “Wind Turbine Design Optimization,”
www.energy.gatech.edu
● “Basic Principles of Wind Resource Evaluation,”
http://www.awea.org/faq/basicwr.html
● “Mechanical Engineering Systems Design,”
Printed lecture notes by T.R. Hsu, San Jose State Unvierasity
San Jose, California, USA