Introduction toDESIGN OF WIND POWER GENERATING STATIONS

presented to

ME 195-3 Senior Design Projects ClassDepartment of Mechanical and Aerospace Engineering

San Jose State Universityby

Tai-Ran Hsu, Professoron

October 28, 2009

Overview of Wind Power Station

121.2 GW = 1.5%worldwide electricityTotal solar PV power generation = 6 GWin 2008

A Promising Fast Growing Clean Power Source

Source: Wikipedia 2009

The Top Ten Wind Power Producing Countries in the World 2008

05000

1000015000200002500030000

USAGerm

any

Spain

China India

Italy

France UKDenm

arkPort

ugal

Countries

Pow

er G

ener

atio

n (M

W)

Source: Wikipedia 2009

0.15 MW

10 m,26 ft

AltamontRegion

Wind Industry Growth Trends• Larger multi-MW turbines• Demand for new innovative technologies• Led by Europeans• Offshore & low wind regime focus in U.S.

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 2030A 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

WindTurbine Gear Box Electric

Generator

PowerElectronics

PowerStorage

Horizontal axis wind turbine

Vertical axis wind turbine Batteries

Capacitor banksGrid power system

Pumped water

Flywheel

Thermal

Superconducting magnetic

WIND

Major Components in Wind Power Plants

Wind Turbogenerator

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

G. Construction of power generating stations

E. Public safety and liability

F. Environmental impacts wildlife protections

A. Site Selection

Flat Plain Hill tops

OffshoreIn NorthSea

Rooftops of (high rise)buildings and structures

Possible sites:

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.

Site Visits:

B. Wind Resource Survey- A major task in wind power generating station design

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.

Viable wind speed for power generation: Minimum threshold speed: 4 m/s Viable speed: 11 m/s

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

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.

Average World Wind Energy Resources(wind velocity at m/s)

5.0 -5.5

5.5 -6.0

6.0 -6.5

6.5 -7.0

Wind speed in SF Bay Area (m/s):

Wind resource in various parts of USA isavailable from US Geological survey

0

500

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0 5 10 15 20

Wind Speed (m/s)

Pow

er/A

rea

(W/m

^2)

Wind Power vs. Wind Speed:

High power output is possible with:

High tower for higher wind speed Long blades for large swept area

Wind power generation: 3VAW ∝

Wind velocity – Why it is important?

(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)

>8.8 (19.7)>800>7.0 (15.7)>400 7

8.0 (17.9)/8.8 (19.7)

600 - 8006.4 (14.3)/7.0 (15.7)

300 - 4006

7.5 (16.8)/8.0 (17.9)

500 - 6006.0 (13.4)/6.4 (14.3)

250 - 3005

7.0 (15.7)/7.5 (16.8)

400 - 5005.6 (12.5)/6.0 (13.4)

200 - 2504

6.4 (14.3)/7.0 (15.7)

300 - 4005.1 (11.5)/5.6 (12.5)

150 - 2003

5.6 (12.5)/6.4 (14.3)

200 - 3004.4 (9.8)/5.1 (11.5)

100 - 1502

<5.6 (12.5)<200<4.4 (9.8)<1001

Speed(b)

m/s (mph)Wind PowerDensity (W/m2)

Speed(b)

m/s (mph)Wind PowerDensity (W/m2)

WindPower Class

50 m (164 ft)10 m (33 ft)

Classes of Wind Power Density at 10 m and 50 m - for evaluationP

refe

rred

for l

arge

sca

le

win

d po

wer

sta

tions

0

500

1000

1500

2000

2500

0 5 10 15 20

Wind Speed (m/s)

Pow

er/A

rea

(W/m

^2)

Distribution of wind speed (red)and energy (blue) for all of 2002 at the Lee Ranch facility in Colorado(Ref: Wikipedia 2009)

Available Wind Energy Density and Wind Speed

Wind speed increases with the height (altitude)– Reason for high tower for wind turbine

( ) ( )n

oo z

zzvzv ⎟⎟⎠

⎞⎜⎜⎝

⎛=

o

o

zzn

vvn

nl

l

=where

v(z) = Extrapolated wind velocity at elevation zv(zo) = measured wind velocity at elevation zo

n = wind shear factor

Extrapolated wind velocity measured at IBM-ARC siteBy SJSU student team in 2009

Formula for extrapolation:

ground cover nsmooth surface ocean, sand 0.1low grass or fallow ground 0.16high grass or low row crops 0.18tall row crops or low woods 0.2high woods with many trees suburbs, small towns 0.3

Conduct wind resource survey on specific site using tower with anemometersfor measuring wind speed:

Wind speed measurements:

:

Wind vane Cup anemometers

Data logger

Thermal sensor

Data logger by solar power

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 at4-6 kHz frequencies.

January 6, 2005 California Wind Generation

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TOTAL Load, MW

Intermittent Nature of Wind Power

Hours in the Day

Wat

ts

Wind power varies randomly in: (a) time of the days, (b) months of the year, (c) by the years

Hou

rs o

f the

Day

Wind speed, m/s

Month of the Year

Ave

rage

Win

d S

peed

, m/s

Required wind energy resource data for wind power generating station design:

Wind Energy on a Selected Site

C. General Design Parameters for Wind Power Generating Station Design

Power electronics efficiency

Generator efficiency

Maximum total annual energy productionGear box efficiency

Power produced at each wind speedBlade coefficients of lift and drag at each wind speed

Torque on gear box at each wind speedFixed or variable speed wind turbine

Optimal blade angle at each wind speedSuitable wind turbine types

Optimal RPM of rotorCapital investment

Optimal generator capacityTotal available wind energy on the site

Optimal rotor diameterAverage annual wind speed

Output-sideInput Variables

(Ref: “Wind Turbine Design Optimization,” Michael Schmidt, Strategic Energy Institute, Georgia Institute of Technology,www.energy.gatech.edu)

Principal selection criteria of wind turbogenerators: The available wind energy on the site Site visit findings Other considerations:

Available Wind power on the Site:Annual Wind Energy on a Selected Site

( )bcVAP 3

21 ρ=

Wind power by the turbine:

kW

where ρ = mass density, kg/m3

A = rotor swept area, m2

V = wind speed, m/sCb = Betz limit < 0.59

( )( )211 2

rrb

VVc −+= with Vr = Vout/Vin

Variable speed rotor

Fixed speed rotor

Wind speed

% of Available windenergy captured 100%

9 →10 m/s

Rotor selection:

Selection of Wind TurbogeneratorHorizontal Axis Wind Turbine

Vertical Axis Wind Turbine

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%.

Horizontal Wind Turbines

Advantages:

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 effective2) The generator and gear box can be placed near ground –

no need to be supported by a tower, and for easy maintenance3) 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 solutions4) Major load on thrust bearings – need frequent replacement – not an easy job

Unique advantages:

1) More suited for roof-top installation2) Optimum height of turbine ≈ 50% of building height

Design of Horizontal Axis Wind Turbines

Basic Structure:

HubNacelle

Tower

Rotor& Blades

Controls, Transformer andPower Electronics

The rotor typically hasthree blades.Blade diameter can be as largeas 40 m

The nacelle yaws or rotates to keepthe turbine faced into the wind

The nacelle also houses the gearbox and generator

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:

pcVAW 3

21 ρ=

where ρ = mass density of airA = rotor swept area V = wind velocitycp = coefficient relating to efficiency

The coefficient cp is:

592.02716

2

112

==⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛+

=in

out

in

out

p

VV

VV

c = Betz limitmeaning the efficiency of HAWTcannot 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 ofWind 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 aerodynamicsprinciple

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 = HighStall!!

Airfoil ShapeJust like the wings of an airplane, wind turbine blades use the airfoil shape to create lift and maximize efficiency.

Tip-Speed RatioTip-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

ΩRVTSR =

Where,Ω = rotational speed in radians /secR = Rotor RadiusV = 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

0.3

0.2

0.1

0.0

Cp

121086420 Tip Speed Ratio

Wind Turbine Structural Design

Loading

STATIC LOADING – Constant in time, e.g. weight

CYCLIC LOADING – Structural vibration induced

STOAHASTIC LOADING – Load varying with timee.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 geometrychange 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

Centrifugal forces from rotation at high speed

Gravitation load in large blades

BLADES

ROTOR

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

MAIN SHAFT

C. Loading on Main Shaft Weight of blades → shear Electromagnetic force of generator → torsion vibration

TOWER

Aerodynamic forces

Uneven centrifugal forces

Rotor weights

Uneven centrifugal forces

Aerodynamic forces

Cyclic tension/compression

Intermittent bending

Intermittent shearing

D. Loading on Tower

Loading on Vertical Axis Wind Turbines

Stochastic aerodynamic loading→ cyclic bending & torsion

Weights → buckling

Friction → wear

DESIGN ANALYSIS

CAD

CFD AnalysisFluid-induced forces

Aerodynamic analysisFlow patterns

Fluid-induced forcesLift/drag coefficients

Stress Analysisusing FEM

Other Input Loads

ComponenetsGeometry & Dimen-

sions

Phenomino-logical Models

MaterialCharacteriza-

tion

Material handbookLab test data

(e.g., fatigue failure models)

Safe/Fail?

Fatigue

Over-stress

Resonant vibration

Solid models

Fatigue Failure of Wind Turbine Blades by Cyclic Stresses:

Mean stress: 2minmax σσσ +

=m Stress amplitude: 2minmax σσσ −

=a Stress range: minmax σσσ −=r

Fluctuating stress

Non-sinusoidal fluctuating stress

Non-fluctuating sinusoidal stress

Sinusoidal fluctuating stress

Repeated stress

Completely reversed sinusoidal stress

Typical Fatigue (S-N) Curves for Ferrous and Non-Ferrous Metals

Note: Calculated stress can be: σm, or σa, or σr

(Laboratory Test Data for Specific Materials)

D. Power Transmission and Storage

January 6, 2005 California Wind Generation

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TOTAL Load, MW

Two Major Cost Factors of Wind Power: Power transmission – involves hundreds miles of transmission from power generating

stations to the consumers. Transmission often require overrugged terrains or over waters.

Power storage – wind power is intermiitent in nature, There is rarely matching betweenthe time of power generations and that of the needs for power

Mid-dayPeak needs by business

& industry

Power storage systems are essential parts of wind power generation

Wind Power

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

SynchronousInverter

Customer’s Distribution

Panel

UtilityMeter

Excess energy fed to the grid for credit

Additional power requirements satisfied by the utility

IBM-ARCCampus

A Viable Energy Storage System- Net metering with local utility power generator

Power generatorand user

To and fromutility, e.g. PG&E

Most utility generators impose limit on how much power may be swapped with the generators – a major design consideration

E. Environmental Impact Study

Noise and vibration

Avian/Bat mortalityAvian fatality < 1 in 10,000

Visual impacts

shadow flicker

Environmental impacts by wind power generation are minor in comparison to other means of power generations.

Major concerns are:

Construction of Wind Power Stations

Turbine blade convoy passing through Edenfield in the UK

Constructionsites

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 UnvierasitySan Jose, California, USA