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
The Top Ten Wind Power Producing Countries in the World 2008 30000 25000 20000 15000 10000 5000 0 Power Generation (M W) USA Germany Spain China India Italy France UK Denmark Portugal Countries Source: Wikipedia 2009
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
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 Large Wind Turbines
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 Turbine Wind Turbogenerator Gear Box Electric Generator Horizontal axis wind turbine Power Electronics Power Storage Vertical axis wind turbine 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: 3 W AV Wind Power vs. Wind Speed: 2500 Power/Area (W/m^2) 2000 1500 1000 500 0 0 5 10 15 20 Wind Speed (m/s) High power output is possible with: High tower for higher wind speed Long blades for large swept area
Classes of Wind Power Density at 10 m and 50 m - for evaluation 10 m (33 ft) 50 m (164 ft) Wind Power Class Wind Power Density (W/m 2 ) Speed (b) m/s (mph) Wind Power Density (W/m 2 ) Speed (b) m/s (mph) 1 <100 <4.4 (9.8) <200 <5.6 (12.5) Preferred for large scale wind power stations 2 3 4 5 6 100-150 150-200 200-250 250-300 300-400 4.4 (9.8)/5.1 (11.5) 5.1 (11.5)/5.6 (12.5) 5.6 (12.5)/6.0 (13.4) 6.0 (13.4)/6.4 (14.3) 6.4 (14.3)/7.0 (15.7) 200-300 300-400 400-500 500-600 600-800 5.6 (12.5)/6.4 (14.3) 6.4 (14.3)/7.0 (15.7) 7.0 (15.7)/7.5 (16.8) 7.5 (16.8)/8.0 (17.9) 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) 2500 Power/Area (W/m^2) 2000 1500 1000 500 0 0 5 10 15 20 Wind Speed (m/s)
Wind speed increases with the height (altitude) Reason for high tower for wind turbine Formula for extrapolation: v ( z) = v( z ) o z z o n where v(z) = Extrapolated wind velocity at elevation z v(z o ) = measured wind velocity at elevation z o n = wind shear factor Extrapolated wind velocity measured at IBM-ARC site By SJSU student team in 2009 n = v ln vo z ln z o ground cover n smooth surface ocean, sand 0.1 low grass or fallow ground 0.16 high grass or low row crops 0.18 tall row crops or low woods 0.2 high woods with many trees suburbs, small towns 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.
400 350 300 250 200 150 100 50 0 Intermittent Nature of Wind Power January 6, 2005 California Wind Generation TOTAL Load, MW 34000 32000 30000 28000 26000 24000 22000 20000 MW 0:00:00 1:00:00 2:00:00 3:00:00 4:00:00 5:00:00 6:00:00 7:00:00 8:00:00 9:00:00 10:00:00 11:00:00 12:00:00 13:00:00 14:00:00 15:00:00 16:00:00 17:00:00 18:00:00 19:00:00 20:00:00 21:00:00 22:00:00 23:00:00 Watts Wind power varies randomly in: (a) time of the days, (b) months of the year, (c) by the years Hours in the Day
Required wind energy resource data for wind power generating station design: Hours of the Day Wind speed, m/s Average Wind Speed, m/s Month of the Year Wind Energy on a Selected Site
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: Total available wind energy on the site Capital investment Suitable wind turbine types Fixed or variable speed wind turbine Blade coefficients of lift and drag at each wind speed Gear box efficiency Input Variables Average annual wind speed Output-side Optimal rotor diameter Optimal generator capacity Optimal RPM of rotor Optimal blade angle at each wind speed Torque on gear box at each wind speed Power produced at each wind speed 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 3 P = ρ AV ( c b ) kw 2 where ρ = mass density, kg/m 3 Rotor selection: % of Available wind energy captured 100% c b A = rotor swept area, m 2 V = wind speed, m/s C b = Betz limit < 0.59 2 ( 1+ V )( 1 V ) r r = with V r = V out /V in 2 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: The coefficient c p is: c p = V 1 + V out in where V 1 V 2 out in W = 1 ρ AV 2 3 c p ρ = mass density of air A = rotor swept area V = wind velocity c p = coefficient relating to efficiency 2 = 16 27 = 0.592 = Betz limit 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. α = low α = medium <10 degrees The Drag Force is parallel to the direction of motion. We want to make this force small. 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 TSR = ΩR V Where, Ω = rotational speed in radians /sec R = Rotor Radius V = Wind Free Stream Velocity R ΩR
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
A. Loading on Blades Loading on Horizontal Axis Wind Turbines 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 B. Loading on Rotor MAIN SHAFT 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 TOWER C. Loading on Main Shaft Weight of blades shear Electromagnetic force of generator torsion vibration
D. Loading on Tower Uneven centrifugal forces Aerodynamic forces Cyclic tension/compression Aerodynamic forces Intermittent bending Uneven centrifugal forces Rotor weights Intermittent shearing
Loading on Vertical Axis Wind Turbines Stochastic aerodynamic loading cyclic bending & torsion Friction wear Weights buckling
DESIGN ANALYSIS Aerodynamic analysis Flow patterns Fluid-induced forces Lift/drag coefficients CAD Solid models CFD Analysis Fluid-induced forces Material Characterization Material handbook Lab test data (e.g., fatigue failure models) Other Input Loads Stress Analysis using FEM Componenets Geometry & Dimensions Phenominological Models Fatigue Safe/Fail? Over-stress Resonant vibration
Mean stress: Fatigue Failure of Wind Turbine Blades by Cyclic Stresses: σ m σ max + σ = min 2 Stress amplitude: σ a σ max σ = min 2 Stress range: σ r = σ max σ min Fluctuating stress Sinusoidal fluctuating stress Non-sinusoidal fluctuating stress Repeated stress Non-fluctuating sinusoidal 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: σ m, or σ a, or σ r
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 350 300 Wind Power 32000 30000 250 28000 MW 200 26000 150 100 24000 50 22000 0 20000 0:00:00 1:00:00 2:00:00 3:00:00 4:00:00 5:00:00 6:00:00 7:00:00 8:00:00 9:00:00 10:00:00 11:00:00 12:00:00 13:00:00 Mid-day 14:00:00 15:00:00 16:00:00 17:00:00 18:00:00 19:00:00 20:00:00 21:00:00 22:00:00 23:00:00 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 Utility Meter Power generator and user 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