Wind Energy Handbook

Transcription

1 Wind Energy Handbook Second Edition Tony Burton Wind Energy Consultant, Powys, UK Nick Jenkins Cardiff University, UK David Sharpe Wind Energy Consultant, Essex, UK Ervin Bossanyi GL Garrad Hassan, Bristol, UK ~WILEY A lohn Wiley and Sons, Ltd., Publication

2 Contents About the Authors Preface to Second Edition Acknowledgements for FirstEdition Acknowledgements for Second Edition List of Symbols Figures Cl and C2 - Co-ordinate Systems xvii xix xxi xxiii xxv xxxv I Introduction I 1.l Historical development I 1.2 Modem wind turbines Scope of the book 6 References 7 Further reading 8 2 The wind resource The nature of the wind Geographical variation in the wind resource IO 2.3 Long-term wind speed variations II 2.4 Annual and seasonal variations Synoptic and diurnal variations Turbulence The nature of turbulence The boundary layer Turbulence intensity Turbulence spectra Length scales and other parameters Asymptotic limits Cross-spectra and coherence functions The Mann model of turbulence 28

3 vi 2.7 Gust wind speeds 2.8 Extreme wind speeds Extreme winds in standards 2.9 Wind speed prediction and forecasting Statistical methods Meteorological methods 2.10 Turbulence in wakes and wind farms 2.11 Turbulence in complex terrain References Aerodynamics of horizontal axis wind turhines Introduction The actuator disc concept Simple momentum theory Power coefficient The Lanchester-Betz limit The thrust coefficient Rotor disc theory Wake rotation Angular momentum theory Maximum power Vortex cylinder model of the actuator disc Introduction Vortex cylinder theory Relationship between bound circulation and the induced velocity Root vortex Torque and power Axial flow field Tangential flow field Axial thrust Radial flow field Conclusions Rotor blade theory (blade-element/momentum theory) Introduction Blade element theory The blade-element/momentum (BEM) theory Determination of rotor torque and power Breakdown of the momentum theory Free-streamlwake mixing Modification of rotor thrust caused by flow separation Empirical determination of thrust coefficient Blade geometry Introduction Optimal design for variable speed operation A simple blade design Effects of drag on optimal blade design Optimal blade design for constant speed operation 74

4 vii 3.8 The effects of a discrete number of blades Introduction Tip-Iosses Prandtl's approximation for the tip-ioss factor Blade root losses Effect of tip-loss on optimum blade design and power Incorporation of tip-loss for non-optimal operation Alternative explanation for tip-loss Stall delay Calculated results for an actual turbine The performance curves Introduction The Cp - ).. performance curve The effect of solidity on performance The C Q -).. curve The C T - ).. curve Constant rotation al speed operation Introduction The K» - 1/).. curve Stall regulation Effect of rotation al speed change Effect of blade pitch angle change Pitch regulation Introduction Pitching to stall Pitching to feather Comparison of measured with theoretical performance Variable speed operation Estimation of energy capture Wind turbine aerofoil design Introduction The NREL aerofoils The Ris~ aerofoils The Delft aerofoils 117 References 119 Websites 120 Further reading 120 Appendix A3 lift and drag of aerofoils 120 A3.1 Definition of drag 121 A3.2 Drag coefficient 123 A3.3 The boundary Iayer 124 A3.4 Boundary layer separation 124 A3.5 Laminar and turbulent boundary Iayers 125 A3.6 Definition oflift and its relationship to circulation 127 A3.7 The stalled aerofoil 130 A3.8 The lift coefficient 131

5 viii A3.9 Aerofoil drag eharaeteristies A3.10 Cambered aerofoils Further aerodynamic topics for wind turbines Introduetion The aerodynamies of turbines in steady yaw Momentum theory for a turbine rotor in steady yaw Glauert's momentum theory for the yawed rotor Vortex eylinder model of the yawed aetuator dise Flowexpansion Related theories Wake rotation for a turbine rotor in steady yaw The blade element theory for a turbine rotor in steady yaw The blade element - momentum theory for a rotor in steady yaw Calculated values of indueed veloeity The method of aeeeleration potential Introduetion The general pressure distribution theory of Kinner The axi-symmetrie pressure distributions The anti-symmetrie pressure distributions The Pitt and Peters model The general aeeeleration potential method Comparison of methods Unsteady flow Introduetion Adaptation of the aeeeleration potential method to unsteady flow Unsteady yawing and tilting moments Quasi-steady aerofoil aerodynamies Introduetion Aerodynamie forees eaused by aerofoil aeeeleration The effeet of the wake on aerofoil aerodynamies in unsteady flow Dynamie stall Computational fluid dynamies 190 Referenees 191 Further reading Design loads for horizontal axis wind turbines ] National and international standards ] Historieal development IEC ] GL rules Basis for design loads Sourees of loading ] Ultimate loads 195

6 ix Fatigue loads Partial safety factors Functions of the control and safety systems Turbulence and wakes Extreme loads Operationalload cases Non-operationalload cases Blade/tower clearance Constrained stochastic simulation of wind gusts Fatigue loading Synthesis of fatigue load spectrum Stationary blade loading Lift and drag coefficients Critical configuration for different machine types Dynamic response Blade loads during operation Deterministic and stochastic load components Deterrninistic aerodynamic loads Gravity loads Deterministic inertia loads Stochastic aerodynamic loads: analysis in the frequency domain Stochastic aerodynamic loads: analysis in the time domain Extreme loads Blade dynamic response Modal analysis Mode shapes and frequencies Centrifugal stiffening Aerodynamic and structural damping Response to deterrninistic loads: step-by-step dynamic analysis Response to stochastic loads Response to simulated loads Teeter motion Tower coupling Aeroelastic stability Blade fatigue stresses Methodology for blade fatigue design Combination of deterrninistic and stochastic components Fatigue prediction in the frequency domain Wind simulation Fatigue cycle counting Hub and low speed shaft loading Introduction Deterministic aerodynamic loads Stochastic aerodynamic loads Gravity loading Nacelle loading Loadings from rotor Cladding loads 278

7 x 5.12 Tower loading Extreme loads Dynamic response to extreme loads Operational loads due to steady wind (deterrninistic component) Operational loads due to turbulence (stochastic component) Dynamic response to operationalloads Fatigue loads and stresses Wind turbine dynamic analysis codes Extrapolation of extreme loads from simulations Derivation ofempirical cumulative distribution function of global extremes Fitting an extreme value distribution to the empirical distribution Comparison of extreme value distributions Combination of probability distributions Extrapolation Fitting probability distribution after aggregation Local extremes method Convergence requirements 305 References 306 Appendix 5: dynamic response of stationary blade in turbulent wind 308 A5.1 Introduction 308 A5.2 Frequency response function 309 A5.2.1 Equation of motion 309 A5.2.2 Frequency response function 309 A5.3 Resonant displacement response ignoring wind variations along the blade 310 A5.3.1 Linearisation of wind loading 310 A5.3.2 First mode displacement response 311 A5.3.3 Background and resonant response 311 A5.4 Effect of across-wind turbulence distribution on resonant displacement response 313 A5.4.1 Fonnula for nonnalised ~o-spectrum 314 A5.S Resonant root bending moment 316 A5.6 Root bending moment background response 318 A5.7 Peak response 319 A5.8 Bending moments at intermediate blade positions 322 AS.8.1 Background response 322 A5.8.2 Resonant response 322 References Conceptual design of horizontal axis wind turbines Introduction Rotor diameter Cost modelling Simplified cost model for machine size optimisation an illustration The NREL cost model 329

8 xi Machine size growth Gravity limitations Machine rating Simplified cost model for optimising machine rating in relation to diameter Relationship between optimum rated wind speed and annual mean Specific power of production machines Rotational speed Ideal relationship between rotational speed and solidity Influence of rotational speed on blade weight Optimum rotational speed Noise constraint on rotational speed Visual considerations Number of blades Overview Ideal relationship between number of blades, rotational speed and solidity Some performance and cost comparisons Effect of number of blades on loads Noise constraint on rotational speed Visual appearance Single-bladed turbines Teetering Load relief benefits Limitation of large excursions Pitch-teeter coupling Teeter stability on stall-regulated machines Power control Passive stall control Active pitch control Passive pitch control Active stall control Yaw control Braking systems Independent braking systems: requirements of standards Aerodynamic brake options Mechanical brake options Parking versus idling Fixed speed, two speed or variable speed Two speed operation Variable slip operation (see also Chapter 8, Section 8.3.8) Variable speed operation Other approaches to variable speed operation Type of generator Historical attempts to use synchronous generators Direct drive generators 367

9 xii Evolution of generator systems Drive train mounting arrangement options l.l Low speed shaft mounting High speed shaft and generator mounting Drive train compliance \3 Rotor position with respect to tower Upwind configuration Downwind configuration Tower stiffness Stochastic thrust loading at blade passing frequency Tower top moment f1uctuations due to blade pitch errors Tower top moment f1uctuations due to rotor mass imbalance Tower stiffness categories Personnel safety and access issues 379 References Component design \ Blades Introduction Aerodynamic design Practical modifications to optimum design Form of blade structure Blade materials and properties Properties of glass/polyester and glass/epoxy composites Properties of wood laminates Blade loading overview Blade resonance Design against buckling Blade root fixings Pitch bearings Rotor hub Gearbox Introduction Variable loading during operation Drive train dynamics Braking loads Effect of variable loading on fatigue design of gear teeth Effect of variable loading on fatigue design of bearings and shafts Gear arrangements Gearbox noise Integrated gearboxes Lubrication and cooling Gearbox efficiency Generator \ Fixed-speed induction generators Variable slip induction generators 439

10 xiii Variable speed operation Variable speed operation using a Doubly Fed Induction Generator (DFIG) Variable speed operation using a Full Power Converter (FPG) Mechanical brake Brake duty Factors governing brake design Calculation of brake disc temperature rise High speed shaft brake design Two level braking Low speed shaft brake design Nacelle bedplate Yaw drive Tower Introduction Constraints on first mode natural frequency Steel tubular towers Steellattice towers Foundations Slab foundations Multi-pile foundations Concrete monopile foundations Foundations for steellattice towers Foundation rotational stiffness 469 References 47\ 8 The controller 8.1 Functions of the wind turbine controller Supervisory control Closed loop control The safety system 8.2 Closed loop control: issues and objectives Pitch control (See also Chapter 3, Seetion 3.13 and Chapter 6, Seetion 6.7.2) Stall control Generator torque control (see also Chapter 6, Section 6.9 and Chapter 7, Section 7.5) Yaw control Influence of the controller on loads Defining controller objectives PT and PID controllers 8.3 Closed loop contro!: general techniques Control of fixed speed, pitch regulated turbines Control of variable speed pitch regulated turbines Pitch control for variable speed turbines Switching between torque and pitch control Control of tower vibration

11 xiv Control of drive train torsional vibration Variable speed stall regulation Control of variable slip turbines Individual pitch control Multivariable control - decoupling the wind turbine control loops Two-axis decoupling for individual pitch control Load reduction with individual pitch control Individual pitch control implementation Further extensions to individual pitch control Commercial use of individual pitch control Feedforward control using lidars 8.4 Closed loop control: analytical design methods Classical design methods Gain scheduling for pitch controllers Adding more terms to the controller Other extensions to classical controllers Optimal feedback methods Pros and cons of model-based control methods Other methods 8.5 Pitch actuators (see also, Chapter 6 Section 6.7.2) 8.6 Control system implementation Discretisation Integrator desaturation References 9 Wind turbine installations and wind farms 9.1 Project development Initial site selection Project feasibility assessment The Measure-Correlate-Predict (MCP) technique Micrositing Site investigations Public consultation Preparation and submission of the planning application 9.2 Landscape and visual impact assessment Landscape character assessment Design and mitigation Assessment of impact Shadow flicker Sociological aspects 9.3 Noise Terminology and basic concepts Wind turbine noise Measurement, prediction and assessment of wind farm noise

12 9.4 Electromagnetic Interference Modelling and prediction of EMI from wind turbines Aviation radar 9.5 Ecological assessment Impact on birds References xv Wind energy and the electric power system Introduction The electric power system Electrical distribution networks Electrical generation and transmission systems Wind farm power collection systems Earthing (grounding) of wind farms Lightning protection Connection of wind generation to distribution networks Power system studies Power quality Voltage flicker Harmonics Measurement and assessment of power quality characteristics of grid connected wind turbines Electrical protection Wind farm and generator protection Islanding and self-excitation of induction generators Interface protection for wind turbines connected to distribution networks Distributed generation and the Grid Codes Grid Code - continuous operation Grid Code - voltage and power factor control Grid Code - frequency response Grid Code - fault ride through Synthetic inertia Wind energy and the generation system Capacity credit Wind power forecasting 604 References 607 Appendix AIOSimple calculations for the connection of wind turbines 609 AlO.1 The Per-unit system 609 AIO.2 Power flows, slow voltage variations and network losses OtTshore wind turbines and wind farms Development of offshore wind energy The offshore wind resource The structure of winds offshore Site wind speed assessment Wakes and array losses in offshore wind farms 617

13 xvi 11.3 Design loads International Standards Wind conditions Marine conditions Wave spectra Ultimate loads: operationalload cases and accompanying wave climates Ultimate loads: non-operationalload cases and accompanying wave climates Fatigue loads Wave theories Wave loading on support structure Constrained waves Analysis of support structure loads 11.4 Machine size optimisation 11.5 Reliability of offshore wind turbines 11.6 Support structures Monopiles Monopile fatigue analysis in the frequency domain Gravity bases lacket structures Tripod structures Tripile structures 11.7 Environmental assessment of offshore wind farms 11.8 Offshore power collection and transmission Offshore wind farm transmission Submarine AC cable systems HVDC transmission 11.9 Operation and access References Appendix All References for table A 11.1 Index