Zusammenfassung Wind Energy PDF

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Zusammenfassung Wind Energy Vorlesung 1: Basic Facts / Introduction

Aerodynamic efficiency in practice < 0.5 (+ mechanical & electrical losses) * reffered to V and A (not a real efficiency) —> referring to streamtube inlet = 16/27 * 3/2 = 0.889 Efficiency = mechanical energy / energy in the flow

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Anatomy:

Differences to aeronautical applications: - dimensions (need low cost materials, large volumes) - reliability / maintenance: performance with simplicity and robustness The Mix: - Wind Energy cannot have same importance in all countries —> different countries should aim at different energy mixes, depending on their specific natural resources —> integrated efficient grid enables management of energy mix within each country (time of day / season) and across borders Capacity and Capacity Factor: —> power plants do not work at full power at all times WTs: - environmental variability (wind; density of air) - availability (f.e. not during maintenance), usually > 95% —> Capacity factor: (produced energy in given period) = (capacity factor) * (theoretical max energy), capacity factor is between 0 & 1 depending on technology (theoretical max energy) = (nameplate power) * (period length) Costs (levelized cost of energy)

Issues Affecting Growth - policy uncertainties - permitting & approval time/delays - grid capacity & transmission limits environmental impacts (wildlife, noise, visual…) - social acceptance - skilled industrial/labor availability - availability of suitable on-shore sites - current technological limits (e.g.: for exploitation of deep-water off-shore resources) - …

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Off-Shore Wind: Reasons: - huge available resources - social acceptability - environmental impacts (e.g. noise) - scale / logistics - potential for reduced LCOE - BUT great technological problems (logistics, maintenance, harsh environment)

Vorlesung 2: Wind Turbine Typology Different models of turbines for different purposes: - Windmills for milling grain or pumping water: Goal: maximize torque; high rotor solidity - Wind turbines for production of electricity: Goal: maximize power capture; Generally higher rotational speed and less solidity Ongoing Wind Mills and Water Pumps Why controls? loads increase with power of 3! Tail fan forces the rotor out of the wind as wind speed increases

Stall regulated wind turbines: - transitional technology - blades did not pitch, aerodynamic stall was used as control method

Modern Wind Turbines: - upwind - 3 blades - pitch control - various typologies depending on generator type, location of main bearings, gearbox, brake Design goal: cheapest cost of energy possible General overview: blades, spinner, pitch mechanism, main bearing(s) for rotor shaft, gearbox, brake, generator, control / power electronics, yaw motor

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tower, foundation, electrical transformer, network connection Almost every component has been removed from various designs: - direct drive —> no gearbox - kite turbines —> no tower

Nacelle: container on top of tower (non rotating)

- rotor produces large bending moments due to gravity and aerodynamics - a main bearing along -

rotor shaft is used to react this load (location different on various design layouts, other models remove aft bearing —> gearbox has to absorb some of off-axis rotor loading) direct coupling of generator to rotor (direct drive): - remove gearbox (and associated problems) and use generator with multiple poles (current frequency = rotation * pole pairs) - generator rotation speed = rotor rotation speed 2 blade upwind turbines: - not yet a commercial success - dynamics / resonance / control issues and aesthetics three bladed downwind turbines: - lower yaw demand - built to withstand typhoon wind storms brake for WTs: - rotor energy too large for mechanical braking - brakes for parking and slowing, most braking performed aerodynamically - brake can be forward of gearbox (low speed shaft LSS, very high torque) or aft of gearbox (high speed shaft HSS, lower torque) Yaw drives (electrical or hydraulic): control orientation of rotor to wind Towers: - mostly steel constructions, bolted together on-site - hybrid towers: segmented (layered rings on top of eachother) concrete towers, top of tower is steel Other concepts: f.e. Vertical axis turbines: - typically lower efficiency than horizontal axis turbines

Vorlesung 3: The Wind Resource Wind Variability: - Spatial: Hemispheres; Climatic regions; Physical geography (land, see, mountains, plains, …); local orography; vegetation (influence roughness, moisture, absorption/reflection of solar radiation) - Temporal: Long term (poorly understood); Yearly (El Nino); Synoptic (due to large scale weather patterns, high/low pressure, cold/warm fronts …); Diurnal; Turbulence

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Wind Motion: - Pressure force (per unit mass): initial driver, due to sun heating - Coriolis force (per unit mass) - Geostrophic wind: tends to align itself to isobars - isobars are typically curved, because of low and high pressure areas —> centrifugal force - friction force (alters this simple model, especially to low altitudes) Wind Speed Distribution: probability distribution function, used to describe distribution of wind speeds over extended period of time

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Wind Direction Distribution: Wind rose diagram: - speed - direction -frequency Vertical Wind Profile (Shear): - Wind speed increases with height with atmospheric boundary layer - strong effect on power production and fatigue loading - Profile models: z_r = reference height z_0 = surface roughness length (crops, trees, buildings, waves, …) alpha = wind shear (power law) exponent (stability, terrain features, wind speed, …) (typically 0.2 on-shore, 0.14 offshore) Flow within Wind Farms: - Wind fluctuations cause power fluctuations at wind turbine level - Wind farms smooth out some fluctuations, yet large and fast fluctuations are still possible - grid operator has to compensate with coal/gas/nuclear (however not immediate and costly) - Fast response compensation technologies for wind fluctuations: - Batteries (Cons: expensive, limited charge/discharge cycles, susceptible to temperature changes, often polluting/difficult to dispose of) - Fly wheels (Cons: high RPM, danger of breakage (underground and/or use of materials that break in small pieces - e.g. carbon), gyroscopic moments due to Earth rotation, magnetic bearings + vacuum for efficiency (expensive)

Vorlesung 4: Wind Turbine Aerodynamics Airfoil Terminology

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Stall: dramatic loss of lift, since separated flow causes higher pressure on upper surface of airfoil Leading Edge vs. Trailing Edge Stall: TE stall is quite gradual, LE stall is more abrupt Viscous effects: - friction drag, due to shear stress, higher for turbulent BL - pressure drag, due to flow separation, higher in case of laminar flow (as turbulent flow tends to separate later) Geometric vs Aerodynamic angle of attack Efficiency (E) of an airfoil: ration of lift to drag Main geometric factors influencing aerodynamic performance: - Camber: Maximum camber, Location of max camber

- thickness to chord ratio, t/c: c_lmax increases with increasing t/c up to t/c of order of 13-15%; at about t/c > 15% c_lmax starts to decrease

- c_dmin increases with increasing t/c - nose radius: large leading edge radius: larger C_lmax, gentle stall break; small leading edge radius: sharp stall break Airfoil design requirements for wind turbine blades: - high lift-to-drag ratio: crucial, strictly related to wind turbine maximum power coefficient - high-enough thickness: important to lower structural weight - low LE roughness sensitivity: contamination agents (dust, dirt, ice, insects) accumulate on blades (V_tip = 300 km/h) and generate roughness to varying degrees - stall characteristics (gentle - abrupt): important for transient phenomena and fatigue, crucial for stallregulated WTs

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Aerodynamic Damping: Vibrations in blades/wings: additional relative motion of airfoil wrt flow Flapwise (orthogonal to flow) are usually much more damped than edgewise (parallel to flow) vibrations 1D Annular Stream Tube Theory: Hypothesis: - stationary flow - constant mass flow rate along stream tube (no interaction among annuli and no mixing at stream tube boundary) - incompressible and inviscid flow - actuator disk (infinite number of blades)

Thrust: Assuming uniform inflow across rotor disk:

Betz Limit: maximum theoretical aerodynamic efficiency = 16/27 (s. 1. Seite ganz oben inkludieren!!!) 1D Annular Stream Tube Theory with Wake Swirl Pressure jump across rotor:

Thrust:

Blade Element Momentum Theory

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Blade section: consider velocity components from momentum theory and aerodynamic force components

Combination of both expressions from momentum theory and at blade element + simplification —> optimal local loading: —> optimal design conditions (1) (2)

Possible strategies: A) Assume const. C_L (and hence alpha), e.g. for maximum efficiency E=C_L/ C_D, and compute optimal chord from (1) and twist from (2) B) Assume constant chord, compute C_L from (1), then it’s corresponding alpha, and finally twist from (2) Remark: power P and thrust T will be the same for strategies A and B when two rotors have the same solidity

Typical Behavior of power and thrust coefficient curves (Q: what will happen approaching stall?)

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Variable Speed Regulation

Initial Rotor Sizing Considerations:

Vorlesung 5: Wind Turbine Regulation

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Regulation: how to govern a WT for varying wind speed: typical regulation goals are to maximize power production for low wind speeds, and to limit it to a preset value (rated power) for higher wind speeds Control: given a regulation strategy, how to implement it using some closed-loop algorithm: on top of the regulation goals, control systems also try to achieve additional goals (load reduction, vibration avoidance, limit actuator usage, etc.) Control System Architecture: Supervisory Control System: Main tasks: - operational managing and monitoring - diagnostics, safety communication, reporting and data logging - … Operational states: - idling - start up - normal power production - normal shut down emergency shut down Main input data: - wind speed - rotor speed blade pitch -electrical power - temperatures in critical area - accelerations - … but also: - stresses, strains (blades, tower) position, speed (yaw, blade, actuators, teetering angle, rotor tilt, …) - fluid properties and levels - electrical systems (voltage, grid characteristics) - icing conditions, humidity … Regulation Strategies: Basic WT regulation strategies and power curves: - constant TSR strategy - constant rotor speed strategy - below and above rated speed control - variable speed pitch-torque regulated WT - stall and yaw/tilt control

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Vorlesung 6: Electrical Aspects Electromagnetic Induction: = main principle for operation of electrical generators - in practice, generators have rotating magnets, while voltage (and currents) are generated on nonrotating part of the generator (stator) (it would be complex to implement transmission of high-power signals from rotor to stator); instead of single loops, stators comprise windings to capture the magnetic flux better Generators in WTs: - generators convert mechanical (rotational) energy to electrical energy

- Induction generators: Squirrel-cage induction generators (SCIG); Wound-rotor induction generators (WRIG)

- Synchronous generators: (Wound-rotor) synchronous generators; Synchronous generators with permanent magnets

- Two types of operation: fixed-speed; variable-speed Synchronous Generators: - stator comprises windings - two types of synchronous generators based on the way the rotor magnetic field is generated: - electro-magnets: - (wound-rotor) synchronous generator (pole sind spulen) - DC current (Gleichstrom) flows through rotor windings - external excitation (creation of rotor magnetic field) needed - permanent magnets: - synchronous generator with permanent magnets - no external excitation needed - lately, the cost of permanent magnets is decreasing - used only for variable-speed WTs - as rotor rotates, magnetic field through stator windings changes - changes of magnetic field, induce voltage in stator windings: - AC voltage is induced Frequency of the induced voltage is a multiple of rotor speed —> synchronous speed - Frequency of produced voltage depends on rotor speed and number of magnetic poles

- the higher the number of poles, the lower rotor speed is required (possible to construct rotors with large number of magnetic poles (over 60)) Induction Generators: - very mature und relatively cheap technology - in general, losses are higher than for synchronous generator - stator comprises windings - two different types of rotors: a) squirrel-cage rotor b) wound-rotor - rotating (stator) magnetic field induced by stator currents (stator has pole pairs…) —> power grid has to supply reactive power to the generator - the power supply (connected to the stator windings) defines the frequency of the output voltage - rotor speed is defined by applied torque, stator voltage and rotor resistance - used both for fixed-speed and variable-speed WTs Fixed-Speed Operation: Pros: - simple, robust and reliable concept - cost of the system is low Cons: - increased loads - uncontrollable reactive power - limited power quality —> Due to decreasing price of power electronics, this concept is being pushed out of the market by variablespeed WTs 13  /  19

Variable-Speed Operation: - Designed to achieve maximum aerodynamic efficiency in a wide range of wind speeds - both induction and synchronous generators can be used - power electronics (frequency converters) are an essential part of variable-speed WTs - Pros: + increased energy capture (optimized over a wide range of wind speeds) + increased power quality + reduced mechanical loads - Cons: - increased cost of equipment - power electronics introduces additional losses Doubly-fed induction generator: -wound-rotor induction generator with both stator and rotor connected to power grid -stator winding connected directly to grid -rotor windings connected to grid over frequency converter Overview:

Vorlesung 7: Wind Turbine Blades Design Criteria: Primary: - cost considerations - WT power output maximization - Lightness aeroelastic stability (flutter, divergence) - dynamics (resonance) - stiffness: a) out-of-plane bending (tower clearance) b) in-plane bending (low-damping, fatigue) c) torsion (aerodynamics, aeroelasticity) Secondary: - ease in handling during fabrication, transport and mounting - buckling of panels noise - lighting protection - disposal - LE erosion, sensitivity to dirt,… Loads acting on a WT blade:

- Wind loads: a) flapwise bending b) edgewise bending c) torsion - gravity: a) edgewise bending b) axial tensile and compressive loads - inertial loads - centrifugal loads - others (gyroscopic, coriolis) - Loads are inherently time dependent: - wind turbulence (change in velocity and direction) - wind shear - tower shadow - rotational effects - start and stop of the WT - Design against ultimate and fatigue loads for lifetime of 20 years - International standards (IEC…) prescribe design load cases (DLCs) to use for WT blade certification 14  /  19

Composite materials: advantages over metals: - higher stiffness and strength to weight ratios - better fatigue behavior - better corrosion resistance - high versatility (anisotropy) - lower electrical conductivity - good manufacturability - low cost of components Disadvantages: - prone to delamination - difficult damage detection (ongoing research) - brittle behavior - expensive and difficult manufacturing processes - possible environmental issues during manufacturing process Structural Components: three main elements: - Aerodynamic shell: a) geometric design for optimal aero performance b) carries torsional loads c) should avoid changes of shape during load (effects on aerodynamics) d) sandwich construction - load carrying beam that provides bending stiffness: a) spar caps: main structural elements, heavy and thick; UD material aligned with blade axis to maximize bending stiffness in flapwise direction b) shear webs: carry shear loading - LE and TE reinforcements: areas far away from edgewise bending neutral axis —> high strains, high fatigue damage —> reinforcement needed with uni-directional (UD) fiber material

Blade pre-bending: - pre-bent away from tower: wind push bends blade backward, increasing diameter (slight increased area, hence power capture) - allows for reduced stiffness, hence reduced spar-cap thickness, hence reduced weight Blade root connection: critical area for WT blade design (strong bending moments act over the circular section): -out-of-plane bending loads - cyclic in-plane bending (gravity and wind) - root is one of the hotspots for fatigue damage - centrifugal axial loads - blade root with steel inserts that are internally threaded: Carrots: + smooth load transfer from root to hub + smaller area required for placement - challenge to obtain sufficient bonding between bushing and composite - T-bolts: + no bonding required + replaceable - higher laminate thickness required Manufacturing: three main possibilities for manufacturing process of large structures made of composites: a) Hand lay up: - low quality - labor intensive - not used anymore b) Vacuum Assisted Resign Transfer Moulding (VARTM): - most common method for WTB - two moulds are created for the aerodynamic shell, with suction and pressure sides - fibers and foams are placed in two half moulds and vacuum bagged - suction is used to pull through the dry fibers - once fibers are all wet, temperature is increased (curing cycle) - spar and shear webs created separately and used to join halves together using large amounts of adhesive - good trade-off between costs and laminate properties c) Pre-preg composites: - used for high quality parts (bicycles, aerospace, …) expensive method - autoclave needed - necessary for CFRP 15  /  19

Testing: to be certified, WT blades must pass several full scale tests defined by the certification agencies: - mode test: intended to measure the natural frequencies and mode shapes of the blade - static test: resistance to ultimate loads and stiffness for tower clearance - challenge: actual loading is continuous along blade, testing needs loading at discrete limited points - fatigue test: simulations return expected fatigue damage for different locations along the span for a lifetime of 20 years - a static test os often performed at the end of fatigue test to simulate ultimate loads at turbine endlife - failure test Transport and Assembly: - Transport: - length of blades limits shipping - chord of large blades often limited because of bridges and roadways width —> trucks have been adapted with pitch regulators - large blades often built near shipping (sea) facilities and used only for offshore applications. Max length for land shipping appx 50 meters - Assembly: - smaller rotors are assembles on ground and connected to the hub through crane - new larger rotors are too heavy and blades are assembled one at a time

Vorlesung 8: Offshore Wind Key advantages: + large areas for installation + higher average wind speeds + huge available resources + social acceptability potential for reduced LCOE + lower turbulence intensity + lower wind shear: Key challenges: - higher costs (installation, maintenance, lower availability) - corrosion problems - more complex dynamic effects Support Structure: Tower, Substructure, Foundation

- Most common: Monopile: steel cylinder (2.5 - 4 m

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diameter), hammered 10 - 20 m in sea bed, drilling may be necessary in presence of hard or rocky soil, transition pi...