• Wind Resources


    European Wind Atlas

    Scotland enjoys high wind energy resource.



    Wind Speed Distribution

    Weibull and Rayleigh Probability Distributions

    It was found by measurements that wind speed at a certain site over one year follow a Weibull or Rayleigh probability distribution.

    It implies that moderate wind speed range has highest probability w. r. t. low or high speed wind.



    Wind Speed vs Height

    Effect of tower height from ground on wind speed

    The wind sheer at ground surface causes wind speed increase with height in accordance with:

  • Vertical vs. Horizontal Axis Wind Turbines


    Vertical Axis Wind Turbine (VAWT)
    Darrieus VAWT

    Image by Salameh, Z. “Renewable energy System Design”, chapter 3

    Design patented 1931.

    The only VAWT manufactured commercially at a wide scale.

    Used for low power application.

    Gearbox located on the ground

    No tower needed for support

    No need for yawing mechanism to turn the rotor against the wind.

    Low efficiency and not self starting

    Requires guy wires for support.




    Vertical Axis Wind Turbine

    Other types of VAWTs

    Image by Salameh, Z. “Renewable energy System Design”, chapter 3



    Vertical Axis Wind Turbine

    Cross-flow VAWT



    Vertical Axis Wind Turbine

    Advantages:
    • Generator and gearbox nearer to the ground and easier to maintain
    • No need for tower support (less cost)
    • No need for yawing mechanism to turn the rotor against the wind


    Disadvantages:
    • Less efficient that HAWTs (closer to ground, lower wind speed).
    • More difficult control and starting.
    • May need support through guy wires
    • Uneven distribution of mechanical stress to blades.
    • Therefore, not suitable for high power (utility scale)



    Horizontal Axis Wind Turbine (HAWT)

    Advantages:
    • Easier to control and start w.r.t. VAWTs
    • Higher efficiency
    • Easier to design for high power ratings at commercial scale.


    Disadvantages:
    • Need for strong tower to support the nacelle and blades
    • Generator and gearbox are up in the nacelle
    • Yawing mechanism needed



    Types of HAWT (blades count)






    Types of HAWT (upwind/downwind)



    Image by Salameh, Z. “Renewable energy System Design”, chapter 3

    Upwind WT has the rotor facing the wind; this is the more popular design.
    Upwind WT avoids the wind shade of the tower.
    Upwind WT needs a yaw mechanism to keep the rotor facing the wind.
    Downwind WT has the wind hitting the turbine first
    The nacelle may be designed to follow wind direction without a yaw mechanism.
    Downwind is not a suitable design for large turbines.

    Types of HAWT (Fixed speed WT)

    The early wind turbine designs were fixed speed
    Traditionally known as the “Danish concept” pioneered by the Danish manufacturer “Vestas”
    Blades rotate at a fixed speed
    In some models two generators with different number of poles are used to allow operation at two different speeds.
    Fixed speed WTs have lower energy harvest than variable-speed WTs
    Mostly limited to small and medium sized wind turbines (<600kW)

    Types of HAWT (Variable speed WT)

    Multi-megawatt WTs are normally variable speed WTs
    Variation of blades rotation speed captures more wind energy.
    Vestas was first to introduce limited variable speed operation with OptiSlip™ technology
    Variable speed wind technology requires power converters and more complex control systems
    Variable speed WTs are the current WT technology standard.

    WT Power ratings

    Consortium of industry organizations, research institutes, universities, and governmental agencies are working to overcome technical challenges of designing 10-20 MW wind turbines.



    WT Power ratings

    Vestas in collaboration with DTU has proposed a 4-rotor wind turbine design proposed for future >10 MW wind turbines.
    • It increases swept area (captured power) with less overall weight and mechanical stresses.
    A proof of concept prototype was build using four V29 225kW nacelles


    Offshore Wind Farms

    Advantages
    • Capture more power from higher offshore winds
    • Little or no visual disturbance when farther from shore
    • Higher capacity factors and load factors due to steadier wind flows

    Disadvantages
    • Higher cost due to complexity of foundation structures
    • More difficult to access for maintenance
    • Submarine power cables required
    • When farther from shore HVDC (high voltage DC) transmission is the only option. An expensive offshore platform must be built for the HVDC power converter station

  • HAWT Structure


    Primary components

    Foundation
    Transformer room
    Support tower
    Rotor hub
    Blades
    Nacelle:
    - Control systems
    - Generator
    - Power converters
    - Drivetrain and Gearbox
    - Instrumentation




    WT blades

    As the air flow strikes the aerofoil blade section, the ‘lift’ and ‘drag’ force components occur.

    The blade is designed and angled such that the ‘lift’ force is significantly greater than the ‘drag’ force.

    In WTs lift-to-drag ratio can reach 120 compared to 15 for airliners.

    WT blades are made of light composite materials to minimize weight.


    The Nacelle

    Schematic view

    Image by Salameh, Z. “Renewable energy System Design”, chapter 3

    Mounted on top of turbine tower

    Contains the gearbox, wind generator, brakes, control and instrumentation systems

    Carries the WT rotor hub and blades

    Rotor hub

    Normally made offiberglass


    Realistic view



    Generators and Power Converters

    Fixed-speed wind turbines:
    - Squirrel cage induction generators (SCIG) , no power converters used.

    Variable speed wind turbines:
    - Wound rotor induction generators with partially-rated power converter. Also called doubly-fed induction generator (DFIG-WT)
    - SCIG with fully-rated converter (FRC)
    - Permanent-magnet synchronous generator (PMSG-WT) with FRC

    We will look in depth at these technologies next lecture!



    HAWT Drivetrain

    Drivetrain transmits mechanical power from the wind turbine rotor hub to the generator with a speed step up.
    Consists of the gearbox, brakes system, low-speed shaft, and highspeed shaft.


    Multi-megawatt HAWT Gearbox.

    Gearbox connects rotor hub (rotating at speeds <25 rpm) to the generator which can be rotating at 1500rpm (4 pole machine)
    2-stage or 3-stage gearbox to achieve high gear ratios (e.g. 1:90)

    WT gearbox is normally of planetary design
    Smaller gearboxes also required for yaw and pitch motor
    Due to stress and wear of moving parts, gearbox requires maintenance or even replacement



    Operational experience show that WT requires gearbox replacement at least once in its lifetime (see video of replacement process).



    This is challenging for offshore wind farm installations.
    That’s why direct drive WT designs avoid use of gearbox by using low speed synchronous generators with high number of permanentmagnet poles (permanent-magnet synchronous generators)



    HAWT Pitch Mechanism

    Used to maintain a certain shaft speed especially at higher wind speeds
    Can be partial-pitch angle control (part of the blade pitched) or full pitch-angle control, where the whole blade is rotated
    A pitch actuator is used to control hydraulic flow and rotate the blades
    In other mechanisms, pitch actuator (electric motor) drives a gearbox for speed transmission.





    Pitching mechanism can also be used as an aerodynamic brake system to stop the turbine in wind gusts, emergency, or planned down time.

    This can be done by pitching the blades to full-feather position, where blade section is parallel to wind flow and no power is captures (90° pitch angle).





    HAWT Yaw Mechanism

    Used to swing the nacelle to remain aligned with wind direction
    Receives a signal from a wind velocity sensor placed on top of the nacelle to sense wind speed and direction.
    The yaw mechanism consists of more than one unit each comprising an electric motor and a gearbox.


    The WT Tower





    Carries the weight of the nacelle, rotor hub and blades.

    Must be strong enough to sustain compression and drag forces

    Designed to allow access for maintenance

    Made of sections to be fixed on top of one another using cranes.

    Made of :
    Tubular steel (most large WTs)
    Lattice (Medium size WTs)
    Guyed pole (small WTs)








    Tower Foundation (onshore)

    Design depends on turbine size, height, mechanical loads, soil type and bedrock.
    Consists of: a hole, steel, concrete and backfill.

    See onshore foundation construction in this time lapse below




    Tower Foundation (offshore)

    Monopile foundation in shallow water up to 30m in depth.
    Jacket/tripod foundation for medium depth (up to 50m)
    Floating structures for deep waters.

    See floating foundation construction in this video





    WT Structure and operation

    See wind turbine structure and operation in this video



    Supplementary videos on WT manufacturing, structure and operation:



    WT installation

    See onshore wind turbine installation




    WF Power Collection Networks

    Each WT has a step up transformer placed in the base of the tower, or in a separate power room, or in some designs in the nacelle.
    The transformer steps up the voltage from 690V or 575V from the generator to typically 33kV for the medium voltage cable network in the wind farm collecting power from WTs
    For larger wind turbines (>5MW), 66kV step up transformer can be used.



    WF Power Collection Networks

    Underground AC cables (onshore) or subsea AC cables (offshore) connect all turbines to a transformer substation to step up the voltage to transmission voltage levels
    Offshore, farther from the shore, AC power has to be converter to DC power for transmission to grid.
    Offshore high voltage DC converter station (HVDC) is built with step up transformer on one or more offshore platforms.



    WF Power Collection Networks





    Offshore substations host step up transformers to step the voltage to transmission levels (e.g. 150kV)

    When HVDC is used, the substation hosts also Power Converter Station.

    At high power wind farms, two substations can be built.

    Platforms require significant and costly civil engineering work.






    See offshore substation construction in these videos



    Cost of WT components



    WT Wake Effect





    Wake refers to the reduced wind speed downstream of a turbine after extraction of energy by an upstream turbine from the incoming wind flow

    This is associated with creation of turbulence by each WT

    As a rule of thump, WTs in a WF are normally spaced about 7 to 9 times rotor diameter in the prevailing wind direction to minimize wake effect.




    See CFD visualization of WT wake in this video



    Expectedly wind changes direction, so WT wake is unavoidable but can be minimized by sufficient spacing between WTs in the prevailing wind direction
    The image shows clouds forming in the wakes of the front row of wind turbines of the Horns Rev wind farm off the coast of Denmark. The downstream wind turbines lose 20% or 30% of their power, and sometimes even more, relative to the front row. The spacing of the turbines is 7 rotor diameters.

  • HAWT Energy Capture and Power Curves


    Available Wind Power




    Power Captured by the WT





    As the wind flows through turbine blades plane, the wind expands and its speed drops from v1 to v2.

    The mean wind speed at the WT rotor plane is:

    As long as v2 is not equal to zero, the wind turbine has not captured all available wind energy.

    It is inconceivable to imagine a wind turbine can stop wind flow (i.e. reduce wind speed to zero).

    So, what is the maximum portion of incoming wind power a WT can capture?



    Derivation of max WT efficiency (Betz’s limit)





    Performance Coefficient





    cp -λ relationship




    Wind Turbine Power Curve (Power/wind speed)

    WT power curve represents WT mechanical power output (or electrical power after deducting all losses) as a function of wind speed.
    Before cut-in wind speed, wind speed is too low to turn the wind turbine efficiently
    From cut-in speed up to WT rated output power, WT controller operates the WT at optimal tip-speed ratio. This is also called “maximum-power point tracking (MPPT)” region
    When rated power is reached at rated wind speed, pitch angle control operated to avoid overloading the WT. The pitching region extends till cut out speed where wind is too high for safe operation and the blades are fully-feathered and mechanical brakes activated.



    Wind Turbine Power Curve (Power/rotor speed)

    WT power curve represents controlled WT mechanical power output (or electrical power after deducting all losses) as a function of WT rotor speed.
    From cut-in speed up to WT rated output power, WT controller operates the WT at optimal tip-speed ratio. This is also called “maximum-power point tracking (MPPT)” region.
    When rated power is reached at rated wind speed, pitch angle control operated to avoid overloading the WT