Wind Turbine Technologies

Dominant Wind Turbine DesignWind turbine technologies have evolved over centuries from traditional wind mills to the slick turbines we know today. Developing a wind turbine requires a number of design decisions, starting from the axis orientation, and number of blades to choice of material.

Over the years, a dominant design has emerged, which consists of 3 blades, mostly up-wind rotor, pitch-control for braking and an integrated gear box.

However, in this context, "dominant" does not necessarily mean "best". The best turbine technology for a given project depends on the site's precise wind profile and other requirements. This is why many variations exist.

Rotor Axis Direction and Number of Blades
Vertical
The rotor axis is perpendicular to the wind direction. Some designs use draf, others use lift forces.

The most widely used application is the wind anemometer with its moving cups. Small-scale roof-top turbines are also often with a vertical axis to avoid gear box.

Lift
Drag
Darrieus
Darrieus Wind Turbine
  tip speed ratio: 4
Savonius
Savonius Wind Turbine
tip speed ratio: <1 
Wind Anemometer
Wind Anemometer
 
  • Omni-direction - wind may come from any direction
  • Easy to mount at ground level, no tower needed
  • Low rotation speed - no gearbox needed - less noise
  • Generally near the ground with low wind speeds
  • Self-starting problems
  • Drag devices capture only ~15% of energy, Darrieus ~40%.
  • No large-scale commercial application

 

Horizontal
The rotor is parallel to the wind direction. All devices use lift forces rather than drag.

Horizontal axis is the most common design for turbines, and is also the design of the traditional wind mills. For electricity generation, mostly 3 blades or less.

   
1 Blade
One Blade Turbine
Tip speed ratio:12
3 Blades
Three Blade Turbine
Tip speed ratio: 6
Water Pumps
Multi blade turbine
Tip speed ratio: 1
  • Tip speed ratio is the ratio of the tangential velocity of the blade (at the tip) to the undisturbed wind speed. Fewer blades means higher tip speed. The optimum tip speed ratio that maximises the lift-to-drag force ratio is around 8. With more than 4 blades, less efficiency because each blade operates in the wake of others.
  • Efficiencies up to 50%
  • Lower cut-in wind speeds than vertical turbines
Axis Orientation and Hub
Up-Wind
Upwind devices have the rotor facing the wind
  • Avoids the wind shade that the tower causes. Fewer fluctuations in the power output.
  • Requires a rigid hub, which has to be away from the tower. Otherwise, if the blades are bending too far, they will hit the tower.
  • This is the dominant design for most wind turbines in the MW- range
Down-Wind
Downwind devices have the rotor on the lee-side.
Down Wind Turbine
  • May be built without a yaw mechanism if the nacelle has a streamlined body that will make it follow the wind.
  • Rotor can be more flexible: Blades can bend at high speeds, taking load off the tower. Allow for lighter build.
  • Increased fluctuations in wind power, as blades are affected by the tower shade.
  • Only small wind turbines.
Control Mechansim

A control mechanism is an overspeed control that allows the rotors to be slowed down or stopped. Its purpose is to

  • optimize aerodynamic efficiency
  • keep the generator with its speed and torque limits and rotor and tower within strength limits
  • enable maintenance
  • educe noise
Stalling
Principle: Increased angle of attack results in decreasing lift-to-drag ratio.      Stall Control
  • Passive: Blades are at a fixed pitch that starts to stall when the wind speed is too high.
  • Active: motor turns the blades towards stall when wind speeds are too high.
  • Hybrid: Pitch can be adjusted manually to reflect site's particular wind regime.
  • Disadvantage: Stalled blades cause large vibration and therefore noise.
Pitch Control
Principle: Decrease angle of attack also results in decreasing lif-to-drag ratio.
Pitch Control

Always active control: Blades rotate out of the wind when wind speeds are too high.

 

 

 

 

 

 

 

 

Furling
Principle: Moving the axis out of the direction of the wind decreases angle of attack and cross-section
Furling

Standard modern turbines all furl in high wind.

  • Requires active pitch control: Pitch angle of the blades needs to be minimised first, otherwise the torque on the rotor would be to big for furling.
  • Active: Vertical furling (as diagram) with hyrdraulic, spring-loaded or electric motor driven.
  • Passive: Horizontal furling with yaw
Other Design Decision
Generator & Drive Train
Direct: No gearbox needed, requires a synchronous generator Less noise than with gearbox, but more expensive. Prominent example: Enercon wind turbines.

Indirect: The rotor drives a gearbox, which drives the asynchronous generator at around 1000rpm.

Blade Material
  • Laminated wood
  • Aluminium
  • Lightweight glass-enforced plastic - this is most common among large blades - 50 to 100m long.

 

 

 

Yaw Mechanism

Yaw mechanism moves the nacelle of a horizontal turbine around its tower into the wind when the wind direction changes.

  • Passive: with a fin attached to the nacelle on the opposite side of the rotor.
  • Active: with a motor
Tower

Steel tube

  • Bending and welding in factory
  • Long experience
  • Steel is expensive.
  • Transporting on the ground subject to size limits 4-5m.
  • Example: REpower

Lattice tower:

  • Less steel than steel tube.
  • Good for high towers
  • Constructed on-site
  • Requires lots of inspection and on-site labour.
  • Example: SeeBa, Suzlon

Concrete tower:

  • Less use of expensive steel
  • Quality problems with on-site concrete. due to changing weather conditions. Alternative: ship segments from manufactury subject to transport limits.
  • Example: Enercon
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