Components and concepts

Question 1

What is the equation for the power of the wind at constant wind speed?

Answer 1

P=1/2(dm/dt) v^2= {dm/dt=\rho *A * v}=1/2\rho A v^3

where A=swept area, \rho is air density, v is wind speed.

Question 2

What are the three forces that act on a wind turbine?

Answer 2

Lift, drag and rotation

Question 3

Describe aerodynamic lift.

Answer 3

The pressure differential between top and bottom surfaces results in a force called aerodynamic lift. In an aircraft wing, this force causes the airfoil to ‘rise’, lifting the aircraft off the ground. Since the blades of a wind turbine are constrained to move in a plane with the hub as its center, the lift force causes rotation about the hub,

Question 4

Draw the 2D model of the forces on an airplane wing.

Answer 4

See picture in slide 23 L02a

Question 5

Draw 2D model of the forces on a wtg and write the equation for the lift.

Answer 5

See picture in slide 24 L02a

Question 6

What are the typical five parts of a wind turbine?

Answer 6

Foundation, Transformer, tower, nacelle, blades.

Question 7

What parts are included in the nacelle with gearbox?

Answer 7

See picture slide 3 L02c. Spinner, rotor hub, rotor blade, main bearing, main axel, gear box, yaw control gear/motor, disc break, maintenance crane, cooling system, generator, tower.

Question 8

What parts are included in a gearless nacelle type I?

Answer 8

See picture slide 4 L02c. Rotor blade pitch regulator, spinner, blade adapter, blade pitch motor,axel pin, brake, machine support, yaw motor, Generator/ stator, generator/ rotor, load winch, tower.

Question 9

What parts are included in a gearless nacelle type II?

Answer 9

See picture slide 5 L02c. Pitch controller, blades, blade joints, rotor, rotor shaft, Electrical circuitry, generator, yaw drive, tower.

Question 10

What material is used for blades with low tip speed?

Answer 10

Steel plates

Question 11

What material is used for blades with high tip speed?

Answer 11

Glass-fiber-reinforced polyester (GFRP) and carbon-fibre-reinforced plastic (CFRP). CFRP is more expensive, but its fatigue strength is three times higher and therefore they are ideal for lightweight design.

Question 12

What are the mechanical breaks used for?

Answer 12

back-up for the aerodynamic braking system (like a parking breaks), ones the turbine is stopped in case of stall controlled turbines.

For pitch controlled turbines the breaks are rarely used (except for maintenance) as the rotor cannot move very much after the rotor blades are pitched 90 degrees.

Question 13

How is the gear box used?

Answer 13

The power from the rotation of the wind turbine rotor is transferred to the generator through the power train, i.e. through the main (low Speed) shaft, the gearbox and the high speed shaft.

The gear box increases the slower speed o the wind turbine to the higher speed required by the generator (1200 rpm for 50 Hz)

Question 14

How is the Yaw System used?

Answer 14

The wind turbine yaw mechanism is used to turn the wind turbine rotor against the wind.

Almost all horizontal axis wind turbines use forced yawing i.e. they use a mechanism which uses electric motors and breaks to keep the turbine yawed against the wind.

Question 15

How is the hydraulic system used?

Answer 15

The hydraulics system might be used to reset the aerodynamic brakes of the wind turbine.

Other applications of wind turbine hydraulics systems might include operation of the mechanical brake system and pitch control.

Question 16

How is the cooling system used?

Answer 16

The cooling unit contains an electric fan which is used tp cool the electrical generator. In addition, it contains an oil cooling unit which is used to cool the oil in the gearbox. Some turbines have water-cooled generators.

Question 17

What is an anemometer and a wind vane?

Answer 17

The electronic signals from the anemometer are used by the wind turbine’s electronic controller to start the wind turbine at cut in speed and stop it at cut out speed.

The wind vane signals are used by the wind turbine’s electronic controller to turn against the wind, using yaw mechanism.

Question 18

What is cut-speed?

Answer 18

The wind speed at which a wind turbine begins to produce power.

Question 19

What is the rated speed?

Answer 19

The ‘rated wind speed’ is the wind speed at which the ‘rated power’ is achieved and generally corresponds to the point at which the conversion efficiency is near its maximum.

In most cases, the power output above the rated wind speed is maintained at a constant level.

Question 20

What is the rated power output?

Answer 20

The power at, or above, the rated speed.

Question 21

What is cut-out speed?

Answer 21

The cutout speed is the wind speed at which teh turbine may be shut down to protect the rotor and drive train machinery from damage or high wind stalling characteristics.

Some manufacturers, using pitch control, slowly control the power output to zero (with increase of wind speed)

Question 22

Compare Stall and pitch control.

Answer 22

Stall: More exact, Almost fail prove (dirt on blades), Easier (no moving parts), But: high loading on support structure.

Pitch: Additional equipment (electrical of hydraulic), Must be fast (to reduce loading during gusts), better in extreme conditions!

Question 23

What are the 4 wind turbine concepts?

Answer 23

Danish concept: type A , Variable resistance: Type B, DFIG: Type C, Full Converter: Type D.

Question 24

Describe and draw a type A turbine.

Answer 24

The danish concept is based on two generators, one small one and one large induction generator. For larger tubines, today pole changing machines are common. Picture Page40 L02c

Question 25

Describe and draw a type C turbine.

Answer 25

The double-Feed Asynchronous Generator (DFIG). Gearbox: i=78.2 for GE Wind 2MW. Usually pitch control. Power electronic converter (IGBT inverter). Controllable Reactive Power Output. Variable Speed operation possible: N=~0.8-~1,2 (GE Wind 2 MW: 13-23 rpm).

Picture slide 39 L02c

Question 26

Draw a type B turbine.

Answer 26

Picture slide 39 L02c

Question 27

Draw a Type D turbine.

Answer 27

See slide 42 L02c. Full converter type. Can use a synchronous or induction generator with a gearbox or without.

Question 28

Compare variable and fixed speed.

Answer 28

Variable speed: Higher Yield, might be lost in Power Electronics, Reduced forces, Enercon:no oil, no gearbox, reduced moving parts- but generator heavy and may be expensive.

Fixed Speed: Easy to build, may be cheaper but higher forces, reduced lifetime? Stall regulation is simpler and may be more precise. No power electronics required.