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Technische Universitt Mnchen

Prof. Dr.-Ing. H.-P. Kau Module Fluid Machinery 12 - 1

Chapter 12

12.Wind power plants

1. Environmental conditions2. Examples

3. Specifications

4. Advantages and Disadvantages

5. Realised designs

6. Design

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Environmental conditions of wind turbines

Approx. 1.5 to 2.5% of the sunss energy is transformed to wind giving a theoretical windpotential

of 35 * 1011

kWh/a for Germany. From this amount of wind energy only 2 * 1011

kWh/a aretechnically usable since the wind velocity has to be between 4 and 24 m/s. This results in a wind

energy of40 to 8000 W/m2.

Resulting from the required wind

velocities the typical operational area of

wind turbines in Germany is north of the

line Aachen-Kassel-Erfurt-Chemnitz-

Dresden.

The wind velocity is more permanent

above a height of 50m over groundwhich eases the efficient use of wind

turbines.

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Wind conditions Europe and Germany

StrombasiswissenNr.109,Informationszentraleder

Elektrizittsw

irtschafte.V.FrankfurtamMain1995

Averaged wind

velocity in 10m

above ground

to

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Munich waste disposal site

Hub height: 67m

Nominal power: 1500kW

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Offshore Windpark Middelgrunden

20 wind turbines (2MW Bonus Energy A/S,

rotor diameter: 76m) 180m distance

4-8m Water Depth

Wind speed at 50m: 7.2m/s

3,4km overall length

Per Year Output: 89 GWh

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Specifications of a wind turbine

Simple design and low manufacturing costs

Rugged construction (gust and storm-proof

design)

Low maintenance

Low-noise operation

High efficiency

Turnable tower

Rotor blades are adjustable at hub

Interconnected operation or possibilities to store

the obtained energy

For example pumped or compressed air

storage plants

Rotor blade

Connection to

power supplyFoundation

Tower

Rotor hub and

blade adjustment Wind tracking

Generator

Nacelle

Gear boxBrake

Switschboard &

control system

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Advantages and disadvantages of a wind turbine

Hydropower plant Wind turbine Oil / coal / gas

power plant

Nuclear power plant

Fuel reserve + Renewable energy:

timely unlimited

+ Renewable energy:

timely unlimited

- approx. 50 years - approx. 50 years =>

fast breeder reactor?

Local existence - Often not at the

location where

energy is needed

- Often not at the

location where energy

is needed

+ easily transportable + Comparatively easy to

transport

Upgradability - Limitied - Limitied + short-term

unlimited

+ short-term hardly

unlimited

Emissions no emission from

power plant, but CO2-

emission from

reservoir

Noise emission,

shadow strike

- Waste heat, CO2-

and NOx-emission

- Waste heat, radiation

near to the plant?

Efficiency + approx. 90% approx. 45% - approx. 35 60% - approx. 35%

Disposal problems + no + no - Disposal of ash and

gypsum

- Final disposal

unsolved!

Accident hazard,

probability

+ very low + very low medium + with corresponding

monitoring effort very

low

Consequences - Dam breaks: very

huge damages,

short-term

+ very low + limited range,

short-term

- Explosion: extreme

consequences, short-

and long-term

+good

medium

-poor

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Advantages and disadvantages of a wind turbine

Hydropower plant Wind turbine Oil / coal / gas power

plant

Nuclear power plant

Land consumption River power station

small, storage powerplant maybe very

high

Comparatively high,

land remains arable

+ small + small

Environmental impact + managable, overall

nearly neutral

+ managable, overall

nearly neutral

- managable, clearly

negative

- ??

Dismanteling + possible, maybe

complex

+ very easy + easy - Very complex

Operation: Contro l + Very fast adjustable - Not adjustable Coal slow, CCGT fast - Only slowly

adjustable

Avail ib il ity + Very high + Very high medium + Very high

Investment costs - Very high + medium + medium high

Amortisat ion Approx. 20 years Approx. 5 to 10 y. Approx. 5 to 15 y. Approx. 15 y.

Maintenance + low + low medium medium - high

Life + 40 to 100 years - 10 to 20 y. - 10 to 30 y. approx. 30 y.

Construction time 2 to 10 years Approx. 1 year 1 to 3 y. 5 to 10 y.

+good

medium

-poor

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Basic classification of wind turbines

Horizontal spool machinery Vertical spool machinery

Lift using machinery Drag using machinery

Most common design: Horizontal spool machinery with lift using blades

a) Mul tiblade converter b) Twoblade turbine c) Threeblade turbine

d) Cased turbine e) Darrieus machine f) H-Rotor

g) Savonius rotor h) Flettner rotor i) Cup rotor

j) Half cased drag rotor

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Arrangement of the geared power train in horizontal machinery

Gearbox and

generator in thenacelle (common

design)

Generator

verticallymounted in the

tower head

Gearbox and

generator in thetower base

Gearbox in the

tower head andgenerator in the

tower base

Generator in the

tower baseGearbox is split

Directly driven

generatorwithout gearbox

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Vertical spool machinery

Parabolic rotorCylindrical rotor

Advantages and

disadvantages in

comparison with a horizontal

spool machinery:

+ less moving parts

+ no driven orientation

necessary

+ smaller forces

- lower efficiency

- poorer performance

Blade

BearingCirculargenerator

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Example Nordex N80 / 2500 kW

1. Rotor blades

2. Hub

3. Engine frame

4. Bearing, established in double-row spherical roller bearings

5. Shaft

6. Twostage planetary drive

7. Disc brake

8. Generator coupling

9. Water-cooled generator.

10. Cooling-system for generator and gearbox

11. Cooling-system for generator

12. Redundant wind measuring system

13. Control

14.Hydraulic system for the hydraulic pressure of the brake cylinders

and the yaw brakes15. Azimuth drive

16. Azimuth bearing

17. Nacelle made of glassfibre reinforced plastic

18. Tubular steel tower

19. Pitchsystem (three independant, electrically driven pitch gear units)

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Data sheet Nordex N80Rotor GeneratorNumber of blades 3 Power 2.500 kW

Speed 10.9 bis 19.1 /min Currency 660V

Rotordiameter 80 m Typ Double-fed induction generator, liquid-

cooledSwept ares 5.026 m2 Speed 740-1.310 /min

Power control Pitch Protection class IP 54

Startwind 3 m/s Mass ca. 12.000 kg

Stopwind 25 m/s

Orientation controlRated power at approx. 15 m/s

Surviving wind speed 65 m/s according tp GL class 1 / 70 m/saccording IEC Tclass 1

Azimuth bearing Ball bearing

Pitch control Single blade pitch Brake hydraulic, Disc brakeMass approx. 50.000 kg Drive Two induction motors with integrated

brakes

Speed approx. 0.5 0/s

Gearbox TowerTyp Threestage planetary spur gearbox Design Modular tubular steel tower, cylindrical /

top segment conical or truss tower, hot-dipped

Gear ratio 02:08.1 Hub height Tubular steel tower 60 m, certificate IEC1a, DIBt 3

Mass approx. 18.500 kg Truss tower 80 m, certificate IEC 1a, NVN1a, DIBt 3

Status: 09/2005 (subject to technical modifications)

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Energy transformation: Stream theory

The wind flowing into the wind turbine (1) with the velocity v1is decelerated. Kinetic energy is withdrawn from the wind to

drive the rotor (A). The deceleration of the air stream in the

rotor causes a stream expansion (dilatation) (2).

A pressure increase occurs downstream at the inside of the

stream. Inside the rotor (A) a pressure decrease takes place.

At some distance upstream and downstream of the rotor atposition (1) and (2) the pressure is equal to the ambient

pressure p0 on the entire cross-section. Hence for a annular

cross-section the relative velocity becomes |w1| = | w2|.

pt

A

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Formulas of the stream theory

Power withdrawn from the stream at the massflow :

Thrust of the rotor:

Actio = Reactio Thrust of the rotor corresponds to

the force which the rotor exerts on the fluid

Average velocity in the rotor plane:

Massflow through the rotor:

Mechanical power of the rotor:

)(2

1 22

2

1 vvmP m

)( 21 vvmF

vvvmvFP )( 21

vvvmvvmP )()(

2

121

2

2

2

1

2

)( 21 vvv

)(21 21 vvAm

))((4

121

2

2

2

1 vvvvAP

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Betz theory about the power of a wind turbine

Betz coefficient:

AvP 3

102

1

1

2

2

1

2

3

1

212

22

1

0

112

1...

2

1

))((41

v

v

v

v

Av

vvvvA

P

PcP

The maximum of the Betz coefficient is at for (Betz factor) for

an ideal, frictionless flow.

593,027

16Pc

3

1

1

2 v

v

Power of the airflow at the cross-section A without any power withdrawn:

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Power characteristics of wind turbines

R

Rotor power as a function of the wind velocity: with cPR = Betz coefficient of the rotor

Betz factor cp

AvcP PRR3

12

Betzcoefficient Fast runner

Theoretic power coefficient of an ideal wind turbine

3-blade

rotor

2-blade rotor

Single blade

rotorSlow runner

Darrieus

rotor

Dutchmans windmill

American w indmil l

Tip speed ratio

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Real characteristics according to the number of blades

Gradual approximation of

the real rotor characteristic

with the help of the theory

23

R

Number of

blades

Powercoefficient

Tip speed ratio at design point

Capability of the air flow

Ideal Betz factor

Swirl losses Profile losses

Finite number of blades

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Power control through pitch

The power generated by the wind turbine has to be controlled due to the technically limited capacity

of the generator, due to safety aspects and to the purpose of keeping the power generation constant.

One possibility is to decrease the pitch angle of the rotor blades (pitch control).

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Power control through stall

The rotor blades stall at a fixed pitch angle and at a

constant circumferential velocity when the wind velocity

raises. This effect limits the power output of the windturbine so this can be used to control the plant (stall

control).

The design of the plant has to be modified:

Increased stiffness and ruggedness of the entire plant isnecessary to withstand the higher aerodynamic loading

Comparatively high generator power necessary to cover

peak power

Improved starting capabilities of the rotor (2-blade rotorneeds an electrical starter)

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Power

Factor Formula

Theoretical maximum possible power coefficient of the rotor:(Betz factor)

Rotor efficiency:

(caused by aerodynamic losses of the rotor)

Betz coefficient:(modern rotor designs obtain a Betz coefficient of approx. 0.5)

Efficiency of all mechanical and electrical components:

From the efficiency of the rotor and all remaining components the effciency

of the wind turbine is calculated:

Effective power of the wind turbine:

(Electrical power of the plant)

593.0Pc

gRPPR cc ,

85.07.0, gR

elmech,

elmechPRPlantP cc ,,

RotorPlantPel AvcP

3

1,2

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Principal sequence of the wind turbine desing

Iterative proces(maximum energy

supply)

Design wind

data

Rotor

diameter

Generator

power

Tower

height

Rotor performancemap

Av. wind velocity and frequency

distribution in hub height

Management

and control

Optimum rotor

speed

effective rotor

characteristic

Mech. and el. losses

in power trainWind data at

installation location

Power characteristic of

plant Pel

Energy supply

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