Unit-4+Wind+Energy+Systems+-2

8/12/2019 Unit-4+Wind+Energy+Systems+-2

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Unit - IV

Wind EnergyGeneration Systems - 2


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Power in theWIND


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Kinetic Energy in the

Wind

Kinetic Energy = Work = mV2

Where:

m= mass of moving object

V = velocity of moving object

What is the mass of moving air?

= density () x volume (Area x distance)

= x A x d

= (kg/m3) (m2) (m)

= kgV

A

d


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Power = Work / t= Kinetic Energy / t

= mV2 / t

= (Ad)V2/t= AV2(d/t)

= AV3

d/t = V

Swept Area = A = R2

Area of the circle swept by

the rotor.

R

d = Displacement


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Parameters

Energy in wind= A v3

Height

above

Ground

Power law

Swept Area


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Wind Turbine Power

Power from a Wind Turbine Rotor = CpAV3

Cp is called the power coef f ic ient.

Cp is the percentage of power in the windthat is converted into mechanical energy.

What is the maximum amount of energy that

can be extracted from the wind?

5926.27

16max, pC Betz Lim i t = 59.3 %


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Tip-Speed Ratio Tip-speed ratio is the ratio of the

speed of the rotating blade tip to the

speed of the free stream wind. There is an optimum angle of attack

which creates the highest lift to dragratio.

Because angle of attack is dependant

on wind speed, there is an optimumtip-speed ratio

R

VTSR, =

Where,

= rotational speed in radians /sec

R= Rotor Radius

V= Wind Free Stream Velocity

R

R

Power Coefficient varies with Tip Speed Ratio

Characterized by Cp vs Tip Speed Ratio Curve


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Maximum Possible Power Coefficient0.60

0.50

0.40

0.30

0.20

0.10

0.00

Cp

109876543210Tip Speed Ratio

Betz - Without Wake Rotation

With Wake Rotation


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Betz Limit All the energy in the windcannot be captured byrotor as the air would be

completely still behindrotor and not allow morewind to pass through.

Theoretical limit of rotorefficiency is 59%

Most modern windturbines are having rotorefficiency in the range of3545%

The Betz equation is

derived from several lawsincluding Conservation ofMomentum, Conservationof Mass and Bernoullisprinciple.


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Airfoil Nomenclature

wind turbines use the same aerodynamic principals as aircraft

= angle of attack i.e., angle between the chord lineand the direction of wind, V

V = wind speed

V


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Lift & Drag Forces

The Lift Forceisperpendicular to thedirection of motion. Wewant to make this force

MAXIMUM.

The Drag Forceisparallel to the directionof motion. We want tomake this force minimum.

= low

= medium


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Airfoil Shape

Just like the wings of an airplane,

wind turbine blades use the

airfoil shape to create lift and

maximize efficiency.

The Bernoulli Effect


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Twist & Taper

Speed through the air of apoint on the blade changes

with distance from hub

Therefore, tip speed ratio

varies as well

To optimize angle of attackall along blade, it must twist

from root to tip

Fast

Faster

Fastest


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Airfoil in stall (with flow separation)

Stall arises due to separation of flow from airfoil

Stall results in decreasing lift coefficient withincreasing angle of attack

Stall behavior complicated due to blade rotation


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Energy Production Terms

Rated Power:Maximum power the

generator can produce.

Cut-inwind speedwhere energy

production begins

Cut-outwind speedwhere energy

production ends. Typical Power Curve

Cut-in: 3-4 m/s Rated: 12-25 m/s Cut-out: 25 m/s


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Yaw control Most WEGs Use forced Yaw

Cable Twist Counter


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Gradual curves

Sharp trailing edge

Round leading edge Low thickness to

chord ratio

Smooth surfaces

Making Good Airfoils

Good

Not so good


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Considerations for Optimum Blade

Optimum blade will have low solidity (10%) and tip speedratio, ,about 5-7 (match speed to generator)

High means lower pitch angle (blade tip is flat to the

plane of rotation).

Lower means higher pitch angle (feathered).

Pitch angles should be equal for all blades.

Optimum blade has large chord and large twist near hub

and gets thinner near the tip.

Optimum blade is only "optimum" for one tip speed ratio.

The optimum blade will have smooth streamlined airfoils.

MODEL GE 3 6 l


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MODEL: GE 3.6 sl

Rated capacity: 3.600 kW

Cut-in wind speed: 3.5 m/sCut-out wind speed: 27 m/s

Rated wind speed: 14 m/s

Wind Class - IEC: 111m

Rotor diameter: 9677 m2

Swept area: 8.515.3 U/minHub height: Site specific

Power Control: Active blade pitch

control


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Heat

storage

Cold

storage

Hydro

Compressed

air

Hydrogen

Large-scale

BatteryFlywheel

Natural

gas

Electric

Vehicles

Curtailment

Too much windNot enough wind OR

too much wind

Present

Fallback option

Near Term

Longer Term

Energy Storage comes in many

forms


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Why

offshore ?


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Why offshore?

V80-2.0 MW, North Hoyle, UK

Advantages

Better wind resources

Less turbulence/low roughnessmore steadyproduction

Layout flexibility

Less resistance from local population

No physical limits for size and weight

Space

Transportation

Disadvantages

More complex site conditions

Geology

Sand waves

Sea, Waves and currents Saline environment

Installation and maintenance are more complicated

and expensive


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Wind Energy 23

Burning questions

What are your most burning questions

about wind energy?

Break into small groups and come up with

two biggest questions per group.


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Wind Energy 25

Human-related bird kills

Wind turbines

Communicationtowers

Pesticides

Vehicles

High-tension lines

Other

Cats

Buildings/windows

http://www.awea.org/faq/wwt_environment.html#Bird%20and%20bat%20kills%20and%20other%20effects


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Wind Energy 26

Bat Kills

Bat fatality at wind turbines has been documentedworldwide in the U.S., Australia, Canada, Germany,Spain, and Sweden.

Bat fatalities have been reported at nearly all wind

energy facilities in the U.S. annual mortality estimated at


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Wind Energy 27

When the wind doesnt blow

Do fossil-fired generating units have to be kept running on astandby basis in case the wind dies down?

No. Wind speeds rise and fall gradually and the system operator hastime to move other plants on and off line as needed.

A 100-MW wind plant requires about 2 MW of conventional capacityto compensate for changes in wind.

Wind can reliably provide 20% or more of our electricity.



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Wind Energy 28

Lifetime environmental impact

Manufacturing wind turbines and building

wind plants does not create large

emissions of carbon dioxide.

When these operations are included, wind

energy's CO2emissions are quite small:

about 1% of coal, or

about 2% of natural gas

(per unit of electricity

generated).http://www.awea.org/faq/wwt_environment.html#Bird%20and%20bat%20kills%20and%20other%20effects


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Wind Energy 30

Shadow flicker

A wind turbine's moving blades can cast amoving shadow on a nearby residence,depending on the time of the year and time ofday.

Normally, it should not be a problem in the U.S.,because at U.S. latitudes (except in Alaska) thesun's angle is not very low in the sky.



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Wind Energy 31

Electrical power quality

Generally not a concern for low penetration

Weak grids and grid reinforcement Problems may occur if a turbine is connected to a

weak electrical grid, which can be reinforced.

Power quality problems caused by wind farms arethe exact mirror-image of connecting a largeelectricity user, (e.g. a factory with large electricalmotors) to the grid.

Electrical flicker Flicker = short lived voltage variations in the

electrical grid which may cause light bulbs to flicker.

Flicker may occur if a wind turbine is connected to aweak grid.

Flicker can be reduced with proper turbine design.

http://www.windpower.org/en/tour/grid/rein.htm


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Wind Energy 32

TV and radio reception

Modern small (residential) wind turbines willnot interfere with communication signals. The materials used to make such machines are

non-metallic (composites, plastic, wood).

Small turbines are too small to create

electromagnetic interference (EMI) by "choppingup" a signal.

Large wind turbines can interfere with radioor TV signals if a turbine is in the "line of

sight" between a receiver and the signalsource. Alleviate the problem by: improving the receiver's antenna

installing relays to transmit the signal around thewind farm


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Unit-4+Wind+Energy+Systems+-2