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Energy Technology & Conservation

Week_07

Instructor: Mr. Adnan Qamar

Mechanical Engineering Department2

Wind Energy

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Wind - Atmospheric air

in motion

Solar radiation differentially

absorbed by earth surface

converted through convective

processes due to temperature

differences.

The kinetic energy stored in the winds is about 7 × 1021

J.

About 1% of absorbed solar radiation, 1200 TW (1200 × 1012 W), is dissipated in this way.

Origin of Wind

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Brief History

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Harvesting wind power isn’t exactly a new idea – sailing ships,

wind-mills, wind-pumps

1st Wind Energy Systems

•Ancient Civilization in the Near East / Persia

•Vertical-Axis Wind-Mill: sails connected to a vertical

shaft connected to a grinding stone for milling

Wind in the Middle Ages

•Post Mill Introduced in Northern Europe

•Horizontal-Axis Wind-Mill: sails connected to a

horizontal shaft on a tower encasing gears and axles

for translating horizontal into rotational motion

Wind in 19th century US

• Wind-rose horizontal-axis water-pumping wind-mills

found throughout rural America

Need for Wind Energy

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• Climate change (Global)

• Environmental concerns (regional and local)

• Security of energy supply (domestic “fuel”)

• Cost Competitiveness with traditional power

generation

• Provides local employment, regional economic

development

• Can be installed fast, compared with

conventional power plants

Current Status

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Onshore wind generation and projection

Wind Power depends on:

• amount of air (volume)

• speed of air (velocity)

• mass of air (density)

flowing through the area of interest (flux)

Power from Wind

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Betz Limit & Power Coefficient

• Power Coefficient, Cp, is the ratio of power extracted by the

turbine to the total contained in the wind resource

Cp = PT/PW• Turbine power output

The Betz Limit is the maximal possible Cp = 16/27

• 59% efficiency is the BEST a conventional wind turbine can

do in extracting power from the wind

http://www.ewea.org/wind-energy-basics/how-a-wind-turbine-works/

Power from Wind

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Wind Power Curve

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• A wind turbine is usually designed to reach full rated power

at wind velocities of around 12–15 m/s.

• It mostly runs at part-load, as the wind is not always strong

enough.

• The turbine should be able to efficiently convert power from

weak winds, therefore it is often designed to reach full

efficiency at around 8 – 10 m/s wind speed.

• Also in stronger winds the turbine must decrease its output

to protect the generator from overloading, i.e. at strong

winds the turbine dumps energy and works at lower

efficiency.

Wind Power Curve

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The power curve of a wind turbine is a graph that indicates

how large the electrical power output will be for the turbine at

different wind speeds.

0 4 8 12 16 20 24… wind speed, m/s

Power output

from the

turbine

Cut-in

rated output

Cut-off

The speed at which the turbine first starts

to rotate and generate power is called the

cut-in speed and is typically between 3

and 4 metres per second.

This is the speed at which the turbine blades are brought to rest to avoid damage from high winds.

Dynamic Matching

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• Capacity Factor (CF)

The fraction of the year the turbine generator is

operating at rated (peak) power

Capacity Factor = Average Output / Peak Output ≈ 30%

• CF is based on both the characteristics of the turbine

and the site characteristics (typically 0.3 or above for a

good site)

Power Generation from

Wind Turbine

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Four Main Variables• Wind Speed Energy content of the wind varies with

the cube of the average wind speed. Operating

Range 8-55 mph. Max power generation at 25-55

mph.

• Blade Radius The vertical disc created by blades.

Larger area will have more yield.

• Tower Height Tall turbines are usually more efficient

• Air Density Air density is a function of altitude. Dense

air, near sea levels drives rotor more effectively.

Types of Wind Turbines

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Types of Wind Turbines

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Vertical axis wind turbines (VAWTs), may be as efficient as current

horizontal axis systems, might be practical , simpler and

significantly cheaper to build and maintain than horizontal axis

wind turbines.

Advantages of VAWTs• They are always facing the wind – no need for steering into the wind.

•Have greater surface area for energy capture

•Are more efficient in gusty winds – already facing the gust

•Can be installed in more locations

•Do not kill birds and wild – life – slow moving and highly visible.

•Can be scaled more easily – from milliwatts to megawatts.

•Can be significantly less expensive to build

•Can have low maintenance downtime – mechanisms at or near ground level

•Produce less noise – low speed means less noise

•Are more esthetically pleasing – to some.

Types of Wind Turbines

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Horizontal axis wind turbines (HAWTs), have the main rotor shaft

and electrical generator at the top of a tower, and may be pointed

into or out of the wind. Small turbines are pointed by a simple wind

vane, while large turbines generally use a wind sensor coupled

with a servo motor.

Advantages of HAWTs

• Variable blade pitch, which gives the turbine blades the

optimum angle of attack.

• The tall tower base allows access to stronger wind in sites with

wind shear. In some wind shear sites, every ten meters up, the

wind speed can increase by 20% and the power output by 34%.

• High efficiency, since the blades always move perpendicularly to

the wind, receiving power through the whole rotation.

Types of Wind Turbines

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Disadvantages of HAWTsDifficult to transport. •

Difficult to install, needing very tall and expensive cranes and •

skilled operators.

Massive tower construction is required to support the heavy blades, •

gearbox, and generator.

Reflections from tall HAWTs may affect side lobes of radar •

installations.

Obtrusively visible across large areas, disrupting the appearance of •

the landscape and sometimes creating local opposition.

Downwind variants suffer from fatigue and structural failure caused •

by turbulence when a blade passes through the tower’s wind

HAWTs require an additional yaw control mechanism to turn the

blades toward the wind.

Types of Wind Turbines

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Types of Wind Turbines

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Power Generation from

Wind Turbine

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http://environment.nationalgeographic.com/environme

nt/global-warming/wind-power-interactive/

http://energy.gov/eere/wind/animation-how-wind-

turbine-works

http://environment.nationalgeographic.com/environment/global-warming/wind-power-interactive/http://energy.gov/eere/wind/animation-how-wind-turbine-works

Terminology

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Drag Force - component in line with the

relative velocity

Lift Force - component perpendicular to

FD. The use of the word ‘lift’ does not

mean FL is necessarily upwards, and

derives from the equivalent force on an

airplane wing.

Angle of Attack The angle which an

object makes with the direction of an air

flow, measured against a reference line

in the object.

Chord Line The reference from which

measurements are made on an aero foil

section

Terminology

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• Solidity - the ratio of the total area of the blades at any onemoment in the direction of the airstream to the swept area

across the airstream. Low solidity (high speed, low torque),

High solidity (low speed, high torque)

• The tip speed ratio (TSR) is given by dividing the speed of

the tips of the turbine blades by the speed of the wind.

• Optimum TSR depends on the number of blades in the wind

turbine rotor. The fewer the number of blades, the faster the

wind turbine rotor needs to turn to extract maximum power

from the wind. A two-bladed rotor has an optimum tip speed

ratio of around 6, a three-bladed rotor around 5, and a four-

bladed rotor around 3.

Problem 1

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Based on average speed data only, estimate the annual

energy production from a horizontal axis wind turbine with a 12

m diameter operating in a wind regime with an average wind

speed of 8 m/s. Assume that the wind turbine is operating

under standard atmospheric conditions (ρ = 1.225 kg/m3).

Assume a turbine efficiency of 0.4.

Problem 2

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A 40 m diameter, three bladed wind turbine produces 700 kW

at a wind speed (hub height) of 14 m/s. The air density is 1.225

kg/m3. Find:

a) The rotational speed (rpm) of the rotor at a tip-speed ratio of

5.0.

b) What is the tip-speed (m/s)?

c) If the generator turns at 1800 rpm, what gear ratio is

needed to match the rotor speed

to the generator speed.

d) What is the efficiency of the wind turbine system (including

blades, transmission,

shafts, and generator) under these conditions?

Problem 2

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Energy Extraction

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Assume constant air density and speed, 1.2 kg m−3 at sea level , and useful power can be harnessed in moderate winds when u0 ∼ 10 m s

−1 and P0 = 600 W m−2 .

Betz Model

Energy Extraction

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Betz Model

Interference factor a is the fractional wind speed

decrease at the turbine.

Energy Extraction

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Betz Model

Power Coefficient

Betz criterion

Energy Extraction

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Estimating wind speed for

a site

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Wind speed measurements from nearby locations•

Wind speed maps and atlases•

Wind flow simulation models (NOABL, WASP)•

Environmental Impacts

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Noise: Mechanical Noise, Aerodynamic Noise

Electromagnetic Interferences: dependant on material

of blades and surface shape of tower.

Visual Impact: physical parameters

Public Attitude

Birds

Cost of Wind Energy

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Economic appraisal of wind energy involves factors

such as :

• Annual energy production from wind turbine

installation (KVmAtT)

• Capital cost of installation

• Annual capital charge rate

• Length of contract with purchaser of electricity

produces

• Number of years over which investment is to be

recovered

• Operation and maintenance cost

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