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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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