Advanced Energy Engineering :- Wind Energy.pptx

MET 458 ADVANCED
ENERGY ENGINEERING
Module-III

Introduction
Wind is air motion derived from solar energy
The Earth continuously releases into the atmosphere
the heat received by the sun, but unevenly.
In the areas where less heat is released(cool air
zones) the pressure of atmospheric gases increases,
whereas where more heat is released, air warms up
and gas pressure decreases.

About 2 % of total solar flux that reaches the earth
surface is transformed into wind energy due to uneven
heating of earths atmosphere
During daytime air over the land mass heats up more
than the air over the oceans.
Hot air expands and rises while cool air from oceans
rushes to fill the space creating local wind.

During night time air over lands cools rapidly
than water over offshore land causing breeze
as shown below.

Since the atmosphere tends to constantly re-
establish the pressure balance, the air moves
from the areas where the pressure is higher
towards those where it is lower; therefore, wind is
the movement of an air mass, more or less quick,
between zones at different pressure.
The profile and unevenness of the surface of the
dry land or of the sea deeply affect the wind and
its local characteristics; in fact the wind blows
with higher intensity on large and flat surfaces,
such as the sea: this represents the main element
of interest for wind plants on and off shore.

 Moreover, the wind gets stronger on
the top of the rises or in the valleys
oriented parallel to the direction of
the dominant wind, whereas it slows
down on uneven surfaces, such as
towns or forests, and its speed with
respect to the height above ground is
influenced by the conditions of
atmospheric stability.

Historically wind energy used
for sailing ship, grinding grain,
water pumping etc.
Wind Energy Conversion
System – converts the kinetic
energy in wind to usable form
of mechanical energy (shaft
power)/electrical energy

History
Wind energy harnessing was introduced
by Egyptians back to 4000 BC when
Egyptians used to wind power to sail
boats in Nile river
In the early days of 10th
century wind
mills were used for grinding corn and for
water pumping
In the present days wind mills are using
for the electricity generation
India being fifth after Germany, USA,
Spain and Denmark in electricity
generation using wind energy

Renewable Energy Installed
capacity in India

Wind power plant
 Wind flow is created as an effect of solar heat,
which creates low and high-pressure regions on the
earth due to heating. Thus wind energy is rightly an
indirect form of solar energy. The flowing wind is
used to rotate the wind turbine, which is also
known as windmill.
 Wind turbines are usually located at the sea shore
or in the sea where there is availability of wind.
 For electric power generation, the average wind
speed required is 5 m/s.

Wind energy is
inexhaustible
Fluctuation of wind
energy is the main
drawback of the
system
Requires negligible
fuel cost but higher
capital cost
Indian wind energy
sector has an
installed capacity
of 32380.18 MW
India is ranked 5th
in
terms of wind
power installed
capacity

Life span is predicted as 20-25 years
however from actual practices 12-15 years
Clean , zero emissions
Reduce fossil fuel dependence
No fuel price volatility

Operation
“Wind turbine operates by
slowing down air and extracts
part of its kinetic energy and
converts it to mechanical/
electrical energy”
This is achieved with the
help of a suitable device
called wind mill

Sources of wind
Winds are natural phenomena in the atmosphere and have
two different origins viz., planetary winds and local winds.
1. Planetary winds – Planetary winds are caused by solar
heating of the earth's surface near the equator than near
the north or south poles. This causes warm tropical air to
rise and flow through the upper atmosphere towards the
poles and cold air from the poles to flow back to the
equator nearer to the earth's surface. The direction of
motion of the planetary winds is affected by the rotation of
the earth.
2. Local winds – Local winds are caused by un-equal
heating and cooling of land and water, and also by hills
and mountain sides. During the day warmer air over land
rises upwards and colder air from lakes, ocean, forest
areas, flows towards warmer zones.

WIND SPEED DISTRIBUTION
 Wind speed is the most critical data
needed to appraise the power potential
of a candidate site.
 The wind is never steady at any site. It
is influenced by the weather system, the
local land terrain, and its height above
the ground surface.
 Wind speed varies by the minute, hour,
day, season, and even by the year.
 Therefore, the annual mean speed
needs to be averaged over 10 year or
more.

This is known as the measure, correlate, and
predict (mcp) technique.
In such situations, the short-term data, for
example, over 1 yr, is compared with long-
term data from a nearby site to predict the
long-term annual wind speed at the site under
consideration.
Such a long-term average gives a greater
confidence in assessing the energy-capture
potential of a site. However, long-term
measurements are expensive and most
projects cannot wait that long.

Working Principle of
Wind turbine
 Wind turbines operate on
a simple principle. Wind
is merely air in motion.
Wind turbines convert
kinetic energy from the
wind that passes over the
rotors into electricity.
 The kinetic energy in the
wind turns two or three
propeller-like blades
around a rotor. The rotor
is connected to the main
shaft, which spins a
generator to create
electricity.

Wind turbines can be used to
produce electricity for a single
home or building, or they can be
connected to an electricity grid
for more widespread electricity
distribution.
Wind turbines harness the power
of the wind and use it to
generate electricity. Simply
stated, a wind turbine works the
opposite of a fan. Instead of
using electricity to make wind,
like a fan, wind turbines use wind
to make electricity.

Components of wind turbine
1. Hub – The blades are attached to the
hub.
2. Rotor – Blades and hub together is
called the rotor. Rotor is attached to
the slow speed shaft.
3. Nacelle – Nacelle is the cover housing
that houses all of the generating
components in a wind turbine,
including the generator, gearbox, drive
train, and brake assembly.
4. Tower– The tower of the wind turbine
carries the nacelle and the rotor.
Towers may be made from steel or
concrete.
Horizontal axis wind power plant
(Propeller type)
Hub
Blade
Gear box
Generator
Tower
Nacelle
5. Gears – Gears connect the low-speed shaft attached to the hub to the
high-speed shaft attached to the generator and increase the rotational
speed.

Parts of Wind Turbine
Anemometer: Measures the wind speed
and transmits wind speed data to the
controller.
Blades: Lifts and rotates when wind is
blown over them, causing the rotor to spin.
Most turbines have either two or three
blades.
Brake: Stops the rotor mechanically,
electrically, or hydraulically, in
emergencies.

Controller: Starts up the machine at wind
speeds of about 8 to 16 miles per hour (mph)
and shuts off the machine at about 55 mph.
Turbines do not operate at wind speeds
above about 55 mph because they may be
damaged by the high winds.
Gear box: Connects the low-speed shaft to
the high-speed shaft and increases the
rotational speeds from about 30-60 rotations
per minute (rpm), to about 1,000-1,800 rpm;
this is the rotational speed required by most
generators to produce electricity. The gear
box is a costly (and heavy) part of the wind
turbine and engineers are exploring "direct-
drive" generators that operate at lower
rotational speeds and don't need gear
boxes.

Generator: Produces 60-cycle AC electricity; it is usually
an off-the-shelf induction generator.
High-speed shaft: Drives the generator.
Low-speed shaft: Turns the low-speed shaft at about 30-
60 rpm.
Nacelle: Sits at top the tower and contains the gear box,
low- and high-speed shafts, generator, controller, and
brake. Some nacelles are large enough for a helicopter
to land on.
Pitch: Turns (or pitches) blades out of the wind to control
the rotor speed, and to keep the rotor from turning in
winds that are too high or too low to produce electricity.
Rotor: Blades and hub together form the rotor.

Tower: Made from tubular steel, concrete,
or steel lattice. Supports the structure of
the turbine. Because wind speed
increases with height, taller towers enable
turbines to capture more energy and
generate more electricity.
Wind direction: Determines the design of
the turbine. Upwind turbines—face into
the wind while downwind turbines face
away.
Wind vane: Measures wind direction and
communicates with the yaw drive to
orient the turbine properly with respect to
the wind.

Yaw drive: Orients upwind turbines
to keep them facing the wind when
the direction changes. Downwind
turbines don't require a yaw drive
because the wind manually blows
the rotor away from it.
Yaw motor:Powers the yaw drive.

Wind turbine Power Plant Operation

Lift and drag
These are two primary mechanisms
producing the forces from the wind
Lift force: forces produced by the
changing velocity of air stream either
side of lifting surfaces
Lift force is always in the perpendicular
direction
Speeding up the air flow process causes
the pressure to drop
While slowing down the air flow causes
the increase in pressure

In other words any change in
velocity generates pressure
difference across the lifting
surfaces
The pressure difference produces a
force that begins to act on the
high pressure side and moves in
low pressure side of lift in surfaces
which is called aerofoil
Lift forces acting in the
perpendicular directions to the air
flow while drag force acts in the
direction of air flow

A good
aerofoil has
high lift to drag
ratio
In some case
this ratio is 30
A lot of
turbulence
ensures the lift
to decrease
and drag to
increase
substantially
This
phenomenon
is called
stalling

For the efficient operation of
wind turbines blades used to
function as much lift as
possible and low drag as
possible because drag
dissipates energy.
As lift involves simple
deflection on air flow
therefore it is an efficient
process

Aerodynamic principle of wind turbines
Lift and drag forces acting on a blade
Lift
Drag
Wind
All the wind turbines
work on two physical
principles (or combination
of these two) in blade
designs by which energy is
extracted from the wind.
These principles are either
(i) drag principle or (ii)
lift principle. Blade
designs operate on either
the principle of drag or lift.

Drag principle – Drag devices are simple
wind machines that use flat, curved or
cup-shaped (unlike aerodynamic shapes
of the lift devices) blades to run the rotor.
In the drag blade design, the wind pushes
the blades out of the way. Drag powered
wind turbines are characterized by slower
rotational speeds and high torque
capabilities.
Lift principle – The lift devices employs the
same principle that enables airplanes,
kites and birds to fly. The blade is
essentially an aerofoil, or wing. The top
surface of a blade aerofoil is more curved
than the bottom surface. When air flows
past the blade, a wind speed and
pressure differential is created between
the upper and lower blade surfaces.

For measurement of wind speed, the basic sensors used are
anemometers and for measurement of direction, wind vanes are
used. The most commonly used anemometer is rotating cup
anemometer. In this type, a vertical shaft supports a cup
assembly. The cup rotates about the vertical axis in proportion
with the incoming wind speed.
E
W
N
S
Anemometer and wind vane
Cup anemometer
Wind vane
The calculation of the power of the
wind energy (Pt
) is based on the kinetic
energy of moving air molecules.
According to Betz' law, wind power,
3
1
2
t
P ρAV

Therefore, wind speed (V) is the most important parameter, as wind energy
(Pt
) is proportional to the cube of wind speed.
Pt = Power available in the wind,
= Density of air,
A = Swept area, and
V = Velocity of wind.
ρ

Classification of wind turbines
I. According to
the orientation of
axis
• 1 horizontal axis
machines
• 2 vertical axis machines
1
II. According to
the application
• 1 lift type machines
• 2 drag machines
2

Classification of wind turbines
Horizontal axis type Vertical axis type
Multi-blade type Sail type Propeller type Savonius type Darrieus type
Wind turbines
1. Horizontal axis wind turbine (HAWT) –The horizontal axis
machines have to face the direction of the wind in order to generate
power. In addition to being parallel to the ground, the axis of blade
rotation is parallel to the wind flow.
2. Vertical axis wind turbine (VAWT) – In vertical-axis wind
turbines, the orientation of the spin axis is perpendicular to the
ground. A vertical axis wind turbine can catch wind in all
directions. So, a vertical axis machine need not be oriented with
respect to wind direction. This means that unlike a HAWT, no
yawing mechanism (adjusting the nacelle about the vertical axis
to bring the rotor facing the wind) is needed for a VAWT. Because
the shaft is vertical, the transmission and generator can be
mounted at ground level allowing easier servicing and a lighter
weight, lower cost tower.

Multi blade type Sail type
Horizontal axis wind turbines
(HAWT)
Commonly found horizontal axis wind turbines are
multi-blade type, sail type and propeller type.
Both the multi-blade and sail-type wind turbines run
at low speeds of 60 to 80 rpm. The propeller type
has two or three aerofoil blades and run at speeds of
300 to 400 rpm.

Horizontal Wind Turbine Operation

Vertical axis wind turbines (VAWT)
Savonius type Darrieus type
The basic vertical axis designs are the Darrieus type, which has
curved blades and efficiency of 35%, and the Savonius type having the
efficiency of 30%.
Savonius type uses drag forces to
create rotation of the shaft. Savonius
windmill consists of a hollow circular
cylinder sliced in half, the two halves
being fixed to a vertical axis with a
gap in between
Darrieus type uses lift forces to create
the rotation of the shaft. Darrieus type
requires much less surface area. It is
shaped like an egg beater and has two
or three blades shaped like aerofoils.

SAVONIUS WIND TURBINE
 Savonius turbines are one of the
simplest turbines.
Aerodynamically, they are drag-
type devices, consisting of two or
three scoops.
 Looking down on the rotor from
above, a two-scoop machine
would look like an "S" shape in
cross section. Because of the
curvature, the scoops experience
less drag when moving against
the wind than when moving with
the wind.

The differential drag causes the
Savonius turbine to spin. Because
they are drag- type devices,
Savonius turbines extract much
less of the wind's power than
other similarly-sized lift-type
turbines.
Much of the swept area of a
Savonius rotor may be near the
ground, if it has a small mount
without an extended post,
making the overall energy
extraction less effective due to
the lower wind speeds found at
lower heights.

DARRIEUS WIND TURBINE
 The Darrieus wind turbine is a
type of vertical axis wind
turbine (VAWT) used to
generate electricity from the
energy carried in the wind.
 The turbine consists of a
number of aerofoils usually—
but not always—vertically
mounted on a rotating shaft or
framework.

GIROMILL WIND TURBINE
 A subtype of Darrieus turbine with
straight, as opposed to curved, blades. The
cycloturbine variety has variable pitch to
reduce the torque pulsation and is self-
starting.
 The advantages of variable pitch are: high
starting torque; a wide, relatively flat
torque curve; a lower blade speed ratio; a
higher coefficient of performance; more
efficient operation in turbulent winds; and
a lower blade speed ratio which lowers
blade bending stresses. Straight, V, or
curved blades may be used.
 Giromill VAWTs are also self-starting.

Upwind and downwind machines
Upwind with tail vane
(passive yaw control)
Wind direction
sensor
Upwind with active yaw control Downwind with free yaw
(active yaw for large turbines)
Tail vane
Shadow area
Yaw control in upwind and downwind machines
In an upwind machine are those machines that have
rotor facing the wind. In these machines the wind
meets the rotor first and then leaves from the
direction in which the nacelle is located. In a
downwind machine, the rotor is located downwind of
(behind) the tower as shown in the figure. This means
the nacelle comes first in the path of the wind and

 As the blades are turned by the
wind, centrifugal forces pull air from
the hallow tower through the blade
tips
 At the same time pressure difference
between tips of the rotor and blade
pedestal also draws air up through
semi vacuum created in the tower
 As the air passes through the tower
and passes through the turbine and
gives energy to it
 No direct coupling between rotor and
power generating equipment

Vertical axis machines
 Axis of the wind turbine is
perpendicular to the motion of the
winds
1. Torque is produced by the pressure
difference between concave and
convex surfaces of the half facing the
wind and by recirculation effects on
the convex surfaces
 It found to be heavy weight per unit
power output
 It requires external input power for
starting

Nomenclature
of WECS
 An aerofoil is the cross-section
view of an aircraft wing.
 Angle of attack-Angle between
chord and relative airflow is known
as angle of attack
 Upstream-away from leading edge
in the air flow direction
 Downstream-away from trailing
edge in the air flow direction
 Swept area- area covered by the
rotating rotor
 Solidity- ratio of blade area to
swept area
 Cut in speed-at which wind
turbines start operates

 Cut out speed- above which
wind turbines stops due to
high winds
 Yaw control- it keeps the axis
of the turbine in wind
direction

Advantages
of Vertical
axis turbine
Omni directional
Components can be mounted at
lower level
Ease of service
Lighter weight towers
Theoretically less amount of
materials to catch the same amount
of wind

Disadvantages
of Vertical axis
turbine
Rotors near to ground where wind
is poor
Centrifugal forces stresses blades
Poor self-starting capabilities
Requires support at the top of
turbine rotor
Requires entire rotor to remove for
changing bearings
Overall poor performance and
reliability

Comparing HWT and VWT
Parameter HWT VWT
Tip speed ratio High and hence noisy. Low and hence less noisy.
Application
Large scale electricity
generation.
Small scale electricity generation.
Yawing
Yawing is required, as
HWTs are dependent of
wind direction.
Yawing is not required, as VWTs
are independent of wind
direction, but are affected by
wind speed.
Torque Low. More at lower wind speeds.
Maintenance Difficult.
Easier, as heavy components can
be located at the ground level.
Stability
More stable and hence
large sized turbines can
be constructed.
Less stable.

Performance of wind turbines
The tip-speed ratio is the ratio
of the rotational speed of the
blade to the wind speed. The
larger this ratio, the faster the
rotation of the wind turbine rotor
at a given wind speed. Lift-type
wind turbines have maximum
tip-speed ratios of around 10,
while drag-type ratios are
approximately 1.
The coefficient of performance is defined as the ratio of the power
delivered by the rotor, P, to the maximum power available, Pt
, in the
wind and is given by the following expression.
3
1
2
p
t
P P
C
P ρAV
 
It is seen that the values of tip speed for the multi-blade and Savonius
types are much lower than the values for the propeller and the Darrieus
types. It is also seen that the highest values of Cp are obtained with the
propeller type.
Multi-blade
Darrieus
(3 blade)
Propeller (2 blade)
Ideal rotor
Savonius
0 1 2 3 4 5 6 7
Tip speed ratio
0.1
0.2
0.3
0.4
0.5
0.6
Cp

Power
Wind turbine
Solar/Wind
controller
Solar panel
(PV array)
DC Load
AC Load
Inverter
Battery
Solar-Wind hybrid energy systems
Rectifier
 Solar-wind hybrid energy system is the combined power generating
system consisting of wind turbines and solar energy panel. It also
includes a battery which is used to store the energy generated from both
the sources. Using this system, power generation by wind turbines when
wind source is available and generation from PV module when light
radiation is available can be achieved. Both units can be generated
power when both sources are available.

Storage
of wind
energy
Wide variations in the speed and direction of winds
Does not possess the requirements of energy
resource – ready availability and long lasting supply
To overcome this wind energy can be stored in
some other form during periods of high winds
Various methods of storage are :
Large wind turbines can be grouped together
into windfarms which can provide bulk power to
electrical grid.
Can be stored in rechargeable batteries

Storage in the form of
potential energy by pumping
water to high level reservoirs
Storage in the form of
mechanical energy –
flywheels
Wind turbines can use excess
power to compress air which
is usually stored in large tanks

Wind
Energy
Farms
Wind energy is first among renewable
energy resources to become an
economically viable source of power
generation.
Energy content in wind in different regions
varies with latitude, land sea disposition,
altitude and season.
A site is considered suitable where the
wind speed is 18 kmph
Maximum wind energy can be tapped
from a site by installing several wind
turbines, connected to a network, and
the whole system is called a “wind farm”

Wind
Resource
Surveys
 Three types of wind survey
projects were undertaken during
1985 by MNRE with Indian
Institute of Tropical Meteorology
1st
category was of a wind monitoring
project to determine windy locations
using 20m mast and a
microprocessor based measuring
instrument, to generate data for wind
power development
2nd
category constituted wind
mapping projects, based on 5m mast,
to establish wind regime in a given
area on an extensive basis
3rd
category projects covered complex
terrain studies in hilly and
mountainous regions to find wind
flow in mountain passes and over
undulated terrains

Assessment of Wind
Availability from
Meteorological Data
 Meteorological data is used to
evaluate:
To identify the areas where highest wind
speeds are available
To measure Mean Annual Wind Speeds
(MAWS) and their variability form year to
year
To record Monthly Mean Wind Speeds to
indicate wind regimes for the area
Measurement of daily mean wind speeds
to understand their variation during
different seasons
 The MAWS is an approximate index
of wind potential at a site.
 Mean Monthly Wind Speed provides
a comprehensive pattern about
variability in wind energy during the
course of the year.

 Daily Mean Wind Speed is the average of winds during
24 hours of the day.
 A threshold speed of 15kmph is the lowest speed needed
to operate the wind electric generators.
 Wind power classification

Estimation
of Wind
Energy
Potential
 Wind speed extrapolation: Wind
Speed data are recorded by data
loggers at a height of 10m and 20m.
Wind speed increases with height as
per power law. Since WEGs are
installed at a greater height, it is
necessary to extrapolate the mean
wind speed measured at one level to
higher levels.
 Methods of calculation:
1. Based on wind data of a specific site
using frequency distribution
2. Based on type of wind energy generator
3. Based on Weibull factors of wind data
and WEG characteristics.

Equations used for Calculations
 Based on wind data:
1. Power law index is calculated from the
equation:
2. Wind power density is calculated from
the equation:
 Based on wind energy generator
(WEG):
1. Machine capacity factor(CF): Ratio of
average power output of a turbine
during a month or a year to the rated
power output

2. Capacity utilisation factor (CUF):
 Capacity factor on the basis of WEG
characteristics and using Weibull
factors.
 Energy likely to be generated is
calculated using the power curve of the
WEG and above equations, based on
frequency distribution.

Wind Resource Assessment in India
Centre for Wind Energy Technology (C-WET),
Chennai conducts wind energy assessment in India in
coordination with state nodal agencies.
India’s wind power potential has been assessed at
45000MW.
But potential for the grid-interactive wind power is
less, around 15000MW.

Technical Planning of a Wind Power Project
 Phase I : land availability, characteristic location and
landscape profile: the proposed capacity of the project
determines land requirement.
 Accessibility to wind project site: Approach roads to the
site are needed for the transportation of wind turbine and
electrical parts, civil construction materials, etc
 Soil characteristics: Soil investigation of the proposed site
has to be carried out for foundation and earthing designs
 State grid: Grid must be available to pump generated
electricity to the electricity board grid.
 Ambient conditions at the proposed site: Temperature,
relative humidity, corrosion factor, sand and salt
concentration in air, etc would affect the WEG
performance.

 Phase II : Micro- siting of the wind electric
generators (WEGs) : siting is necessary to
optimise the power output
Visual inspection of the land helps in
understanding the topography of the terrain.
WEGs are located at highest level of the land
in the region of least turbulence.
Array efficiency should normally be above
95%, which depends on specific
configuration and orientation. Minimum loss
due to shadow effect should be ensured
 A schematic layout of a 10MW wind power plant
having 50 nos. of 225kW WEGs is given:

Annual Energy Output
 The power curve of a 225 kW WEG as a function of wind
speed distribution pattern ( follows the Weibull probability
density function):

 Annual generation at wind farms with different wind speeds:
 The capacity of a wind generator is optimised to suit the site by
having theoretical energy projections.
 A right choice of WEG reduces the generation cost.

SITE SELECTION OF WIND POWER PLANT
 High, exposed sites.
 Not suitable sites in highly populated residential
areas.
 Avoid roof mounted turbines.
 Power transmission loss
 Distance between the turbine and the nearest
obstacle
 Connection with national power grid

Advantages of Wind Power
 Environmental
No air pollution
No greenhouse gasses
No water needed for operations
 Resource Diversity & Conservation
Domestic energy source
Inexhaustible supply
Small, dispersed design reduces supply risk
 Cost Stability

Economic Development
Expanding Wind Power development brings jobs to
rural communities
Increased tax revenue
Purchase of goods & services
 Wind turbines can be used for both distributed generation
or grid interactive power generation using on-shore or off
shore technologies.
 Ranges of power producing turbines are available. Micro-
turbines are capable of producing 300 W to 1MW and
large wind turbines have typical size of 35 kW–3 MW.
 It can be made available easily in many off-shore, on-
shore and remote areas; thus, helpful in supplying
electric power to remote and rural areas.
 It is a non-polluting and environment friendly source of
energy.

 It is an important renewable and sustainable source
of energy, available free of cost.
 The scope of wind resource, globally, is enormous
and is less dependent on latitude than other solar
based renewable energy technologies.
 Power generation is cheaper as there is no shortage
of input cost and recurring expenses are almost nil.

Disadvantages of wind power
 It has low energy density.
 Electricity production depends on- wind speed,
location, season and air temperature. Hence
various monitoring systems are needed and may
cost expensive.
 High percentage of the hardware cost (for large
wind turbine) is spent on the tower designed to
support the turbine
 It is variable, unsteady, irregular, intermittent,
erratic and sometimes dangerous.
 Wind turbine design, manufacture and installation
have proved to be complex due to widely varying
atmospheric conditions in which they have to
operate.

 Wind farms can be located only in vast open
areas in locations of favorable wind. Generally,
such locations are away from load centers.
 The appearance of wind turbines on the
landscape and their continual whirling and
whistling can be irritating.

Environmental impacts of
wind power
Conventional technologies have regional and
global impacts due to their emissions , however
the impacts of wind energy systems are local
 Erosion
 Bird and bat kills
 Visual impacts
 Noise

THE POWER IN THE WIND
Wind mills converts the kinetic energy of the wind to mechanical energy. The total
power of the wind stream is equal to the time rate of kinetic energy.
Kinetic energy = 1/2 mv2
...(9.1)
The amount of air passing in unit time, through an area A. with velocity V = AV.
Mass m = PAV
...(9.2)
where 'p' is the density of air particles (kg/m²)
Kinetic energy per unit volume = From equation (9.1) and (9.2)
Kinetic energy = PAV³ Watts

All this power cannot be extracted because for this wind velocity would
have to
be reduced to zero which means that the wind mill would accumulate
static air
around it which would prevent the wind mill operation.
It is clear that the power output of a windmill varies as cube of the wind
velocity. directly proportional to wind density and directly proportional
to area of stream A So, the wind mill produces maximum power at high
wind velocity. Wind velocities below 5 m/s and above 25 m/s are not
suitable for wind turbine. At lower speed, very large turbine-rotor is
required and at higher wind speed, the stresses on turbine blade and
shaft are very high.
From equation (9.5) wind power is proportional to the intercept area.
Thus windmill with a large swept area has higher power. Normally, area is
circular with diameter D, thus

A = (Π * D2
)/4
then
Pt = ½ PV3Π D2
/4 = 1/8ρ Π D2
V3
From equation (9.6) it is clear that power is proportional to the square of
the diameter of swept area. The combined effects of wind speed and rotor
diameter variation shown in Fig. 9.3.

Most commonly used wind turbine is horizontal axis, propeller type.
Consider this wind turbine.
Let
a = Inlet plane
b = Exit plane
P = Incoming wind pressure
₁
V = Incoming wind velocity P = Wind pressure at exit from blades C
V= Wind velocity at exit from blades (m/s)
v = Specific volume = 1/ρ
Ve is less than Vi because Kinetic enregy extracted by the
turbine.Assuming no energy loss and no change in air density. The
pressure and velocity changes are plotted as shown in fig 9.4

Applying total energy equation
Similarly, for exit area
The wind velocity decreases from a to b, because kinetic energy
is converted to mechanical work. Therefore,
Vi > Va
Vb > Ve
Pa > Pi
Pb <Pe

From equations (9.8) and (9.9)
Assume that at exit end away from the turbine at e, can be
assumed to ambient i.e..
P = P V = V = V
₁ ₁ ₁
then from equation (9.10).
If 'A' is the projected area of wind mill perpendicular to the wind
stream, the
axial force F, is given by

Axial force also equal to the change of
momentum
Equating the equations (9.12) and (9.13)

Now consider the total thermodynamic system
bounded by i and e. The general energy equation
now reduces to the steady flow work w' and
kinetic energy terms.

This is the condition for maximum power V opt is the optimum
exit velocity. Put the value of Ve opt in equation

The maximum efficiency also called power coefficient
is given by
where Ptoal is the total power in the wind stream and put this
value from equation (9.5). From equation (9.21) it is clear
that maximum efficiency of a propeller type turbine is 59%.
The factor 59 is known as Betz limit. Actual efficiency is less
than the maximum efficiency. Actual efficiency is generally
50% to 70% of maximum efficiency.
n = Actual efficiency
= 0.6 × 0.59
= 0.354.

Therefore, the actual efficiency is approximately 35%. Fig.
9.6 shows the power coefficient for different types of rotors
verses the tip speed ratio (which is the ratio of tip
(peripheral) velocity of rotor to the wind speed).

FORCES ON BLADES AND TORQUE OF WINDMILL
The torque or circumferential force causing the rotation of
the wind turbine shafts depends on the turbine rated power
output and rotor angular velocity. Thus
T = Torque (N)
ω= Angular velocity of turbine wheel (m/s)
N = Speed of the wheel
D = Diameter of turbine wheel (m)

Efficiency = P/Pt
P = η * Pt
Put the value of P, from equation (9.6)
From the equation 9.22
The value of the torque will be maximum at maximum
efficiency. Put the value of maximum efficiency from (9.21)

Axial force is given by equation
(9.12)
Put the value of A in above equation

Maximum axial force will occur at maximum efficiency.
The condition for maximum efficiency is
Ve = 1/3 Vi
Put this value in equation (9.27)
From equation (9.28) it is clear that the axial force is directly
proportional to the square of the diameter. Therefore, there is an
upper limit of diameter of the wheel.
The ratio of peak rated wind velocity to average wind velocity is an
important parameter which governs the overall performance of the
windmill system. For a generator of a given rated power, a low peak
to average velocity ratio requires a large rotor windmill while a high
peak to average velocity ratio requires a small rotor windmill. A large
rotor mill is more expensive but gives greater average output and,
therefore, a balance between the two is necessary.

1. A propeller type, horizontal shaft wind turbine having
following wind characteristics.
Speed of wind 10 m/s at 1 atm and 15 °C.
The turbine has diameter of 120 m and its operating
speed is 40 rpm at maximum efficiency. Calculate
(i) the total power density in the wind stream (ii) the
maximum obtainable power density assuming eta = 40 %
(iii) total power produced (in kW) and
(iv) the torque and axial thrust.

Solution.
where,
Air density p= P RT
P= Pressure of air, Pat
T = Temperature of air, K
R= Gas constant
R = 0.287 KJ/kg K
1atm = 1.01325 * 10 ^ 5 * P * a
T= 15+ 273 288 K

Advanced Energy Engineering :- Wind Energy.pptx

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