BHAW NATH JHA
Wind energy is a form of solar energy. Sun alleviates
the atmospheric temperature and pressure difference
is produced due to uneven solar heating of the earth’s
While the sun heats air , water and land on one side of
the earth , the other side is cooled by thermal
Much more solar energy is absorbed near the equator
than at the poles .
Warmer , lighter air rises at the equator and flows
towards the poles , while cooler heavier air returns
from the poles to replace it.
The volume of air that passes through an imaginary
Both energy and power are proportional to
the cube of the wind speed.
Air density ‘ρ’ can be calculated by
z = the location's elevation above sea level in meter
Value of “η” commonly ranges from 0.10 to 0.50.
Wind speed variation with height
Wind speed uz
Approximate scale of
z0 - the roughness length
d - zero plane displacement
WIND MACHINE CHARACTERISTICS
Three factors determine the output of a wind
machine. They are:
1. the wind speed
2. the cross-section of the wind swept
3. the overall conversion efficiency of
the rotor, transmission system and
generator of pump
K.E. of wind = ½ mV2
But, m = AV
K.E. = ½ AV3
The available wind energy is directly proportional to the
cube of the wind speed.
Since A = /4 D2, the energy is proportional to the square
of the diameter of the rotor.
Hence higher the wind speed and diameter higher is the
The relation can also be seen from the graph in next slide.
Dependence of wind rotor power on wind speed and rotor diamete
Vertical axis machines are of simple design as
compared to the horizontal axis type.
Horizontal axis wind machines are further
classified as single bladed,multibladed and by-
cycle multiblades types.
Vertical axis wind machines are sub divided into
Savonious (low velocity wind) and Darrieus(high
velocity wind) based on the working speed and the
velocity required by the machine for operation.
One of the simplest of the
modern types of wind energy
conversion system. (work likes a
Invented by S.J Savonius in the
year 1920 and has become popular
since it requires relatively low
velocity winds for operation.
It consists of two half cylinders
facing opposite directions in such
a way that as to have almost an S-
shaped cross section and is
mounted on a vertical axis
perpendicular to the wind
direction with a gap at the axis
between the two drums.
Invented originally and
patented in 1925 by G.J.M.
Darrieus,a French engineer.
It is a vertical axis machine with
a modern rotating propeller
type windmill, by use of an
efficient airfoil, effectively
intercepts large area of wind
with a small blade area.
It has two or three,curved(egg
beater) blades with airfoil cross
section and constant chord
length. Both ends of blades are
attached to a vertical shaft.
Multi blade wind rotor
As name suggests, it contains
a number of wind blades.
It has higher torque
coefficient and lower power
Due to high starting torque
they are more suitable for
It is rarely used in electricity
generation due to low speed.
If to be used gear reduction
becomes a must to increase
High wind speeds
Less aerodynamic losses
Less starting difficulties
Lower cut-in wind speeds
Lower cost-to-power ratio
Ability to furl rotor out of
Active yaw drive
Access to power train
No yaw mechanism
Good access to
Lower wind speeds
Difficult to control overspeed
WIND MACHINE PERFORMANCE
No machine can extract 100 % of wind energy as the
wind have to be brought to hault.
Most possible for a rotor is to decelerate wind by one
third of its free velocity.
Only 60% of the available energy can be converted to
mechanical energy by a 100% efficient aerogenerator.
More losses occur in gears, transmission systems and
Hence, the turbine efficiency reduces to only about
The overall conversion efficiency, of an aero
generator of the general type is the ratio of useful
output power to that of wind power input.
Useful output power
Wind input power
= A. O. G. C. Gen
A = efficiency of the aeroturbine
G = efficiency of the gearing
C = efficiency of the mechanical
Gen = efficiency of the generator
The Savonious rotor and the American multiblade
(farm windmill) are intended for low speed
The modern two blade type and Darrieus rotor
type are intended for high speed operation more
compatible with generating electric energy.
It may be concluded that a two blade propeller has
potentially the best performance of the system
This explains why virtually all large scale systems
built in the past employed two or three bladed
The multiblade types with high starting torque on the
other hand are more suitable for pumping water.
In practice, it is impossible to build wind driven
generators capable of operating at the same efficiency
at all wind speeds.
There is wind speed below which no power can be
generated because of friction losses.
As the wind speed increases, the extracted power is
held constant by turning the blades on their axis to
reduce effective area.
This decreases the power extracted and stabilizes the
The extraction of power, and hence energy from the
wind depends on creating certain forces and applying
them to rotate (or to translate) a mechanism.
two primary mechanisms for producing forces from
the wind; lift and drag.
By definition lift forces act perpendicular to the air
flow, while drag forces act in the direction of flow.
Lift forces are produced by changing the velocity of the air
stream flowing over either side of the lifting surface
speeding up the air flow causes the pressure to drop, while
slowing the air stream down leads to increase in pressure.
change in velocity generates a pressure difference across
the lifting surface.
This pressure difference produces a force that begins to act
on the high pressure side and moves towards the low
pressure side of the lifting surface which is called an airfoil
The cross-section of the blade has a streamlined
The blade profile is a hollow profile usually formed by
two shell structures glued together
To make the blade sufficiently strong and stiff, so-
called webs are glued onto the shells in the interior of
this web will act like a beam
It is important that the blade sections near the hub are
able to resist forces and stresses from the rest of the
So the blade profile near the root is both thick and
Further, along the blade, the blade profile becomes
thinner so as to obtain acceptable aerodynamic
At the root, the blade profile is usually narrower and
tubular to fit the hub.
The determination of the number of blades involves design
•system reliability, and
Aerodynamic efficiency increases with number of blades but
with diminishing return.
Increasing the number of blades from one to two yields a six
percent increase in aerodynamic efficiency, whereas increasing
the blade count from two to three yields only an additional
three percent in efficiency.
Further increasing the blade count yields minimal
improvements in aerodynamic efficiency and sacrifices too
much in blade stiffness as the blades become thinner.
Previously: wood and canvas were used
But use of these materials limits the shape of blade to a
flat plate which has higher drag and thus reduces the
So nowadays composite materials such as fibre glass,
aluminium, carbon fibres, FRP etc are used
The hub is the fixture for attaching the blades to the
Consists of nodular cast iron.
transmits the rotational energy from the rotor hub to
the gearbox or directly to the generator.
Supports the main shaft and transmits the reactions
from the rotor loads to the machine frame.
the spherical roller bearing type is often used
act as a speed increaser and to transmit energy between
the rotor and the generator.
Spur gear is most oftenly used.
Mechanical brakes are usually used as a backup system for
the aerodynamic braking system of the wind turbine.
transforms mechanical energy into electric power.
Both synchronous and asynchronous generator can be used
It is necessary that the turbine should be installed with
certain minimum ground clearance and without any
obstacle within the area with respect to the horizontal plane
of the turbine.
In general, it is considered that a wind turbine should be at
least 10m above any wind obstacle.
Above all, the tower must be strong enough to hold the
system during extreme environmental condition.
1. Lattice tower
2. Guyed tower and
3. Solid tower
Battery Charge Controller
The main purpose of the charge controller is to check
the battery and generator voltage and act accordingly.
After the battery voltage reaches the upper threshold
voltage (which is predefined) it disconnects the wind
turbine generator and connects to the ballast load.
The battery is normally connected to the primary load.
When the battery condition falls below the lower
threshold voltage the primary load is now
disconnected and the battery is connected to the wind
The wind turbine is always designed considering cut in and cut
out wind speeds.
The turbine normally start turning at the wind speed of 3m/s.
The wind turbine must furl or turn out of the wind direction
above the maximum wind speed (normally 16 m/sfor small
wind turbine) .
So, in order to protect the turbine from breakage at higher
wind speeds, a tail with the proper yaw angle (angle between
wind direction and turbine axis) and furling is designed.
Normally, a tail is electrically driven for large wind turbine and
free yaw for small turbines which helps in keeping the turbine
facing towards the wind direction.
Wind Energy Resources
Assessment in Nepal
Well established wind anemometers at 2 m and 3
m heights are available at 264 meteorological
stations in Nepal.
Monitoring of the effect, due to wind, on the
paddy growing in agricultural process.
These heights are not effective for wind power
Establishment of Wind Anemometers
Establishment of 5 anemometer stations by Water and
Energy Commission Secretariat (WECS) in 2001;
Okhaldhunga (Site elevation: 1720m)
Nagarkot (Bhaktapur) (Site elevation : 2163m)
Butwal (Rupandehi), Palpa (Site elevation : 230m)
Kagbeni (Mustang) (Site elevation : 2820m)
Thini (Mustang) (Site elevation : 2645m)
Currently AEPC is monitoring these stations and
collecting the data
Data already collected: Butwal (2000 to 2003); Nagarkot
(2001 to 2004); Thini (2001 to 2004); Kagbeni (2001 to
2004); Palpa (2003 to 2004); Okhaldhunga (2002 to
contains two anemometers: at 10m and at 20m above
automatically records wind data every hour by NRG
9300 data logger.
data record of up to 6 months can be stored in the
NRG’s PCMCIA FLASH memory card (capacity 256
KB to 2 MB)
uploaded data can be analyzed by NRG 9300
Microsite system in computer.
Three blade HAWT is a best option in order to produce
electricity and multi blade wind turbine can be used
where there is a need of high starting torque like
HAWT seems to be better wind turbine than VAWT.
Slight change in a wind velocity changes the extracted
energy to a large extent.
Wind energy can be a contributing source of energy in
the today’s scenario of load shedding in Nepal.
There is a lot of potential of wind energy in Nepal.
Guidelines for Design of Wind Turbines, Det Norske
Development of Technical Specifications and Standards
for Small Scale Wind Energy, Kathmandu Alternative
Power and Energy Group, Dhulikhel, Nepal