WIND ENERGY ESSAY QUESTIONS

1. Explain the different types of winds?

Planetary Winds:

The winds blowing throughout the year from one latitude to another in response to

latitudinal differences in air pressure are called “planetary or prevailing winds”. They
involve large areas of the globe.

:Local Winds

Local winds are the ordinary winds. They are influenced by various landforms such as
vegetation, hill, plains, water bodies, mountains and so on. The blow variedly and the changes
are because of different temperatures and pressure regions during the night and day.

Local winds are the kind of winds that are focused as part of daily weather by the
meteorological department on broadcast media such as radio and TV. The speeds of local winds
range from mild to strong but just for a few hours, and they only blow over short
distances. Common examples of local winds are the land and sea breezes, and valley and
mountain breezes.

2. Derive the expression for maximum power that can be extracted

from the wind?
3. Explain the site selection for WECS?
 The power available in the wind increases rapidly with the speed, hence
wind energy conversion machines should be located preferable in areas
where the winds are strong and persistent. Although daily winds at a given
site may be highly variable, the monthly and especially annual average are
remarkably constant from year to year.
The major controbution to the wind power available at a given site is
actually made by winds with speeds above the average. Nevertheless, the
most suitable sites for wind turbines would be found in areas where the
annual average wind speeds are known to be moderately high or high.
The site choice for a single or a spatial array of WECS is an important
matter when wind electrics is looked at from the systemspoint of view of
aeroturbine generators feeding power into a convertional electric grid.
If the WECS sites are wrongly or poorly chosen the net wind electrics
generated energy per year may be sub optimal with resulting high capital
cost for the WECS apparatus, high costs for wind generated electric energy,
and low Returns on Investment. Even if the WECS is to be a small
generator not tied to the electric grid, the sitting must be carefully chosen if
inordinately long break even times are to be avoided. Technical, Economic,
Evironmental, Social and Other actors are examined before a decision is
made to erect a generating plant on a specific site.

1. High annual average wind speed:

The speed generated by the wind mill depends on cubic values of velocity of
wind, the small increases in velocity markedly affect the power in the wind. For
example, Doubling the velocity, increases power by a factor of 8. It is obviously
desirable to select a site for WECS with high wind velocity. Thus a high average
wind velocity is the principle fundamental parameter of concern in initially
appraising WESCS site. For more detailed estimate value, one would like to have
the average of the velocity cubed.
2. Availability of anemometry data:
It is another improvement sitting factor. The aenometry data should be available
over some time period at the precise spot where any proposed WECS is to be built
and that this should be accomplished before a sitting decision is made.
3. Availability of wind V(t) Curve at the proposed site:
This important curve determines the maximum energy in the wind and hence is the
principal initially controlling factor in predicting the electrical output and hence
revenue return o the WECS machines.
It is desirable to have average wind speed „V‟ such that V>=12-16 km/hr (3.5 – 4.5
m/sec) which is about the lower limit at which present large scale WECS
generators „cut in‟ i.e., start turning. The V(t) Curve also determines the reliability
of the delivered WECS generator power, for if the V (t) curve goes to zero there be
no generated power during that time.
If there are long periods of calm the WECS reliability will be lower than if the
calm periods are short. In making such realiability estimates it is desirable to have
measured V(t) Curve over about a 5 year period for the highest confidence level in
the reliability estimate.
4. Wind structure at the proposed site:
The ideal case for the WECS would be a site such that the V (t) Curve was flat, i.e.,
a smooth steady wind that blows all the time; but a typical site is always less than
ideal. Wind specially near the ground is turbulent and gusty, and changes rapidly in
direction and in velocity. This depature from homogeneous flow is collectively
referred to as “the structure of the wind”.
5. Altitude of the proposed site:
It affects the air density and thus the power in the wind and hence the useful
WECS electric power output. Also, as is well known, the wind tend to have higher
velocities at higher altitudes. One must be carefully to distinguish altitude from
height above ground. They are not the same except for a sea level WECS site.
6. Terrain and its aerodynamic:
One should know about terrain of the site to be chosen. If the WECS is to be
placed near the top but not on the top of a not too blunt hill facing the prevailing
wind, then it may be possible to obtain a „speed-up‟ of the wind velocity over what
it would otherwise be. Also the wind here may not flow horizontal making it
necessary to tip the axis of the rotor so that the aeroturbine is always perpendicular
to the actual wind flow.
It may be possible to make use of hills or mountains which channel the prevailing
wind into a pass region, thereby obtaining higher wind power.
7. Local Ecology
If the surface is base rock it may mean lower hub height hence lower structure
cost. If trees or grass or vegetation are present, all of which tend to destructure the
wind, the higher hub heights will be needed resulting in larges system costs that the
bare ground case.
8. Distance to road or railways:
This is another factor the system engineer must consider for heavy machinery,
structure, materials, blades and other apparatus will have to be moved into any
choosen WECS site.
9. Nearness of site to local centre/users:
This obvious criterion minimizes transmission line length and hence losses and
cost. After applying all the previous string criteria, hopefully as one narrows the
proposed WECS sites to one or two they would be relatively near to the user of the
generated electric energy.
10. Nature of ground:
Ground condition should be such that the foundation for a WECS are secured.
Ground surface should be stable. Erosion problem should not be there, as it could
possibly later wash out the foundation of a WECS, destroying the whole system.
11. Favourable land cost:
Land cost should be favourable as this along with other siting costs, enters into the
total WECS system cost.
12. Other conditions such as icing problem, salt spray or blowing dust should not
present at the site, as they may affect aeroturbine blades or environmental is
generally adverse to machinery and electrical apparatus.

4. Explain the basic components of WECS with neat sketch?

Wind turbines harness the power of the wind and use it to generate
electricity. Simply stated, wind turbines work the opposite of a fan.
Instead of using electricity to make wind—like a fan—wind turbines
use wind to make electricity. The wind turns the blades, which in
turn spins a generator to create electricity. This illustration provides
a detailed view of the inside of a wind turbine, its components, and
their functionality.
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 atop 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 (shown here), 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—like
the one shown here—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.

5. Write the classification of WECS?

It is a mechanical machine that converts kinetic energy of the fast moving winds into electrical energy.
On the basis of axis of rotation of the blades, it is divided into two parts.
 1. Horizontal axis wind turbine (HAWT)

2. Vertical axis wind turbine (VAWT)

1. Horizontal Axis Wind Turbine (HAWT)

It is a turbine in which the axis of rotation of rotor is parallel to the ground and also parallel to wind
direction.

They are further divided into two types

(i) Upwind turbine

(ii) Downwind turbine

(i) Upwind Turbine

The turbine in which the rotor faces the wind first are called upwind turbine.

 Today most of the HAWT is manufactured with this design.

 This turbine must be inflexible and placed at some distance from the tower.
 The basic advantage of this turbine is that, it is capable of avoiding wind shade behind the tower.
 It requires yaw mechanism, so that its rotor always faces the wind.

(ii) Downwind Turbine

The turbine in which the rotor is present at the downside of the tower is called downwind turbine. In these
types of wind turbines, the wind first faces the tower and after that it faces the rotor blades.

 Yaw mechanism is absent in this turbine. The rotors and nacelles are designed in such a way that the
nacelle allows the wind to flow in a controlled manner.
 It receives some fluctuation in wind power because here the rotor passes through the wind shade of the
tower. In other words the rotor is present after nacelle of the tower and this create fluctuation in the wind
power.

Advantages and Disadvantages of HAWTs

The various advantages and disadvantages of the horizontal axis types of wind turbines are:

Advantages
 It has self-starting ability. It does not require any external power source to start.
 It has high efficiency as compared with the HAWT.
 Capable of working in high wind speed condition.
 In the case of slow wind condition, its angle of attack can be varied to get maximum possible efficiency.
 Since all blades of this turbine work simultaneously, so it is capable of extracting maximum energy form
the wind.

Disadvantages
 Its initial installation cost is high.
 It requires large ground area for its installation.
 Because of its giant size of blades and towers, it becomes difficult to transport it to the sites.
 High maintenance cost.
 Creates noise problem.
 It cannot be installed near human population.
 It is not good for the bird‟s population. They are killed by its blades rotation.

2. Vertical Axis Wind Turbine (VAWT)

It is a turbine in which the axis of rotation of the rotor is perpendicular to the ground and also
perpendicular to the wind direction.

 It can operates in low wind situation.

 It is easier to build and transport.
 These types of Wind turbines are mounted close to the ground and are capable of handling turbulence in
far better way as compared with the HAWT.
 Because of its less efficiency, it is used only for the private purpose.
VAWTs are further classified as

(i) Darrieus turbine

(ii). Giromill turbine

(iii) Savonius turbine

(i) Darrieus Turbine

Darrieus turbine is type of HAWT. It was first discovered and patented in 1931 by French aeronautical
engineer, Georges Jean Marie Darrieus. It is also known as egg beater turbine because of its egg beater
shaped rotor blades.

 It consists of vertically oriented blades which are mounted on a vertical rotor. It is not a self-starting
turbine and hence a small powered motor is required to start its rotation.
 First the Darrieus turbine is rotated by using a small powered motor. Once it attains sufficient speed, the
wind flowing across its blades generates lift forces and this lift forces provides the necessary torque for
the rotation. As the rotor rotates, it also rotates the generator and electricity is produced.

(ii) Giromill Turbine:

It is similar to the Darrieus turbine but the difference is that, it has H-shaped rotor. It works on the same
principle of Darrieus turbine.

 This turbine has H- shaped rotor. Here Darrieus design which has egg beater shaped rotor blades are
replaced by straight vertical blades attached with central tower with horizontal supports. It may consists
of 2-3 rotor blades.
 Giromill turbine is cheap and easy to build as compared with Darrieus turbine. It is less efficient turbine
and requires strong wind to start. Same as darrieus types of wind turbines, it is also not self- starting and
requires small powered motor to start. It is capable of working in turbulent wind conditions.

(iii) Savonius Turbine

Savonius turbine is HAWT.It was first discovered in 1922 by a Finnish Engineer Sigurd Johannes
Savonius. It is one of the simplest turbine among all known turbines.

 It is a drag-type device and consists of two or three scoops. If we look it from above than it looks „S‟
shape in cross section. The scoops of these turbines have curvature shape and because of that, it
experiences less drag when it moves against the wind instead of moving with the wind.
 Since it is a drag-type machine, it is capable of extracting very less amount of wind power as compared
with other similar sized lift-type turbines.

Advantages and Disadvantages of Vertical

Axis Wind Turbine
Advantages
 It is simple in design and easy to construct and transport.
 It can be easily installed to desired location.
 It requires less ground area for its installation.
 Initial installation cost is very less as compared with the HAWT.
 It can work in turbulent wind condition.
 It is omni-directional and hence do not need to track winds.
 They are smaller in size and hence can be used for domestic or private purpose easily.
 They have low maintenance cost as compared with the HAWT.

Disadvantages
 It is less efficient. The efficiency of this turbine is about 30-35%.
 They are not self-starting. A small powered motor is needed to start it.
 Guy wires may required to support this turbine.
6. Explain the components of wind electric system with neat
sketches?

7. a. Explain the schemes of electric generation in WECS?

b.Explain the environmental aspects of WECS?

Wind power plant as a renewable energy source has slight influence on environment,
nevertheless people reluctantly agree to construct power plants in their
neighbourhood.

One of the most common environmental aspect regard noise. The noise has two
sources: mechanical (inherent in the gearing system – excluding double feed system)
and the second one related with aerodynamics of the rotor blade. The first one is
possible to eliminate but the second one not. The aerodynamic noise arise when the
rotating blade passing the tower. That effect calls the tower thumb. The most influence
on the level of loud has wind speed. When the wind blow fast the background noise is
enough loud to drown out the tower thumb effect but when the wind is blowing lightly
the tower thumb effect is audible in long distance. [3]

Other environmental problem is electromagnetic interference. Large structures like

wind rotor and tower can cause objectionable electromagnetic interference in the
performing of a nearby transmitters or receivers. Moreover the rotating blades can also
reflect signals what make experience interference at the blade passage frequency. The
highest influence on that effect strength has location. This problem is important for
onshore technology but in some of case large offshore power plants the problem could
be higher. The offshore power plant can interference with radar and flight paths to
airfields.
Next one environmental problem is effect on birds. The wind turbine blade is a lethal
weapon against any avian population. The birds may be killed or at least injured if the
collide with blade. The most often situation the suction draught created by wind
flowing to a turbine caught the birds and poke them into air stream headed for the
blades. Extremely dangerous are turbines with lattice towers, where the bird has
possibility to nest. Therefore often wind farms have to be sited away avian flight paths.

The last one environmental problem is visual impact mainly for onshore wind farms.
The problem is related with property owners around the wind farm, who do not allow
for wind power plant installation in their neighbourhood. However that problem also
regard offshore technology for farms installed near resorts.