Post Page Advertisement [Top]

What is Oral Communication? Advantages and Disadvantages of Oral Communication.

Communication as a way of exchanging or transfer of ideas, emotions or thoughts, has various forms. These forms differ depending upon the availability of feedback, use of verbal or non-verbal signs etc. The process of communication begins with the sender. The sender is the first agent to conceive an idea or select a message. He chooses suitable matter as the message keeping in mind the knowledge and experience level of the receiver. The success of communication depends a lot on the sender's knowledge, experiences and abilities. One of the most popular mode of communication is Oral Communication.

Oral Communication implies communication through mouth. It includes people conversing with one another, be it direct speech or telecommunication speech. Speeches, presentation, discussions area unit all types of speech. speech is usually counseled once the communication matter is of temporary kind or wherever an on the spot interaction is needed. Face to face communication (meeting, lectures, conferences, interviews, etc.) is important thus on build a rapport and trust.


 ORAL COMMUNICATION

Oral communication is that mode of communication in which messages are transmitted in the spoken form. The term 'oral' means anything pertaining to the mouth. In oral communication, the sender and the receiver exchange their ideas through speech either in face to face talk or through some mechanical or electronic device. The face to face mode of oral communication is rated very high as we can see all signs of body language along with immediate exchange of words. The message can be orally transmitted also through computer voice mail and telephoning which is considered not so effective as face to face communication but they do provide vocal hints and an opportunity of immediate feedback.

Oral Communication

Media: Various media of oral communication are face to face conversation, meetings and conferences, lectures, interviews, telephonic talks, seminars, radio broadcasts, television telecasts etc.


ADVANTAGES OF ORAL COMMUNICATION

[1]. Faster Mode of Communication

Oral communication is faster as compared to written communication. It saves a lot of time. The sender does not have to spend much time to prepare the message. Oral communication is advantageous especially when the sender wants a feedback immediately. The sender can pass on insturctions to the receiver either personally or over the telephone to get his work done in time. Speech is more effective in getting the job done. That is why project leaders or managers prefer to transmit the message orally. He can use voice modulation and his body language to convey his message to his subordinates or juniors to make his message ore effective.


[2]. Immediate Feedback

Oral communication is more effective because of direct contact between the communication (sender) and the communicatee (receiver). As the sender and the receiver are present in person physically or stay connected over the telephone, there is an immediate response from both the sides.


[3]. More Flexible

Oral communication provides a greater flexibility since no record is kept on at daily basis. The sender can immediately correct or modify a faulty message event after sending it to the receiver. He can clarify and check the response given by the receiver on the spot. He can shape his message according to knowledge level and the background of the receiver.


[4]. Less Expensive

Oral communication is relatively less costly as compared to written communication. Here the sender does not have to spend much on preparing his message using different writing materials in written and printed forms. Oral communication does not use a lot of manpower as in the case of post office or courier agencies.


DISADVANTAGES OF ORAL COMMUNICATION

[1] Not Suitable for Long and Bulky Messages

Oral communication is not convenient for sending lengthy messages as the listner (receiver) will not be able to understand the whole information. There is every possibility that the receiver might miss out important information. It becomes very difficult to hold the audiences' (receiver) attention for hours together. If the lengthy message is sent to the receiver in written form he can go through it at his own convenience, in parts and would understand it completely.


[2] No Legal Importance

In case of written communication, the written message can be produced as a piece of evidence in the court of law. But in oral communication the spoken words do not have any importance and hence has no legal validity. Hence, statements given orally cannot be accepted as a legal document.


[3] Cannot Serve as a Permanent Record

Oral communication cannot be kept as a permanent record for any future reference. An oral message cannot be treated as an authentic record unless it is audio or video recorded. The oral messages stay in our memory for a very short duration of time.. Certain important steps taken orally might be forgotten and one has to face similar problems again and again.


[4] Cannot Fix Responsibility

It is easier to assign responsibility in case of written communication but it is difficult to hold somebody responsible in oral communication. Orders given by the sender orally may not be executed by the receiver. The receiver may refuse to give a feedback if the sender gives a message orally.

Read more ...

What is Written Communication? Advantages and Disadvantages of Written Communication?

WHAT IS WRITTEN COMMUNICATION?

The communication in which information or message is exchanged in the written or printed form is known as written communication. It is one of the basic ways of communicating. It is the most formal mode of communication.

Written Communication

Media: It is transmitted by words in the forms of letters, memo's, circulars, bulletins, instruction cards, reports, mannuals, handbooks, magazines etc. In the present age of computers, the written mode of communication also includes fax and e-mails. 


ADVANTAGES OF WRITTEN COMMUNICATION

[1]. Faultless and Precise (Sender)

Written communication is usually composed with great care. Since the sender has ample time to prepare the message, he tries to make the message free from mistakes or faults. He gives a sincere thought to his ideas and tries to present them in an ordered manner. Once he writes the message, he revises and edits it keeping in view the knowledge level of the receiver. This makes written communication accurate and precise.


[2]. Promotes Better Understanding (Receiver)

In the written form of communication, the receiver can understand the message from the sender better. He reads the message according to his own convenience. He has much time to read and reread the message till he understands it fully. He does the see any part of the message. As a result, he is able to give a clear feedback to the sender and completes the communication process successfully.


[3]. Acts as Permanent Record

Written communication can be kept as a permanent record of any individual or organisation. It can be very useful for future reference. Old orders and decisions can serve as references while making new decisions. The written records of the previous year can be of great help in framing new policies and fixing current targets. The preserved record of any organisation help overcoming crisis at a later stage.


[4]. Acts as a Legal Document

In Written communication can be treated as a legal document in the court of law. there is a dispute, the written document will be valid to be produced in the court for awarding justice to the victim. In almost all organisations, both government and private, the administrators prefer to do work in the written form.


[5]. Fixing of Responsibility

It is only through the written communication that it becomes easier to fix responsibility. If a mistake is committed through oral communication, it is very difficult to find out the person who has committed the mistake. For example: A junior officer may not carry out the order given orally by a senior officer. But if he is given an order in writing, he is bound to do it. For this reason written communication helps to run an organization in a smooth way. Managers may sometimes try to shift responsibility for a mistake, to the lower staff. This is precisely the reason why the lower staff members demand a written order in carrying out any work.


[6]. Wide Coverage

Written communication has a wide coverage facility. It has been made possible due to the large postal network. A written message can reach any corner of the world by post or courier services. The modern system of e-mails and faxes have made access between the sender and the receiver, staying wide apart, easier and effective.


DISADVANTAGES OF WRITTEN COMMUNICATION

[1]. Time Consuming

Written communication consumes a lot of time especially when the sender and receiver are far away from one another. A letter may take several days to reach the receiver who stays in a place where their is no telephone, fax or internet facilities. The sender also has to wait for many days to get a feedback. There is every possibility that by the time the sender receives the reply, the purpose of the message might get lost. Apart from these, if the written message has some mistakes, it takes a lot of time for clarification.


[2]. More Costly

Written communication in the form of letters exchanged through the postal services and couriers is more expensive. The manpower involved in processing the letters and sending them to the desired destinations in very huge. various modes of transport are used to send these written documents. All these make written communication more costly.


[3]. Delay in Feedback or Response

In case of written communication, getting a feedback or reply takes a lot of time. The sender writes the letter and posts it. The letter travels through a lot of postal stations before it reaches the final receiving point. The receiver receives it and sends a reply which again has to travel back through the same elaborate (multiple) channels before it reaches the sender. As a result, feedback is delayed. Besides, if the message contains some fault, the response time is almost doubled.


[4]. No Chance of Immediate Clarification

After sending the letter or the message, if the sender wants to make some modification in the message, he cannot make any immediate clarification. He will have to write back and wait for the response of the receiver for a long time. At the other end, the receiver, on getting a faulty message might get confused and may take a lot of time to give a feedback.

Read more ...

What is Wind Energy? Wind Turbine and Its Classification.

Movement of air is called wind. About 1 to 3% of solar energy falling on earth gets converted into wind energy. Wind energy is dilute form of energy like solar energy. Uneven heating of the earth's atmosphere is the main cause of wind. The cause of uneven heating of earth's surface is the equatorial regions which receive more solar radiation than the polar region. Warm air at the equatorial region rises and comes down at about 30° North and 30° South latitude.

Wind Energy


During day time, air above the land mass heats up more than air above water bodies likes lakes, seas, oceans. Hot air expands and rises up while cool air over water bodies rushes to fill the space and is called local winds. At night, the process is reversed.


HARNESSING WIND ENERGY

Harnessing of wind energy is not new. Wind energy has been used for centuries to sail vessels, pump water and grind grain. At present, Germany, USA, Denmark, Spain and India account for about 80% of worlds wind power with installed capacity of 7,93,000 MW. India ranks fifth in world and has wind potential of 45,000 MW. States harnessing wind energy are Tamil Nadu, Gujarat, Andhra Pradesh, Maharastra, Karnataka, Kerala, Lakshadweep, Rajasthan, Madhya Pradesh, Odisha, Uttar Pradesh, Andman and Nicobar. Among these states, Tamil Nadu has highest potential of wind energy. Asia's largest wind power plant named Pawan Shakti is located at Lamba near Porbandar in Gujarat.


POTENTIAL WIND AREAS

Wind potential of any geographical region depends on wind speed and wind energy density. Power can be extracted from a Wind Turbine Generator (WTG) if wind speed lies between cut in speed and cut out speed. However rated, output power obtained if the wind speed range lies between 14 m/s to 25 m/s. Areas having these speed range wind potential. According to Beaufort wind scale it is evident that areas with code 4, 5 and 6 have good wind potential. Above cut out speed, pitch control comes into action to produce rated output power.

Tropical regions are at 30° north and south of the equator. These regions have seasonal wind systems, like the monsoon and trade winds. These are high pressure belts. India is dominated by monsoon type of flow. Equator is the high temperature and high humidity region. Wind blows from sub-tropical belts towards the equator and are known as trade winds. The areas in these regions with rich wind potential are open sea, coastal areas, hills, valleys, terrace, saddle and khals (low depression).

In coastal regions, sea breezes and land breezes are the prevailing winds which occur due to temperature difference. During day times, land is hotter than sea and air above the land is heated and rises upward. The hot air over land has low pressure. The cooler air above the sea has high pressure. This causes a cooler air called sea breeze to blow from sea to land. At night, the land cools off more quickly than the sea. When the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land. This causes a cooler air called land breeze to blow from land to sea.

Hilly areas have high wind potential. Hill experiences high wind speeds due to acceleration over the hill. Monsoon winds from Arabian sea and the bay of Bengal strike Himalaya mountain ranges and have high wind potential.


WIND TURBINE

To harness the energy of the wind for production of electric power we need a wind turbine. A wind turbine is a device which converts wind energy into mechanical energy. When wind blows, low pressure air forms on the downward side of blade. This air pulls the blade towards it causing the rotor to rotate. This is called lift force. Thus, lift force exerted by wind in a direction perpendicular to the direction of wind flow. The force exerted by wind on blade in the direction of wind is called drag force. The lift and drag causes rotor to rotate.


Main Parts of Wind Turbine

1. Blades: The front and rear sides of a wind turbine blade have shape similar to that of a long rectangle, with edges bounded by the leading edge, the trailing edge, blade tip and blade root. The blade root is bolted to the hub. The radius of blade is distance from the rotor shaft to the outer edge of blade tip. Wind turbine blades are light weight and are made of glass fibre reinforced polyster. They posses adequate strength and suitable geometrical structure to create lift when air flow over them. Blades capture wind energy and convert it into rotational kinetic energy.

2. Hub: The blades of wind turbines are bolted to the hub which is mounted on a shaft.

3. Nacelle: It houses the generator, gear box and yawing mechanism. Nacelle is placed on the top of the tower.

4. Power Transmission System: The main parts of transmission system are main shaft, gear box, bearing and coupling. Gear box is placed between main shaft and generator. The mechanical power of rotor blades is transmitted to generator by a two stage gear box. It increases the slow speed of rotor blades to generator speed of 1000 or 1500 RPM.

5. Generator: It converts mechanical power input into electrical power. AC or DC generator is used depending on type of service rendered. Direct current can be generated by a brushless DC generator attached to the rotor shaft. Large induction generator type of wind turbine generators (WTG) are used for grid connected system. These are asynchronous AC generators. They draw reactive power from grid and supply active power to grid in a grid connected system, during the period of no wind, the induction generator will operate as a motor and this will draw power from the system and drags the rotor blades as a large electric fan. Therefore, the wind turbine is disconnected from the grid during calm period. Likewise, it is also disconnected during low wind speed.

6. Yaw Control: The yaw control turns the rotor and nacellee to face the wind. Modern large wind turbines are controlled to face the wind direction measured by a wind vane situated on the back of the nacelle. By minimizing the aw angle (the misalignment between wind and turbine pointing direction), the power output is maximised. This is done by yawing motor which continuously tracks and keeps the rotor axis in the direction of wind.

7. Controller: The controller is basically a microcomputer. A series of sensors measure the conditions in wind turbine. Mechanical and electrical parameters are measured by these sensors. For example, wind speed is measured by anemometer which helps in taking decision by microcomputer to stop as soon as wind speed exceed rated speed. Similarly, other parameters are measured constantly by sensors and help to control the operation of wind turbine.

8. Braking: It becomes necessary to stop wind turbine during adverse situations by applying brakes. The mechanical brake is a disc brake placed on the gearbox on the high speed shaft.

9. Tower: Tower provides stuctural support to wind turbine. It should be capable to withstand gravity load and wind loads of turbine. Tower height depends on wind speed and rotor design. It costs about 15% of turbine.


Classification of Wind Turbine

Wind turbine are generally grouped into two types:

1. Vertical Axis Wind Turbine (VAWT): The axis of rotation of rotor is vertical, i.e., perpendicular to the stream of wind. Savonius and Darrius wind turbines are vertical axis type. The Darrius turbine works on lift force and the Savonius turbine works on drag force. The vertical type turbine systems need no special infrastructure and can be kept on the ground.

2. Horizontal Axis Wind Turbine (HAWT): The axis of rotation of this turbine is horizontal, i.e., parallel to the stream of wind. This type of turbine requires special infrastructure to keep generator, gearbox, etc. at the hub height. As the height from ground increases the wind speed increases. Thus high power can be generated using horizontal axis wind turbine.

This type of turbine has five - systems :

  • Rotor: Consisting of two or three blades mounted on a hub and also has pitch control system.

  • The Transmission System: Including gearbox, hydraulic system, shafts, braking system and naccelle.

  • Yaw System: This helps in positioning of rotor blade perpendicular to wind stream.

  • Electrical and Electronic System: Consisting of generator, protective relays, circuit breakers, cables, control and electronic devices and sensors.

  • Tower: This supports nacelle.


CLASSIFICATION OF WIND POWER PLANT

Different types of wind power plant are classified according to rating and are given in Table 1. Also type of generator generally used in each case is indicated.

Table 1

Rating Type Generator Used
0.5 to 1 kW Very Small PM Type DC Generator
1 to 15 kW Small PM Type DC Generator
15 to 200 kW Medium PM Type DC Generator Induction Generator
200 kW to 1 MW Large Induction Generator (three phase) Synchronous Generator (three phase)
1 to 6 MW Very Large Induction Generator (three phase) Synchronous Generator (three phase)


TERMS AND DEFINITIONS

Cut in Speed: The speed at which turbine starts to rotate and generate power is called cut in speed. It is about 3 to 4 m/s.

Rated Output Speed: Wind speed at which output power reaches rated power is called rated output speed. It lies between 12 to 17 m/s. Above this speed there is no further increase in output power. This is done by adjusting the blade angle.

Cut Out Speed: At a speed there is risk of damage to the rotor and braking system is employed to bring the rotor to stand still. This speed is called cut out speed. It is around 25 m/s.

Wind Farm: A wind farm is an area where a large number of wind turbine generators (WTGs) are installed for developing wind power. Generally, a wind farm has 5 to 50 units of wind turbine generator (WTG) sets. These areas must have steady wind speed range of 6 m/s to 30 m/s with average annual wind speed of 10 m/s.

Angle of Attack (AOA): or α: It is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow.

Tip Speed Ratio (λ): It is the ratio of speed of outer blade tip to speed of wind


λ = Tip speed of blade / Wind speed

λ = ωr / v


Where,

ω = angular velocity of blade in radian per second

r = radius of blade

v = wind speed


Swept Area: It is the ar area of the circle created by rotating blade.

Swept area A = πr²

Solidity (σ): It is the ratio of blade area to swept area. Solidity represents fraction of swept area covered by metal part of blade. Turbine has low solidity if the number of blade is less. In such case, rotor should moves faster to capture wind energy otherwise major part of the wind energy would be lost through the large gap between blades. Turbine with low solidity must operates at high tip speed ratio. On the other hand, if the number of blades is more then the turbine has high solidity. These turbines are operated at low tip speed ratio. 

It is evident that number of blade decreases with increase of tip speed ratio and is given in the Table 2.

Tip Speed Ratio Number of Rotor Blades
1 8-18
2 6-12
3 4-10
4 3-8
5 3-5
8 3-4
10 above 1-2

Table 2


Pitch Angle: It is the angle between blade and the plane of blade

Pitch Control of Blade: A system where the pitch angle of blade changes according to wind speed for efficient operation. Any change in speed is sensed by a governor and blade pitch angle is automatically adjusted to get constant frequency power with different speed.

Yaw Angle: The angle between wind direction and turbine pointing direction is called yaw angle.

Yaw Control: As the direction of wind changes frequently, the yaw control is provided to steer the axis of the turbine in the direction of wind. It keeps the turbine blade in the plane perpendicular to wind. There is a wind vane at the back of nacelle which monitors the wind direction and gives signal to the control system so that rotor blade face the direction of wind.

Anemometer: An instrument used to measure wind speed.


WIND ENERGY AND POWER

Wind Energy (E)

Kinetic energy of wind is given by

E = 1/2 mv²

where m = wind mass and v = wind velocity


m = Avt ρ,

where A = swept area through which wind is flowing, ρ = air density and t = time

E = 1/2 (Avtρ)v² = 1/2 Atρ v³

Air Density (ρ)

Air density (ρ) is directly proportional to air pressure and inversely proportional to air temperature in degree Kelvin and is expressed as

ρ = P/RT

where R is gas constant, P is air pressure temperature in degree Kelvin.


R = 287 J/kg.K

P = 1.01325 x 10⁵ Pascal

For 15°C temperature

T = 15 + 273 = 288° Kelvin 

Substituting the above values in the expression of air density gives ρ= 1.226 kg/m³

Air density is maximum increase of altitudes.

Wind Power (P)

Wind power P is energy per unit and is given by 

P = E / t

= 1/2 Aρv³

Wind power is proportional to third power of wind speed. Therefore, wind turbines need to be efficient at greater wind speeds. If the wind speed doubles, then wind power increases to eight times. Also wind power is proportional to swept area (the area covered by rotating blades). Hence, the rotor blades should be long to make swept area large to get more wind power. Wind power is also proportional to air density (ρ). A wind generator will produce less power in summer than in winter at the same wind speed as air has low density in summer than in winter. Similarly, a wind generator will produce lesser power at higher altitudes as air density decreases gradually with increase of altitudes. 

The air density is 1.22 kg/m³ under standard temperature (25°C) and pressure (760 mm of Hg), therefore the simple formula for the power is.

P = 0.6 Av³ watts

Wind speed increases at higher altitudes. It is proportional to 1/7th power of altitude. In an open flat area away from cities and forests, wind speed is given by

V ∝ H^1/7

where V = wind speed and H= height

Doubling the tower height of a wind turbine increases wind speeds by 10% and the expected power by 34%. Modern wind turbines have tower height of 50m to capture more power.

For a generator of given rated power, a low average wind speed requires a large turbine rotor, whereas a high average wind speed requires a small turbine rotor.

Bitz Limit

Total wind power available could be captured only if the wind speed is reduced to zero. But this is practically impossible as the captured air must leaves the turbine. The maximum theoretical efficiency is the ratio of maximum power output to total wind power available. It is also called power coefficient Cp.

Cp = Pmax / Ptotal = 0.593

The factor 0.593 is called Bitz limit.

Therefore, the maximum achievable extraction of wind power by a wind turbine is 59% of the total theoretical power. 

The efficiency of wind turbine is further reduced due to the following factors:

• Rotor blade friction and drag
• Gearbox losses 
• Generator losses

Wind Power Density (WPD)

Wind power density (WPD) is the ratio of wind power per unit area of wind through which it is passing. WPD is the yardstic used to determine the best locations for wind energy.

Therefore, wind power density = P / A

= 1/2 ρv³

Maximum power density = Pmax / A

Actual power density = Efficiency of turbine × Total power density 

Power output of turbine P = Actual power density × Swept area

Torque

Torque (T) on rotor shaft depend on angular velocity (w) of rotor and power output (P) of turbine.

P = ωT

T = P / ω

= ηt . 1/2 Aρv³ . 1/ω

Where ηt is efficiency of turbine.


LIFT AND DRAG

Energy of wind can be extracted by wind turbine by two forces acting on it, namely, lift force and drag force. When wind passes over the rotor blade it produces a pressure difference across its surface. The force acting from a high pressure side to low pressure side due to pressure difference is called lift force. This force acts perpendicular to the direction of air flow. A lift force based wind turbine has medium torque and high rotational speed (rpm). So these type of turbines are mainly used for electricity generation.

Drag is the force which is exerted on the turbine blade due to airflow across them. Drag force acts in the same direction as that of air flow. Drag force causes axial thrust on the blades. Wind turbines based on drag force provide high torque and low rotational speed (rpm). So they are used for water pumpling and grinding type of applications.


WIND ENERGY STORAGE

Wind energy can be stored in the following ways:

Pumped Water Storage

This system requires two reservoirs, one at the lower side and other at the upper side with sufficient head. Water from the lower reservoir is fed to the upper one by pump operated by wind turbine generator (WTG). The stored water in the upper reservoir is used to run a micro hydro turbine generator which produces electricity as and when required. So this type of storage is a hybrid of WTG and micro hydro system. It is suitable where both wind and water are available sufficiently. The size of this system is less than 100 kW.

Compressed Air Storage

Wind energy can be stored in the form of compressed air. When wind is not blowing energy stored in the storage tank can be used to drive the wind turbine to generate electricity by an electric generator as and when required.

Battery Storage

In this system, electrical energy generated by WTG is given to charge a battery bank which stores electrical energy in the form of chemical energy. The stored energy can be utilized at any time. However, an inverter system is required to convert de supply of battery to required ac voltage.

Lead acid batteries are commonly used for storage because they have high efficiency and low cost. Since high capacity batteries are not available at present, so a large number of batteries is required to store energy. Huge amount of energy cannot be stored because of high capital cost and high maintenance cost.


LIMITATIONS IN USING WIND ENERGY 

There are certain limitations in using wind energy as an alternative energy source.

1. The locations of wind energy generators are limited to the areas where strong and dependable winds are available most of the time. 

2. Since, the sufficiently strong winds are not available all the time, the energy produced by the wind energy generators is also intermittent. So, the wind energy generators are required to be supported by a back-up supply. This enhances the overall cost of the system.

3. Because of the great heights of turbine tower and large rotor blades, it is difficult to repair and maintain them. 

4. The wind turbines must be strong to withstand storms and lightning.


BLOCK DIAGRAM OF CONTROL PANEL OF WIND ENERGY SYSTEM

Wind energy in large scale can be obtained from a group of wind turbine generator system working in an area is called wind farm.

It is microprocessor based control system of a grid connected Wind Energy Generator (WEG). It is provided to change setting and to adjust parameters to get maximum output. The controller receives input of wind speed and direction along with load requirement at given voltage and frequency. It gives signal to the turbine for yaw control blade pitch control and to apply brake in case of high wind.


OFF-GRID WIND TURBINE SYSTEM

When the wind turbine is operated in isolation from the grid system, then it is called off-grid or stand alone system. It supplies to a group of local consumers. The generating capacity must be matched with the demand of the consumer. This system is mostly used for. (i) power supply for domestic use, (ii) battery charging, (iii) power supply for water pump for irrigation and drinking for purposes.

The variable ac supply obtained from WEG is first rectified into de in control unit (CU) and then given to charging of battery. With the help of inverter de is again converted into ac at desired voltage and frequency and fed to the local consumers. In the absence of wind, the battery can give supply to the consumer. It has the advantage of supplying load to the local consumers mainly in remote area where no grid connectivity is available. Further, it has no transmission losses as load is near to the WEG.


VOLTAGE REGULATION IN WIND TURBINE GENERATOR (WTG)

Wind energy is one of the most rapidly growing sources of electricity generation all over the world. It is predicted that 12% of the total world electricity demands will be supplied from wind energy by 2020. This encourages researcher to develop various control techniques for better performance of wind turbine within a wind speed range and to generate constant frequency power and to keep the terminal voltage of generator within the prescribed limit. WTG based on induction generators have no inherent voltage control. WTGs installation is connected to a grid substation. This substation has a variety of equipment to control voltage such as capacitor, tapping transformers, etc. Some WTG use power electronic converters which regulate reactive power to achieve desire voltage control.


WIND TURBINE NOISE

Wind turbine generates two types of noise: aerodynamic and mechanical. Aerodynamic noise is generated when blades are rotating through air. Noise level depends on turbine size and wind speed. Estimated noise level in dB of some standard make turbine is given in Table 3.

Turbine Size Wind Speed (m/s) Noise Level (dB)
900 W 5 83
10 91
10 kW 5 87
7 96
10 105

Table 3


Mechanical noise is generated by turbine's internal gear. It is irritating. Excessive exposure to noise causes hearing loss and sleep disorder.


SUITABLE SITE FOR WIND TURBINE GENERATOR (WTG)

The main factors governing selection of site of WTG are:

1. Availability of Wind: Site of WTG should be selected considering the wind data for that location. For that data from wind observatories may be used. The site should be such that it has high wind potential and wind power density. The area for this are open seas, coastal areas, hilly areas, etc. Wind speed range for power production lies between 14 m/s to 25 m/s.

2. Availability of Land: The height of the wind turbine should be between 10 m to 100 m above the ground. Therefore, the land area for installation of wind turbine should be free from obstacles buildings, towers, poles, trees, etc. Also the terrain and soil condition should be suitable for establishment of wind farms.

3. Accessibility: The site should be accessible for transport of machinery and construction material. Also the site should be accessible for maintenance.

4. Grid Proximity: The site should be near the power grid so that its power generated from WTG can be fed to the grid without transmission losses.


ADVANTAGES AND DISADVANTAGES OF WIND ENERGY

Advantages of Wind Energy

1. The wind is free and with modern technology it can be captured efficiently.

2. This energy does not produce green house gases or other pollutants.

3. Wind turbines mounted over tall tower which occupies a small plot of land. Thus land below can still be used in the case of agricultural areas where farming can still continued.

4. Electricity generated by wind turbines can be used to supply to remote areas that are not connected to grid.

5. Conventional energy sources will be exhausted in future. Wind energy is a renewable and will play main role in future.

6. Wind turbines are available in a range of size which benefit a wide varieties of consumers.

Disadvantages of Wind Energy

1. The wind speed is not constant and it varies from zero to that of storm. This means that wind turbines do not produce the same amount of electricity all the time. There will be times when they produce no electricity at all.

2. Many people feel that these large structures create a bad landscape.

3. Wind turbine are noisy. So the wind power plants should be located away from residential areas.

4. There is some pollution at the time of manufacturing of wind turbines.

5. Large wind farms are needed to provide power supply for entire area.

6. The limitation of wind power is that no electricity is produced when the wind is not blowing.

7. Limited to windy areas.

8. May affect endangered birds, however tower design can reduce impact.

Read more ...

What is Power Station? What are the types of Power Plant?

In this modern world, the dependence on electricity is so much that it has become a -part and parcel of our life. The ever increas ing use of electric power for domestic, commercial and industrial purposes necessitates to provide bulk electric power economically. This is achieved with the help of suitable power producing units, known as Power plants or Electric power generating stations. The design of a power plant should incorporate two important aspects. Firstly, the selection and placing of necessary power generating equipment should be such so that a maximum of return will result from a mini mum of expenditure over the working life of the plant. Secondly, the operation of the plant should be such so as to provide cheap, reliable and continuous service. In this chapter, we shall focus our attention on various types of generat ing stations with special reference to their advantages and disadvantages.


Generation of Electrical Energy

The conversion of energy available in different forms in nature into electrical energy is known as generation of electrical energy.

Electrical energy is a manufactured commodity like clothing, furniture or tools. Just as the manufacture of a commodity involves the conversion of raw materials available in nature into the desired form, similarly electrical energy is produced from the forms of energy available in nature. However, electrical energy differs in one important respect. Whereas other commodities may be produced at will and consumed as needed, the electrical energy must be produced and transmitted to the point of use at the instant it is needed. The entire process takes only a fraction of a second. This instantaneous production of electrical energy introduces technical and economical considerations unique to the electrical power industry.

Energy is available in various forms from different natural sources such as pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc. All these forms of energy can be converted into electrical energy by the use of suitable arrangements. The arrangement essentially employs an alternator coupled to a prime mover. The prime mover is driven by the energy obtaimed from various sources such as burning of fuel, pressure of water, force of wind etc. For example, chemical energy of a fuel (e.g., coal) can be used to produce steam at high temperature and pressure. The steam is fed to a prime mover which may be a steam engine or a steam turbine. The turbine converts heat energy of steam into mechanical energy which is further converted into electrical energy by the alternator. Similarly, other forms of energy can be converted into electrical energy by employing suitable machinery and equipment.

Power Station Power Plant


What is Power Station?

Bulk electric power is produced by special stations or plants known as Power Station or Power Plant. Power Station is also known as Generating Station.

A power station or generating station essentially employs a prime mover coupled to an alternator for the production of electric power. The prime mover (e.g.. steam turbine, water turbine etc.) converts energy from some other form into mechanical energy. The alternator converts mechanical energy of the prime mover into electrical energy. The electrical energy produced by the generating station is transmitted and distributed with the help of conductors to various consumers. It may be emphasised here that apart from prime mover-alternator combination, a modern generating station employs several auxiliary equipment and instruments to ensure cheap, reliable and continuous service.


What are the types of Power Plant or Power Station?

Depending upon the form of energy converted into electrical energy, the power stations are classified as under: 


(i) Steam Power Stations

(ii) Hydroelectric Power Stations

(iii) Diesel Power Stations

(iv) Nuclear Power Stations


Steam Power Station (Thermal Power Plant)

A generating station which converts heat energy of coal combustion into electrical energy is known as a steam power station.

A steam power station basically works on the Rankine cycle. Steam is produced in the boiler by utilising the heat of coal combustion. The steam is then expanded in the prime mover (i.e., steam turbine) and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.

Steam Power Station


Advantages

(i) The fuel (i.e., coal) used is quite cheap. (ii) Less initial cost as compared to other generating stations.
(iii) It can be installed at any place irrespective of the existence of coal. The coal can be transported to the site of the plant by rail or road.
(iv) It requires less space as compared to the hydroelectric power station. 
(v) The cost of generation is lesser than that of the diesel power station.

Disadvantages

(i) It pollutes the atmosphere due to the production of large amount of smoke and fumes. 
(ii) It is costlier in running cost as compared to hydroelectric plant.


Schematic Arrangement of Steam Power Station

Although steam power station simply involves the conversion of heat of coal combustion into electri cal energy, yet it embraces many arrangements for proper working and efficiency. The schematic arrangement of a modern steam power station is shown in Fig. The whole arrangement can be divided into the following stages for the sake of simplicity: 


1. Coal and ash handling arrangement

2. Steam generating plant

3. Steam turbine

4. Alternator

5. Feed water

6. Cooling arrangement


1. Coal and Ash Handling Plant : The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal is delivered to the coul handling plant where it is pulverised (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air. The pulverised coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal.

It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a modern thermal power station. A 100 MW station operating at 50% load factor may burn about 20,000 tons of coal per month and ash produced may be to the tune of 10% to 15% of coal fired i.e., 2,000 to 3,000 tons. In fact, in a thermal station, about 50% to 60% of the total operating cost consists of fuel purchasing and its handling.


2. Steam Generating Plant : The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilisation of flue gases.


(i) Boiler : The heat of combustion of coal in the boiler is utilised to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through super heater, economiser, air pre-heater and are finally exhausted to atmosphere through the chimney.

(ii) Superheater : The steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated (i.e., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principal benefits. Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the superheater is fed to steam turbine through the main valve.

(iii) Economiser : An economiser is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economiser before supplying to the boiler. The economiser extracts a part of heat of flue gases to increase the feed water temperature.

(iv) Air Preheater : An air preheater increases the temperature of the air supplied for coal burn ing by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are: increased thermal efficiency and increased steam capacity per square metre of boiler surface.


3. Steam Turbine : The dry and superheated steam from the superheater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation.


4. Alternator : The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators.


5. Feed Water : The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the boiler is heated by water heaters and economiser. This helps in raising the overall effi ciency of the plant.


6. Cooling Arrangement : In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed* by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser.


Choice of Site for Steam Power Stations

In order to achieve overall economy, the following points should be considered while selecting a site for a steam power station :


(i) Supply of Fuel : The steam power station should be located near the coal mines so that transportation cost of fuel is minimum. However, if such a plant is to be installed at a place where coal is not available, then care should be taken that adequate facilities exist for the transportation of coal.


(ii) Availability of Water : As huge amount of water is required for the condenser, therefore, such a plant should be located at the bank a river or near a canal to ensure the continuous supply of water..


(iii) Transportation Facilities : A modern steam power station often requires the transportation of material and machinery. Therefore, adequate transportation facilities must exist i.e., the plant should be well connected to other parts of the country by rail, road. etc.


(iv) Cost and Type of Land : The steam power station should be located at a place where land is cheap and further extension, if necessary, is possible. Moreover, the bearing capacity of the ground should be adequate so that heavy equipment could be installed.


(v) Nearness to Load Centres : In order to reduce the transmission cost, the plant should be located near the centre of the load. This is particularly important if d.c. supply system is adopted. However, if a.c. supply system is adopted, this factor becomes relatively less important. It is because a.c. power can be transmitted at high voltages with consequent reduced transmission cost. Therefore, it is possible to install the plant away from the load centres, provided other conditions are favourable.


(vi) Distance from Populated Area : As huge amount of coal is burnt in a steam power station, therefore, smoke and fumes pollute the surrounding area. This necessitates that the plant should be located at a considerable distance from the populated areas.


Conclusion : It is clear that all the above factors cannot be favourable at one place. However, keeping in view the fact that now-a-days the supply system is a.c. and more importance is being given to generation than transmission, a site away from the towns may be selected. In particular, a site by river side where sufficient water is available, no pollution of atmosphere occurs and fuel can be transported economically, may perhaps be an ideal choice.


Efficiency of Steam Power Station

The overall efficiency of a steam power station is quite low (about 29%) due mainly to two reasons. Firstly, a huge amount of heat is lost in the condenser and secondly heat losses occur at various stages of the plant. The heat lost in the condenser cannot be avoided. It is because heat energy cannot be converted into mechanical energy without temperature difference. The greater the temperature dif ference, the greater is the heat energy converted into mechanical energy. This necessitates to keep the steam in the condenser at the lowest temperature. But we know that greater the temperature difference, greater is the amount of heat lost. This explains for the low efficiency of such plants.


(i) Thermal Efficiency : The ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion of coal is known as thermal efficiency of steam power station.

The thermal efficiency of a modern steam power station is about 30%. It means that if 100 calories of heat is supplied by coal combustion, then mechanical energy equivalent of 30 calories will be available at the turbine shaft and rest is lost. It may be important to note that more than 50% of total heat of combustion is lost in the condenser. The other heat losses occur in flue gases, radia tion, ash etc.


(ii) Overall Efficiency : The ratio of heat equivalent of electrical output to the heat of combus tion of coal is known as overall efficiency of steam power station.

The overall efficiency of a steam power station is about 29%. It may be seen that overall eff ciency is less than the thermal efficiency. This is expected since some losses (about 1%) occur in the alternator. The following relation exists among the various efficiencies. Overall efficiency Thermal efficiency × Electrical efficiency


Equipment of Steam Power Station

A modern steam power station is highly complex and has numerous equipment and auxiliaries. However, the most important constituents of a steam power station are:


1. Steam generating equipment 

2. Condenser

3. Prime mover

4. Water treatment plant

5. Electrical equipment.


1. Steam Generating Equipment : This is an important part of steam power station. It is concerned with the generation of superheated steam and includes such items as boiler, boiler furnace superheater, economiser, air pre-heater and other heat reclaiming devices.


(i) Boiler : A boiler is closed vessel in which water is converted into steam by utilising the head of coal combustion. Steam boilers are broadly classified into the following two types:

(a) Water tube boilers

(b) Fire tube boilers

In a water tube boiler, water flows through the tubes and the hot gases of combustion flow over these tubes. On the other hand, in a fire tube boiler, the hot products of combustion pass through the tubes surrounded by water. Water tube boilers have a number of advantages over fire tube boilers viz, require less space, smaller size of tubes and drum, high working pressure due to small drum, less liable to explosion etc. Therefore, the use of water tube boilers has become universal in large capacity steum power stations.


(ii) Boiler Furnace : A boiler furnace is a chamber in which fuel is burnt to liberate the beat energy. In addition, it provides support and enclosure for the combustion equipment i.e., burners. The boiler furnace walls are made of refractory materials such as fire clay, silica, kaolin etc. These materials have the property to resist change of shape, weight or physical properties at high temperatures. There are following three types of construction of furnace walls: 

(a) Plain refractory walls 

(b) Hollow refractory walls with an arrangement for air cooling

(c) Water walls

The plain refractory walls are suitable for small plants where the furnace temperature may not be high. However, in large plants, the furnace temperature is quite high and consequently, the refrac tory material may get damaged. In such cases, refractory walls are made hollow and air is circulated through hollow space to keep the temperature of the furnace walls low. The recent development is we water walls. These consist of plain tubes arranged side by side and on the inner face of the refractory y walls. The tubes are connected to the upper and lower headers of the boiler. The boiler water is made to circulate through these tubes. The water walls absorb the radiant heat in the furnace which would otherwise heat up the furnace walls.


(iii) Superheater : A superheater is a device which superheats the steam i.e., it raises the tempera ture of steam above boiling point of water. This increases the overall efficiency of the plant. A superheater consists of a group of tubes made of special alloy steels such as chromium-molybdenum. These tubes are heated by the heat of flue gases during their journey from the furnace to the chimney.

The steam produced in the boiler is led through the superheater where it is superheated by the heat of flue gases. Superheaters are mainly classified into two types according to the system of heat transfer from flue gases to steam viz.

(a) Radiant superheater

(b) Convection superheater


The radiant superheater is placed in the furnace between the water walls and receives heat from the burning fuel through radiation process. It has two main disadvantages. Firstly, due to high furnace temperature, it may get overheated and, therefore, requires a careful design. Secondly, the temperature of superheater falls with increase in steam output. Due to these limitations, radiant superheater is not finding favour these days. On the other hand, a convection superheater is placed in the boiler tube bank and receives heat from flue gases entirely through the convection process. It has the advantage that temperature of superheater increases with the increase in steam output. For this reason, this type of superheater is commonly used these days.


(iv) Economiser : It is a device which heats the feed water on its way to boiler by deriving heat from the flue gases. This results in raising boiler efficiency, saving in fuel and reduced stresses in the boiler due to higher temperature of feed water. An economiser consists of a large number of closely spaced parallel steel tubes connected by headers of drums. The feed water flows through these tubes and the flue gases flow outside. A part of the heat of flue gases is transferred to feed water, thus raising the temperature of the latter.


(v) Air Pre-heater : Superheaters and economisers generally cannot fully extract the heat from flue gases. Therefore, pre-heaters are employed which recover some of the heat in the escaping. gases. The function of an air pre-heater is to extract heat from the flue gases and give it to the air being supplied to furnace for coal combustion. This raises the furnace temperature and increases the thermal efficiency of the plant. Depending upon the method of transfer of heat from flue gases to air, air pre-heaters are divided into the following two classes :

(a) Recuperative type

(b) Regenerative type

The recuperative type air-heater consists of a group of steel tubes. The flue gases are passed through the tubes while the air flows externally to the tubes. Thus heat of flue gases is transferred to air. The regenerative type air pre-heater consists of slowly moving drum made of corrugated metal plates. The flue gases flow continuously on one side of the drum and air on the other side. This action permits the transference of heat of flue gases to the air being supplied to the furnace for coal combustion.


2. Condensers : A condenser is a device which condenses the steam at the exhaust of turbine. It serves two important functions. Firstly, it creates a very low pressure at the exhaust of turbine, thus permitting expansion of the steam in the prime mover to a very low pressure. This helps in converting heat energy of steam into mechanical energy in the prime mover. Secondly, the condensed steam can be used as feed water to the boiler. There are two types of condensers, namely:

(i) Jet condenser

(ii) Surface condenser

In a jet condenser, cooling water and exhausted steam are mixed together. Therefore, the temperature of cooling water and condensate is the same when leaving the condenser. Advantages of this type of condenser are: low initial cost, less floor area required, less cooling water required and low maintenance charges. However, its disadvantages are: condensate is wasted and high power is re quired for pumping water.

In a surface condenser, there is no direct contact between cooling water and exhausted steam. It consists of a bank of horizontal tubes enclosed in a cast iron shell. The cooling water flows through the tubes and exhausted steam over the surface of the tubes. The steam gives up its heat to water and is itself condensed. Advantages of this type of condenser are: condensate can be used as feed water, less pumping power required and creation of better vacuum at the turbine exhaust. However, disadvantages of this type of condenser are : high initial cost, requires large floor area and high maintenance charges.


3. Prime Movers : The prime mover converts steam energy into mechanical energy. There are two types of steam prime movers viz., steam engines and steam turbines. A steam turbine has several advantages over a steam engine as a prime mover viz., high efficiency, simple construction, higher speed, less floor area requirement and low maintenance cost. Therefore, all modern steam power stations employ steam turbines as prime movers.

Steam turbines are generally classified into two types according to the action of steam on moving blades viz.

(i) Impulse Turbine

(ii) Reactions turbines

In an impulse turbine, the steam expands completely in the stationary nozzles (or fixed blades), the pressure over the moving blades remaining constant. In doing so, the steam attains a high velocity and impinges against the moving blades. This results in the impulsive force on the moving blades which sets the rotor rotating. In a reaction turbine, the steam is partially expanded in the stationary nozzles, the remaining expansion takes place during its flow over the moving blades. The result is that the momentum of the steam causes a reaction force on the moving blades which sets the rotor in motion.


4. Water Treatment Plant : Boilers require clean and soft water for longer life and better efficiency. However, the source of boiler feed water is generally a river or lake which may contain suspended and dissolved impurities, dissolved gases etc. Therefore, it is very important that water is first purified and softened by chemical treatment and then delivered to the boiler.

The water from the source of supply is stored in storage tanks. The suspended impurities are removed through sedimentation, coagulation and filtration. Dissolved gases are removed by aeration and degasification. The water is then 'softened' by removing temporary and permanent hardness through different chemical processes. The pure and soft water thus available is fed to the boiler for steam generation.


5. Electrical Equipment : A modern power station contains numerous electrical equipment. However, the most important items are:


(i) Alternators : Each alternator is coupled to a steam turbine and converts mechanical energy of the turbine into electrical energy. The alternator may be hydrogen or air cooled. The necessary excitation is provided by means of main and pilot exciters directly coupled to the alternator shaft.


(ii) Transformers : A generating station has different types of transformers, viz., 

(a) main step-up transformers which step-up the generation voltage for transmission of power.

(b) station transformers which are used for general service (e.g., lighting) in the power station.

(c) auxiliary transformers which supply to individual unit-auxiliaries.


(iii) Switchgear : It houses such equipment which locates the fault on the system and isolate the faulty part from the healthy section. It contains circuit breakers, relays, switches and other control devices.


Hydro - electric Power Station

A power station or generating station which utilises the potential energy of water at a high level for the generation of electrical energy is known as hydro - electric power station.

Hydro-electric power stations are generally located in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. In a hydro-electric power station, water head is created by constructing a dam across a river or lake. From the dam, water is led to a water turbine. The water turbine captures the energy in the falling water and changes the hydraulic energy (i.e.. product of head and flow of water) into mechanical energy at the turbine shaft. The turbine drives the alternator which converts mechanical energy into electrical energy. Hydro-electric power stations are becoming very popular because the reserves of fuels (i.e., coal and oil) are depleting day by day. They have the added importance for flood control, storage of water for irrigation and water for drinking purposes.

Hydroelectric Power Stations


Advantages

(i) It requires no fuel as water is used for the generation of electrical energy.

(ii) It is quite neat and clean as no smoke or ash is produced.

(iii) It requires very small running charges because water is the source of energy which is avail able free of cost.

(iv) It is comparatively simple in construction and requires less maintenance. 

(v) It does not require a long starting time like a steam power station. In fact, such plants can be put into service instantly.

(vi) It is robust and has a longer life.

(vii) Such plants serve many purposes. In addition to the generation of electrical energy, they also help in irrigation and controlling floods.

(viii) Although such plants require the attention of highly skilled persons at the time of construction, yet for operation, a few experienced persons may do the job well.


Disadvantages

(i) It involves high capital cost due to construction of dam.

(ii) There is uncertainty about the availability of huge amount of water due to dependence on weather conditions.

(iii) Skilled and experienced hands are required to build the plant.

(iv) It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.


Schematic Arrangement of Hydro-electric Power Station

Although a hydro-electric power station simply involves the conversion of hydraulic energy into electrical energy, yet it embraces many arrangements for proper working and efficiency. The sche matic arrangement of a modern hydro-electric plant is shown in Fig.

The dam is constructed across a river or lake and water from the catchment area collects at the back of the dam to form a reservoir. A pressure tunnel is taken off from the reservoir and water brought to the valve house at the start of the penstock. The valve house contains main sluice valves and automatic isolating valves. The former controls the water flow to the power house and the latter cuts off supply of water when the penstock bursts. From the valve house, water is taken to water turbine through a huge steel pipe known as penstock. The water turbine converts hydraulic energy into mechanical energy. The turbine drives the alternator which converts mechanical energy into electrical energy.

A surge tank (open from top) is built just before the valve house and protects the penstock from bursting in case the turbine gates suddenly close due to electrical load being thrown off. When the gates close, there is a sudden stopping of water at the lower end of the penstock and consequently the penstock can burst like a paper log. The surge tank absorbs this pressure swing by increase in its water level.


Choice of Site for Hydro-electric Power Stations

The following points should be taken into account while selecting the site for a hydro-electric power station:


(i) Availability of Water : Since the primary requirement of a hydro-electric power station is the availability of huge quantity of water, such plants should be built at a place (e.g., river, canal) where adequate water is available at a good head.


(ii) Storage of Water : There are wide variations in water supply from a river or canal during the year. This makes it necessary to store water by constructing a dam in order to ensure the generation of power throughout the year. The storage helps in equalising the flow of water so that any excess quantity of water at a certain period of the year can be made available during times of very low flow in the river. This leads to the conclusion that site selected for a hydro-electric plant should provide adequate facilities for erecting a dam and storage of water.


(iii) Cost and Type of Land : The land for the construction of the plant should be available at a reasonable price. Further, the bearing capacity of the ground should be adequate to with stand the weight of heavy equipment to be installed.


(iv) Transportation Facilities : The site selected for a hydro-electric plant should be accessible by rail and road so that necessary equipment and machinery could be easily transported. It is clear from the above mentioned factors that ideal choice of site for such a plant is near a river in hilly areas where dam can be conveniently built and large reservoirs can be obtained.


Constituents of Hydro-electric Plant

The constituents of a hydro-electric plant are (1) hydraulic structures (2) water turbines and (3) electrical equipment. We shall discuss these items in turn.


1. Hydraulic Structures : Hydraulic structures in a hydro-electric power station include dam, spillways, headworks, surge tank, penstock and accessory works.


(i) Dam : A dam is a barrier which stores water and creates water head. Dams are built of concrete or stone masonary, earth or rock fill. The type and arrangement depends upon the topography of the site. A masonary dam may be built in a narrow canyon. An earth dam may be best suited for a wide valley. The type of dam also depends upon the foundation conditions, local materials and transportation available, occurrence of earthquakes and other hazards. At most of sites, more than one type of dam may be suitable and the one which is most economical is chosen.


(ii) Spillways : There are times when the river flow exceeds the storage capacity of the reservoir. Such a situation arises during heavy rainfall in the catchment area. In order to discharge the surplus water from the storage reservoir into the river on the down-stream side of the dam, spillways are used. Spillways are constructed of concrete piers on the top of the dam. Gates are provided between these piers and surplus water is discharged over the crest of the dam by opening these gates.


(iii) Headworks : The headworks consists of the diversion structures at the head of an intake. They generally include booms and racks for diverting floating debris, sluices for by-passing debris and sediments and valves for controlling the flow of water to the turbine. The flow of water into and through headworks should be as smooth as possible to avoid head loss and cavitation. For this purpose, it is necessary to avoid sharp corners and abrupt contractions or enlargements.


(iv) Surge Tank : Open conduits leading water to the turbine require no protection. However, when closed conduits are used, protection becomes necessary to limit the abnormal pressure in the conduit. For this reason, closed conduits are always provided with a surge tank. A surge tank is a small reservoir or tank (open at the top) in which water level rises or falls to reduce the pressure swings in the conduit.

A surge tank is located near the beginning of the conduit. When the turbine is running at a steady load, there are no surges in the flow of water through the conduit i.e., the quantity of water flowing in the conduit is just sufficient to meet the turbine requirements. However, when the load on the turbine decreases, the governor closes the gates of turbine, reducing water supply to the turbine. The excess water at the lower end of the conduit rushes back to the surge tank and increases its water level. Thus the conduit is prevented from bursting. On the other hand, when load on the turbine increases, addi tional water is drawn from the surge tank to meet the increased load requirement. Hence, a surge tank overcomes the abnormal pressure in the conduit when load on the turbine falls and acts as a reservoir during increase of load on the turbine.


(v) Penstocks : Penstocks are open or closed conduits which carry water to the turbines. They are generally made of reinforced concrete or steel. Concrete penstocks are suitable for low heads (< 30 m) as greater pressure causes rapid deterioration of concrete. The steel pen stocks can be designed for any head; the thickness of the penstock increases with the head or working pressure.

Penstock


Various devices such as automatic butterfly valve, air valve and surge tank are provided for the protection of penstocks. Automatic butterfly valve shuts off water flow through the penstock promptly if it ruptures. Air valve maintains the air pressure inside the penstock equal to outside atmospheric pressure. When water runs out of a penstock faster than it enters, a vacuum is created which may cause the penstock to collapse. Under such situations, air valve opens and admits air in the penstock to maintain inside air pressure equal to the outside air pressure.


2. Water Turbines : Water turbines are used to convert the energy of falling water into mechanical energy. The principal types of water turbines are:

(i) Impulse Turbines

(ii) Reaction Turbines


(i) Impulse Turbines : Such turbines are used for high heads. In an impulse turbine, the entire pressure of water is converted into kinetic energy in a nozzle and the velocity of the jet drives the wheel. The example of this type of turbine is the Pelton wheel. It consists of a wheel fitted with elliptical buckets along its periphery. The force of water jet strik ing the buckets on the wheel drives the turbine. The quantity of water jet falling on the turbine is controlled by means of a needle or spear placed in the tip of the nozzle. The movement of the needle is controlled by the governor. If the load on the turbine decreases, the governor pushes the needle into the nozzle, thereby reducing the quantity of water striking the buckets. Reverse action takes place if the load on the turbine increases.

Pelton Wheel


(ii) Reaction Turbines : Reaction turbines are used for low and medium heads. In a reaction turbine, water enters the runner partly with pressure energy and partly with velocity head. The important types of reaction turbines are:

(a) Francis turbines

(b) Kaplan turbines


A Francis turbine is used for low to medium heads. It consists of an outer ring of stationary guide blades fixed to the turbine casing and an inner ring of rotating blades forming the runner. The guide blades control the flow of water to the turbine. Water flows radially inwards and changes to a downward direction while passing through the runner. As the water passes over the "rotating blades" of the runner, both pressure and velocity of water are reduced. This causes a reaction force which drives the turbine.

A Kaplan turbine is used for low heads and large quantities of water. It is similar to Francis tur bine except that the runner of Kaplan turbine receives water axi ally. Water flows radially inwards through regulating gates all around the sides, changing direction in the runner to axial flow. This causes a reaction force which drives the turbine.


3. Electrical Equipment : The electrical equipment of a hydro - electric power station includes alternators, transformers, circuit breakers and other switching and protective devices.


Diesel Power Station

A generating station in which diesel engine is used as the prime mover for the generation of electri cal energy is known as diesel power station.

In a diesel power station, diesel engine is used as the prime mover. The diesel burns inside the engine and the products of this combustion act as the "working fluid" to produce mechanical energy. The diesel engine drives the alternator which converts mechanical energy into electrical energy. As the generation cost is considerable due to high price of diesel, therefore, such power stations are only used to produce small power.

Diesel Power Station


Although steam power stations and hydro-electric plants are invariably used to generate bulk power at cheaper cost, yet diesel power stations are finding favour at places where demand of power is less, sufficient quantity of coal and water is not available and the transportation facilities are inad equate. These plants are also used as standby sets for continuity of supply to important points such as hospitals, radio stations, cinema houses and telephone exchanges.


Advantages

(i) The design and layout of the plant are quite simple.

(ii) It occupies less space as the number and size of the auxiliaries is small.

(iii) It can be located at any place.

(iv) It can be started quickly and can pick up load in a short time.

(v) There are no standby losses.

(vi) It requires less quantity of water for cooling.

(vii) The overall cost is much less than that of steam power station of the same capacity. 

(viii) The thermal efficiency of the plant is higher than that of a steam power station.

(ix) It requires less operating staff.


Disadvantages

(i) The plant has high running charges as the fuel (i.e., diesel) used is costly. 

(ii) The plant does not work satisfactorily under overload conditions for a longer period.

(iii) The plant can only generate small power. 

(iv) The cost of lubrication is generally high.

(v) The maintenance charges are generally high.


Schematic Arrangement of Diesel Power Station

Fig. shows the schematic arrangement of a typical diesel power station. Apart from the diesel generator set, the plant has the following auxiliaries:


(i) Fuel Supply System : It consists of storage tank, strainers, fuel transfer pump and all day fuel tank. The fuel oil is supplied at the plant site by rail or road. This oil is stored in the storage tank. From the storage tank, oil is pumped to smaller all day tank at daily or short intervals. From this tank, fuel oil is passed through strainers to remove suspended impurities. The clean oil is injected into the engine by fuel injection pump.


(ii) Air Intake System : This system supplies necessary air to the engine for fuel combustion. It consists of pipes for the supply of fresh air to the engine manifold. Filters are provided to remove dust particles from air which may act as abrasive in the engine cylinder.


(iii) Exhaust System : This system leads the engine exhaust gas outside the building and dis charges it into atmosphere. A silencer is usually incorporated in the system to reduce the noise level.


(iv) Cooling System : The heat released by the burning of fuel in the engine cylinder is partially converted into work. The remainder part of the heat passes through the cylinder walls, piston, rings etc. and may cause damage to the system. In order to keep the temperature of the engine parts within the safe operating limits, cooling is provided. The cooling system consists of a water source, pump and cooling towers. The pump circulates water through cylinder and head jacket. The water takes away heat form the engine and itself becomes hot. The hot water is cooled by cooling towers and is recirculated for cooling.


(v) Lubricating System : This system minimises the wear of rubbing surfaces of the engine. It comprises of lubricating oil tank, pump, filter and oil cooler. The lubricating oil is drawn from the lubricating oil tank by the pump and is passed through filters to remove impurities. The clean lubricating oil is delivered to the points which require lubrication. The oil coolers incorporated in the system keep the temperature of the oil low.


(vi) Engine Starting System : This is an arrangement to rotate the engine initially, while starting, until firing starts and the unit runs with its own power. Small sets are started manually by handles but for larger units, compressed air is used for starting. In the latter case, air at high pressure is admitted to a few of the cylinders, making them to act as reciprocating air motors to turn over the engine shaft. The fuel is admitted to the remaining cylinders which makes the engine to start under its own power.


Nuclear Power Station

A power station or generating station in which nuclear energy is converted into electrical energy is known as a nuclear power station.

In nuclear power station, heavy elements such as Uranium (U²³⁵) or Thorium (Th²³²) are subjected to nuclear fission in a special apparatus known as a reactor. The heat energy thus released is utilised in raising steam at high temperature and pressure. The steam runs the steam turbine which converts steam energy into mechanical energy. The turbine drives the alternator which converts mechanical energy into electrical energy.

Nuclear Power Station


The most important feature of a nuclear power station is that huge amount of electrical energy can be produced from a relatively small amount of nuclear fuel as compared to other conventional types of power stations. It has been found that complete fission of 1 kg of Uranium (U²³⁵) can produce as much energy as can be produced by the burning of 4,500 tons of high grade coal. Al though the recovery of principal nuclear fuels (i.e., Uranium and Thorium) is difficult and expensive, yet the total energy content of the estimated world reserves of these fuels are considerably higher than those of conventional fuels, viz., coal, oil and gas. At present, energy crisis is gripping us and, therefore, nuclear energy can be successfully employed for producing low cost electrical energy on a large scale to meet the growing commercial and industrial demands.


Advantages

(i) The amount of fuel required is quite small. Therefore, there is a considerable saving in the cost of fuel transportation.

(ii) A nuclear power plant requires less space as compared to any other type of the same size. 

(iii) It has low running charges as a small amount of fuel is used for producing bulk electrical energy.

(iv) This type of plant is very economical for producing bulk electric power.

(v) It can be located near the load centres because it does not require large quantities of water and need not be near coal mines. Therefore, the cost of primary distribution is reduced.

(vi) There are large deposits of nuclear fuels available all over the world. Therefore, such plants can ensure continued supply of electrical energy for thousands of years.

(vii) It ensures reliability of operation.


Disadvantages

(i) The fuel used is expensive and is difficult to recover.

(ii) The capital cost on a nuclear plant is very high as compared to other types of plants. 

(iii) The erection and commissioning of the plant requires greater technical know-how.

(iv) The fission by-products are generally radioactive and may cause a dangerous amount of radioactive pollution.

(v) Maintenance charges are high due to lack of standardisation. Moreover, high salaries of specially trained personnel employed to handle the plant further raise the cost.

(vi) Nuclear power plants are not well suited for varying loads as the reactor does not respond to the load fluctuations efficiently.

(vii) The disposal of the by-products, which are radioactive, is a big problem. They have either to be disposed off in a deep trench or in a sea away from sea-shore.


Schematic Arrangement of Nuclear Power Station

The schematic arrangement of a nuclear power station is shown in Fig. The whole arrangement can be divided into the following main stages :

(i) Nuclear reactor

(ii) Heat exchanger

(iii) Steam turbine

(iv) Alternator


(i) Nuclear Reactor : It is an apparatus in which nuclear fuel (U²³⁵) is subjected to nuclear fission. It controls the chain reaction that starts once the fission is done. If the chain reaction is not controlled, the result will be an explosion due to the fast increase in the energy released.

Nuclear Reactor

A nuclear reactor is a cylindrical stout pressure vessel and houses fuel rods of Uranium, moderator and control rods. The fuel rods constitute the fission material and release huge amount of energy when bombarded with slow moving neutrons. The moderator consists of graphite rods which enclose the fuel rods. The moderator slows down the neutrons before they bombard the fuel rods. The control rods are of cadmium and are inserted into the reactor. Cadmium is strong neutron absorber and thus regulates the supply of neutrons for fission. When the control rods are pushed in deep enough, they absorb most of fission neutrons and hence few are available for chain reaction which, therefore, stops. However, as they are being withdrawn, more and more of these fission neutrons cause fis sion and hence the intensity of chain reaction (or heat produced) is increased. Therefore, by pulling out the control rods, power of the nuclear reactor is increased, whereas by pushing them in, it is reduced. In actual practice, the lowering or raising of control rods is accomplished automatically according to the requirement of load. The heat produced in the reac tor is removed by the coolant, generally a sodium metal. The coolant carries the heat to the heat exchanger.


(ii) Heat Exchanger : The coolant gives up heat to the heat exchanger which is utilised in raising the steam. After giving up heat, the coolant is again fed to the reactor.


(iii) Steam Turbine : The steam produced in the heat exchanger is led to the steam turbine through a valve. After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.


(iv) Alternator : The steam turbine drives the alternator which converts mechanical energy into electrical energy. The output from the alternator is delivered to the bus-bars through transformer, circuit breakers and isolators.


Selection of Site for Nuclear Power Station

The following points should be kept in view while selecting the site for a nuclear power station :


(i) Availability of Water : As sufficient water is required for cooling purposes, therefore, the plant site should be located where ample quantity of water is available, e.g., across a river or by sea-side.


(ii) Disposal of Waste : The waste produced by fission in a nuclear power station is generally radioactive which must be disposed off properly to avoid health hazards. The waste should either be buried in a deep trench or disposed off in sea quite away from the sea shore. Therefore, the site selected for such a plant should have adequate arrangement for the dis posal of radioactive waste.


(iii) Distance from Populated Areas : The site selected for a nuclear power station should be quite away from the populated areas as there is a danger of presence of radioactivity in the atmosphere near the plant. However, as a precautionary measure, a dome is used in the plant which does not allow the radioactivity to spread by wind or underground waterways.


(iv) Transportation Facilities : The site selected for a nuclear power station should have adequate facilities in order to transport the heavy equipment during erection and to facilitate the move ment of the workers employed in the plant.


From the above mentioned factors it becomes apparent that ideal choice for a nuclear power station would be near sea or river and away from thickly populated areas.


Gas Turbine Power Plant

A power station or generating station which employs gas turbine as the prime mover for the generation of electrical energy is known as a gas turbine power plant.

In a gas turbine power plant, air is used as the working fluid. The air is compressed by the compressor and is led to the combustion chamber where heat is added to air, thus raising its temperature. Heat is added to the compressed air either by burning fuel in the chamber or by the use of air heaters. The hot and high pressure air from the combustion chamber is then passed to the gas turbine where it expands and does the mechanical work. The gas turbine drives the alternator which converts mechanical energy into electrical energy.

It may be mentioned here that compressor, gas turbine and the alternator are mounted on the same shaft so that a part of mechanical power of the turbine can be utilised for the operation of the compressor. Gas turbine power plants are being used as standby plants for hydro-electric stations, as a starting plant for driving auxiliaries in power plants etc.


Advantages

(i) It is simple in design as compared to steam power station since no boilers and their auxiliaries are required.

(ii) It is much smaller in size as compared to steam power station of the same capacity. This is expected since gas turbine power plant does not require boiler, feed water arrangement etc.

(iii) The initial and operating costs are much lower than that of equivalent steam power station.

(iv) It requires comparatively less water as no condenser is used.

(v) The maintenance charges are quite small. 

(vi) Gas turbines are much simpler in construction and operation than steam turbines.

(vii) It can be started quickly form cold conditions.

(viii) There are no standby losses. However, in a steam power station, these losses occur because boiler is kept in operation even when the steam turbine is supplying no load.


Disadvantages

(i) There is a problem for starting the unit. It is because before starting the turbine, the com pressor has to be operated for which power is required from some external source. How ever, once the unit starts, the external power is not needed as the turbine itself supplies the necessary power to the compressor.

(ii) Since a greater part of power developed by the turbine is used in driving the compressor, the net output is low.

(iii) The overall efficiency of such plants is low (about 20%) because the exhaust gases from the turbine contain sufficient heat.

(iv) The temperature of combustion chamber is quite high (3000°F) so that its life is comparatively reduced.


Schematic Arrangement of Gas Turbine Power Plant

The schematic arrangement of a gas turbine power plant is shown in Fig. The main components of the plant are:

(i) Compressor

(ii) Regenerator

(iii) Combustion chamber

(iv) Gas turbine

(v) Alternator

(vi) Starting motor

Gas Turbine Power Station


(i) Compressor : The compressor used in the plant is generally of rotatory type. The air at atmospheric pressure is drawn by the compressor via the filter which removes the dust from air. The rotatory blades of the compressor push the air between stationary blades to raise its pressure. Thus air at high pressure is available at the output of the compressor.


(ii) Regenerator : A regenerator is a device which recovers heat from the exhaust gases of the turbine. The exhaust is passed through the regenerator before wasting to atmosphere. A regenerator consists of a nest of tubes contained in a shell. The compressed air from the compressor passes through the tubes on its way to the combustion chamber. In this way, compressed air is heated by the hot exhaust gases.


(iii) Combustion Chamber : The air at high pressure from the compressor is led to the combus tion chamber via the regenerator. In the combustion chamber, heat* is added to the air by burning oil. The oil is injected through the burner into the chamber at high pressure to ensure atomisation of oil and its thorough mixing with air. The result is that the chamber attains a very high temperature (about 3000°F). The combustion gases are suitably cooled to 1300°F to 1500°F and then delivered to the gas turbine.


(iv) Gas Turbine : The products of combustion consisting of a mixture of gases at high tempera ture and pressure are passed to the gas turbine. These gases in passing over the turbine blades expand and thus do the mechanical work. The temperature of the exhaust gases from the turbine is about 900 F.


(v) Alternator : The gas turbine is coupled to the alternator. The alternator converts mechanical energy of the turbine into electrical energy. The output from the alternator is given to the bus-bars through transformer, circuit breakers and isolators.


(vi) Starting Motor : Before starting the turbine, compressor has to be started. For this purpose, an electric motor is mounted on the same shaft as that of the turbine. The motor is energised by the batteries. Once the unit starts, a part of mechanical power of the turbine drives the compressor and there is no need of motor now.

Read more ...

Bottom Ad [Post Page]