Electrical Power Generation, Hydro, Thermal, Nuclear

Factor Consideration for Site Selection in HEPP
1. Water Availability
A hydroelectric power plant utilizes the potential energy of the flowing water to convert it into mechanical energy and thereby converting the mechanical energy into electrical energy with help of a generator. Water is the main fuel for such plants. Hence, selecting hydroelectric power plants near the site with adequate water supply, the good head seems to be important and necessary.
2. Storage of Water
During the dry seasons, the availability of water in the rivers decreases significantly. So, in order to have a continuous supply of water during the dry seasons, dams and water reservoirs are constructed. The study of the site’s geological and topographical conditions has to be done for the construction of such structures. The site under the selection of construction of hydroelectric plants should have a feasible geographical and geological condition.
3. Site Selection for Hydroelectric Power Plant According to the Availability of Water Head
As mentioned earlier, a hydroelectric power plant primarily utilizes the potential energy of the flowing water. The available water head is directly proportional to the potential energy. So, the more the head, more will be the potential energy available and the more will be the generation of electricity. The availability of water heads also has a considerable significance on the cost and economy of the plant. With a high water head, a given hydroelectric power plant can be constructed with higher generation capability and lower cost.
4. Distance from the Load Center
The load center is the location or the place where the electricity produced by the plants will be consumed. Hydroelectric power plants are generally constructed in places far away from the load centers. So, a high amount of cost is required to erect the transmission lines in order to transmit the power from these generating stations to the load centers. The route of these transmission lines must be selected in advance and wisely to reduce the transmission cost.
5. Accessibility of the Site
For transportation of the goods, manpower required for the construction and operation of the hydroelectric plant, the site should be accessible by road. The accessibility of the site will make the transportation of machinery and required equipment easy.
6. Pollution of Water Resource
The polluted water resource consists of various foreign particles. The foreign particles in the water will corrode the hydroelectric power plant structure such as the blades of the turbine. So, the site selection of the hydroelectric power plant must be done where there is the availability of good quality water.
7. Sedimentation
The flowing water of the river carries fine sand, clay, or other material (also called silt). This silt gets deposited in the reservoir and as time progresses, more and more silt deposition occurs, which ultimately reduces the reservoir’s storage capacity. Also, these particles strike with

1. Factor Consideration for Site Selection in HEPP 1. Water Availability A hydroelectric power plant utilizes the potential energy of the flowing water to convert it into mechanical energy and thereby converting the mechanical energy into electrical energy with help of a  generator. Water is the main fuel for such plants. Hence, selecting hydroelectric power plants near the site with adequate water supply, the good head seems to be important and necessary. 2. Storage of Water During the dry seasons, the availability of water in the rivers decreases significantly. So, in order to have a continuous supply of water during the dry seasons, dams and water reservoirs are constructed. The study of the site’s geological and topographical conditions has to be done for the construction of such structures. The site under the selection of construction of hydroelectric plants should have a feasible geographical and geological condition.

2. 3. Site Selection for Hydroelectric Power Plant According to the Availability of Water Head As mentioned earlier, a hydroelectric power plant primarily utilizes the potential energy of the flowing water. The available water head is directly proportional to the potential energy. So, the more the head, more will be the potential energy available and the more will be the generation of electricity. The availability of water heads also has a considerable significance on the cost and economy of the plant. With a high water head, a given hydroelectric power plant can be constructed with higher generation capability and lower cost. 4. Distance from the Load Center The load center is the location or the place where the electricity produced by the plants will be consumed. Hydroelectric power plants are generally constructed in places far away from the load centers. So, a high amount of cost is required to erect the transmission lines in order to transmit the power from these generating stations to the load centers. The route of these transmission lines must be selected in advance and wisely to reduce the transmission cost.

3. 5. Accessibility of the Site For transportation of the goods, manpower required for the construction and operation of the hydroelectric plant, the site should be accessible by road. The accessibility of the site will make the transportation of machinery and required equipment easy . 6. Pollution of Water Resource The polluted water resource consists of various foreign particles. The foreign particles in the water will corrode the hydroelectric power plant structure such as the blades of the turbine. So, the site selection of the hydroelectric power plant must be done where there is the availability of good quality water. 7. Sedimentation The flowing water of the river carries fine sand, clay, or other material (also called silt). This silt gets deposited in the reservoir and as time progresses, more and more silt deposition occurs, which ultimately reduces the reservoir’s storage capacity. Also, these particles strike with the blades of the turbine thereby corroding the blades .

4. 8. Large Catchment Area For a continuous supply of water during dry seasons, dams and reservoirs are constructed in hydroelectric power plants. The catchment area of the reservoir should be large enough so that the water level in the reservoir will not fall below the minimum level during the dry season. 9. Site Selection for Hydroelectric Power Plant According to the Availability of Land A large area of land is required for the construction of the hydroelectric power plants and build its constituent structures such as a powerhouse etc. For economic consideration, the land should be available at a cheap cost and should have a proper geological condition in order to withstand the weight of the structures and machinery of the power plants.

5. The main components are •  Water reservoir • Dam • Spillway • Gate • Pressure tunnel • Surge tank • Penstock • Water turbine • Draft tube • Tail race level • Powerhouse

6. In a reservoir the water collected from the catchment area during the rainy season is stored behind a dam. Catchment area gets its water from rains and streams. Continuous availability of water is a basic necessity for a hydroelectric power plant. The level of the water surface in the reservoir is called the Headwater level. The water head available for power generation depends on the reservoir height. the purpose of the dam is to store the water and to regulate the out going flow of water. The dam helps to store all the incoming water. It also helps to increase the head of the water. In order to generate a required quantity of power, it is necessary that a sufficient head is available. Excess accumulation of water endangers the stability of dam construction. Also in order to avoid the overflow of water out of the dam especially during rainy seasons spillways are provided. This prevents the rise of the water level in the dam. Spillways are passages that allow the excess water to flow to a different storage area away from the dam.

7. 4 ) A gate is used to regulate or control the flow of water from the dam. 5 ) It is a passage that carries water from the reservoir to the surge tank. 6 ) A surge tank is a small reservoir or tank in which the water level rises or falls due to sudden changes in pressure. There may a sudden increase of pressure in the penstock pipe due to sudden backflow of water, as the load on the turbine is reduced. This sudden rise of pressure in the penstock pipe is known as water hammer. 7 ) Penstock pipe is used to bring water from the dam to the hydraulic turbine. Penstock pipes are made up of steel or reinforced concrete. The turbine is installed at a lower level from the dam. Penstock is provided with a gate valve at the inlet to completely close the water supply. It has a control valve to control the water flow rate into the turbine. Water turbine or hydraulic turbine (Prime mover): The hydraulic turbine converts the energy of water into mechanical energy. The mechanical energy (rotation) available on the turbine shaft is coupled to the shaft of an electric generator and electricity is produced. The water after performing the work on the turbine blade is discharged through the draft tube .

8. 8) Draft tube is connected to the outlet of the turbine. It converts the kinetic energy available in the water into pressure energy in the diverging portion. Thus, it maintains a pressure of just above the atmospheric at the end of the draft tube to move the water into a tailrace. Water from the tailrace is released for irrigation purposes. 9) Tailrace is a water path to lead the water discharged from the turbine to the river or canal. The water held in the tailrace is called the Tailrace water level . 10) The powerhouse accommodates the water turbine, generator, transformer, and control room. As the water rushes through the turbine, it spins the turbine shaft, which is coupled to the electric generator. The generator has a rotating electromagnet called a rotor and a stationary part called a stator. The rotor creates a magnetic field that produces an electric charge in the stator. The charge is transmitted as electricity. The step-up transformer increases the voltage of the current coming from the stator. The electricity is distributed through power lines.

9. Classification of Hydroelectric power plant Hydroelectric power plants are usually classified according to the available of head of water. • High head power plants •Medium head power plants • Low head power plants High head power plants:  When the operating head of water exceeds 70 meters, the plant is known as High head power plant. Pelton wheel turbine is the prime mover used. Medium head power plants:  When the water ranges from 15 to 70 meters, then the power plant is known as a Medium head power plant. It uses Francis Turbine. Low head power plants:  When the head is less than 15 meters, the plant is named as Low head power plant. It uses Francis or Kaplan turbine as the prime mover.

10. Advantages of hydroelectric power plant The water source is perennially available. No fuel is required to be burnt to generate electricity. It is aptly termed as ‘the white coal’. Water passes through turbines to produce work and downstream its utility remains undiminished for irrigation of farms and quenching the thirst of people in the vicinity. The running costs of hydropower installations are very low as compared to thermal or nuclear power stations. In thermal stations, besides the cost of fuel, one has to take into account the transportation cost of the fuel also. There is no problem with regard to the disposal of ash as in a thermal station. The problem of emission of polluting gases and particulates to the atmosphere also does not exist. Hydropower does not produce any greenhouse effect, cause the pernicious acid rain and emit obnoxious NO. The hydraulic turbine can be switched on and off in a very short time. In a thermal or nuclear power plant the steam turbine is put on turning gear for about two days during start-up and shut-down.

11. The hydraulic power plant is relatively simple in concept and self-contained in operation. Its system reliability is much greater than that of other power plants. Modern hydropower equipment has a greater life expectancy and can easily last 50 years or more. This can be compared with an effective life of about 30 years of a thermal or nuclear station. Due to its great ease of taking up and throwing off the load, hydropower can be used as the ideal spinning reserve in a system mix of thermal, hydro, and nuclear power stations. Modern hydro-generators give high efficiency over a considerable range of load. This helps in improving the system efficiency. Hydro-plants provide ancillary benefits like irrigation, flood control, afforestation, navigation, and aqua-culture. Being simple in design and operation, the hydro-plants do not require highly skilled workers. Manpower requirement is also low.

12. Disadvantages of Water Power Hydro-power projects are capital-intensive with a low rate of return. The annual interest of this capital cost is a large part of the annual cost of hydropower installations. The gestation period of hydro projects is quite large. The gap between the foundation and completion of a project may extend from ten to fifteen years. Power generation is dependent on the quantity of water available, which may vary from season to season and year to year. If the rainfall is in time and adequate, then only the satisfactory operation of the plant can be expected. Such plants are often far away from the load center and require long transmission lines to deliver power. Thus the cost of transmission lines and losses in them are more. Large hydro-plants disturb the ecology of the area, by way of deforestation, destroying vegetation and uprooting people. Strong public opinion against. The erection of such plants is a deterrent factor. The emphasis is now more on small, mini and micro hydel stations.

13. Types and Size of Hydro Electric Power Plants There are three types of hydropower facilities: I mpoundment Types. D iversion or Run-of-River Type Pumped S torage Type. Some hydropower plants use dams and some do not. Large Hydro Power Plant Although definitions vary, DOE defines large hydropower plants as facilities that have a capacity of more than 30 megawatts (MW). Small Hydro Power Plant Although definitions vary, DOE defines small hydropower plants as projects that generate between 100 kilowatts and 10 MW. Micro Hydro Power Plant A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro hydroelectric power system can produce enough electricity for a single home, farm, ranch, or village .

14. Prime Mover: Prime movers rotate around themselves and assist in the generation of energy for others. Because one of the characteristics of the prime mover is that the mechanical energy is taken from the natural mover, it cannot be called a prime mover if you turn it around and help others generate energy. Steam turbines, gas turbines, water turbines, and inbound and outbound engines are examples. In a power plant, the prime mover normally rotates the shaft of the AC generator. An electric motor, on the other hand, turns various pumps, lathe machines, boring bars, and other items, but it is not a primary mover because it is powered by electricity rather than natural energy . “A prime mover is a device that takes energy from natural sources and turns it into mechanical energy .” The literal meaning of the term “prime mover” is a primary source of power. It refers to all of the machinery that generates power in order to do various mechanical activities. It is a set of devices that convert energy from thermal, electrical, or pressure to mechanical form, which can subsequently be used to perform mechanical work in a variety of ways. Turbines and engines are two examples of such machines.

15. Types of Turbines based on energy exchanged between the water and the machine Impulse Turbines: Pelton Turgo & Cross-Flow Reaction Turbines: Francis, Kaplan & Deriaz 2) Types of Turbine based on Fluid Direction Through the Machine Radial Flow Turbine i.e. Inward Radial Flow Turbine Tangential or Peripheral Flow Turbine Axial Flow Turbines Mixed Flow Turbines 3) Types of Turbines based upon the Hydraulic Operating range Low Head Turbines Medium Head Turbines High Head Turbines 4) Types of Turbines based upon Specific Speed Low Specific Speed Turbine Medium Specific Speed Turbine High Specific Speed Turbine 5) Introduction of Widely Used Industrial Water Turbines and their classification Pelton Cross-Flow Francis Kaplan

16. Impulse Turbines If the turbine wheel is driven by the kinetic energy of the fluid that strikes the turbine blades through the nozzle or otherwise, the turbine is known as an impulse turbine. In these types of turbines, a set of rotating machinery operates at atmospheric pressure. Impulse turbines are usually suitable for high head and low flow rates. Reaction Turbines If the sum of potential energy and kinetic energy of water which are due to the pressure and velocity, respectively cause the turbine blades to rotate, the turbine is classified as a reaction turbine. In these types of turbines, the entire turbine is immersed in water and changes in water pressure along with the kinetic energy of the water cause power exchange. Applications of reaction turbines are usually at lower heads and higher flow rates than impulse turbines.

17. Radial Flow Turbines: If the flow in the runner moves radially, the turbine is radial flow. These turbines are divided into two types: Inward radial flow and outward radial flow. Francis turbines can be in the form of radial flow turbines. Tangential or Peripheral Flow Turbines In these turbines, water flows in a tangential direction to the runner. Pelton belongs to this category of turbines . Axial Flow Turbines In this type of turbine, the fluid flows parallel to the turbine shaft (turbine axis). Kaplan is one of these turbines. Mixed Flow Turbines  A turbine in which the flow enters the turbine radially and leaves it axially is a mixed flow turbine, like modern Francis turbines .

18. Low Head Turbines  Hydraulic turbines operating in the head range of fewer than 45 meters are considered low-head turbines. Kaplan turbine is one of these turbines. If the head is less than 3 meters, it is considered an ultra-low head. Medium Head Turbines The working range for heads of 45 to 250 meters is known as medium heads. Francis turbines generally operate in such conditions. High Head Turbines  Turbines with heads higher than 250 meters are known as high-head turbines, Like the Pelton Turbine.

19. Low Specific Speed Turbine  The values between 1 and 10 are low specific speeds. Impulse turbines operate in this range. For example, the Pelton turbine usually operates at a specific speed of about 4. Medium Specific Speed Turbine  Turbines that operate in the specific speed range of 10 to 100, such as Francis, have a medium specific speed. High Specific Speed Turbine  Specific speeds above 100 are considered high values. Kaplan turbine works at a high specific speed.

20. Hydropower Turbine Type Typical Site Characteristics Archimedean Screw Low heads (1.5 – 5 metres )Medium to high flows (1 to 20 m 3 /s ). For higher flows multiple screws are used. Crossflow turbine Low to medium heads (2 – 40 metres ), Low to medium flows (0.1 – 5 m 3 /s) Kaplan turbine Low to medium heads (1.5 – 20 metres ), Medium to high flows (3 m 3 /s – 30 m 3 /s), For higher flows multiple turbines can be used. Pelton/ Turgo turbine High heads (greater than 25 metres ), Lower flows (0.01 m 3 /s – 0.5 m 3 /s) Waterwheels Low heads (1 – 5 metres ) – though turbines often more appropriate for higher heads, Medium flows (0.3 – 1.5 m 3 /s) Francis turbines No longer commonly used except in very large storage hydropower systems, though lots of older, smaller turbines are in existence and can be restored. For older turbines : Low to medium heads (1.5 – 20 metres ), Medium flows (0.5 – 4 m 3 /s ).

21. The  Francis turbine  was developed in 1848 by  by British-American engineer James B. Francis and it’s the most commonly used hydraulic turbine. A centripetal flow turbine, as the water reaches the impeller through a spiral conduit, adjustable palettes attached to the stationary part direct the flow so that it strikes the impeller blades. It’s used for medium height differences, from 10 up to 400 meters, and from 2 to 100 cubic meters per second  of water capacity. FRANCIS Turbine

22. The  Pelton turbine  was introduced in 1879 by American carpenter and inventor Lester Allan Pelton. Its principle of operation recalls the classic paddle wheel of old watermills redesigned to increase efficiency. The water is channeled into a penstock with a nozzle at the end. This bottleneck directs the water by increasing its velocity. The jet of water that comes out of the nozzle strikes the impeller’s spoon-shaped blades. In order to achieve higher velocity, Pelton turbines are used for  large height difference s, from 300 up to 1,400 meters, and  less than 50 cubic meters per second  of water capacity. PELTON Turbine

23. PELTON Turbine KAPLAN Turbine

24. The  Kaplan turbine,  invented in 1913 by Austrian professor Viktor Kaplan, follows the principle of operation of a boat propeller. Kaplan turbines are  axial.  The flow of water that turns the propeller blades enters and exits axially in relation to the propeller’s axis of rotation. As the blades’ angle of incidence is adjustable, this type of turbine ensures optimum performance with  small height differences , but also with  large variations in capacity  (200 cubic meters per second and above). KAPLAN Turbine

25. Q. 1 A hydro-electric generating station is supplied from a reservoir of capacity 5 × 10 ⁶ cubic metres at a head of 200 metres . Find the total energy available in kWh if the overall efficiency is 75 %. Sol. Weight of water available is W = Volume of water × density W = (5 × 10 ⁶ ) × (1000) (mass of 1m³ of water is 1000 kg) W = 5 × 10 ⁹ kg = 5 × 10 ⁹ × 9·81 N Electrical energy available = W × H × η overall = (5 × 10 ⁹ × 9·81) × (200) × (0·75) watt sec 9.81 200 1000) kWh = 2.044 x 10 ⁶ kWh  

26. Q. 2 It has been estimated that a minimum run off of approximately 94 m³ /sec will be available at a hydraulic project with a head of 39 m. Determine ( i ) firm capacity (ii) yearly gross output. Assume the efficiency of the plant to be 80%. Sol. Weight of water available, W = 94 × 1000 = 94000 kg/sec, Water head, H = 39 m, Work done/sec = W × H = 94000 × 9·81 × 39 watts = 35, 963 × 10³ W = 35, 963 kW This is gross plant capacity. Firm capacity = Plant efficiency × Gross plant capacity = 0·80 × 35,963 = 28,770 kW Yearly gross output = Firm capacity × Hours in a year = 28,770 × 8760 = 252 × 10 ⁶ kWh.

27. Q. 3 Water for a hydro-electric station is obtained from a reservoir with a head of 100 metres . Calculate the electrical energy generated per hour per cubic metre of water if the hydraulic efficiency be 0·86 and electrical efficiency 0·92. Sol. Water head, H = 100 m ; Discharge , Q = 1 m³ /sec ; η overall = 0·86 × 0·92 = 0·79 Wt. of water available/sec, W = Q × 1000 × 9·81 = 9810 N, Power Produced = W × H × η overall = 9810 × 100 × 0·79 watts = 775 × 10³ watts = 775 kW, ∴ Energy generated/hour = 775 × 1 = 775 kWh

28. Selection of Site for Thermal Power Plant Supply of Fuel (Coal) The thermal plant should be located near the coal mines so that the transportation cost is minimum. Although, if the thermal power plant is to be installed at a place where coal is not available near the site, then care should be taken that adequate facilities exist for the transportation of coal. Water Availability Since in a thermal power plant, huge amount of water is required for the operation. Hence, a thermal power plant should be located near a river or canal to ensure the continuous supply of water. Transportation Facilities As a modern thermal power plant requires transportation of material (ex. coal) and machinery. Hence, the power plant should be well connected to the other parts of the country by rail, road, etc. so that adequate transportation facilities are available .

29. Type and Cost of Land The thermal power should be located at a place where land is cheap and the further extension is possible. As in a thermal power plant, heavy equipment are to be installed, therefore the bearing capacity of the land should be adequate . Near to the Load Centres The thermal power plant should be located near to the centres of the load, so that the transmission cost is reduced. It is more important if DC supply system is adopted rather than AC supply system . Distance from the Populated Area The thermal power plant should be located at a considerable distance from the populated area. Because, a large amount of coal is burnt in a thermal power station, which produces smoke and fumes that pollute the surrounding environment and may have adverse effects on the health.

30.   Availability of labor: Large men power is needed during the construction of plant. Therefore, labor should be available near the construction site at cheap cost.  Ash disposal facilities:   Huge amount of hot ash comes out of the coal based thermal power plants which is hazardous to human and plant life. It is corrosive and polluting in nature. Therefore, there must be sufficient space and facilities available to dispose of large quantity of ash . Future expansion:   The site selected should be such that it allows economic extensions of the plant with the estimated growth of load . Pollution:   The thermal power plants should be located away so as to avoid any nuisance from smoke, fly ash, noise and heat discharged from the plant. Away from air fields:   The power plant should be located away from the densely populated and industrial area by taking into account the aerial warfare.

31. Layout of Thermal Power Plant: In the thermal power plant, the electrical energy is transformed from heat energy. Heat energy can be derived from different heat sources like; coal, diesel, biofuel, solar energy, nuclear energy, etc. The power plant that uses coal to generate heat is known as the thermal power plant. The thermal power plant is a conventional power plant. Sometimes, the thermal power plant is also known as a steal-turbine power plant or coal power plant. A thermal Power Plant Works on the Rankine Cycle.

32. Components of Thermal Power Plant In a thermal power plant, various components are used in the cycle. Here we have listed, main components of the thermal power plant. Boiler Turbine Super-heater Condenser Economizer Feed water pump Alternator Chimney Cooling tower

33. Boiler The pulverized coal is fed to the boiler with preheated air. The boiler is used to produce high-pressure steam. The boiler in the thermal power plant is used to convert the chemical energy of coal into thermal energy or heat energy. During the combustion of coal, a high temperature is produced inside the boiler. This temperature is high enough to convert water into steam.    The size of the boiler depends on the amount of heat required for the thermal power plant. And there are several types of boiler used in a thermal power plant like; Haycock and wagon top boiler, firetube boiler, Cylindrical fire-tube boiler, water-tube boiler, etc . Turbine The high pressure and high-temperature steam are fed to the boiler. This superheated steam is a strike on the turbine blade. And the turbine starts rotating. The turbine is a mechanical device that is used to convert the heat energy of steam into rotational energy or kinetic energy. The turbine is mechanically coupled with an alternator via a shaft. When the steam release from the turbine, the temperature and pressure is reduced. And this steam is passed to the condenser .

34. Super-heater In a steam turbine, super-heated steam is used to rotate the turbine. The wet and saturated steam is supplied to the super-heater. And it is a device that converts it into dry and superheated steam. The super-heater’s temperature is the highest among all components of the thermal power plant. In the thermal power plant; there are three types of superheaters used; convection, radiant, and separately fired. The superheater is used to increase the temperature of the steam generated from the boiler. This will increase the thermal energy of the steam . Condenser When the steam release from the turbine, the temperature and pressure is decreased. The exhaust steam of the turbine reuse in the cycle.  To increase the turbine efficiency, we need to condense this steam to maintain a proper vacuum. The condenser decreases the operating pressure. So, the vacuum is increased. And this will increase the volume of steam that results in more amount of work available at the turbine. And due to this, the plant efficiency will increase with the increase in turbine output .

35. Economizer The economizer is a heat exchanger device that is used to reduce energy consumption. In the boiler, flue gases are exhausted into the atmosphere. These gases have a high temperature. So, the economizer uses the heat of flue gases to heat the water.   The water release from the condenser is again used in the cycle. With the help of a feedwater pump, this water is transferred to the economizer. An economizer uses the heat of flue gases to increase the temperature of the water. The economizer uses the waste heat of flue gases. Hence, it is used to increase the efficiency of the entire cycle . Feedwater pump A feedwater pump is used to supply water into the boiler. The water may be from the condenser or freshwater. This pump is used to pressure the water. Generally, the feedwater pump is a centrifugal type or positive displacement type of pump. Alternator The alternator and turbine are connected on the same shaft. The turbine rotates with the flow of steam and the turbine rotates. The rotor of the alternator rotates and generates electrical energy. Therefore, the alternator is a device that converts kinetic energy or rotational energy into electrical energy .

36. Chimney  In most of the thermal power plants, coal is used as fuel. During the combustion of coal, the flue gases are generated in the boiler. The chimney provides a path to the flue gas and exhaust to the atmosphere . The chimney works based on the natural draft and stack effect. The hot air is light in weight and it goes up. The height of the chimney is high. The taller height, the more draft or draught is created . Cooling tower As the name suggests, the cooling tower is used to reject waste heat in the atmosphere. Different heat transfer methods are used in the cooling tower. The heat of the water evaporates into the atmosphere. And remains cool water that further use in the cycle.  The condenser converted steam into water. And the water that comes from the condenser is supplied to the cooling tower. Generally, forced flow cooling towers are used in the thermal power plant. And the air is circulated from bottom to top of the tower .

37. Efficiency of Thermal Power Plant In a thermal power plant, the electrical energy is generated from two energy conversion. The chemical energy of coal is converted into thermal energy of heat energy. After the thermal energy is converted into kinetic energy or mechanical energy. And finally, the mechanical energy is converted into electrical energy. So, due to the number of energy conversions, the efficiency of thermal power plants is very low around 20-29%. The efficiency of a thermal power plant is also depending on the size of the plant and the quality of coal. In a thermal power plant, the heat energy is lost in the condenser. There are two types of efficiency in thermal power plants . Thermal efficiency: Thermal efficiency is defined as the ratio of heat equivalent mechanical energy available at the turbine to the heat energy available at the combustion of coal in the boiler . The thermal efficiency of the thermal power plants is 30% approx. Most of the heat energy (approx. 50%) is wasted in the condenser. The rest of the heat energy is wasted in the flue gases, ash etc.  

38. Overall efficiency: The overall efficiency is defined as the ratio of heat equivalent of electrical output to the heat of combustion of coal . The overall efficiency includes the losses that occur at all stages of a cycle. It also includes the efficiency of an alternator. The overall efficiency of a thermal power plant depends on its size and rating of a power plant in MW. Lower the capacity, lower the efficiency .   Installed Plant Capacity Average Overall Thermal Efficiency Upto 1 MW 4 % 1 MW to 10 MW 12 % 10 MW to 50 MW 16 % 50 MW to 100 MW 24 % More than 100 MW 27 %

39. Advantages The advantages of a thermal power plant are listed below. The initial cost of this plant is less compared to other power plants. The cost of fuel is less. Running cost is less compared to a diesel power plant. It is less depending on the seasons. Most thermal power plants running throughout the year. The coal is easily available and easy to transport in bulk quantity. Compared to hydropower plants, it requires less space. The maintenance of thermal power plants is less compared to the other power plants. This plant can be installed in any location where adequate transport facility and bulk water is available .

40. Disadvantages The disadvantages of the thermal power plants are as listed below. The thermal power plant uses coal as a fuel. And it is a conventional source of energy. So, we need to use less conventional sources for a better future. Coal is a commodity. So, the price of coal depends on the commodity market and it varies day by day. The running cost of this plant is high compared to a hydro power plant. It produces ash as a by-product. And it is necessary to dispose of ash without harming the environment. Due to the combustion of coal, flew gases and smoke are released into the atmosphere. Therefore, this plant is not environmentally friendly. It produces harmful noise in the surrounded area. So, it affects workers and the people leave nearby areas. The overall efficiency of the thermal power plant is very less. It is approx. 30%.

41. Q. 1 A Steam Power Station of 100 MW capacity uses coal of calorific value 6400 kcal/kg. The thermal efficiency of the station is 30% and electrical generation efficiency is 92%. Find the coal require per hour when plant is working at full load. Sol: Energy output in one hour = 100 x 10 ⁶ x 3600 = 36 x 10¹⁰ Watt sec Energy input in one hour = (36 x 10 ¹ ⁰) / 0.3 x 0.92 = 130.43 x 10¹⁰ 1 Watt Sec = 1 Joule Energy Input =130.43 x 10 ¹ ⁰ Joules = 130.43 x 10¹ ⁰ x 0.239 Calories = 31.17 x 10¹ ⁰ Calories = 31.17 x 10 ⁷ kcal Coal Required = 31.17 x 10⁷ / 6400 = 48703.1 kg per hour

42. Nuclear Power Plants In a nuclear power plant, many of the components are similar to those in a fossil-fueled plant, except that the steam boiler is replaced by a Nuclear Steam Supply System (NSSS). The NSSS consists of a nuclear reactor and all of the components necessary to produce high pressure steam, which will be used to turn the turbine for the electrical generator . Principle of Nuclear Power Generation Nuclear Fission Nuclear fission is a nuclear reaction in which the nucleus of an atom is bombarded with low energy neutrons which split the nucleus into smaller nuclei. An abundant amount of energy is released in this process. Nuclear fission reactions are used in nuclear power reactors since it is easy to control and produces large amounts of energy . When uranium-235 is bombarded with slow-moving neutrons, the heavy nucleus of the uranium splits and produces krypton-89 and barium-144 with the emission of three neutrons.

43. Nuclear Fusion Nuclear Fusion is a reaction that occurs when two or more atoms combine together to form to a single heavier nucleous . An enormous amount of energy is released in this process, much greater than the energy released during the nuclear fission reaction . Fusion occurs in the sun where the atoms of (isotopes of hydrogen, Hydrogen-3, and Hydrogen-2) Deuterium and Tritium combine in a huge pressure atmosphere with extremely high temperatures to produce an output in the form of a neutron and an isotope of Helium. Also, the amount of energy released in fusion is way greater than the energy produced by fission.

44. Nuclear Fission Nuclear Fusion When the nucleus of an atom splits into lighter nuclei through a nuclear reaction the process is termed nuclear fission. Nuclear fusion is a reaction through which two or more light nuclei collide with each other to form a heavier nucleus. When each atom split, a tremendous amount of energy is released The energy released during nuclear fusion is several times greater than the energy released during nuclear fusion. Fission reactions do not occur in nature naturally Fusion reactions occur in stars and the sun Little energy is needed to split an atom in a fission reaction High energy is needed to bring fuse two or more atoms together in a fusion reaction Atomic bomb works on the principle of nuclear fission Hydrogen bomb works on the principle of a nuclear fusion bomb.

45. Factors for Site Selection Availability of Water Supply Distance from a P opulated Area Township for Staff Availability of space for Disposal of Waste Nearness to the Load Centre Transportation Facilities Accessibility Type of Land

46. Availability of water supply Inside the reactor of the nuclear power plant, water is used as a moderator for cooling the heat generated by fission reaction and for other cooling purposes also so the requirement of water in a nuclear power plant is very high (that is twice the requirement of water in thermal power plant) so the plant must be located in the area where availability of water is plenty that is near the river, lake or by the seaside. Distance from a populated area As we know that for fuel we use for the nuclear power plant is radioactive materials like uranium, thorium, etc., and though we take all the safety measures, we cannot make all the environment around the power plant radioactive free, so there should be a reasonable distance between the nuclear power plant and the nearest populated area from the safety point of view. However, as a precautionary measure, a dome structure is used in the plant, which does not allow radioactivity to spread through underground waterways or wind. Also, it is highly undesirable to choose a site adjacent to the chemical industry, oil refineries, hospitals, or schools. So the power plant must be located away from a populated area.

47. Transportation facilities We have studied in a steam power plant that we need railway lines to transfer the tones of coal. Still, in the case of nuclear power plants, the fuel requirement is very less. Hence, we require the transportation facility during the nuclear power plant construction only. In contrast, we may not require all the transportation facilities like steam power plants for normal working days . Nearness to the load center The transmission cost of electricity depends upon the nearness of the plant to the load center more the distance more the transmission cost; hence practically, a power plant should always be located near the load center, but we cannot place nuclear power plant that closes to the load center so for this authorities have to erect a nuclear power plant by taking all safety consideration.

48. Availability of space for disposal of waste Like the other power plants, we cannot dispose of the wastage of nuclear power plant, after all the use of fuel to generate electricity. Even after decaying that fuel, it still remains radioactive and hazardous for us. So while selecting the location of the nuclear power plant, one must think of the availability of space for the disposal. Accessibility There should be reasonable accessibility for plant personnel, haul equipment, and dispatch and receive heavily shielded radioactive materials. Type of land Reactors used for a nuclear power plant may weigh 100,000 tonnes and impose bearing pressure around 50 tones/m 2 . While selecting the site for nuclear power plant, one should select the land having high sustainability to withstand all the burdens of reactors and other heavy types of machinery.

49. General Layout of Nuclear Power Plant

50. Main Components of Nuclear Power Plants Moderators: In any chain reaction, the neutrons produced are fast moving neutrons. These are less effective in causing fission of U 235  and they try to escape from the reactor. It is thus implicit that speed of these neutrons must be reduced if their effectiveness is carrying out fission is to be increased. This is done by making these neutrons collide with lighter nuclei of other materials, which does not absorb these neutrons but simply scatter them. Each collision causes loss of energy and thus the speed of neutrons is reduced. Such a material is called a ‘Moderator ’. The neutrons thus slowed down are easily captured by the fuel element at the chain reaction proceeds slowly . Reflectors: Some of the neutrons produced during fission will be partly absorbed by the fuel elements, moderator, coolant and other materials. The remaining neutrons will try to escape from the reactor and will be lost. Such losses are minimized by surrounding (lining) the reactor core with a material called a reflector which will reflect the neutrons back to the core. They improve the neutron economy. Example: Graphite, Beryllium.

51. Shielding: During Nuclear fission  α , β , γ - particles and neutrons are also produced. They are harmful to human life. Therefore it is necessary to shield the reactor with thick layers of lead, or concrete to protect both the operating personnel as well as environment from radiation hazards . Cladding: In order to prevent the contamination of the coolant by fission products, the fuel element is covered with a protective coating. This is known as cladding . Control rods are used to control the reaction to prevent it from becoming violent. They control the reaction by absorbing neutrons. These rods are made of boron or cadmium. Whenever the reaction needs to be stopped, the rods are fully inserted and placed against their seats and when the reaction is to be started the rods are pulled out . Nuclear Reactor: A nuclear reactor may be regarded as a substitute for the boiler fire box of a steam power plant. Heat is produced in the reactor due to nuclear fission of the fuel U 235 . The heat liberated in the reactor is taken up by the coolant circulating through the core. Hot coolant leaves the reactor at top and flows into the steam generator (boiler).

52. Coolant : The main purpose of the coolant in the reactor is to transfer the heat produced inside the reactor. The same heat carried by the coolant is used in the heat exchanger for further utilization in the power generation . Some of the desirable properties of good coolant are listed below: It must not absorb the neutrons. It must have high chemical and radiation stability. It must be non-corrosive. It must have high boiling point (if liquid) and low melting point (if solid ). It must be non-oxidizing and non-toxic . It must also have high density, low viscosity, high conductivity and high specific heat. These properties are essential for better heat transfer and low pumping power . The above-mentioned 5 properties are essential to keep the reactor core in safe condition as well as for the better functioning of the content . The water, heavy water, gas (He, CO2), a metal in liquid form (Na) and an organic liquid are used as coolants . The coolant not only carries large amounts of heat from the core but also keeps the fuel assemblies at a safe temperature to avoid their melting and destruction.

53. Radiation hazards and Shielding: The reactor is a source of intense radioactivity. These radiations are very harmful to human life. It requires strong control to ensure that this radioactivity is not released into the atmosphere to avoid atmospheric pollution . Steam Generator: The steam generator is fed with feed water which is converted into steam by the heat of the hot coolant. The purpose of the coolant is to transfer the heat generated in the reactor core and use it for steam generation. Ordinary water or heavy water is a common coolant . Turbine: The steam produced in the steam generator is passed to the turbine and work is done by the expansion of steam in the turbine . Coolant Pump & Feed Pump: The steam from the turbine flows to the condenser where cooling water is circulated. Coolant pump and feed pump are provided to maintain the flow of coolant and feed water respectively.

54. Advantages of nuclear power plant It can be easily adopted where water and coal resources are not available. The nuclear power plant requires very small quantity of fuel. Hence fuel transportation cost is less. Space requirement is less compared to other power plants of equal capacity. It is not affected by adverse weather conditions . Fuel storage facilities are not needed as in the case of the thermal power plant. Nuclear power plants will converse the fossils fuels (coal, petroleum) for other energy needs. Number of workmen required at nuclear plant is far less than thermal plant. It does not require large quantity of water . Disadvantages Radioactive wastes, if not disposed of carefully, have adverse effect on the health of workmen and the population surrounding the plant .   It is not suitable for varying load condition . It requires well-trained personnel .   It requires high initial cost compared to hydro or thermal power plants.

55. Types of Nuclear Reactor Pressurized Water Reactor (PWR) Boiling Water Reactor (BWR) Pressurized Heavy Water Reactor(PHWR) CANdium Deuterium Uranium (CANDU) Reactor Advanced Gas Cooled (AGC) Reactors Light Water Graphite Moderated Reactor (LWGMR) Fast Neutron Reactors (FNR)

56. This is the most common type, with about 300 operable reactors for power generation and several hundred more employed for naval propulsion. The design of PWRs originated as a submarine power plant. PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine . A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tones of uranium. Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressurizer (see diagram). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit. The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit . Pressurized Water Reactor (PWR)

57. This type of reactor has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there. BWR units can operate in load-following mode more readily than PWRs. The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived, so the turbine hall can be entered soon after the reactor is shut down. A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that more steam in the top part reduces moderation Boiling Water Reactor (BWR)

58. The PHWR reactor has been developed since the 1950s in Canada as the CANDU, and from 1980s also in India. PHWRs generally use natural uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D 2 O ).  The PHWR produces more energy per kilogram of mined uranium than other designs, but also produces a much larger amount of used fuel per unit output. The Moderator is in a large  tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure (about 100 times atmospheric pressure) in the primary cooling circuit, typically reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit. It is also less costly to build than designs with a large pressure vessel, but the tubes have not proved as durable. Pressurized Heavy Water Reactor

59. A CANDU fuel assembly consists of a bundle of 37 half meter long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat (though this circuit is not shown on the diagram above). Newer PHWR designs such as the Advanced Candu Reactor (ACR) have light water cooling and slightly-enriched fuel. CANDU reactors can accept a variety of fuels. They may be run on recycled uranium from reprocessing LWR used fuel, or a blend of this and depleted uranium left over from enrichment plants. About 4000 MWe of PWR might then fuel 1000 MWe of CANDU capacity, with addition of depleted uranium. Thorium may also be used in fuel. CANDU Reactor

60. These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide as primary coolant. The fuel is uranium oxide pellets, enriched to 2.5 - 3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel (hence 'integral' design). Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant. The high temperature gives it a high thermal efficiency – about 41%. Refuelling can be on-load . The AGR was developed from the Magnox reactor. Magnox reactors were also graphite moderated and CO 2  cooled, used natural uranium fuel in metal form, and water as secondary coolant. The UK's last Magnox reactor closed at the end of 2015.  Advanced Gas Cooled Reactor

61. Light Water G raphite Moderated Reactor: The main LWGR design is the RBMK, a Soviet design, developed from plutonium production reactors. It employs long (7 meter) vertical pressure tubes running through graphite moderator, and is cooled by water, which is allowed to boil in the core at 290°C and at about 6.9 MPa, much as in a BWR. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 meters long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorption without inhibiting the fission reaction, and a positive feedback problem can arise, which is why they have never been built outside the Soviet Union. See appendix on RBMK Reactors for further information . Fast Neutron Reactor: Some reactors do not have a moderator and utilize fast neutrons, generating power from plutonium while making more of it from the U-238 isotope in or around the fuel. While they get more than 60 times as much energy from the original uranium compared with normal reactors, they are expensive to build. Further development of them is likely in the next decade, and the main designs expected to be built in two decades are FNRs. If they are configured to produce more fissile material (plutonium) than they consume they are called fast breeder reactors (FBR). See also pages on Fast Neutron Reactors and Small Nuclear Power Reactors papers.

62. Reactor Type Fuel Coolant Moderator Pressurized Water Reactor (PWR) Enriched Uranium Water Water Boiling Water Reactor (BWR) Enriched Uranium Water Water Pressurized Heavy Water Reactor (PHWR) Natural Uranium Heavy Water Heavy Water Light Water Graphite Reactor (LWGR) Enriched Uranium Water Graphite Advanced Gas-Cooled Reactor (AGCR) Natural & Enriched Uranium CO₂ Graphite Fast Neutron Reactor (FNR) Plutonium & Uranium (Natural & Enriched) Liquid Sodium None High Temperature Gas Cooled Reactor (HTGR) Enriched Uranium Helium Graphite