Power Plant Engineering

1.1 Introduction

Power plant engineering deals with the study of energy, its sources

and utilization of energy for power generation. The power is generated by

prime movers (example Hydraulic turbines, steam turbines, diesel engines).

Large amount of power is generated using prime movers in a site or layout

called power plants, where all the equipments and machineries required for

power generation is located.

Energy: Energy may be defined as the capacity to do work. Energy

exists in various forms, such as Mechanical Energy, thermal energy,

electrical energy, solar energy etc. Electricity is the only form of energy,

which is easy to produce, easy to transport, easy to use and easy to control.

Electricity consumption per capita is the index of the living standard people

of a place or country i.e. the utilization of energy is an indication of the

growth of the nation.

1.2 Power and Power Plant:

Power is primarily associated with mechanical work and electrical

energy. Therefore, power can be defined as the rate of flow of energy and

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can state that a power plant is a unit built for production and delivery of a

flow of mechanical or electrical energy. In a common usage, a machine or

assemblage of equipment that produces and delivers a flow of mechanical or

electrical energy is a power plant. Hence an internal combustion engine is a

power plant; a water wheel is a power plant, etc. However, what we

generally mean by the term power plant is that assemblage of equipment,

permanently located on some chosen site which receives raw energy in the

form of a substance capable of being operated on in such a way as to

produce electrical energy for deliver from the power plant.

1.3 Sources of energy

The basic sources of energy for power generation are coal, oil, nuclear

fuels and gas. These sources are known as “conventional sources of energy”.

These sources of energy will one day be used up and are exhaustible.

The most reassuring and promising energy, which is abundant in

supply and is inexhaustible is “Non – conventional sources of energy” such

as solar, wind, tidal, geothermal etc.

Energy resources can be broadly classified as follows

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Energy Resources

Conventional sources of energy Non conventional sources of

energy

(Or) or

Non – renewable sources of energy Renewable source of energy

Examples: - Examples:-

Fuels like coal, oil, Sun, wind, waves, tides,

energy

Natural gas, nuclear fuels etc. From earth core, hydro

electric

Power etc.

1.3.1 Non – renewable sources:

Most of the energy we use are from source like coal, oil, natural gas

and nuclear fuels. These primary energy sources are called Non – renewable

sources because once they have been used up, they cannot be replaced.

1.3.2 Renewable sources:

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Sources of energy that can be used over and over again are called

renewable sources. These sources can be used to produce electricity. Some

of the renewable sources are:

Energy from the sun (Heat and light energy)

Energy from the wind (Kinetic energy)

Energy from the waves and tides (Kinetic energy)

Energy from earth’s core (Geothermal energy)

1.4 Classification of power plants

A power plant makes use of any one of the energy sources to produce

power. Depending on the type of energy source the power plants are

classified as

Thermal power plant (It makes use of coal)

Internal combustion engine plants (makes use of petrol or diesel)

Gas turbine power plant (makes use of a permanent gas)

Nuclear power plant (makes use of nuclear fuels)

Solar power plant (makes use of the suns radiation heat)

Tidal power plant (makes use of the power of tides in the sea)

Hydro electric power plant (makes use of the potential energy of

water)

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Wind power (makes use of energy available in wind)

Geothermal power plant (makes use of heat energy available under

the ground)

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1.5 Working principle of Steam power plants

1.5.1 Introduction

Steam power plant is also known as Thermal power plant.

A steam power plant converts the chemical energy of the fossil fuels (coal,

oil, gas) into mechanical / electrical energy. This is achieved by raising the

steam in the boilers, expanding it through the turbines and coupling the

turbines to the generators which convert mechanical energy into electrical

energy as shown in fig. 1.1.

The following two purposes can be served by a steam power plant:

1. To produce electric power

2. To produce steam for industrial purposes besides producing electric

power. The steam may be used for varying purposes in the industries

such as textiles, food manufacture, paper mills, sugar mills and

refineries.

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Fig. 1.1. Production of Electric energy by steam power plant

1.5.2 Classification of steam power plants

The steam power plants may be classified as follows:

1. Central stations

2. Industrial power stations or captive power stations

1. Central stations. The electrical energy available from these stations

is meant for general sale to the customers who wish to purchase it.

Generally, these stations are condensing type where the exhaust

steam is discharged into a condenser instead of into the atmosphere.

In the condenser the pressure is maintained below the atmospheric

pressure and the exhaust steam is condensed.

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2. Industrial power stations or captive power stations. This type of

power station is run by a manufacturing company for its own use and

its output is not available for general sale. Normally these plants are

non-condensing because a large quantity of steam (low pressure) is

required for different manufacturing operations.

In the condensing steam power plants the following advantages

accrue:

(i) The amount of energy extracted per kg of steam is increased

(a given size of the engine or turbine develops more power).

(ii) The steam, which has been condensed into water in the

condenser, can be recirculated to the boilers with the help of

pumps.

In a non – condensing steam power plants a continuous supply of fresh

feed water is required which becomes a problem at places where there is a

shortage of pure water.

1.5.3 Layout of modern steam power plant

A steam power plant must have the following equipments:

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A furnace to burn the fuel.

Steam generator or boiler containing water. Heat generated in the

furnace is utilized to convert water into steam.

Main power unit such as an engine or turbine to use the heat energy

of steam and perform work.

Piping system to convey steam and water.

The general layout of the thermal power plant consists of mainly 4

circuits as shown in fig. 1.2. The four main circuits are:

1. Coal and ash circuit

2. Air and gas circuit

3. Feed water and steam flow circuit

4. Cooling water circuit

Coal and ash circuit: - This circuit consists of coal storage, ash storage,

coal handling and ash handling systems. The handling system consists of

belt conveyors, screw conveyors etc. Coal arrives at the storage yard and

after necessary handling, passes on to the furnaces through the fuel feeding

device. Ash resulting from combustion of coal collects at the back of the

boiler and is removed to the ash storage yard through ash handling

equipment. The Indian coal contains 30 to 40% of ash and a power plant of

100MW produces normally 20 to 25 tones of hot ash per hour.

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Fig.1.2 Layout of Steam Power plant

Air and gas circuit: - This circuit consists of air filter, air preheater, dust

collector and chimney. Air is taken in from the atmosphere to the air

preheater through the action of a forced or induced draught fan or by using

both. The dust from the air is removed by means of using air filter before

supplying to the combustion chamber. The exhaust gases carrying sufficient

quantity of heat and ash are passed through air preheater where the exhaust

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heat of the gases is given to the air and then it is passed through dust

collectors where most of the dust is removed before exhausting the gases to

the atmosphere through chimney.

Feed water and steam flow circuit: - This circuit consists of boiler feed

pump, boiler, turbine and feed heaters. The steam generated in the boiler is

fed to the steam prime mover to develop the power. The steam coming out

of prime mover is condensed in the condenser and then fed to the boiler with

the help of pump. The condensate is heated in the feed heaters using the

steam tapped from different points of the turbine. The feed heaters may be

of mixed type or indirect heating type.

Some of the steam and water is lost passing through different

components of the system, therefore, feed water is supplied from external

source to compensate this loss. The feed water supplied from external

source is passed through the purifying plant to reduce the dissolved salts to

an acceptable level. The purification is necessary to avoid the scaling of the

boiler tubes.

Cooling water circuit: - This circuit consists of circulating water pump,

cooling water pumps and cooling tower. The quantity of cooling water

required to condense the steam is considerably large and it is taken from

lake, sea or river. The cooling water is taken from the upper side of the river,

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it is passed through the condenser and heated water is discharged to the

lower side of the river. Such system of cooling water supply is possible if

adequate cooling water is available through the year. This system is known

as open system. When the adequate water is not available, then the water

coming out from the condenser is cooled either in cooling pond or cooling

tower. The cooling is effected by partly evaporating the water. This

evaporative loss (this includes evaporation and carryover) is nearly 2 to 5 %

of the cooling water circulated in the system. To compensate the evaporative

loss, the water from the river is continuously supplied. When the cooling

water coming out of the condenser is cooled again and supplied to the

condenser, then the system is known as closed system. When the water

coming out from the condenser is discharged to river downward side directly,

the system is known as open system. Open system is economical than closed

system provided adequate water is available throughout the year.

The different components, which are used in thermal power plants, are

listed below:

Boiler, steam turbine, generator, condenser, cooling towers, circulating

water pump, boiler feed pump, induced/forced draught fans, ash

precipitators etc.

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1.5.4 Working of the thermal power plant

Steam is generated in the boiler of the thermal power plant using the

heat of the fuel burned in the combustion chamber. The steam generated is

passed through steam turbine where part of its thermal energy is converted

into mechanical energy, which is further used for generating electric power.

The steam coming out of the steam turbine is condensed in the condenser

and the condensate is supplied back to the boiler with the help of the feed

pump and the cycle is repeated.

The function of the boiler is to generate the steam. The function of

condenser is to condensate the steam coming pout of steam turbine at low

pressure. The function of the steam turbine is to convert part of heat energy

of steam into mechanical energy. The function of the pump is to raise the

pressure of the condensate from the condenser pressure (0.015 bar) to

boiler pressure (200 bar). The other components like economiser,

superheater and steam feed heaters (steam from different points of turbine

is fed to the heaters to heat the condensate to a higher temperature) are

used in the primary circuit to increase the overall efficiency of the thermal

power plant.

1.5. 5 Characteristics of steam power plant:

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Higher efficiency

Lower cost

Ability to burn coal especially high ash content, inferior coals

Reduced environmental impact in terms of air pollution

Reduced water requirement

Higher reliability and availability

1.5.6 Advantages (merits) of thermal power plant

1. The initial cost of construction of the plant is low compared to hydro

electric plant

2. The power plant may be located near the load centre, so that the cost

of transmission and the losses due to transmission are considerably

reduced.

3. The quantity of water in hydroelectric plant depends on nature, such

as rain and rivers. This is not so in the case of thermal power plants.

4. The construction and commissioning of thermal power plant takes

lesser period when compared to hydro electric power plant.

1.5.7. Disadvantages (demerits) of thermal power plant

1. The fuel (coal or oil) used in thermal power plant will one day get

exhausted since it is a non renewable source of energy that is used.

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2. It cannot be used as peak load plant, as its part load efficiency

decreases very rapidly with decreasing load.

3. The transportation of fuel is a major problem for power plants located

away from coal fields.

4. The cost of power generation is considerably high compared to hydro-

electric power plant.

5. The smoke produced by the burning fuel when exhausted into the

atmosphere causes air pollution.

6. The life of thermal power plant according to the Electricity supply act is

25 years and that of hydroelectric plant is 35 years. The efficiency

decreases to less than 10% after its life period. Hydroelectric power

plant can have a life of even 100 to 125 years.

7. The turbines in thermal power plants run at a speed of 3000 to 4000

rpm and they require special material and rigid construction as

compared to hydro electric plant which has a low running speed of 300

to 400 rpm.

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1.6 Gas Turbine Power plant

A gas turbine power plant may be defined as one “in which the

principal prime mover is of the turbine type and the working medium is a

permanent gas”.

A simple gas turbine plant consists of the following:

1. compressor

2. Intercooler

3. Regenerator

4. Combustion chamber

5. Gas turbine

6. Reheating unit

1. Compressor:

In gas turbine plant, the axial and centrifugal flow compressors are

used. In most of the gas turbine power plant, two compressors are used. One

is low pressure compressor and the other is high pressure compressor. In low

pressure compressor, the atmospheric air is drawn into the compressor

through the filter. The major part of the power developed by the turbine

(about 66%) is used to run the compressor. This low pressure air goes to the

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high pressure compressor through the intercooler. Then the high pressure air

goes into the regenerator.

2. Intercooler:

The intercooler is used to reduce the work of the compressor and it is placed

in between the high pressure and low pressure compressor. Intercoolers are

generally used when the pressure ratio is very high. The energy required to

compress the air is proportional to the air temperature at inlet. The cooling

of compressed air in intercooler is generally done by water.

3. Regenerator:

Regenerators are used to preheat the air which is entering into the

combustion chamber to reduce the fuel consumption and to increase the

efficiency. His is done by the heat of the hot exhaust gases coming out of the

turbine.

4. Combustion chambers

Hot air from regenerator flows to the combustion chambers and the fuel like

coal, natural gas or kerosene are injected into the combustion chamber.

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After the fuel injection, the combustion takes place. This high pressure, high

temperature products of combustion are passed through the turbine.

Fig.1.3 Layout of Gas turbine power plant

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5. Gas turbine

Two types of gas turbines are used in gas turbine plant.

1. High pressure turbine

2. Low pressure turbine

1. High Pressure turbine

In the beginning, the starting motor runs the compressor shaft. The

burnt gases (product of combustion) expand through the high pressure

turbine. It is important to note that when the turbine shaft rotates it infact

drives the compressor shaft which is couples to it. Now, the high pressure

turbine runs the compressor and the starting motor is stopped. About 66% of

the power developed by the turbine is used to run the compressor and only

34% of the power developed is used to generate electric power.

2. Low pressure turbine

The purpose of the low pressure turbine is to produce electric power.

The shaft of the LPT is coupled with the generator. The burnt fuel (gases)

after leaving the HPT is again sent to a combustion chamber where it further

undergoes combustion.

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Even if there is any left out unburnt fuel from the previous turbine it

gets fully burnt in the combustion chamber. The burnt gases run the low

pressure turbine (LPT). The shaft of the turbine is directly coupled with the

generator for producing electricity.

The exhaust hot gases after leaving the LPT passes through the

regenerator before exhausted through the chimney into the atmosphere. The

heat from the hot gases is used to preheat the air leaving the HPC before it

enters the combustion chamber. This preheating of the air improves the

efficiency of the combustion chamber.

6. Reheating unit

In this unit, the additional fuel is added to the exhaust gases coming

out from the high pressure turbine, and the reheated combustion products

goes into the low pressure turbine.

1.6.1 Working of Gas Turbine Power plant

The working of gas turbine plant is shown in fig.1.3. The atmosphere

air is drawn into the low pressure compressor through the air filter and it is

compressed.

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The compressed low pressure air goes into the high pressure

compressor through the intercooler. Here, the heat of the compressed air is

removed. Then the high pressure compressed air goes into the combustion

chamber through the regenerator. In the combustion chamber, the fuel is

added to the compressed air and the combustion of the fuel takes place.

The product of the combustion goes into the high pressure turbine. The

exhaust of the high pressure turbine goes to another combustion chamber

and the additional fuel is added and it goes to the low pressure turbine.

After the expansion in the low pressure turbine, the exhaust is used to

heat the high pressure air coming to the combustion chamber through the

regenerator. After that, the exhaust goes to the atmosphere.

1.6.2..Advantages of Gas turbine power plant

1. For a gas turbine plant, Natural gas is a very suitable fuel. It would be

ideal to install gas turbine plants near the site where natural gas is

readily available.

2. Gas turbine plants can work economically for short running hours.

3. Storage of fuel requires less area and handling is easy.

4. Gas turbine plant is small and compact in size as compared to steam

power plants.

5. It can be started quickly and can be put on load in a very short time.

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6. The cost of maintenance is less.

7. It is simple in construction. There is no need for boiler, condenser and

other accessories as in the case of steam power plants.

8. The gas turbine can operate at high speed since there are no

reciprocating parts

9. Cheaper fuel such as kerosene, paraffin, benzene and powdered coal

(cheaper than petrol and diesel) can be used.

10. Gas turbine plants can be used in water scarcity areas

11. Less pollution and less water is required.

1.6.3 Disadvantages of gas turbine power plant

1. 66% of the power developed is used to drive the compressor; the gas

turbine unit has a low thermal efficiency

2. The running speed of the gas turbine is in the range of (40, 000 to 1,

00,000 rpm) and the operating temperature is as high as 2000 0C, for

this reason special metals and alloys have to be used for the various

parts of the turbine.

3. Special cooling methods are required for cooling the turbine blades.

4. It is difficult to start a gas turbine as compared to a diesel engine in a

diesel power plant

5. The life of a gas turbine plant is upto 10 years, after which its

efficiency decreases to less than 10 percent.

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1.6.4 Applications of gas turbine power plant

To drive generators and supply loads in steam, diesel or hydro plants

To work as combination plants with conventional steam boilers

Thermal process industries

Petro chemical industries

Power generation (used for peak load and as stand by unit)

Aircraft and ships for their propulsion. They are not suitable for

automobiles because of their very high speeds.

1.6.5 Classification of Gas Turbine

Gas turbines may be broadly classified as:

(i) Open cycle gas turbine

(ii) Closed cycle gas turbine

Open cycle gas turbine

In the open cycle gas turbine, fig.1.4, air is drawn into the compressor

from the atmosphere. The compressed air is heated by directly burning the

fuel in the air at constant pressure inside the combustion chamber. The high

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pressure hot gases from the combustion chamber drive the turbine and the

power is developed when the turbine shaft rotates.

Gas turbines are not self starting. A starting motor drives the compressor till

fuel is injected inside the combustion chamber, once the turbine starts

gaining speed the starting motor is disengaged.

Part of the power developed by the gas turbine (about 60%) is used to

drive the compressor and the remaining is used to drive a generator or other

machinery.

Fig. 1.4 Open cycle gas turbine

In the open cycle, system, the working fluid i.e. air and the fuel must

be replaced continuously as they are exhausted into the atmosphere. Thus

the entire flow comes from the atmosphere and is returned to the

atmosphere, hence it is called “open cycle”.

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Closed cycle gas turbine

In this the compressed air from the atmosphere is heated in air heater

(heat exchanger). Heat is added to the air heater from some external source

(oil or coal) at constant pressure. High pressure working fluid expands

through the turbine and power is developed. The exhaust working fluid is

cooled in a pre cooler before the same fluid is sent into the compressor

again.

Fig. 1.4 Closed cycle gas turbine

In a closed cycle gas turbine the same working fluid is continuously

circulated. The fuel required for adding heat from an external source can be

any fuel ranging from kerosene, to heavy oil and even peat and coal slurry

without reducing the efficiency.

1.7 Diesel power plant

1.7.1 Introduction

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This is a fossil fuel plant since diesel is a fossil fuel. Diesel engine power

plants are installed where supply of coal and water is not available in

sufficient quantity.

(i) These plants produce the power in the range of 2 to 50 MW.

(ii) They are used as standby sets for continuity of supply such as

hospitals, telephone exchanges, radio stations, cinema theatres and

industries.

(iii) They are suitable for mobile power generation and widely used in

railways and ships.

(iv) They are reliable compared to other plants.

(v) Diesel power plants are becoming more popular because of

difficulties experienced in construction of new hydel plants and

thermal plants

1.7.2 Layout of Diesel Power plant

The essential components of diesel power plant are

Diesel engine

Air filter and super charger

Engine starting system

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Fuel system

Lubrication system

Cooling system

Governing system

Exhaust system

Diesel engine:

This is the main component of a diesel power plant. The engines are

classified as two stroke engine and four stroke engines. Engines are

generally directly coupled to the generator for developing power. In diesel

engines, air admitted into the cylinder is compressed. At the end of

compression stroke, fuel is injected. The fuel is burned and the burning

gases expand and do work on the piston. The shaft of the engine is directly

coupled to the generator. After the combustion, the burned gases are

exhausted to the atmosphere.

Air filter and supercharger

The air filter is used to remove the dust from the air which is taken by

the engine. Air filters may be of dry type, which is made up of felt, wool or

cloth. In oil bath type of filters, the air is swept over a bath of oil so that dust

particles get coated. The function of the supercharger is to increase the

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pressure of the air supplied to the engine and thereby the power of the

engine is increased.

Engine starting system

Diesel engine used in diesel power plants is not self starting. Engine

starting system includes air compressor and starting air tank. This is used to

start the engine in cold conditions by supplying the air.

Fuel system

It includes the storage tank, fuel pump, fuel transfer pump, strainers and

heaters. Pump draws diesel from the storage tank and supplies it to the

small day tank through the filter. Day tank supplies the daily fuel need for

the engine. The day tank is usually placed high so that diesel flows to engine

under gravity.

Diesel is again filtered before being injected into the engine by the fuel

injection pump. The fuel injection system performs the following functions.

Filter the fuel

Meter the correct quantity of the fuel to be injected

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Time the injection process

Regulate the fuel supply

Secure fine atomization of fuel oil

Distribute the atomized fuel properly in the combustion;

chamber.

The fuel is supplied to the engine according to the load on the plant.

Lubrication system

It includes oil pumps, oil tanks, coolers and pipes. It is used to reduce

the friction of moving parts and reduce wear and tear of the engine parts

such as cylinder walls and piston. Lubrication oil which gets heated due to

the friction of the moving parts is cooled before recirculation.

In the lubrication system the oil is pumped from the lubricating oil tank

through the oil cooler where the oil is cooled by the cold water entering the

engine. The hot oil after cooling the moving parts return to the lubricating oil

tank.

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Fig. 1. 5 Diesel Power plant

Cooling system

The temperature of the burning fuel inside the engine cylinder is in the

order of 15000 C to 20000 C. In order to lower this temperature, water is

circulated around the engine. The water envelopes (water jacket) the engine,

the heat from the cylinder, piston, combustion chamber etc, is carried by the

circulating water. The hot water leaving the jacket is passed through the

heat exchanger. The heat from the heat exchanger is carried away by the

raw water circulated through the heat exchanger and is cooled in the cooling

tower.

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Governing system

It is used to regulate the speed of the engine. This is done by varying

the fuel supply according to the engine load.

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Exhaust system

The exhaust gases coming out of the engine is very noisy. In order to

reduce the noise a silencer (muffler) is used.

1.7.3 Working of Diesel Power Plant

The air and fuel mixture act as a working medium in diesel engine

power plant. The atmosphere air enters inside the combustion chamber

during the suction stroke and the fuel is injected through the injection pump.

The air and fuel is mixed inside the engine and the charge is ignited due to

high compression inside the engine cylinder. The basic principle in diesel

engine is that, the thermal energy is converted into mechanical energy and

this mechanical energy is converted into electrical energy to produce the

power by using generator or alternator.

1.7.4 Applications of Diesel Engines in Power Field

The diesel electric power plants are chiefly used in the following

field.

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(a) Peak load plant: Diesel plants can be used in combination with

thermal or hydro-plants as peak load units. They can be easily started or

stopped at a short notice to meet the peak demand.

(b) Mobile plant: Diesel plants mounted on trailers can be used for

temporary or emergency purposes such as for supplying power to large civil

engineering works.

(c) Standby unit: If the main unit fails or cannot cope up with the demand,

a diesel plant can supply the necessary power. For example, if water

available in a hydro-plant is not adequately available due to less rainfall, the

diesel station can operate in parallel to generate the short fall in power.

(d) Emergency plant: During power interruption in a vital unit like a key

industrial plant or a hospital, a diesel electric plant can be used to generate

the needed power.

(e) Nursery station: In the absence of main grid, a diesel plant can be

installed to supply power in a small town. In course of time, when electricity

from the main grid becomes available in the town, the diesel unit can be

shifted to some other area which needs power on a small scale. Such a diesel

plant is called a "nursery station".

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(f) Starting stations: Diesel units can be used to run the auxiliaries (like

FD and ID fans, BFP, etc.) for starting a large steam power plant.

(g) Central stations :Diesel electric plants can be used as central station

where the capacity required is small

1.7.5 Advantages and disadvantages of diesel power plant

Following are the advantages of diesel electric stations.

1. It is easy to design and install these electric stations.

2. They are easily available in standard capacities.

3. They can respond to load changes without much difficulty.

4. There are less standby losses.

5. They occupy less space.

6. They can be started and stopped quickly.

7. They require less cooling water.

8. Capital cost is less.

9. Less operating and supervising staff required.

10. High efficiency of energy conversion from fuel to

electricity.

11. Efficiency at part loads is also higher.

12. Less of civil engineering work is required.

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13. They can be located near the load centre.

14. There is no ash handling problem.

15. Easier lubrication system.

1.7.6 Disadvantages in installing diesel units for power

generation.

1. High operating cost.

2. High maintenance and lubrication cost.

3. Capacity is restricted. Cannot be of very big size.

4. Noise problem.

5. Cannot supply overload.

6. Unhygienic emissions.

7. The life of the diesel power plant is less (7 to 10 years) as

compared to that of a steam power plant which has a life span

of 25 to 45 years. The efficiency of the diesel plant decreases

to less than 10% after its life period.

1.8 Hydroelectric power plant

1.8.1 Working principle

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Hydroelectric power plant (Hydel plant) utilizes the potential energy of

water stored in a dam built across the river. The potential energy of the

stored water is converted into kinetic energy by first passing it through the

penstock pipe. The kinetic energy of water is then converted into mechanical

energy in a water turbine. The turbine is coupled to the electric generator.

The mechanical energy available at the shaft of the turbine is converted into

electrical energy by means of the generator.

Because gravity provides the force which makes the water fall, the

energy stored in the water is called gravitational potential energy.

Fig. 1.6 Principle of Hydro electric plant

1.8.2 Layout of Hydro electric power plant

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Fig.1.7 shows the schematic representation of a Hydro electric power

plant. The main components are

Water reservoir

Dam

Spillway

Gate

Pressure tunnel

Surge tank

Penstock

Water turbine

Draft tube

Tail race level

Power house

Water reservoir: In a reservoir the water collected from the catchment

area during 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 water surface in the

reservoir is called Head water level. The water head available for power

generation depends on the reservoir height.

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Dam: 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.

Spillway: 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 water level in the dam. Spillways are passages which allow the excess

water to flow to a different storage area away from the dam.

Gate: A gate is used to regulate or control the flow of water from the dam.

Pressure tunnel: It is a passage that carries water from the reservoir to the

surge tank.

Surge tank: 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 sudden

increase of pressure in the penstock pipe due to sudden backflow of water,

as load on the turbine is reduced. This sudden rise of pressure in the

penstock pipe is known as water hammer.

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Fig. 1.7 Layout of Hydro electric Power plant

A surge tank is introduced between the dam and the turbine and serves

the following purposes:

1. To reduce the distance between the free water surface in the dam and

the turbine, thereby reducing the water hammer effect. Otherwise,

penstock will be damages by the water effect.

2. To serve as a supply tank to the turbine when the water in the pipe is

accelerated during increased load conditions and as a storage tank

when the water is decelerating during reduced load conditions.

Penstock: Penstock pipe is used to bring water from the dam to the

hydraulic turbine. Penstock pipes are made up of steel or reinforced

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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 turbine blade is discharged through the draft tube.

The prime movers which are in common use are Pelton wheel, Kaplan

turbine, Francis turbine.

Draft tube: 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 above the

atmospheric at the end of the draft tube to move the water into a tail race.

Water from the tail race is released for irrigation purposes.

Tail race level: Tail race is a water path to lead the water discharged from

the turbine to the river or canal. The water held in the tail race is called Tail

race water level.

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Power House: The power house 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.

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1.8.3 Classification of Hydro electric power plant

Hydro electric 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 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 prime

mover.

1.8.4 Advantages of hydro electric power plant

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1. 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.

2. The running costs of hydropower installations are very low as

compared to thermal or nuclear power stations. 1n thermal stations,

besides the cost of fuel, one has to take into account the

transportation cost of the fuel also.

3. There is no problem with regards 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.

4. 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. .

5. 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.

6. Modern hydropower equipment has a greater life expectancy and can

easily last 50 years or more. This can be compared with the effective

life of about 30 years of a thermal or nuclear station.

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7. Due to its great ease of taking up and throwing off the load, the hydro-

power can be used as the ideal spinning reserve in a system mix of

thermal, hydro and nuclear power stations.

8. Modern hydro-generators give high efficiency over a considerable

range of load. This helps in improving the system efficiency.

9. Hydro-plants provide ancillary benefits like irrigation, flood control,

afforestation, navigation and aqua-culture.

10. Being simple in design and operation, the hydro-plants do not

require highly skilled workers. Manpower requirement is also low.

1.8.5 Disadvantages of Water Power

1. 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.

2. 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.

3. 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.

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4. Such plants are often far way from the load centre and require long

transmission lines to deliver power. Thus the cost of transmission lines

and losses in them are more.

5. Large hydro-plants disturb the ecology of the area, by way of

deforestation, destroying vegetation and uprooting people. Strong

public opinion against. Erection of such plants is a deterrent factor. The

emphasis is now more on small, mini and micro hydel stations.

1.8.6 Hydro electric power plant in India

Srisailam Hydel power plant – AP – 770 MW

Upper sileru Hydor electric project – AP - 120

Kodayar hydro electric power plant – TN – 100 MW

Iddiki hydel project – Kerala – 800 MW

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1.9 Nuclear power plant

1.9.1. Introduction

As large amounts of coal and petroleum are being used to produce

energy, time may come when their reserves may not be able to meet the

energy requirements. Thus there is tendency to seek alternative sources of

energy. The discovery that energy can be liberated by the nuclear fission of

materials like uranium (U), Plutonium (Pu), has opened up a new source of

power of great importance. The heat produced due to fission of U and Pu is

used to heat water to generate steam which is used for running

turbogenerator.

It has been found that one kilogram of U can produce as much energy

as can be produced by burning 4500 tonnes of high grade coal. This shows

that nuclear energy can be successfully employed for producing low cost

energy in abundance as required by the expanding and industrializing

population of future.

Some of the factors which go in favor of nuclear energy are as follows:

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1. Hydro electric power is of storage type and is largely dependent of

monsoons. The systems getting power from such plants have to shed

load during the period of low rainfall.

2. Oil is mainly needed for transport, fertilizers and petrochemicals and

thus cannot be used in large quantities for power generation.

3. Coal is available only in some parts of the country and transportation

of coals requires big investments.

4. Nuclear power is partially independent of geographical factors, the

only requirement being there should be reasonably good supply of

water. Fuel transportation networks and larger storage facilities are not

needed and nuclear power plant is a clean source of power which does

not pollute the air if radioactive hazards are effectively prevented.

5. Large quantity of energy is released with consumption of only a small

amount of fuel.

1.9.2 Nuclear fission

The fuel inside the reactor is a metal called uranium. Uranium exists as

an isotope in the form of U235, U234 and U238. Out of these isotopes U235 is more

unstable. When a neutron is captured by a nucleus of an atom of U235, it

splits up roughly into two equal fragments and about 2.5 neutrons are

released and a large amount of energy (nearly 200 million electron volts

MeV) is produced. This is called fission process. The neutrons so produced

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are very moving neutrons and can be made to fission other nuclei of U235

thus enabling a chain reaction to take place. When a large number of fission

occurs, enormous amount of heat is produced. The following fig.1.8. shows

the chain reaction

Fig. 1.8 Nuclear Fission

It may be observed from the fig.1.8 that 2.5 neutrons are released in

fission of each nucleus of U235, out of these one neutron is used to sustain the

chain reaction, 0.9 neutrons is absorbed by U238 and becomes Pu239. The

remaining 0.6 neutrons escape from the reactor. Moderators are provided to

slow down the neutrons from the high velocities but not to absorb them. The

moderators which are commonly used are ordinary water and Heavy water.

The fig.1.9 shows how the reactor is put on and off.

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Fig. 1.9 Control rods

Control rods limit the number of fuel atoms that can split. They are

made up of a material that absorbs neutrons. To turn on the reactor some

rods are pulled out. The rods are made of boron or cadmium.

1.9.3 Main components of a nuclear power plant

The main components of a nuclear power plant are

Nuclear fuel

Nuclear reactor

Steam generator

Moderator

Control rods

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Reflector

Turbine

Condenser

Shielding

Nuclear Fuel: Fuel of a nuclear reactor should be fissionable material which

can be defined as an element or isotope whose nuclei can be caused to

undergo nuclear fission by nuclear bombardment and to produce a fission

chain reaction. It can be one or all of the following U235, U233 and Pu239

Nuclear reactor: A nuclear reactor may be regarded as a substitute for the

boiler furnace of a steam power plant. Heat is produced in the reactor due to

nuclear fission of the fuel. During the fission process, the large amount of

heat is liberated. This large amount of heat is absorbed by the coolant and it

is circulated through the core.

The various types of reactors used in nuclear power plant is

1. Boiling water reactor

2. Pressurised water reactor

3. Fast breeder reactor

Steam generator: The heat liberated in the reactor is taken up by the

coolant circulating through the core. The purpose of the coolant is to transfer

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the heat generated in the reactor core and use it for steam generation.

Ordinary water or heavy water is a common coolant.

Moderator: It is used to reduce the kinetic energy of fast neutrons into slow

neutrons and to increase the probability of chain reaction. Graphite, heavy

water and beryllium are generally used as moderator. A moderator should

possess the following properties:

1. It should have high thermal conductivity

2. It should be available in large quantities in pure form

3. It should have high melting point in case of solid moderators and low

melting point in case of liquid moderators. Solid moderators should

also possess good strength and machinability.

4. It should provide good resistance to corrosion

5. It should be stable under heat and radiation

6. It should be able to slow down neutrons

Control rods: They regulate the rate of a chain reaction. They are made of

boron, cadmium or other elements which absorb neutrons. Control rods

should posses the following properties:

1. They should have adequate heat transfer properties

2. They should be stable under heat and radiation

3. They should be corrosion resistant

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4. They should be sufficient strong and should be able to shut down the

reactor almost instantly under all conditions.

5. They should have sufficient cross sectional area for the absorption.

Reflector: The neutrons produced during the fission process will be partly

absorbed by the fuel rods, moderator, coolant or structural material etc.

Neutrons left unabsorbed will try to leave the reactor core and will be lost.

Such loss is minimized by surrounding the reactor core by a material called

reflector which will send the neutrons back into the core. The returned

neutrons can then cause more fission and improve the neutrons economy of

the reactor. Generally the reflector is made up of graphite and beryllium.

Turbine: The steam produced in the steam generator is passed to the

turbine. Work is done by the expansion of stem in the turbine.

Condenser: The exhaust steam from the turbine flows to the condenser

where cooling water is circulated. The exhaust steam is condensed to water

in the condenser by cooling. The condensate is pumped again into the steam

generator by the feed pump.

Shielding: The reactor is a source of intense radioactivity. These radiations

are very harmful and shielding is provided to absorb the radioactive rays. A

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thick concrete shielding and a pressure vessel are provided to prevent the

radiations escaped to atmosphere.

1.9.4 Working of a Nuclear Power plant

The reactor of a nuclear power plant is similar to the furnace of steam

power plant. The heat liberated in the reactor due to the nuclear fission of

the fuel is taken up by the coolant circulating through the reactor core. Hot

coolant leaves the reactor at top and then flows through the tubes of steam

generator (boiler) and passes on its heat to the feed water. The steam

produced is passed through the turbine and after work has been done by

expansion of steam in the turbine, steam leaves the turbine and flows to

condenser. Pumps are provided to maintain the flow of coolant, condensate

and feed water.

1.9.5 Boiling water reactor (BWR)

In this reactor, enriched uranium (enriched uranium contains more

fissionable isotope U235 than the naturally occurring percentage 0.7% as

nuclear fuel and water is used as coolant. Water enters the reactor at the

bottom. It takes up the heat generated due to the fission of fuel and gas

converted into steam. Steam leaves the reactor at the top and flows into the

turbine. Water also serves as moderator. India’s first nuclear power plant at

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Tarapur has two reactors (each of 200 MW capacity) of boiling water reactor

type.

Fig. 1.10 Boiling water reactor

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1. 9.6 Pressurised Water reactor (PWR)

Fig. 1.11 Pressurised Water Reactor

A pressurised water nuclear plant is shown in fig. It uses enriched

uranium as fuel. Water is used as coolant and moderator. Water passes

through the reactor core and takes up the heat liberated due to nuclear

fission of the fuel. In order that water may not boil (due to its low boiling

point 2120 F at atmospheric conditions) and remain in liquid state, it is kept

under a pressure of about 1200 p.s.i.g in the pressuriser. This enables water

to take up more heat from the reactor. From the pressuriser, water flows to

the steam generator where it passes on its heat to the feed water which in

turn gets converted into steam.

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1.9.7 Fast breeder reactor (FBR)

In this reactor the core containing U235 is surrounded by a blanket (a

layer of fertile material placed outside the core) or fertile material U238. In

this reactor no moderator is used. The fast moving neutrons liberated due to

fission of U235 are absorbed by U238 which gets converted into fissionable

material Pu239, Pu239 is capable of sustaining chain reaction. Thus the reactor

is important because it breeds fissionable material from fertile material U238

available in large quantities. This reactor uses two liquid metal coolant

circuits. Liquid sodium is used as primary coolant when circulated through

the tubes of intermediate heat exchanger transfer its heat to secondary

coolant sodium potassium alloy. The secondary coolant while flowing through

the tubes of steam generator, transfer its heat to feed water.

Fast breeder reactors are better than conventional reactor both from

the point of view of safety and thermal efficiency. For India which already is

fast advancing towards self reliance in the field of nuclear power technology,

the fast breeder reactor becomes inescapable in view of the massive

reserves of thorium and the finite limits of its uranium resources.

1.9.8 Advantages of nuclear power plant

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1. The fuel used in nuclear power plant is uranium; it does not release

chemical or solid pollutants into the air during use.

2. Space required is less when compared with other power plants.

3. Fuel consumption is very less.

4. Fuel transportation cost is low and no large storage area for fuel is

required.

5. The plant is not affected by weather conditions. The plant can function

throughout the year (Hydel power plants depends on monsoon)

6. By using nuclear fuel we can conserve the fossil fuels like coal, oil, gas

etc for other purposes. For example coal can be used to power steam

engines, oil can be used for running vehicles, and gas be used for

cooking.

7. Number of workers required is less.

8. Nuclear power plant is the only source which can meet the increasing

demand of electricity.

9. A nuclear power plant uses much less fuel than a fossil fuel plant

1 metric ton of uranium fuel = 3 million metric tons of coal = 12 million

barrels of oil

1.9.9 Disadvantages of nuclear power plant

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1. Nuclear plants cost more to build than thermal or hydro electric

power plants of the same capacity.

2. Radioactive wastes must be disposed carefully, otherwise it will

adversely affect the health of workers and the environment as a

whole.

3. Maintenance cost of the plant is high.

4. Not suitable for varying load conditions

5. Well trained persons are required to operate the plant.

1.9.10 Nuclear power stations in India

Tarapur Nuclear power station (Bombay) – Boiling water reactor –

200 MW

Rana Pratap Sagar Nuclear power station (Kota in Rajasthan) – Two

200 MW

Kalpakkam Nuclear power station – Two 235 MW – Pressurised

water reactor

Narora Nuclear power station (Uttar Pradesh) – Two 235 MW –

CANDU reactor

Kakarpur Nuclear power plant (Gujarat) – 4 235 MW CANDU type

reactor

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