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October 01, 2018

The Dead Weight safety valve consists of a valve V which is made of gun metal to prevent rusting. It rests on the gun metal seat S and is fixed to the top of a vertical steam pipe P. The pipe has a flange F at the bottom for fixing at the top of the boiler shell.

A weight carrier C is suspended from the top of the boiler. It carries cast iron rings (i.e., weight W). the total weight must be sufficient to the keep the valve on its seat against the normal working pressure.

Working of Dead Weight safety valve:

When the steam pressure in the boiler exceeds the normal working pressure, it lifts the valve with its weight. The excess steam therefore escapes through the pipe to the atmosphere, until the pressure reaches its normal value.

It is the simplest type of safety valve; it is suitable for stationary boilers only, because it cannot withstand the jerks and vibration of mobile (marine) boilers. Another disadvantage of this valve is the heavy weight required to balance the steam pressure. Hence, it is not suitable for high pressure boilers.




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September 30, 2018
The primitive man had to use his muscle power to carry out different works in his attempt of obtaining the necessities of life. The demand for more power and the scarcity of man power and the ready availability of natural resources like wind, water, etc., might have formed the reason why the ancient man turned his attention to the utilization of these resources as sources of power; thus originated the early prime movers namely wind mill and water wheel.

Principle of wind mill:
Wind flow is created as an effect of solar heat. Winds are caused due to the absorption of solar energy on the earth surface and the rotation of earth about its own axis and around the sun.because of this, alternate heating and cooling occurs and difference in pressure is obtained and the air movement is caused.

The flowing wind which has kinetic energy is used to rotate the wind turbine which is also known as wind mill. Although wind mills have been used for more than a dozen centuries for pumping water and grinding grains, interest in large scale power generation had developed over the past 50 years.

The earth’s atmosphere is thus a marvelous solar driven heat engine. It is estimated that roughly 10 million MW of energy is continuously available in the earth’s winds.

Types of winds mills
Wind mills are classified as

●      Horizontal axis type and
●      Vertical axis type,  Depending on their axis of rotation.


Horizontal axis wind mills are further classified as single-bladed, double-bladed and multi-bladed types.

Horizontal axis wind mill with double blade rotor:
In the horizontal axis single blade type, the blade is of propeller type with counter weight arrangement. The double blade type gives a better performance than the single blade type.

In the double-blade type, the blade has thick cross section of an aerofoil. At the tip of the blade, the velocity is about six times the wind velocity. The blade is set at right angles to the direction of the wind. Ideally the blades should be twisted, but because of construction difficulties this is not always achieved.
The blade rotor drives a generator through a gear box. It is mounted on a bed plate attached on the top of the tower.
Blade material: the blades are made from aluminum or sheet metal. Not a single large wind turbine – generator with metal blades has operated for longer than one year without a blade failure. The suitable material required is a major problem in developing wind mills of higher power generation capacity.
With rotor, the tower is also subjected to the wind loads which may cause serious damage. Hence the structure of the tower should be so designed to withstand the wind load.
The best sites for the wind energy are found off-shore and sea coast. The second best sites are in mountains. The lowest level of wind energy is found in plains, where values are generally three or four times lower than that at the coast.
Multiblade rotor:
It is shown in image below. Rotors with more than two blades will produce high power.
Sail type blades:
It is of recent origin. The blade surface is made from cloth, nylon or plastic arranged as sail wings. There is also variation in the number of sails used.
The horizontal axis types generally have better performance than vertical types. They are mainly used for power generation and pumping water. The biggest wind mill erected for power generation is of 2500kW capacity in U.S.A.
Advantages of Wind Power Generation:
1.    The wind energy is a renewable source of energy. It is free and inexhaustible.
2.    The power requirements for irrigation, lighting and small industrial units can be fulfilled with the use of wind energy. Power generation on large scale using wind energy is not yet so successful, but the small wind mills will play a vital role in the present condition of power shortage.
3.    It does not need transportation.
4.    Like all forms of solar energy, wind power systems are non-polluting.


Disadvantages of Wind Power Generation:
1.    Wind velocity is fluctuating which makes the complications in designing a wind power plant.
2.    Some form of storage of wind energy is essential to maintain a constant supply of power.
3.    The wind is a very hazardous, treacherous and unpredictable source of energy. Blowing in strong gusts from varying directions causing hurricanes, the wind may smash the whole plant within no time. To avoid this, special and costly designs and controls are always required.
4.    It is considered suitable and economical to generate power on a small scale of order of 2 MW.
5.    Power production cost with the present technology available may not compete with the conventional power generating systems. This is because, 1000s of units are required to provide the power output of one fossil-fueled plant.
6.    Wind energy systems are noisy in operation. A large unit can be heard many kilometers away.





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September 29, 2018
Principle of Tidal power generation:      
Tide or wave is periodic rise and fall of water level of the sea. Tides occur due to the attraction of sea water by the moon. Tides contain large amount of potential energy which is used for power generation. When the water is above the mean sea level, it is called flood tide. When water level is below the mean level it is called ebb tide.

Working of Tidal power generation:
The arrangement of this system is shown in image. The ocean tides rise and fall and water can be stored during the rise period and it can be discharged during fall. A dam is constructed separating the tidal basin from the sea and a difference in water level is obtained between the basin and sea.

During high tide period, water flows from the sea into the tidal basin through the water turbine. The height of tide is above the tidal basin. Hence the turbine unit operates and generates power, as it is directly coupled to a generator.


During low tide period, water flows from tidal basin to sea, as the water level in the basin is more than that of the tide in the sea. During this period also, the flowing water rotates the turbine and generates power.
The generation of power stops only when sea level and the tidal basin level are equal. For the generation of power economically using this source of energy requires some minimum tide height and suitable site. Kislaya power plants in France are the only examples of this type of power plant.

Advantages of tidal power plants

1.    It is free from pollution as it does not use any fuel.
2.    It is superior to hydro-power plant as it is totally independent of rain.
3.    It improves the possibility of fish farming in the tidal basins and it can provide recreational facilities to visitors and holiday makers.

Disadvantages of tidal power plants:
1.    Tidal power plants can be developed only if natural sites are available on the bay.
2.    As the sites are available on the bays which are always far away from load centers, the power generated has to be transmitted to long distances. This increases the transmission cost and transmission losses.
3.    The supply of power is not continuous as it depends upon the timing of tides.
4.    The navigation is obstructed.
5.    Utilization of tidal energy on small scale is not economical.

 




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September 28, 2018

Flat plate collector:

In a flat plate collector, the radiation energy of sun falls on a flat surface coated with black paint having high absorbing capacity. It is placed facing the general direction of sun. The materials used for the plate may be copper, steel or aluminum. The thickness of the plate is 1 mm to 2 mm. tubing of copper is provided in thermal contact with the plate.
Heat is transferred from the absorber plate to water which is circulated in the copper tubes through the flat plate collector.
Thermal insulation is provided behind the absorber plate to prevent heat losses from the rear surface. Insulation material is generally fiber glass or mineral wool. The front cover is made up of glass and it is transparent to the in-coming solar radiation.

Cylindrical parabolic concentrator collector:
Concentrator collectors are of reflecting type utilizing mirrors. The reflecting surface may be a parabolic mirror. The solar energy falling on the collector surface is reflected and focused along a line where the absorber tube is located. As large quantity of energy falling on the collector surface is collected over a small surface, the temperature of absorber fluid is very much higher than in flat plate collector.
While flat plate collectors may be used to heat water upto 80’C (low temperature), the concentrating type of collectors are designed to heat water to medium and high temperature ranges.
Butane boilers:
The water heated in flat plate solar collector to 80’C is used for boiling butane at high pressure in the butane boiler. Boiling point of butane is about 50’C.
Turbine:
The butane vapour generated at high pressure in the boiler is used to run the vapour turbine which drives the electrical generator.
The vapour coming out of the turbine at low pressure is condensed in a condenser using water. The condensed liquid butane is fed back to the butane boiler using feed pump.

Tower concept for power generation:


Steam is generated in the boiler, which may attain a temperature upto 2000’K.
Electricity is generated by passing steam through the turbine coupled to a generator. A 50 KW plant based on this concept has been built and successfully operated in Italy.

Advantages of solar power plant:

1.    Sun is essentially an infinite source of energy. Therefore solar energy is very large inexhaustible and renewable source of energy and is freely available all over the world.
2.    It is environmentally very clean and is hence pollution-free.
3.    It is dependable energy source without new requirements of a highly technical and specialized nature for its wide spread utilization.

Disadvantages of solar power plant:

1.    It is available in a dilute form and is at a low potential. The intensity of solar energy on sunny day in India is about 1.1 KW/Square meter area. Hence very large collecting areas are required.
2.    Also the dilute and diffused nature of the solar energy needs large land area for the power plant; for instance, about 30 square kilometer area is required for a solar power station to replace a nuclear plant on a 1 square kilometer site. Hence capital cost is more for the solar plant.
3.    Solar energy is not available at night or during cloudy or rainy days.

 



 


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September 26, 2018

Basic principle of solar energy:

The sun gives out 3.7*10 ^20 MW of energy into space, out of which earth intercepts one 1.7*10^11 MW. Solar radiation is reduced in intensity in the atmosphere by clouds, dust, haze, and fog. The energy emitted by the sun in three minutes is equivalent to the world energy consumption during a year. Most of the energy we receive from the sun comes in the form of light, a short wave radiation, not all of which is visible to human eye. When this radiation strikes a solid or liquid, it is absorbed and transformed into heat energy.
The heat energy is collected in a fluid, generally water. This heat energy can be used in a solar heater or drier or can be used to run an engine. Flat plate collector or parabolic concentrated type collector is issued to collect and concentrate the solar energy and increase the temperature of the working fluid.

Applications of solar energy enjoying most success today are:

• Solar engines for water pumping.
• Solar water heaters.
• Solar cookers
• Solar furnaces.
• Photo-voltaic conversion (solar cells).
• Solar power generation.

Arrangement of solar power plant:


The Image above shows a solar power plant with a low temperature solar engine using heated water from flat plate solar collector and butane as the working fluid. This was developed in France for lift irrigation.






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September 24, 2018
Working Principle:

A nuclear power plant differs from a conventional steam power plant only in the steam generating part. There is no change in the turbo-alternator and the condensing system.

The nuclear fuel which is at present in commercial use is uranium. Scientists say that 1 kg of uranium can produce as much energy as can be produced by burning 4500 tonnes of high grade coal.

To understand how nuclear fission works, refer the below image.

Uranium exists in the isotopic form of U235 which is unstable. When a neutron enters the nucleus of U235, the nucleus splits into two equal fragments and also releases 2.5 fast moving neutrons with a velocity of 1.5*10^7 meters/sec producing a large amount of energy, nearly 200 million electron-volts. This is called “nuclear fission”.

Chain reaction:

The neutrons released during the fission can be made to fission other nuclei of U235 causing a chain reaction. A chain reaction produces enormous amount of heat, which is used to produce steam.

The chain reaction under uncontrolled conditions can release extremely large amounts of energy causing “atomic explosion”.

Energy liberated in chain reaction, according to Einstein law, is E=mv^2, where E=energy liberated, m=mass in grams, v= speed of light = 3*10^10 cm/sec.

Out of 2.5 neutrons released in fission of each nuclei of U235, one neutron is used to sustain the chain reaction, 0.9 neutron is converted into fissionable material Pu239 and 0.6 neutron is absorbed by control rod and coolant moderator.

Function of the moderator is to reduce the energy of neutrons evolved during fission in order to maintain the chain reaction. The moderators which are commonly used are ordinary water and heavy water.

Main components of a nuclear power plant

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 U235. 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).

Radiation hazards and shielding:

The reactor is a source of intense radio-activity. These radiations are very harmful to human life. It requires strong control to ensure that this radio-activity is not released into the atmosphere to avoid atmospheric pollution. A thick concrete shielding and a pressure vessel are provided to prevent the escape of these radiations to atmosphere.

Types of reactors:

●      Pressurized water reactor
●      Boiling water reactor
●      Heavy water-cooled reactor.

Steam generator:

The steam generator is fed with feed water which is converted into steam by 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 pumps and 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 the coolant and feed water respectively.

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 case of the thermal power plant.
●      Nuclear power plant will conserve the fossil 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 of nuclear power plant

●      Radio-active wastes, if not disposed carefully, have adverse effect on the health of workmen and the population surrounding the plant.
●      It is not suited for varying load conditions.
●      It requires well-trained personnel.
●      It requires high initial cost compared to hydro or thermal power plants.




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September 23, 2018

Working principle:

 

Hydro-electric power plant utilizes the potential energy of water stored in a dam built across the river. The potential energy of the water is used to run water turbine to which the electric generator is coupled. The mechanical energy available at the shaft of the turbine is converted into electrical energy by means of the generator.

 

General arrangement of a hydro-electric power plant:

 

Image below shows the schematic representation of the hydro-electric power plant.

Water reservoir:

Continuous availability of water is the basic necessity for a hydro-electric plant. Water collected from catchment area during rainy season is stored in the reservoir. Water surface in the storage reservoir is known as head race.

Dam:

The function of a dam is to increase the height of water level behind it, which ultimately increases the reservoir capacity. The dam also helps to increase the working head of the power plant.

Spillway:

Water in the dam after a certain level in the reservoir overflows through spillway without allowing the increase in water level in the reservoir during rainy season.

Pressure tunnel:

It carries water from the reservoir to surge tank.

Penstock:

Water from surge tank is taken to the turbine by means of pen stocks, made up of reinforced concrete pipe or steel.

Surge tank:

There is sudden increase of pressure in the penstock due to sudden backflow of water, as load on the turbine is reduced. The sudden rise of pressure in the penstock is known as water hammer. The surge tank is introduced between the dam and the power house to keep in reducing the sudden rise of pressure in the penstock. Otherwise penstock will be damaged by the water hammer.

 

Water turbine:

Water through the penstock enters into the turbine through an inlet valve. Prime motors which are in common use are pelton turbine, francis turbine and kalpan turbine. The potential energy of water entering the turbine is converted into mechanical energy. The mechanical energy available at the turbine shaft is used to run the electric generator. The water is then discharged through the draft tube.

Draft tube:

It is connected to the outlet of the turbine. It allows the turbine to be placed over tail race level.

Tail race:

Tail race is a water way to lead the water discharged from the turbine to the river. The water held in the tail race is called tail race water level.

Step-up transformer:

Its function is to rasie the voltage generated at the generator terminal before transmitting the power consumers.

Power house:

The power house accommodates the turbine, generator, transformer and control room.

Classification of hydro-power plants

Hydro-plants are classified according to the head of water under which they work.

When the operating head of water exceeds 70 meters, the plant is known as “high head power plant”. Peloton turbine is used as prime mover in such power plants.

When the head of water range is from 15 to 70 meters then the power plant is known as “medium head plant”. It uses francis turbine.

When the head is less than 15 meters the plant is named as “low head plant”. It uses francis or Kaplan turbine as prime mover.

Advantages of hydro-electric power plants

1.    Water is a renewable source of energy. Water which is the operating fluid, is neither consumed or converted into something else..

2.    Water is the cheapest source of energy because it exists as a free gift of nature. The fuels needed for thermal, diesel and nuclear plants are exhaustible and expensive.

3.    There are no ash disposable problems as in case of thermal power plant.

4.    Hydro-plant does not pose the problem of air pollution as in the case of thermal plant or radiation hazards as in the case of nuclear plant.

5.    Variable loads do not affect the efficiency in the case of a hydro-plant.

6.    Life of hydro-plant is very long (1 or 2 centuries) compared with thermal plant ( 3 to 4 decades). This is because the hydro-plants operate at atmospheric temperature, whereas thermal plants operate at very high temperature (about 500 to 800’c).

7.    Hydro plants provide additional benefits like irrigation, flood control, fishery and recreation.

8.    The water storage of hydro-plant can also be used for domestic water supply.

9.    Auxiliaries needed for the hydro-plant are less compared to thermal plant of equal capacity.

10.  It requires less supervising staff.

11.  Maintenance cost is low.

Disadvantages of hydro-electric power plant:

1.    Hydro-plants are generally situated away from the load centres. Hence long transmission lines are required for delivery of power. This increases the cost of transmission lines and also transmission losses. But a thermal plant can be located near the load centre, thereby the transmission cost and transmission losses are considerably reduced.

2.    The power produced by hydro-plant depends upon the quantity of water which in turn is dependent upon the rainfall. The dry year affects the hydro power generation considerably.

3.    Initial cost of the plant is high.

4.    Erection of hydro-plant (construction of dam) usually takes a long period of time.

 






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