TORQUE OF A DC MACHINE

When a dc machine is loaded either as a motor or as a generator,the rotor conductors carry current .These conductors lie in the magnetic field of the air gap.Thus each conductor experiences a force.The conductors lie near the surface of the rotor at a common radius from its center. Hence a torque is produced around the circumference of the rotor and the rotor starts rotating.
When the machine operates as a generator at constant speed,this torque is equal and opposite to that provided by the prime-motor.When the machine is operating as a motor the torque is transferred to the shaft of the rotor and drives the mechanical load.
The expression for the torque is the same for the generator and the motor.It can be deduced as follows:
The voltage equation of a d.c motor is
V=E+IaRa------(1)

Multiplying both the sides of Eq. (1) we obtain
VIa=EIa + (Ia*Ia)*Ra-------(2)
But
VIa = electrical power input to the armature
 (Ia*Ia)*Ra=copper loss in the armature
we also known that input=output+losses-----(3)
comparison of Eq(2) and  Eq(3) show that
EIa=electrical equivalent of gross mechanical power developed by the armature (electromagnetic power)
Let T= average electromagnetic torque developed by the armature in newton meters(Nm)
At this value of torque the electromechanical power conversion takes place.
mechanical power developed by the armature,
Pm=wT=2πnT
Therefore 
Pm=EIa=wT=2πnT---------(4)
but E=(nPϕZ)/A
Therefore
(nPϕZ)*Ia/A=2πnT
T=(PZϕIa)/2πA---------(5)
Eq(5)is called the torque equation of d.c motor.
For a given d.c.machine,P,Z and A are constant,therefore (PZ/2πA) is also constant.
Let 
(PZ/2πA)=k------(6)
T=kϕIa------(7)
T∝ϕIa------(8)
Hence the torque developed by a d.c. motor is directly proportional to the flux per pole and armature current..

Direct-Current Motors

INTRODUCTION

A motor is a machine that converts electrical energy into mechanical energy.The d.c motor is very similar to a d.d generator in construction.Generations are usually operated in more protected locations and therefore their construction is generally of the open type.On the other hand,motors are generally used in locations where they are exposed to dust,moisture,fumes and mechanical damage.Thus,motors requires protective enclosures for example,drip-proof,fire-proof etc.,according to the requirements.

MOTOR PRINCIPLE 

When a conductor carrying current is put in a magnetic field,a force is produced on it.The effect of placing a current-carrying conductor in a magnetic field . Let us consider one such conductor placed in a slot of armature and suppose that it acted upon by the magnetic field from a north pole of the motor.By applying left-hand rule it is found the conductor has a tendency to move to the left-hand side.since the conductor is in a slot on the circumference of the rotor,the force Fc acts in a tangential direction to the rotor.Thus, a torque(turning effect) is developed on the rotor.similar torques are produced on all the rotor conductors.since the rotor is free to move,it starts rotating  in the anticlockwise direction.
The torque produced on the rotors is transferred to the shaft of the rotor and can be utilized to drive a mechanical  load.

BACK E.M.F.
When the motor armature rotates,its conductors cut the magnetic flux.Therefore,the e.m.f of rotation Er is induced in them.In case of a motor,the e.m.f of rotation is known as back e.m.f. or counter e.m.f.The back e.m.f opposes the applied voltage.Since the back e.mf. is induced due to generator action its magnitude is,therefore,given  by the same expression as that for the generator e.m.f.in a d.c generator.That is,

                       E=(NPϕZ)/60A

where the symbols have their usual meanings. 

TYPE OF TARIFFS

Flat Demand Rate Tariff
The flat demand rate tariff is expressed in the from,C=Ax.Here the bill depends only on the maximum demand.It is independent of the energy consumed.This system is used in street lighting,sign lighting,signal system and irrigation tube wells.In all such systems the amount of connected load and hours of their use are known and the rates of charges  are made accordingly.Thus, metering is not required in this system of tariff.

Straight-Line Meter Rate Tariff
This type of tariff is given by the relationship C=By.In this system the bill depends only on the amount of energy consumed.In this system,different types of consumers are charged at different rates.The rate for each type of consumption is decided by taking into consideration the load factor and diversity factor of the load.For example,the flat rate for light and fan loads is higher than that for power loads.For this purpose,separate energy meters are to be installed for light and power loads.

Block Meter Rate Tariff
In this system,the energy consumption is divided into blocks and  the price per unit in is fixed in block.The price per unit in the first block is the highest and  decreases for the succeeding blocks.Generally the price and energy consumption are divide into three blocks.The first few units of energy at a certain rate,the next few at a slightly lower rate and the remaining units at a still lower rate.Theoretically,this form of tariff does not encourage energy conservation.The energy crisis in India has compelled several states to adopt the reverse form of this tariff.In order to discourage to use more energy,the first few kWh are charged at a certain rate,the next few at a slightly higher rate,the next few at a still higher rate,and so on. For example,the first 100 units or less may be charged at the rate of Rs. 1.43 per unit,the next 100 units at the rate or Rs. 1.63 per unit,and the remaining additional units may be charged at the rate of Rs. 1.83 pr unit.This system of Tariff is used  for majority of domestic and commercial consumers.

Tariffs

INTRODUCTION
By tariffs is meant the schedule of rates framed by the supplier for the supply of electrical energy to various types of consumers.Thus,different methods of charging a consumer or rates of payments by the consumer,for consuming electrical energy are known as tariffs.The following factors are taken into account to decide the tariff:
Type of load(domestic,commercial,or industrial)
Maximum demand
Time at which load is required
power factor of the load
Amount of energy used.
The manner in which consumers pay for electrical energy varies according to their requirements.Industrial consumers use more energy for longer times than domestic consumers. Tatiff should be such as to cover total cost of producing and supplying electrical energy plus a reasonable profit.
The cost of electrical energy supplied by a generating station depends on the installed capacity of the plant and kWh generated.Increase in maximum demand increases the installed capacity of the generating station.
The time at which maximum demand occurs is also important in plant economics.If the maximum demand of the consumer coincides with the maximum demand on the system,additional plant capacity is required .However,if the maximum demand of the consumer occurs during off-peak hours,the load factor is improved and no extra plant capacity is needed.Thus,the overall cost per kWh generated is reduced.
Power factor is also important from the point of view of plant economics.At a low power factor,the load current is large.Consequently,the current to be supplied form the generating station is also large.This large current is responsible for greater (I*I)/R losses in the system and larger voltage drops.Thus,the regulation becomes poor.In order to supply the consumer's voltage within permissible limits,power factor correction equipment is to be installed.Therefore,the cost of generation increases.
The cost of electrical energy is reduced by using a large amount of energy for longer periods.

MHD SYSTEM

The broad classification of MHD system is as follows:
(i)Open-cycle systems
(ii)Closed-cycle systems

Open-cycle MHD  System

An elementary open-cycle MHD system.The fuel,coal (or natural gas)is burned in a combustor. The products of combustion with highly preheated air are seeded with an alkali metal,such as cesium or potassium having a low ionization potential.This increases the conductivity of the gas.  The gas is them expanded through a nozzle to increases its velocity.The electrically ionised gas passes through a duct at about 2500-3000K. This duct has two sides supporting a strong transverse magnetic field of flux density 5-7 teslas at right angles to the flow of the gas.The other sides form the faces of electrodes which are connected to the load.As the hot ionised gas or plasma enters the MHD duct,the kinetic energy of the plasma is decreased due to the effect of the strong magnetic field.The electrons and positive ions move to the electrodes and thus,there is a current flow though the load.This is direct current (dc).An inverter is used to change it to change it to ac.the high temperature exhaust flowing out of the MHD generator passes through air preheater for heating the inlet air,since the products of combustion are exhausted  to atmosphere,the oxides and hydroxides of the seeding element may cause severs air pollution.An electrostatic precipitator is used in the recovery of the seed,which can be used again.After going through the pollution control mechanism,the gas is then discharged to the atmosphere through the stack.The high-temperature exhaust from the MHD duct may also be used to generate steam to run a conventional steam-turbine generator as a bottoming plant.

Closed-cycle MHD System

A closed-cycle MHD system it has three interconnected loops.The first loop is the external heating loop.Coal is gasified and the gas is burned in a combustor to produce heat.
This heat is transferred to argon(the working fluid of the MHD cycle) in the heat exchange HX1.
The combustion products are fed to the air preheater to recover part of the heat of combustion products are discharged to the atmosphere through the stack.
The second loop is the MHD loop.Hot argon gas from the heat exchange HX1 is seeded with cesium and passed through the MHD  generator at a very high speed.The dc power output of the MHD generator is converted to ac by a inverter .
The third loop in this system is the steam loop.This is used for further recovering the heat of working fluid and converting this heat into electrical energy.The hot working fluid is slowed down to a low subsonic speed in a diffuser.The hot working fluid then enters another heat exchanger HX2 which serves as a waste-heat boiler.steam generated in HX2 is used to run two steam turbines ST1 and ST2.The turbine ST1 drives an alternator.The output of the alternator is also supplied to the grid.The steam turbine ST2 runs the compressor.The working fluid argon is fed back to the heat exchanger HX1 after being passed through compressor and intercooler.  

INDICATING,RECORDING AND INTEGRATING INSTRUMENTS

Indicating instruments are those which indicate the magnitude of the quantity being measured at that time.These indications are given by pointers moving over calibrated scales.Instruments like ammeters,voltmeters,watt meters,etc.belong to this class.

Recording instruments are those which give a continuous record of the variation of the quantity,such as current voltage or power over a selected period of time.The moving system of the instrument carries an inked pen which rests lightly on a chart or graph sheet carried by a revolving drum moves at a uniform speed in a direction perpendicular to the direction of the deflection of the pen.The path traced out by the pen on the sheet of paper gives a continuous record of the variations in the quantity being recorded.Recording voltmeters are in common use to record the variation of voltage by the suppliers at the terminals of the consumers during 24 hours in case of their complaints of low voltage.

Integrating instruments register by a set of dials and and pointers either total ampere hours (Ah) or the total amount of electrical energy (kWh) supplied to a circuit in a given time.Household energy meter is an integrating instrument. Ampere hour meter is another example of integrating meter.
 

Wind energy

Energy from the wind is a form of solar energy.Winds are turbulent masses of air rushing to even out the differences in atmospheric pressure created when the sun heats the air move in one place than in another.
The power in the wind is proportional to the cube of the wind speed.Turbine type wind energy generators transform the kinetic energy of the wind into the rotary shaft motion,and in turn,into electrical energy.
The advantages of using wind energy are that it is plentiful,inexhaustible and non-polluting.It is an intermittent source of energy.Wind power is advantageous for small power requirements at isolated sites.However,for continuity of supply,it should be used in conjunction with other method of power generation.
with the advent of the oil crisis in 1973,signification strides have been made in the wind energy programme covering   different aspects of wind energy utilization.The wind potential for power generation in India has been estimated to be more than 20000 MW.Wind power plants are located side by side in suitable location where wind velocity is of the order of 30 km per hour.The site is known as wind farm. Nine wind form projects of total capacity of 10.10 MW have so far been installed in Gujarat,Tamil Nadu,Maharashtra and Andhra Pradesh.
   

Electric power system

INTRODUCTION

An electric power system has the following basic functions:
1.Generation of electric energy economically with minimum ecological disturbance.
2.Transfer of this energy to consumers with maximum efficiency and high reliability at practically constant frequency.
Large amount of power is generated at the generating stations.The present trend is to install bigger size of alternators to generate large amount of power to cater the required increasing demand.The site of the power station depends upon the type of power station. The new thermal station are being constructed at pit heads(near the coal mines) because of the higher cost of transportation of coal.Hydro-power station sites are governed by the availability of water resources.The nuclear plants are also situated remote from the centers of consumption due to safety reasons.Thus,the difficulty of getting power station sites near the consuming centers make it inevitable to transfer bulk of electrical energy through longer distances.Long-distance,bulk power transfer is only possible by high voltage transmission system.Extra high voltage (EHV) and ultra high voltage ( UHV)  transmission systems have been developed in most of the countries for transporting  energy from remote power stations.By  EVH is meant the voltage above 220 kV. The voltages above 760 kV are called ultra high voltages(UHV).

BASIC STRUCTURE OF AN AC POWER SYSTEM

Electrical energy generated at generating stations is transported to remote load centers. Between a generating station and a consumer we have transmission,sub transmission and distribution levels of voltage.since the long distance transmission at high voltage is cheap and,low voltage are required for utility purposes,the voltage level goes on decreasing from the transmission system to the distribution system.An electrical power system may be divide into three main components,namely,the generating system,the transmission system,and the distribution system.

DISTRIBUTION VOLTAGE LEVEL
The component of an electrical power system connecting all the consumers in an area to the bulk power sources is called a distribution system.Bulk power substations are connected  to generating stations by transmission lines.They feed a number of substation which are usually situated at convenient point near the load centres. A substation distributes the power to domestic,commercial and relatively small consumers.The consumers requiring larger blocks of power are usually supplied at sub transmission or  even transmission levels.A bulk power station or a generating station usually supplies power to a subtransmission system.

SUBTRANSMISSION LEVEL

The operation of a subtransmission system is similar to that of a distribution system.It differs from a distribution system in the following manner:
1.A subtransmission system has a higher voltage level than a distribution system.
2.It supplies only bigger loads.
3.It supplies only few substations as compared to a distribution system which supplies a number of loads.
 TRANSMISSION LEVEL
A transmission system is quite different from either a subtransmission or a distribution system.It has a higher  voltage level than a subtransmission system.It supplies only large blocks of power to bulk power stations or very big consumers.The third and the main consideration in which a transmission system differs from either a subtransmission or a distribution system is that it interconnects the neighbouring generating stations into a power pool.Thus, a transmission line performs two functions: First,the transport of energy from generating stations to bulk receiving stations,and second,the interconnected of two or more generating stations.The interconnection of two neighbouring subtransmission system is also done by transmission lines. 

Limitations of Hydropower plants

The main limitations associated  with hydro plants are as follows:
1.The generation of power in a hydro plant is dependent  upon the availability of water in its reservoir.Since more than 70% of the annual run-off in India takes place during 3-4 monsoon months,it is necessary to build suitable storage reservoir schemes so as to store water during monsoon period for utilization in non monsoon months.Hydroelectric projects involve huge amount of civil engineering works.
2. Sites for hydro projects are specific.A hydro project requires a very large area for which land is to be acquired from the local populations.This creates many problems like rehabilitation of persons whose lands are acquired and to provide them alternative jobs.
3.Forest clearances pose considerable problems.The flora and fauna in the area is greatly affected.
4.There is a loss of land suitable for agriculture.
5.The time required for the completion of a hydro power projects is very long because of the factors mentioned above.
6.The capital cost of a hydro plant is much higher as compared to thermal plant.
7.The inter-state disputes in sharing water resources are responsible for delaying hydroelectric development.      

HYDROELECTRIC POWER PLANTS

The hydroelectric power plant converter the energy stored in water to electrical energy by the use of water turbine coupled with an electric generator,water from the river or a reservoir at a suitable head is taken to turbine through a penstock (conduit system).The turbine converts the potential energy of water into mechanical energy which drives the generator.Hydroelectric power generation is the oldest and cheapest method of power generation.
The main advantages of hydroelectric power station are as follows:
1.Hydro power is the cheapest,cleanest and most reliable source of energy   for bulk generation of electricity.
2.The cost of generation per kWh is lower than the energy generated at thermal power stations or combined cycle gas turbines.Hydro plants have very low maintenance cost. The auxiliaries needed in a hydro plant are less as compared to other plants of equal rating.
3.Hydro stations increases the system reliability because of their quick start and quick stop operations.Thus,they are ideal for peak loads.They can be started and connected to power network within five minutes in contrast to many hours in case of thermal plants.
4.Hydro power uses a renewable source of energy whereas fossil fuels are finite in nature and, therefore,need to be conserved in the long-term interest of the country.
5.Hydro projects do not require and feed stock,unlike thermal power projects for which coal has to be transported to a long distance from coal mimes to thermal plant sites. This puts an extra burden on country's railway system.
6.Hydro power is a clean source of energy.It is free from either air or water pollution which is not the case for nuclear,thermal or gas power plants.
7.Hydro plants are multipurpose in nature.In addition to power generation they are also useful for irrigation,flood control,navigation and supply of drinking water for domestic and industrial purposes. Reservoir lakes can also be made which can be used for recreation.
8.The life of a hydro plant is very long of the order of 35-40 years.Moreover,its efficiency does not reduces much with age.
9.When hydro projects are set up, the infrastructure,roads facilities,communication and other facilities greatly improve in the region.The local employment potential increases and the economy is improved.
It is due to these reasons that hydro plants have a decided advantage over other methods of   power generation.     

Three-phase Induction Motors

INTRODUCTION

Three phase induction motor is the most popular type of a.c motor.It is very commonly used for industrial drives since it is cheap,robust,efficient and reliable.It has good speed regulation and high starting torque.it requires little maintenance.It has a reasonable overload capacity.

CONSTRUCTION
A three-phase induction motor essential consists of two parts:the stator and the rotor.The stator is the stationary part and the rotor is the rotating part.The stator is build up of high-grade alloy steel lamination to produced to reduce eddy-current losses.The laminations are slotted on the inner periphery and are insulated from each other.These laminations are supported in a stator frame of cast iron or fabricated steel plate.The insulated stator conductors are placed in these slots.The stator conductors are connected to form a three-phase winding.The phase winding  may be either star or delta-connected.
The rotor is also built up of thin laminations of the same material as stator. The laminated cylindrical core is mounted directly on the shaft or a spider carried by the shaft.These laminations are slotted on their outer periphery to receive the rotor conductors.There are two types of  induction motor rotors:
(a)Squirrel-cage rotor or simply cage rotor.
(b)Phase wound or wound rotor.Motor using this type of rotor are also called slip-ring motors. 

BRAKING OF A VEHICLE

The term 'brake' may be defined as a device to apply artificial frictional resistance,to a moving body,in order to retard or stop it. In a moving vehicle,the brake are applied either on the pair of rear wheels,front wheels or on both pairs of the front and rear wheels.Now we shall discuss the braking of a vehicle in all the cases one by one.
1.Braking on rear pair of wheels only.It is a common way of braking the vehicle,in which friction force acts in the rear pair of wheels only.
2.Braking on front pair of wheels only.It is a very way of braking the vehicle,in which the frictional force acts in the front pair of wheels only.
3.Braking on both the pair of wheels.It is the most common way of breaking the vehicle,in which the frictional force acts on both the rear and front pair of wheels.
In all the above mentioned three types of braking,the frictional force acts in the backward direction(i.e.,opposite to the tractive force or direction of motion of the vehicle).A little  consideration will show,that the breaking distance(i.e.,distance in which the vehicle is brought to rest after the application of brakes)will be the least when the brakes are applied on both the pair of wheels.it is due to this reason that the breaks are applied on both the pairs of wheels(or sometimes termed as all the four wheels of a four-wheeled vehicle) these days.such vehicles are generally used in hilly areas or army operations.

Work,power and Energy

Whenever a force acts on a body,and the body undergoes some displacement,then work is said to be done.e.g.,if a force P, acting on a body, causes it to move through a distance s .
Then work done by the force P=Force X Distance
                                                 =p x S
sometimes,the force p does not act in the direction of motion of the body,or in other words,direction of motion of the body,or in other words,the body does not move in the direction of the force
Then work done by the force P
                                                = component of force in the direction of motion X Distance 
                                                = P cos θ X s
UNIT OF WORK

We have already discussed that the work is the product of force and distance,through which the body moves due to action of the force.Thus the units of work depend upon the units of the force and distance.The units of work(or work done) are:
1.One N-m.It is the work done by a force of 1 N,when it displaces the body through 1 m.It is called joule(briefly written as J),Mathematically.
                                                    1 joule=1 N-m
One kN-m.It is the work done by a force of 1 kN,when it displaces the body through 1 m.

It is also called kilojoule(briefly written as kj).Mathematically.
                                              1 kilo-joule=1 kN-m

EQUILIBRIUM SPEED FOR SUPERELEVATION

In last article,we have obtained a relation for the superelevation in railways. The superelevation obtained,by this relation,is popularly known as equilibrium superelevation. It has been experienced,all over the world,that different trains pass over the curve with different speeds.It is therefore obvious that superelevation provided for a particular speed would not suit any other speed.A little consideration will show,that as the superelevation increases with the square of the speed,therefore at higher speeds there is a tendency for the train to overturn.similarly,at lower speeds,there is a tendency for the train to derail.
It will be interesting to know,that there are so many theories prevalent for calculation of superelevation all over the world.In most of the countries(including India) superelevation is provided in such a way that the faster trains  may run safely without the danger of overturning or discomfort to the passengers due to insufficient superelevation. And at the same time,the slower trains may also travel safely without the danger of derailment due to excessive superelevation. These days,the superelevation is,usually,provided for equilibrium speed or weighted average speed under average conditions. 

SUPERELEVATION

In our day to day life,we see that whenever a roadway(or railway) is laid on a curve or curved path,then both the edges are not the same level, In such a case, the outer,the outer edge is always higher than the inner one.This  is done to keep the vehicle in equilibrium while going on the curved path.
we Know that the body,moving along a curved path,is subjected to the following forces:
1. Its own weight,and
2. Centrifugal force.

EFFECT OF SUPERELEVATION IN ROADWAY

To counterbalance this effect and maintain equilibrium of the vehicle,the surface of the path is made perpendicular to the resultant by keeping the inner edge level and raising the *outer edge of the roadway or (outer rail of the railway). The amount by which the outer edge of rail is raised is known as cant or superelevation.
In case of roadways, the outer edge is raised with respect to the inner edge of the road surface.The amount,by which the outer edge is raised is known as cant or superelevation. The process of providing superelevation is known as banking of the road.The general practice,to define the superelevation,is to mention the angle of inclination of the road surface.
Let
m=Mass of the body in tonnes,
r=Radius of circular path in m,
v=velocity of the body in m/s,and
θ=Angle of the bank.
we have seen in the pervious articles,that whenever a body is moving along circumference of a circle,it is subjected to the following forces:
1. Its own weight =mg
2. Centrifugal force=m(v*v)/r
 tanθ=centrifugal force/weight of the vehicle=(m(v*v)/r)/mg
        =(v*v)/gr

   

FLYWHEEL

It is a  circular heavy wheel,generally,fitted to a rotating engine shaft to control variation in its speed during each cycle. Strictly speaking,it serves as a reservoir to store and restore energy by virtue of its inertia.Or in other words,it shears its energy during the period,when the supply of energy is more than the requirement; and releases it during the period when the supply of energy is less than the requirement.
It will be interesting to know that in all types of steam engines and I.C. engines,the power generated and supplied to the shaft is variable.As a matter of fact,the flywheel speed increases when it absorbs energy and decreases when it releases energy.Thus a flywheel is designed to keep the engine speed within the prescribed limits during each cycle.
The difference between the maximum and minimum speeds of an engine during a cycle is called the function of speeds.And the ratio of fluctuation  of speed,to the mean engine speed,is called coefficient of fluctuation of speed.
Let                                 M=Mass of the flywheel,
                                       I=Mass moment of Inertia of the flywheel,

Picture of Flywheel

  

MAGNETOHYDRODYNAMIC (MHD) GENERATION

MAGNETOHYDRODYNAMIC (MHD) generation of power is the direct conversion of heat into electrical energy.In this method gases at about 3000°C are passed through the MHD duct across which a strong magnetic field is applied.Since the gases are hot and partly ionised,they form an electrically conducting medium(conductor) moving in a magnetic field.Thus a voltage is generated.The electrical power generated by an MHD generator is in the direct current(dc)form.The principle of operation of a dc MHD generator is the same as that of a conventional dc generator.The essential difference between the two is as follows:
1.In an MHD generator an Ionised gas,or plasma,serves the purpose of conductor.In a dc generator a prime mover is needed but in an MHD generator,the flow of ionised gas replaces the prime mover.
2.In a conventional dc generator the current is carried to the external load through brushes.while in the MHD generator the same function is performed by electrodes.
3.The MHD generator performs the function of turbine and generator both.
4.The MHD generator is well suited for high power and high temperature.

OCEAN THERMAL ENERGY CONVERSION (OTEC)

The generation of electric power from ocean thermal energy conversion (OTEC) is based on the utilization of the natural temperature difference between the warm sea water at the surface and the cold sea water at some depth.The temperature of water at the surface of sea is about 28°C and the temperature of the cold deep sea water is about 5°C to 7°C at the depths of about 800 m  to 1000 m in tropical seas. Thus,in tropical waters there are two infinite heat reservoirs.One is the heat source at the surface at about 28°C and the other is the depth of about 1000 m. OTEC  systems are economically attractive where temperature difference of about 20°C exists between top and bottom sea water.
There are two OTEC systems. The open-cycle system uses the sea water as the working fluid. The warm surface water is "flash evaporated"in a chamber maintained under high vacuum and the generated vapour drives a low pressure  turbine connected with the generator.The exhaust steam is condensed using sea water.
In the closed-cycle system,low boiling liquid such as Freon or ammonia is the working fluid.The fluid is evaporated using the warm surface sea water.After the vapour drives the turbine,it is condensed by cold sea water.This condensate is pumped back to the evaporator.
While the open-cycle system eliminates the need for a surface heat exchanger; its disadvantage is that steam is generated at very low pressure.This requires,a large volume of steam.Hence the diameter of the steam turbine is about 12 m for 1 MW plant. The maintenance of vacuum in the pumps need very large vacuum pumps.
In a closed-cycle system expensive fluids like Freon and ammonia are needed.But it would not require vacuum pumps as the fluids evaporate at around 25°C and the diameter of the ammonia turbine will be much smaller,not more than 0.6 m.Technically,fabrication of such a turbine is easier than that of large steam turbine used in the open-cycle system.
The efficiency of an OTEC system is about 3% because of the small temperature difference between warm and cold sea waters.The major advantage of the OTEC for power generation is that ocaen energy is a renewable source.since no fuel is used,the OTEC system may become more economical in future. 

TIDAL POWER PLANTS

The three main components of a tidal power plant are as follows:
(a)The dam or dyke (low wall)to form pool or basin.
(b)sluice ways from the basin to the sea and vice versa.
(c)Power house.
The function of a dam or dyke is to form a barrier between the sea and the basin or between one basin or the other in case of multiple basins.
The sluice ways are gate controlled devices.They are used to fill the basin during the high tide or empty the basin during the low tide.
The turbine,generators and other auxiliary equipment are the main components of a power house.
usually the power house,sluice ways are in alignment with the dam.

CLASSIFICATION OF TIDAL PLANTS

Tidal power plants can be classified on the basis of basins used for power generation.There are two types of basin systems.
1.Single basin system.
2.Double basin system.

Single-basin system
This is the simplest system to generate tidal power.In this system there is only one basin which is separated  from the sea by a dam or dyke.sluices (gates)are  provided in the dam.when the tides are high,water is stored in the basin and the sluice ways are closed.When the tides are falling,sluices are opened to allow the water to pass through the turbine to generated power.The generation of power continues till the level of falling tides coincides with the level of the next rising tide.The power output from single-basin system is intermittent in nature and highly variable.

Double basin system
The turbines are located in the dam separating the two basins.The sluice gates are provided in the dam.The high-level basin gates are called inlet gates and the low-level gates as outlet gates.At the beginning of the tides the turbines are closed ,the inlet gates opened and the outlet gates closed. The upper basin is filled with water.when the water level in basin A provides a sufficient difference of head between the two basin A. When the tide reaches its peak value, the water level in basin A is maximum, the inlet sluices are then closed.The water flows from the upper basin to the lower basin through the turbines.Therefore ,the water level in the  upper basin falls and that in the lower basin through the turbines. Therefore,the water level in the upper basin falls and that in the lower basin rises.When the rising level in basin B becomes equal to the level of the falling tide,the outlet sluices are  opened.When the tide reaches its lower most level, the outlet gates are closed.After sometime the tide rises.When its level becomes equal to the low level of the upper basin,the inlet gates are opened.consequently the level of water in basin A starts rising.Thus,the cycle is repeated.

TIDAL ENERGY

Tide is a periodic rise and fall of water level of the sea caused by the forces of attraction of the sun and the moon.The rotation of earth causes two high tides and two low tides to occur daily.However, the effect of revolution of the moon around earth increases the intervals between two high tides from 12 hours to 12 hours 25 minutes.The difference between high and low water levels is called the range  of the tide.
Tides of higher ranges are known as spring tides.They occur on full moon and new moon days,when the force of the sun and moon on earth add together.When the gravitational forces due to sun and moon on earth oppose each other,tides of minimum range are obtained.such tides are known as Neap Tides Neap  tides occur at the first and third quarter of the moon.
In the open sea the range of the tide is about 1 m.At some location on the coasts,the tidal range is 20 m or even more. The tides can be used to generate electrical energy which is known as tidal energy.However,tidal energy can only be utilized economically for power generation where the tidal range is more than 5 m.
In a tidal project, a structure is constructed at the mouth of the creek or gulf where tidal ranges are more than 5 m. The reservoir formed by its construction is known as Basin.It is filled during high tides and emptied during low tides. The potential energy of water on the two sides of the structure is used to drive a turbine,which is turn drives a generator to generate electrical energy.

SOLAR PHOTOVOLTAICS

Salar photo voltaic (SPV)  system enables the conversion of solar energy directly to electrical energy.Energy conversion devices used to convert sunlight to electrical energy are called  solar cells.Thus, the basic building block of a photo voltaic system is the solar cell.
A solar cell is basically a semiconductor diode capable of developing a voltage of 0.5 to 1.0 V.A solar cell can be made from various semiconductor materials which are processed with special additives.This makes them photosensitive.When photons(particles of energy in sunlight)strike the surface of the solar cell, electron hole pairs are generated and current starts flowing. conductive layers are provided at the top and bottom of the solar cell to collect current.
Solar cells are connected together in series and parallel suitably  to provide desired voltage and power output.This group of solar cells is put in a special protective enclosure called a module.Several modules grouped together form a panel.When a number of panels are interconnected to supply higher voltage and power output,the arrangement is known as an array.
The performance  and quality of a photovoltaic devices is represented by its efficiency,which is the ratio of electrical energy produced to the solar energy incident on its surface.
Most commercial solar cells today are made of silicon.Efficiencies of about 24% have been realised with crystalline silicon solar cells in the laboratory.Efficiencies of commerically produced solar cells range between 13-15%.Higher efficiencies have been achieved using concentrated sunlight.Several alternative materials and techniques have been investigated to reduce the costs  of PV devices.Amorphous silicon technology has achieved some success.Here ,thin layers of silicon are deposited on glass or stainless steel substrates.These layers do not possess a crystalline structure and, therefore,they are called amorphous films.other thin film materials being used are  copper indium diselenide(CuInse2) and camium telluride (CdTe).The main advantages of thin film technology are the small quantity of materials needed and the lower energy consumption.      

NUCLEAR POWER PLANT

A nuclear is that part of nuclear plant in which nuclear fuel is subjected to controlled nuclear fission.In the process of fission heat is generated which is used to heat a coolant.The coolant transfers heat to the heat exchanger to produce steam.The steam generated in the heat exchanger is admitted to the turbine which is coupled to the alternator.The alternator converts mechanical energy into electrical energy.The steam after expansion through the turbine is condensed in the condenser.The condensate pump sends the condensate back to the heat exchanger thus forming a closed-feed system.The reactor  heat exchanger are equivalent to the furnace and the boiler in a conventional coal-fired steam power plant.
MAIN PARTS OF A REACTOR
The main parts of a reactor are as follows:
1.Reactor core.
2.Moderator.
3.Reflector.
4.Shielding.
5. Reactor control.
6.Cooling system.

1.Reactor Core.The reactor consists of a number of fuel rods made of fissile material.
2.Moderator.This material in the reactor core is used to moderator,or reduce,the neutron speeds to a value that increases the probability of fission occurring.
3.Reflector:this completely surrounds the reactor within the thermal shielding arrangement and helps to bounce escaping neutrons back into the core.this conserves the nuclear fuel,as the low-speed neutrons thus returned are useful in continuing the chain reaction.

HYDRO POWER IN INDIA

India has a tremendous hydro potential.The Himalayas spreading across a length of about 3200 km and a width of 640 km has falls ranging from a few meters to a few thousand meters.The great Indus,the Ganga and the Brahmaputra rivers with their innumerable tributaries originating from Himalayas constitute about 70% of the country's hydro potential.The peninsular plateau,flanked on one side by the Eastern Ghats and on the other by the Western Ghats is a respectable of enormous hydro power.
As per the assessment of hydro potential carried out by the central Electricity Authority,our country's economically exploitable hydro potential of major and medium hydro schemes is 84044 MW at 60% load factor which will yield an annual energy generation of 442 billion units.With seasonal energy,the total energy,the total energy potential is assessed to be 600 billion units per year.The hydro  potential of 84044 MW at 60% load factor would be equivalent to an installed capacity of over 150,000 MW on the basis of probable load factor.
In addition to be above,56 sites have been identified for development of pumped storage schemes with total installed capacity of 93000 MW.
The potential of small schemes which can be developed on small rivers/hilly streams,canal head and canal drops is about 7000 MW in terms of installed capacity from about 1450 schemes.
As on 31.3.1996,the total hydro installed capacity in the capacity in the country was 20976 MW representing 14.84% of available hydro potential at 60% load factor.This included 1494 MW of pumped storage capacity from 7 pumped storage schemes. The share of hydro capacity in the total installed capacity of 83288 MW(from coal,gas,nuclear and hydro stations) was 25.19%.seventy seven (77) more schemes with an aggregate capacity of 17675 MW have been sanctioned by the Govt.and are at various stages of execution.These schemes account for 6.99% of available hydro potential at 60% load factor.The bulk of potential amounting to 78.17% thus remains to be developed.

SOLAR THERMAL ENERGY

The first step in solar energy utilization is the collection of this energy.This is done through solar collectors.A solar collector is a device used to collect solar radiation and transfer the energy to a fluid passing in contact with it.The surface of a solar collector is designed for  high absorption and low emission.Solar collectors are of two types,namely,non-concentrating or flat-plate type and concentrating or focusing type collectors.

Flat-plate collector are used where temperature below 90°C are required.These collectors could be of liquid heating or air heating types.The liquid heating collectors are often used for heating water whereas the air heating collectors are used for drying of agricultural produce,space heating,heating of green houses ,etc.

Concentrating collectors employ optical system in the form of reflectors or refractors to concentrate the energy of direct solar radiation on the absorbing surface.The reflectors may be flat mirrors or in the shape of a parabolic trough or a paraboloidal dish.An array of reflectors can be used to concentrate solar radiation on a single absorbing surface.
Concentrating collectors enable the generation of much higher temperature compared to flat-plate collectors.Hence they are used for applications which need high grade heat (above 300°C)such as power generation.However ,concentration collectors need tracking arrangements which move the collectors constantly so that they receive the direct beam from the sun throughout the day and in all seasons.
In a solar power plant,solar energy is collected by large parabolic collectors.This energy is used to heat water or any other fluid.the heat is transferred to feed water to convert it into steam.The steam is used to run a turbine which drives an alternator.The heat of cooling water of the condenser may be used for some other purpose.
Solar thermal energy has the following main application:

1.Heating and cooling of buildings.
2.Solar water heating.
3.Solar cookers.
4. solar engines for water pumping water.
5.Solar drying of agricultural and animal products.
6.Food refrigeration.
7.Solar furnaces.
8.Solar electric power generation.
9.water desalination.
10.Solar greenhouses.

SOLAR ENERGY

Solar energy is a very large,inexhaustible source of energy.The earth receives nearly 4000 trillion kWh of energy every day in the form of electromagnetic radiation from the sun.this is about a hundred times the total energy consumption of world in a year.Solar energy is cheap and free from pollution.It is virtually inexhaustible and is usually expressed in watts per square meter.The total energy incident over a period of time (such as a day) is expressed in kilowatt hours per square meter.Most parts of India receive 4 to 7 kWh of solar radiation per square meter per day. these are low values from the point of view of technological utilization.large collecting areas are required in many applications.Hence initial investments required for collection and storage of solar energy are large.Another problem associated with the use of solar energy is that its availability varies widely with time. The daily and seasonal variation in the energy available may required storage or a combination with other energy sources.
consequently,the energy collected when the sun is shining must be stored for use during the periods when it is not available.

Of the various forms in which solar energy can be converted,its conversion in the form of electricity is  considered to be most desirable. Presently,two technologies are being developed for conversion of solar energy to electrical energy.
1.Solar thermal energy technology.
2.Solar photovoltaic technolgy. 

ECONOMISERS,AIR PREHEATER

ECONOMISERS

An economiser is a heat exchanger  which raises the temperature of the feed water leaving the highest pressure feed water heater to about the saturation temperature corresponding to the boiler pressure.This is done by means of hot flue gases leaving the superheater at a temperature varying from 370°C to 620 °C.It consists of a series of steel tubes through which feed water flows.Hot flue gases pass over the tubes and transfer some of their heat to the feed water.The advantages of an economiser are as follows:
1.It reduced the consumption of coal.
2.It improves thermal efficiency.
 Although an economizer requires  additional floor area ,extra investment and more maintenance cost,it has become an integral part of all modern thermal power plants.

AIR PREHEATER

 The heat carried with the flue gases coming out of economiser is further utilised for preheating air before supplying to the combustion chamber.It has been found that an incease of 20°C in the air temperature increases tha boiler efficiency by one percent.In modern power plants a preheater is a necessary  equipment for supplying hot air to dry the coal in pulverised fuel systems to facilitate grinding and stisfactory combustion of fuel in furnace.The main advantages of air preheaters are as follows:
(i) improve combustion
(ii)effective use of low grade(high ash content) fuel
(iii)saving in fuel combustion
(iv) increased steam generation capacity of the boiler
Air preheaters are usually installed with steam generators that burn solid fuels.They are rarely used with gas or oil fired units.Air preheaters are not essential for the operation of steam generator.Their installation depends upon the economic advantage.on the other hand economisers are used for most boiler burning coal, liquid or gas whether or not air preheater is provided.

SUPERHEATER

The steam produced in the boiler is nearly saturated.The superheater is a heat exchanger to remove the last traces of moisture(1 to 2 %) from the saturated steam coming out of the boiler and to increase its temperature sufficiently above saturation temperature. superheating increases the overall efficienty. In addition,it reduces the moisture content in the last stages of the turbine.Therefore ,there is lesser erosion of turbine blades.the heat of combustion gases is utilized for superheating.The steam is superheated by transferring the heat of combination gases either by convention,or by radition,or by combined convention and radiation. 

POWER PLANTS

MAIN SOURCES OF ELECTRICAL ENERGY

The energy requirement is increasing with the growth of the world's population.The demand of electrical energy is  doubling every ten years.Therefore it has become essential to harness more energy resources with utmost economy so as to keep pace with the requirement of the world's population.

The main sources of generating power on large scale are as follows:
1.Thermal 
2.Hydro 
3.Nuclear

The thermal and Nuclear source of energy come under the category of non-renewable energy sources while the hydel energy is grouped under the renewable energy source.other significant forms of electrical energy sources include the following:
1.Solar energy 
2.wind energy 
3.Tidal energy 
4.Geothermal energy
5. Magneto-hydrodynamic (MHD)generation 
6. Ocen thermal energy conversion (OTEC)
7. Wave energy 

THERMAL POWER PLANTS

conventionally the thermal power plants can be classified as follows:
(a)Conventional coal-fired steam turbine based,
(b)Oil or gas-fired steam turbine based,
(c)Oil/gas fired diesel generator set
(d)Oil/gas fired gas turbines with waste heats recovery and steam turbine units.

SALIENT FEATURES OF A MODERN COAL-FIRED STEAM POWER PLANT

In a coal fired steam power plant,coal is burned in a furnace.The combustion products heat water in a boiler which converts water into steam.The steam drives a turbine which is mechanically coupled to an alternator.
coal is taken from storage.It is then crushed into small pieces and fed to a  pulveriser to make it into fine powdered form.It is mixed with preheated air and blown into the furnace,where it is burned.The modern large-size boiler are all water-tube boilers. In these boilers water flows inside the tubes and hot flue gases flow outside them.The circulation of water through the tubes of the boiler is forced circulation through the action of pumps.
The common equipment used in modern coal-fired steam power plants to increase the thermal efficiency are economizers, super heaters and preheaters.

The combustion in the boiler requires supply of sufficient quantity of air and removal of exhaust gases.Air is taken from the atmosphere by the action of forced and induced draft fans.The air is preheated in the preheater by the heat of flue gases before being supplied to the furnace.
An economiser is used to heat the feed water on its way to boiler by utilizing heat from the flue gases.Steam generated in the boiler is further super heated in a super heater.A superheater raises the temperature of steam to about 620 °C. This superheated  steam is supplied to the turbine which acts as a prime mover.After expansion through the turbines,the steam goes to the condenser. In order to improve the power plant efficiency,sufficiently low pressure(vacuum)is maintained in the condenser.

CIRCULATING WATER PUMPS 

circulate cooling water through the condenser tubes and condense the exhaust steam into water which is pumped back to the boiler.The same water is used over and over again in the boiler.
Water flowing through the tubes gets heated and is cooled again. A condenser needs huge quantity of water to condense the exhaust steam. Modern power plants use larger size of cooling towers(more than 100 m high) for cooling water coming out from the condenser. After cooling in the cooling towers,cooled water is recirculated through the condenser tubes.