Electrical Energy Generation
“The conversion of available energy in different forms in nature to electrical energy is known as electrical energy generation.”
Electrical energy can be obtained from various forms like pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc. All these forms of energy can be converted into electrical energy by the use of suitable arrangements. The arrangements essentially employs alternator coupled with prime mover. The prime mover is driven by the energy obtained from various sources such as burning fuels, water pressure, wind force etc. For example, chemical energy of fuel can be used to produce steam at high temperature and pressure. The steam is fed to a prime mover which may be a steam engine or steam turbine. The turbine converts heat energy of steam into mechanical energy which is further converted into electrical energy by employing suitable machinery and equipment.
Sources Of Energy
As electrical energy is produced from energy available in various forms in nature, these sources of energy can be
- The Sun
- The Wind
- Nuclear Energy
At present, water, fuels and nuclear energy are primary sources used for the generation of electrical energy.
Electrical power energy is produced at special plants known as generating stations or power plants.
A generating station essentially employs a prime mover coupled to an alternator for the production of electric power. The prime mover converts energy from some other forms into mechanical energy. The alternator converts mechanical energy of the prime mover in to electrical energy. The electrical energy produced by the generating station is transmitted and distributed with the help of conductors to various consumers.
Depending upon the form of energy converted in to electrical energy, the generating stations are classified as under:
- Steam power stations
- Diesel power stations
- Hydroelectric power stations
- Nuclear power stations
Steam Power Station
“A generating station which converts heat energy of coal combustion into electrical energy is known as a steam power station”
A steam power station basically works on Rankine cycle. Steam is produced in the boiler by utilizing the heat of coal combustion. The steam is then expanded in the prime mover and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts the mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.
- The fuel used is quite cheap.
- Less initial cost as compared to other generating stations.
- It can be installed at any place irrespective of the existence of coal. The coal can be transported to the site of the plant.
- It requires less space as compared to the hydroelectric power station.
- The cost of generation is lesser than that of the diesel power station.
- It pollutes the atmosphere due to the production of large amount of smoke and fumes.
- It is costlier in running cost as compared to hydroelectric plant.
Important Components Of Steam Power Station
Although steam power station simply involves the conversion of heat of coal combustion into electrical energy, yet it embraces many arrangements for proper working and efficiency. The following are the important components of a steam power station:
- Coal and ash handling arrangement
- Steam turbine
- Feed water
- Steam generating plant
- Cooling arrangement
Coal and ash handling arrangement
The coal is transported to the power by road or rails and is stored in the coal storage plant. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal.
Steam generating plant
The steam generating plant consist of a boiler for the production of steam and other auxiliary equipment for the utilization of flue gases.
The heat of combustion of coal in the boiler is utilized to convert water into steam at high temperature and pressure. The flue gases from the boiler makes their journey through super heater, economizer, air pre-heater and are finally exhausted to atmosphere through the chimney.
The steam produced in the boiler is wet and is passed through a super heater where it is dried and super heated by the flue gases on their way to chimney. Super heating provides two principal benefits. Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine is avoided. The super heated steam from the super heater is fed to steam turbine through the main valve.
An economizer is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economizer before supplying to the boiler. The economizer extracts a part of heat of flue gases to increase the feed water temperature.
- Air preheater
An air preheater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced drought fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are increased thermal efficiency and increased steam capacity per square meter of boiler surface.
- Steam turbine
The dry super heated steam from the super heater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation.
The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformers, circuit breakers and isolators.
- Feed Water
The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the boiler is heated by water heaters and economizer. This helps in raising the overall efficiency of the plant.
- Cooling arrangement
In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed by means of a condenser. Water is drawn from a natural source of supply and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling tower where it is cooled. The cold water from the cooling tower is reused in the condenser.
Efficiency Of Steam Power Station
The overall efficiency of a steam power station is quite low ( about 29%) due mainly to two reasons. Firstly, a huge amount of heat is lost in the condenser and secondly heat losses occur at the various stages of the plant. The heat lost in the condenser cannot be avoided. It is because heat energy cannot be converted into mechanical energy without temperature difference. The greater the temperature difference, the greater is the heat energy converted in to mechanical energy. This necessitates to keep the steam in the condenser at the lowest temperature. But we know that greater the temperature difference, greater is the amount of heat loss. This explains for the low efficiency of such plants.
- Thermal Efficiency
“The ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion of coal is known as thermal efficiency of steam power station. “
Thermal efficiency, ηefficiency = (Heat equivalent of mech. energy transmitted to turbine shaft/Heat of coal combustion)
The thermal efficiency of a modern steam power station is about 30%.
- Overall Efficiency
“The ratio of heat equivalent of electrical output to the heat of combustion of coal is known as overall efficiency of steam power station.”
Overall efficiency, ηefficiency = (Heat equivalent of electrical output/Heat of combustion of coal)
The overall efficiency of a steam power station is about 29%.
The following relation exists among the various efficiencies.
Overall efficiency = Thermal efficiency × Electrical efficiency
Hydro Electric Power Station
“A generating station, which utilizes the potential energy of water at a high level for the generation of electrical energy is known as a hydro electric power station.”
Hydro electric power station’s are generally built in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. From the dam, water is led to a water turbine. The water turbine captures the energy in the falling water and changes the hydraulic energy into mechanical energy at the turbine shaft. The turbine drives the alternator which converts mechanical energy into electrical energy.
- It requires no fuel as water is used for the generation of electrical energy.
- No smoke or ash is produced.
- It requires very small running charges.
- Requires less maintenance.
- It can be be put in to service instantly, requires no long starting time.
- It is robust and has a longer life.
- In addition to electricity generation, they also help in irrigation and controlling floods.
- It involves high capital cost due to construction of dam.
- Skilled and experience hands are required to build the plant.
- It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.
Hydro Electric Power Station Arrangement
Although a hydro electric power station simply involves the conversion of hydraulic energy into electrical energy, yet it embraces many arrangement for proper working and efficiency. The schematic arrangement of a modern hydro electric plant is shown below:
The dam is constructed across a river or lake and water from the catchment area collects at the back of the dam to form a reservoir. A pressure tunnel is taken off from the reservoir and water brought to the valve house at the start of the pen-stock. The valve house contains main sluice valves and automatic isolating valves. The former controls the water flow to the power house and the latter cuts off supply of water when the pen-stock bursts. From the valve house, water is taken to water turbine through a huge steel pipe known as pen-stock. The water turbine converts hydraulic energy in to mechanical energy. The turbine drives the alternator which converts mechanical energy in to electrical energy.
A surge tank is built just before the valve house and protects the pen-stock from bursting in case the turbine gates suddenly close due to electrical load being thrown off. When the gates close, there is a sudden stopping of water at the lower end of the pen-stock and consequently the pen-stock can burst. The surge tank absorbs this pressure swing by increase in its level of water.
Water turbines are used to convert the energy of falling water into mechanical energy. The principal types of water turbines are:
Such turbines are used for high heads. In an impulse turbine, the entire pressure of water is converted into kinetic energy in a nozzle and the velocity of the jet drives the wheel. The example of such type of turbine is the Pelton Wheel. It consist of a wheel fitted with elliptical buckets along its periphery. The force of water jet striking the buckets on the wheel drives the turbine. The quantity of water jet falling on the turbine is controlled by means of a spear placed in the tip of the nozzle. The movement of the spear is controlled by the governor. If the load on the turbine decreases, the governor pushes the needle into the nozzle, thereby reducing the quantity of water striking the buckets. Reverse action takes place if the load on the turbine increases.
Reaction turbines are used for low and medium heads. In a reaction turbine, water enters the runner partly with pressure energy and partly with velocity head. The important types of reaction turbines are:
- Francis turbines
- Kaplan turbines
A Francis turbine is used for low to medium heads. It consists of an outer ring of stationary guide blades fixed to the turbine casing and an inner ring of rotating blades forming the runner. The guide blades control the flow of water to the turbine. Water flows radially inwards and changes to a downward direction while passing through the runner. As the water passes over the rotating blades of the runner, both pressure and velocity of water are reduced. This causes a reaction force which drives the turbine.
A kaplan turbine is used for low heads and large quantities of water. It is similar to Francis turbine except that the runner of kaplan turbine receives water axially. Water flows radially inwards through regulating gates all around the sides, changing direction in the runner to axial flow. This causes a reaction force which drives the turbine.
Diesel Power Station
“An electricity generating station in which diesel engine is used as the prime mover for the generation of electrical energy is known as diesel power station.”
In a diesel power station, diesel engine is used as the prime mover. The diesel burns inside the engine and the products of this combustion act as the “working fluid” to produce mechanical energy. The diesel engine drives the alternator which converts mechanical energy into electrical energy. As the generation cost is considerable due to high price of diesel, therefore, such power stations are only used to produce small power.
Although steam power stations and hydro electric plants are invariably used to generate bulk power at cheaper cost, yet diesel power stations are finding favor at places where demand of power is less, sufficient quantity of coal and water is not available and the transportation facilities are inadequate. These plants are also used as standby sets for continuity of supply to important points such as hospitals, radio stations, cinema houses and telephone exchanges.
- The design and layout of the plant are quite simple.
- It occupies less space as the number and size of the auxiliaries is small.
- It can be located at any place.
- It can be started quickly and can pick up load in a short time.
- There is no standby losses.
- It requires less quantity of water for cooling.
- The overall cost is much less than that of steam power station of the same capacity.
- The thermal efficiency of the plant is higher than that of a steam of power station.
- It requires less operating staff.
- The plant has high running charges as the fuel.
- The plant does not work satisfactorily under overload conditions for a longer period.
- The plant can only generate small power.
- The cost of lubrication is generally high.
- The maintenance charges are generally high.
Nuclear Power Station
” A generating station in which nuclear energy is converted into electrical energy is known as a nuclear power station.”
In nuclear power station, heavy elements such as Uranium (U235) or Thorium (Th232) are subjected to nuclear fission in a special apparatus known as a reactor. The heat energy thus releases is utilized in raising steam at high temperature and pressure. The steam runs the steam turbine which converts mechanical energy into electrical energy.
The most important feature of a nuclear power station is that huge amount of electrical energy can be produced from a relatively small amount of nuclear fuel as compared to other conventional type of power stations. At present, energy crisis is gripping us and , therefore, nuclear energy can be successfully employed for producing low cost electrical energy on a large scale to meet the growing commercial and industrial demands.
- The amount of fuel required is quite small. Therefore, there is a considerable saving in the cost of fuel transportation.
- A nuclear power plant requires less space as compared to any other type of the same size.
- It has low running charges as a small amount of fuel is used for producing bulk electrical energy.
- This type of plant is very economical for producing bulk electric power.
- It can be located near the load centers because it does not require large quantities of water and need not be near coal mines. Therefore, the cost of primary distribution is reduced.
- It ensures reliability of operation.
- The fuel used is expensive and is difficult to recover.
- The capital cost on a nuclear plant is very high as compared to other types of plants.
- The erection and commissioning of the plant requires greater technical know how.
- The fission by products are generally radioactive and may cause a dangerous amount of radioactive pollution.
- Maintenance charges are high due to lack of standardization.
- Nuclear power stations are not suitable for varying loads as the reactor does not respond to the load fluctuations effficiently.
- The disposal of the by products, which are radioactive, is a big problem. They have either to be disposed off in a deep trench or in a sea away from sea shore.
Important Components Of Nuclear Power Station
The important components of a nuclear power station are:
- Nuclear Reactor
- Heat Exchanger
- Steam Turbine
It is an apparatus in which nuclear fuel (U235) is subjected to nuclear fission. It controls the chain reaction that starts once the fission is done. If the chain reaction is not controlled, the result will be an explosion due to the fast increase in the energy released. A nuclear reactor is a cylindrical stout pressure vessel and houses fuel rods of Uranium, moderator and control rods. The fuel rods constitute the fission material and release huge amount of energy when bombarded with slow moving neutrons. The moderator consists of graphite rods which enclose the fuel rods. The moderator slows down the neutrons before they bombard the fuel rods. The control rods are of cadmium and are inserted into the reactor. Cadmium is strong neutron absorber and thus regulates the supply of neutrons for fission. When the control rods are pushed in deep enough, they absorb most of the fission neutrons and hence few are available for chain reaction which, therefore, stops. However, as they are being withdrawn, more and more of these fission neutrons cause fission and hence the intensity of chain reaction is increased. Therefore, bu pulling out the control rods, power of the nuclear reactor is increased, whereas by pushing them in, it is reduced. The heat produced in the reactor is removed by the coolant, generally a sodium metal. The coolant carries the heat to the heat exchanger.
The coolant gives up the heat to the heat exchanger which is utilized in raising the steam. After giving up heat, the coolant is again fed to the reactor.
The steam produced in the heat exchanger is led to the steam turbine through a valve. After doing useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.
The steam turbine drives the alternator which converts mechanical energy into electrical energy. The output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators.
Variable Load On Power Station
“The load on a power station varies from time to time due to uncertain demands of the consumers and is known as variable load on the station.”
A power system is designed to meet the load requirements of the consumers. An ideal load on the station, from stand point of equipment needed and operating routine, would be one of constant magnitude and steady duration. The load demand of one consumer at any time may be different from that of the consumer. The result is that load on the power station varies from time to time.
“The curve showing the variation of load on the power station w.r.t time is known as a load curve.”
The load on a power station is never constant, it varies from time to time.
The daily load curves have attained a great importance in generation as they supply the following information readily.
- The daily load curve shows the variations of load on the power station during different hours of the day.
- The area under the daily load curve gives the number of units generated in the day.
Units generated/day = Area (Kwh) under daily load curve
- The highest point on the daily load curve represents the maximum demand on the station of that day.
- The area under the daily curve divided by the total number of hours gives the average load on the station in the day.
Average Load = Area (kwh) under daily load curve/24 hours
- The ratio of the area under the load curve to the total area of rectangle in which it is contained gives the load factor.
Load factor = Average load/Maximum demand = Average load × 24 / Maximum load × 24
= Area (Kwh) under daily load curve/ Total area of rectangle in which the load curve is contained
- The load curve helps in selecting the size and number of generating units.
- The load curve helps in preparing the operation schedule of the station.
Important Terms And Factors
The variable load problem has introduced the following terms and factors in power plant engineering.
“It is the sum of continuous ratings of all the equipment’s connected to supply system.”
“It is the greatest demand of load on the power station during a given period.”
“It is the ratio of maximum demand on the power station to its connected load.”
Demand factor = Maximum demand/ Connected load
The value of demand factor is usually less than 1.
“The average of loads occurring on the power station in a given period is known as average load or average demand.”
Daily average load = No. of units (Kwh) generated in a day/24 hours
Monthly average load = No. of units (Kwh) generated in a month/Number of hours in a month
Yearly average load = No. of units (Kwh) generated in a year/8760 hours
“The ratio of average load to the maximum demand during a given period is known as load factor.”
Load factor=Average load/Maximum demand
If the plant is in operation for T hours,
Load factor = Average load×T/Maximum demand×T
= Units generated in T hours/Maximum demand×T hours
“The ratio of the sum of individual maximum demands to the maximum demand on power station is known as diversity factor.”
Diversity factor = Sum of individual maximum demands/Maximum demand on power station
Plant Capacity Factor
“It is the ratio of actual energy produced to the maximum possible energy that could have been produced during a given period.”
Plant capacity factor = Actual energy produced/Maximum energy that could have been produced
= Average demand×T /Plant capacity×T
= Average demand/Plant capacity
Thus if the considered period in one year,
Annual plant capacity factor= Annual kWh output/Plant capacity×8760
The plant capacity factor is an indication of the reserve capacity of the plant. A power station is so designed that it has some reserve capacity for meeting the increased load demand in future. Therefore, the installed capacity of the plant is always somewhat greater than the maximum demand on the plant.
Reserve capacity = Plant capacity – Maximum demand
Plant Use Factor
“It is ratio of kWh generated to the product of plant capacity and the number of hours for which the plant was in operation.”
Plant use factor = Station output in KWh/Plant capacity ×Hours of use
Units Generated Per Annum
It is often required to find the kWh generated per annum from maximum demand and load factor. The procedure is as follows:
Load factor = Average load/Maximum demand
Average load = Maximum demand×load factor
Units generated/annum = Average load(kW)×Hours in a year
=Maximum demand(kW)×load factor×8760
Base Load And Peak Load On Power Station
The changing load on the power station makes its load curve of variable nature.
It is clear that load on the power station varies from time to time. However, a close look at the load curve reveals that load on the power station can be considered in two parts
- Base Load
- Peak Load
“The unvarying load which occurs almost the whole day on the station is known as base load.”
“The various peak demands of load over and above the base load of the station is known as peak load.”