Various Plants used for Generation of Electric Power : Hydroelectric, Nuclear and Thermal Energy Plants Parts & Working
The different plants used for generation of electric power are the following.
1. Hydroelectric power plant.
2. Nuclear power plant.
3. Thermal power plant.
Hydroelectric Power Plant
Hydro electric power plants convert energy available from flowing water into electricity. These power stations are suitable where water resources with sufficient head are available.
The layout of a typical hydroelectric power plant and description of all major parts are given below.
- Reservoir : The basic requirement of a hydro electric power plant is a good reservoir where large quantity of water is stored during flood season and used during dry season.
- Dam : A reservoir is built by constructing a dam across a river. The function of the dam is to increase the height of water level which in turn increases the reservoir capacity. the dam also helps to increase the working head of the power plant.
- Penstock : A pipe between the surge tank and the power house is known as pen stock. A pen stock is a conductor that takes water from the reservoir to the power house. Usually steel, RCC pipes are used. Penstocks are usually equipped with head gates at the inlet which can be closed during the repair of penstocks.
- Valve House : The valve house contains valves to control the water flow into the power house during normal conditions. the automatic isolating valves in the valve house cuts off the water supply when penstocks bursts.
- Surge Tank : Surge tank is a small additional storage facility near the power house. It is required when there is considerable distancle between the power house and the reservoir. When the distance is more non-uniform water intake to the power house results in the bursting of penstocks. In the absence of surge tank, the excess water rushes at the lower end causing the penstock to burst. However in the presence of a surge tank and can be used whenever there is any water shortage. Thus the surge tank acts as a shock absorber or a pressure regulator tank.
- Power House : A power house houses the turbine and the generator. The turbine rotates the turbine shaft which in turn rotates the generator shaft, which is coupled to the turbine shaft. Thus the turbine converts hydraulic energy into mechanical energy and the generator converts mechanical energy into electrical energy. The power house is usually at the foot of the dam.
working of turbine
Advantages of Hydroelectric Power Plant
- It also serves irrigation and flood control.
- It requires no fuel.
- Low start up time.
- No pollution problem.
- The generation cost is very low.
- It uses simple technology.
- Generally hydro electric power plants have longer life.
Disadvantages of Hydroelectric Power plant
- Cost of construction is high.
- Large area is required for installation.
- It affects the ecological balance of the area.
- The generation is weather dependent.
- Long gestation period.
Nuclear Power Plants
If nuclear fission reaction is made to occur in a controlled manner, then the energy released can be used for constructive purposes like electricity generation. The arrangement or equipment used to carry out fission reaction under controlled conditions is called a nuclear reactor. The energy produced in a controlled manner can be used to produce steam which can run turbines and produce electricity. This arrangement is employed in a nuclear power plant to generate electricity.
The schematic and main parts associated with a nuclear power plant are given below.
- Nuclear Reactor : The nuclear fission reaction occurs in a nuclear reactor. The enriched fuel is used in the reactors in the form of rods. To control the fission reaction, movable rods made of cadmium or boron is suspended between the fuel rods. These rods control the fission process by absorbing excess neutrons. These rods are called the control rods.Moderators are substances used to slow down fast moving neutrons so that they are easily captured by the fuel and bring about further fission reaction. Usually graphite or heavy water is used as moderators.
- Protective concrete shield : The nuclear reactor is enclosed in a thick, massive, concrete, shield so as to protect the surrounding from radiation.
- Heat Exchanger : In order to absorb the heat produced during fission, a liquid called coolant is circulated in the reactor core and heat exchanger. Generally heavy water is used as coolant. In the heat exchanger, the steam is generated using the heat transferred from the reactor. The large amount of heat generated during the nuclear fission of U-235 is used to convert water into steam. This steam is then used to drive the turbines which are connected to electricity generators. In this way, electricity is generated.
Disadvantages of Nuclear Power Plant
- Radiation hazard is a major problem.
- Disposal of radio active wastes creates difficulty.
- It requires high initial cost.
- The fuel is expensive and not abundant.
- It requires high technical knowledge for its operation.
Advantages of Nuclear Power Plant
- Power generation is independent of weather conditions.
- It requires less area for installation.
- The quantity of fuel involved is less.
- Pollution is less when compared to thermal power plants.
Thermal Power Plants
Thermal generation has a central role to play in supplying electric power, and we are striving for the development of power generation technology that is even more efficient.
In a thermal power plant steam is produced and used to spin a turbine that operates a generator. A conventional thermal power plant uses coal, oil or natural gas as fuel to boil water to produce the steam. There are three major types of thermal power plants and they are the following.
- Steam turbine power plants.
- Gas turbine power plants.
- Diesel engine power plants.
- steam turbine power plants use coal, oil or gas as fuel whereas gas turbines use oil or gas only as fuel. In diesel power plants, diesel oil is used as the fuel.
Steam Turbine power Plant
Steam power plant facilities constitute a means of power generation that uses the expansion power of steam. Fuel is burned inside a boiler to heat water and generate steam. This steam is then used to drive turbines which in turn drive the power generators to make electricity. This steam is suitable for the use of thermal energy of relative low temperature (below 600 degree celsius). The steam coming out of the turbine is condensed to water and is recycled.
In order to improve the efficiency of the steam power plant, some additional functional parts such as super heater, economizer etc. is also included in the plant. A schematic and different parts are shown below.
- Coal and Ash System : In this system, the coal from the coal storage is fed to the boiler through coal handling equipment for the generation of steam. Ash so produced due to ash storage through ash-handling system
- Air and Gas System : Air is supplied to the combustion chamber of the boiler through a draught fan. The exhaust gases carrying sufficient quantity of heat and ash are passed through the air heater and then passed into the boiler. the flue gases coming out of the boiler is of the order of 1000 degree celsius. This hot gas is passed through different stages like super heater, economizer, air pre-heater and at last, it is released out through the chimney.
- Boiler House : Boilers burn the fuel transferred from the tank and use the resulting heat to convert water into steam. Inside the boilers are tens of thousands of water carrying tubes. When combustion commences, the temperature inside the boilers rises to between 1,100 and 1,500 degree celsius, the water inside the tubes is turned into high temperature and high pressure steam, and the steam is transferred to the steam turbines. Coal and air in correct proportion is sent to the boiler house for combustion to take place. Water is supplied to the boiler through the boiler feed pump. This water is converted to steam in the boiler house.
- Super Heater : Super heater is a heat exchanger that increases the steam pressure and also eliminates moisture in the steam by using the high temperature flue gases from the boiler.
- Economizer : The water from the condenser is passed to the economizer which heats up the water and sends it to the boiler.
- Air Pre-heater : Air pre-heater acts as heat exchanger. It uses heat from flue gases and heats up the air coming from the air intake system. Then this heated air is send to boiler. This ensures proper combustion and it helps to improve the overall efficiency of the power plant.
- Cooling System : The steam produced in the boiler is sent to the turbine through the super heater. The steam coming out of the turbine is condensed to water using the condenser and cooling tower arrangement. The condensed water can be recycled throughout the process. the feed water pump sends this water to the boiler through the economizer. The steam is cooled by seawater in condenser, restored to water, and then returned to the boiler for reuse. This cycle of water, steam to water is repeated over and over again.
- Turbine and Alternator : The steam produced in the boiler is sent to the turbine through the super heater. This steam drives the turbine. The steam rotates the turbine blades at high speed of 3,000 rpm. The turbines convert the heat energy into mechanical energy. This turns the alternator, which is directly connected to the turbines. Alternators convert this mechanical energy into electrical energy.
Gas Turbine Power Plant
Gas turbines use natural gas, oil etc. as fuel. There are two types of gas turbine cycles namely open cycle and combined cycle. In open cycle, the fuel is burned in compressed air and the exhaust gas produced is used to drive the gas turbine.
Layout and main components of a Gas turbine power plant are given below
- Compressor : The high flow rate of turbine coupled with relatively moderate pressure ratios necessitates the use of rotary compressors. These are of two types. Centrifugal compressor and Axial flow compressor. The axial flow compressor are commonly used in gas turbines because of their higher efficiency (90-95%), even though they are less rugged in construction and have lower pressure ratio, per stage of compression. The air, at atmospheric pressure, is drawn by the compressor through an air filter. the blades are made of air foil section and set in such a way that on rotation air moves towards the trailing edge of the blades. Diffusion takes place as air enters the next set of moving blades, fixed on the stator. In each successive stage compression takes place and air is available at high pressure at the output of the compressor.
- Regenerator : Exhaust gas from the turbine have a substantial heat content which can be utilized for heating the compressed air to be supplied for combustion in the combustion chamber. the device where this heat exchange takes place is known as regenerator. It consists of a nest of tubes in a shell. The exhaust gases flow inside the tube whereas air flows outside the tubes inside the shell, in opposite direction and heated up.
- Combustion Chamber : The air, at high pressure from the regenerator is fed to the combustion chamber where it is heated up by burning oil. the oil is injected through the burner into the chamber, at high pressure to ensure its atomization and through mixing with air. The temperature of chamber may go up to 3000 degree Fahrenheit and the combustion gases are cooled to about 1500 degree Fahrenheit before being delivered to the gas turbine.
- Gas Turbine : The mixture of gases at high temperature and pressure, from the combustion chamber, moves to the gas turbine where it expands and delivers mechanical energy. The temperature of the exhaust gases from the turbine is about 900 F. The gas turbines are normally of axial flow type. the basic requirements of the turbine are light weight, longer life and high efficiency.
- Alternator : The gas turbine is coupled to an alternator. The mechanical energy of the gas turbine is converted into electrical energy by the alternator.
- Starting Motor : Compressor must be driven before it supplies compressed air to the turbine. A motor is operated using either battery supply in case of an isolated system or regular supply. In combined cycle power plants, the hot gas is used to produce steam as well and there is a steam turbine in addition to the gas turbine. Since there is better energy utilization in this case, the overall efficiency increases considerably.
Diesel Electric Power Plant
In the diesel electric power plant, diesel oil is used as fuel. The block schematic and different parts of diesel electric plants are explained below.
- Diesel Engine : This is the main component of the diesel electric power plant which develops power. They may be 4 strokes or 2 stroke engine. 4 stroke engines has lower fuel consumption, more flexibility, better scavenging and higher efficiency than 2 stroke. Cylinders are arranged in V shape to make the engine more compact. 6 to 8 cylinders are commonly used. Speed is in the range of 500-1000 rpm. The diesel engines are compression ignition type. Diesel engines are available in sizes from 75kW to 3750kW.
- Air filter and Supercharger : The function of air filter is to remove the dust from the air, which is taken by the engine. The function of supercharger is to increase the pressure of air supplied to the engine to increase the power of the engine. The supercharger is driven by the engine.
- Exhaust system : This includes silencer and connecting ducts. silencer is required in between the engine and the intake system, since the noise may be transmitted back to outside air via the air intake system. The temperature of exhaust gases are really high that heat is used for heating the oil or air supplied to the engine.
- Fuel System : This includes fuel storage tank, fuel pump, fuel transfer pump, strainers and heaters. the fuel is supplied according to the load on the plant. Strainers are provided to remove the suspended impurities. Heaters are required to heat the oil, especially during winter seasons.
- Cooling System : This includes oil pumps, oil tanks, filters, coolers and connecting pipes. The function of the lubricating system is to reduce the friction of moving parts and to reduce the wear and tear of the engine parts. The life of engine and its efficiency largely depends on the lubricating system.
- Starting System : This includes compressed air tanks. The function of this system is to start the engine from cold by supplying the compressed air.
- Governing System : Their function is to maintain the speed of the engine constant irrespective of load on the plant. This is done by varying fuel supply to the engine according to load.
- Diesel Engine Generator : The generators used in diesel power plants are of rotating field, salient pole construction, speed ranging from 214 to 1000 rpm and capacities ranging from 25-5000 kVA at 0.8 power factor lagging. Generators are coupled directly to diesel engine. They are provided with voltage regulators to allow voltage regulation. The excitation is usually provided at 115 to 230 V from a DC exciter, usually coupled to the engine shaft through a belt.
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