A power station which uses the gas turbine as the prime mover for the alternator, to produce an electrical energy is called gas turbine power station.
In a gas turbine power plant, air is used as a working fluid. The compressor is used to compress the air. The compressed air is taken to the combustion chamber. This adds heat to the air. This increases the temperature of the air. The air is heated either by burning the fuel or using the air heaters.
The hot and high pressure air is then transferred to the gas turbine. Here the air is expanded and drives the turbine. Thus the heat energy is converted to the mechanical energy. The rotating turbine drives an alternator which is coupled to the same shaft. The alternator converts the mechanical energy into an electrical energy. The output of an alternator is given to the bus bar through the transformer, circuit breakers and isolators.
The energy conversion involved in the gas turbine plant is shown in the Fig. 1.
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| Fig. 1 Energy conversion |
This plant can be situated anywhere and is not having any specific requirements for the selection of site.
1.1 General Arrangement of Gas Turbine Power Plant
The Fig. 2 shows the schematic arrangement of gas turbine power plant.
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| Fig. 2 Schematic arrangement of gas turbine power station |
The main components of the plant are,
1. Compressor 2. Regenerator 3. Combustion chamber
4. Gas turbine 5. Alternator 6. Starting motor.
1.2 Compressor
The function of the compressor is to produce high pressure air. The compressor draws the air which is atmospheric pressure and increases its pressure. To remove the dust and other impurities from the air while taking in the compressor, it is passed through a filter. Thus at the output of the compressor, a very high pressure pure air is available, which is necessary for further processes. Generally a rotary type of compressor is used.
1.3 Regenerator
The exhaust gases from the turbine are given to a generator. The regenerator is a device which recovers the heat from the exhaust gases of the turbine, before the gases are released to the atmosphere. A regenerator consists of a shell and a tube. The exhaust gases are made to flow inside the nest of the tubes while the air flows outsides the tubes in the shell in opposite direction. Thus the air is heated up due to the heat given by the exhaust gases. Thus the compressed air is heated before entering the combustion chamber. The regenerator reduces the requirement of the fuel for the combustion to produce heat and improves the pant efficiency. The heated compressed air is taken to the combustion chamber.
1.4 Combustion Chamber
The function of the combustion chamber is to add large amount of heat to the compressed air. In the combustion chamber the heat is produced by burning the fuel like oil. The oil is injected in the chamber through a burner, at a high pressure. Due to this, the temperature of the chamber becomes very high about 3000 Fo. Thus the air also attains very high temperature in a combustion chamber. Before taking to the gas turbine, the air is suitably cooled, to 1500 Fo.
1.5 Gas turbine
The air at high pressure and temperature, available at the output of the compressor, is given to the gas turbine. These gases while passing over the blades expand and do the mechanical work. This drives the turbine and thus the heat energy is converted to a mechanical energy. The temperature of the exhaust gases leaving the turbine is about 900 Fo.
1.6 Alternator
An alternator is coupled to the gas turbine. Thus when the turbine starts rotating, it drives the alternator. The alternator converts the mechanical energy into an electrical energy. The output of the alternator is given to the bus bars through the transformer, circuit breaker and isolators.
1.7 Starting Motor
The compressor is required to be started first before turbine starts. Hence an electric motor is mounted on the same shaft of turbine and compressor. The motor is operated by the batteries. Once the compressor starts, the entire unit starts operating and the turbine starts rotating. Then the turbine drives the compressor and hence motor can be switched off. Thus in running condition, the motor is not required hence it is called starting motor.
1.8 Advantages
1. Much compact and less space required compared to steam power plant.
2. Simple in construction compared to the steam power plant, due to absence of boiler and other equipments.
3. It can be started very quickly and has very short starting time.
4. The fuel consumption during starting and shutting down is very low.
5. The water required is less as condenser is absent.
6. Due to the absence of reciprocating masses such as piston, there are no vibrations and not much noise.
7. The initial and operating costs are much smaller.
8. The maintenance charges are small.
9. There is much higher output per unit.
10. The shaft speeds can be much higher.
11. The standby losses are absent because the parts like boiler are absent which are on through the steam turbine is on no load.
12. The number of workers required is very less.
13. The partial or full automation is possible.
14. Very heavy foundations and structures are not required.
15. The components and circuits can be arranged so as to give most economical results.
1.9 Disadvantages
1. The compressor is required to start first hence an extra motor additional power is required.
2. The part of the turbine work is used to drive the compressor hence net output is low.
3. The overall efficiency is very low as large amount of heat gets wasted through exhaust gases through regenerator is used.
4. The life of the combustion chambers is less due to very high temperature.
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