Compressors, combustion chambers, and turbines are the basic components of a gas turbine power plant. The working principle of the gas turbine power plant is explained along with the overview of open and closed-cycle operation. Gas turbine plant components like regenerator, intercooler, and reheater contribute to the energy efficiency of the plant. The working and applications of these components are discussed here.
Site Selection for Gas Turbine Power Plant
The main criteria for site selection are the availability of land at low cost, a good amount of water supply, less distance from load centre, and the plant should be away from the crowded neighbourhood. Each of these is explained in how to select a site for a power plant.
Working Principle of Gas Turbine Power Plant
The gas turbine unit draws air from the atmosphere and passed it to the compressor. In the compressor, the pressure of air is raised several times that of the atmosphere. After that compressed air goes to the combustor where fuel is injected and burned. The burning of fuel increases the temperature of the compressed air. The product of the combustor passed through the turbine. In a turbine high-temperature, compressed air expand and develops motive power. This motive power results in the rotation of the turbine motor.
A part of the power developed by the turbine is used to turn the compressor and auxiliaries while the rest is used to generate electrical energy. After expansion and development motive power gases leave the turbine at atmospheric pressure.
The temperature of exhaust gases is in the range of about 475oC to 550oC. In the compressor, a part of the air is delivered ahead of burning fuel. This cools hot combustion products from 1600oC to the range of 650oC to 800oC. If hot combustion products are allowed to enter the turbine then the turbine blade will get damaged.
In this above simple arrangement, about 65% of the power drives the compressor and net output amounts to only about 20% of the energy in form of fuel. By using the following components of a gas turbine power plant energy efficiency of the plant can be improved significantly.
Methods to Improve Thermal Efficiency of Gas Turbine Plant
Regenerator
In an open cycle, the temperature of exhaust gases is very high temperature. If we can transfer the heat of these exhaust gases to compressed air, then the amount of fuel could be saved. The exchange of heat takes place in the re-generator. Re-generator is of shell and tube construction. Exhaust gasses are made to flow inside the nest of the tube, whereas compressed air flows outside the tube in the shell. Compressed air flows in the opposite directions of exhaust gases and the heat will get transferred.
There is a reduction in fuel consumption and an improvement in cycle thermal efficiency by the use of a re-generator. There is no effect of the re-generator on the working of the compressor and turbine. To improve heat transfer from the re-generator we can either increase the surface area or increase the turbulence flow of the gases.
Inter-Cooling
Air compression can be done in two stages. In this case, an inter-cooler will be introduced. For a high-pressure ratio, more stages can be used. The number of stages used for compression depends on the presser ratio. The two-stage compressor structure results reduction in the volume of cooled air in the intercooler. Inter-cooler cools the air after it has partially compressed and hence improvement in thermal efficiency, air rate, and work ratio. When an intercooler is employed, the size of the compressor and the turbine can be reduced for the same output or the same size plant can give greater work.
Reheating
Like a two-stage compressor in an inter-cooler, we can use two turbines. In the first high-pressure turbine, gases partially expand. After that gases are reheated and allowed to expand further in other turbines and may produce additional work. As the inter-cooler improves compressor efficiency, the re-heater improves output for the turbine by multiple heating.
Working of Open and Closed Cycle Gas Turbine Power Plant
Open Cycle: Complete view of an open-cycle gas turbine power plant with components is shown below figure. Fresh Air is drawn in the compressor and its pressure is raised using a two-stage compressor setup. After that heat is added to the compressed air in the combustor by burning fuel and air temperature is raised. Gases coming out of the combustor pass through the turbine where they expand to do mechanical work.
In an open cycle, the gases are exhausted into the atmosphere. So, the working medium has to be replaced continuously. The temperature of exhaust gases is quite high so they need to be released at a certain height. So that it will not harm the individuals living near the plant.
Closed Cycle: In the case of closed-cycle, working fluid is fed into the compressor. Compressed air goes through the combustor where its temperature increases. High-temperature air is expanded in the turbine. After that, the exhaust gases passed to the regenerator to transfer the heat to compressed air. There is one more stage “re-cooler” added in closed-cycle operation. In re-cooler, the temperature of exhaust gases is cooled to the original temperature in re-cooler. Then it re-enters to the compressor to begin the cycle. In this way, the same working medium circulates through the working part of the system. A complete closed-cycle gas turbine power plant is shown below.
What are the Applications of Gas Turbine Plant
- Useful as an auxiliary power plant for the thermal station.
- Unlike a diesel power plant, gas turbine plant can handle peak loads.
- Used in combination with a conventional steam boiler.
- Stand-by unit for peak loads in power generation.
- Drives pumps, compressor, high-speed cars etc.
Advantages of Gas Turbine Plant
- Capital cost is lower than the steam turbine plant.
- There are no standby losses.
- Can be located very near to load.
- Foundation and building are simple and cheap unlike hydroelectric power plants.
- Like thermal power plant there is no ash handling problem.
- Require fewer additional components.
- Emission of gasses is less.
Disadvantages of Gas Turbine Plant
- External driving power is required to start the compressor, before starting the unit.
- Required more area as compared to the diesel power plant.
- The life cycle is less as compared to other plants.
- High operating and maintenance cost.