In this article we will discuss about the various methods used for improving the thermal efficiency of a simple gas turbine power plant.
The efficiency of a simple gas turbine power plant can be improved by employing regenerator, intercooler and re-heater. Regenerator is usually of shell and tube construction. The exhaust gases are made to flow inside the nest of tubes while air flows outside the tubes in the shell in the counter flow and heated up by the heat given out by the exhaust gases. Thus the regenerator utilises the heat of exhaust gases to heat the compressed air before it is sent to the combustion chamber, reduces the fuel consumption of the plant and improves the cycle thermal efficiency.
It is noteworthy that addition of regenerator in the circuit makes no change in the duties/work of the compressor and turbine but the quantity of fuel supplied is substantially reduced as the temperature of the air entering the combustion chamber is raised. The other noteworthy point is that the gain in efficiency is greater at lower pressure ratios.
The heat transfer from the regenerator can be improved either by increasing the surface area or by increasing the flow turbulence. Increasing of surface area of regenerator involves higher initial cost, while the increasing of flow turbulence involves increased pressure drop. Thus the design of a regenerator is a compromise between the gains in heat recovery on the one hand and higher initial cost and operating cost on the other.
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However, for short time operation such as peak loads, the cost of regenerator may not justify its use in gas cycle.
A greater part (about two-third) of power developed by the turbine is used in driving the compressor. This requirement of power, however, can be reduced if the compression of air could be done in two or more stages and an intercooler is introduced between the two. This is because of reduction in volume of air due to cooling in the intercooler.
Intercooling means the removal of heat from compressed air between the stages of compression. This necessitates the use of compressors with two stages viz., low pressure and high pressure compressors. The intercooler is heat exchanger which cools the partly compressed air in order to reduce volume and increase density. The intercooling results in improvement of thermal efficiency, air rate and work ratio. By use of intercooling the size of turbine and compressor for the same output is reduced.
The number of stages to be employed for compression is generally decided on the basis of the saving in work done and additional investment required for the equipment (compressors and intercoolers). Maximum advantage of intercooling is had when pressure ratio for each stage is the same and high, compressor efficiency is low and regeneration is employed.
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In reheating, the combustion gases are not expanded in one turbine only but in two turbines. The exhaust of the high pressure turbine is reheated in a re-heater and then expanded in a low pressure turbine. Reheating improves the output from the turbines due to multiple heating in the same way as intercooling improves the performance of the compressor. However, the cost of additional fuel may be heavy unless a heat exchanger is also employed.
This process necessitates a compounding of the turbine. It is specially advantageous for improving the efficiency of plants having high pressure ratio and moderate cycle temperature.
Open cycle gas turbine power plant with a regenerator. Both compressors (high pressure compressor and low pressure compressor) and both the turbines (HP turbine and LP turbine) and mounted on a common shaft. The alternator is coupled to the main shaft through gears so that the speed is reduced to the synchronous speed of the alternator. The efficiency of the plant can be improved to about 30% by using regenerator, intercooler and re-heater.