Closed Cycle Gas Turbine- Components, Working, Advantages, Disadvantages, and Applications
Closed Cycle Gas Turbine is an internal combustion engine that operates on rotary motion instead of reciprocating motion.
This turbine works on the principle of the Brayton Cycle.
Closed Cycle Gas Turbine is an extended version of Open Cycle Gas Turbine. A cooler is added to the Open Cycle Gas Turbine to make it a Closed Cycle Gas Turbine.
A closed-cycle means that the working fluid used in this cycle will be repeatedly used in a cyclic manner and will not be escaped to the atmosphere.
Working fluid in this cycle will be in gaseous form. The working fluid commonly used are air, argon, helium, etc
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Components Of Closed Cycle Gas Turbine:
1 Compressor:
In the compressor, the isentropic process will be carried out i.e the process will be adiabatic and reversible. No heat will be transferred during this process.
2 Burner or Combustion Chamber:
After passing the compressor, the air will enter into the burner or the
Combustion Chamber. In this burner, there will be a constant pressure process. Pressure will be constant but the temperature will increase due to heat addition and volume will also increase.
3 Turbine:
When the hot air from the combustion chamber will reach the turbine and will strike the blade of the turbine, the air will expand, and the turbine will rotate. This expansion of air in the turbine will be an isentropic process.
The turbine is connected to the turbine with a shaft. Both the compressor and the turbine have the same shaft.
When the turbine rotates it also transfer its power to the compressor and help it to rotate. The turbine also transfers its power to some auxiliaries.
The turbine is connected with a generator through a coupling. As the turbine will rotate, the generator will produce electricity using the work done by the turbine.
4) Cooler or Pre Cooler:
The air after providing work done in the turbine is sent to the cooler. The cooler will cool the air to bring it back to its original or initial state.
The cooling process in the cooler will be carried out under constant pressure.
Working Of Closed Cycle Gas Turbine:
Process 1-2:Isentropic Compression
Let assume air is the working fluid. When the air will enter the compressor, the air will be compressed. Due to compression, the pressure and temperature will increase but the entropy will remain constant.
In the P-V diagram, the pressure (P) will increase from P1 to P2 and the volume will decrease from V1 to V2 due to compression.
In the T-S diagram, the temperature (T) will increase from T1 to T2 and the entropy will remain constant as it is an isentropic process.
Process 2-3: Constant pressure heat addition
From 2 to 3 the compressed air from the compressor will enter the burner. In the burner, the external fuel provide in the combustion chamber will mix with air and the mixture will burn inside the combustion chamber after mixing with each other.
After burning the temperature will increase and expansion of air will occur at constant pressure. SO in the combustion chamber, constant pressure heat addition will occur.
In the P-V diagram, the pressure (P) will remain constant and the volume will increase from V2 to V3.
In the T-S diagram, the temperature (T) will increase from T2 to T3 and the entropy will also increase from S2 to S3.
Process
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3-4: Isentropic Expansion:
This process will be the opposite of process 1-2. The heated air from the burner or combustion chamber will reach the turbine and the air will expand inside the turbine and some work will be done due to expansion.
This process will be an isentropic process and no heat will be transferred during this process. In this process volume will increase due to expansion and pressure will decrease.
In the P-V diagram, the volume (V) will increase from V3 to V4, and pressure (P) will decrease from P3 to P4.
In the T-S diagram, the temperature (T) will decrease from T3 to T4 and the entropy (S) will remain constant as it is an isentropic process.
Process 4-1: Constant pressure heat rejection
This process is the opposite of the process 2-3.
In this process, the hot air from the turbine is again cooled and brought to its initial state so that it can be used again.
In the P-V diagram, the pressure (P) will remain constant and volume (V) will decrease from V4 to V1.
In the T-S diagram, the temperature (T) will decrease from T4 to T1 due to cooling, and entropy (S) will also decrease from S4 to S1 due to heat rejection.
Advantages Of Closed Cycle Gas Turbine:
1) In a closed-cycle gas turbine, erosion of the turbine blade due to the contaminated gases and fouling of compressor blades due to dust does not happen. Hence the life of the turbine and compressor extends.
2) The closed cycle does not have atmospheric backpressure at the turbine exhaust like an open cycle gas turbine.
3) In the closed cycle gas turbine there is no need to filter the air coming to the compressor.
4) Working fluid density can be maintained high by increasing the internal pressure range. Hence, the compressor and the turbine are smaller for their rated output.
5) Inferior oil or solid fuel can be used because direct heating is done in a closed-cycle gas turbine. These fuels are of very low cost. Hence the process is more economical.
6) It has a higher output and efficiency.
7) Maintenance cost is low.
Disadvantages Of Closed Cycle Gas Turbine:
1) Due to high internal pressure in the system, all the components used have complicated designs which increase the cost significantly.
2) Its response to load variation is poor as compared to an open cycle gas turbine.
3) It requires very big heat exchangers as the heating of the working fluid is done indirectly.
4) An extra component pre-cooler is added in the closed cycle gas turbine which requires a considerable quantity of cool water.
Applications Of Closed Cycle Gas Turbine:
A closed-cycle gas turbine has the potential to serve as a power conversation system for a wide range of energy sources such as fossil fuel, concentrated solar power, nuclear, biomass, and waste heat.
