What is a Closed Cycle Gas Turbine & Its Working

A closed-cycle gas turbine method is adopted to overcome the disadvantages of an open cycle gas turbine method.  The corrosion and erosion of turbine blades is the main drawback in an open cycle. This drawback can be overcome by using superior quality of working medium (air or helium, argon, hydrogen or neon) where it doesn’t mix with the fuel in the combustion chamber. The other advantage of using a closed cycle method is, the rejection of heat of exhaust gases takes place in a re-cooler or re-heaters or heat exchangers. This article discusses an overview of this turbine, working, advantages, and disadvantages.


What is a Closed-cycle Gas Turbine?

A closed-cycle gas turbine can be defined as a gas turbine, which overcomes the drawbacks of the open cycle gas turbine. In this type of turbine, the air is circulated continuously within the gas turbine with the help of a compressor, heat chamber, gas turbine, and cooling chamber. The ratios of pressure, temperature, and air velocities will be constant in this type. It performs a thermodynamic cycle, which means working fluid is circulated and used continuously again and again without leaving the system.

Closed Cycle Gas Turbine
Closed Cycle Gas Turbine

A closed-cycle gas turbine diagram is very simple and contains components like a compressor, heat chamber, and a gas turbine. The generator, compressor, and cooling chamber are driven by the gas turbine. The diagram of this is shown below.


  • The gas is compressed in the compressor.
  • The compressed gas is heated in the heating chamber.
  • The gas turbine helps to generate electricity.
  • Electricity is generated by the generator with the use of gas turbine
  • The cooling of gases passed from the turbine gets cooled in the cooling chamber.

Efficiency

The efficiency of a closed cycle gas turbine can be explained with the help of the T-S diagram as shown below.

T-S Diagram
T-S Diagram

The efficiency of this can be given as,

n = (available network) / input heat


n = Cp(Wt – Wc) / input heat

n= 1 – [(T4-T1) /(T3-T2)]

Where ‘Wt’ = work is done by the gas turbine per kg of air = Cp(T2-T3)

‘Wc’ = work is done by the compressor per kg of air = Cp(T1-T4)

‘Cp’ constant pressure is taken in KJ or Kg

‘T’ = temperature

Input heat = Cp (T3-T2)

The efficiency of this turbine is higher than the open cycle gas turbine

Closed Cycle Gas Turbine Working Principle

The closed-cycle gas turbine working principle is based on the Brayton cycle or Joule’s cycle.
In this type of gas turbine, the compressor is used to compress the gas isotropically and the resultant compressed gas flows into the heating chamber. The rotor type compressor is preferred in this turbine.

An external source is utilized to heat the compressed air and then passed over the turbine blades.
When the gas is flowing over the turbine blades, it gets expanded and it is allowed to pass into the cooling chamber and gets cooled down. The gas gets cooled by using the circulation of water at constant pressure to its initial temperature.

  • Again the gas is passed into the compressor and the process is repeated.
  • In this turbine, the same gas is circulated repeatedly.
  • The complexity of the system and the cost would increase if the working fluid/medium used in the turbine is other than air. This may lead to problems and it is difficult to resolve.

Difference between Open Cycle and Closed Cycle Gas Turbine

Heat source, type of fluid used for working, circulated air, turbine blades capacity, cost of maintenance and installation, etc gives the difference between the open cycle and closed gas turbine. The circulation of working fluid is the main difference.

Open Cycle Gas Turbine

Closed Cycle Gas Turbine

In this type, the combustion chamber is used for heating compressed air. Due to the mixing of products in the combustion chamber and heated air, the gas doesn’t remain constant.In this type, the heating chamber heats the compressed air, which is compressed firstly before heating. When an external source heats the air, then the gas remains constant.
The amount of gas that came out from the turbine is exhausted in the atmosphereThe amount of gas came out from the gas turbine is allowed to pass into the cooling chamber.
Replacement of working fluid is continuesCirculation of working fluid continues.
The working fluid is airFor better thermodynamic properties, helium is used as a working fluid
As the air in the combustion chamber gets contaminated, results in the earlier wearing of turbine bladesAs there is no contamination of enclosed gas while passing through the heating chamber, results in no earlier wearing of turbine blades
Mainly used for moving vehiclesMainly used for stationary installation and marine applications.
The cost of maintenance is lowThe cost of maintenance is high
Installation mass per KW is lessInstallation mass per KW is more.

Advantages

The closed-cycle gas turbine advantages are

  • High thermal efficiency at any temperature limit and pressure ratio
  • Any type of working fluid can be used with low caloric value. For example helium.
  • No corrosion.
  • Internal cleaning is not required.
  • Re-heaters can be used to heat the water for the supply of hot water for domestic and industrial purposes.
  • The size of the gas turbine is small
  • An increase in pressure gives a better heat transmission coefficient in the exchanger
  • Fluid friction loss is less.

Disadvantages

The closed-cycle gas turbine disadvantages are

  • As the whole system work under high pressure with a working fluid (medium), it increases the cost.
  • It requires a large air heater and it is not enough when the combustion chamber is used in the open cycle.
  • Not used in aeronautical engines because this type of gas turbine uses cooling water.
  • Complex system and should resist at high pressure.

Applications

The closed-cycle gas turbine applications include the following.

  • Used in the generation of electric power
  • Used in many industrial applications
  • Used in marine propulsion, locomotive propulsion, automotive propulsion
  • Used in aviation to provide power to the jet Propulsion

Thus, this is all about the closed cycle gas turbine – diagram, working, efficiency, and differences, advantages, disadvantages, and applications. Here is a question for you, “What are the disadvantages of the open cycle gas turbine? “

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