Difference between Open Cycle Gas Turbine and Closed Cycle Gas Turbine


A type of internal combustion engine which converts the energy of natural gas or other fuel gas into mechanical rotational energy is called a gas turbine. In simple words, a gas turbine is a type of turbine that driven by expanding hot gases produced by the combustion of fuel gas.

The gas turbines are broadly classified into two types, viz −

  • Open cycle gas turbine

  • Closed cycle gas turbine

This article provides a comprehensive understanding of differences between open cycle gas turbine and closed cycle gas turbine by considering various parameters such as basic definition, working fluid, heat input method, efficiency, cost, etc.

What is an Open Cycle Gas Turbine?

A type of gas turbine in which the hot gases produced by the combustion of liquid fuel to rotate the turbine and the remaining heat will be exhausted to the atmosphere is called an open cycle gas turbine. The schematic diagram of an open loop gas turbine is shown in Figure-1.

The main parts of an open cycle gas turbine are: a compressor, a combustion chamber and a turbine unit. In open cycle gas turbine, the compressor receives the fresh air from the atmosphere and compresses it, then this compressed air is mixed with the fuel and injected into the combustion chamber. In the combustion chamber, the burning of fuel rises the temperature of compressed air. These high pressure hot gases are passed through the turbine blades to spin the turbine.

Finally, after rotating the turbine, the gases are exhausted into the atmosphere and the compressor is supplied with the fresh atmospheric air repeat the process.

What is a Closed Cycle Gas Turbine?

A closed cycle gas turbine is the type of gas turbine in which the air is continuously circulated within the turbine. Hence, in case of closed loop gas turbine, the fuel gas forms a closed thermodynamic system in which the exhaust gases are not released into the atmosphere but instead cooled by a cooling chamber and are returned to the compressor.

A closed cycle gas turbine consists of a compressor, a heating chamber, a gas turbine and a cooling chamber. This type of gas turbine works on the Brayton cycle. In which the working fluid used repeatedly without leaving the turbine system.

In the closed cycle gas turbine, the gas or air is compressed by a compressor which then flows into a heating chamber. In the heating chamber, the compressed air or gas is heated with the help of heat supplied from external source before it is passed over the turbine blades. When these hot and compressed gases flow over the turbine blades, they expand and rotate the turbine and finally enter the cooling chamber, where they are cooled down and the gases is again passed into the compressor to repeat the cycle.

Difference between Open Cycle and Close Cycle Gas Turbine

The major differences between an open cycle gas turbine and a closed cycle gas turbine are listed in the following table −

Basis of Difference Open Cycle Gas Turbine Closed Cycle Gas Turbine
Definition The type of gas turbine in which the working fluid (hot gases) are exhausted into the atmosphere after rotating the turbine is called an open cycle gas turbine. A closed cycle gas turbine is the one in which the working fluid (hot gases) circulates repeatedly without exhausting into the atmosphere.
Heating of compressed air In an open cycle gas turbine, the compressed air is heated in the combustion chamber. In the closed cycle gas turbine, the compressed air is heated in the heating chamber by suppling heat from an external source.
Amount of gas Open cycle gas turbine involves the mixing of products of combustion and compressed hot air. Therefore, the amount of gas does not remain the same throughout the cycle. In closed cycle gas turbine, the heat is supplied from an external source. Therefore, the amount of gas remains the same throughout the cycle.
Exhausted gas In an open cycle gas turbine, the gas after rotating the turbine is exhausted into the atmosphere. In a closed cycle gas turbine, the gas after passing through the turbine is not exhausted into the atmosphere, but instead passes into the cooling chamber.
Circulation of working fluid The working fluid is continuously replaced by the fresh one in case of open cycle gas turbine. Closed cycle gas turbine involves the continuous circulation of working fluid.
Working fluid The atmospheric air is used as the working fluid in an open cycle gas turbine. In a closed cycle gas turbine, any fluid with good thermodynamic properties can be used such as air, helium, carbon dioxide, nitrogen, etc.
Thermal efficiency The thermal efficiency of an open cycle gas turbine is less than closed type. Closed cycle gas turbine has more thermal efficiency.
Loss in working fluid Since cycle is open, hence there is a loss in the working fluid. No loss in working fluid in a closed cycle gas turbine.
Mass of power plant The mass of the power plant, or mass per kW is less when open cycle gas turbine is used. The mass of power plant is more in case of closed cycle gas turbine.
Life of turbine blades The turbine blades get damaged earlier because they come in direct contact with the products of combustion. Hence, the blade’s life is shorter. In closed cycle gas turbine, the life of turbine blades is longer because they do not come in direct contact with the combustion products.
Part load efficiency Open cycle gas turbine has less part load efficiency. The part load efficiency is higher for a closed cycle gas turbine.
Operating pressure The open cycle gas turbines operate at low pressure. The closed cycle gas turbine operates at higher pressure.
Maintenance cost Open cycle gas turbine requires less maintenance cost. The maintenance cost of a closed cycle gas turbine is comparatively high.
Control In open cycle turbine, it is easy to control the load or power production. It is difficult to control the load and power production in case of closed cycle gas turbine.
Construction & cost Open loop gas turbine has simple construction and less cost. The closed loop gas turbine are complicated and expensive.
Suitability Open cycle gas turbines are best suited for moving vehicles where the turbine is discharged into the atmosphere. Closed cycle gas turbines are suitable for stationary applications such as gas turbine power plants.

Conclusion

The most significant difference between open and closed cycle gas turbines is that in an open cycle gas turbine, the discharge is exhausted into the atmosphere; whereas in a closed cycle gas turbine, the exhaust is cooled down and again supplied to the compressor to repeat the cycle.

Updated on: 03-Aug-2022

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