Operating principle - Brayton cycle

Gas turbine plants are of two types

1) open cycle power plant

2) Closed cycle power plant

The line diagram of such power plant is shown below.

Open cycle power plant

It consists of a compressor coupled with turbine on the same shaft. For turbines to operate at fixed load and fixed speed condition, use of single shaft is sufficient.

The air is sucked at the inlet of compressor and compressed by it. Then it is sent to combustion chamber where it meets the fuel and combustion takes place. Large heat is generated on combustion which along with the combustion products reaches the turbine and strikes the blade of turbine. The turbine starts rotating which in turn is coupled with generator. The generator converts mechanical energy to electrical energy. From here electrical energy is distributed as per need.

Heat exchangers can also be added in the arrangement for improving the thermal efficiency of the gas power plant. But its major draw back is that there is pressure drop in the heat exchanger which reduces the power output by 10 percent.

The arrangement below shows the addition of heat exchanger to the gas turbine plant.

For the same temperature limit, open cycle has less thermal efficiency.

Closed cycle gas turbine

It is distinguished from open cycle gas turbine by the fact that the working fluid is re circulated in the system inspite of expelling it to atmosphere. Hence it helps to maintain clean surrounding. Other than this it provides high thermal efficiency. Unlike open cycle turbine which uses air-fuel mixture as fuel, this type of turbine uses other combination of fuel such as carbon dioxide and helium mixture. 

These plants are generally smaller in size as compared to open cycle power plants. In these plants combustion does not take place inside the plant rather heat can be supplied from any source like solar heat, nuclear reaction heat etc which is transferred to the plant by heat exchanger. As heat is not produced by burning fuel, no steam will strike the turbine blades and hence turbine blades have larger life. The only demarit of such plant is that it is costly and complex than open cycle plants.

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