In closed cycle gas turbine plant, the working fluid does not come in contact with the atmospheric air. The working fluid is air or some other suitable gas.
The cold gas at point A is compressed in the compressor at the expense of compressor work, Wc. The heat energy to compressed gas is indirectly supplied in the heat exchanger called source.
The hot gases at high pressure at point C enter the turbine. These hot gases expand upto the inlet pressure of the compressor.
During expansion of the gases, the work developed is WT. Part of the work developed WT will be used to drive the compressor and network of (WT-Wc) is available as shaft work.
Exhaust gases of the turbine at point D are cooled in a heat exchanger or sink rejecting the heat to cooling media. The gas is gets back to its initial stage 1.
In this cycle, the same working fluid is recirculated and it remains uncontaminated by the products of combustion.