As we know that in open cycle gas turbine power plant, the fuel is mixed with air in the combustion chamber and the combustion gases are expanded in the gas turbine which causes erosion and corrosion of turbine blades and therefore it is necessary to use fuel superior quality in the combustion chamber in order to minimize erosion and corrosion. This negative effect overcome in case closed gas turbine power plant. In the closed gas turbine power plant, the same air or working fluid is circulated over and over again. The working medium is not mixed with fuel, but it is heated by the burning of fuel in a separate supply of air in the combustion chamber and the transferring this heat to the working fluid which passes through tubes fitted in the combustion chamber. The working fluid does not come in to direct contact with products of combustion. The other disadvantage of open cycle gas turbine gas turbine plant is that the turbine exhaust is discharged into the atmosphere resulting in rejection of heat of exhaust gases to the atmosphere. In case of closed cycle these heat are recovered in a heat exchanger or recooler.
The working fluid (air or any other suitable gas such as helium, argon, hydrogen, and neon) coming out from compressor is heated in the heat exchanger (heater) by an external source at constant pressure. The high temperature and high pressure air coming out from the external heater is passed through the gas turbine. The working fluid coming out from the turbine is cooled to its original temperature in the heat exchanger (cooler) using external heating source before passing in the compressor. In the closed cycle, the working fluid is continuously circulated through compressor, cooler, heater and turbine without its change of phase, the required heat addition and rejection taken place in the heater and cooler respectively.
The performance characteristics, component elements and analysis for open cycle apply equally as well as to the closed cycle gas turbine.
Advantages of closed cycle over open cycle:
1) In the closed cycle gas turbine, the working medium is heated externally and the fuel does not mix with it, hence any inexpensive solid fuel such as coal can be used to heat the working fluid. Also need for filtration of the incoming air is completely eliminated.
2) They are kept free from product of combustion. Hence absence of corrosion and abrasion of the interiors of turbines and compressors extends the life of the plant and maintains the efficiency of the plant constant throughout its life.
3) The working medium is at relatively high internal pressure. So, specific volume is less and the dimensions of the compressors and turbines can be reduced and the maximum unit capacity can be increased.
4) A working medium such as helium, argon, hydrogen, neon may be used which has very good physical properties compared to those of air. For example thermal conductivity of hydrogen is about 6.8 times that of air and therefore require smaller heat exchanger.
5) The closed cycle can be operated with highest efficiencies in comparison to open cycle plants at an equal initial temperature of working fluid.
6) The power output at constant speed can be varied by adding or substracting the working fluid and thus altering the weight of the charge. This gives improved part load efficiency as compared to open cycle gas turbine.
7) High heat transfer can be possible
Disadvantages of closed cycle:
1) In closed cycle gas turbine considerable quantity of cooling water is required.
2) Heat addition to the working medium takes place through heat exchanger, hence full heat of fuel fired is not utilized.
3) Additional equipments such as externally fired heater, cooler are required, the plant and its operation makes additional complexity.
