Closed cycle gas turbine plant was originated and developed in Switzerland. In the year 1935, J. Ackeret and C. Keller first proposed this type of machine and first plant was completed in Zurich in 1944. It used air as working medium and had a useful output of 2 MW. Since then, a number of closed cycle gas turbine plants have been built all over the world and largest of 17 MW capacity is at Gelsenkirchen, Germany and has been successfully operating since 1967. In closed cycle gas turbine plant, the working fluid (air or any other suitable gas) coming out from compressor is heated in a 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 fluid coming out from the turbine is cooled to its original temperature in the cooler using external cooling source before passing to the compressor. The working fluid is continuously used in the system without its change of phase and the required heat is given to the working fluid in the heat exchanger. The arrangement of the components of the closed cycle gas turbine plant is shown in Figure.
(A) Advantages
The inherent disadvantage of open cycle gas turbine is the atmospheric backpressure at the turbine exhaust. With closed cycle gas turbine plants, the backpressure can be increased. Due to the control on backpressure, unit rating can be increased about in proportion to the backpressure. Therefore the machine can be smaller and cheaper than the machine used to develop the same power using open cycle plant.
The closed cycle avoids erosion of the turbine blades due to the contaminated gases and fouling of compressor blades due to dust. Therefore, it is practically free from deterioration of efficiency in service. The absence of corrosion and abrasion of the interiors of the compressor and turbine extends the life of the plant and maintains the efficiency of the plant constant throughout its life as they are kept free from the products of combustion.
The need for filtration of the incoming air which is a severe problem in open cycle plant is completely eliminated.
Load variation is usually obtained by varying the absolute pressure and mass flow of the circulating medium, while the
pressure ratio, the temperatures and the air velocities remain almost constant. This result in velocity ratio in the compressor and turbine independent of the load and full load thermal efficiency maintained over the full range of operating loads.
The density of the working medium can be maintained high by increasing internal pressure range, therefore, the compressor and turbine are smaller for their rated output. The high density of the working fluid further increases the heat transfer properties in the heat exchanger.
As indirect heating is used in closed cycle plant, the inferior oil or solid fuel can be used in the furnace and these fuels can be used more economically because these are available in abundance.
Finally the closed cycle opens the new field for the use of working medium (other than air as argon, CO2, helium) having more desirable properties. The ratio γof the working fluid plays an important role in determining the performance of the gas turbine plant. An increase in γfrom 1.4 to 1.67 (for argon) can bring about a large increase in output per kg of fluid circulated and thermal efficiency of the plant.
The theoretical thermal efficiencies of the monoatomic gases will be highest for the closed cycle type gas turbine. Further, by using the relatively dense inert gases, such as argon, krypton and xenon, the advantage of smaller isentropic heat fall and smaller cross-sectional flow areas would be realized: Whether CO2 or Helium should be adopted as working medium is matter of controversy at present.
Blade material poses a problem to use helium as working fluid. In case of CO2, a new kind of compressor must be designed to compress the fluid. The main advantage of CO2 is that it offers 40% efficiency at 700°C whereas helium would need 850°C or more to achieve the same efficiency. A helium turbine would also need to run faster imposing larger stresses on the rotor.
The maintenance cost is low and reliability is high due to longer useful life.
The thermal efficiency increases as the pressure ratio (Rp) decreases. Therefore, appreciable higher thermal efficiencies are obtainable with closed cycle for the same maximum and minimum temperature limits as with the open cycle plant.
Starting of plane is simplified by reducing the pressure to atmospheric or even below atmosphere so that the power required for starting purposes is reduced considerably.
(B) Disadvantages
The system is dependent on external means as considerable quantity of cooling water is required in the pre-cooler.
Higher internal pressures involve complicated design of all components and high quality material is required which increases the cost of the plant.
The response to the load variations is poor compared to the open-cycle plant,
It requires very big heat-exchangers as the heating of workings fluid is done indirectly. The space required for the heat exchanger is considerably large. The full heat of the fuel is also not used in this plant. The closed cycle is only preferable over open cycle where the inferior type of fuel or solid fuel is to be used and ample cooling water is available at the proposed site of the plant.
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