Closed Cycle Gas Turbine Power Plants and Its Advantages

In this article we will discuss about the closed cycle gas turbine power plants and its advantages.

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 becomes essential to use fuel of superior quality in the combustion chamber. This drawback is overcome in case closed cycle power plant is adopted, in which fuel is not mixed with the working medium (air or any other gas such as helium, argon, hydrogen and neon).

The other draw­back of the open cycle plant is that the turbine exhaust is discharged into the atmosphere resulting in rejection of heat of exhaust gases to the atmosphere. In closed cycle plant this drawback is overcome by rejecting heat of exhaust gases in a heat exchanger or re-cooler.

In a closed cycle plant, the medium is heated externally and is continuously circulated through the compressors, heat exchangers, intercoolers, re-heaters and gas re-coolers. The pressure ratios, the temperatures and air velocities in the system remain practi­cally constant.

The advantages of a closed cycle power plant are given below:

1. The risk of corrosion and abrasion of the interiors of turbines is eliminated as the turbines are kept free from the combustion products.

2. Since the working medium is heated externally and the fuel is not mixed with it, any fuel (inferior type of fuel or solid fuel) can be used.

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 with physical properties supe­rior to those of air (such as helium, argon, hydro­gen, neon) may be used. The heat conductivity of hydrogen is about 6.8 times that of air, so with the use of hydrogen as working medium, the size of heat exchangers is reduced.

5. There is an improvement in the rate of heat transmission.

6. There is a reduction in fluid friction loss due to higher Reynolds’s number.

7. These plants can be operated with highest efficiencies in comparison to open cycle plants at an equal ini­tial temperature of working fluid.

8. The power output at constant speed can be varied by adding or subtracting the working fluid and thus altering the weight of the charge. This gives im­proved part-load efficiency as compared to open cycle gas turbine.

However, all the advantages of closed cycle operation are offset by the additional complexity of the plant which includes externally fired heater, gas re-cooler and various heat exchangers.

If the working medium is other than air, it is necessary that the system be gas tight. This adds to the cost and in­creases the engineering problems.

Closed cycle gas turbine plant would be preferable over open cycle one only when inferior type of fuel or solid fuel is to be used and ample cooling water at site is available. Closed cycle plants have not as yet been employed for gen­eration of electrical power because of requirement of large size heat exchangers.

Some gas plants have been designed with a combination of open and closed cycles. Such plants are known as semi-closed cycle plants.