Gas turbine power plant- construction and working

This is The Mechanical post and today we’ll see What is Gas turbine power plant? Its types, construction, and working along with advantages and limitations of gas turbine power plant.  

You can download the PDF on the gas turbine power plant at the end of the article!

What is a Gas turbine power plant?

A gas turbine power plant is a type of power plant which works by converting the kinetic energy of the hot gases produced by combustion into rotational mechanical energy. This mechanical energy is then converted into electrical energy with the help of a generator.  

The gas turbine power plant is similar to a steam power plant, however, as the name suggests instead of steam, hot gases produced by the combustion of fuel are used to rotate the turbine.  

Working principle of Gas turbine

The gas turbine works on the principle of the Brayton cycle in which air is compressed by a compressor and then heated by combustion of fuel at constant pressure and is allowed to expand through a turbine to produce work.   

The Gas turbine power plant is mainly classified into 2 types

• Open cycle gas turbine power plant
• Closed cycle gas turbine power plant

Let us now see them in detail starting off with the open cycle gas turbine power plant.  

Open cycle gas turbine

The open cycle gas turbine is the basic type of gas power plant which intakes air, compresses it, heats it, and then is allowed to expand in the turbine to perform work.

However, after the expansion, it is exhausted to the atmosphere instead of being reused hence, the name open cycle.  

Components of Open cycle Gas turbine power plant

The following are the major components of an open cycle gas turbine power plant:

• Compressor
• Combustion chamber
• Gas Turbine
• Starting motor
• Generator

Working of Open cycle gas turbine

The starting motor is used to initially start the compressor. The compressor then intakes fresh air and compresses it to a higher pressure.   

At the same time, the temperature of the air also increases. This compressed air is then passed to the combustion chamber.  

Liquid fuels like kerosene etc or natural gas are the types of fuel used in open cycle gas turbines.  

The combustion chamber increases the heat energy of the compressed air at a constant pressure in the gas turbine power plant.  

The hot high-pressure gas is then sent to the turbine. The high pressure of the gas is converted into kinetic energy with the help of a nozzle. 

The jet of hot gases coming from the nozzle is allowed to strike the turbine blades which rotate the turbine blades. Thus, the kinetic energy is converted into rotational mechanical energy.

The gas is then exhausted to the atmosphere through the chimney.   

Now, since the compressor and the turbine are mounted on a common shaft, out of the work produced, some amount is consumed by the compressor to compress the air, whereas the rest of the energy is supplied to the generator.  

The generator then converts the rotational mechanical energy into electrical energy. This is how electric power is generated in an open cycle gas turbine.  

Closed cycle gas turbine

The closed-cycle gas turbine makes use of the exhaust gas from the turbine by sending it to the heat exchanger instead of exhausting it to the atmosphere.   

The heat supplied to the gas is indirect i.e. a heat exchanger called the air heater is used to heat the air through surface contact.

Thus, the combusted fuel never comes in contact with the gas. Only the heat is transferred to the gas via a heat exchanger.  

Components of Closed cycle gas turbine

The following are the components of closed cycle gas turbine:

• Air Compressor
• Air heater
• Turbine
• Heat exchanger
• Generator
• Starting motor

Working of Closed cycle gas turbine

Initially, the starting motor helps rotate the compressor so that the power plant could function.   The air compressor compresses the air to high pressure and temperature coming from the heat exchanger.   

This high-pressure compressed air is then sent to a heat exchanger called the air heater. Air, helium, carbon dioxide, and nitrogen are commonly used working fluids in the closed cycle gas turbine.  

The air is heated at constant pressure. After that, it is sent to the turbine where the gas is allowed to expand in the turbine through a nozzle.   

The high-velocity jet of gas strikes the turbine, thus, converting the kinetic energy of the gas into rotational mechanical energy.  

The turbine and the compressor are mounted on the same shaft, therefore part of the energy is consumed by the compressor, and the rest is sent to the generator to produce electrical energy.

The exhaust gas from the turbine is then sent to the heat exchanger where the gas is cooled i.e. heat rejection takes place at a constant pressure.   

The gas is then sent to the compressor and the cycle begins once again. Because of this closed circuit, this type of power plant is known as the closed cycle power plant.  

P-V and T-S diagram of Closed cycle gas turbine

Process 1-2 = Adiabatic compression of gas.

Process 2-3 = Heat addition at a constant pressure.

Process 3-4 = Adiabatic expansion of gas in the turbine.

Process  4-1 = Heat rejection at constant pressure.

Explanation of P-V and T-S diagram of Closed cycle gas turbine

Process 1-2    

The air enters the compressor and is compressed adiabatically hence for processes 1-2 you can see a rise in pressure in the PV diagram.

At the same time, a straight line can be seen in the TS diagram as the adiabatic compression increases the temperature while there is no change in entropy.  

Process 2-3

The compressed air is sent to the combustion chamber where it is heated at constant pressure, thus we can see a straight line in the PV diagram and we get a curve in the TS diagram due to heat addition.  

Process 3-4  

The hot gases are allowed to expand in the turbine which results in a pressure drop at constant entropy, therefore we see a decreasing curve in the PV diagram and a straight line in the TS diagram.

Process 4-1  

The Gas is then passed through the Heat exchanger where heat is rejected at constant pressure hence a straight line in the PV diagram and since heat is rejected, entropy and temperature decrease, thus, the drop in the TS diagram.

Advantages of Gas turbine power plant

The following are the advantages of the gas turbine power plant:

• Poor quality fuel can be used for combustion.
• The size of the power plant is small and less in weight.
• The initial cost is low.
• Easy to construct and can be quickly started.
• Running costs are low.

Limitations of Gas turbine power plant

The following are the disadvantages of a gas turbine power plant:

• The power plant needs an external source for starting i.e. they are not self starting.
• The thermal efficiency of gas power plants is low.
• Overall efficiency is low as part of the power developed is consumed by the compressor itself.
• Solid fuels cannot be used for such type of power plant.
• Turbine blades require special cooling due to the high temperature of the gases entering the turbine.
• Due to extremely high temperatures, the turbine blades are made up of special alloys which drive up the cost. 

Difference between Open cycle and closed cycle gas turbine power plants:

Open cycle Gas turbine	Closed cycle Gas turbine
The gas is heated directly in the combustion chamber in an open cycle gas turbine	However, in the closed cycle, the gas is heated indirectly via a heat exchanger known as the air heater.
The thermal efficiency of the open cycle gas turbine is lower.	The thermal efficiency of the closed-cycle gas turbines is higher.
The output of the open cycle gas turbine is comparatively lower.	The output of the closed cycle is higher as compared to open cycle gas turbines.
The operating pressure in open cycle gas turbines is lower.	The operating pressure in closed cycle gas turbines is higher
Open cycle gas turbines are smaller in size.	Closed cycle gas turbines are comparatively larger in size.
In the open cycle, the air and fuel are combusted together.	The air and fuel don’t come in contact in a closed cycle
In the open cycle gas turbine, the exhaust gas from the turbine is let into the atmosphere.	The gas in the closed cycle gas turbine is recirculated.
In an open cycle turbine, the blade life suffers as the gas coming from the combustion chamber may contain pollutants or other foreign particles leading to wear of the blade	This is not in the case of a closed cycle, thus the blade life is comparatively longer.
The maintenance cost is lower.	Maintenance cost is comparatively higher.

Some FAQ on Gas turbines

What is used in gas turbine power plant?

High pressure and high-temperature gases are used in the gas turbine power plant to rotate the turbine and produce work. The gas is first compressed by a compressor and then heated to a high temperature before expanding in the turbine.  

How do you start a gas turbine power plant?

The gas turbine power plant is started by a  motor called the starting motor which brings the compressor into motion initially so that the power plant could get started.  

What is the difference between steam turbine and gas turbine?

The initial cost and running costs of the gas turbine power plant are less when compared to a steam power plant. Also, the gas turbine power plant can be started quickly. It is easier to control or govern the gas turbine as compared to the steam turbine.  

Which fuel is used in gas turbine?

Natural gas is widely used for combustion in gas turbines. However, other gases and liquid fuels are also used. For eg. Kerosene, gasoline, etc.                   

That’s all about the Gas turbine power plant. Will see you again until then Keep learning!

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