There are two types of gas turbine open cycle and closed cycle. The thermodynamic cycle used in a gas turbine is the Brayton cycle
The air is used as a working fluid in the Brayton cycle. The compressor pressurizes the air and then lets it ignited by spraying fuel over it. The generated high temperature gas is further expanded in the gas turbine to net work output.
The Brayton cycle consists of four significant processes given in the table below,
|Process 1-2||Isentropic compression (In Compressor)|
|Process 2-3||Constant pressure heat addition (In combustion chamber)|
|Process 3-4||Isentropic expansion (In turbine)|
|Process 4-1||Constant pressure heat rejection (exhaust)|
In the gas turbine cycle, the widely used cycle is a closed-cycle gas turbine. There are few methods employed to increase the performance of the cycle. The gas turbine power plant can give quick output power as compared to coal based thermal power plants.
Components of gas turbine cycle
There are four main components of the gas turbine cycle. The additional components are used to increase performance
- 1. Compressor
- 2. Combustion chamber or combustor
- 3. Turbine
- 4. Condenser
- 5. Regenerator heat exchanger
- 6. Intercooler
- 7. Reheater
The function of every component is predefined in gas turbine cycle. In an open cycle gas turbine, the atmospheric air is compressed by a compressor. The temperature of the air is raised enough to ignite fuel in the combustor. After combustion, the high temperature gas is supplied to the turbine. The turbine blade is getting rotated due to the expansion of this gas. The turbine shaft is rotated with constant output.
The closed-cycle gas turbine is working on the principle of the Brayton cycle (Joule’s cycle). In a gas turbine cycle, the type of compressor used is rotary to pressurize the air isentropically. This higher pressure air is supplied to Combustor. In combustor, the temperature of air is raised at constant pressure. There are two types of combustors available for gas turbine.
1) Radial or annular type 2) Can type
The heated air from the combustor is let expand in turbine for power generation. The electric generator is used with a turbine to transfer mechanical energy in the electrical energy.
The expansion process is carried out at constant entropy (isentropic). After expansion, the gas is getting cooled into the condenser. The condenser is one type of heat exchanger with water as a coolant.
The cooled gas is again reaching the compressor. This process will get repeated continuously for constant power generation.
Gas turbine cycle with regenerator
The regenerator is one of the proper methods to increase the efficiency of the gas turbine cycle.
The counter flow heat exchanger (regenerator) is utilized to exchange heat from exhaust gases of turbine to pressurized air leaving the compressor.
The thermal energy of the gas turbine cycle is increased due to the reuse of exhaust heat. We can say that regeneration decreases the fuel required (by reducing heat input). The regeneration method can increase the thermal efficiency of the gas turbine plant in the range of 35 to 40%. The regenerator causes minor pressure loss in the system. The power output slightly decreased due to pressure loss.
Though the cost and maintenance of the regeneration cycle are required, the overall benefit is more likely. Compared to fuel cost, the regeneration gas turbine cycle is highly beneficial.
Practical example of a closed cycle gas turbine
The closed-cycle gas turbine has the potential to supply quick and continuous power supply by utilizing the following heating sources.
- Fossil fuel
- Biomass energy
- Solar energy (Concentrated solar energy)
- Nuclear energy source
- Waste heat recovery
- Geothermal energy
- Hybrid energy source
- Renewable fuel
The gas turbine cycle can be clubbed with any above listed heating source. The other components like compressor, turbine, and condenser in the gas turbine cycle remain the same. The heating source can be varied from the above examples as per the requirement of power and energy. The widely used fuel for a gas turbine is natural gas or LPG (liquefied petroleum gas). These natural gases are well-known to be utilized because of their properties of combustion and purity. The turbine-like 400 GE is operating on the fuel naphtha, crude oil, or heavy fuel.
The present technology also focuses on the reduction of carbon emissions. The hydrogen powered turbine has been developed to reduce pollutions. As we know, hydrogen has a vast potential for future energy. This turbine is flexible to be utilized in existing as well new power plants to reduce emissions.
Intercooling and reheating in a gas turbine cycle
The Intercooling and the reheating is an additional arrangement to the gas turbine cycle.
The air is cooled in between two stages of compression in inter cooling. This process can reduce the compression work and the output of the gas turbine cycle. In reheating, the hot flue gas from the turbine is again reheated to get expand in another turbine.
The reheating is superior to increase the turbine work. The reheating and intercooling are method for improving the specific power output and thermal efficiency of the gas turbine cycle.
Why are intercoolers used in the compressors?
The intercooler is a valuable component in-between stages of compressors.
In various stages of the compressor, the high temperature of gas from the first stage can reduce the performance of the second stage of the compressor.
The intercooler is installed in between the two stages of the compressor. The hot air from the first stage is cooled in the intercooler and then supplied for second stage compression.
The high temperature occupies more volume of the compressor due to more intermolecular distance. The function of this device is to decrease this volume. The reduction in volume is more beneficial to rise in pressure.
During the intercooling, the water vapors are formed due to the cooling of air. It is required to separate that water vapors from the air. It is also a prime function of the intercooler to supply dry air to the second stage.