The topic Brayton cycle Vs Rankine cycle gives us an idea that they both must be similar in some aspects. Both cycles are used to generate mechanical energy out of thermal energy.
The major difference between these cycles is the working fluid used. Rankine cycle uses liquid (mostly water) as working fluid whereas Brayton cycle uses gas (mostly air) as working fluid. This article does a comparative analysis on Brayton cycle vs Rankine cycle.
Major components used in Brayton cycle
Every cycle needs a set of machinery that helps achieve the desired output.
Brayton cycle consists of following working parts-
- Compressor- Compresses the air isentropically.
- Mixing chamber- Heat is added to the compressed air that increases the temperature isobarically.
- Turbine- Air is expanded in turbine, as turbine shaft rotates the air pressure reduces and temperature reduces. This process is isentropic expansion.
Working of Brayton cycle
Brayton cycle generally uses atmospheric air as its working fluid. It takes minimum three processes to complete this cycle (An open cycle has three processes and closed cycle has minimum four processes).
The different processes that the working fluid undergoes in closed Ideal Brayton cycle are-
- Isentropic compression- Ambient air is drawn inside the compressor and compressed isentropically.
- Isobaric heat addition- Heat is added to the compressed air at constant pressure.
- Isentropic expansion- Air is expanded in a turbine isentropically.
- Isobaric heat rejection- Heat is rejected from the system at constant pressure.
Isentropic compression and expansion processes denote an ideal cycle. Usually, the process is not completely isentropic due to irreversibilities and friction losses in turbine and compressor. The isentropic efficiency of turbine and compressor denote the magnitude of useful output that can be obtained from given conditions.
Parts used in Rankine cycle
Rankine cycle produces mechanical energy from thermal energy of the working fluid. This is achieved by many components working in harmony.
The working components used in Rankine cycle are-
- Pump- The low pressure liquid is pumped to boiler increasing its pressure.
- Boiler- Heat is added to the working liquid inside the boiler. The heat addition process is isobaric. The high pressure liquid gets converted to high pressure steam inside the boiler.
- Turbine- Steam is expanded in turbine. The high pressure steam is responsible for producing mechanical energy which is achieved by turbine shaft rotation.
- Condenser- The low pressure steam is condensed inside the condenser. Condenser is nothing but a heat exchanger that extracts heat from the steam to convert it into liquid.
Working of Rankine cycle
Rankine cycle is used to produce mechanical energy from thermal energy of working fluid (in this case water) which in turn is used for generating electricity (shaft power of turbine is used to produce electricity).
Rankine cycle also works on four major processes. They are-
- Isentropic compression (process 1-2): Pressure of working fluid increases in this process.
- Isobaric heat addition (process2-3): High pressure liquid is subjected to heat inside the boiler where it gets converted into steam. The steam exits at high pressure and enters the turbine at point 3.
- Isentropic expansion (process 3-4): The high pressure steam rotates the turbine propellers as a result turbine shaft starts rotating. During this process, the high pressure steam gets converted to low pressure steam. The low pressure steam enters the condenser.
- Isobaric heat rejection- The steam gets converted back to liquid state inside the condenser. The heat is rejected from the steam at constant pressure as a result of which the steam gets converted to liquid.
Note that condenser and boiler are devices that change the state of working fluid without changing the temperature and pressure.
Rankine cycle and Brayton cycle efficiency
Efficiency is the measure of cycle’s effectiveness. The amount of output a cycle can deliver in a given amount of input is called the efficiency of a cycle.
Before discussing the efficiencies of Rankine cycle and Brayton cycle, lets have a look on turbine and compressor efficiency-
Turbine isentropic efficiency is given by-
Compressor isentropic efficiency is given by-
The comparison of Rankine cycle efficiency and Brayton cycle efficiency is given below-
|Subject of comparison||Rankine cycle||Brayton Cycle|
Image credits: Rankine cycle by Home IITK
How to increase efficiency of Brayton cycle and Rankine cycle?
Efficiency is the ratio of output to input. To increase the efficiency of any cycle, one needs to increase output at constant input or decrease input for constant output or increase the output while reducing the input.
In both the cycles, same methods can be used to improve the efficiency. These methods are-
- Regeneration– Steam from condenser is passed through turbine to increase inlet temperature before the steam enters the boiler.
- Reheat- A secondary turbine is used that results in more work output.
- Intercooling– Intercooler cools the gas after compression thereby making it available to be compressed again. This way the compressor work is reduced.
- Combined regeneration, intercooling and reheat cycle– This cycle uses combination of regenerative cycle, reheat cycle and intercooling.
What are the two main types of Brayton cycle?
Brayton cycle may use regeneration, reheat, intercooling or sometimes all of them. But the foundational cycle in which such methods can be used are of two types.
The two basic forms of Brayton cycle are-
- Open Brayton cycle- In Open Brayton cycle, the exhaust gases are spat out to the atmosphere. Each cycle uses new set of gas or working fluid.
- Closed Brayton cycle- In Closed Brayton cycle, the exhaust gases are cooled and sent back to the compressor to be used again. This forms a complete cycle.
What is a combined cycle?
One combines two things to get more output or increase the efficiency of particular system. In a combined cycle, both Brayton and Rankine cycles are combined to derive more output from a given set of input.
Brayton cycle produces more power so it is called as topping cycle. The exhaust gases from this cycle are so hot enough that it can be used as source for a comparatively low power producing cycle that is Rankine cycle. In this case, it is also known as bottoming cycle.
The heat from the exhaust gases is recovered by waste heat recovery boiler in bottoming cycle. The steam/water gets heated to complete Rankine cycle.
This way the waste exhaust gases from one cycle can be used as source for another cycle.