Presentation of the exercise
This exercise extends that on the gas turbine model.
Firstly, we build the exergy balance of the gas turbine studied previously, using the spreadsheet presented in the session S06En, which you should refer to.
We also suggest to download the document on the exergy balances provided in the resources (click on the link below): Cycles / Exergy Balances.
This document gives some explanations on the difficulties that exist when you seek to build up the exergy balance of a cycle a bit complex.
Secondly we will insert a regenerator in the machine, which will increase its performance.
Course reference:
- “Thermodynamics Fundamentals / Second Law”
- “Cycles / Exergy Balances”
To follow the presentation, go to the next step
(Session realized on 06/16/11 by Renaud Gicquel)GT exergy balance
Exergy balance analysis
The exergy balance shows an exergy efficiency of 26.5%.
The distribution of overall losses shows that, except for the combustion irreversibilities that are inevitable, it is the exhaust losses that are the most important (42.6%).
Ways to improve gas turbines are deduced from the exergy balance:
- increase the cycle temperature to increase the combustion efficiency. We are faced at this level with technological limitations it is difficult to overcome.
- recover the exergy available in the exhaust.
The gas turbine outlet temperature being equal to 600 ° C, and that of the air leaving the compressor at 206 ° C, it is possible to preheat the compressed air with the exhaust gas before entering the combustion chamber.
The regenerative cycle that we will study now implements this solution.
Cycle improvements
- regeneration
Regeneration cycle
Regeneration cycle
Construction of the heat exchanger
If you followed step by step the instructions which were given orally, the construction of the heat exchanger should have been done without problem.
If for some reason or another you have a problem, you can repeat your construction by operating as follows:
- build a cycle diagram similar to that presented to you
- transfer the elements in the simulator using the interface accessible from the Special menu in the diagram editor
- open the two exchange processes that you created, set point 5with 300 ° C and 5 bar and point 6 with 400 ° C and 1 bar, and calculate the two processes
- then double-click twice on the blue link of the heat exchanger from the diagram editor, and save the exchanger.
In principle, this construction should work well
Regeneration cycle
Exergy balance analysis
The exergy balance shows an exergy efficiency of 43.2 %.
The exhaust losses are greatly reduced and represent only 25.5%. However, the gases come out even at 300 ° C, which represents a lost energy potential.
Only the combined cycle can cool the gas at lower temperature, as discussed in a subsequent session (S41En).
The losses of the compressor, the turbine and the regenerator are all the same order of magnitude.
Thermoptim regenerative cycle files and the spreadsheet to calculate the exergy balances are given in the downloadable archive below, as well as the result files used in this session