Presentation of the exercise
This exercise extends that on the steam power plant cycles.
Firstly, we build the exergy balance of the simple steam power plant 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 study the extraction and reheat cycle exergy balance, which will increase its performance.
Course reference:
“Thermodynamics Fundamentals / Second Law”
“Cycles / Exergy Balances”
Simple cycle exergy balance
Exergy balance analysis
The exergy balance shows an exergy efficiency of 48.8%. Taking into account an exergy efficiency boiler of about 70%, the value would be only 34%.
The distribution of overall losses shows that, except for the combustion irreversibilities that are not taken into account, it is the economizer losses that are the most important (48.2%). This is because the gases at 1000 ° C are used for heating water at low temperatures (from about 30 ° C)
Ways to improve steam cycles are deduced from the exergy balance:
- increase the average temperature of the cycle by conducting reheats;
- heat water at low temperature by internal regeneration with steam partially expanded, whose enthalpy is still high enough so that a small extraction can heat the bulk flow.
Extraction and reheat cycle exergy balance
Exergy balance analysis
The exergy balance shows an exergy efficiency of 50.5%. Taking into account an exergy efficiency boiler of about 70%, the value would be only 35%.
The distribution of overall losses shows that, except for the combustion irreversibilities that are not taken into account, it is the economizer losses that are still the most important (28.6%), although they have declined significantly as compared to the simple cycle. The irreversibility of mixing, not negligible, shows that the choice of the extraction temperature is not optimal. In a real plant, we would use 6-9 extractions at different temperature levels, being careful to minimize mixing irreversibilities.
The losses of the other heat exchangers are almost similar, but their total is considerable, given the difference in temperature between the heat source and steam.
Only the combined cycle today allows one to give value to the potential exergy of combustion, as discussed in a subsequent session (S41En).
Thermoptim result files of both cycles used by the spreadsheet to calculate the exergy balances are given in the downloadable archive below.