Interpretation of Entropy Calculations in Energy Conversion Systems
Round 1
Reviewer 1 Report
Four energy conversion processes: (1) a biomass torrefaction process where
torrefied solid fuel is produced by first drying and then torrefying raw feedstock, (2) a cryogenic air separation system which splits ambient air into oxygen and nitrogen while consuming electrical energy, (3) a cogeneration process whose desirable outcome is to produce both electrical and thermal energy, and (4) a thermochemical hydrogen production system were analysed in the paper. The conditions at which minimum entropy
production may lead to an improved performance were discussed for each of the four systems.
The paper is very interesting and may be published in Energies journal in the submitted form.
Author Response
Thank you very much for taking the time and reviewing the manuscript. The positive recommendation of the reviewer is gratefully appreciated.
Reviewer 2 Report
The study presented is very interesting, although merely theoretical.
Its main strength is that its breakthroughs can be considered in the design of more energy-efficient thermodynamic systems, as long as they belong to one of the 4 analyzed.
1) The main weakness is the fact that no link is made between the presented theory and its application in engineering practice. I think it would be interesting to present, at the end of the analysis of each of the 4 situations, practical examples in which the relationship between the minimization of entropy generation and the maximization of the system's energy efficiency is verified and not verified.
2) In view of what is exposed in the manuscript, the title is too generic. I suggest something like "Interpretation of entropy calculations in energy conversion systems where heat-to-work conversion is not a primary goal".
3) They are many variables which meaning is not defined in the text. Then, a table of Nomenclature must be added.
4) A direct correspondence between the variables that appear in the equations and those that are marked in the figures is recommended, which is not always the case (e.g., Figure 3 and equation (25)).
5) Text after equation (1). Error in description of TL and TH. TL represents Tcold and TH represents Thot. Please, indicate TL and TH units.
6) Text after equation (13). It would be interesting to specify/highlight the conditions that must be ensured for the coefficient of Yv in equation (10) to be guaranteed positive.
7) “Cryogenic air separation process” section. In this section the variables for "na" moles are represented by the same symbols as the variables for 1 mole. This is misleading.
8) Figure 2. In Figure 2, the MAC compressor is represented by the turbine symbol. Please, correct.
9) Text before equation (24). The number of the equation in question is missing.
Author Response
Comment: The study presented is very interesting, although merely theoretical. Its main strength is that its breakthroughs can be considered in the design of more energy-efficient thermodynamic systems, as long as they belong to one of the 4 analyzed.
Response: I would first like to thank the reviewer for examining the manuscript and providing many helpful comments and suggestions.
It must be stressed that the four systems have arbitrarily chosen to demonstrate the relation between performance indicator and the entropy generation of energy system is conditional. The last paragraph of the Conclusions has been modified to reflect the main objective.
Comment: The main weakness is the fact that no link is made between the presented theory and its application in engineering practice. I think it would be interesting to present, at the end of the analysis of each of the 4 situations, practical examples in which the relationship between the minimization of entropy generation and the maximization of the system's energy efficiency is verified and not verified.
Response: As precisely stated by the reviewer, the framework of this article is theoretical. However, the systems examined are processes that can be found in engineering practice. These systems have been studied previously by numerous authors. Additional references are cited for this purpose – please see the highlighted Refs.
One may find countless published articles on the application of the first and second laws (theoretical analyses) to variety of thermal processes. As noted at the beginning of the Abstract, and now in the last paragraph of the Introduction, nearly all entropy-based analyses merely present entropy calculation without demonstrating how/if they can be used to improve the performance indicator of the system studied.
In each of the four energy systems studied, the conditions for equivalence of minimization of entropy generation and maximization of system efficiency have been discussed. The different scenarios presented can be viewed as examples of conditions at which the performance indicator may or may not correspond inversely with the entropy production.
Comment: In view of what is exposed in the manuscript, the title is too generic. I suggest something like "Interpretation of entropy calculations in energy conversion systems where heat-to-work conversion is not a primary goal".
Response: Thank you for the suggestion. The above title is relatively long whereas the current title is more concise but cohesive. Given that the discussion begins with a summery of past studies on heat engines (heat-to-work conversion systems). it is still believed that the current present accurately reflects the goal and scope of the article.
Comment: They are many variables which meaning is not defined in the text. Then, a table of Nomenclature must be added.
Response: A Nomenclature has been included in the revised manuscript.
Comment: A direct correspondence between the variables that appear in the equations and those that are marked in the figures is recommended, which is not always the case (e.g., Figure 3 and equation (25)).
Response: Equation (27) included in the revised manuscript reveals that the enthalpy of water vapor at the exit of the steam generator, shown in Fig. 3, is implicitly accounted for in Eq. (26).
Comment: Text after equation (1). Error in description of TL and TH. TL represents Tcold and TH represents Thot. Please, indicate TL and TH units.
Response: Thank you for the careful observation. The corrections have now been made.
Comment: Text after equation (13). It would be interesting to specify/highlight the conditions that must be ensured for the coefficient of Yv in equation (10) to be guaranteed positive.
Response: It would, indeed, be interesting. To determine under what conditions the coefficient of Yv in Eq. (10) becomes positive requires a more comprehensive analysis, for instance, using a process model developed in Refs. [24, 28], as there are several parameters in the bracketed coefficient. Nevertheless, this would be beyond the scope of the present article. A note is added at the end of Section 2.
Comment: “Cryogenic air separation process” section. In this section the variables for "na" moles are represented by the same symbols as the variables for 1 mole. This is misleading.
Response: The analysis had been presented first per unit molar flowrate of the air. To avoid any confusion, this section has now been modified by first analyzing the case that accounts for the molar flowrate of the air followed by a discussion and analysis per unit flowrate of the air; see Eq. (25).
Comment: Figure 2. In Figure 2, the MAC compressor is represented by the turbine symbol. Please, correct.
Response: Thank you for the careful observation. The figure has now been modified.
Comment: Text before equation (24). The number of the equation in question is missing.
Response: As the ASU section has been modified, the above typo no longer exits.
