A Simple Method of Finding New Dry and Isentropic Working Fluids for Organic Rankine Cycle
Round 1
Reviewer 1 Report
This work presents a simple method to find the suitability of working fluids for organic Rankine cycles by determining if they can behave as dry, wet or isentropic.
The paper is well written and after a few changes could be published in Energies.
The method presented in this work is based in a classification scheme recently developed by the authors and coworkers (reference 17 of the paper). The main result of the paper is the simple rule that the dry and isentropic fluids defined according to ref. 17 are those for which the molar isochoric specific heat capacity at a reduced temperature T/Tc=0.74 is higher than 80 J/(mol.K).
Some comments/suggestions:
1) I do believe that the only really ‘novel’ results of the paper are those presented in Section 4. This should be clearly stated in the Introduction. Furthermore, in lines 55-56 where it reads ‘A short review of the novel classification will be given in this paper’ it should read ‘A short review of the novel classification will be given in Section 3’.
2) Perhaps it is a matter of taste but I do not like the term ‘isentropic fluid’ for a fluid that ‘behaves as isentropic’. If we have a look at the large panel in Fig. 2, for me it is quite clear that the shape of the saturation curve in a T-s diagram is that of a ‘dry fluid’ that depending of the type of isentropic expansion (green, red, or blue) behaves as dry, isentropic, or wet. Although the authors have made an effort to convince the reader that fluids ANCMZ should be termed as isentropic, I’m not sure that the average reader would agree with this. Instead it is clear that ANCMZ fluids can behave as isentropic. Some changes should be made in the manuscript (specially Section 3) to make clear this subtle point.
3) I understand that Figure 4 is obtained for cv of many fluids but I cannot help comparing with Figure 6 (for c_p^ig of butane) of the work of Garrido et al. (ref. 12). Is there any relation?
Rules similar to those of lines 240-241 but involving the critical molar volume have been recently presented in ref. 26. This should be commented in the manuscript.
Author Response
Dear Reviever,
Please find our answers in the attached pdf-file
Let me close by expressing our thanks for dealing with this manuscript.
Sincerely Yours
Attila Imre
Author Response File: 
Author Response.pdf
Reviewer 2 Report
1) Some brief explanation should be provided about why no experimental nor theoretical examples of AZCM and ACNZM fluids exist. Or if no explanation can be provided, then those two possibilities should be simply deleted from the paper. There is no point in having categories of fluids which do not actually occur.
2) The manuscript refers to Supplementary Material, but it was not included. Supplementary Material should be reviewed!
3) Instead of only referencing [7] on line 178, a brief explanation should be provided to readers about why "molecules with higher degrees of freedom are probably dry or isentropic" because this is a key point leading to Fig. 4. Readers should not have to look up a separate paper to understand such an important point. One or two sentences of explanation would help.
4) It is no surprise (at least to me) that the correlation is with heat capacity per mole rather than per kilogram.
5) On line 237, a brief explanation needs to provided about why a reduced temperature of 0.74 was selected. (Why not 0.75 or 0.52 or whatever?) It seems too arbitrary. How would the results change if some other value had been selected?
Author Response
Dear Reviever,
Please find our answers in the attached pdf-file
Let me close by expressing our thanks for dealing with this manuscript.
Sincerely Yours
Attila Imre
Author Response File: Author Response.pdf
