Supercritical CO2 Mixtures for Advanced Brayton Power Cycles in Line-Focusing Solar Power Plants
Round 1
Reviewer 1 Report
In the reviewer’s experience, the REFPROP does not correctly give critical temperature of many mixtures as well as the properties near the critical point. The authors should at least put an effort to show that the mixture properties calculated from REFPROP do not have substantial errors that can change the conclusion of the paper. Furthermore, it is not just the critical point that is an issue, the pseudo critical line will be also very important for the cycle optimization. The authors should discuss how the pseudo critical line changes with the mixture composition. The reviewer is not sure how inner pinch issue of sCO2 power cycle can be avoided by using UA method. The authors should acknowledge the issue of inner pinch in sCO2 power cycle, and discuss how that cannot be an issue in this study. Minimizing turbomachinery dimension may not be the direction for the best cycle. If the turbomachinery becomes too small the system becomes very unrealistic to construct. Thus, the authors should not consider minimizing the turbomachinery. However, minimizing heat exchanger size has a substantial value. Also, minimizing volumetric flowrate or maximizing pressure ratio may have more impact in reality. It is not clear if the CSP plant analyzed in this article does not have an energy storage system. From figure 1, it seems that the sCO2 mixture is directly receiving heat from the solar field. In this case, are the analyzed hydrocarbons stable under high temperature? PHX wall temperature will be very high and locally the mixture will experience very high temperature near the wall. The authors should check and quickly mention if the analyzed fluids are stable in the temperature range which the authors are exploring.Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The review concerns the article entitled: Supercritical CO2 Mixtures for Advanced Brayton Power Cycles in Line-Focusing Solar Power Plants.
The article concerns an interesting topic, but there is no visible scientific thread.
The content of the article suggests that the authors have put their propositions of mixtures into a ready-made calculation tool and analyze the results.
It seems that such a calculation tool, based on basic equations, is probably owned by people working at Solar Power Plant. So they can do such analyzes themselves without much problem.
Even the abstract indicates a lack of innovation at work, and the authors cite the statements of other people in their conclusions.
Cost analysis is interesting and adds value. But this is not enough for publication in a scientific journal.
Additional remarks:
-Line 42, 123, ... passive text applies to scientific papers
-Line 60-63 tool used for calculations should be described in more detail, indicating its complexity and uniqueness. The current description suggests only reproductive work.
-Line 292 Conclusions are not transparent and reference to research of others is not justified at this point.
According to the reviewer, the current version of the article is not suitable for publication in the Journal.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The paper analyses the effect of using different supercritical CO2 mixtures on the performance of a recompression cycled powered by a line-focussing solar power plant. The results provided might be of interest for potential readers, however, some major modifications should be carried out to improve the quality of the current work:
1.- The novelty of the work should be clearly stated in the introduction and the importance of the study carried out should be justified.
2.- Table 2: Are the values of the heat exchanger UA correct in the table? In the table the values are 2500, 5000 and 7500 kW/K, however, in the text these values are 5000, 7500 and 15000 kW/K.
3.- Line 93: “The optimal efficiency of the s-CO2 recompression Brayton cycle is for a CIT (Compressor Inlet Temperature) equal to the critical temperature and a CIP (Compressor Inlet Pressure) above the 90 critical pressure [20]; these values are 45.4% (CIP=7.41 MPa and UA=5000 kW/K), 48.4% 91 (CIP=7.43 MPa and UA=10000 kW/K), and 49.4% (CIP=7.44 MPa and UA=15000 kW/K). Something similar happens with the s-CO2-based mixture; however, in this work, the study of the behavior of 93 the cycle was performed with a CIT and CIP just above the critical point”. Why are not the optimal values used?
4.- Line 100: “We have worked with molar fractions of the additives He, Kr and CH4 to obtain mixtures with critical temperatures close to 0 °C”. Why is this temperature selected?
5.- Figure 4: Why is the mole fraction of sCO2/He analysed only up to 0.10? Please, justify the range of variability of all parameters analysed.
6.- Line 112: “UA = 15000”. Please include units.
7.- Line 119: “The heat exchangers’ heat transfer coefficients are closely related to the Cp”. The dependence of the convection coefficient on Cp is via the Prandtl number Pr. Therefore, the effect of using different mixtures on the convection coefficient cannot be evaluated analysing exclusively the variation of Cp. According to the definition of the Prandtl number, Pr=\mu·Cp/k, the thermal conductivity and viscosity of the mixtures have also an effect. Thus, the values of these parameters should also be considered in Table 2. However, the variation of Cp also affects the temperatures in the heat exchangers and the heat rate, but this is a separate effect to that on the convection coefficient. These two effects seem to be mixed in this paragraph.
8.- Line 121: “The ethane mixture is the solution for optimizing the Cp”. I would say “for maximizing” instead of “for optimizing” since there no optimal value for Cp. This is also the case for density in line 131.
9.- Line 138: “However, for future works, it would advisable to continue” -> “However, for future works, it would be advisable to continue”.
10.- Lines 145 and 236: “UA = 15000 W/K”. Shouldn’t the units be kW/K?
11.- Figure 6. The calculation procedure used to get the results shown in Figure 6 should be explain in detail or a reference to a work describing the procedure should be provided.
12.- Figures 9 to 21: The paper has a total of 21 figures and 5 tables in 18 pages (excluding references). Some of the figures are obviously not properly discussed. For figures 9 to 21, these are the only comments in the manuscript:
Line 200: “In contrast, a better efficiency was obtained if working with higher compressor inlet temperatures of 318.15, 323.15, 328.15 and 333.15 K (cf. Figure 9, Figure 13, Figure 16 and Figure 19).”
Line 235: “The influence of the parameters wmc/wt, wrc/wt and Δha/wt on the cycle efficiency that works with UA = 15000 W/K and CIT = 318.15K can be seen in Figure 10.”
Line 241: “The savings in the installations of PTC could reach up to 7 million USD for CIT = 318.15K, 11 million USD for CIT = 323.15K, 9 million USD for CIT = 328.15K, and 10 million USD for CIT = 333.15 K (cf. Figure 11, Figure 14, Figure 17 and Figure 20, respectively).”
Line 246: “Alternatively, in the case of the installations of linear Fresnel, the savings could reach 6 million USD for CIT = 318.15K, 8 million USD for CIT = 323.15K, 7 million USD for CIT = 328.15K, and 7 million USD for CIT = 333.15K (cf. Figure 12, Figure 15, Figure 18 and Figure 21, respectively).”
If this is the only important information about these figures, they can be removed from the manuscript and the results can be highlighted and commented in the text. Then, 8 figures and 5 tables would remain in the manuscript, which seems to be a reasonable number. Furthermore, removing these figures the authors would gain space to improve the discussion of the 8 remaining figures.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
.
Reviewer 2 Report
The authors introduced many changes and improve the quality of the article.
In the current version, the reviewer recommends the document for publication.
Reviewer 3 Report
The quality of the manuscript has improved after the reviewing process.
