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Special Issue “Physics and Mechanics of New Materials and Their Applications 2021”
 
 
Article
Peer-Review Record

High-Performance Solid Medium Thermal Energy Storage System for Heat Supply in Battery Electric Vehicles: Proof of Concept and Experimental Testing

Appl. Sci. 2022, 12(21), 10943; https://doi.org/10.3390/app122110943
by Volker Dreißigacker * and Lukas Hofer
Reviewer 1:
Reviewer 2: Anonymous
Reviewer 3:
Appl. Sci. 2022, 12(21), 10943; https://doi.org/10.3390/app122110943
Submission received: 5 September 2022 / Revised: 10 October 2022 / Accepted: 26 October 2022 / Published: 28 October 2022
(This article belongs to the Section Applied Thermal Engineering)

Round 1

Reviewer 1 Report

The work seems very interesting, but is presented in a confusing / difficult to follow manner to a reader. My suggestions for improvement are primarily aimed to address how easily the paper can be read and understood. After that review, I'm happy to comment again on the scientific value, design of experiments etc. Suggestions below: 

1. Figure 1 can be made bigger, at least the fonts, the text is small and difficult to read. 

2. Figure 2 can use some captions to explain to the reader what they're seeing, a scale bar, an explanation of the difference between the item on the left and the right. 

3. Figure 3, what are the components of this rig? Can you link this to Figure 1 for the reader to follow more easily? 

4. Figure 4, the text and subscripts are too small, unreadable. 

5. Table 2 seems to have issues with formatting. Also captions don't need to be only formulas - a table should be easy to understand on its own, without forcing the reader to go back to a different figure or formula to know what specific value they're following. Adding more text would help. 

6. Figure 5: I'm not sure I follow the data: there's two lines and two sets of points, connected by lines. In my experience, experimental data is reported as single points, not connected by a line, to show that the measurement occurred at discrete points along the way. Results from a model can be reported as a continuous line because there is a known relationship between input and output. What's being reported here? Is it two sets of experimental results and accompanying models? Clarification needed. 

7. Figure 6: why are those lines there, is there a model to support a relationship to gain insight between experimental points? Otherwise you can't draw a line there. 

8. Figure 7: same issue, although the model line can be continuous, I agree. 

9. Figure 8: how do you get such a wide range for an experimental result? 

10. Table 3: why include the line for a power of 1 kW if there are no values to report. Also, is the data different between Figure 9 and Table 3? If not, pick just one to show. 

11. Table 4: what are the inputs and what are the outputs? Why are Touts missing for the bottom right half? 

12. Figure 10: same issue with experimental results shown with points connected, incorrect. 

13. Figure 11: same as figure 10. 

14. Figure 12: same as 10 and 11. 

15. Figure 15: same. 

16. Overall the text is very hard to read, with multiple run on sentences. As an example, the first sentence in the conclusion is a run on sentence with a mismatch in singular / plural for the subject and verb. I'm pasting it here: "A key element for improving efficiency and flexibility are thermal energy storage systems, which efficiently store heat available or generated in processes at times of low thermal energy demand (charging period) and provide it at times of increased thermal energy demand (discharging period)." This is only one example, but there are multiple. The authors should take the time and re-write the text with stronger opening sentences for each paragraph, that deliver the message of the paragraph succinctly, and make it easy to navigate the paper without reading every single paragraph in detail. 

Happy to review again, in a format that delivers the message more clearly to the reader. 

Author Response

Comments and Suggestions for Authors

The work seems very interesting, but is presented in a confusing / difficult to follow manner to a reader. My suggestions for improvement are primarily aimed to address how easily the paper can be read and understood. After that review, I'm happy to comment again on the scientific value, design of experiments etc. Suggestions below: 

Dear Reviewer, Thank you for your detailed comments and remarks. I have updated the annotations to the figures as far as possible and made language improvements. I look forward to your feedback on the scientific content. Please find below the responses to your remarks. The updates in the manuscript were made in Word in the track change mode.

  1. Figure 1 can be made bigger, at least the fonts, the text is small and difficult to read. 

à Response: new figure with larger texts was integrated

  1. Figure 2 can use some captions to explain to the reader what they're seeing, a scale bar, an explanation of the difference between the item on the left and the right. 

à Response: additional explanations were integrated in the figure subtext

  1. Figure 3, what are the components of this rig? Can you link this to Figure 1 for the reader to follow more easily? 

à Response: new figure was integrated

  1. Figure 4, the text and subscripts are too small, unreadable. 

à Response: text has been increased to font size 18

  1. Table 2 seems to have issues with formatting. Also captions don't need to be only formulas - a table should be easy to understand on its own, without forcing the reader to go back to a different figure or formula to know what specific value they're following. Adding more text would help. 

à Response: thanks for your comment; table description was updated including some texts explaining the values

  1. Figure 5: I'm not sure I follow the data: there's two lines and two sets of points, connected by lines. In my experience, experimental data is reported as single points, not connected by a line, to show that the measurement occurred at discrete points along the way. Results from a model can be reported as a continuous line because there is a known relationship between input and output. What's being reported here? Is it two sets of experimental results and accompanying models? Clarification needed. 

à Response: Dear reviewer, two experimental results are compared each with simulation results. For clarification, the legend was extended. Regarding the line/marker style: model results are illustrated with continuous lines and experimental results with symbols. Additionally, dashed lines were used for temporal (experimental) results due to the high amount of data points (time step size of 10 s). This illustration style was used throughout the publication. I hope it matches a compromise between visual clarity and scientific standard.

  1. Figure 6: why are those lines there, is there a model to support a relationship to gain insight between experimental points? Otherwise you can't draw a line there. 

à Response: lines illustrate the simulative results, the symbols the experimental results. For clarification additional descriptions were integrated in the figure text

  1. Figure 7: same issue, although the model line can be continuous, I agree. 

à Response (see point 6): Regarding the line/marker style: model results are illustrated with continuous lines and experimental results with symbols. Additionally, dashed lines were used for temporal (experimental) results due to the high amount of data points (time step size of 10 s).

  1. Figure 8: how do you get such a wide range for an experimental result? 

à Response: Dear reviewer, the explanation for the wide range in the experimental results is described in the text under figure 8. This text section is pasted here: “The significantly higher spread in the experimental results is due to the fact of only three radially distributed measurement locations (see figure 4), whereby the outermost measuring zone was also integrated close to the shell-side thermal insulation. As a result, heat loss effects in combination with a limited number of thermocouples are disproportionately visible leading to elevated temperature spread inside the experimental results.”

  1. Table 3: why include the line for a power of 1 kW if there are no values to report. Also, is the data different between Figure 9 and Table 3? If not, pick just one to show. 

à Response: Dear reviewer, the results summarized in Table 3 refer to the point in time for activating the active cooling (experiment vs. simulation). These additional results are not presented graphically but in tabular form. Since the active cooling was not activated at the operating point with 1 kW heating power, this was marked with a "-". This row has been removed from the table for better understanding.

  1. Table 4: what are the inputs and what are the outputs? Why are Touts missing for the bottom right half? 

à Response: Dear Reviewer, the values shown include the boundary condition (mass flow rate) and the target values (inventory temperature and constant mixing temperature). These three values were varied independently of each other, resulting in a total of 4 x 4 x 2 = 32 measurement points. For this reason there are no entries in the table below on the right.

  1. Figure 10: same issue with experimental results shown with points connected, incorrect. 

à Response: see point 6

  1. Figure 11: same as figure 10. 

à Response: see point 6

  1. Figure 12: same as 10 and 11. 

à Response: see point 6

  1. Figure 15: same. 

à Response: see point 6

  1. Overall the text is very hard to read, with multiple run on sentences. As an example, the first sentence in the conclusion is a run on sentence with a mismatch in singular / plural for the subject and verb. I'm pasting it here: "A key element for improving efficiency and flexibility are thermal energy storage systems, which efficiently store heat available or generated in processes at times of low thermal energy demand (charging period) and provide it at times of increased thermal energy demand (discharging period)." This is only one example, but there are multiple. The authors should take the time and re-write the text with stronger opening sentences for each paragraph, that deliver the message of the paragraph succinctly, and make it easy to navigate the paper without reading every single paragraph in detail. 

à Response: Dear Reviewer, thank you for your comments and helpful input. I revised the document, shortened or simplified long sentences. The revisions are mainly focused in the introduction and conclusion section, but were conducted in this manner throughout the manuscript. I hope this helps you with readability.

Happy to review again, in a format that delivers the message more clearly to the reader. 

 

Reviewer 2 Report

The English writing and other typing issues should be thoroughly revised and improved, mainly due to the following problems (although the following list presents the most important issues, an independent and careful survey of the manuscript should be performed for other issues not listed here):

(1)   Use bracketing commas to separate additional information in a sentence, e.g., in l. 13-14, “which enable due to their time-decoupled operation increases in systemic efficiency and flexibility” should be modified to “which enable, due to their time-decoupled operation, increases in systemic efficiency and flexibility”. This type of situation is very common throughout the manuscript and should be corrected accordingly.

(2)   Omission of definite article “the” and indefinite article “a”, e.g., in l. 145, “During the thermal discharge operation” (the article is missing in the manuscript), and in l. 57, “as a separate component”. The correct use of articles should be reviewed.

(3)   Very long sentences should be split into smaller ones to aid legibility, e.g., in l. 99-103, “Such a storage system with honeycomb-shaped ceramic solids was investigated in [26] with vehicle-typical boundary conditions showing under neglection of the containment mass high systemic storage densities of up to 285 Wh/kg with constant electrical charging and thermal discharging powers of more than 5 kW.” In some cases, a better use of commas would also help the readability of long sentences.

(4)   Some sentences are very confusing and should be clarified (modified), e.g., l. 148-151, “Thereby, the bypass path encloses the thermal insulation on the shell side to integrate during discharging period heat losses within the insulation into the cold entering air and optional to preheat during charging period the interior through a slight active cooling.”

(5)   Variable symbols should be consistent. Sometimes variables are first presented in italic and after in roman format, e.g. T_B-out is typed in italic in the mains text but in roman in figure 5. It is worth noting that normally variable names are represented in italic, and although not mandatory, this is a good practice, and the authors represent many variables in roman format, e.g. variables r, z, and L.

(6)   All variables should be defined, e.g. variable t in figure 5 is not defined (although it is an obvious one, it still has to be defined), and variables r, z, and L in p. 7.

(7)   Spelling mistakes, such as “season” in Figure 1 or “therefor” in l. 112.

Author Response

The English writing and other typing issues should be thoroughly revised and improved, mainly due to the following problems (although the following list presents the most important issues, an independent and careful survey of the manuscript should be performed for other issues not listed here):

 

Dear Reviewer, Thank you for your detailed comment. Your comments specifically on language improvements helped me a lot. In fact, some formulations are very “tiring” to read. I have revised the document accordingly. The answers to the points you raised separately can be found below. The updates in the manuscript were made in Word in the track change mode.

  • Use bracketing commas to separate additional information in a sentence, e.g., in l. 13-14, “which enable due to their time-decoupled operation increases in systemic efficiency and flexibility” should be modified to “which enable, due to their time-decoupled operation, increases in systemic efficiency and flexibility”. This type of situation is very common throughout the manuscript and should be corrected accordingly.

à Response: Dear reviewer, commas were integrated and sentences were shortened here and throughout the whole document.

  • Omission of definite article “the” and indefinite article “a”, e.g., in l. 145, “During the thermal discharge operation” (the article is missing in the manuscript), and in l. 57, “as a separate component”. The correct use of articles should be reviewed.

à Response: corrected and here and throughout the whole document.

  • Very long sentences should be split into smaller ones to aid legibility, e.g., in l. 99-103, “Such a storage system with honeycomb-shaped ceramic solids was investigated in [26] with vehicle-typical boundary conditions showing under neglection of the containment mass high systemic storage densities of up to 285 Wh/kg with constant electrical charging and thermal discharging powers of more than 5 kW.” In some cases, a better use of commas would also help the readability of long sentences.

à Response: sentence was shortened here and throughout the whole document.

  • Some sentences are very confusing and should be clarified (modified), e.g., l. 148-151, “Thereby, the bypass path encloses the thermal insulation on the shell side to integrate during discharging period heat losses within the insulation into the cold entering air and optional to preheat during charging period the interior through a slight active cooling.”

à Response: sentence was clarified and shortened here. Similar confusing sentences in the manuscript was revised in the same manner.

  • Variable symbols should be consistent. Sometimes variables are first presented in italic and after in roman format, e.g. T_B-out is typed in italic in the mains text but in roman in figure 5. It is worth noting that normally variable names are represented in italic, and although not mandatory, this is a good practice, and the authors represent many variables in roman format, e.g. variables r, z, and L.

à Response: Dear reviewer, the figures were created in Matlab, with some of the latex interpreters having to be used to create the special characters. Unfortunately, I haven't managed to set all the characters to one format. However, in order to achieve a uniform typeface here, I have updated the symbols throughout the manuscript to a comparable layout. I hope you agree with this.

  • All variables should be defined, e.g. variable t in figure 5 is not defined (although it is an obvious one, it still has to be defined), and variables r, z, and L in p. 7.

à Response: Description of the variables r, t and z were introduced in line 257 and 259 and the variable L in line 207.

  • Spelling mistakes, such as “season” in Figure 1 or “therefor” in l. 112.

à Response: corrected; figure 1 was updated

Reviewer 3 Report

The thermal energy storage system for heat supply in battery electric vehicles is very interesting. But some parts in the paper also needs to be modified.

(1)   The background introduction in the abstract needs to be condensed.

(2)   Table format needs attention. For example: Table2, Table 4.

(3)   The principle of heat storage and heat release is unclear. How to discharge the heat by Al2O3?

Author Response

The thermal energy storage system for heat supply in battery electric vehicles is very interesting. But some parts in the paper also needs to be modified.

Dear reviewer, thank you for your feedback and comments. I am very pleased that you found the contents of the manuscript interesting. You will find my answers to your individual points below. The updates in the manuscript were made in Word in the track change mode.

  • The background introduction in the abstract needs to be condensed.

à Response: Dear reviewer, the introduction was condensed by means of shortened sentences and deleted text sections.

(2)   Table format needs attention. For example: Table2, Table 4.

à Response: Table formats were corrected. For your information: the variable summarized in these tables were independent from each other varied inside the measurement campaign. This is also the reason for the “gaps” in the tables.

(3)   The principle of heat storage and heat release is unclear. How to discharge the heat by Al2O3?

à Response: Dear reviewer, the heat is transferred convectively from the previously electrically heated Al2O3 honeycombs to the cold air flow. Additional explanations have been added to this (below figure 1 and at the beginning of section 4.3).

 

 

Round 2

Reviewer 1 Report

Very nice work in addressing the comments and making the paper more interesting and accessible to a wider audience. The paper is now ready for publication. 

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