Charging of an Air–Rock Bed Thermal Energy Storage under Natural and Forced Convection

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

The authors presented results of the simulation study on rock bed thermal energy storage with air. Natural and forced convection effects are studied. Comments are provided below:

Abstract should be revised; the last sentence should be better integrated.

The citation of the first 10 references is very brief.

The authors mention in the review that sensible thermal energy storage (STES) has found practical application from 120 to 1250 ° C. However, it should be noted that when we have solar energy systems for the preparation of hot water, STES are often used at temperatures below 100 ° C. Because specific applications are not explained.

The authors very briefly discuss existing studies in the field. Research in this area should be discussed in more detail, showing the scientific gap and the relevance of the problem.

The presentation of the subject should not (Figure 1) be in the Introduction section.

Is turbulence assessed in the modelling? If so, in what way? This should be described.

The conclusions should be rewritten. There should be some kind of summary in this section, rather than just writing individual results.

The authors should also make some recommendations arising from their work.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

1. Please revise the abstract. The numerical validation section must be presented prior to the presentation of the major numerical results obtained from the CFD calculations.

2. Do not exceed six keywords.

3. Avoid grouping multiple references together unless it is a review. For example: [1-6], [7-10], [10,17,23]. However, you can briefly provide an overview of each reference related to your state-of-the-art.

4. What is the Rayleigh number (Ra) associated with your CFD modeling? Please mention it in the revised version of your article.

5. Calculating the Rayleigh number will justify the nature of the flow (laminar or turbulent). Therefore, it is necessary to determine whether to activate the turbulence model to capture or not capture the vortex structures that accompany your modeled heat and mass transfer.

6. Please clearly rework the main objective of your contribution (numerical and experimental parts) from line 100 to line 121.

7. Mention all dimensions of your storage tank in a clear diagram similar to Figure 2. This will facilitate the identification of dimensions and the CAD reproducibility of your geometric model.

8. Figure 4 illustrating the mesh is not very representative. It is necessary to zoom in on the different areas of the tank to present the effort made to ensure a good transition of mesh size. Additionally, zoom in on the areas where you have performed significant refinement. Furthermore, if it is necessary to redo the calculations with a turbulence model after calculating the Rayleigh number, then it will be necessary to add a boundary layer mesh figure.

9. Calculating the Reynolds number (re) is also desirable to determine the nature of the air flow in your tank and the necessity of using a turbulence model in your CFD modeling.

10. A figure illustrating the boundary conditions must be added to your revised version.

11. The presentation of your governing equations needs to be reworked. Start with the mass conservation equation, followed by the momentum equation, and then the energy equation.

12. List all the assumptions you have made for your CFD calculations point by point before presenting your governing equations for heat and mass transfer.

13. The numerical methodology should be represented in a concise and precise text or in a diagram.

14. The discussion section should be further enriched with recent references.

 

15. The results of your experimental and CFD study should be rigorously presented point by point in your conclusion, in order to reflect the originality of your contribution compared to the existing state of the art.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This article explored a rock bed thermal storage system designed for small-scale solar power generation and analyzed with computational fluid dynamics using ANSYS. Performance analysis was conducted under various porosities, considering natural and forced convection. However, the article has major issues: 1) missing important information about the experiment and the simulation model such as the HTF and the helical coiled (size, material, length, flow rate , etc); 2) the methodology section needs to be rearrange completely; and 3) the difference between experimental and simulated results is large (even at the first 15 hours) which may indicate a not validated model. Therefore, I do not think it is suitable for publication in the current version. The following specific points needs to be addressed to help developing this paper:

1.        Line 62: "It is worth noting that no phase change occurs within this temperature range". I believe this sentence should be moved up to line (56) after "120C to 1250C".

2.        Line 89: Your different approach is that "rock-bed TES is considered to be directly heated at the bottom by an arrangement of solar reflectors or by using waste heat materials". You need here to elaborate why do you think this approach will enhance the rock-bed TES?  

3.        Please describe the HTF and the helical coiled (size, length, flow rate, etc.).

4.        Line 126: What are the materials for the helical tube and the heat transfer fluid?

5.        Line 129: "0.45 m wide cylinder". Is this the diameter?

6.        Table 1: Please add the thermo-physical properties of the HTF and the stainless steel.

7.        Figure 4: What is "Rec" in this figure?

8.        Table 2: Please redesign the table to remove unnecessary repeated columns.

9.        2.2. Boundary conditions: Most detail in this section is not related to the boundary conditions. You may need to rearrange the subsections.

10.   Line 243: Could you explain what do mean by the second assumption: "Equal temperatures in air and rocks (local thermal equilibrium)".

11.   Line 247: Which boundary is this?

12.   The difference between experimental and simulated results is large (even at the first 15 hours), why? This may not be validated model.

13.   I do not see experimental results for the forced convection to be used to validate the forced convection simulation cases.

14.   please add details (add pictures) of the experiment like the fan types and the diameter of the air ports.

15.   Line 346: If air is pushed into the inlet port on one side and the outlet is at the other side, the air will not move easily to the top left corner (Figure 11) which means heat might transfer by conduction. Did you check the air flow direction inside the container?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Kindly provide me with a revised version that will be marked in red. This will facilitate the identification of all your modifications compared to the initial version. Additionally, I recommend that you mention the page number and the specific line affected by your changes in your response document attached to your revised article. Best of luck!

Author Response

The revised manuscript has been uploaded with track changes to clearly show the modifications made based on the reviewers' comments. Thank you for your kind wishes.

Reviewer 3 Report

Comments and Suggestions for Authors

1. Figure 11 appears to replicate the grid sensitivity analysis from the initial manuscript (Figure 3 in the previous version). Could you confirm if this is accurate, especially considering that Table 3 in the revised manuscript has different values compared to Table 2 in the earlier version?

2. In the validation of natural convection, Figure 13 in the new version differs from Figure 5 in the previous version. Given that the model has been updated, I would expect the figures in the Results section to also reflect these changes. However, Figures 15 to 18 in the new version appear to be identical to those in the old version. Could you clarify why this is the case? For instance, Figure 13 in the new version shows the temperature of the top surface reaching approximately 90°C after 50 hours, whereas Figure 5 in the old version shows it reaching 160°C. Consequently, I would anticipate that Figure 15 in the new version might also differ from its previous counterpart.

3. Could you please highlight or color-code the changes made in the revised version? This will help me efficiently review the modifications without needing to examine the entire document again, which is inconvenient.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have submitted a revised version that presents the major changes they have made to their manuscript. Could you please ask them to also submit a clean revised version without the deleted text (strikethrough text) to facilitate an efficient evaluative reading?

Author Response

The revised manuscript, without track changes, has been uploaded. Significant improvements have been made in response to the feedback from the various reviewers. Thank you.

Reviewer 3 Report

Comments and Suggestions for Authors

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Author Response

The revised manuscript, without track changes, has been uploaded. Significant improvements have been made in response to the feedback from the various reviewers. Thank you.

Round 4

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have adeptly addressed all the comments and have incorporated nearly all the modifications I proposed. Consequently, I recommend the publication of this manuscript following the prompt integration of these three minor modifications:

1. Kindly incorporate the table, currently inserted as a screenshot in Figure 2, as a well-defined table within the body of the text.

2. The temperature contours in Figure 15 are presented in Kelvin, whereas those in Figure 20 are in °C; please standardize the units to enhance the quality of the presentation of your results.

 

3. Please reformulate your conclusion to succinctly present your major numerical results in bullet points.

Author Response

Comments 1: Kindly incorporate the table, currently inserted as a screenshot in Figure 2, as a well-defined table within the body of the text.

Response 1: Thank you for bringing this to our attention. We agree with your comment. In the updated manuscript, we revised the table as per your suggestion. It originally showed component labels for the TES drawing, but we've now replaced it with a more effective representation.

Comments 2: The temperature contours in Figure 15 are presented in Kelvin, whereas those in Figure 20 are in °C; please standardize the units to enhance the quality of the presentation of your results.

Response 2: Thank you for bringing this to our attention. We agree with your comment. In the updated manuscript, both figures are now presented in °C following your important recommendation.

Comments 3: Please reformulate your conclusion to succinctly present your major numerical results in bullet points.

Response 3: In the revised manuscript, the key numerical results are now presented as bullet points in the conclusion, following the recommendation.