Thermal Balance of a Water Thermal Accumulator Based on Phase Change Materials

Round 1

Reviewer 1 Report (Previous Reviewer 1)

The authors addressed all raised comments and I have no other feedback.

Author Response

Dear reviewer, thank you for your approval of our article.

Reviewer 2 Report (Previous Reviewer 3)

 

This is the first revision of a paper about the design of a water thermal accumulator (WTA) on the basis of phase change materials (PCMs) by means of the numerical investigation.

The results obtained, as well as the methodology employed are quite interesting. In the first round of revision, I recommended some improvements in the manuscript, which were almost done with one exception. Therefore, I repeat the recommendation here:

In Section “3. Simulation in the thermal regime”, I strongly recommend to add two paragraphs regarding “Grid independence verification” and “Validation of results” respectively. In particular, a criterion for appropriate mesh size should be given and verified, such as the following (where e is a suitable threshold):

  | (q(j,max) -|q(j+1,max)) / |q(j,max)  |<e

In my opinion, after this addition, the manuscript should be ready for publication.

Author Response

Dear Reviewer,

Unfortunately, we are unable to perform the operation you suggested because COMSOL selects the grid steps automatically without our participation.

If you select the grid step manually, this causes errors in the calculations due to the divergence of the series and leads to a significant increase in the calculation time.

As a result, the program calculates for a long time and at the end gives an error about the impossibility to complete the calculation.

Round 2

Reviewer 2 Report (Previous Reviewer 3)

I understand the reply from the authors.

Therefore, I recommend to accept the manuscript in the present form.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

- What is the effect of carbon particle doping? It is instructive to elaborate on this in the main text.

-Phase-change materials have shown significant interest in thermal energy applications. I suggest the authors include more recent articles (such as [Nanophotonics 9.5 (2020): 1189-1241]) to enlighten the broad interest of journal readers.

- It is good that the authors identify the types of PCMs for this application. Which specific material properties are more of interest?

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Reviewer 2 Report

 

The manuscript describes the simulation of the heat transfer phenomenon from a flowing water at 90°C to a thin layer of a PCT composite material at 20°C as a function of time. The PCT material is a mixture of wax (paraffin) and thermally reduced graphene oxide of different composition. The PCT phase is placed in the space between a copper tube containing the flowing hot water and a thermally insulating plastic coating. This devices can be used as thermal accumulator based on phase change materials. In the thermal balance simulation the thermal conductivity of copper is assumed as infinity, while the thermal conductivity of the external polymeric coating is zero. Longitudinal and transversal temperature profiles have been obtained at different times by using the code set COMSOL.

This manuscript does not include experimental results but only some mathematical simulation information. It seems just an exercise to test the potentiality of the code set COMSOL in the engineering problem of thermal accumulators based on PCT materials. The manuscript cannot be accepted for publication because it is lacking of novelty.

English style and grammar is of good quality.

Reviewer 3 Report

This work is concerned with the design of a water thermal accumulator (WTA) on the basis of phase change materials (PCMs) by means of the numerical investigation.

The results obtained, as well as the methodology employed are quite interesting. In my opinion, the paper could be accepted after major revisions/improvements in the manuscript. My main suggestions are presented below:

-  First, an extensive editing of the English language and style is required throughout the whole paper.

- Please avoid useless hyphens like in “ex-change” (line 76), “pro-duction” (line 85), “ac-cumulating” (line 106), “co-efficient” (line 241), etc.

- Research background, problem statement, objectives of the study and research gap are elucidated in the abstract properly. However, there is lack of clear-cut statement in the results section coverage of the abstract. Please improve appropriately the novelty part.

- In the introduction section, I recommend the insertion of more recent references in order to reinforce the novelty of the present paper. As for this, one option is to cite the following papers, which fall in the same domain:

Nazzi Ehms,J.H.,  De Césaro Oliveski,R., Rocha,L.A.O., Biserni,C., “Theoretical and numerical analysis on phase change materials (PCM): A case study of the solidification process of erythritol in spheres”, International Journal of Heat and Mass Transfer, pp. 523-532, Vol. 119, 2018.

Nazzi Ehms,J.H.,  De Césaro Oliveski,R., Rocha,L.A.O., Biserni,C., Garai,M., “Fixed Grid Numerical Models for Solidification and Melting of Phase Change Materials (PCMs)”, Applied Sciences, 4334 pp. 1-22, Vol. 9, 2019.

- In the numerical treatment, I strongly recommend to add two paragraphs regarding “Grid independence verification” and “Validation of results” respectively. In particular, a criterion for appropriate mesh size should be given and verified, such as the following (where e is a suitable threshold):

| (q(j,max) -|q(j+1,max)) / |q(j,max)  |<e

- The quality of Figures 2 and 3 must be drastically improved.

- Finally, the conclusion section should be revised to reinforce the new contribution of the paper and future perspective of employment of the method used in this study in other engineering or physical problems.

An extensive editing of the English language and style is required throughout the whole paper.

Please avoid useless hyphens like in “ex-change” (line 76), “pro-duction” (line 85), “ac-cumulating” (line 106), “co-efficient” (line 241), etc.