A Review on Geothermal Heat Exchangers: Challenges, Coating Methods, and Coating Materials

Round 1

Reviewer 1 Report

-In the line of 34, what is 'figures'?

-Fig. 1 were not cited in the text. Cite it and explain what it is.

-In the line of 55, 'Error! Reference source not found' correct it.

-Coating methods have been highly emphasized in this review, but the main emphasis of the study should be on coating materials. The types of coating applied and the anti-wear and corrosion surface coating elements should be highlighted. Rather than the coating methods, which coating types are used should be emphasized in detail. In addition, emphasis should be placed on which parameters in the methods used reveal what kind of coating results. The study should be strengthened by giving SEM, EDS and corrosion wear results of the coating types in literature.

Minor editing of English language required

Author Response

Reviewer1

Thank you for your suggestion and the changes have been incorporated into the manuscript.

-In the line of 34, what is 'figures'?

Response: We have changed the word to statistics

-Fig. 1 were not cited in the text. Cite it and explain what it is.

Response: We have mentioned it in lines 61 - 62.

-In the line of 55, 'Error! Reference source not found' correct it.

Response: We have corrected it

-Coating methods have been highly emphasized in this review, but the main emphasis of the study should be on coating materials. The types of coating applied and the anti-wear and corrosion surface coating elements should be highlighted. Rather than the coating methods, which coating types are used should be emphasized in detail. In addition, emphasis should be placed on which parameters in the methods used reveal what kind of coating results. The study should be strengthened by giving SEM, EDS and corrosion wear results of the coating types in literature.

Response: We have added the relevant information in the section 3.2 and also altered the review paper title to ‘A review on geothermal heat exchangers: Challenges, coating methods and coating materials.’

Information related to coating materials and their performance has been added in the subsection 3.2 from lines 649 – 771.

Reviewer 2 Report

Coatings for Geothermal Heat exchangers

In this submission titled “Coatings for Geothermal heat exchangers” the authors have given due importance to the geothermal energy and the heat exchangers in the geothermal plant. This review discussed the challenges in the geothermal energy conversion like erosion, silica scales and corrosion are the techniques to overcome them by surface techniques. The characteristic features of the geothermal plant are discussed. A table comprising the advantages and disadvantages of different coating techniques is appreciated. The authors claim that the research papers in this area are meager.

In this connection, I would welcome the authors to answer the queries raised here.

Major issues

1.      The authors shall outline the differences between fouling and scaling, technically. On Page No. 5, a sub-heading on Fouling and scaling was given, but scaling is not dealt. If silica scaling is a part of Fouling as shown on Page No. 3 Line No. 139, the sub-heading shall be changed accordingly and some information can be added.

2.      The concept of spalling is quite important. It shall be included in the challenges section. Actually it is shown in the Figure 2 a, but not in the context.

3.      The authors shall also consider fatigue cracks, hydrogen attacks, pitting and their effects as the challenges encountered in heat exchangers.

4.      A protective coating to carbon steel was suggested by the authors for the geothermal heat exchangers to reduce cost. Whether the coating would be compatible for the carbon steels and will this affect the working (heat transfer) of the heat exchanger in any way?

5.      What kind of technique was used to find the rate of corrosion post coating?

6.      The title is too short and general. It shall be refined including the word “review”

7.      The authors shall briefly outline what the review focuses upon and what does this review paper deliver and conclude?

8.      The authors say that, to investigate coatings for geothermal heat exchangers, Google Scholar, Web of Science and Scopus were used. Although literature specifically focusing on coatings for geothermal heat exchangers is limited, less than 25 articles, published between 2000 and 2023, I would suggest the authors to search with different terms so that some other literature related to this study can be found and reviewed. For example, the search string may be, Surface coatings in geothermal heat exchangers, Surface modifications on geothermal heat exchangers,

9.      In the coatings for geothermal heat exchangers, the authors shall add the role of phase change materials and nanofluids for coatings.

Minor issues

1.      The expansion for the acronym IEA shall be given in the first line of the Introduction. In the same sentence, the unit Gt shall be given in brackets, atleast in once.

2.      On Page No. 2, Line No. 55, The message “ Error ! Reference source 55 not found shall be removed or changed” . For example, it may be changed as “Internet sources show geothermal plants in Buyuk Menderes Graben (BMG) in ………”.

3.      On Page No. 13, Line No. 417, “advantageous” shall be replaced as “advantages”.

4.      On Page No. 13, Line No. 461, the word “respectively” shall be added for conveying the appropriate reduction in corrosion rate.

5.      Minor English check can improve the paper.

The authors shall improve the paper considering some suggestions given in the reviewer comments.

Some Minor English corrections are suggested for the authors. Kindly see the reviewers comments to the authors.

Author Response

Reviewer 2

Thank you for your suggestion and the changes have been incorporated into the manuscript.

In this submission titled “Coatings for Geothermal heat exchangers” the authors have given due importance to the geothermal energy and the heat exchangers in the geothermal plant. This review discussed the challenges in the geothermal energy conversion like erosion, silica scales and corrosion are the techniques to overcome them by surface techniques. The characteristic features of the geothermal plant are discussed. A table comprising the advantages and disadvantages of different coating techniques is appreciated. The authors claim that the research papers in this area are meager.

In this connection, I would welcome the authors to answer the queries raised here.

Major issues

1. The authors shall outline the differences between fouling and scaling, technically. On Page No. 5, a sub-heading on Fouling and scaling was given, but scaling is not dealt. If silica scaling is a part of Fouling as shown on Page No. 3 Line No. 139, the sub-heading shall be changed accordingly and some information can be added.

       Response: We have modified the ‘fouling and scaling’ subheading in the section 2.1 (line 132) to ‘fouling’ and added more description (lines 137–147 and 164-165) which highlights scaling as being a type of fouling. 

 2    The concept of spalling is quite important. It shall be included in the challenges section. Actually, it is shown in the Figure 2 a, but not in the context.

Response: It was a spelling mistake from our part (It is scaling instead of scalling) in the caption of figure 2 (a).

3. The authors shall also consider fatigue cracks, hydrogen attacks, pitting and their effects as the challenges encountered in heat exchangers.

      Response: We have added additional information about hydrogen embrittlement, stress corrosion cracking, pitting corrosion under the section 2.1 subheading ‘corrosion’ from lines 183-187 and 193–232.

4. A protective coating to carbon steel was suggested by the authors for the geothermal heat exchangers to reduce cost. Whether the coating would be compatible for the carbon steels and will this affect the working (heat transfer) of the heat exchanger in any way?

       Response: We have made the necessary changes (lines 326 – 327). The sentence refers to coatings in a general sense and does not specify any particular coatings. The effect of any coating and its performance depends on the coating material and the microstructure/morphology attained. 

5. What kind of technique was used to find the rate of corrosion post coating?

Response: We have added the information asked in to the manuscript in lines 183–187.

6. The title is too short and general. It shall be refined including the word “review”

Response:  We have changed the title to ‘A review on geothermal heat exchangers: Challenges, coating methods and coating materials.’

7. The authors shall briefly outline what the review focuses upon and what does this review paper deliver and conclude?

       Response:  We have included it under the Introduction section (lines 77 – 86)

8. The authors say that, to investigate coatings for geothermal heat exchangers, Google Scholar, Web of Science and Scopus were used. Although literature specifically focusing on coatings for geothermal heat exchangers is limited, less than 25 articles, published between 2000 and 2023, I would suggest the authors to search with different terms so that some other literature related to this study can be found and reviewed. For example, the search string may be,Surface coatings in geothermal heat exchangers, Surface modifications on geothermal heat exchangers,

      Response:  In accordance with the suggestion, we searched for more literature using the technical terms recommended by the reviewer along with other terms. However, we failed to find any more articles on ‘coatings applied in geothermal heat exchangers’ than those already mentioned in the manuscript.

      Conversely, we have also removed the precise number of articles (25) as mentioned previously.

9. In the coatings for geothermal heat exchangers, the authors shall add the role of phase change materials and nanofluids for coatings.

 Response:  To the best of our knowledge phase change materials are mostly used for thermal energy storage and we were unable to find any studies related specifically to heat exchangers for geothermal environment.

We have added some works done using nano-fluids to improve heat transfer efficiency, under the section 3.2, lines 743 – 768.

Minor issues

1. The expansion for the acronym IEA shall be given in the first line of the Introduction. In the same sentence, the unit Gt shall be given in brackets, atleast in once.

 Response: We have changed it.

2. On Page No. 2, Line No. 55, The message “Error! Reference source 55 not found shall be removed or changed”. For example, it may be changed as “Internet sources show geothermal plants in Buyuk Menderes Graben (BMG) in ………”.

  Response: We have changed it.

3. On Page No. 13, Line No. 417, “advantageous” shall be replaced as “advantages”.

  Response: We have changed it.

4. On Page No. 13, Line No. 461, the word “respectively” shall be added for conveying the appropriate reduction in corrosion rate.

  Response: We have changed it.

5. Minor English check can improve the paper.

      Response: We worked on the manuscript

Reviewer 3 Report

This review provides information about the geothermal sources, their application for electricity production, and the challenges for heat exchangers and solutions. The authors summarized engineering techniques with focus on coatings to address the challenges. This review relates to the current interest in environmental-friendly alternative energies. It might be helpful for beginners in this field. The text of this paper is in the scope of this Journal. The manuscript is written properly. Thus, I’d like to suggest acceptance of this manuscript for publication in Coatings after some improvements

A careful check is required.

Author Response

Reviewer 3

This review provides information about the geothermal sources, their application for electricity production, and the challenges for heat exchangers and solutions. The authors summarized engineering techniques with focus on coatings to address the challenges. This review relates to the current interest in environmental-friendly alternative energies. It might be helpful for beginners in this field. The text of this paper is in the scope of this Journal. The manuscript is written properly. Thus, I’d like to suggest acceptance of this manuscript for publication in Coatings after some improvements

Response: Thank you for reviewing our manuscript and for your feedback.

Reviewer 4 Report

In this review article, the authors systematized information about the main equipment used in the production of geothermal heat, mainly about the variety of heat exchangers and their features, the problems that arise during the processes (fouling, scaling, corrosion) in the heat exchange equipment and their effective solutions, focusing mainly on the application of various coating technologies in geothermal heat exchangers.

I believe that such summarized information will help researchers working in relevant fields of science more effectively develop existing and create new technological solutions for the formation of coatings, and also it will encourage the spreading of new coating technologies in practice.

Although the authors reviewed a large number of already existing publications, in my opinion, this new review article should be expanded with information on the application of paints (common or specific) in geothermal heat exchangers, highlighting the advantages and disadvantages of paint coatings, as well as the peculiarities of their formation methods.

Also, before publication minor corrections should be made to the submitted manuscript. My suggestions and comments are as follows:

1) Some abbreviations are not preceded by their full name when they are first mentioned in the text: IEA – in line 30; ORC – in line 57; FPS – in line 422; PTFE – in line 511; CNTs, HTC, MWNT, PPS  – in the last unnumbered table;

2) I would recommend presenting the chemical formulas next to the names of chemical compounds mentioned for the first time in the text, and for frequently repeated compounds to continue using only chemical formulas in the text, e.g. stibnite, silica, zirconia, alumina, titania etc.;

3) The sequence and numbering of some figures mentioned in the text should be rearranged, e.g. Figure 4 (in line 115) is mentioned before Figures 2 and 3, although it is presented after Figures 2 and 3; the wrong figure number is specified (see line 123);

4) In the body of the text is a lack of reference to Table 2; the last table is not numbered at all and has no name; the number of a non-existent table is indicated (see line 564);

5) The name of section "3.1.6 Electrochemical method" should be changed to "Electroless deposition method" or "Electroless plating method", as this section exclusively deals with electroless deposition, but not electrodeposition (using electrolysis); also, the term "electrochemical" should be changed to "electroless" (in line 481) and to "Electroless deposition" (in Table 2);

6) In the last table, the formulas of the chemical compounds should be revised by moving the numbers to the subscript.

Author Response

Reviewer4

Thanks for your feedback!

In this review article, the authors systematized information about the main equipment used in the production of geothermal heat, mainly about the variety of heat exchangers and their features, the problems that arise during the processes (fouling, scaling, corrosion) in the heat exchange equipment and their effective solutions, focusing mainly on the application of various coating technologies in geothermal heat exchangers.

I believe that such summarized information will help researchers working in relevant fields of science more effectively develop existing and create new technological solutions for the formation of coatings, and also it will encourage the spreading of new coating technologies in practice.

Although the authors reviewed a large number of already existing publications, in my opinion, this new review article should be expanded with information on the application of paints (common or specific) in geothermal heat exchangers, highlighting the advantages and disadvantages of paint coatings, as well as the peculiarities of their formation methods.

Response:  Information about paints and related studies conducted on geothermal heat exchangers have now been added in the manuscript to the section 3.2.

Also, before publication minor corrections should be made to the submitted manuscript. My suggestions and comments are as follows:

• Some abbreviations are not preceded by their full name when they are first mentioned in the text: IEA – in line 30; ORC – in line 57; FPS – in line 422; PTFE – in line 511; CNTs, HTC, MWNT, PPS – in the last unnumbered table;

Response:  All the abbreviations are now expanded in the manuscript.

• I would recommend presenting the chemical formulas next to the names of chemical compounds mentioned for the first time in the text, and for frequently repeated compounds to continue using only chemical formulas in the text, e.g. stibnite, silica, zirconia, alumina, titania etc.;

Response:  Chemical formulas are added along with the names of the chemical compounds.

• The sequence and numbering of some figures mentioned in the text should be rearranged, e.g. Figure 4 (in line 115) is mentioned before Figures 2 and 3, although it is presented after Figures 2 and 3; the wrong figure number is specified (see line 123);

Response: Changes have been made in the manuscript according to the suggestion.

• In the body of the text is a lack of reference to Table 2; the last table is not numbered at all and has no name; the number of a non-existent table is indicated (see line 564);

Response: We have now made all the required and suggested changes.

• The name of section "3.1.6 Electrochemical method" should be changed to "Electroless deposition method" or "Electroless plating method", as this section exclusively deals with electroless deposition, but not electrodeposition (using electrolysis); also, the term "electrochemical" should be changed to "electroless" (in line 481) and to "Electroless deposition" (in Table 2);

Response: Changes have been made according to the suggestion.

• In the last table, the formulas of the chemical compounds should be revised by moving the numbers to the subscript.

Response: Changes have been made according to the suggestion.

Reviewer 5 Report

Manuscript ID: coatings-2665175 entitled:

Coatings for Geothermal Heat Exchangers

Authors

Arunima Bhuvanendran Nair Jayakumari , Nigar Gul Malik , Garima Mittal , David Martelo , Namrata Kale , Shiladitya Paul

General comment

This paper is a review that contain recent details upon critical role coatings play in improving the efficiency and service life of heat exchangers in geothermal power plants.

Some recommendations and observation remain:

1. Insert in a subparagraph 2.1 few information about the principal species that can lead to erosion, scale degradation processes, and corrosion in geothermal fluids.

2. Also, before paragraph 3.1 it is beneficial to insert a sub-paragraph on the most common types of protective coatings for Geothermal Heat Exchangers.

3. To complete the review insert few comments upon the evaluation and characterization of coatings insert in a subparagraph entitled for example. Characterization of surface coatings (electrochemical techniques, scanning electron microscope (SEM), mechanical properties, water‐drop contact angle, thermal conductivity measurements etc).

Author Response

Reviewer 4

Thank you for reviewing our manuscripts and for the feedback.

This paper is a review that contain recent details upon critical role coatings play in improving the efficiency and service life of heat exchangers in geothermal power plants.

Some recommendations and observation remain:

1. Insert in a subparagraph 2.1 few information about the principal species that can lead to erosion, scale degradation processes, and corrosion in geothermal fluids.

Response: Figure 5 highlights the factors and species leading to corrosion. This information is also mentioned in the text from line 180 – 183.

2. Also, before paragraph 3.1 it is beneficial to insert a sub-paragraph on the most common types of protective coatings for Geothermal Heat Exchangers.

Response: This information has been added in sub-section 3.2; lines 646 – 768.

3. To complete the review insert few comments upon the evaluation and characterization of coatings insert in a subparagraph entitled for example. Characterization of surface coatings (electrochemical techniques, scanning electron microscope (SEM), mechanical properties, water‐drop contact angle, thermal conductivity measurements etc).

Response: We have included this information in sub-section 3.2; lines 646 – 768.

Round 2

Reviewer 1 Report

Thanks for satisfactory revision.

Author Response

Please see the attachment