An Analysis of the Heat Transfer Characteristics of Medium-Shallow Borehole Ground Heat Exchangers with Various Working Fluids

Round 1

Reviewer 1 Report (Previous Reviewer 3)

The authors have studied the effect of the working fluid type on the performance of medium-shallow BGHEs. The topic is quite interesting and in the scope of the Journal. The developed model is reliable and useful. The followings are some minor errors needed to be solved, and then the manuscript can be accepted.

1. For table 1, unit K should be replaced by ℃, because in the following discussion, only℃ is used.

2. In the abstract, please avoid using “we”.

3. Q is used for heat rate and flow rate simultaneously. Please use a new symbol for the flow rate.

4. The conclusion is too long. I suggest that the authors should condense it appropriately.

Author Response

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Reviewer 2 Report (New Reviewer)

It is an interesting study. It is well-written and well-structured. I have some minor comments as below:

*Line 24: please revise it.

*Line 26: R600a? It is not defined in the text so far.

*Line 29: water

*Line 46: geothermal is not zero-carbon! 

*Line 110: you can add references regarding the affecting parameters for heat extraction from geothermal systems:

#Thermo-hydro-mechanical modeling of an enhanced geothermal system in a fractured reservoir using carbon dioxide as heat transmission fluid-A sensitivity investigation.

#Simulations and global sensitivity analysis of the thermo-hydraulic-mechanical processes in a fractured geothermal reservoir.

*Line 133: in a similar application, CO2 shows better performance than water as the working fluid and you can use this study to support your findings:

#Comparative study on heat extraction from Soultz-sous-Forêts geothermal field using supercritical carbon dioxide and water as the working fluid.

*Line 143: please cite the related paper.

*Please use references for the equations!

*All figures should be mentioned int he text. Please check this issue. Also, it is better to put them after their explanation in the text.

*Line 248-250: it is not clear.

*Line 269: transferred heat?

*It is better to use "Equation" instead of the "Formula" through the text.

*Figure 8: you can not connect the error points to each other, because you do not have a continuous data. Just show them with points.

*Line 543: law or trend?

*Line 594: not clear.

*Line 606: show or showed?

Minor editing of English language required.

Author Response

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Reviewer 3 Report (New Reviewer)

Manuscript ID: sustainability-2563808

Title: Analysis of Heat Transfer Characteristics of Medium-Shallow Borehole Ground Heat Exchangers with Various Working Fluids

This study investigates the impact of different working fluids on the performance of medium-shallow borehole ground heat exchangers (BGHEs). A validated heat transfer model is used to compare and analyze various factors such as heat transfer performance, pressure loss, economy, and environmental benefits. The results suggest that CO2 is the most suitable working fluid among organic fluids considered, providing higher heat transfer efficiency than water. These findings contribute to selecting appropriate working fluids and improving BGHEs' heat transfer performance for medium-shallow geothermal energy applications. The authors should answer the following comments:

1. Provide more details on the research accuracy and limitations.

2. Elaborate on the specific experimental test data used to validate the numerical model, including information on sample size, conditions, and any potential sources of error or uncertainty.

3. Provide a comprehensive explanation of how the comparison and analysis were conducted for the heat transfer performance of different working fluids in BGHEs, including a clear description of parameters measured and statistical methods employed.

4. in conclusion: Clearly define and explain the "heat transfer index" mentioned in relation to the burial depth of boreholes and its implications for heating versus cooling capacity.

5. Recently, the use of ultrasonic vibrations has been introduced as another way to improve the efficiency of thermal systems. It is suggested to mention this in the introduction for readers’ information. Referring to the following articles will be helpful: Doi: 10.1007/s40997-023-00667-5; 10.1016/j.icheatmasstransfer.2022.106098

6. Present a detailed discussion on why R600a and CO2 are considered more suitable working fluids than water based on their heat transfer performance, pump power consumption, and other relevant factors.

7. Expand upon the environmental benefits associated with different working fluid types in medium-shallow BGHEs by providing quantitative data or references supporting claims about carbon emissions reduction potential.

8. Provide further insights into economic analysis by explaining how investment cost calculations were made, considering factors such as equipment costs, maintenance expenses, energy efficiency gains over time, etc., alongside payback period estimations.

9. Justify recommendations regarding operating mode preferences (heating mode in winter/heat dissipation mode in summer) for medium-shallow casing buried pipe systems based on their impact on long-term heat transfer characteristics and energy efficiency considerations.

10. Expand upon future research directions by providing more context around investigating inlet pressure effects and phase changes of working fluids within BGHEs to gain deeper insights into their impact on heat performance.

Minor editing of the English language is required.

Author Response

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Round 2

Reviewer 3 Report (New Reviewer)

All the comments are addressed.

Minor editing of the English language is required.

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

(1) The abstract requires some quantitative data to support the results.  In this way, the results in the abstract are more comprehensive and informative.

(2) Is the research in this manuscript about in-situ geothermal development? If so, I suggest that the authors highlight this point in their manuscript, as it is an innovative point. If it is not in-situ mining, is it still related to groundwater seepage and thermal convection?

(3) In Figure 5, there are three solid red lines in the legend, which one is the temperature curve? Please check the entire manuscript to ensure that the data or curves in the charts are clearly expressed. In addition, we can see from the validation results in Figure 5 that there is an error of up to about 5% in some parts. What are the reasons for these high errors? 

(4) Section "4.1. Introduction to the working fluid" should not be separated into a single section and can be merged with the next section. Moreover, its significance as a separate part of the content is not significant.

(5) Now, the tense energy situation has become a consensus, and the development of new energy is imperative. Some references should be cited to support the statement in Line 34-35.  ①https://link.springer.com/article/10.1007/s11053-023-10202-7; ② https://link.springer.com/article/10.1007/s11356-023-26279-9.

(6) R134a, R152a, R227ea, R245fa, and R600a are abbreviations for certain substances in the manuscript. Readers in the field of chemistry will be very aware of this, but readers in other fields will not be aware of it. Therefore, it is recommended that authors use parentheses to indicate their scientific and full names for the first time.

The language of the manuscript is acceptable, just pay attention to some details and make slight language adjustments.

Author Response

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Reviewer 2 Report

In their manuscript “Investigation on the Heat Transfer Characteristics of Borehole Ground Heat Exchangers of Medium-Shallow Strata Based on Various Working Fluids”, Wang et al. tried to convince us that their research results in this paper could provide a theoretical basis and technical guidance for the rational selection of the working fluid and the improvement in the heat transfer performance of BGHEs, which could promote the development and application of medium-shallow geothermal energy sources.

In the introduction, the authors attempt to put forward the benefits of their technology compared to others. Nevertheless, the difference between their approach and others remains unclear.

Concerning the evaluation of the scenarios, the parameters evaluated seem interesting but it might have been interesting to have more details on the dynamic models and the type of simulations used.

It would benefit from a thorough revision from a native English speaker. 

Author Response

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Reviewer 3 Report

Comments for author File: Comments.pdf

Author Response

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Round 2

Reviewer 1 Report

Overall, the quality of the manuscript has greatly improved, reaching a level where it can be accepted and published. However, the paper still needs to cite the literature mentioned in the first round of review in the appropriate position (https://link.springer.com/article/10.1007/s11356-023-26279-9 ). Because it is indeed a great work in this field. At the same time, language requires the authors to polish the entire manuscript, and there are too many complex and confusing sentences.

Language requires the authors to polish the entire manuscript, and there are too many complex and confusing sentences.

Author Response

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Reviewer 2 Report

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Author Response

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Round 3

Reviewer 2 Report

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Author Response

Based on your comments from the previous two rounds, corresponding modifications have been made to the manuscript.