Thermal Performance Analysis and Multi-Factor Optimization of Middle–Deep Coaxial Borehole Heat Exchanger System for Low-Carbon Building Heating
Round 1
Reviewer 1 Report
This paper proposed a multi-factor optimization and performance analysis of a MDBHE system. It compared the impact of different parameters through orthogonal experimentation method. The paper is well-written and well-structured, however, the following comments and concerns shall be addressed before publication:
1. It is highly recommended to add a nomenclature section to the paper.
2. Please add future works to the conclusion section.
3. What are the limits of the proposed method?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The present study has lack of sufficient novelty and the manuscript shows an unacceptable level of overlap with prior publications.
The level of English throughout the manuscript does not meet the journal’s required standard. You may wish to ask a native speaker to check your manuscript for grammar, style and syntax.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The title of the paper is slightly misleading: no real optimisation is yet possible with more information about actual heat pump and fluid pump performances.
This study appears more as a preliminary parametric exercise, performed with the help of a simulation model previously developed and previously validated.
Here are some specific comments and suggestions:
1. Introduction
You should make better appear what is new here, by reference to the many previous studies.
Are you considering heating only? No cooling?
"by superposing the solutions of heat transfer in soil without geothermal gradient and that in soil with only geothermal gradient[9]": this is not very clear...
"based on the known temperature distribution of the borehole wall[10]": not very clear...
" assumes that the thermal physical property parameters of underground soil are the same, and the heat transfer load of buried pipes per unit depth is equal,": not very clear...
2. Mathematical model
2.1 Model strategy
"(b) The surface temperature is fixed, i.e. does not change over time": not clear...
2.2 Model details
Is it useful to repeat all equations already given in references 23 and 24 (ad probably also in other references)?
If you decide to repeat them, you must give a complete list of symbols, definitions and units.
You should anyway explain us how the convective heat transfer coefficients "h1" and "h2" (and corresponding frictions factors) are actually identified!
2.3 Model validation
If you decide to repeat here what is already said in reference 25, you should tell us much more:
- An error on temperature expressed in % has no meaning. You should speak about a temperature difference compared with some reference.
How is the "error" identified in relationship with Figure 1?
By the way, why are the actual high frequency oscillations lost in the simulation?
3. Results and analysis
We are now back to pure simulation, without any new experimental verification!
3.1 Single factor analysis
3.1.1 Borehole depth
Figure 1 disserves to be better explained. Are these results obtained by integration over 120 days only? What about pressure drops and pump consumption?
3.1.2 Mass flow rate
Same questions with even more interest to pressure drops...
3.1.3 Inlet temperature
"there are risks such as freezing if the inlet temperature is too low.": of course, but it would also produce a high degradation of heat pump COP...
3.1.4 Buried tube radial structure parameters
(a) Buried tube radial structure parameters s0
The thickness of the inner tube should also disserve more attention, because of the thermal short circuit unavoidable in this sort of heat exchanger.
Of course, this has to be carefully considered in relationship with actual convective coefficients h1 and h2...
(b) Buried tube radial structure parameters s2
Same questions about actual convective transfer coefficients and pressure drops...
3.1.5. Thermal conductivity of the selected material
To be considered in relationship with "s1", "h1" and "h2"!
Figure 10: to be compared with the impact of "h2"!
3.3. Variable flow operation simulation analysis
Again here, what about pressure drops and pump consumption?
3.3.1. Variable flow operating system model
This disserves to be much more developed!
Impossible for the reader to catch the difference between both diagrams of Figure 14. Should be presented at more appropriate scale and/or completed with help of some table...
Table 5. Simulation results under different operating modes
At the end, something is said here about pump consumption!
Of course, this has to be combined with heat pump performances...
4. Discussion
Impossible to have a satisfactory discussion without tracking the convective heat transfer coefficients and the corresponding pressure drops!
5. Conclusions
Impossible to conclude...
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
No further comments
Reviewer 2 Report
It's wonderful to hear that the revised manuscript has shown significant improvement and is now considered acceptable in its current format. This indicates that the changes made have addressed previous issues and improved the overall quality of the document. If all the necessary criteria for acceptance have been met, then it seems like the manuscript is ready for publication. However, it's always a good practice to double-check with any specific Journal guidelines.
