A Finite Element Model for Investigating Unsteady-State Temperature Distribution and Thermomechanical Behavior of Underground Energy Piles

Round 1

Reviewer 1 Report

The paper present a finite element model used to analyze the temperature evolution and the thermomechanical behavior of energy piles.

The manuscript is interesting and well written. Publication is recommended after minor review.

Comments and suggestions

Please find a pdf with comments attached to the review.

As a suggestion, the abstract can include a very short summary of the findings.

Reference [1] does not seem to discuss the reduction of carbon dioxide emissions.

Please consider to replace the reference.

"This is unlikely to provide any other significant function to the building". This consideration does not seem to be brought up in reference [2]. Please consider to review the reference.

"energy pile can provide more efficient heat transfer capacity" What is the term of comparison? Water-filled boreholes, grouted boreholes? Please clarify.

As a suggestion, the peculiarities of the study compared to the available literature can be further stressed in the introduction.
The use of gradient and divergence symbol is strongly suggested over "grad", "div" in all equations, also for consistency with equations in all sections.

Transformation of Eq.2 into 3 requires also the thermal conductivity to be constant and uniform.

"The main reason for the error is that the effect of heat convection in free water in soil pores is not considered" Are there other reasons? Was the groundwater flow monitored during the experiment?

The terms "heat load", "heating load", "cold load" and "cooling load, "heat", "hot" are used throughout the text. It is suggested the use of heating load and cooling load for consistency.

"If not intervened, it will not only reduce the heat transfer efficiency of the system, but also seriously affect the sustain able utilization. Generally, when the heating load is dominant, auxiliary measures should be adopted, such as adding cooling towers; when the cooling load dominates, joint heating measures such as solar energy can be taken.". If additional installations should be considered in case of unbalanced loads, are energy pile solutions are (economically) not viable unless the yearly load is balanced?

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

This is an interesting manuscript, which reports publishable results. However, a number of basic heat transfer aspects and related terminology must be adjusted, as follows: 

·       ● “Temperature evolution” and “evolution of temperature field” should preferably be changed to “unsteady-state temperature distribution”. “Temporal and spatial evolution of temperature field” should thus become “temporal and spatial temperature distribution”.

·       ● On page 7, lines 146-147, “heat flux density” is redundant and should be “heat flux” only. 

·       ● “Specific heat” should everywhere be renamed as “specific heat capacity”. 

·       ● On page 4, lines 136-137, “volume specific heat … J/(kg.℃)” is obviously erroneous and should be “mass specific heat capacity”. “Material” is too generic and must be specified. 

·       ● While Eq. (2) reflects the nonlinear heat transfer problem with temperature-dependent thermophysical properties, Eq. (3) characterises the linear case with constant properties (and specifically thermal conductivity). Is this the real physical situation in water-containing grounds for the investigated temperature ranges? 

·      ● While Eq. (3) is valid for isotropic properties and heat transfer scenarios, Eq. (18) implies an anisotropic case, which constitutes an unacceptable mismatch, thereby confusing the readership and creating doubts in the results reported! 

·       ● On page 5, line 199, the expression “Kxx, Kyy and Kzz are thermal conductivity of materials” must be modified to explain that Kyy and Kzz are thermal conductivities in 3 different directions. “Materials” is too generic and must be specified.

The liaison between the proposed model and the simulations carried out remains unclear. More details are needed about the employed (commercial and/or custom-made) software and its performance to generate numerical results.

 

The manuscript can merit publication after substantial improvements, mandatorily taking into consideration the aforementioned remarks.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors did a number of terminological and technical improvements of the manuscript in accordance with the reviewer’s comments (points 1-4 and 7). 

For the transition from Eq. (2) to Eq. (3), the authors merely mentioned that the thermal conductivity is constant, without giving any reason for this assumption. The answer to the reviewer (point 5) explains that the latter is made for simplicity only, but this argument is neither included in the paper itself, nor convincing. There are well-established mathematical models and software tools to handle non-linear heat conduction problems, so the non-linearity could not be a real obstacle. 

The authors are trying to explain (point 6) that the heat transfer problem is isotropic, while the thermomechanical one is anisotropic. This mismatch remains absolutely unclear and makes the model suspicious. Such a contradiction is highly confusing and totally unacceptable, unless a reasonable rationale is provided in the paper itself. 

The details about the software tools communicated to the reviewer (point 8) must be included in the paper text. Remember that research papers are written to serve the broad international readership!

Author Response

Please see the attachment

Author Response File: Author Response.pdf