Improvement in Operation Efficiency of Shallow Geothermal Energy System—A Case Study in Shandong Province, China
Round 1
Reviewer 1 Report
The paper needs major revisions:
1-Optimization of operation efficiency is done without optimization algorithm application. "Optimization' term in the title would reader perplexed
2-Which optimization algorithm was used to provide "Simulation of the optimized operation schemes" section???
3-Boundary condition should be comprehensively revised.
4-Motivation and innovation should be detailed in the introduction: "Most of studies about the sustainability of SGE system focused on system thermodynamics, the optimization of installation and operation stages[9, 10],".
5-The research organization of introduction in the last paragraph should be detailed.
Author Response
1-Optimization of operation efficiency is done without optimization algorithm application. "Optimization' term in the title would reader perplexed
Accepted: In this paper, the “optimization algorithm” couldn’t be considered. Therefore, the word “optimization” could be substituted by the word “improvement” in this paper.
2-Which optimization algorithm was used to provide "Simulation of the optimized operation schemes" section???
Clarified: "Simulation of the optimized operation schemes" is changed to “ "Simulation of the improved operation schemes" . In order to improve the operation efficiency, an example of the SGE system in Shandong province of China, is analyzed by the hydro-thermal coupling simulated mode on the different condition of pumping and injected well.
3-Boundary condition should be comprehensively revised.
Clarified: The boundary condition of the model was comprehensively introduced in the “3.1. Conceptual model of hydrogeology” and in the “3.2.3. Model condition setting and solution” as follow:
3.1. Conceptual model of hydrogeology
According to the borehole profile, the formation lithology is mainly clay, gravel and limestone (showed in Figure 2). The aquifer is confined with a depth of 45.8 m. The main water-bearing medium is gravel and is generalized to be an isotropic aquifer. The pumping and injection wells are completely penetrated wells.
The groundwater flows from the South to the North, i.e., from the 4# well to the 1# well. Based on the pumping test, the range of influence for pumping well is less than 81.5m, therefore, the range of conceptual model is suggested as a size of 1000m (length)×1000m (width)×140m (depth). And the SGE system locates in the center of the square. The distance between the boundary and the SGE system is 500m, which is not affected by the pumping wells. The south and north boundaries are considered as constant head boundary with a head value of 29.74m and 28.09m, respectively. The east and west boundaries are considered as impermeable boundary. The effect of rainfall is ignored in this simulation. During the operation period, the fluxes of the pumping and injection wells are constant.
For the thermal model, the upstream boundary is set to constant temperature of 21℃, and the other boundary conditions are considered as heat exchange boundaries. The temperature of the pumping wells is constant at 21℃, while the temperature of the injection wells during the cooling and heating periods is constant at 26℃ and 16℃, respectively.
3.2.3. Model condition setting
Grid dissection is performed using the Grid Builder program, and the subsurface water flow and heat transport model is solved by the finite element calculation software Feflow. According to the conceptual model, the model is divided into 6 vertical layers, and the finite element mesh generation of the model followed the rule that the size of mesh is 1m×1m in the region, while 0.2m×0.2m for the wells.
(1) Boundary condition
For the steady flow model, the water head at upstream and downstream boundary are 29.74m and 28.09m, and the west and east boundaries are no-flow boundaries, with no recharge at both the upper and lower boundaries, shown in the formulas (3.2.3-1) . The flux of each pumping well is 1512m3/d, while, 453.6m3/d or 756 m3/d for each injection well according the Table 1.
For the thermal transport model, the temperature at the upstream boundary is 21°C, while the other boundaries with free heat exchange. During the cooling and heating operation periods, the temperatures of injection are 16 ℃ and 26 ℃, respectively, shown in the formulas (3.2.3-2).
(2) Initial conditions
The water head of initial conditions for the unstable three-dimensional model are obtained from the stable three-dimensional model which is the same as the unstable 3D model expect for the wells. While the initial temperature is set to 21 ℃, while the initial temperatures for cooling and heating periods are 26 ℃ and 16 ℃ ,respectively, shown in the in the formulas (3.2.3-3).
4-Motivation and innovation should be detailed in the introduction: "Most of studies about the sustainability of SGE system focused on system thermodynamics, the optimization of installation and operation stages[9, 10],".
Clarified: How to improve the operational efficiency of SGE systems is vital to in the sustainably utilizate geothermal energy. The sustainability of SGE system was widely focused on system thermodynamics, the optimization of installation and operation stages by researchers and engineers, which could affected the energy exchange efficiency of the SGE system during the operation period. Andrea Aquino, et al. showed that the factors on the sustainability of SGE system were mainly the climate and ground properties, the installation operations, the maintenance of refrigerant circuit and the serviced building and so on, and the optimization of installation and operation stages could improve the SGE system[9], while the efficiency of the SGE system could be mainly controlled by the quality and quantity of the groundwater, the choose of aquifers and filter pipes, the distance between hot and cold wells, the location of wells[10]. Chang and Kin[11] performed the thermal conductivity of the vertical closed U-loop ground heat exchanger increases with the increase of the larger diameter of tube. A new innovative borehole heat exchanger structure of one outlet and three inlet pipes was proposed, and the experimental results showed that the average circulating water temperature of the structure was 1 ℃ and 3.7 ℃ than the traditional single-U, double-U, respectively.[12]. The thermal conductivity of materials transferring the heat from the pipe and earth was analyzed that the temperature rose on the borehole wall of the inclined pipe could be 10-35% lower than the vertical pipe, showing that the the structure could affect the SGE system long-term performance[13]. The installation costs are evaluated by the validated capacitance-resistance numerical model, considering the factors of the borehole diameter of the helical coil, depth of helical coil, spacing[14]. The parameter of COP was optimized by a methodology which would operate on both heating and cooling models, and the maximum COP for only heating and cooling operations were 4.25 and 3.32, respectively[15]. The method of life cycle assessment (LCA) was applied to assess the impact of the SGE system on the environment, mainly about the manufacturing, transportation,operation,energy consumption, and air emission to environment system[16].
5-The research organization of introduction in the last paragraph should be detailed.
Clarified: The research organization of introduction in the last paragraph was detailed as followed:
Firstly, the meteorology, hydrology, hydrogeology and operation of the SGE system were introduced. Secondly, the mathematical model containing groundwater flow and thermal transport codes, and the simulation scheme were both shown. Then the simulation results of the current operation scheme and the improved operation scheme were analyzed. Lastly, the degree of heat transfixion and the accumulated minable shallow geothermal energy were discussed, and the main conclusions were advised, which might be valuable to the operations of the SGE system.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
(1) In section "Study area ", one Figure showing the details of the study area (Location and strata profile) needs to be added. In this way, the details of the study area can be displayed more clear. In addition, Figure 1 should also be beautified, at least the size and other parameters of the model should be explained.
(2) Model condition setting and solution are in the section of mathematical model. Therefore, these contents should be given in the form of mathematical formulas. And,
(3) The numerical model presented in manuscript is 3D model. However, the model in the case study is indeed two-dimensional. The mathematical model does not correspond to the case model. Please explain this issue.
(4) What is the applicability of the mathematical model developed in the section 3.2? The reviewer thinks that the applicability of the mathematical model should be verified.
(5) All figures and tables should be stated in manuscript. If not, the reviwers and readers will be confused about the content of the manuscript. Moreover, the authors should polish the language of the manuscript.
(6) In the "References" section, some references should be added to support the statement in the manuscript: One of the effective ways to realize the above goal is to Vigorously develop SGE The SGE could make a significant contribution on reducing CO2 emissions and gradually replace the fossil fuels, while ensuring sustainable socio-economic development. After searching, the reviewers think the following references are excellent and classic: https://doi.org/10.3390/w15040677, https://doi.org/10.1007/s40948-022-00396-0, https://doi.org/10.1007/s11356-021-18169-9, https://doi.org/10.3390/min12111355.
Author Response
（1）In section "Study area ", one Figure showing the details of the study area (Location and strata profile) needs to be added. In this way, the details of the study area can be displayed more clear. In addition, Figure 1 should also be beautified, at least the size and other parameters of the model should be explained.
Accepted: A new figure 1 was added as follows.
（2）Model condition setting and solution are in the section of mathematical model. Therefore, these contents should be given in the form of mathematical formulas. And,
Cleared: 3.2.3. Model condition setting
3.2.3. Model condition setting
Grid dissection is performed using the Grid Builder program, and the subsurface water flow and heat transport model is solved by the finite element calculation software Feflow. According to the conceptual model, the model is divided into 6 vertical layers, and the finite element mesh generation of the model followed the rule that the size of mesh is 1m×1m in the region, while 0.2m×0.2m for the wells.
(1) Boundary condition
For the steady flow model, the water head at upstream and downstream boundary are 29.74m and 28.09m, and the west and east boundaries are no-flow boundaries, with no recharge at both the upper and lower boundaries, shown in the formulas (3.2.3-1) . The flux of each pumping well is 1512m3/d, while, 453.6m3/d or 756 m3/d for each injection well according the Table 1.
For the thermal transport model, the temperature at the upstream boundary is 21°C, while the other boundaries with free heat exchange. During the cooling and heating operation periods, the temperatures of injection are 16 ℃ and 26 ℃, respectively, shown in the formulas (3.2.3-2).
(2) Initial conditions
The water head of initial conditions for the unstable three-dimensional model are obtained from the stable three-dimensional model which is the same as the unstable 3D model expect for the wells. While the initial temperature is set to 21 ℃, while the initial temperatures for cooling and heating periods are 26 ℃ and 16 ℃ ,respectively, shown in the in the formulas (3.2.3-3).
（3）The numerical model presented in manuscript is 3D model. However, the model in the case study is indeed two-dimensional. The mathematical model does not correspond to the case model. Please explain this issue.
Cleared: The model in the case study is 3D model. In fact, the two-dimensional results of temperature distributions for the first aquifer. In addition, the three-dimensional results of temperature distributions were added in the paper.
（4）What is the applicability of the mathematical model developed in the section 3.2? The reviewer thinks that the applicability of the mathematical model should be verified.
Cleared: The software Feflow is the most comprehensive, well-tested and reliable programs for the simulation of flow, groundwater age, mass- and heat-transport processes in porous media, coupling the equations of groundwater flow and thermal transport. The flow and heat-transport processes was simulated successfully and widely in the word, such as in the references of 19-21, as follows:
[19]Zhou Yanzhang; Zhou Zhifang; Wu Rong, et al. Simulation study of the stage-characteristics of groundwater thermal transport in aquifer medium for GWHP System. Hydrogeology & Engineering Geology, 2011, 38(5):128-134.
[20]Bulte, Manon; Duren, Thierry; Bouhon, Olivier; et al. Numerical Modeling of the Interference of Thermally Unbalanced Aquifer Thermal Energy Storage Systems in Brussels (Belgium). Energies, 14(19):6241. doi:org/10.3390/en14196241.
[21]Lo Russo, Stefano; Gnavi, Loretta; Roccia, E; et al. Groundwater Heat Pump (GWHP) system modeling and Thermal Affected Zone (TAZ) prediction reliability: Influence of temporal variations in flow discharge and injection temperature. Geothermics, 2014, 51:103-112. DOI:10.1016/j.geothermics.2013.10.008.
Then, add the three references in this manuscript as follows:
3.2. Mathematical model
To analyze the operation efficiency of the SGE system under different schemes, the mathematical models of groundwater flow and heat transport were coupled in numerical simulations [19-21].
(6) All figures and tables should be stated in manuscript. If not, the reviwers and readers will be confused about the content of the manuscript. Moreover, the authors should polish the language of the manuscript.
Cleared: Only the main figures and tables could be shown in the manuscript, because the number of manuscript was limited. The writing has been improve in the manuscript.
(6) In the "References" section, some references should be added to support the statement in the manuscript: One of the effective ways to realize the above goal is to Vigorously develop SGE The SGE could make a significant contribution on reducing CO2 emissions and gradually replace the fossil fuels, while ensuring sustainable socio-economic development. After searching, the reviewers think the following references are excellent and classic: https://doi.org/10.3390/w15040677, https://doi.org/10.1007/s40948-022-00396-0, https://doi.org/10.1007/s11356-021-18169-9, https://doi.org/10.3390/min12111355.
Cleared: According to the DOI of the four references suggested, the four references have little correlation with this manuscript.
https://doi.org/10.3390/w15040677: Weifang Qiao and Xinyi Wang. Parallel Factor Analysis with 3DEEMS of Dissolved Organic Matter from Deep Porous Medium Reservoirs in the City of Kaifeng. Water, Water 2023, 15(4), 677.
The abstract as follows: The deep geothermal water found within Kaifeng City, Henan province, China, is mainly contained within a loose-pore geothermal reservoir in the Minghuazhen Formation (Neogene Period). To understand the role and composition of Dissolved Organic Matter (DOM) in geothermal water, water samples collected from 13 geothermal wells at different depths were studied using three-dimensional (3D) excitation-emission matrix-parallel factor (EEM-PARAFAC) analysis. Fluorescent components were analyzed according to depth, and DOM in geothermal water between 800 m and 1600 m was classified. The results show that the fluorescence index (FI), biological index (BIX), and humification index (HIX) of DOM differ among geothermal water from different thermal reservoirs. Based on these three indices, the humification degree of DOM in deep geothermal water in Kaifeng City is low and is mainly derived from an endogenous source, which is closely related to microbial activities in thermal reservoirs. The fluorescent components of DOM in geothermal water from depths less than 1200 m are mainly tryptophan, tyrosine, and fulvic acid-like. The fluorescent components of DOM in geothermal water from depths greater than 1200 m are more complex, with tryptophan, tyrosine, humic acid, and fulvic acid-like components. Therefore, the characteristics of DOM composition in the geothermal water from different reservoirs in Kaifeng can also be used to infer and explain that the quality of deep geothermal water has not been affected by human activities, and there is no obvious hydraulic connection between the geothermal water of each thermal reservoir.
https://doi.org/10.1007/s40948-022-00396-0: Li Q , Wu J . Factors affecting the lower limit of the safe mud weight window for drilling operation in hydrate-bearing sediments in the Northern South China Sea[J]. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2022, 8(2):-.
The abstract as follows: Maintaining wellbore stability is the basis and prerequisite for efficient methane production from offshore gas hydrates. Nevertheless, owing to the invasion and disturbance by drilling mud, gas hydrates around wellbore gradually dissociate, resulting in the rapid reduction of sediment strength, thereby the borehole collapse. Accurate design of mud density based on appropriate strength criterion is a feasible measure to avoid this case, but the traditional Mohr-Coulomb (MC) criterion was commonly used now. Herein, the self-designed low temperature triaxial experimental equipment was set up to explore the strength of sediment with different saturation of hydrate, and a new strength criterion was developed. Moreover, impact law and mechanism of several critical factors on the lower limit of the safe mud weight window (abbreviated as MDlow value) was then investigated. The investigation results showed that the MDlow value calculated by the new criterion is higher (or more conservative), which is more conducive for prevention of borehole collapse. Further analysis reveals that the increase of cohesion or hydrate saturation enhances the reservoir strength, resulting in the smaller MDlow value. In this way, the wider window is more conducive to reasonable mud density design. Furthermore, the safe mud weight window for hydrate reservoir is temperature sensitive, and it varies obviously when the drilling fluid temperature is between 288.14 K and 288.45 K. Finally, the investigation in this study also shows that the MDlow value was still affected by stress difference. The preferred design strategy is to orient the azimuth of wellbore axis at 90 degrees if horizontal wellbore is drilled for hydrate development.
[3]https://doi.org/10.1007/s11356-021-18169-9: Li Q , Wang F , Forson K , et al. Affecting analysis of the rheological characteristic and reservoir damage of CO2 fracturing fluid in low permeability shale reservoir[J]. Environmental Science and Pollution Research, 2022(25):29.
The abstract as follows: The fracturing property of liquid CO2 fracturing fluid varies greatly due to the rheology of fracturing fluid during fracturing process. The main objective of this investigation is to study the rheology property of thickened liquid CO2 by measuring the viscosity of thickened liquid CO2 in different physical parameters of this prepared thickener and explain the causes of rheological changes. The results show that thickener content, branching content, and molecular weight of a thickener for all could significantly improve the rheology of liquid CO2; the consistency coefficient K increased as they rose, but the rheological index n presented a decreased trend. Meanwhile, the mesh structure is proposed as a model to explain the rheological changes, and the large wetting angle means an excellent backflow, low reservoir damage, and low adsorption property. These results herein provide a basic reference to improve the CO2 fracturing technology and molecular design of CO2 thickener.
[4]https://doi.org/10.3390/min12111355: Fan, Bowen; Shi, Peng; Wan, Zhijun; et al. Simulation Study on the Disaster-Causing Mechanism of Geothermal Water in Deep High-Temperature Heat-Damaged Mines. MINERALS, 2022,12(11):1355.
This paper takes the bottom pumping roadway of 33190 machine roadway in the No.10 mine of China PingMeiShenMa Group as the engineering background. This mine is a hydrothermal mine, with strong heat conduction and thermal convection activities between the surrounding rock and geothermal water. This forms a geothermal anomaly area, making the overall temperature of the surrounding rock temperature field increase and affecting the mine thermal environment. According to the measured field data and the engineering geological conditions of the roadway, a roadway seepage-heat transfer model is constructed using the comsol numerical simulation software, emulating the effect of geothermal water upwelling to the roadway through random cracks in the surrounding rock at different temperatures and pressures, which has an impact on the airflow temperature field of the roadway. The study shows that the evolution law of the airflow temperature field in the roadway under different water upwelling temperatures and pressures is roughly the same, and the temperature at the entrance of the roadway is almost unchanged: the heating rate is 0, and then increases linearly. The variation in the airflow outlet temperature is analyzed, both under the conditions of same temperature but different pressure, and under the same pressure but different temperature. The water upwelling temperature and the cooling efficiency are positively correlated, and the overall growth rate of the airflow temperature is positively correlated with the water upwelling temperature and pressure; however, the effect of temperature is far greater than that of pressure. The upwelling temperature of geothermal water is the main influencing factor on the temperature field of the airflow in the roadway. Therefore, it is possible to reduce the temperature of upwelling water by laying heat insulation materials on the bottom plate, evacuating geothermal water and circulating cold-water by injection, so as to improve the thermal environment of water-heated mines and increase their production efficiency.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Accept as is
Author Response
The Reviewer 1 Comments is "Accept as is", therefore, there is ok for the manuscript, without modification.
Reviewer 2 Report
(1) In Section 3.2. Mathematical model, a complicated mathematical formula is given, but the whole research is based on Feflow software. How significant are these mathematical formulas in the mathematical model for the whole study? It is of little practical significance to simply list some formulas that even the author cannot understand.
(2) Linking up the above question 1 , what is the method of discretization of this mathematical model, and what is the result of discretization, please write it down? Also, please give the iterative scheme of the mathematical model in question 1.
(3) This study is a numerical simulation, but even the geometric model of the model cannot be found in the manuscript? This is a very low-level problem for the papers of numerical simulation.
(4) Line 208: what is this of "Bookmark not defined". some low-level problems like this, please check the whole article and correct it.
(5) The authors are suggested to cite the four references mentioned in the first round of review.
(6) Although the author has made a simple modification to the language of the manuscript, there are still some grammatical problems. The language has not yet reached the level of publication, and still needs to be polished by native English speakers or professional institutions, and proof of polishing is attached.
Author Response
（1）In Section 3.2. Mathematical model, a complicated mathematical formula is given, but the whole research is based on Feflow software. How significant are these mathematical formulas in the mathematical model for the whole study? It is of little practical significance to simply list some formulas that even the author cannot understand.
Response 1: The finite element calculation software Feflow, which could simulate flow, mass-and heat-transport processes in porous media. In this paper, only groundwater flow and heat-transport processes are simulated for the study about improvement of operation efficiency of shallow geothermal energy system , therefore, only the main mathematical model of groundwater flow and thermal transport are listed.
By the way, what is the mean of “It is of little practical significance to simply list some formulas that even the author cannot understand.”?
（2）Linking up the above question 1 , what is the method of discretization of this mathematical model, and what is the result of discretization, please write it down? Also, please give the iterative scheme of the mathematical model in question 1.
Response 2: The mathematical model is discretized by finite element method. According to the discretization of the model in Feflow software is introduced in so many references, i.g., the references of 19,20, and 21. So some of information about the mathematical model (introduced in the three references) are simplified in this paper.
（3）This study is a numerical simulation, but even the geometric model of the model cannot be found in the manuscript? This is a very low-level problem for the papers of numerical simulation.
Response 3: The geometric model is added.
Line 208: what is this of "Bookmark not defined". some low-level problems like this, please check the whole article and correct it.
Response 4: The parameters in the equations have been defined or explained in above.
The authors are suggested to cite the four references mentioned in the first round of review.
Response 5: The four references have no or little relationship to this manuscript, so no one is cited, which was explained in the first round.
(6) Although the author has made a simple modification to the language of the manuscript, there are still some grammatical problems. The language has not yet reached the level of publication, and still needs to be polished by native English speakers or professional institutions, and proof of polishing is attached.
Response 6: The language of the manuscript has been modified.
Author Response File: Author Response.pdf
