Numerical Investigation of Techno-Economic Multiobjective Optimization of Geothermal Water Reservoir Development: A Case Study of China

Round 1

Reviewer 1 Report

The manuscript is a case study of geothermal heating in Xinji, China is presented. While the abstract clams a new approach, the study itself is unfortunately rather bland. While this might be in some part driven by a lack of structure (that makes it currently impossible to understand what exactly was done in the method section), the results presented reveal no particularly new information and I can’t really see whom this study would be useful for (besides of course any Xinji energy managers). Additionally there are quite a bunch of more specific areas that need to be reconsidered, especially in regards to the economic ‘optimization’. Thus I recommend to reject the article in its current form. However, after some major restructuring and an additional focus on economic feasibility the manuscript will be worth to reconsider. Several more detailed comments are given in the following

Line 62-77: this is a long list of studies on geothermal energy – how do each of those relate to what you are doing? Line 88/89: It needs to become way clearer what exactly the new aspects of your study are and how they will further the field. Chapter 2.1) – please give some information on the study area and the current systems in place (e.g. its size, how long it has been in use, the heating demand…). Chapter 3 – this needs to be restructured. I would put Figure 3 at the beginning and after that describe the sub-steps in details. Also put in a schematic drawing of the setup so I know what you are talking about when you mention distance etc. Line 173 – why is it an important step? Line 185 – for the heating load – are WC and Wb given per sqm of residential/commercial are or per unit area of a residential/commercial building – these are not the same thing, are they? Isn’t one just in general the entire area used for residential/commercial space including roads etc. while the other one is only areas in buildings, but also considering the number of floors? Line 217 how did you determine CI and CO? Line 227 – 235 – I have no clue what you are talking about It is not clear at all how the different sections (3.1., 3.2 and 3.3) are connected. Table1 – these are not all of your input parameters – what about e.g. construction cost? Uncertainties should be include! Figure 4 – the systems are apparently only meant to be used for 50 years. Thus the economic evaluation needs to include this e.g. life cycle analysis. Discussion is pretty much missing, please compare your results to previous studies. Line 363 onwards: Is what you’re are presenting really an optimization? You are testing three extraction rates, temperatures and distances, but could you mathematically just determine the exact parameters that optimize the system?

Author Response

Dear professor:

Thanks for your review. Based on your suggestions, we have modified the article. The details are shown in article. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Reviewer Comments

The article “Numerical investigation on techno-economic multi-objective optimization of geothermal water reservoir development: a case study of China” geothermal heating system for the application of Xinji reservoir. The authors objective here is to develop a new approach considering heating demand, geothermal water resource, and economics of the geothermal project. The authors have taken into account parameters like reinjection temperature, reinjection rate, distance between wells, and few other parameters for the study. Following are my comments:

Abstract:

I would suggest authors add more quantitative data in the abstract. Information of the numerical model or the software used is missing and needs to be included.

Introduction

Introduction is well written. However, I would suggest authors include the schematics or a pictorial diagram of the geothermal systems analyzed in the study. Since authors are carrying a numerical study, I suggest authors to add some recent advancements in the numerical modelling w.r.t geothermal heat extraction like

Pandey, S. N., and A. Chaudhuri. "The effect of heterogeneity on heat extraction and transmissivity evolution in a carbonate reservoir: A thermo-hydro-chemical study." Geothermics 69 (2017): 45-54.

Bagalkot, Nikhil, Alireza Zare, and G. S. J. E. Kumar. "Influence of fracture heterogeneity using linear congruential generator (lcg) on the thermal front propagation in a single geothermal fracture-rock matrix system." Energies 11.4 (2018): 916.

Project Background

More information should be given on the type of subsurface region (fractured/porous media/both), and rock properties. The type of subsurface region defines the numerical model to be applied.

Heating Strategy

Well written section, but I am believer of picture presents more info than words. So, I suggest replacing the line 125-131 with a flow diagram.

Mathematical Model and methodology

This section is completely opposite of the introduction section. Not well presented, missing information, and numerical model completely missing.

First no information is given about the type of porous media (fractured/homogeneous), the authors needs to give this info. The authors do not give information on if the equations are for 1D/2D/3D (mentioned later in the results, this has to be clearly stated before the mathematical model is decribed). Authors have to list out the assumptions taken in the model. Authors have to list out the physical process considered. Where is the equation representing sink and source term (qh). What does the rock velocity physically mean? And is this velocity in the same direction of fluid velocity? Do the authors mean deformation of rock? In line pressure in in Pa, however the pressure in table 1 is in m. I don’t understand m as a dimension of pressure. Is it a hydrostatic pressure? How did the authors obtain the rock properties like thermal conductivity, porosity, and others? Authors have to provide the source. If no source, then authors have to say it as assumed and justify the assumption. The whole numerical model is missing. Only the mathematical model has been presented and no talk of numerical model. To put it in simple words “how did the authors solve the governing equations”. If the authors are using a software, then the information of the software and the numerical method used by the software has to be given. Why there is no validation/verification of the model. On what basis the readers should believe these results?

Results

Figure 4a: Why does there is early breakthrough for 200m well spacing? I want to see what would happen at 300 m well spacing, just to have picture of intermediate option. I think the authors should provide with an envelope of parameters that constitute optimal operation. In the Figure 5, what would happen if production rate is altered at low injection temperature? In all the Figures, there are only observation and discussion (why) is absent. I think without why there is not discussion part. Can the authors provide the temperature vs depth profile for the system?

Author Response

Dear professor

Thanks for your review. Based on your suggestions, we have modified the article. The details are shown in article. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

See the text attached with same of the comments. 

Major revision both English and presentation of the topic is need.

Comments for author File: Comments.pdf

Author Response

Dear professor:

Thanks for your review. Based on your suggestions, we have modified the article. Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments (Round 2)

I have found the response by the authors to round-1 comments unsatisfactory. There are many questions still unanswered below are my comments. Unless, the authors satisfactorily respond to important comment-1 below, I wound not recommend the article to be published.

Comment-1: Both Equation (1) and (4) are dimensionally not balanced. Therefore, I doubt the results of the whole systems. For example in the Equation (4) the first term on the LHS has a dimension of (kg m-3 s-1) and second term on LHS has (Kg m-2 s-1), while the RHS has (kg h-1). Similarly, in Equation (1), and may be in other (I have not checked). How can this be possible? Authors needs to address this, else prove that I am wrong?

Comment-2: Regarding Numerical Model: I think either I was not clear or authors is unaware of the content to be put in the numerical model. The response to Numerical model comment in line 163-169 is highly unsatisfactory. Kindly refer to page 11 of https://www.opengeosys.org/books/geoenergy-modeling-ii/. This one of the sources that the software OGS depends on. In future I advise the authors describe the numerical model in the above manner.

Comment-3: Line166-167: What does the author mean when they say FEM for special analysis and FDM for temporal. Does it mean that they use FDM for source term and FEM for others? I don’t understand this at all, I hope authors may be more elaborate to explain if the model is so complex.

Comment-4: Line 97-102. Authors fail to correctly respond the comment on the type of the subsurface system. Is it homogeneous (no fractures) or heterogeneous (with fractures). The governing equations are dependent on the type of the subsurface system.

Comment-5: Authors have not provided justifications to any assumptions taken. Neither authors provide a list of assumptions. For example, authors have taken only diffusion and convection. Why the authors have ignored dispersion, even after good flow rates? The author needs to give justification to each and every assumption. Making an assumption is not enough, you have to justify to show the credibility of then model. Similarly, you have to justify (line 137) the assumption of no water/heat loss.

Author Response

Dear professor：

Thanks for your review. Based on your suggestions, we have modified the article. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript has improved impressively. I reccomend It pubblication in its new shape and cleareness.

Author Response

Dear professor：

Thanks for your review.

Round 3

Reviewer 2 Report

I am satified with the response from the authors. I suggest the editor to publish the article in the present form.