Optimising a Biogas and Photovoltaic Hybrid System for Sustainable Power Supply in Rural Areas

Round 1

Reviewer 1 Report

Dear Authors,
Kindly address the following,
1. In the abstract "The results obtained in this case may vary under certain conditions.".  Add your final observation rather than making a generic statement.
2. Introduction and Literature review: These two content can be combined as a Introduction part and can be explained in brief.
3.Kindly change your headings like "Materials", give an appropriate one.
4.Line 385-416 . All these detailed steps can be added as an appendix. Term can be explained in brief alone.
5.Line 437-443. A flow chart can be used to explain your optimization process.
6. Line 461 and 469, no need to mention "In this Paper".
7. Line 549. Can you add some insights into the 12 year period for IRR and how can you maintain a 20% savings continuously.

Author Response

We are very grateful to the reviewers and editors for the constructive comments and suggestions on our article. They have helped us to considerably improve the quality of the manusctipt. We have carefully revised the article and have considered all the reviewers' comments and suggestions. The changes are highlighted in yellow in the revised document, and each point is specifically addressed below. We sincerely hope that the quality of this article is now suitable for publication in Applied Sciences.

Reviewer #1

Dear Authors,

Kindly address the following,

1. In the abstract "The results obtained in this case may vary under certain conditions.". Add your final observation rather than making a generic statement.

Thank you for your insightful comment. We understand the importance of highlighting the conclusions of our work in the abstract. Based on your feedback, we have revised the abstract to include specific results and conclusions from our analysis. Additionally, we have noted that the numerical results may vary depending on certain variables, which we have thoroughly discussed in the sensitivity analysis section of the paper. We hope this revised version provides a clearer picture of our findings and their implications.

Please refer to the abstract in the revised version to check the changes included (lines 12-24).

2. Introduction and Literature review: These two content can be combined as a Introduction part and can be explained in brief.

Thank you for your feedback. We appreciate your suggestion to combine the Introduction and Literature Review into a single section. After carefully considering your advice, we have made the necessary revisions to merge these two sections into a concise and clear Introduction. Additionally, we have made sure to succinctly summarize the relevant literature and provide a table to further clarify this analysis, as suggested by another reviewer. We believe these changes will greatly enhance the overall readability and organization of the paper. Thank you again for your valuable input.

Please refer to the Introduction in the revised version to check the changes included (lines 27-200).

3. Kindly change your headings like "Materials", give an appropriate one.

The section ‘Materials and methods’ was split into two sections in the original manuscript. According to your suggestion, we have renamed the first part of this section as 'Hybrid System Components' to better reflect the content discussed in the section. This should provide better clarity for the readers and help in the overall understanding of our work. We hope this modification addresses your concerns and enhances the quality of our manuscript.

Please refer to the second section in the revised version to check the changes included (line 201).

4. Line 385-416. All these detailed steps can be added as an appendix. Term can be explained in brief alone.

Thank you for this suggestion. All the authors totally agree that there was too much information in these lines. The definitions have been left alone and the complementary explanations have been moved to an appendix to simplify the text and structure. We believe that this will simplify the main text and make it easier to understand. By including these detailed steps in an appendix, readers who are interested in the methodology can refer to it for a more in-depth understanding, while others can simply focus on the essential information presented in the main text. We believe that this change will greatly improve the overall readability and structure of the paper

Please refer to lines 365-378 and Appendix A in the revised version to check the changes included (lines 564-598).

5. Line 437-443. A flow chart can be used to explain your optimization process.

The flowchart was in figure 5. However, according to the suggestions received, the authors have reallocated it and cited it properly in the text to clarify these steps and the whole process. Thank you for your suggestion. We hope that these modifications meet your expectations and contribute to the overall readability and understanding of the optimization process.

Please refer to fig. 5 in the revised version to check the changes included (lines 403-412).

6. Line 461 and 469, no need to mention "In this Paper".

This has been corrected. Thank you for carefully reading and reviewing these details.

Please refer to lines 431 and 438 in the revised version to check the changes included.

7. Line 549. Can you add some insights into the 12 year period for IRR and how can you maintain a 20% savings continuously.

In the original manuscript there was some little confusion regarding the savings, so this has been rewritten including the suggestion provided here. A 12 year period is sometimes used in this kind of installations because the lifetime can by up to twice this value. Therefore, if the investment is profitable and attractive in this period of time, then there is quite a big confidence for investors due to the margin over the lifetime of these equipment.

The use of IRR is justified because not only indicates profitability but it also quantifies the value of the investment.

Regarding the 20% mentioned in the text, it has been clarified and the authors apologize for the deficient explanation of the original manuscript. In the case study developed in this work, a group of neighbours create an energy community and invest some money to install and manage the facilities. In return, they have a discount in the electricity bill of 20% in energy prices. With this discount, the IRR is calculated for a period of 12 years, resulting in the values shown in the paper. On the one hand, thanks to the installation of the renewable resources, the energy community has to buy much lower quantities of energy from the main system. On the other hand, the neighbours have a discount in electricity that allows them to recover their investments in a reasonable period of time to have benefits for several years.

Please refer to lines 388-394 and the conclusions in the revised version to check the changes included (lines 527-530).

We apologize for any confusion caused in the original manuscript and hope that the revisions address the concerns raised by the reviewer. Once again, we appreciate the time and effort taken by the reviewer in providing such a thorough analysis and suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper, the Biogas as an electricity generating element combined with a photovoltaic (PV) plant for a small municipality was analyzed. The topic of this research is very interesting I would suggest the publication of this article if the following minor issues are addressed.

-         -  Line 230, Valencian Community (Spain), the location of the municipality should be mentioned precisely in the text.

-       -   In equation2, please keep the same symbol for the thickness e or e’

-       -    In figure 2, the unit of power should be checked.

-       -   Line 394, the investment needed to build the PV plant Ip=1.5€/W, the reference should be added. Since the investment is variable within the technologies of the PV system (type of solar panels, inverters,..)

-       -   Eq(16), the unit of stored gaz should be in Nm³

-       -   The sizing of PV installation should be specified (area of solar field, type of PV panels and characteristics, inverters specifications,…)

-       -     PV energy is not available at night when electricity is needed by habitants, so the BG remains producing electricity. Please add more details on working hours during the whole day.

-        -  Daily electricity production for combined plant (BG-PV) should be depicted for a typical day

 

Author Response

We are very grateful to the reviewers and editors for the constructive comments and suggestions on our article. They have helped us to considerably improve the quality of the manusctipt. We have carefully revised the article and have considered all the reviewers' comments and suggestions. The changes are highlighted in yellow in the revised document, and each point is specifically addressed below. We sincerely hope that the quality of this article is now suitable for publication in Applied Sciences.

Reviewer #2

In this paper, the Biogas as an electricity generating element combined with a photovoltaic (PV) plant for a small municipality was analyzed. The topic of this research is very interesting I would suggest the publication of this article if the following minor issues are addressed.

• Line 230, Valencian Community (Spain), the location of the municipality should be mentioned precisely in the text.

Thank you for your suggestion. Due to confidentiality reasons, it has been agreed with the Major of this municipality to remove the name of it from the manuscript. However, we have added the coordinates of the municipality. This way it can be seen that it is a rural area, with small villages located in the North-West of Valencia.

Please refer to lines 202-208 in the revised version to check the changes included (lines 12-24).

• In equation2, please keep the same symbol for the thickness e or e’

Thank you for your very detailed analysis. After analysing it carefully, we realized that after equation 2 there was a comma (,), as suggested by the editors of this journal, since the next sentence was continuing in the next line. We have added a blank space so that it does not seem like e’ but the final version will be discussed with the editor to carefully follow their instructions regarding the style. We apologize for any confusion caused in the original submission. Our goal is to present clear and accurate information, and your feedback has been instrumental in helping us achieve this goal. We appreciate your time and efforts in reviewing our work.

Please refer to equation 2 in the revised version to check the changes included (lines 270-271).

• In figure 2, the unit of power should be checked.

Thank you for your suggestion. The units were ‘Power per unit’ (Power pu). This unit is common in studies in which the total value is not the important parameter, since what is being analysed is the comparison of the values in different moments. Sometimes, this is also expressed as power in percentage (%) of the rated value. We have clarified it in the figure and in its title. We appreciate your attention to detail and hope that the revised version meets your expectations.

Please refer to fig. 2 in the revised version to check the changes included (lines 322-323).

• Line 394, the investment needed to build the PV plant Ip=1.5€/W, the reference should be added. Since the investment is variable within the technologies of the PV system (type of solar panels, inverters,..)

Thank you for pointing out the missing reference on line 394. The value for the investment needed to build the PV plant was taken from a previous study and has now been properly cited for clarity and transparency. We appreciate your attention to detail and your efforts to enhance the quality of our manuscript.

Please refer lines 368-370 and the References in the revised version to check the changes included (lines 761-763).

• Eq(16), the unit of stored gaz should be in Nm³

All the authors totally agree with this comment. It has been corrected in the revised version.

Please refer Eq. 16 in the revised version to check the changes included (lines 424-425).

• The sizing of PV installation should be specified (area of solar field, type of PV panels and characteristics, inverters specifications, …)

These specifications have been added in a new appendix to allow a better understanding of the proposed facility. This addition also enhances the transparency of the proposed installation. As suggested by other reviewers, there are now two appendixes to clarify information, making the text simpler and more appealing. We hope that the addition of this appendix has improved the readability and clarity of the manuscript. Thank you again for your invaluable feedback.

Please refer to lines 477-480 and Appendix B in the revised version to check the changes included (lines 599-605).

• PV energy is not available at night when electricity is needed by habitants, so the BG remains producing electricity. Please add more details on working hours during the whole day.
• Daily electricity production for combined plant (BG-PV) should be depicted for a typical day

Thank you for your insightful comments. We have added a figure to the results section that shows the daily electricity production for the combined BG-PV plant for a typical day, including working hours during the whole day (both for sunny and cloudy days). This figure demonstrates the complementary nature of both plants and highlights the role of the BG in meeting electricity demand during night-time hours. We believe this information will provide a clearer understanding of the plant's operation and how it meets the electricity needs of the inhabitants. It is also interesting to notice that the size of the biogas plant is smaller than the maximum size to feed the whole demand and in some hours, some energy must be bought to the system due to lack of stored gas or limited power, but the system results to be the optimal configuration in terms of IRR. Thank you again for your contribution to the improvement of this paper.

Please refer to the new fig. 10 in the revised version to check the changes included (lines 477-482).

The authors hope that the new version clarifies all the aspects mentioned in the detailed review provided by the reviewer and want to acknowledge once again the effort of the reviewer and all the thorough analysis and suggestions provided. We tried to explain every change according to these suggestions while following the suggestions of the other reviewers, so that the final version is significantly improved from all the points of view.

Author Response File: Author Response.pdf

Reviewer 3 Report

In the paper, the authors proposed a method to analyse a hybrid system formed by a biomass power plant and a PV plant. They determined the optimal configuration composition of the hybrid system that allows the maximum possible savings in energy costs. The obtained results are interesting. However, there are some aspects which the authors should consider to improve the structure and quality of the paper.

1.       In the abstract part, the novelty and key idea of the proposed method should be described. The authors only described that “This paper presents a method for analyzing a hybrid system consisting of a biomass power plant and a photovoltaic plant, supported by the electrical network, to determine the optimal configuration for maximum savings in energy costs. The method is applied to a small rural municipality in the Valencian Community of Spain, under the structure of an energy community.” The novelty and key idea are not clear. Of course, the reviewer can understand the aim, but the originality of the method is missing.

2.       In the first part of the paper, the authors present various approaches from the literature. Maybe, a synthesis of the solutions proposed in the literature depending on the type of analysis, which to highlight more clearly the advantages and disadvantages, is useful for readers. This synthesis can be given as a table.

3.       Are the values in Table 1 estimated by the authors or taken from a bibliographic source? Please detail.

4.       For all the variables/parameters presented in the mathematical models, the measurement units should be indicated.

5.       The authors asserted that "Actual curves recorded in the population will be used. The records for the entire year 2021 are available” Where are these data available? How was the type profile from Fig. 3? Is it taken from a bibliographic source or was it processed by the authors? Please enter details about this process.

6.       On the other hand, if these data are available and the generation profiles can be uploaded from the PVGIS program, why do the authors work with typical profiles? The results which can be obtained using the daily profiles can be different compared with the use of typical profiles.

7.       Which is the reference for the price curve from Fig. 4? Please provide details.

8.       Why was the IRR criterion chosen and not another? Is the criterion agreed upon in Spain for such calculations?

9.       The authors considered various scenarios regarding electricity prices and weather conditions (sunny or cloudy conditions). Studies related to the combination of hypotheses related to electricity prices and weather conditions would also be indicated, to which hypotheses related to the required load (increase/decrease) should be added to ensure that the best solution has been obtained.

 

Author Response

We are very grateful to the reviewers and editors for the constructive comments and suggestions on our article. They have helped us to considerably improve the quality of the manusctipt. We have carefully revised the article and have considered all the reviewers' comments and suggestions. The changes are highlighted in yellow in the revised document, and each point is specifically addressed below. We sincerely hope that the quality of this article is now suitable for publication in Applied Sciences.

Reviewer #3

In the paper, the authors proposed a method to analyse a hybrid system formed by a biomass power plant and a PV plant. They determined the optimal configuration composition of the hybrid system that allows the maximum possible savings in energy costs. The obtained results are interesting. However, there are some aspects which the authors should consider to improve the structure and quality of the paper.

1. In the abstract part, the novelty and key idea of the proposed method should be described. The authors only described that “This paper presents a method for analyzing a hybrid system consisting of a biomass power plant and a photovoltaic plant, supported by the electrical network, to determine the optimal configuration for maximum savings in energy costs. The method is applied to a small rural municipality in the Valencian Community of Spain, under the structure of an energy community.” The novelty and key idea are not clear. Of course, the reviewer can understand the aim, but the originality of the method is missing.

Thank you for your constructive feedback. We appreciate your insights and have revised the abstract to better emphasize the novelty and key idea of the proposed method. Our method offers a unique approach to analyzing a hybrid energy system composed of a biomass power plant and a photovoltaic plant, and determines the optimal configuration for maximum cost savings. The method's originality lies in its comprehensive consideration of various parameters such as the energy demand, generation and cost, to arrive at the most cost-efficient solution. We believe that this revised abstract now more clearly communicates the significance and innovation of our work.

Please refer to the abstract in the revised version to check the changes included (lines 12-24).

2. In the first part of the paper, the authors present various approaches from the literature. Maybe, a synthesis of the solutions proposed in the literature depending on the type of analysis, which to highlight more clearly the advantages and disadvantages, is useful for readers. This synthesis can be given as a table.

Thank you very much for your suggestion. The introduction section has been restructured to include the literature review, according to some suggestions receives in the review process. Furthermore, it has been shortened and the literature review has been clarified using a summary table, as suggested here. This will hopefully provide the readers with a clearer understanding of the state-of-the-art. We believe these modifications will improve the clarity and coherence of the paper. Thank you again for your constructive criticism.

Please refer to the new table 1 in the revised version to check the changes included (lines 180-181).

3. Are the values in Table 1 estimated by the authors or taken from a bibliographic source? Please detail.

Thank you very much for pointing this out. These data were provided by the municipality but, due to confidentiality concerns, they have not been cited or published in any other format. The obtained data were processed by the authors and Table 1 shows the total values calculated with the data provided by the municipality. We have now included additional clarification in the text to better explain the origin of these values.

Please refer to the new table 2 (lines 236-240) and the references in the revised version to check the changes included (line 757).

4. For all the variables/parameters presented in the mathematical models, the measurement units should be indicated.

Thank you for your insightful comment. We appreciate your attention to detail and agree with you that including units for all variables and parameters in the mathematical models is essential. We have thoroughly reviewed the manuscript and added the missing units where necessary. We apologize for any oversight in this regard and are committed to ensuring the accuracy and completeness of the information presented in the paper. Thank you for your assistance in making our work better.

This change affects several lines in the text.

5. The authors asserted that "Actual curves recorded in the population will be used. The records for the entire year 2021 are available” Where are these data available? How was the type profile from Fig. 3? Is it taken from a bibliographic source or was it processed by the authors? Please enter details about this process.

Thank you for bringing this to our attention. We have revised the statement to clarify the source of the data. The data used for the study was recorded by the authors using a data logger over a period of several years and the records for the entire year 2021 have been selected for analysis. These data are not publicly available due to confidentiality restrictions. However, the method presented in this study can easily be applied to other data sets. These details have been included in the revised version of the text for clarity. Thank you for your valuable input.

Please refer to lines 209-210 and section 2.3 in the revised version to check the changes included (lines 325-329).

6. On the other hand, if these data are available and the generation profiles can be uploaded from the PVGIS program, why do the authors work with typical profiles? The results which can be obtained using the daily profiles can be different compared with the use of typical profiles.

Thank you for bringing this to our attention. We appreciate your insightful suggestion. The authors agree that using daily profiles can provide different results compared to using typical profiles, with a higher accuracy level. Our aim was to propose a method for sizing a hybrid system using easily obtainable data in rural areas, where data availability and accuracy can many times be a challenge. In this regard, we have measured the solar irradiance using laboratory equipment to obtain typical profiles and to ensure the robustness of our proposed method.

However, as future work, we are now studying the effect of using daily profiles on the optimal configuration of the hybrid system. We understand that this would be an important extension to our work and will keep the scientific community informed if we find any interesting results in this regard. The sensitivity analysis we carried out showed that variations in external conditions do not lead to significant changes in the optimal configuration for the hybrid system.

Thank you again for your constructive feedback, we hope that the revised version addresses your concerns and provides a clearer understanding of our work.

Please refer to the conclusions in the revised version to check the changes included (lines 539-542).

7. Which is the reference for the price curve from Fig. 4? Please provide details.

We apologize for forgetting to cite the corresponding reference. It has been included in the title of the figure. It was already cited in the text, but this clarification substantially improves the consistency of this work. Thank you for your suggestion.

Please refer to the Methods section in the revised version to check the changes included (lines 335-342).

8. Why was the IRR criterion chosen and not another? Is the criterion agreed upon in Spain for such calculations?

Thank you for raising this point. The selection of IRR as the evaluation criterion was made considering its ability to not only measure profitability but also the time value of money. IRR is a commonly used indicator in project evaluation and it is especially useful for comparing different investment scenarios, as demonstrated in the study. However, it is important to note that other metrics such as Net Present Value (NPV) may also be used for evaluating such projects. In Spain, IRR is widely accepted as a standard evaluation criterion for projects and is considered to be a reliable indicator of project viability.

Please refer to lines 388-394 and the conclusions in the revised version to check the changes included (lines 527-530).

9. The authors considered various scenarios regarding electricity prices and weather conditions (sunny or cloudy conditions). Studies related to the combination of hypotheses related to electricity prices and weather conditions would also be indicated, to which hypotheses related to the required load (increase/decrease) should be added to ensure that the best solution has been obtained.

Thank you for your feedback. We appreciate your suggestions and the time you took to review our work. We understand your request for a more comprehensive study on electricity prices, weather conditions, and required load. However, we believe that the method is robust enough for the scope of the proposed paper. A more exhaustive study would go beyond the aim and objectives of the article and from your suggestion, some insights have been added as proposed future work. Analysing the proposed case study, it is clear that variations in the considered conditions have a very low impact in the optimal configuration of the hybrid system. Therefore, we hope that the results presented in our paper can still contribute to the field and provide useful insights for future research. Thank you again for your comment.

Please refer to the conclusions in the revised version to check the changes included (lines 539-542).

The authors hope that the new version clarifies all the aspects mentioned in the detailed review provided by the reviewer and want to acknowledge once again the effort of the reviewer and all the thorough analysis and suggestions provided. We tried to explain every change according to these suggestions while following the suggestions of the other reviewers, so that the final version is significantly improved from all the points of view.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors,

 Thanks for addressing all the comments.

Reviewer 3 Report

The authors performed changes to the initial manuscript. New explanations, elaborations of details, and revisions have been added. I have not other observations.