Decentralized versus Clustered Microgrids: An Energy Systems Study for Reliable Off-Grid Electrification of Small Islands

Round 1

Reviewer 1 Report

This paper evaluated the applicability of cluster microgrids to the Polio group of islands in the Philippines. Although the evaluation results are very detailed, this paper is an evaluation report rather than a research paper. It lacks in originality as well as availability for publication as a research paper in a journal.

[Originality]

Only the originality of this paper is the evaluation of cluster microgrids applicability to Polio islands. As shown in Fig. 1, all the optimization methods used in the simulation were existing tools and had no originality. While the authors might have made some technical efforts to apply the tools, there are no explanation in Section 2.3-2.5. This paper lacks in the originality in terms of research and development component.

[Availability]

Although the simulations were detailed, their targets are too limited. Academic value is very small even though the results might be useful for policy or investment decision making in Philippine. There are so many islands in the world where cluster microgrids could be applied. The reader are usually interested in how to apply the simulation methods to other islands with different situations rather than the detailed simulations results for Polio islands.

[Here are some other concerns that the authors need to address.]

Section 2.3-2.5 should be drastically improved. The evaluation methods should be shown even if they are existing tools. For example, nothing is explained how LCOE was calculated in Section 2.3. How the initial cost, the maintenance cost, the durable life, discount rate, etc. were considered?

Resiliency evaluation is different from reliability evaluation. This paper evaluate reliability. If the authors intend to evaluate resiliency, transmission faults as well as generation faults should be considered. Moreover, synchronous stability and frequency stability should be evaluated.

It is so natural that the reliability of cluster microgrids is higher than that of standalone microgrids. Because cluster microgrids are interconnected with one another. The resiliency considering interconnected line faults should be evaluated in order to clarify the superiority of cluster microgrids to standalone microgrids.

Author Response

This paper evaluated the applicability of cluster microgrids to the Polio group of islands in the Philippines. Although the evaluation results are very detailed, this paper is an evaluation report rather than a research paper. It lacks in originality as well as availability for publication as a research paper in a journal. [Originality] Only the originality of this paper is the evaluation of cluster microgrids applicability to Polio islands. As shown in Fig. 1, all the optimization methods used in the simulation were existing tools and had no originality. While the authors might have made some technical efforts to apply the tools, there are no explanation in Section 2.3-2.5. This paper lacks in the originality in terms of research and development component The authors acknowledge the optimization methods are existing ones. The authors would like to note, however, that the novelty of this paper lies in its methodology. This paper proposes a framework for analyzing interconnection using existing energy systems software. The authors added this as an additional objective at the end of the Introduction (Section 1). The authors also acknowledge that the Materials and Methods (Section 2) was unclear. We have edited the General Approach (Section 2.1) to clarify the scope of each calculation (i.e. a “system” pertaining to a cluster of islands vs. a “configuration” referring to the four islands in the case study). Section 2.3 was edited to discuss system-level calculations, while Section 2.4 discusses configuration-level calculations. The scope of the reliability calculations involved only “systems”, and Section 2.5 was revised to clarify this. [Availability] Although the simulations were detailed, their targets are too limited. Academic value is very small even though the results might be useful for policy or investment decision making in Philippine. There are so many islands in the world where cluster microgrids could be applied. The reader are usually interested in how to apply the simulation methods to other islands with different situations rather than the detailed simulations results for Polio islands. The authors appreciate the suggestion. While the case study deals with Philippine islands, the importance of this paper is the framework for analyzing interconnection between off-grid islands. This framework may be applied to other off-grid islands as well. The Introduction (Section 1) to cater to a more international audience. The first paragraph was revised to minimize discussions regarding the Philippines, while the remaining paragraphs were edited to discuss about off-grid islands in general (i.e. not only about the Philippines). The objectives of the study (last paragraph of the Introduction) were updated and now mention the proposal of a modeling framework. Studies about other islands are mentioned as future work in the Conclusion (Section 5). [Here are some other concerns that the authors need to address.] Section 2.3-2.5 should be drastically improved. The evaluation methods should be shown even if they are existing tools. For example, nothing is explained how LCOE was calculated in Section 2.3. How the initial cost, the maintenance cost, the durable life, discount rate, etc. were considered? The authors appreciate the suggestion. As previously mentioned, the Materials and Methods section was heavily revised for clarity. The equation for the LCOE was explained in Section 2.3.2, but the reader is referred to the HOMER Pro User Manual for the calculation of other parameters to avoid derailing the discussion. These parameters (capital costs, operating costs, etc.) are deemed less important as they are often found in engineering economics courses. Resiliency evaluation is different from reliability evaluation. This paper evaluate reliability. If the authors intend to evaluate resiliency, transmission faults as well as generation faults should be considered. Moreover, synchronous stability and frequency stability should be evaluated. The authors appreciate the clarification. All instances of the term “resiliency” were removed, and the calculations were referred to as “reliability” only. This affects the literature review in the Introduction (Section 1), Reliability (Section 2.5), Reliability (Section 3.3), Discussion (Section 4), and Conclusion (Section 5). It is so natural that the reliability of cluster microgrids is higher than that of standalone microgrids. Because cluster microgrids are interconnected with one another. The resiliency considering interconnected line faults should be evaluated in order to clarify the superiority of cluster microgrids to standalone microgrids. The authors appreciate the suggestion. As previously mentioned, the paper was revised to focus in reliability instead of resiliency. The additional (resiliency) calculations are therefore unnecessary. Moreover, the simulation of interconnection line faults is also the same as not interconnecting the islands (i.e. lower degree of interconnection), which was demonstrated in this work.

Author Response File: Author Response.docx

Reviewer 2 Report

In the study "Decentralized versus Clustered Microgrids: A GIS-based Energy Systems Modelling for Resilient Rural Electrification of Small Islands", the authors investigate the economic viability of the hybrid renewable energy systems for rural electrification of three different Islands in Philippines. They applied life cycle cost of electricity approach and carried out the simulation using HOMER Pro simulation tool. Here, author(s) proposed the concept of electrifying these islands considering resiliency. 

After reading the full paper, I feel that the concept proposed in the manuscript, is not a new one, but the application area and proposed interlink between the rural electrification as well resiliency is interesting. Overall, the manuscript can be of suitable for MDPI Energies, but there is room for improvements before a decision is made. There are some major concerns with the manuscript which needs to be resolved carefully and they are given below:

1. I do not have any problem with the title, but I feel there is no clear justification of title in the manuscript. Especially the resiliency approach and the contributions related to GIS use.

Abstract:

1. In the abstract Philippine "off-grid islands" is bit confusing especially for the non-scientific people. So it can be revised.
2. The quoted statement in the abstract "resulting in costly and intermittent supply of electricity". Here, the statement is related to Diesel Generator (DG), I understand that it is costly, but how come the DG can be intermittent.
3. There is another quoted statement "The archipelagic nature of the country also impedes rural electrification". I think the use of off-grid electrification would be apt word here rather than the "rural electrification"
4. Overall, the abstract needs to be revised.

Introduction

1. The introduction section and the literature review seems to be together. But it is not sufficient and in its current form the research gap is not clear.
2. The energy resiliency literature needs further improvements

Methods

1. In section 2.3, description about simulation is given but it is very lame. This section needs major improvements
2. Section 2.5, the method of resiliency with respect to change in LCOE is not clear. Clarity is missing here.
3. The A* algorithm is not described anywhere, it is needed to validate on the model and results
4. Case studies needs further improvements, especially load profile, and system specifications are needed.

Results

1. Reasons for failure scenarios should be mentioned
2. Discussion and comparison is needed.

Conclusion

1. Conclusion section should be improved further by provided the main points in bullet form
2. Future scope can be included

Reference are very clear and appropriate. 

Suggested reference "Hybrid Renewable Energy Microgrid for a Residential Community: A Techno-Economic and Environmental Perspective in the Context of the SDG7"

Author Response

In the study "Decentralized versus Clustered Microgrids: A GIS-based Energy Systems Modelling for Resilient Rural Electrification of Small Islands", the authors investigate the economic viability of the hybrid renewable energy systems for rural electrification of three different Islands in Philippines. They applied life cycle cost of electricity approach and carried out the simulation using HOMER Pro simulation tool. Here, author(s) proposed the concept of electrifying these islands considering resiliency. After reading the full paper, I feel that the concept proposed in the manuscript, is not a new one, but the application area and proposed interlink between the rural electrification as well resiliency is interesting. Overall, the manuscript can be of suitable for MDPI Energies, but there is room for improvements before a decision is made. There are some major concerns with the manuscript which needs to be resolved carefully and they are given below: 1. I do not have any problem with the title, but I feel there is no clear justification of title in the manuscript. Especially the resiliency approach and the contributions related to GIS use. The authors acknowledge this concern. The contribution of GIS was to simply view the bathymetric data and the location of the islands. The A* algorithm for finding the interconnection paths was also implemented in Python 3 and not GIS. Considering the minimal contribution of GIS to this work, the authors removed GIS from the title. Abstract: 1. In the abstract Philippine "off-grid islands" is bit confusing especially for the non-scientific people. So it can be revised. The authors appreciate the suggestion, but the authors have decided to retain the usage of the term “off-grid island.” The term has been used extensively in literature and changing this term may only cause confusion, 2. The quoted statement in the abstract "resulting in costly and intermittent supply of electricity". Here, the statement is related to Diesel Generator (DG), I understand that it is costly, but how come the DG can be intermittent. The authors appreciate the clarification. The Abstract was revised such that diesel is not “intermittent” and that “fuel supply disruptions” occur instead. 3. There is another quoted statement "The archipelagic nature of the country also impedes rural electrification". I think the use of off-grid electrification would be apt word here rather than the "rural electrification" The authors appreciate this comment. The term “rural electrification” was replaced with “off-grid island electrification” in all parts of the manuscript for consistency. The changes are found in the Abstract and Introduction. 4. Overall, the abstract needs to be revised. The authors appreciate the suggestion. The Abstract was revised to reflect the clarifications in the methodology. The term “system” was used to refer to an island (decentralized system or HRES) any cluster of islands (clustered systems or HRES), while the term “configuration” refers to all four islands in Polillo. The numerical figures in the Abstract now refer to configuration-level metrics (details in the Materials and Methods) unless otherwise stated. Introduction 1. The introduction section and the literature review seems to be together. But it is not sufficient and in its current form the research gap is not clear. The authors appreciate the suggestion. The research gap (lack of techno-economic feasibility and reliability calculations in clustered HRES) has been identified in the 4th and 5th paragraphs of the Introduction. The literature review was also modified to emphasize that feasibility and reliability studies do not focus on clustered HRES. The caption of Table 1 was edited to specify that the studies are based on off-grid islands, while the scope of the studies (microgrid, city-wide grid, etc.) in Table 2 were mentioned. 2. The energy resiliency literature needs further improvements The authors appreciate the suggestion. Table 2 was modified to show the scope of each study. This clarifies that previous studies did not focus on clustered HRES. The remarks about each study was also improved. Methods 1. In section 2.3, description about simulation is given but it is very lame. This section needs major improvements The authors appreciate the suggestion. The entire Materials and Methods (Section 2) was heavily revised. The General Approach (Section 2.1) was modified to distinguish between the term “system” (also used in the acronym HRES), which pertains to any group of islands, and “configuration”, which refers to all four Polillo islands. This should make the rest of the subsections clearer. Section 2.3 is allocated for system-level calculations. It includes a discussion on the A* algorithm, the HOMER Pro optimizer, and the clustered HRES simulation methodology. Configuration-level calculations are discussed in Section 2.4. 2. Section 2.5, the method of resiliency with respect to change in LCOE is not clear. Clarity is missing here. The authors appreciate the suggestion. The description of the reliability metric was revised. It is now more clearly defined as the “percent change in LCOE when a disturbance is introduced in comparison to the HRES LCOE when the disturbance is absent” 3. The A* algorithm is not described anywhere, it is needed to validate on the model and results The authors appreciate the suggestion. A brief description of the A* algorithm was added to Section 2.3.1. 4. Case studies needs further improvements, especially load profile, and system specifications are needed. The authors appreciate the suggestion. As previously mentioned, Section 2.1. was revised to clarify the scope of the case studies and calculations. The techno-economic assumptions and load profiles were added to Appendix B and C, respectively. We decided not to present these in the Materials and Methods section to avoid derailing the discussion. Results 1. Reasons for failure scenarios should be mentioned The authors appreciate the suggestion. The rationale behind the failure scenarios were added in Section 2.5. 2. Discussion and comparison is needed. The authors appreciate the suggestion. The Results (Section 3) was also revised to focus on comparisons. The comparison between diesel-only systems and HRES was moved to Appendix A to emphasize results about interconnection. Section 3.1. compares the techno-economic aspects of clustered and decentralized systems, while Section 3.2. compares those at varying degrees of interconnection at the configuration-level. The data presentation has also been revised to emphasize the comparisons. Conclusion 1. Conclusion section should be improved further by provided the main points in bullet form The authors appreciate the suggestion. Insights from the case studies were summarized in bullet form in the Conclusions (Section 5). 2. Future scope can be included The authors appreciate the suggestion. Future works were added in the Conclusions (Section 5). Reference are very clear and appropriate. Suggested reference "Hybrid Renewable Energy Microgrid for a Residential Community: A Techno-Economic and Environmental Perspective in the Context of the SDG7"

Author Response File: Author Response.docx

Reviewer 3 Report

I this work a comparative study on decentralized and clustered microgrids in the Polillo group of islands, Philippines has been performed. The work is well written and it discusses exhaustively the hot topic.

I suggest to implement the following minor changes before publication:

1. Please avoid the use of acronyms in the Abstract;
2. The introduction of a Nomenclature section could be very useful to improve the text readability;
3. Please enrich the Conclusion section by summarising the main contents and the future improvements in a bulleted list;
4. As a general consideration, it can be very interesting to compare the “new” proposed system with the existing one by means of an energy and environmental analysis.
5. English language and style are fine/minor spell check required
6. After this minor changes I recommend the publication on paper.

Author Response

I this work a comparative study on decentralized and clustered microgrids in the Polillo group of islands, Philippines has been performed. The work is well written and it discusses exhaustively the hot topic.

I suggest to implement the following minor changes before publication:

1. Please avoid the use of acronyms in the Abstract;

The authors appreciate the suggestion. The Abstract has been revised and acronyms were minimized. The acronym LCOE (levelized cost of electricity) was retained, however, due to its frequent usage.

1. The introduction of a Nomenclature section could be very useful to improve the text readability;

The authors appreciate the suggestion. A nomenclature section was added between the Abstract and the Introduction.

1. Please enrich the Conclusion section by summarising the main contents and the future improvements in a bulleted list;

The authors appreciate the suggestion. The key insights from the case study are summarized as bullet points in the Conclusion, but the future works are short and were briefly mentioned in a few lines.

1. As a general consideration, it can be very interesting to compare the “new” proposed system with the existing one by means of an energy and environmental analysis.

The authors appreciate the suggestion. The authors included a comparison between the proposed system (HRES) and the existing one (diesel-only) in Appendix A. The advantage of HRES over diesel-only systems has been established in literature, so there is little merit in including this in the main parts of the manuscript..

1. English language and style are fine/minor spell check required

The authors have checked the manuscript for spelling and grammatical errors.

1. After this minor changes I recommend the publication on paper.

Thank you.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors handled the reviewers' comments adequately.

Author Response

The authors would like to thank the Reviewer for the helpful comments.

Reviewer 2 Report

The revised version is much improved when compared to the previous version. Author(s) have considered the most of the given comments and in overall I feel the results are clearly represented. However, few things still needs improvements in the current version, they are given below:

1. Please check the line no. 20 in the abstract, "lithium-ion storage". It sounds odd and unclear, try modifying it to lithium-ion battery energy storage.
2. Line no. 21, "Clustered systems comprising multiple islands were simulated by also .... ". I think, be more specific in terms of giving the no. of island, clustered microgrids etc. 
3. The introduction can further be improved considering the latest research articles published in the areas of Resilience, Reliability, and Sustainability of off-grid and hybrid systems for remote electrification.
4. From line no. 58, the energy reliability and the resilience studies were provided, but it seems this section can be improved considered the suggested references that can be cited: https://www.mdpi.com/1996-1073/13/16/4193 (Power Resilience Enhancement of a Residential Electricity User Using Photovoltaics and a Battery Energy Storage System under Uncertainty Conditions) https://www.mdpi.com/1996-1073/13/15/3978 (A Hybrid vs. On-Grid Photovoltaic System: Multicriteria Analysis of Environmental, Economic, and Technical Aspects in Life Cycle Perspective)
5. It would better to have the support of data or the cases of power failures and other disruptions that actually recorded in Philippines country. This can be added to improve the content between line nos. 58 to 68.
6. The table 2, title and in-text statement does not have consistency. For. e.g., resiliency, and reliability. Please be consistent here.
7. Section 2.5, is still very weak, here, the author(s) have just mentioned PV cell cracking and have given a reference that is dated to 1981. I feel this section can be improved further. A reference "https://www.sciencedirect.com/science/article/pii/S2214157X19304083 (Risk priority number for understanding the severity of photovoltaic failure modes and their impacts on performance degradation) can be used for improving this and can be used as citation.

Author Response

The revised version is much improved when compared to the previous version. Author(s) have considered the most of the given comments and in overall I feel the results are clearly represented. However, few things still needs improvements in the current version, they are given below:

1. Please check the line no. 20 in the abstract, "lithium-ion storage". It sounds odd and unclear, try modifying it to lithium-ion battery energy storage.

The authors appreciate the suggestion. The term “lithium-ion storage” was replaced with “lithium-ion battery energy storage”

1. Line no. 21, "Clustered systems comprising multiple islands were simulated by also .... ". I think, be more specific in terms of giving the no. of island, clustered microgrids etc. 

The authors acknowledge the suggestion. The authors changed the line from "Clustered systems comprising multiple islands” to “Clustered systems encompassing multiple islands in the island group” to emphasize that the clustering takes place only among the Polillo group of islands. This should give the reader an idea about the number of islands considered. The authors chose not to expound on this matter because it would be cumbersome to discuss the clustering methodology in detail within the Abstract.

1. The introduction can further be improved considering the latest research articles published in the areas of Resilience, Reliability, and Sustainability of off-grid and hybrid systems for remote electrification.

The authors appreciate the suggestion. The literature review was revised by adding more studies about off-grid electrification. The studies were described in more detail by discussing how reliability was quantified and stating important conclusions. Moreover, studies that are not closely related, such as microgrid-level studies, were excluded from the review.

1. From line no. 58, the energy reliability and the resilience studies were provided, but it seems this section can be improved considered the suggested references that can be cited: https://www.mdpi.com/1996-1073/13/16/4193 (Power Resilience Enhancement of a Residential Electricity User Using Photovoltaics and a Battery Energy Storage System under Uncertainty Conditions) https://www.mdpi.com/1996-1073/13/15/3978 (A Hybrid vs. On-Grid Photovoltaic System: Multicriteria Analysis of Environmental, Economic, and Technical Aspects in Life Cycle Perspective)

The authors appreciate the suggestion. The specified works were cited in the introduction of system reliability and were added to the literature review in Table 2.

1. It would better to have the support of data or the cases of power failures and other disruptions that actually recorded in Philippines country. This can be added to improve the content between line nos. 58 to 68.

The authors appreciate the suggestion. A reported power outage case in a Philippine off-grid island was added to the introduction of system reliability. The authors chose not to expound on this case, however, to avoid derailing the discussion.

1. The table 2, title and in-text statement does not have consistency. For. e.g., resiliency, and reliability. Please be consistent here.

The authors appreciate the suggestion. The term was correctly replaced with “reliability.” The authors also ensured that no instance of “reliability” appeared in the document.

1. Section 2.5, is still very weak, here, the author(s) have just mentioned PV cell cracking and have given a reference that is dated to 1981. I feel this section can be improved further. A reference "https://www.sciencedirect.com/science/article/pii/S2214157X19304083 (Risk priority number for understanding the severity of photovoltaic failure modes and their impacts on performance degradation) can be used for improving this and can be used as citation.

The authors appreciate the suggestion. The authors cited the suggested work and other recent works about the failure of other energy components (Li-ion and diesel). Section 2.5 was also improved by providing the equation describing the reliability and by specifying the failure scenarios and their rationale in tabular format.

Author Response File: Author Response.docx