Assessing the Optimal Contributions of Renewables and Carbon Capture and Storage toward Carbon Neutrality by 2050

Round 1

Reviewer 1 Report

I would like to commend the authors for having written an article that is very relevant currently. I like the overall flow of the article, the methodology is sound and the results and conclusion are logical. 

My biggest concern is with Figure 14, which I believe is a repitition of Figure 13. Please use correct figure. 

My specific comments (mostly recommendations) are included in the attached file. 

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the suggestive comments of the reviewer. The manuscript has been carefully revised, taking into account all comments of the reviewer, where all changes are highlighted in the blue color. In the following, we would like to response point-by-point to the reviewer's comments.

1. Figure 14 is a repetition of Figure 13. I think this is not the correct figure. Please provide correct figure. Also, both Figures 13 and 14 do not have a y-axis label.  

Our response: Thanks for your careful reading. The corrected figure has been added which is Figure 18 in the revision. 

2. (Lines 264-270): Please explain in greater detail, how the value of 8 yen/kwh was selected.

Our response: Additional explanations have been added to lines 279-282 in the revision. 

3. Figures 3-12: I would suggest replacing these 3-D plots with line plots with the amount on y-axis and the year on x-axis and having different colors for the different prefectures.

Our response: 3-D figures are kept in the revision, because there are 47 prefectures that make 2-D figures hard to see. However, following your suggestion in Comment 9, new figures have been added for a specific prefecture throughout four scenarios to show individual deployment of solar, wind and CCS.

4. Figure 1: Please improve the quality of the figure. It looks a bit hazy.

Our response: It probably was due to the conversion to pdf file. It has been fixed. 

5. (Lines 278-281): Please provide a short justification on why these four scenarios were selected for this analysis.

Our response: Additional explanations have been added to lines 305-310 in the revision.

6. Please make sure that all mentions of “CO2” have the 2 as a subscript.

Our response: Thanks for your careful reading. This typo has been fixed throughout the manuscript.

7. (Lines 144-148): It would benefit the reader to have a more involved discussion of the resilience constraint (used from Ref. 13) in the article here, especially as it a major part of the scenarios selected.

Our response: Thanks for your comment. Additional explanations have been added to lines 151-153 in the revision.

8. (Lines 503-505): Please provide range of prefectural incomes that is considered “high” or “low” for this analysis.

Our response: Thanks for your comment. Additional explanations have been added to lines 545-548 in the revision.

9. In general, I think it would be great to have an example “Scenario 1” figure for a specific prefecture over how the different technologies are deployed over time from 2018-2050.

Our response: Thanks for your suggestion. In the revision, we have added new figures for a specific prefecture, Hokkaido, for all scenarios.

10. I think the article would benefit from a section that discusses the characteristics of this analysis that were unique for Japan and what could be applied directly to other geographic regions.

Our response: Thanks for your comment. Additional explanations have been added to lines 253-257 in the revision.

Reviewer 2 Report

Comments on Japanese energy model with CCS

This is a serious article using well understood linear modeling to test various approaches to mixes of renewable energy (RE) and carbon capture and storage (CCS) in Japan. If viewed as a follow-on or expansion of previous work that did not include CCS, the approach of the authors is understandable. There is no doubt that adding CCS allows better potential solutions than ignoring it.

However, if viewed as practical policy guide, I wonder why the reactivation of nuclear plants is not considered – is that ruled out a priori? Combining nuclear with solar and wind would reduce carbon even more than the various scenarios. I understand this is a contentious political issue.

If electricity storage costs fall sharply in this decade and practical duration of storage increases, as many expect, the expansion of wind and solar should become more economical since cheap RE can be “saved” for periods when they are not producing. This would also make the grid more resilient. Current global RE costs are 5-6 cents (8-10 yen) per kWh and projected storage costs could be as low as 5-6 cents per kWh in the 2030’s. It is unlikely that CCS except in selected locations could compete with those costs. With a carbon tax, the appeal of RE including storage vs CCS is essentially unchanged.

I do not know if activities like air travel, shipping, or cement making are in the model. These are hard to electrify and may require green hydrogen or green ammonia made with RE or nuclear. (Alternatives like biodiesel are hard to scale.) The costs of these inputs are falling and should be competitive with current fossil hydrogen or ammonia by the end of this decade or shortly thereafter, even without subsides. Depending on carbon taxes or subsidies, they may be competitive sooner. If this model looks only at electricity costs, this is irrelevant, as green hydrogen is unlikely to be used for electricity generation.

In a global context, ( https://www.iea.org/reports/renewables-2022/executive-summary ) there was $1.7 trillion invested in green capacity last year compared to just $1 trillion in fossil capacity. (This very rapid growth in green investment is pushing costs down faster than anticipated, though supply chain issues did cause a slight uptick in RE costs to 5-6 cents per kWh for utility scale projects. Much of the energy transition may be largely accomplished in 10 years for new capacity (not the stock of cars or fleet of generators) if the exponential growth continues, even at a slower annual growth rate. (Wind and solar are projected on a global basis to overtake coal as a source of electricity by 2027 according to the International Energy Agency. If nuclear is included, no carbon power is then close to half of all electricity.) Combined with a rapid growth in electric vehicles, where the costs of even initial purchase are now close – let alone life-cycle costs which are already lower. Given this momentum, the model seems conservative in timing, even if its basic finding – that combining all possible tools gives a least cost or highest profit solution – is correct.

Knowing how hard it is to change models, the only suggestion is to see if putting in reactivated nuclear changes a new Scenario 5 much compared to Scenarios 1-4. This scenario can be presented as a political decision, not a recommendation. The introduction of small, modular nuclear reactors can be ignored for this article, even though they may become important in the next decade.

Author Response

The authors would like to thank the suggestive comments of the reviewer. The manuscript has been carefully revised, taking into account all comments of the reviewer, where all changes are highlighted in the blue color. In the following, we would like to response point-by-point to the reviewer's comments.

This is a serious article using well understood linear modeling to test various approaches to mixes of renewable energy (RE) and carbon capture and storage (CCS) in Japan. If viewed as a follow-on or expansion of previous work that did not include CCS, the approach of the authors is understandable. There is no doubt that adding CCS allows better potential solutions than ignoring it.

Our response: Thank you for your kind comments here – we appreciate all of your feedback and have addressed your kind comments in order below.

However, if viewed as practical policy guide, I wonder why the reactivation of nuclear plants is not considered – is that ruled out a priori? Combining nuclear with solar and wind would reduce carbon even more than the various scenarios. I understand this is a contentious political issue.

Our response: Thank you for this kind comment – here we do exclude nuclear a priori, and have now made this clearer in the aim statement and methodology section – this is, as you have assessed, largely a politically driven issue, and in our case we are seeking to investigate the additional/new deployment of energy and complementary solutions, whereas nuclear is ‘existing’ in Japan and new builds are unlikely.

If electricity storage costs fall sharply in this decade and practical duration of storage increases, as many expect, the expansion of wind and solar should become more economical since cheap RE can be “saved” for periods when they are not producing. This would also make the grid more resilient. Current global RE costs are 5-6 cents (8-10 yen) per kWh and projected storage costs could be as low as 5-6 cents per kWh in the 2030’s. It is unlikely that CCS except in selected locations could compete with those costs. With a carbon tax, the appeal of RE including storage vs CCS is essentially unchanged.

Our response: Your analysis here is correct.

I do not know if activities like air travel, shipping, or cement making are in the model. These are hard to electrify and may require green hydrogen or green ammonia made with RE or nuclear. (Alternatives like biodiesel are hard to scale.) The costs of these inputs are falling and should be competitive with current fossil hydrogen or ammonia by the end of this decade or shortly thereafter, even without subsides. Depending on carbon taxes or subsidies, they may be competitive sooner. If this model looks only at electricity costs, this is irrelevant, as green hydrogen is unlikely to be used for electricity generation.

Our response: Thank you for this comment, here we focus on electricity generation.

In a global context, ( https://www.iea.org/reports/renewables-2022/executive-summary ) there was $1.7 trillion invested in green capacity last year compared to just $1 trillion in fossil capacity. (This very rapid growth in green investment is pushing costs down faster than anticipated, though supply chain issues did cause a slight uptick in RE costs to 5-6 cents per kWh for utility scale projects. Much of the energy transition may be largely accomplished in 10 years for new capacity (not the stock of cars or fleet of generators) if the exponential growth continues, even at a slower annual growth rate. (Wind and solar are projected on a global basis to overtake coal as a source of electricity by 2027 according to the International Energy Agency. If nuclear is included, no carbon power is then close to half of all electricity.) Combined with a rapid growth in electric vehicles, where the costs of even initial purchase are now close – let alone life-cycle costs which are already lower. Given this momentum, the model seems conservative in timing, even if its basic finding – that combining all possible tools gives a least cost or highest profit solution – is correct.

Our response: Thank you for this kind comment, you are right, the model is conservative in its timing, as we do not consider the contribution of existing technologies in our analysis – although, as you say, the basic finding is correct, I have added some discussion to clarify this and include your keen analysis here.

Knowing how hard it is to change models, the only suggestion is to see if putting in reactivated nuclear changes a new Scenario 5 much compared to Scenarios 1-4. This scenario can be presented as a political decision, not a recommendation. The introduction of small, modular nuclear reactors can be ignored for this article, even though they may become important in the next decade.

Our response: In line with your and other reviewer’s comments, we agree that nuclear may play a future role, however we have excluded it from our analysis as an ‘existing’ technology – in line with the improved discussion and future work statement in the conclusions, we hope that the exclusion of an additional scenario at this time is acceptable.

Reviewer 3 Report

1.      The novelty and originality shall be justified that the manuscript contains sufficient contributions to the new body of knowledge from the international perspective in addition to focusing on the issues of one country/region. The knowledge gap needs to be clearly addressed.

2.      Literature review is insufficient to present the most updated status for further justification of the originality of the manuscript. The authors should carry out a thorough literature review of papers published in a range of top energy and resource journals in the last three/four years so as to fully appreciate the latest findings and key challenges relating to the topic addressed in the manuscript and to allow the authors to more clearly present their contributions to the pool of existing knowledge.

3.      The discussion should be specific and detailed based on results found in this paper. The discussion usually contains a synthesis of the findings, the practical implications, the theoretical implications, the strengths and limitations of the research, and the future research directions. However, present discussion is really poor, and many contents are missing in this paper.

4.      The implications are really general and have little guiding significance. The authors should improve this part.

Author Response

The authors would like to thank the suggestive comments of the reviewer. The manuscript has been carefully revised, taking into account all comments of the reviewer, where all changes are highlighted in the blue color. In the following, we would like to response point-by-point to the reviewer's comments.

1. The novelty and originality shall be justified that the manuscript contains sufficient contributions to the new body of knowledge from the international perspective in addition to focusing on the issues of one country/region. The knowledge gap needs to be clearly addressed.

Our response: We thank the reviewer for this comment. New explanations have been added to the lines 250-254 in the revised manuscript. Particularly, the proposed linear optimization model is in fact general and is not limited to Japan only. This model can be applied to any geographical regions, subject to the availability of data associated to the variables and parameters in the model. This constitutes the contribution of the current research.

Explanations have also been added to the end of Section 2, line 158 in the revision to show the research gap and our contribution to address such gap. 

2. Literature review is insufficient to present the most updated status for further justification of the originality of the manuscript. The authors should carry out a thorough literature review of papers published in a range of top energy and resource journals in the last three/four years so as to fully appreciate the latest findings and key challenges relating to the topic addressed in the manuscript and to allow the authors to more clearly present their contributions to the pool of existing knowledge.

Our response: Thanks for the suggestion. Indeed, the literature on integration of renewable energy sources and on CCS research is huge. However, to concisely present the research, we have selected the most related studies to the scope of the current work. We hope to have the understanding of the reviewer on this point. 

3. The discussion should be specific and detailed based on results found in this paper. The discussion usually contains a synthesis of the findings, the practical implications, the theoretical implications, the strengths and limitations of the research, and the future research directions. However, present discussion is really poor, and many contents are missing in this paper.

Our response: We indeed provided all the necessary discussions in Section 4, where results of each scenario and their implications to the environment, economy, and policy suggestions were discussed in detail. In the revision, we have added more discussions to clarify the obtained results. We would appreciate if the reviewer could take a closer look at Section 4 again. 

4. The implications are really general and have little guiding significance. The authors should improve this part.

Our response: We believe that results on the effects to the environment and economy were presented and discussed in detail with specified number values, hence clearly showcase the meanings of each investigated scenario. On the other hand, for policy implication, it is difficult to have something similar, because it is a different discipline compared to natural sciences. 

Reviewer 4 Report

REFEREE'S REPORT ON

"Assessing the optimal contributions of renewables and carbon

capture and storage toward carbon neutrality by 2050"

Comments: This paper needs improvement as has been listed below:

1.      Abstract

Ø  The policy proposal of the study's originality and importance should be written in this section.

Ø  A brief description of the model and method of the study should be written.

2.      Introduction

Ø  The hypothesis should be discussed with graphics or tables.

Ø  The main motivation of the study should be explained correctly in this section.

Ø  The study's importance, purpose and theoretical framework should be discussed in detail.

Ø  Theoretical literature explanations regarding the main topic is quite insufficient. It should be developed with reference to current studies. Please read and check this study.

https://doi.org/10.1007/s11356-022-22674-w

3.      Literature

Ø  The difference of the study from the literature and its contribution to the literature should be explained in detail under this title.

4.   Data and Methods

Ø  Analyzes and the findings are successfull but have not been adequately discussed.

5.      Conclusion

Ø  The results obtained should be discussed. The contribution of the study to the literature and which studies it contradicts and supports should be given critically, along with its justifications. Also plenty of policy proposals should be presented.

Author Response

The authors would like to thank the suggestive comments of the reviewer. The manuscript has been carefully revised, taking into account all comments of the reviewer, where all changes are highlighted in the blue color. In the following, we would like to response point-by-point to the reviewer's comments.

1. Abstract

Ø  The policy proposal of the study's originality and importance should be written in this section.

Ø  A brief description of the model and method of the study should be written.

Our response:  A brief description of the model and research methodology was already included in the abstract, lines 15-17. Due to the concise requirement of the abstract, policy suggestions are not included in the abstract.

2. Introduction

Ø  The hypothesis should be discussed with graphics or tables.

Our response:  It is unnecessary to include graphics or tables there.

Ø  The main motivation of the study should be explained correctly in this section.

Our response:  The motivation for our study was already there, in the lines 48-52.

Ø  The study's importance, purpose and theoretical framework should be discussed in detail.

Our response:  The aim of research was already there, lines 58-63 (lines 58-66 in the revision). Theoretical framework was discussed in detail in Section 3 of methodology, not in the introduction.

Ø  Theoretical literature explanations regarding the main topic is quite insufficient. It should be developed with reference to current studies. Please read and check this study.

https://doi.org/10.1007/s11356-022-22674-w

Our response:  Thanks for suggesting this reference, but the scope of the suggested reference is not related to that of the current study. Hence, the mentioned reference was not added to the revision.

3. Literature

Ø  The difference of the study from the literature and its contribution to the literature should be explained in detail under this title.

Our response:  Additional explanation on the originality of the current research has been added to line 155 (line 158 in the revision).

4. Data and Methods

Ø  Analyzes and the findings are successful but have not been adequately discussed.

Our response:  Thanks for the comment. More results and discussions have been added to all scenarios in the revision to clarify the findings.

5. Conclusion

Ø  The results obtained should be discussed. The contribution of the study to the literature and which studies it contradicts and supports should be given critically, along with its justifications. Also plenty of policy proposals should be presented.

Our response:  Thanks for the comment. However, please note that obtained results were discussed in details in Section 4 and policy discussions were discussed in Section 5, so they were only briefly summarized in the conclusion section. Contribution of the current study was described in the introduction, hence was not recalled in the conclusion. Similarly, discussions on the current work related to the literature were detailed in the introduction. The conclusion section purpose is to summarize the main results and limitations of the current study, and hence, show directions for future research. 

Reviewer 5 Report

I thank the authors for their outstanding contribution with this interesting and current research. The title of the manuscript is fully adequate to the content and purpose of the manuscript. The abstract contains all the necessary elements. The introductory part fully corresponds to the writing standard and introduces the readers to the research issues in a very meaningful way. The goal of the research is emphasized, as well as the introduction to the next paragraph. The review of literature dealing with a similar topic is adequate and sufficient, although I suggest strengthening the references.

The linear programming model was used and explained in great detail in the methodology chapter, through a mathematical construct and through figures. The results explain each scenario individually through the text but also through the graphic representation, which is very clear and precise. I do not propose additions or changes to this part of the manuscript.

The discussion chapter is very extensive and gives us an overview and significance of the results, however, I suggest that the conclusion be expanded a bit. If the authors believe that the text should not be transferred to concluding considerations, they can certainly supplement that chapter with the future implications of the results in theory and practice, as well as limiting circumstances. I suggest expanding the references.

Suggested references:

Stereotypes and Prejudices as (Non) Attractors for Willingness to Revisit Tourist-Spatial Hotspots in Serbia. Sustainability,15, 5130. https://doi.org/10.3390/su15065130

Risks in the Role of Co-Creating the Future of Tourism in "Stigmatized" Destinations. Sustainability 2022, 14, 1-19, 15530. https://doi.org/10.3390/su142315530

Minimal corrections, after which I am happy to propose that the manuscript be published.

Author Response

The authors would like to thank the suggestive comments of the reviewer. The manuscript has been carefully revised, taking into account all comments of the reviewer, where all changes are highlighted in the blue color. In the following, we would like to response point-by-point to the reviewer's comments.

I thank the authors for their outstanding contribution with this interesting and current research. The title of the manuscript is fully adequate to the content and purpose of the manuscript. The abstract contains all the necessary elements. The introductory part fully corresponds to the writing standard and introduces the readers to the research issues in a very meaningful way. The goal of the research is emphasized, as well as the introduction to the next paragraph. The review of literature dealing with a similar topic is adequate and sufficient, although I suggest strengthening the references.
The linear programming model was used and explained in great detail in the methodology chapter, through a mathematical construct and through figures. The results explain each scenario individually through the text but also through the graphic representation, which is very clear and precise. I do not propose additions or changes to this part of the manuscript.
The discussion chapter is very extensive and gives us an overview and significance of the results, however, I suggest that the conclusion be expanded a bit. If the authors believe that the text should not be transferred to concluding considerations, they can certainly supplement that chapter with the future implications of the results in theory and practice, as well as limiting circumstances. I suggest expanding the references.
 
Suggested references:
Stereotypes and Prejudices as (Non) Attractors for Willingness to Revisit Tourist-Spatial Hotspots in Serbia. Sustainability,15, 5130. https://doi.org/10.3390/su15065130
 
Risks in the Role of Co-Creating the Future of Tourism in "Stigmatized" Destinations. Sustainability 2022, 14, 1-19, 15530. https://doi.org/10.3390/su142315530
 
Minimal corrections, after which I am happy to propose that the manuscript be published.

Our response: We thank the reviewer for reviewing this manuscript. Following the suggestion of the reviewer, the conclusion section has been improved. The suggested references are not related to the scope of the paper, so they cannot be included in the revision.