Analysis of EU-DEMO WCLL Power Conversion System in Two Relevant Balance of Plant Configurations: Direct Coupling with Auxiliary Boiler and Indirect Coupling

Round 1

Reviewer 1 Report

This paper dealt with a preliminary feasibility assessment of two possible configurations supposed for the DEMO WCLL BB BoP, it is very meaningful for the safe and effective operation of the Balance of Plant (BoP) Power Conversion System. However, there are still minor issues that should be addressed before its acceptance：

1. Two BoP solutions are carried out with GateCycle^TM code, and the adopted parameters are optimized, but the advantages of the solutions given in this paper compared with the references are not explained.
2. In the conclusion of the paper, the average electricity production, average gross efficiency, and the average net electric efficiency of the two solutions are described, but there is no comparison, and the suggestions on the use of the two solutions can be given according to the actual situation.

Author Response

Many thanks to the reviewers for their useful comments and suggestions that certainly enhanced the paper quality. We improved the paper following their reports. All the modifications are described in the following and evidenced in the revised manuscript by using “Track Changes” function.

 

Reviewer #1:

This paper dealt with a preliminary feasibility assessment of two possible configurations supposed for the DEMO WCLL BB BoP, it is very meaningful for the safe and effective operation of the Balance of Plant (BoP) Power Conversion System. However, there are still minor issues that should be addressed before its acceptance:

 

1. Two BoP solutions are carried out with GateCycleTM code, and the adopted parameters are optimized, but the advantages of the solutions given in this paper compared with the references are not explained.

Thank you for the comment. Table 10 has been added in the text to compare pros and cons of the studied configurations with respect to reference one. The reasons why DCD concept was selected as reference BoP configuration were discussed. The following addendum to the original text was introduced:

 

Section 4, ‘Discussion, page 17, rows 653-660:

 

‘Although such solution is considered the most promising one, further R&D efforts are required to cover the remaining technical issues (e.g. thermomechanical analysis of BoP components).

Parameter	WCLL DCD [6]	WCLL DCD AUXB	WCLL ICD
Gross power Pulse (MW)	791.6	892.9	757.7
Gross power Dwell (MW)	62.4	40.3	764.2
Average Gross efficiency	34.9%	35.7%	36.3%
Average net electric efficiency	31.0%	33.6%	33.6%

Table 10. DEMO WCLL PCSs comparison.

 

 

 

 

 

 

 

 

The DCD concept is presently considered the most viable option due to the heat storage size reduction as well as the lack of the AUXB. An extended comparison among the available options of the BoP it is reported in [4].

 

2. In the conclusion of the paper, the average electricity production, average gross efficiency, and the average net electric efficiency of the two solutions are described, but there is no comparison, and the suggestions on the use of the two solutions can be given according to the actual situation.

Thank you for the remark. Such information was entered in Table 10, which was added to the text to compare the BoP configurations studied with the reference one, as explained above. Moreover, the need to also continue with the R&D related to the ICD option was highlighted, above all in an electricity production scenario more and more characterized by the spread of renewable (and unpredictable) energy sources.

To underline this aspect a further sentence was introduced in the text:

 

Section 5, ‘Conclusions, page 18, rows 686-690:

 

‘In the electricity production scenario expected for the long-term future, characterized by very-high percentage of renewable (and not predictable) energy sources, the capability of a large heat storage, such as the one included in the ICD option, could be used in a re-verse way to accommodate the grid spike requests, enhancing the plant flexibility. This justifies the need for further R&D related to an ICD configuration as alternative solution.’

Author Response File: Author Response.docx

Reviewer 2 Report

This paper investigates analysis of EU-DEMO WCLL power conversion system in two relevant Balance of Plant configurations: direct coupling with auxiliary boiler and indirect coupling. In general, I find that the paper is well written and the topic is interesting. Here, there are some concerns of this reviewer: 
1 The deficiencies of existing approaches and the main contributions of this paper should be further summarized and clearly demonstrated. 
2 The Energy Storage System is an important item for the analysis of the power conversion system in this work. Please note that the up-to-date of references will contribute to the up-to-date of your manuscript. The studies [doi.org/10.1016/j.energy.2018.07.200], [doi.org/10.1109/TIA.2021.3106573], [doi.org/10.1016/j.enconman.2020.112526] can be included to improve the literature survey.
3 What are the limitations of the presented approach in practical applications? Please clarify.

4 Although the manuscript is well written in terms of English, there are some (very few, indeed) grammatical and expression errors. It is suggested to proofread the paper. 

Author Response

Reviewer #2:

This paper investigates analysis of EU-DEMO WCLL power conversion system in two relevant Balance of Plant configurations: direct coupling with auxiliary boiler and indirect coupling. In general, I find that the paper is well written and the topic is interesting. Here, there are some concerns of this reviewer:

1. The deficiencies of existing approaches and the main contributions of this paper should be further summarized and clearly demonstrated.

 

Thank you for the comment. The reference operative regime of DEMO is based on a pulse period of 2 hours and a dwell time of 600 s. During pulse, 2 GW_th must be removed from the reactor Breeding Blanket, while, during dwell, the power deposited in the same component approaches the 1% of the nominal value. This generates a major concern related to the unconventional operation of the DEMO BoP. The configurations of this system already exploited in existing (e.g. thermal or nuclear fission) power plants are not suitable for application in nuclear fusion power plant. They have been developed to manage a constant power production. Thus, also the component design and the system architecture has been optimized for this purpose. Such BoP solutions lack the operational flexibility needed to withstand the power fluctuations induced by the pulsed plasma regime. For this, ad hoc BoP configurations have been developed in the framework of the EUROfusion research activity and they are currently under study. To better highlight this point, the following sentence was added to the text:

 

Section 1, ‘Introduction’, page 2, rows 65-70:

 

‘The BoP configurations already exploited in existing (e.g. thermal or nuclear fission) power plants are not suitable for this application. They have been developed to manage a constant power production. Thus, also the component design and the system architecture has been optimized for this purpose. Such BoP solutions lack the operational flexibility needed to withstand the power fluctuations induced by the pulsed plasma regime.’

 

2. The Energy Storage System is an important item for the analysis of the power conversion system in this work. Please note that the up-to-date of references will contribute to the up-to-date of your manuscript. The studies [doi.org/10.1016/j.energy.2018.07.200], [doi.org/10.1109/TIA.2021.3106573], [doi.org/10.1016/j.enconman.2020.112526] can be included to improve the literature survey.

Thank you for the suggestion. Among the proposed references the one that better matches the content and the purpose of the current paper is the third. It has been added to the reference list and integrated in the text by using the following sentence:

 

Section 1, ‘Introduction’, page 2, rows 97-99:

 

‘It is worth to be noted that storage systems already proved to be effective in improving the operational flexibility of integrated energy systems, for example in combination with renewable energy sources, as reported in [9].’

 

With [9] corresponding to:

 

Li, Y.; Wang, C.; Li, G.; Wang, J.; Zhao, D.; Chen, C. Improving operational flexibility of integrated energy system with un-certain renewable generations considering thermal inertia of buildings. Energy Convers. Manage. 2020, 207, 112526. https://doi.org/10.1016/j.enconman.2020.112526.

 

3. What are the limitations of the presented approach in practical applications? Please clarify.

Thank you for the observation. The work described in this paper must be considered as a preliminary system analysis focused on the evaluation of the feasibility of two BoP configurations suitable for the peculiar operative regime characterizing a nuclear fusion power plant. The sizing and calculation outcomes have been important to identify pros and cons of each option and converge to a solution merging the main strengths of both configurations. Although, many analyses must be furtherly performed and will be carried out in the next years within the framework of the EUROfusion project. To clarify this aspect, the following sentence was added to the text:

 

Section 4, ‘Discussion’, page 17, rows 653-655.

 

‘Although such solution is considered the most promising one, further R&D efforts are required to cover the remaining technical issues (e.g. thermomechanical analysis of BoP components, fatigue stress analysis for the steam turbine).’

 

4. Although the manuscript is well written in terms of English, there are some (very few, indeed) grammatical and expression errors. It is suggested to proofread the paper.

Thank you for the advice. The authors performed a check on the paper, some grammatical errors were corrected and the text was improved.

Reviewer 3 Report

The article is devoted to a preliminary assessment of the feasibility of two solutions for the station balance energy conversion system. The study's relevance is since, among the critical project integration issues recently selected for the demonstration Fusion Power Plant, the operation of the station's balance energy conversion system was recognized as a crucial aspect due to the typical pulse mode. As part of the DEMO concept of a water-cooled lead-lithium propagating coating, it was recognized that three BoP solutions could solve this problem. They are based on different connection options between primary heat transfer systems and PCS: indirect connection design with intermediate heat transfer and energy storage systems, direct connection design with the auxiliary boiler, and DCD with low ESS. In this article, the authors conduct a preliminary assessment of the feasibility of the first two solutions. The analysis performed with the GateCycleTM code refers to the preliminary design phase, dedicated to sizing the main components, and the second phase, dedicated to cycle optimization. Both concepts are capable of producing good electricity with acceptable efficiency. For both solutions, the main strengths and weaknesses are compared and discussed.

Despite the satisfactory quality of the article, some shortcomings need to be corrected.

1. The abstract should be rewritten. The numerical results obtained by the authors should be added. 
2. The aim of the research should be defined.
3. The state-of-the-art methods, models, and approaches should be separated from the ones proposed by the authors.
4. The discussion section should include a comparative analysis of obtained results with other research.
5. The scientific novelty of the research should be highlighted.
6. The question addressed in the article is interesting. However, the contribution of the authors is not clear. It is not clear from the article what is proposed by the authors and is original solutions, and what known methods are simply applied.Therefore, I believe that the structure of the article does not allow a clear opinion about the authors' contribution and the results obtained.
7. The conclusions are consistent with the experiments, but the results are not compared with existing studies.

 

Author Response

Reviewer #3:

The article is devoted to a preliminary assessment of the feasibility of two solutions for the station balance energy conversion system. The study’s relevance is since, among the critical project integration issues recently selected for the demonstration Fusion Power Plant, the operation of the station’s balance energy conversion system was recognized as a crucial aspect due to the typical pulse mode. As part of the DEMO concept of a water-cooled lead-lithium propagating coating, it was recognized that three BoP solutions could solve this problem. They are based on different connection options between primary heat transfer systems and PCS: indirect connection design with intermediate heat transfer and energy storage systems, direct connection design with the auxiliary boiler, and DCD with low ESS. In this article, the authors conduct a preliminary assessment of the feasibility of the first two solutions. The analysis performed with the GateCycleTM code refers to the preliminary design phase, dedicated to sizing the main components, and the second phase, dedicated to cycle optimization. Both concepts are capable of producing good electricity with acceptable efficiency. For both solutions, the main strengths and weaknesses are compared and discussed.

Despite the satisfactory quality of the article, some shortcomings need to be corrected.

1. The abstract should be rewritten. The numerical results obtained by the authors should be added.

Thank you for the remark. A sentence was added to the abstract following your indication.

 

Section ‘Abstract’, page 1, rows 20-23:

 

‘They are able to produce a satisfactory average electric power (>700 MW) with an acceptable average net electric efficiency (33.6% for both concepts). For each solution, the main strengths and weaknesses are compared and discussed.’

 

2. The aim of the research should be defined.

Thank you for the comment. The reference operative regime of DEMO is based on a pulse period of 2 hours and a dwell time of 600 s. During pulse, 2 GW_th must be removed from the reactor Breeding Blanket, while, during dwell, the power deposited in the same component approaches the 1% of the nominal value. This generates a major concern related to the unconventional operation of the DEMO BoP. The configurations of this system already exploited in existing (e.g. thermal or nuclear fission) power plants are not suitable for application in nuclear fusion power plant. They have been developed to manage a constant power production. Thus, also the component design and the system architecture has been optimized for this purpose. Such BoP solutions lack the operational flexibility needed to withstand the power fluctuations induced by the pulsed plasma regime. For this, ad hoc BoP configurations have been developed in the framework of the EUROfusion research activity and they are currently under study. To better highlight this point, the following sentence was extended in the text:

 

Section 1, ‘Introduction’, page 2, rows 64-71:

 

‘As mentioned before, the main design challenge related to the DEMO BoP is the coupling with a pulsed power source. The BoP configurations already exploited in existing (e.g. thermal or nuclear fission) power plants are not suitable for this application. They have been developed to manage a constant power production. Thus, also the component design and the system architecture has been optimized for this purpose. Such BoP solutions lack the operational flexibility needed to withstand the power fluctuations induced by the pulsed plasma regime. To cope with this peculiarity, three options for the WCLL BB BoP are under evaluation [5].’

 

3. The state-of-the-art methods, models, and approaches should be separated from the ones proposed by the authors.

Thank you for the advice. As stated before, existing BoP solutions able to cope with a pulsed plasma regime were not available. Throughout the work presented in this paper, the authors developed two alternative BoP options capable of facing the challenging requirements deriving from the operation of a fusion reactor. To better highlight this difference the sentence reported in Point 2 was added to the text. In addition, further references were added in the text:

 

Moscato, I.; Barucca , L.; Bubelis, E.;  Caruso, G.; Ciattaglia, S.; Ciurluini, C.; Del Nevo, A.; Di Maio, P. A.; Giannetti, F.; Hering, W.; Lorusso P.; Martelli, E.; Narcisi, V.; Norrman, S.; Pinna, T.; Perez-Martin, S.; Quartararo, A.; Szogradi, M.; Tarallo, A.; Vallone, E.; Tokamak cooling systems and power conversion system options. Fusion Eng. Des. 2022, 178, 113093. https://doi.org/10.1016/j.fusengdes.2022.113093

 

Barucca, L.; Hering, W.; Perez Martin, S.; Bubelis, E.; Del Nevo, A.;Di Prinzio, M.;Caramello, M.; D’Alessandro, A.; Tarallo, A.; Vallone, E.; Moscato, I.; Quartararo, A.; D’Amico, S.; Giannetti, F.; Lorusso, P.; Narcisi, V.; Ciurluini, C.; Montes Pita, M-J.; Śanchez, C.; Rovira, A.; Santana,D.; Gonzales, P.; Barbero,R.; Zaupa, M.; Szogradi, M.; Normann, S.; Vaananen, M.; Ylatalo, J.; Lewandowska, M.; Malinowski, L.; Martelli, E.; Froio, A.; Arena, A.; Tincani, A.; Maturation of critical technologies for the DEMO balance of plant systems Fusion Eng. Des. 2022, 179, 113096. https://doi.org/10.1016/j.fusengdes.2022.113096

 

4. The discussion section should include a comparative analysis of obtained results with other research.

Thank you for the comment. Table 10 has been added in the text to compare pros and cons of the studied configurations with respect to reference one. The reasons why DCD concept was selected as reference BoP configuration were discussed. The following addendum to the original text was introduced:

 

Section 4, ‘Discussion, page 17, rows 653-660:

 

‘Although such solution is considered the most promising one, further R&D efforts are required to cover the remaining technical issues (e.g. thermomechanical analysis of BoP components, fatigue stress analysis for the steam turbine).

Parameter	WCLL DCD [6]	WCLL DCD AUXB	WCLL ICD
Gross power Pulse (MW)	791.6	892.9	757.7
Gross power Dwell (MW)	62.4	40.3	764.2
Average Gross efficiency	34.9%	35.7%	36.3%
Average net electric efficiency	31.0%	33.6%	33.6%

Table 10. DEMO WCLL PCSs comparison.

 

 

 

 

 

 

 

 

The DCD concept is presently considered the most viable option due to the heat storage size reduction as well as the lack of the AUXB. An extended comparison among the available options of the BoP it is reported in [4].

 

5. The scientific novelty of the research should be highlighted.

Thank you for the observation. The work described in this paper must be considered as a preliminary system analysis focused on the evaluation of the feasibility of two BoP configurations suitable for the peculiar operative regime characterizing a nuclear fusion power plant. The sizing and calculation outcomes have been important to identify pros and cons of each option and converge to a solution merging the main strengths of both configurations. Although, many analyses must be furtherly performed and will be carried out in the next years within the framework of the EUROfusion project. To clarify this aspect, the following sentence was extended in the text:

 

Section 4, ‘Discussion’, page 17, rows 648-655.

 

‘The present study has demonstrated the feasibility of such configurations, highlighting strengths and weaknesses of each concept. Starting from these outcomes, an R&D activity (still ongoing) has highlighted the possibility to merge advantages of the two solutions, while limiting the main drawbacks, in the WCLL DCD BoP, where the PHTSs are directly coupled with the PCS and a small electrically heated ESS provides power to operate the plant at around 10% of the nominal power [5]. Although such solution is considered the most promising one, further R&D efforts are required to cover the remaining technical issues (e.g. thermomechanical analysis of BoP components, fatigue stress analysis for the steam turbine).’

 

6. The question addressed in the article is interesting. However, the contribution of the authors is not clear. It is not clear from the article what is proposed by the authors and is original solutions, and what known methods are simply applied. Therefore, I believe that the structure of the article does not allow a clear opinion about the authors’ contribution and the results obtained.

Thank you for your useful recommendation. Because of it, a sentence was added in the introduction to clearly underline the lack of existing BoP solutions able to cope with a pulsed plasma regime (it is reported in the point 2).

The work described in this paper consists in the preliminary design of this system. The relevancy of the simulation outcomes presented and discussed lies in the capability of the current layouts to respect all the challenging requirements deriving by the operations of the fusion reactor. The analysis carried out by the authors evidenced in a clear way the pros and cons of each BoP configurations studied. To this purpose, a sentence was added in the text to also consider the impact of the different BoP solutions from the perspective of the current effort to reduce the emission of greenhouse gases:

 

Section 4, ‘Discussion’, page 17, rows 641-647.

‘Furthermore, such solution has a big issue related to the greenhouse emissions associated to the gas combustion in the AUXB. This is not aligned to the DEMO policy aimed to de-sign a “fully renewable” fusion power plant able to cope with the efforts of generating large amount of electricity in a zero-emission scenario. This problem could be solved us-ing green-H2 or CH4, instead of natural gas in the AUXB. As a drawback of this solution, the integration of an auxiliary plant able to produce the green fuel must be considered.’.

 

Starting from these considerations, a reference BoP configuration (DCD with small ESS) was developed by Ansaldo Energia and identified as reference to be coupled with a DEMO WCLL fusion reactor.

 

7. The conclusions are consistent with the experiments, but the results are not compared with existing studies.

Thank you for the comment. The result comparison was entered in the text by adding Table 10. In such table, the simulation outcomes performed by the authors (and referred to ICD and DCD AUXB) were evaluated against the one present in literature and related to DCD small ESS option (the reference one). Other studies related to such topic, i.e. the design of BoP configurations for fusion power plant exploiting a Water-Cooled Lead-Lithium blanket, are not currently available.

Round 2

Reviewer 2 Report

Thanks to the careful revision and detailed response made by the authors. All my concerns have been well addressed, and the revised manuscript has been much improved. No other comments. Therefore, I think this paper deserves to be published in its current form.

Reviewer 3 Report

Thanks for the authors for concidering comments and recommendations. Now the article can be accepted.