Energy Storage for 1500 V Photovoltaic Systems: A Comparative Reliability Analysis of DC- and AC-Coupling

Round 1

Reviewer 1 Report

Dear Authors,

Thank You for the opportunity of reading this article. It concerns a reliability analysis of energy storage systems for PV systems. The idea and concept of the article are correct. The strong element of this article is a literature review. In the introduction 17 positions are presented to indicate the lack of previous papers. This clearly justifies the need of conducted research. The results are clearly presented and introduced. The quality of the figures and tables is sufficient.

However, I indicate some elements that demand minor revision. They are as follows:

#1

When the novelty of this article is considered, the drawn conclusion is that this article is a case study. It uses known methods (Miner’s rule, Monte-Carlo, Reliability Block Diagram) to solve problems based on 160 kW PV system. This must be highlighted in the introduction. Maybe in line 64 the additional text that it’s a case study article (not new methodology preposition) should be added.

#2

Line 89 indicates “in this paper, which was recorded in the year of 2019 in Denmark with a sampling rate of 1 min/sample”. Please justify why 1 min was selected? Maybe add an additional explanation how this time selection affects results.

#3

The results are clearly presented. However, there is a lack of discussion of the results. Please add the section “Discussion”. This section is should contain as indicated in Energies journal temple:

• “Authors should discuss the results and how they can be interpreted in perspective of previous studies and of the working hypotheses. The findings and their implications should be discussed in the broadest context possible. Future research directions may also be highlighted.”

Especially please discuss the obtained results of these articles in point of previous articles and general knowledge.

Technical elements to improve:

#4

Please do not use “-“ between value and unit. Please change in the whole text, as well as, in the title. It should be “1500 V” not “1500-V”.

Author Response

Thank You for the opportunity of reading this article. It concerns a reliability analysis of energy storage systems for PV systems. The idea and concept of the article are correct. The strong element of this article is a literature review. In the introduction 17 positions are presented to indicate the lack of previous papers. This clearly justifies the need of conducted research. The results are clearly presented and introduced. The quality of the figures and tables is sufficient.

However, I indicate some elements that demand minor revision. They are as follows:

Thank you very much for reviewing the paper.

Comment 1: When the novelty of this article is considered, the drawn conclusion is that this article is a case study. It uses known methods (Miner’s rule, Monte-Carlo, Reliability Block Diagram) to solve problems based on 160 kW PV system. This must be highlighted in the introduction. Maybe in line 64 the additional text that it’s a case study article (not new methodology preposition) should be added.

Thanks a lot for your comment and suggestion. In the revised paper, we have added the text to clarify this point per your suggestion (Line 66-67).

Comment 2: Line 89 indicates “in this paper, which was recorded in the year of 2019 in Denmark with a sampling rate of 1 min/sample”. Please justify why 1 min was selected? Maybe add an additional explanation how this time selection affects results.

Thanks a lot for your comment and suggestion. 1 min/sample is the maximum sampling rate available in the database from [21]. A high sampling rate (e.g., 1 s to 1 min per sample) is recommended to obtain as much information as possible in the mission profile translation process. In contrast, a lower sampling rate (e.g., 5 min/sample) may introduce a certain uncertainty in the lifetime results. This point has been elaborated in the revision per your suggestion (Line 96-100).

Comment 3: The results are clearly presented. However, there is a lack of discussion of the results. Please add the section “Discussion”. This section is should contain as indicated in Energies journal temple: “Authors should discuss the results and how they can be interpreted in perspective of previous studies and of the working hypotheses. The findings and their implications should be discussed in the broadest context possible. Future research directions may also be highlighted.” Especially please discuss the obtained results of these articles in point of previous articles and general knowledge.

Many thanks for your comments and suggestions. In the revision, we added the Discussion Section, where more discussions with respect to evaluation results are included as you suggested.

Comment 4: Technical elements to improve: please do not use “-“ between value and unit. Please change in the whole text, as well as, in the title. It should be “1500 V” not “1500-V”.

Thank you for the comment. In the revision, this point has been corrected in the title and the whole text, as well as between other values and corresponding units.

Reviewer 2 Report

Dear authors,

I would suggest replacing 1500-V in the title by “Medium Voltage” and in the text 1500-V by “MV”.

Figure 2: Shorten the label to “Schematic of the system modeling for DC- and AC- coupling. Move the symbols in line 88 just before displaying Figure 2.

As demonstrate in the publication, the reliability of the PV-BESS system should be carried out at the system level. It seems that using IGBT modules and without redundancy, the AC-coupled systems performance is better than its DC counterpart.

What if SiC modules which have high Junction Temperature (greater than 175 °C with low switching loss and reduced temperature dependence have been used for the comparison. I suggest that the authors address such comparison because most of PV and BESS commercial systems are now being commercialized with SiC modules.

Author Response

Comment 1: I would suggest replacing 1500-V in the title by “Medium Voltage” and in the text 1500-V by “MV”.

Thank you for the suggestion. To the author’s best knowledge, different technical standards have different categories regarding voltage classes. Take 1500 VDC as an example, it is considered medium voltage in ANSI standards, while according to IEC standards, it is the maximum voltage in the low voltage DC directive, which is one of the main reasons for adopting this voltage as a new standard voltage level of large-scale PV systems. Going to this maximum voltage will enable a considerable reduction in currents (compared to the 1000 V PV system with the same power level), and thus achieve loss reduction and cost decrease (less cabling and equipment on the DC side). On the other hand, above this level, expensive medium-voltage components need to be used. However, the increased DC‐link voltage requires careful consideration in the design and control of the power converters within the PV system. Considering this, this work investigates the BESS of DC- and AC-coupling for this particular PV application with an emphasis on the reliability comparison, and thus uses 1500 V both in the title and text. We Ihope the reviewer would agree with this point. In the revision, we improved the explanation of this point in the revision (Line 22 - 24) and added more references correspondingly.

Comment 2: Figure 2: Shorten the label to “Schematic of the system modeling for DC- and AC- coupling. Move the symbols in line 88 just before displaying Figure 2.

Sorry for this. It has been corrected in the revision (Page 3).

Comment 3: As demonstrate in the publication, the reliability of the PV-BESS system should be carried out at the system level. It seems that using IGBT modules and without redundancy, the AC-coupled systems performance is better than its DC counterpart. What if SiC modules which have high Junction Temperature (greater than 175 °C with low switching loss and reduced temperature dependence have been used for the comparison. I suggest that the authors address such comparison because most of PV and BESS commercial systems are now being commercialized with SiC modules.

Thanks a lot for your comments and suggestions. The authors agree that the SiC modules have been utilized for many PV and BESS commercial systems. Commercially available SiC modules exhibit lower switching losses, faster-switching speed, higher voltage blocking capability, and higher allowable operating temperatures than their Si counterparts, which are very suitable for being applied in 1500 V PV applications. However, compared with the Si devices, the SiC MOSFETs have different failure modes and mechanisms as discussed in [37], and new lifetime models are needed to properly analyze their reliability, which is still challenging and out of the scope of this paper. This point has also been elaborated in the Discussion Section of the revised paper (Line 276-282).  The lifetime evaluation of SiC-based 1500-V PV converters will be further investigated on the basis of this paper as the reviewer suggested. More discussions regarding the challenges in the lifetime prediction of SiC devices are have been provided in [37], which has also been added in the reference.

Reviewer 3 Report

The paper proposed the Energy Storage for 1500-V Photovoltaic Systems: A Comparative Reliability Analysis of DC- and AC-Coupling. The quality of the article is good. Thus, the reviewer believes the article needs to be accepted revision at this level.

Author Response

The paper proposed the Energy Storage for 1500-V Photovoltaic Systems: A Comparative Reliability Analysis of DC- and AC-Coupling. The quality of the article is good. Thus, the reviewer believes the article needs to be accepted revision at this level.

Thank you very much for your appreciation.