Evaluating the Applicability of Mainstream Wave Energy Converters in the South China Sea

Round 1

Reviewer 1 Report (Previous Reviewer 1)

The authors' efforts to improve the quality of their work are acknowledged. However, despite these efforts, the manuscript still contains numerous inconsistencies, according to the reviewer's opinion. It is advised that the authors take the time to thoroughly reconsider their investigation and the methods employed. By addressing the inconsistencies and reevaluating their approach, the authors can enhance the overall quality and reliability of their research.

Regarding the Sharp Eagle Wanshan, the authors mention that  experimental data were provided by the Guangzhou Institute of Energy Conversion (Table 2?). However, it is crucial that this aspect is properly referenced and included.

A range of significant wave height between 1 and 4 m was assumed for all wave energy converters. However, in reference [29], the operational sea states for the RM5 technology are stated to be between 0.75 and 6 m. What about the rest of technology? is this information also missing?

The RM5 is not an "oscillating water column (OWC)", but a oscillating SURGE wave energy converter.

Since the efficiency of Wanshan and RM5 is related to wave direction, the methodology for assessing power production is not convicing, as it does not consider wave directionality. The power matrices are solely a function of height and period, as shown in Eq. (14). Consequently, the results and conclusions are inaccurate and inconsistent. How can the authors claim that "the Wanshan device has the strongest economic competitiveness" when their assessment of power production is based on such a vague assumption of wave unidirectionality?

 

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report (Previous Reviewer 2)

All my remarks have been taken into account in the article. Article recommended for publication.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report (New Reviewer)

The paper deals with the applicability of mainstream wave converters in South China Sea. It should cover the resource assessment in China and the available Wave conversion technology.

Concerning the wave resource assessment an unbalanced presentation has been made. At the same time as a detailed presentation is made of some resource assessments in Spain (refs 4, 6, 7, 8 ,9) other areas are left without representation such as for example:

http://dx.doi.org/10.1016/j.cageo.2014.03.008

https://doi.org/10.1016/j.renene.2018.05.037

https://doi.org/10.1016/j.energy.2018.10.002

Including in Atlantic islands

http://dx.doi.org/10.1016/j.renene.2016.08.040

https://doi.org/10.1016/j.renene.2020.01.011

The studies in China, which should be the main concern are not well covered, also missing some relevant contributions

http://dx.doi.org/10.1016/j.renene.2015.12.060

https://doi.org/10.1016/j.renene.2019.01.011

 

It should be possible to correct this limitation, but afterwards it is important to have a discussion of the different approaches adopted to the resource assessments and also on the levels of available resources in the various areas.

This is particularly important because the authors chose ERA 5 to characterise the wave resource. However, ERA 5 is an oceanic hindcast, not providing very good assessment near the coast, where local effect start being important. In fact, this is the reason for the various dedicated studies mentioned in the Introduction.

Economic considerations related with the cost of transmitting the converted energy to shore through electric cables, makes it more attractive for WECS to be located very close to shore, in areas where local effect of the coast and bottom are important. Thus it is important to recognise this limitation of the data used by the authors

 

Concerning the existing technology, the authors mention some concepts that are no longer being considered because their companies went bankrupt, such as AquaBuoy, and Archimedes Wave Swing (AWS), while some current devices are not considered:

https://doi.org/10.1016/j.renene.2019.11.097

https://doi.org/10.1115/1.4050268

 

Having the wave resource and locations, the authors conduct a reasonably good study by looking at how to combine the available information to lead to a conclusion about the best locations and converter type. Although this is a good discussion, the  authors may want to check also one reference that did a similar study several years ago:

https://www.mdpi.com/1996-1073/6/3/1344

 

 

Minor improvements are possible

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (Previous Reviewer 1)

The power matrices are paramount. If these are not provided nor properly referenced, the study lacks any scientific rigour.

RM5 was deleted. However, WaveStar is still included when it is not an OWC...

Comparing the production of wave energy converters which are influenced by wave direction (which is clearly oversestimated) with wave energy converters that are not (point absorbers), is not convicing. The production of the former is clearly inflated.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report (New Reviewer)

Improvements have been made. Although some problems were not fully resolved, I think this version can be accepted

Author Response

Dear Reviewer,
Thank you for your recognition of our research and allowing us to publish it in the Journal of Marine Science and Engineering. We appreciate your professional opinions and suggestions, which have improved the quality of our manuscript and made its content more complete. Once again, we thank you for your support and assistance with our manuscript!

Best Regards.
Sincerely,
Dr. Yong Wan

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

 

· Lines 33 to 35. “[…] Wave energy is a renewable energy that is easy to directly utilize and has attracted the attention of many coastal countries due to the advantages it offers, such as its wide availability, lack of pollution and high energy output […]”. This sentence should be revised and/or justified.

· The authors mention that several studies on wave energy resource assessment have been carried out over the past years in Spain, but there are studies in all around the world, including Portugal, Australia, Italy and many other countries that are omitted:

Mota, P., & Pinto, J. P. (2014). Wave energy potential along the western Portuguese coast. Renewable Energy, 71, 8-17.

Hughes, M. G., & Heap, A. D. (2010). National-scale wave energy resource assessment for Australia. Renewable Energy, 35(8), 1783-1791.

Liberti, L., Carillo, A., & Sannino, G. (2013). Wave energy resource assessment in the Mediterranean, the Italian perspective. Renewable Energy, 50, 938-949.

· As for the wave energy converters, the authors include some failed technology such as the Pelamis and/or reference the power matrices that have not been made public, but exclude others that have been tested and simulated. One example is the CECO concept:

Ramos, V., López, M., Taveira-Pinto, F., & Rosa-Santos, P. (2018). Performance assessment of the CECO wave energy converter: Water depth influence. Renewable Energy, 117, 341-356.

· SWH

- Line 126. “[…] of the SWH H_s and […]”. The text is confusing. What is SWH? What is the difference with H_s? Please, clarify.

- Line 144. “[…] X_{i_buoy} is the SWH or energy period of buoy […]”. Here SWH is used as a synonym of the energy period, which is contradictory with the previous statement.

- Line 201. “[…] where ?? and ?? are the series of energy periods and the series of SWHs, respectively.” Again, the term is confusing in this sentence.

 · Comprehensive indicator CI

- How the proportion coefficients were assessed? A detailed explanation is expected along with an analysis of the possible values.

 · Equation (6)

- According to the text (line 180), “Pe refers to the energy or power generated by a device during operation, […]”, which is defined in Equation (6). The terms nT and nH in the equation are defined as “the series of energy periods and the series of SWHs”. This definition is not clear nor consistent. Commonly, wave datasets include time series of sea-states (usually corresponding to a 3-h duration) and one or various values of the wave height and the wave period are provided.

- Please, clarify how Pe has been obtained. Should the concept of the wave energy matrix be introduced?  

- As it is mentioned in the text, depending on the wave energy converter, the wave direction related to the alignment of the devices can be paramount to production. Therefore, how does Pe account for the variation in the wave propagation direction?

 ·  Equation (10)

- From the expression it seems that for the calculation of Pw, an average value of E was used. The same applies for Te and the rest of the wave parameters. Does it correspond to the averaged value in each sea state? Please, confirm or clarify.

- P* is defined in the manuscript as the “intermediate variable”, what is the physical meaning of this variable?

- The wave energy flux (“power density” in the manuscript) is commonly obtained as the wave energy plus the wave group celerity. Please, relate the applied method to the standard or redefine.

· Lines 263 to 266. “When the SWH is less than 1 m, the energy value absorbed by the device is less than the economic value of the input. When the SWH exceeds 4 m, however, the wave energy device may be damaged. Therefore, the wave height range selected in this study is 1 m ≤ Hs ≤ 4 m”. This statement is not properly justified. Does it apply to all the wave energy converters? Is this information available? If so, include the references.

· “Lines 268 to 270. “As the wave energy incident direction is orthogonal to the device layout direction, the device can effectively capture the wave energy and achieve the maximum energy absorption efficiency”. This statement is true for some technology. In case of point absorbers, the efficiency is independent of the wave direction.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper the authors study the problem of mainstream wave energy converters in the South China Sea. The ocean offers an abundant source of renewable energy, such as wave and tidal energy. This paper proposes a method a regional classification method was established that comprehensively considered the three factors that impact wave energy resource reserves, the suitable water depth of the wave energy conversion device, and the device layout mode that affects energy absorption efficiency.
The introduction should place the proposed approach on the background of existing and known solution presented in literature. Also the importance of the research field should be stressed.
In my opinion, the article should describe in detail the mode of operation of the device for converting wave energy in various modes. To do this, it is necessary to describe how much electrical energy will be generated by each type of device in the nominal operating mode. How much energy will be generated in the absence of waves and during a storm? What is the efficiency of each of the device for converting wave energy.
It is also necessary to describe what type of electrical energy consumers will be connected to wave energy conversion devices. Will this system work autonomously or will there be a backup connection to the public electrical network? What is the term of the decay of the device for converting wave energy?
The presented report is at a very high scientific level. I believe that the present study has a significant scientific and applied contribution, which is strongly emphasized in the basically reporting volume. A slight clarification can be made in the abstract part, where the quality of the research can be enhanced.
In the conclusions, it is necessary to describe what is the economic efficiency of using an energy conversion device? How reliable are these devices? How much electricity is expected to be generated in 1 year?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The author's response to reviewers has not been provided. Some relevant comments have not been considered by the authors or their changes were not convincing or rigorous, and particularly to some relevant comments (just a few):

1) As for the wave energy converters, the authors include some failed technology such as the Pelamis and/or reference the power matrices that have not been made public, but exclude others that have been tested and simulated.

NO CHANGES MADE.

2) Lines 263 to 266. “When the SWH is less than 1 m, the energy value absorbed by the device is less than the economic value of the input. When the SWH exceeds 4 m, however, the wave energy device may be damaged. Therefore, the wave height range selected in this study is 1 m ≤ Hs ≤ 4 m”. This statement is not properly justified. Does it apply to all the wave energy converters? Is this information available? If so, include the references.

THE REFERENCES INCLUDED DO NOT JUSTIFY THIS METHOD.

3) “lines 268 to 270. “As the wave energy incident direction is orthogonal to the device layout direction, the device can effectively capture the wave energy and achieve the maximum energy absorption efficiency”. This statement is true for some technology. In case of point absorbers, the efficiency is independent of the wave direction.

NO CHANGES MADE.