Proof of Concept of a Breakwater-Integrated Hybrid Wave Energy Converter Using a Composite Modelling Approach

Round 1

Reviewer 1 Report

The authors are proposing the concept of a hybrid type wave power generator, which is the content of a topic that can be accepted as a research paper. However, the following is requested to be supplemented.

- The main specifications of the proposed model are not clearly indicated.

- It is argued that the LCOE of the proposed model is better, but the evidence is not clear.

- The numerical model and method are not shown in detail.

- A time series by CFD analysis is proposed, but a snapshot such as wave elevation needs to be presented.

- Val_Test is not clearly explained. Comparisons were made for very limited analysis conditions.

- On page 12, the reason why the numerical analysis results are not quasi-steady and some oscillatory results should be explained.

- On page 14, what is the reason why the numerical analysis and model test show a big difference after 45 seconds of the water level inside the OWC?

- It is necessary to compare the analysis results through dimensionlessness.

- It is necessary to statistically process the analyzed results and make them dimensionless to compare and discuss the results.

- In conclusion, new finding is insufficient and it is difficult to find meaningful conclusions.

Author Response

We would like to thank the thorough review of our manuscript by Reviewer 1. Comments and suggestions were highly appreciated and have contributed to improve the quality of our work previously submitted. Herein below is the detailed response to each comment individually.

• The main specifications of the proposed model are not clearly indicate

Thank you for the comment. More details on the characteristics of the innovative device are included in Section 5 'Proof of concept of the HWEC-integrated breakwater' of the updated version of the paper. Namely e.g. lines #297 to 299 on where the machinery room is located, lines#326 to 330 on the characteristics of the wave-maker and number of waves in the physical model testing, and lines #420 to 423 on the numerical model set-up and equations.

• It is argued that the LCOE of the proposed model is better, but the evidence is not clear

The integration of the WEC device into breakwater has shown already many advantages in the past in terms of construction and operation costs compared to stand-alone WEC devices, according to several authors (Falcão et al., 2016; Vicinanza 2014; 2019 - see references below) due to the sharing cost of the harbour defence substructure that has to be constructed regardless of the installation of the WEC. However, the energy produced by the OTD and OWC integrated into breakwater is still low and the hydraulic performances of the technology must be improved. To increase the efficiency of the WEC integrated into breakwater, thus reducing the relative LCoE cost, the HWEC is proposed in this work. The novel WEC system incorporates the advantages of each WEC, whilst mitigating their inherent weaknesses. Further tests will be carried out in coming months on large scale with measure of the energy production which will give more detailed information about the LCoE cost of the technology. Text in the conclusions regarding the LCoE is rephrased accordingly. Thank you for the valuable comment.

References:

Falcão, A.F. and Henriques, J.C., 2016. Oscillating-water-column wave energy converters and air turbines: A review. Renewable Energy, 85, pp.1391-1424.

Vicinanza, D., Lauro, E.D., Contestabile, P., Gisonni, C., Lara, J.L. and Losada, I.J., 2019. Review of innovative harbor breakwaters for wave-energy conversion. Journal of Waterway, Port, Coastal, and Ocean Engineering, vol 145-45.

Vicinanza, D., Contestabile, P., Nørgaard, J.Q.H. and Andersen, T.L., 2014. Innovative rubble mound breakwaters for over-topping wave energy conversion. Coastal Engineering, 88, pp.154-170.

• The numerical model and method are not shown in detail

Thank you for this remark. More details regarding the numerical model were added in section 5.3 Model Set-up (see e.g. lines #410 to 423 and lines #467 to 477). Namely, on the details concerning the equations used by Fluent, the discretization schemes and the numerical methods used as well as some more details were included regarding the boundary conditions.

• A time series by CFD analysis is proposed, but a snapshot such as wave elevation needs to be presented

Thanks for the suggestion. Two snapshots were included in Figure 13 and Figure 14. We selected to depict Val_Test02 because it corresponds to the highest wave height across the validation tests and therefore depicting a more obvious and meaningful representation of the free surface elevation.

• Val_Test is not clearly explained. Comparisons were made for very limited analysis conditions

Thank you for your comment. The scope of this paper is the investigation of the performance of an innovative HWEC design in a proof of concept stage. The analysis conditions covered both regular and irregular waves and we believe that the comparative figures presented and the RMSE error presented in Table 1 are sufficient for validation purposes, since all the necessary information during this phase of the project is presented. However, following your recommendation, additional information was added in the manuscript revision in paragraph 5.4.

• On page 12, the reason why the numerical analysis results are not quasi-steady and some oscillatory results should be explained

Indeed. The small oscillations of the free surface elevation that appear in Figure 9 are attributed to the expected reflection from the HWEC device itself. This explanation was also added in the text. Thank you for the suggestion.

• On page 14, what is the reason why the numerical analysis and model test show a big difference after 45 seconds of the water level inside the OWC?

You are absolutely right. After 45 sec the comparison is not as so good as before. This is attributed to the fact that the model is two-dimensional and is not able to capture three dimensional phenomena occurring in the physical model tests. In the experiments it was observed that after some time the 3d effects were not insignificant inside the OWC. More specifically, inside the OWC chamber waves in the transverse direction were created. These waves are responsible for the unsteady pattern of the black wave signal observed after the 45 sec in Figure 16 and cannot be captured by the numerical model. Regarding the wave flume in front of the device the three-dimensional effects during the experiments were not that pronounced after the 45 sec, thus the comparison with the numerical results presented in Figure 15 is much better. This is a paragraph added in the text as well.

• It is necessary to compare the analysis results through dimensionlessness

Thanks for the suggestion. In the results section some extra figures with dimensionlessness variables were added. These figures present the amplification coefficient vs time for all cases compared. The amplification coefficient is derived by dividing the free surface elevation with the significant wave height.

• It is necessary to statistically process the analyzed results and make them dimensionless to compare and discuss the results

While we appreciate your comment, in the manuscript timeseries are compared instead of statistical parameters. Since we aim at replicating the movement of the wavemaker for wave generation, the results are compared wave by wave over time which, in this instance, makes for a more appropriate comparison. It is beyond the scope and the purpose of the manuscript to simulate very long timeseries which would be needed for a statistically processing the results. With the maximum 35 waves that we have analysed this is neither feasible nor more accurate than comparing the wave by wave analysis over time.

• In conclusion, new finding is insufficient and it is difficult to find meaningful conclusions

We respect your comment, but we believe that the modifications done in the manuscript following your insightful suggestions have contributed to clarify the new findings and demonstration the potential of this new concept.

Again, thank you very much for your comments and suggestions, which have much contributed to the bettering of our manuscript.

Reviewer 2 Report

Journal of Marine Science and Engineering

"Proof of concept of a breakwater-integrated Hybrid Wave Energy Converter using a composite modelling approach"

by Theofano Koutrouveli, Enrico Di Lauro, Luciana das Neves, Tomás Calheiros-Cabral, Paulo Rosa-Santos, Francisco Taveira-Pinto

Reviewer’s report

The paper presents a numerical study on a novel wave energy converter embedded in a port breakwater that integrates an Oscillating Water Column device with an Overtopping Device. Furthermore, the paper reports a review of the main technology of WECs embedded with port breakwater.

The argument is of some interest for the scientific community and can be published after a major revision.

Specific comments:

1. L.146-150 - The authors asses that “the wave forces on OWC-integrated caisson breakwaters are smaller than those acting on conventional vertical caisson breakwaters” and that “Goda’s formulae used in conventional structures, tends to underestimate the overturning momentum, which can affect the overall stability”. The two sentences above seem to be at odds with each other. Please clarify such a point.
2. Section 3 – The review of OTD devices lacks recent documents on the performance of this type of device. Hereafter a relevant paper which could be referenced:
   • Iuppa, C., Cavallaro, L., Musumeci, R. E., Vicinanza, D., & Foti, E. (2019). Empirical overtopping volume statistics at an OBREC. Coastal Engineering, 152 doi:10.1016/j.coastaleng.2019.103524
   • Cavallaro L., Iuppa C., Castiglione F., Musumeci R.E., Foti E. A Simple Model to Assess the Performance of an Overtopping Wave Energy Converter Embedded in a Port Breakwater. J. Mar. Sci. Eng. 2020, 8, 858, doi: 10.3390/jmse8110858.
3. L. 350 and L. 372 – The authors refer to two papers by Cabral et al. (Cabral et al., 2020a and b). However, there is only one paper by Cabral et al. within the references. Please check such a point.
4. Section 5.1 and 5.2 – Please provide more details about the physical and numerical model setup. Is the wave tank equipped with a reflection absorbing system at the wave generator? How long are the performed tests?
5. Section 5.3 – Please provide a detailed sketch of the tested device and of the physical and numerical tank
6. L. 455 – The symbol “α“ is repeated twice.
7. L. 498-500 – Please explain in detail the differences found between the results obtained from the physical and numerical model.
8. Figure 14 – Why is the resolution of the grid at the top of the domain so different between Geometry B and C?
9. Section 5.6 – A more detailed analysis of results is needed. How much energy can be produced by the volumes of fluid accumulated in the different tanks?

Author Response

Firstly, we would like to thank the work carried out by the manuscript Reviewer 2. The made comments and suggestions were highly appreciated by the authors and have contributed to improve the quality of the work previously submitted. The detailed responses to the reviewers are presented are presented herein below:

• L.146-150 - The authors asses that “the wave forces on OWC-integrated caisson breakwaters are smaller than those acting on conventional vertical caisson breakwaters” and that “Goda’s formulae used in conventional structures, tends to underestimate the overturning momentum, which can affect the overall stability”. The two sentences above seem to be at odds with each other. Please clarify such a point.

Thank you for the comment. A difference is indeed found from literature regarding the comparisons between measured forces and momentum on the OWC caisson and the Goda formula.

Liu et al. (2011) conducted a two-dimensional physical model test, providing a focus on the stability analysis of the OWC-integrated vertical breakwater. The results from laboratory confirm that the wave forces on OWC caissons are smaller than those acting on vertical caissons. Similar results were obtained by Kuo et al. (2015). The authors conducted physical model tests on an OWC under regular waves, and the results indicated that Goda’s formulas give an overestimation of the maximum resultant forces, compared with the measured data on OWC caissons. Conversely, according to the authors, Goda’s formulation underestimates the momentum due to a different measured pressure distribution over the structure, which could affect the overall OWC caissons stability against the overturning.

The section is modified as follows from line 144-150:

“New design methods for the calculation of the wave forces on OWC front wall were presented by Thiruvenkatasamy et al. (2005), Patterson et al. (2009), Huang et al. (2010). Liu et al. (2011). conducted physical model test, providing a focus on the stability analysis of the OWC-integrated vertical breakwater. The results from these researchers generally confirm that the wave forces on OWC-integrated caisson breakwaters are smaller than those acting on conventional vertical caisson breakwaters. Similar results were obtained by Kuo et al. (2015) who conducted physical model tests on an OWC under regular waves, confirming that Goda’s formulas give an overestimation of the maximum resultant forces, compared with the measured data on OWC caissons. Conversely, according to Kuo et al. (2015), Goda’s formulation underestimates the momentum due to a different measured pressure distribution over the structure, which could affect the overall OWC caissons stability against the overturning”.

The following paper has been also added in the Reference list:

Kuo, Y.S., Lin, C.S., Chung, C.Y. and Wang, Y.K., 2014. Wave loading distribution of oscillating water column caisson breakwaters under non-breaking wave forces. Journal of Marine Science and Technology.

• Section 3 – The review of OTD devices lacks recent documents on the performance of this type of device. Hereafter a relevant paper which could be referenced: Iuppa, C., Cavallaro, L., Musumeci, R. E., Vicinanza, D., & Foti, E. (2019). Empirical overtopping volume statistics at an OBREC. Coastal Engineering, 152 doi:10.1016/j.coastaleng.2019.103524 Cavallaro L., Iuppa C., Castiglione F., Musumeci R.E., Foti E. A Simple Model to Assess the Performance of an Overtopping Wave Energy Converter Embedded in a Port Breakwater. J. Mar. Sci. Eng. 2020, 8, 858, doi: 10.3390/jmse8110858.

Thank you for the suggestion. The relevant papers you suggested are inserted in the new version of the paper.

• L. 350 and L. 372 – The authors refer to two papers by Cabral et al. (Cabral et al., 2020a and b). However, there is only one paper by Cabral et al. within the references. Please check such a point.

Thank you. The following paper has been correctly added in the Reference list:

Cabral, T., Clemente, D., Santos, P.R., Pinto, F.T., Morais, T. and Cestaro, H., 2020b. Evaluation of the annual electricity production of a hybrid breakwater-integrated wave energy converter, Energy, Elsevier, 213, 118845, ISSN: 0360-5442. 15 Dec. doi: 10.1016/j.energy.2020.118845.

• Section 5.1 and 5.2 – Please provide more details about the physical and numerical model setup. Is the wave tank equipped with a reflection absorbing system at the wave generator? How long are the performed tests? The wave generation system used in the physical model test is a single piston-type paddle (HRWallingford, Oxfordshire, UK).

The wave basin in the physical model testing is equipped with a multi-element wave generator from HR Wallingford of 12 wave-paddles having a 0.75m width each. As indicated in Figure 9 the wave basin is partitioned to a wave flume of 1-paddle width. Wave reflections were minimized by a dynamic reflection absorption system integrated in the wavemaker (Cabral et al., 2020a). The duration of the tests was long enough to contain around 100 waves for regular wave tests and around 1000 waves for tests with irregular sea state (Rosa-Santos et al., 2019). The above information’s are included in the new version of the paper. Thank you for the comment. Regarding the numerical model, we have replicate exactly the wave maker motion from the laboratory. The numerical simulations, though, contain from 15-35 waves regarding the case due to high computational cost of CFD simulations. This information was included in the text. Also more, details regarding the numerical model were added in section 5.3 Model Set-up. The details concern the equations used by Fluent, the discretization schemes and the numerical methods used as well as some more details were included regarding the boundary conditions.

• Section 5.3 – Please provide a detailed sketch of the tested device and of the physical and numerical tank

Thank you for the useful suggestion. The numerical model was set in this work to fully replicate the geometry of the wave flume and the integrated HWEC device tested, as described in Cabral et al., 2020a. Figure 9 contains a representation of the physical model setup in the channel built inside the wave basin with the indication of the positions of the wave gauges and device model, while Fig.17 shows a sketch of the different geometries of the HWEC system analysed for the geometrical optimization. Since the experimental work was comprehensively described in previous papers, in order to avoid duplication of contents, we recommend the reader to get more information about the experimental facility and further details of the geometry of the HWEC device tested in laboratory in, for instance, Cabral et al., 2020a. Regarding the numerical model, Figure 4 shows the full numerical model domain, while Figure 6 focuses on the HWEC itself. Also within the text (lines 430-444 and lines 459-467) specific dimensions are mentioned. Figure 6 contains a ruler that can be used also if more dimensions wanted to be found. Thank you for the indications which helped to make the manuscript clearer to the reader.

• L. 455 – The symbol “α“ is repeated twice.

Thank you. We have fixed the typo.

• L. 498-500 – Please explain in detail the differences found between the results obtained from the physical and numerical model.

Thanks for the comment. More things explaining the differences were added in the section 5.4 Model validation. For instance in lines 568 to 589.

• Figure 14 – Why is the resolution of the grid at the top of the domain so different between Geometry B and C?

Thanks for the comment! It was a matter of different zooming in. The figure was replaced and the same zoom for the two geometries is presented now.

• Section 5.6 – A more detailed analysis of results is needed. How much energy can be produced by the volumes of fluid accumulated in the different tanks?

Thank your comment, Section 5.6 has been expanded according. With respect to the energy produced by the HWEC module and parts, while we appreciate the pertinence of your comment we make reference to the following two papers authored by some of the present co-authors and referenced to in the manuscript:

Cabral, T., Clemente, D., Rosa-Santos, P., Taveira-Pinto, F., Morais, T., Belga, F. and Cestaro, H., 2020a. Performance As-sessment of a Hybrid Wave Energy Converter Integrated into a Harbor Breakwater. Energies, 13(1), p.236.

Cabral, T., Clemente, D., Santos, P.R., Pinto, F.T., Morais, T. and Cestaro, H., 2020b. Evaluation of the annual electricity production of a hybrid breakwater-integrated wave energy converter, Energy, Elsevier, Vol.213, 118845, ISSN: 0360-5442. 15 Dec. doi: 10.1016/j.energy.2020.118845.

Again, thank you very much for your pertinent comments and suggestions, which overall contributed to a much improved revised manuscript.

Round 2

Reviewer 1 Report

Your conclusions are still a bit general. It is necessary to present conclusions in more quantitative terms. How good is the proposed model in terms of efficiency? It is necessary to show how much the LCOE is improving. You can see that the model you proposed is better than the existing OWC, but the conclusion is rather general.

Author Response

Firstly, we would like to thank the work carried out by manuscript Reviewer 1; whose comments and suggestions were highly appreciated and have contributed to improve the quality of the work previously submitted.

The detailed responses to the comments received are presented below and shown in the document enclosed in track changes.

Your conclusions are still a bit general. It is necessary to present conclusions in more quantitative terms.

Thank you very much for your comments. We fully agree that conclusions were not sufficiently detailed in the last version of the manuscript. Therefore, after reading the manuscript again, its main topics were summarized and the conclusions enlarged, highlighting the major achievements and their relationship with previous works of the authors. The conclusions have now 700 words, which compares with the 498 of the previous version of the manuscript.

How good is the proposed model in terms of efficiency?

This is a point that is important to highlight. By combining the two wave energy conversion concepts, i.e., the oscillating water column with the overtopping principle, we can have better overall efficiencies than with the individual concepts.

Since the efficiency subject was already discussed in two recent publications (Cabral et al., 2000a,b ) based on the results of experimental tests, we have initially decided to give much attention to it the previous publication. However, we agree that this is an important point that should be mentioned in this manuscript.

Thank you very much for your suggestion.

References:

Cabral, T., Clemente, D., Rosa-Santos, P., Taveira-Pinto, F., Morais, T., Cestaro, H., 2020. “Evaluation of the annual electricity production of a hybrid breakwater-integrated wave energy converter”. Energy, Elsevier, Vol.213, 118845, ISSN: 0360-5442. 15 Dec. doi: 10.1016/j.energy.2020.118845.

Cabral, T., Clemente, D., Rosa-Santos, P., Taveira-Pinto, F., Morais, T., Belga, F., Cestaro, H., 2020. “Performance Assessment of a Hybrid Wave Energy Converter Integrated into a Harbour Breakwater”. Energies, MDPI, Vol.13, Issue 1, 236, pp.1-22; ISSN: 1996-1073. doi: 10.3390/en13010236.

It is necessary to show how much the LCOE is improving.

The LCOE values is in fact a key variable to assess the economic viability of a renewable energy harvesting technology. Nevertheless, as you certainly agree, in a young sector as this one, it is very difficult to assess objectively the LCoE. In this case, it is even more difficult because we are dealing with an innovative wave energy conversion technology.

Based on the expertise of the authors, affiliated to both a consultancy company and also an university, relevant reductions are expected in the LCOE, by the reasons explained in the paper. Nevertheless, due to the uncertainties related mostly with the cost of the civil engineering works (construction of novel structures and adaptations of the breakwater), the authors are not comfortable presenting any figure. Please note also that this manuscript, as well as previous research, is based on the performance of the hybrid WEC converting wave energy.

New experimental works are being prepared as we speak that will provide important contributions to have a realistic estimation of the LCoE in the future, since they include the measurement of pressures in the key parts of the hybrid WEC and a high resolution characterization of the impact of the WEC in the breakwater. Since we do recognize the importance of clarifying these aspects, we have stressed the importance this detailed assessments in the paper.

Thank you very much for you comment.

You can see that the model you proposed is better than the existing OWC, but the conclusion is rather general.

Thank you for noticing that. As mentioned before, we have improved and completed the conclusions section following your comments and suggestions, which were highly appreciated.

Thank you for your time reading again our manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors replied to all my previous issues. Therefore, the paper can be published in present form.

Author Response

We would like to thank the thorough review of our manuscript by Reviewer 2. English language and style has been checked in the revised manuscript.