Experimental Analysis of a Novel Adaptively Counter-Rotating Wave Energy Converter for Powering Drifters
Round 1
Reviewer 1 Report
This paper is well written. The proposed work is novel and original. The introduction is structured very well. However, there are some major concerns regarding the experiments. The following comments may help the authors to improve the quality before its publication:
1. In Page 2, ‘Several articles[,,]’, here the references are not shown. The same problems are found in page 3, as well as some other pages, which should be modified.
2. In page 3, ‘Control strategies for wave energy conversion systems also have been studied in several papers.’ Please put few references related to WEC control, e.g. [1] Maximization of energy absorption for a wave energy converter using the deep machine learning. Energy 165, pp 340-349. [2] Wave force prediction effect on the energy absorption of a wave energy converter with real-time control. IEEE Transactions on Sustainable Energy, DOI: 10.1109/TSTE.2018.2841886.
3. ‘Howerer, when the depth reaches 15 m or over, the fluctuation of water will become extrmely small and it will be equal to the still water in the vertical direction if the depth is even’. This statement is less rigorous and it is not reasonable. In wave theory, the wave length to water depth ratio is usually used to assess whether the wave particle motion is negligible or not. The authors should read more relevant papers.
4. ‘According to research results of Basom[40], the amplitude of fluctuation is about halved at a depth of about 1/9 the wavelength’. Again, when you consider the wavelength, in the real sea, it is usually hundreds meters long. 1/9 of the wavelength is not small at all, which could even larger than your device dimension.
5. ‘3. In the working process, only buoy works on the surface of the sea, while PTO works in the hydrostatic water deep in the sea, which effectively avoids the damage to PTO from complicated sea conditions on the sea surface.’ This statement is your speculation, which is not supported by any data. What is the draft of the PTO? What is the dimension of your floater? What is the length of the cable?
6. ‘The role of Stabilizer vane is to ensure that the motion of PTO is always kept in a vertical direction in the water and prevent PTO from falling due to the phase difference between Buoy and PTO at some point during the movement.’ Again, this statement is your speculation, which is not supported by any data. Did you compare the cases with / without the Stabilizer? When you have some vertical plates, you will have horizontal forces, as well as the yaw moment which twists the cables. Please comment on it.
7. ‘From the previous analysis, it can be known that, the movement of PTO is equal to a vertical motion in still water.’ Please show the data to support it. Otherwise, I suspect this conclusion.
8. In Section 2.2, the second, third, fourth and fifth paragraph are not well written in terms of the language. I could not understand the working principle of the system. Some sentences should be re-written,
9. Figure 7 and the paragraph above are not questionable. When you discuss the submerged depth and the wave particle motion, you should always provide the dimension and wave frequency (wavelength).
10. ‘When the PTO is at a sufficient depth, the motion of the particle representing the PTO can be considered as a linear motion in the vertical direction.’ Again, you should refer to my comments above.
11. My first major concern on experimental test is the full-scale test. It is very challenging as the scale of your device dimension is different from the wavelength scale in real seas. It will be very difficult to generate real ocean waves in your very small tank. Please comment on how you achieve the real-sea wavelength in the small tank.
12. ‘The typical wave spectrum in the ocean of sea state 3 is 0.14–1.0 Hz.’ Please provide which spectrum was used and show the relevant graph on sea state 3.
13. Another major concern is about the real-sea wave height. Please comment on what is the wave height in real sea, e.g. Sea State 3, and how you generate the wave height in full scale in a small tank.
14. My third major concern is the water depth in the wave tank. It is a very shallow water tank. To conduct the full-scale test, how the vertical dimension of the device is achieved? Please provide the full dimension of the device.
15. As the wave-length is large, and the water depth is small. The shallow water theory should be applied. The previous discussions on wave motion are not valid in the shallow water case.
Please address my above concerns, especially 11-14. I will comment more on the results in the next revision. But if the authors could not address the above concerns, my comments on results are not really necessary. The paper is well written anyway. I look forward to see the revisions.
Author Response
Dear Editors and Reviewers:
Thank you for your comments concerning our manuscript entitled “Experimental Analysis of a Novel Adaptively Counter-rotating Wave Energy Converter for Powering Drifters”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researchers. We have studied every suggestion carefully and have made corrections which we hope will meet with approval. Revised portion are highlighted and marked in red in the paper, and all responses and corrections are listed here.
Responses to reviewer 1:
1、 In Page 2, ‘Several articles[,,]’, here the references are not shown. The same problems are found in page 3, as well as some other pages, which should be modified.
◆Response: Thank you for your corrections. Due to some errors occurred during the format conversion of the manuscript, many marked numbers cannot be displayed in the paper. These formatting errors have been corrected.
◆Correction:
Several articles[7,8,9] have studied the feasibility for supplying power to floaters with thermal power.
And several research papers relating to ocean wave energy conversion have been emerged in an endless stream[17,18,19,20].Control strategies for wave energy conversion systems also have been studied in several papers[21,22,23,24].
2、 In page 3, ‘Control strategies for wave energy conversion systems also have been studied in several papers.’ Please put few references related to WEC control, e.g. [1] Maximization of energy absorption for a wave energy converter using the deep machine learning. Energy 165, pp 340-349. [2] Wave force prediction effect on the energy absorption of a wave energy converter with real-time control. IEEE Transactions on Sustainable Energy, DOI: 10.1109/TSTE.2018.2841886.
◆Response: Thank you for your suggestions. According to your comments, the corresponding literatures are changed to more appropriate pioneer work you provided.
◆Correction:
[21] Liang Li, Zhiming Yuan, Yan Gao, Maximization of energy absorption for a wave energy converter using the deep machine learning, Energy, Volume 165, Part A,2018,Pages 340-349.
[22] L. Li, Z. Yuan, Y. Gao and X. Zhang, "Wave Force Prediction Effect on the Energy Absorption of a Wave Energy Converter With Real-Time Control," in IEEE Transactions on Sustainable Energy, vol. 10, no. 2, pp. 615-624, April 2019.
[23]. L Wang, J Isberg, E Tedeschi. Review of control strategies for wave energy conversion systems and their validation: the wave-to-wire approach. Renewable and Sustainable Energy Reviews 81, 366-379. 2018.
[24]. Romain Genest, Josh Davidson, John Vincent Ringwood. Adaptive control of a wave energy converter. IEEE Transactions on Sustainable Energy. DOI: 10.1109/TSTE.2018.2798921. 2018.
3、 ‘Howerer, when the depth reaches 15 m or over, the fluctuation of water will become extrmely small and it will be equal to the still water in the vertical direction if the depth is even’. This statement is less rigorous and it is not reasonable. In wave theory, the wave length to water depth ratio is usually used to assess whether the wave particle motion is negligible or not. The authors should read more relevant papers.
◆Response and ◆Correction: We’re sorry for not making it clear enough.
The point absorber typically use a steel cable to secure one end to the sea floor or to a stationary platform, which are considered the reference coordinates of the system. When the ocean waves drive the buoy to move relative to the reference coordinates, the wave energy in the waves is captured and converted into mechanical energy. Similarly, the root cause of the counter–rotating self–adaptable WEC is the interaction between water particles and absorber blades. Only when there is relative motion between the water particles and the blades, the blades can obtain torque under the push of water particles. The phrase " the fluctuation of water will become extrmely small and it will be equal to the still water in the vertical direction if the depth is even " is to illustrate that the absorber in the deeper position has a relative motion with the surrounding water particles.
As shown in Figure 1, assuming that the wave is an ideal deep water wave, the displacement of the wave surface particle in the vertical direction is H, and the displacement of the water particle at the depth L is h in the vertical direction. The displacement of the PTO in the vertical direction driven by the buoy is about H, and the displacement of the PTO relative to the surrounding water particles is . Thus water particles passing through the PTO can push the PTO blades to generate torque. In the case where the wave height H and the period are constant, when the water depth L is larger, h is smaller, and is larger, the more the water particles passing through the PTO, the more the absorber absorbs the wave energy.
Figure 1. Motion of a particle and WEC in an ocean wave
4、 ‘According to research results of Basom[40], the amplitude of fluctuation is about halved at a depth of about 1/9 the wavelength’. Again, when you consider the wavelength, in the real sea, it is usually hundreds meters long. 1/9 of the wavelength is not small at all, which could even larger than your device dimension
◆Response and ◆Correction: We’re sorry for not making it clear enough.
Ocean waves can be considered as the superposition of many cosine waves, the characteristics of the individual wave components can be analyzed by the wave spectrum. As shown in Fig. 2, the typical wave spectrum of the wind wave shows that the spectral density at the lower and higher circular frequencies is very low. According to the Dispersion Relation of deep water waves: . When the sea state is low, the wave component with a longer wavelength has a lower wave height and less influence on the underwater absorber.
Figure 2. The typical wave spectrum of the wind wave
When there is a wave with a large wavelength and a high wave height, the sea state is higher. The relative displacement of the PTO to the surrounding water particles is not so significant. The proportion of such cases is relatively low, and in most cases the absorber is able to capture enough energy for the SVP drifter.
5、 ‘3. In the working process, only buoy works on the surface of the sea, while PTO works in the hydrostatic water deep in the sea, which effectively avoids the damage to PTO from complicated sea conditions on the sea surface.’ This statement is your speculation, which is not supported by any data. What is the draft of the PTO? What is the dimension of your floater? What is the length of the cable?
◆Response and ◆Correction: Thank you for your suggestions. We’re sorry for the imprecision of our statement. Since the absorber is connected to the buoy through the steel cable, the PTO will definitely be affected by the waves. Due to the large number of objects involved, the damage relationship between the PTO and the waves is complicated, and it is difficult to draw a clear conclusion through simple discussion, and this is not the focus of this article, so delete the statement.
The buoy is a sphere of radius 625px, the size of the absorber is shown in figure 3.
Figure 3. Wave energy-powered SVP drifter concept with counter–rotating self–adaptable WEC
6、 ‘The role of Stabilizer vane is to ensure that the motion of PTO is always kept in a vertical direction in the water and prevent PTO from falling due to the phase difference between Buoy and PTO at some point during the movement.’ Again, this statement is your speculation, which is not supported by any data. Did you compare the cases with / without the Stabilizer? When you have some vertical plates, you will have horizontal forces, as well as the yaw moment which twists the cables. Please comment on it.
◆Response and ◆Correction: Thank you for your suggestions. We’re sorry for the unreasonable design of PTO.
After removing the stabilizer vane, the power generation experiment was carried out again in the wave tank. But the effect of the stabilizer vane on the power generation performance was not obvious because the experimental environment was not conducive to direct observation of the role of the stabilizer vane. The function of the stabilizer vane is analyzed by simulation, and through simulation comparison, it can be found that the addition of stabilizer vane does not enhance the performance of the whole converter, and even weakens the performance slightly.
Figure 4. Comparative simulation results on the stabilizer vane of the absorber
The installed stabilizer vane exacerbate the blocking effect of impinging water flow. The stabilizer vane divides the recirculating flow region into four sub-recirculating flow regions. The eddy currents in these sub-recirculating flow regions aggravate the flow pattern change of the impinging water flow and cause a larger loss of the input kinetic energy of the downstream absorber.
Therefore, all the diagrams about the absorber have been changed:
Figure 4. Wave energy-powered SVP drifter concept with counter–rotating self–adaptable WEC
Figure 6. Design concept and mechanical structure:
Figure 7. Working principle: (a, c) Rising process; (b) Sinking process
Figure 10. Experimental platform in a wave tank.
7. ‘From the previous analysis, it can be known that, the movement of PTO is equal to a vertical motion in still water.’ Please show the data to support it. Otherwise, I suspect this conclusion.
◆Response: We’re sorry for the imprecision of our statement. Whether the reciprocating motion of the PTO can be approximated as the vertical motion in the still water is related to the wave parameters and the depth of the PTO. Only when the wavelength of the surface wave is short and the depth of PTO is greater than the significant wavelength of the wave, the movement of water particles near the PTO is very small, the water near the PTO can be approximated as a still water.
The buoy is affected by the horizontal component of the wave force and will deviate from the origin position in the horizontal direction. Assuming that the horizontal position of the PTO is unchanged, there is an angle between the tether and the vertical direction.
Therefore, increasing the length of the tether can mitigate the effect of the horizontal component of the wave motion on the WEC. The wave height range in sea state 3 is 0.5-1.25m, and the depth of the PTO is 30m, then the maximum angle is .
It can be seen that the horizontal component of the wave motion has little effect on the PTO. In addition, the lateral cross-sectional area of the drogue is large, and the lateral damping is large in the seawater. Therefore, it is difficult for the wave to change the horizontal position of the PTO through the buoy and cable. The moving direction of the PTO can be approximated as vertical. The assumption about the horizontal position of the PTO is reasonable.
Figure 5. The effect of wave motion on the position of PTO.
◆Correction:
8. In Section 2.2, the second, third, fourth and fifth paragraph are not well written in terms of the language. I could not understand the working principle of the system. Some sentences should be re-written,
◆Response: We’re sorry for not making it clear enough.
◆Correction:
The absorber captures wave energy mainly through a group of blades composed of of 8 blades. Each blade can swing freely around its own axis over a range of angles. As shown in Figure 8(a), the is the free swing angle range of one of the blades, which is about . The angular direction of the blade is always the same as the direction in which the water flow impacts the blade group. The direction of the upper blade group and lower blade group is clockwise and counterclockwise, as shown in Figure 8(b). When the water flows through the absorber, the two groups of blades are respectively driven to rotate clockwise and counterclockwise, and their difference in rotational speed is the input speed of the generator. Detailed structural design is introduced in the next section.
Figure 6. Basic function and form of the blade
The Buoy moves up and down under the action of wave force and it drives underwater PTO to move in a heave motion through Power cable. The whole working process is divided into two processes.
Process 1 is the process of the buoy being pushed by the wave from the trough to the crest. The underwater PTO is dragged by the Power cable to rise with the surface Buoy. All blades are adaptively swung down to the maximum deflection angle under the influence of water flow, as shown in Figure 11(a). Due to the limitation of the adjustable ring, the blades keep in the inclined state after reaching the maximum angle of inclination. The water body continues to impact the inclined blades and produces thrust to push the blades forward. Since the blades are circumferentially arranged, the UBG rotates clockwise. Likewise, the lower blade group (LBG) rotates counterclockwise.
Process 2 is the process of the whole device descending from the crest to the trough under its own gravity. The Buoy sinks, the underwater PTO also sinks because of gravity force. All blades are adaptively swung up to the maximum deflection angle under the influence of water flow, as shown in Figure 11(b). The water body continues to impact the inclined blades and produce thrust to push the blades forward. The UBG continues to rotate clockwise without changing direction. Likewise, the LBG keeps rotating counterclockwise.
When the whole device hits another crest, as shown in Figure 7(c), the process 1 will be repeaded. When the surface Buoy comes to a balanced position (The maximum of the crest and the minimum of the trough.), the blades are in the swinging process and unable to provide thrust. Therefore, the UBG and LBG will maintain the clockwise and counterclockwise rotation direction respectively because of inertia.
Based on the above analysis, it can be drawn that, no matter in rising process or sinking process, the PTO can passively adjust the angle of blades to ensure upper and lower blade group rotating in clockwise and counterclockwise respectively, thus ensuring the PTO’s stability of absorption process. Meanwhile, the double–layered blade groups can automatically balance the overall torque of the underwater PTO. This WEC takes three main steps to absorb wave energy. (1) Buoy absorbs the kinetic energy of the waves and transforms it into kinetic energy of the PTO’s heaving motion; (2) The kinetic energy of the PTO transforms into rotating mechanical energy of UBG and LBG; (3) The UBG and LBG drive the generator to generate electricity.
9. Figure 7 and the paragraph above are not questionable. When you discuss the submerged depth and the wave particle motion, you should always provide the dimension and wave frequency (wavelength).
◆Response and ◆Correction: Thank you for your suggestions.
Figure 7. Working principle: (a, c) Rising process; (b) Sinking process
10. ‘When the PTO is at a sufficient depth, the motion of the particle representing the PTO can be considered as a linear motion in the vertical direction.’ Again, you should refer to my comments above.
◆Response and ◆Correction: We’re sorry for the imprecision of our statement. Please refer to the explanation of question 7.
11. My first major concern on experimental test is the full-scale test. It is very challenging as the scale of your device dimension is different from the wavelength scale in real seas. It will be very difficult to generate real ocean waves in your very small tank. Please comment on how you achieve the real-sea wavelength in the small tank.
◆Response: Thank you for your suggestions. We still can't solve this problem now.
Because the deep-sea environment in which the drifters are located is very special, it is difficult to simulate a deep-sea environment in the laboratory. Due to space and cost constraints, most wave-making pools are smaller in size and generate waves with lower wave heights.
12. ‘The typical wave spectrum in the ocean of sea state 3 is 0.14–1.0 Hz.’ Please provide which spectrum was used and show the relevant graph on sea state 3.
◆Response and ◆Correction: Thank you for your suggestions. Currently, the theoretical spectrum of ocean waves commonly used includes: Bretschneider Spectrum, Pierson-Moskowitz(P-M) Spectrum and JONSWAP Spectrum. These spectra are usually fitted according to the measured wave data in a specific sea area, which can better express the wave energy distribution in some sea areas. In this paper, the classification of sea states is based on Pierson-Moskowitz spectrum.
Pierson-Moskowitz Sea Spectrum
Sea state | Significant wave height (meters) | Average Length of Waves (meters) | Significant Range of Periods (sec) | Average Period (sec) |
1 | 0 - 0.1 | 4 | 1 - 4 | 1.5 |
2 | 0.1 - 0.5 | 9 | 1.5 - 6 | 3 |
3 | 0.5 – 1.25 | 17 | 2 – 7.5 | 4 |
4 | 1.25 - 2.5 | 27.5 | 2.5 – 9.5 | 5 |
5 | 2.5 - 4 | 39 | 3 - 11 | 6.5 |
6 | 4 - 6 | 68 | 4 – 15.5 | 8 |
7 | 6 - 9 | 130 | 5.5 - 22 | 11 |
Figure 8. Sea State Wave Characteristics and Average Global Distribution
13. Another major concern is about the real-sea wave height. Please comment on what is the wave height in real sea, e.g. Sea State 3, and how you generate the wave height in full scale in a small tank.
◆Response: We’re sorry for not making it clear enough.
The wave height in the real ocean is irregular. Referring to the P-M spectrum, when the sea state is 3, the wave height in the real ocean is 0.5-1.25 meters. The maximum wave height that can be generated in the wave tank is 0.6 meters, so it cannot generate the wave height in full scale in a small tank. In order to enable the wave tank to simulate the corresponding wave height of the sea state 3, the geometry of the buoy and the absorber is reduced by 50%. According to the similarity principle of fluid, the wave height generated by the wave tank can be approximated as a proportional reduction of sea state 3 wave height.
Pierson-Moskowitz Sea Spectrum
Sea state | Significant wave height (meters) | Average Length of Waves (meters) | Significant Range of Periods (sec) | Average Period (sec) |
3 | 0.5 – 1.25 | 17 | 2 – 7.5 | 4 |
4 | 1.25 - 2.5 | 27.5 | 2.5 – 9.5 | 5 |
5 | 2.5 - 4 | 39 | 3 - 11 | 6.5 |
14. My third major concern is the water depth in the wave tank. It is a very shallow water tank. To conduct the full-scale test, how the vertical dimension of the device is achieved? Please provide the full dimension of the device.
◆Response and ◆Correction: We’re sorry for not making it clear enough. We still can't solve this problem now.
Due to cost and space constraints, it is difficult to achieve full-scale experiments in wave tank, especially full-scale simulation experiments in the vertical direction. As shown in Figure 9, in order to increase the credibility of the experiment as much as possible, while reducing the size of the prototype, the wave tank was also modified. At the end close to the wave machine, a pit with a length of 2 meters and a depth of 0.8 meters was dug. Therefore, the vertical length of the WEC can be increased to 1.2 meters. Of course, this is far from the ideal vertical size, and I hope to carry out the full-scale experiment in the subsequent sea experiments.
Figure 9. Experimental schematic.
15. As the wave-length is large, and the water depth is small. The shallow water theory should be applied. The previous discussions on wave motion are not valid in the shallow water case.
◆Response and ◆Correction: We’re sorry for the imprecision of our statement. The theory of deep water wave is adopted in the paper, which is mainly selected according to the sea conditions of the SVP drifters. It is difficult to simulate the actual sea states in the test, and the actual boundary conditions are different. The wave tank experiment is only used to prove the feasibility of the WEC in the power generation and to find some key parameters that have a great impact on performance. Therefore, no additional theoretical analysis of the wave cell test has been performed.
Thank you for all your suggestions again.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors presented the article "Experimental Analysis of a Novel Adaptively Counter-rotating Wave Energy Converter for Powering Drifters" to the journal JMSE and in my opinion it is within the aim of the journal. For that reason I consider that it can be accepted for the review process.
The subject is very interesting and actual and brings a significative advance in the actual knowledge about these sort of technologies.
I compared both versions of the article and at this point V2 is much more improved and more close to be in the publication form.
I don't feel qualified enough to judge the english language level. However small arrangements can be done and small corrections are need as well.
Authors must try to better explain in the introductory section the importance of this study and the impact that it may have in future developments.
This must be reinforced in the conclusions as well.
The state-of-the-art section must be improved and some recent references are missing.
Author Response
Dear Reviewer:
Thank you for your comments concerning our manuscript entitled “Experimental Analysis of a Novel Adaptively Counter-rotating Wave Energy Converter for Powering Drifters”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researchers. We have studied every suggestion carefully and have made corrections which we hope will meet with approval. Revised portion are highlighted and marked in red in the paper, and all responses and corrections are listed here.
1. However small arrangements can be done and small corrections are need as well.
◆Response: Thank you for your suggestions. Due to some errors occurred during the format conversion of the manuscript, many marked numbers cannot be displayed in the paper. These formatting errors have been corrected.
About arrangements:
Before the proposal was put forward, the sea states in which the drifters were located was analyzed.
Added a small section in the second chapter. （2.1 Basic mechanism.）
Some sentences have been re-written about the working principle of the system.
◆Correction:
Several articles[7,8,9] have studied the feasibility for supplying power to floaters with thermal power.
And several research papers relating to ocean wave energy conversion have been emerged in an endless stream[17,18,19,20].Control strategies for wave energy conversion systems also have been studied in several papers[21,22,23,24].
2. Authors must try to better explain in the introductory section the importance of this study and the impact that it may have in future developments.
◆Response: Thank you for your suggestions.
◆Correction:
Figure 4. Sea State Wave Characteristics and Average Global Distribution
Referring to Figure 2 and Figure 4, the distribution of SVP drifters and the sea state at their location can be known. The sea state at most drifters location is greater than 2, so when the wave energy device can generate a certain amount of electricity under low sea state to meet the needs of the drifter, the wave energy device can solve the problem of the drifter's battery life.
Considering the working environment and energy demands of drifters, this paper proposes a counter–rotating self–adaptable wave energy converter (WEC) for powering drifters. The Wave energy-powered SVP drifter concept is illustrated in Figure 4 for a counter–rotating self–adaptable PTO module is fixed under the drogue. The WEC uses an underwater absorber to convert the reciprocating motion in the vertical direction into the relative rotation of the two blade groups of the absorber to drive the generator to generate electricity. The buoy is a sphere of radius 625px, the size of the absorber is shown in the figure. The drogue can keep the drifter follows the movements of the water and is unaffected by wind and instruments, reduce slippage between the drifter package and the water[4], The drifter is mainly used to investigate ocean currents and other parameters like temperature or salinity, common electrical devices include radio frequency transmitter, atmospheric pressure sensor, temperature sensor and salinity sensor. Therefore, the WEC placed at a depth of about 30 meters from the ocean surface will hardly affect the measurement results of sensors near the wave surface.
The WEC is a point absorber with the advantages of easy installation on SVP drifters and simple power generation process. Through this simple and effective integration, it will provide continuous power supply to SVP drifter and will hardly affect the monitoring results. It is expected to be an efficient way for improving the sampling intervals of drifters on the base of easy integration method.
Figure 5. Wave energy-powered SVP drifter concept with counter–rotating self–adaptable WEC
Thank you for all your suggestions again.
Author Response File: Author Response.pdf
Reviewer 3 Report
Pag. 1 line 35-36: the authors cannot start the sentence with a reference. Please check all the document.
Pag. 3 line 79-82-85-88-92: [] reference is missing. Please check all the document.
Pag. 4 line 120-124: wave basin and not wave pool.
Pag. 8 line 241: the wave length could be 100 meters. The PTO is located at one wave length ?
In my opinion the mechanical structure (WEC) strongly affect the lagrangian measurements. This could be only verified in real field comparing a standard drifter with this innovative one. The wave tank cannot give us this answer. Please introduce all the problems related with this in the discussion. The system will produce energy but this will affect wave and currents measurements.
See the following paper:
Design of oscillating-water-column wave energy converters with an application to self-powered sensor buoys
J.C.C. Henriques, J.C.C. Portillo, L.M.C. Gato, R.P.F. Gomes, D.N. Ferreira, A.F.O. Falcao
Energy
Volume 112, 1 October 2016, Pages 852-867
dx.doi.org/10.1016/j.energy.2016.06.054
For wave buoy under lagrangian drifter program http://gdp.ucsd.edu/ldl_drifter/index.html inclusing wave measurements see:
Contestabile, P., Ferrante, V., Di Lauro, E., Vicinanza, D. (2016). "Prototype Overtopping Breakwater for Wave Energy Conversion at Port of Naples", Proceedings of the 26th International Conference ISOPE, Rhodes, Greece, ISBN 978-1-880653-88-3; ISSN 1098-6189, pg. 616-621.
Centurioni, L., Di Lauro, E., Braasch L., Contestabile, P., De Leo, F., Casotti, R., Franco, L., Vicinanza, D. (2017). "A new strategic wave measurement station off Naples port main breakwater", Proceedings of the 35 International Conference on Coastal Engineering, ISBN: 978-0-9896611-3-3, ISSN: 2156-1028, pg. 1-12, Antalya, Turkey, 2016. https://doi.org/10.9753/icce.v35.waves.36 and https://journals.tdl.org/icce/index.php/icce/article/view/8144
Contestabile, P., Ferrante, V., Di Lauro, E., Vicinanza, D. (2017). "Full-scale prototype of an overtopping breakwater for wave energy conversion", Proceedings of the 35 International Conference on Coastal Engineering, ISBN: 978-0-9896611-3-3, ISSN: 2156-1028, pg. 1-12, Antalya, Turkey, 2016. https://doi.org/10.9753/icce.v35.structures.12 and https://journals.tdl.org/icce/index.php/icce/article/view/8119
Author Response
Dear Reviewer:
Thank you for your comments concerning our manuscript entitled “Experimental Analysis of a Novel Adaptively Counter-rotating Wave Energy Converter for Powering Drifters”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researchers. We have studied every suggestion carefully and have made corrections which we hope will meet with approval. Revised portion are highlighted and marked in red in the paper, and all responses and corrections are listed here.
1. Pag. 1 line 35-36: the authors cannot start the sentence with a reference. Please check all the document. Pag. 3 line 79-82-85-88-92: [] reference is missing. Please check all the document.
◆Response: Thank you for your corrections. Due to some errors occurred during the format conversion of the manuscript, many marked numbers cannot be displayed in the paper. These formatting errors have been corrected.
◆Correction:
Several articles[7,8,9] have studied the feasibility for supplying power to floaters with thermal power.
And several research papers relating to ocean wave energy conversion have been emerged in an endless stream[17,18,19,20].Control strategies for wave energy conversion systems also have been studied in several papers[21,22,23,24].
2. Pag. 4 line 120-124: wave basin and not wave pool.
◆Response: Thank you for your corrections.
◆Correction:
These spelling errors are corrected.
Page 5, paragraph 1, line 10, changing “pool” to “basin”;
Page 5, paragraph 1, line 14, changing “pool” to “basin”;
Page 11, last paragraph, line 7, line 8, line 9, changing “pool” to “basin”;
3. Pag. 8 line 241: the wave length could be 100 meters. The PTO is located at one wave length ?
◆Response: We’re sorry for the imprecision of our statement.
◆Correction:
The depth of the PTO is about 30 meters.
Figure 1. Wave energy-powered SVP drifter concept with counter–rotating self–adaptable WEC
As shown in Figure 1, assuming that the wave is an ideal deep water wave, the displacement of the wave surface particle in the vertical direction is H, and the displacement of the water particle at the depth L is h in the vertical direction. The displacement of the PTO in the vertical direction driven by the buoy is about H, and the displacement of the PTO relative to the surrounding water particles is . Thus water particles passing through the PTO can push the PTO blades to generate torque. In the case where the wave height H and the period are constant, when the water depth L is larger, h is smaller, and is larger, the more the water particles passing through the PTO, the more the absorber absorbs the wave energy.
Figure 2. Motion of a particle and WEC in an ocean wave
4. In my opinion the mechanical structure (WEC) strongly affect the lagrangian measurements. This could be only verified in real field comparing a standard drifter with this innovative one. The wave tank cannot give us this answer. Please introduce all the problems related with this in the discussion. The system will produce energy but this will affect wave and currents measurements.
See the following paper:
Design of oscillating-water-column wave energy converters with an application to self-powered sensor buoys
J.C.C. Henriques, J.C.C. Portillo, L.M.C. Gato, R.P.F. Gomes, D.N. Ferreira, A.F.O. Falcao
Energy Volume 112, 1 October 2016, Pages 852-867
dx.doi.org/10.1016/j.energy.2016.06.054
◆Response and ◆Correction: We’re sorry for the imprecision of our statement.
Due to cost and space constraints, it is difficult to achieve full-scale experiments in wave tank, especially full-scale simulation experiments in the vertical direction.
Figure 3. Experimental schematic.
The wave tank can't test the influence of WEC device on drifters. Since the absorber is connected to the buoy through the steel cable, the buoy will definitely be affected by the PTO. Due to the large number of objects involved, the comparative study of the standard drifter and a standard drifter with this innovative one need a lot of experimentation and data analysis, our current experimental conditions and data are not enough, it is difficult to draw a clear conclusion through simple discussion. In this article, we only tested the power generation effect of WEC and hope to solve the integration problem between WEC and drifters in ocean experiments.
The SVP drifter is mainly used to investigate ocean currents and other parameters like temperature or salinity, common electrical devices include radio frequency transmitter, atmospheric pressure sensor, temperature sensor and salinity sensor. Therefore, the WEC placed at a depth of about 30 meters from the ocean surface will hardly affect the measurement results of sensors near the wave surface. Of course, in order to eliminate the influence of the absorber on the sea surface buoy as much as possible, the integration method shown in Figure 4 can also be selected. The SVP drifter and the WEC device respectively have their own buoys, and the two buoys are connected by a cable to transmit electricity.
Figure 4. Wave energy-powered SVP drifter concept with counter–rotating self–adaptable WEC
Thank you for all your suggestions again.
Author Response File: Author Response.pdf
Reviewer 4 Report
A lot of the references are missing in the text body. I believe this may be an unintended oversight. This should be rectified
I am concerned about the ability of the drifter to gather adequate data(wave height and period) since it will be affected by the overall device (PTO. cable motion). A study should be shown for example to see the data capturing of a stand alone device and one that is connected to a PTO and see the deviation
Recent studies show that buoy motion and power capture are affected by cable length, hence the optimal length needs to be adequately calculated
I am also concerned about the PTO reliability during resonance. Was this investigated?
Author Response
Dear Reviewer:
Thank you for your comments concerning our manuscript entitled “Experimental Analysis of a Novel Adaptively Counter-rotating Wave Energy Converter for Powering Drifters”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researchers. We have studied every suggestion carefully and have made corrections which we hope will meet with approval. Revised portion are highlighted and marked in red in the paper, and all responses and corrections are listed here.
1. A lot of the references are missing in the text body. I believe this may be an unintended oversight. This should be rectified
◆Response: Thank you for your corrections. Due to some errors occurred during the format conversion of the manuscript, many marked numbers cannot be displayed in the paper. These formatting errors have been corrected.
◆Correction:
Several articles[7,8,9] have studied the feasibility for supplying power to floaters with thermal power.
And several research papers relating to ocean wave energy conversion have been emerged in an endless stream[17,18,19,20].Control strategies for wave energy conversion systems also have been studied in several papers[21,22,23,24].
2. I am concerned about the ability of the drifter to gather adequate data(wave height and period) since it will be affected by the overall device (PTO. cable motion). A study should be shown for example to see the data capturing of a stand alone device and one that is connected to a PTO and see the deviation
◆Response: Thank you for your suggestions. SVP drifters only have a few specific sensors that cannot be used to measure data such as wave heights and periods.
◆Correction:
The drifter is mainly used to investigate ocean currents and other parameters like temperature or salinity, common electrical devices include radio frequency transmitter, atmospheric pressure sensor, temperature sensor and salinity sensor. SVP drifters only have a few specific sensors that cannot be used to measure data such as wave heights and periods. Therefore, the WEC placed at a depth of about 30 meters from the ocean surface will hardly affect the measurement results of sensors near the wave surface.
Of course, in order to eliminate the influence of the absorber on the sea surface buoy as much as possible, the integration method shown in Figure 1 can also be selected. The SVP drifter and the WEC device respectively have their own buoys, and the two buoys are connected by a cable to transmit electricity.
Figure 1. Wave energy-powered SVP drifter concept with counter–rotating self–adaptable WEC
3. Recent studies show that buoy motion and power capture are affected by cable length, hence the optimal length needs to be adequately calculated. I am also concerned about the PTO reliability during resonance. Was this investigated?
◆Response: Thank you for your suggestions.
◆Correction:
As shown in Figure 1, assuming that the wave is an ideal deep water wave, the displacement of the wave surface particle in the vertical direction is H, and the displacement of the water particle at the depth L is h in the vertical direction. The displacement of the PTO in the vertical direction driven by the buoy is about H, and the displacement of the PTO relative to the surrounding water particles is . Thus water particles passing through the PTO can push the PTO blades to generate torque. In the case where the wave height H and the period are constant, when the water depth L is larger, h is smaller, and is larger, the more the water particles passing through the PTO, the more the absorber absorbs the wave energy. So in an ideal situation, the longer the cable, the better.
Thank you for all your suggestions again.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The quality of the paper has been improved. However, my major concern still holds. When generating the waves under sea state 3 in your ‘small and shallow’ wave tank, the waves must be regarded as shallow water waves. Your discussions and conclusions based on the tank test are only valid in your specific testing condition in your tank. They are not valid generally in real sea conditions. In the other words, you could not demonstrate its feasibility by using the test data. Please put these comments to your manuscript. I will leave the comments to the editor to decide whether this paper can be published or not.
Author Response
Dear Reviewer:
Thank you for your comments concerning our manuscript entitled “Experimental Analysis of a Novel Adaptively Counter-rotating Wave Energy Converter for Powering Drifters”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researchers. Revised portion are highlighted and marked in red in the paper, and responses and corrections are listed here.
1、 The quality of the paper has been improved. However, my major concern still holds. When generating the waves under sea state 3 in your ‘small and shallow’ wave tank, the waves must be regarded as shallow water waves. Your discussions and conclusions based on the tank test are only valid in your specific testing condition in your tank. They are not valid generally in real sea conditions. In the other words, you could not demonstrate its feasibility by using the test data. Please put these comments to your manuscript. I will leave the comments to the editor to decide whether this paper can be published or not.
◆Response:
Thank you for your comments and suggestions. We are sorry that the last reply did not eliminate your major concerns. Because the water depth of the wave tank is small, the waves in the tank are actually shallow waves. So the actual experimental conditions are indeed different from the application imagination and analysis of the WEC. Due to our limited experimental conditions, we are currently only able to test in ‘small and shallow’ wave tank with semi-scale WEC model. (In the previous manuscript, we made a mistake in the introduction of the WEC prototype. The prototype is a semi-scale model, not a full-scale model.) Therefore, we can only do small-scale prototype power generation experiments in shallow water waves.
The power generation principle of the novel adaptively counter-rotating wave energy converter is based on the difference in the vertical displacement of the water particles at a surface wave and the water particles at a certain depth. Shallow water waves and deep water waves have some similar properties, for example, the movement of water particles has a significant attenuation in the vertical direction with the increase of water depth. According to the power generation principle of the novel adaptively counter-rotating wave energy converter, the results of wave tank experiments should reflect the feasibility of the WEC in principle.
Figure 1. WEC's 3D design model and test prototype
Of course, the wave tank as a test site for ACWEC has great limitations and lacks suitable means to simulate real ocean waves. For example, the wave tank can hardly simulate deep water waves, hardly eliminate the influence of the wave tank wall on the waves. The length of the water tank is limited. After the wave hits the end wall, it will bounce back to affect the waveform. Limited water depth makes it impossible to connect a long rope between the buoy and the absorber. After completing a larger experimental prototype and obtaining better experimental support, we plan to conduct power generation tests of the new WEC in the ocean in order to obtain more detailed and rigorous experimental results.
Thanks again for your corrections and suggestions. We have written these comments on the manuscript.
◆Correction:
Due to the limitation of the size of the wave tank, the wave generated in the wave tank is different from the ocean wave, for example, deep water waves cannot be generated in the wave tank, and the water depth of the wave tank limits the position that the absorber can only be placed close to the water surface. The semi-scale WEC experiment in the wave tank is difficult to simulate the case of ACWEC in a real ocean. Experimental conclusions are based on the specific wave conditions in the wave tank. The wave tank experiment can only qualitatively verify the feasibility of the ACWEC and the influence of key parameters on the power generation performance of ACWEC. In order to obtain more detailed and rigorous experimental results, the experiment of the full-scale ACWEC in the ocean needs to be carried out after completing a larger experimental prototype and obtaining better experimental support.
Author Response File: Author Response.pdf
