Wind–Water Experimental Analysis of Small SC-Darrieus Turbine: An Approach for Energy Production in Urban Systems

Round 1

Reviewer 1 Report

This manuscript can be accepted for publication after major revisions, see the followings:

*The introduction should be improved.
*English should be improved.
*The Abstract should be improved.
*The References should be updated.
*Better description and explanation of figures 10 to 15.
*There are some typing errors and inaccuracies in the manuscript. Please, check the paper again for any possible misprints.

*The quality of all figures should be improved.

*The validation section should be added.
* The conclusion should be improved.

Author Response

*The introduction should be improved.

- The introduction section has been shorted and improved.

*English should be improved.

- The English of the article has been reviewed.

*The Abstract should be improved.

- Yes, it has been improved, highlighting the main findings of the investigation.

*The References should be updated.

- Most of the references are from the previous five years. Also, four other references have been included in the introduction section.

*Better description and explanation of figures 10 to 15.

- According to your suggestion, the explanation of figures from 10 to 15 has been clarified in the new version.

*There are some typing errors and inaccuracies in the manuscript. Please, check the paper again for any possible misprints.

-Yes, you are right, in the revised version, all typing errors have been eliminated.

*The quality of all figures should be improved.

-All figures have been replaced with high resolution ones.

*The validation section should be added.

- The validation has been performed thanks to a published research (Patel et al. 2017) that has been performed with a turbine of similar solidity and blade profile (NACA-0015).  Patel's results in open-waters are close to those obtained in the wind tunnel and water flume experimental tests, in this case, eliminating the effect of blockage. The validation has been included in the results section (Figure. 15) due to the importance of the discussion about the different characteristics.

*The conclusion should be improved.

The conclusion section has been improved highlighting the main findings of the investigation.

Reviewer 2 Report

The authors studied a prototype model of SC Darrieus turbine both in wind and water and tried to demonstrate the effect of Blockage Ratio on power coefficient. 

Though similar studies (separately for wind and water) can be found in the literature, this manuscript can still be considered for publication. However, I request the authors to address the following issues: 

It is suggested to add a Table that will summarize all the test cases for easy referencing. I found it a bit difficult to follow all the experimental cases that are run in wind and waves.

The values for performance coefficient in water at various blockage ratios, especially for higher values, appear a bit unrealistic to me (considering other similar studies found in the literature).  I was wondering how the authors ensured that their measurements are accurate enough. It will be good if some information on the calibration of the instruments is added. Besides, figure 15 shows the corrections for the 20% blockage ratio only. It will be interesting to see how the corrected curves change for other blockage ratios.

It is expected that the turbine will produce higher Cp at higher velocities and for higher blockage ratios. Therefore, it is expected that the authors highlight the contributions of their work explicitly (other than these two obvious facts).

 

 Line 464 to 470: I guess, the authors are trying to explain the fact why the power curves for lower velocities are not extended over the lower RPM regions. It should have been written in a much proficient way.

In fact, I strongly suggest that the article should be revised to improve the English. A few examples where improvements are necessary are as follows:

Line 47:Part of this investment will be destinated to research and develop systems based on hydrokinetic turbines for it use at ocean and flowing currents.

It can be written as 'A portion of this investment will be allocated for research and development of hydrokinetic turbines at ocean currents.'

Line 49: These systems will harness the great potential that exists in those locations which, in the case of marine currents, is worldwide estimated in 800 TWh/year.

Line 53: Versality.   It should be versatility.

Line 54: Recently, and related with Smart Cities, some proposals based on microgeneration  using turbines are being developed

Line 102: These designs base their design on aerodynamical profiles used at the wind industry.

Line 388: should be width, instead of wide.

Line 414: To test the turbine, it has been designed and built a water-resistant methacrylate box.

and so on...

Author Response

The authors studied a prototype model of SC Darrieus turbine both in wind and water and tried to demonstrate the effect of Blockage Ratio on power coefficient. Though similar studies (separately for wind and water) can be found in the literature, this manuscript can still be considered for publication. However, I request the authors to address the following issues:

*It is suggested to add a Table that will summarize all the test cases for easy referencing. I found it a bit difficult to follow all the experimental cases that are run in wind and waves.

- According to your suggestion, a table which summarizes tests conditions at wind and water has been included at the end of the test procedure section (Table 1).

*The values for performance coefficient in water at various blockage ratios, especially for higher values, appear a bit unrealistic to me (considering other similar studies found in the literature). I was wondering how the authors ensured that their measurements are accurate enough. It will be good if some information on the calibration of the instruments is added.

In fact, for the existence of blockage (more than 30%), the power coefficient values are typically more than Betz limit (Cp=0.59) as the flow accelerates around the turbine rotor while the blockage increases. This could be found also in other investigation inside water channel with maximum power coefficients around 1.0 for blockage ratios more than 25% (reference 27). Also, the information of the torque sensor calibration and accuracy has been included in the text from line 406 to 409:

“The torque transducer is an integrated high precision torque and rotational speed sensor Magtrol TS-103 (0.5 Nm of rated torque, accuracy <0.1% and 1,5000 rpm max. speed, accuracy <0.015%) being calibrated in accordance with the standards of the Swiss Federal Institute of Metrology (METAS)”.

*Besides, figure 15 shows the corrections for the 20% blockage ratio only. It will be interesting to see how the corrected curves change for other blockage ratios.

Actually, by correcting other blockage values with the same formulas, the results in the open field would be similar. But strictly the comparison between wind tunnel and water flume results should be done at the same Reynolds number. That is the reason why we select the value of 20% blockage.

 

*It is expected that the turbine will produce higher Cp at higher velocities and for higher blockage ratios. Therefore, it is expected that the authors highlight the contributions of their work explicitly (other than these two obvious facts).

The main objectives of this research are two: firstly, evaluate the operation of a Squirrel cage Darrieus turbine type in both wind and water realistic operation conditions that can be found in the urban system; secondly, demonstrate that, the curves obtained in the wind tunnel (open field conditions) are similar to those obtained in the water current flume (confined conditions) as long as the same Reynolds number is maintained, and the blockage corrections are applied. These two objectives have been clarified in the introduction and the conclusion sections.

 

 

*Line 464 to 470: I guess, the authors are trying to explain the fact why the power curves for lower velocities are not extended over the lower RPM regions. It should have been written in a much proficient way.

Yes, this part has ben rewritten from line 475 to line 480.

 

*In fact, I strongly suggest that the article should be revised to improve the English.

The English of the article has been reviewed

A few examples where improvements are necessary are as follows: Line 47:Part of this investment will be destinated to research and develop systems based on hydrokinetic turbines for it use at ocean and flowing currents. It can be written as 'A portion of this investment will be allocated for research and development of hydrokinetic turbines at ocean currents.'

 

That sentence has been modified according to your suggestion in line 48 and 49.

 

Line 49: These systems will harness the great potential that exists in those locations which, in the case of marine currents, is worldwide estimated in 800 TWh/year.

That sentence has been modified according to your suggestion in line 50 and 51.

 

Line 53: Versality.   It should be versatility.

This sentence has been rewritten eliminating that expression (lines 52-53).

Line 54: Recently, and related with Smart Cities, some proposals based on microgeneration  using turbines are being developed

This sentence has been rewritten eliminating that expression (lines from 53 to 54).

Line 102: These designs base their design on aerodynamical profiles used at the wind industry.

The sentence has been rewritten (lines 96-97).

Line 388: should be width, instead of wide.

This word has been changed in the water flume description section (line 372).

Line 414: To test the turbine, it has been designed and built a water-resistant methacrylate box.

The sentence has been rewritten (line 398).

Reviewer 3 Report

Dear authors:

I have found it a very interesting and current work, any contribution related to the achievement of sustainability in cities is welcome.

Below I highlight the positives and those that need improvement.
Research work that performs an experimental evaluation of a Darrieus type vertical axis turbine.
Positive aspects:
- The bibliography used as the basis for the study is correct and sufficient.
-The structure of the work is coherent, very well explained with solid results.
-Visual objects support the theoretical explanation.
-The conclusions are proportional to the results of the study.

Minor aspects to modify:

-What is your contribution? It must be perfectly explained
-The structure of the article must be in the introduction.
-The figures that are not your own, indicate the source in the title. For example figure 2.
-Figures 5 and 7, include who are "a" and "b" in the title
-Figure 9, explain in the text, in addition to TSR, power (P) and what "i" means.

Author Response

I have found it a very interesting and current work, any contribution related to the achievement of sustainability in cities is welcome.

Below I highlight the positives and those that need improvement.

Research work that performs an experimental evaluation of a Darrieus type vertical axis turbine.

Positive aspects:

- The bibliography used as the basis for the study is correct and sufficient.

-The structure of the work is coherent, very well explained with solid results.

-Visual objects support the theoretical explanation.

-The conclusions are proportional to the results of the study.

 

Minor aspects to modify:

-What is your contribution? It must be perfectly explained.

The main objectives of this research are two: firstly, evaluate the operation of a Squirrel cage Darrieus turbine type in both wind and water realistic operation conditions that can be found in the urban system; secondly, demonstrate that, the curves obtained in the wind tunnel (open field conditions) are similar to those obtained in the water current flume (confined conditions) as long as the same Reynolds number is maintained, and the blockage corrections are applied. These two objectives have been clarified in the introduction and the conclusion sections.

 

-The structure of the article must be in the introduction.

*At the end of the introduction a paragraph has been included to describe the article structure (lines from 173 to 176):

“The article has been structured in five main sections: introduction, theoretical bases of the experiments (in both wind and water), materials and methods (include the description of the rotor, wind tunnel, water flume and testing procedure), results and discussion, and finally the conclusions section”.

-The figures that are not your own, indicate the source in the title. For example figure 2.

References have been included in figures 2,3 and 4.

-Figures 5 and 7, include who are "a" and "b" in the title

Ok, the numbers have been included in the caption.

-Figure 9, explain in the text, in addition to TSR, power (P) and what "i" means.

The explanation has been included in lines from 414 to 433:

“The characterization procedure (Figure 9) starts with an initial measurement where no braking force is applied, so the rotor has its maximum rotational speed but without generating power. This step is called “no-load condition”. Subsequently, the resistive torque of the electric brake is sequentially changed, in different “i” steps, through voltage variations at power supply device, obtaining different extracted powers (P_i) and tip speed ratios (λ_i)”.

Round 2

Reviewer 1 Report

This article can be accepted.

Reviewer 2 Report

Thanks for revising the manuscript. I have no further comments. Just one minor note for the future - changes done in the revised manuscript should be highlighted for easy referencing.