Wind Farm Control for Improved Battery Lifetime in Green Hydrogen Systems without a Grid Connection

Round 1

Reviewer 1 Report

The paper is well written and describes the results well and the conclusions are supported by the results. The methodology used have some assumptions which might limit the applicability of the method but the assumptions are clearly stated (line 408-412) and thus, make this paper acceptable for publication. 

I would advise the authors to make one significant change. The paper is about 'Control of Wind Farm Power...' but the control design is not given prominence in the paper. WFC and supervisory controller are part of the 'model description'. The WFC equations are described with an inline equation and important information like Tau_b = 1 sec is also inline. The value of the gain used in supervisory control is not provided. These should be given more prominent positions; Ideally in a separate control design section, but if that's not possible then with numbered equations which are referred back to later (e.g in table 3).

Author Response

Many thanks for your review comments for our paper.  We have Repeated the comments in the next section for clarity and provide our responses in the subsequent section.

Original Comments

The paper is well written and describes the results well and the conclusions are supported by the results. The methodology used have some assumptions which might limit the applicability of the method but the assumptions are clearly stated (line 408-412) and thus, make this paper acceptable for publication.

I would advise the authors to make one significant change. The paper is about 'Control of Wind Farm Power...' but the control design is not given prominence in the paper. WFC and supervisory controller are part of the 'model description'. The WFC equations are described with an inline equation and important information like Tau_b = 1 sec is also inline. The value of the gain used in supervisory control is not provided. These should be given more prominent positions; Ideally in a separate control design section, but if that's not possible then with numbered equations which are referred back to later (e.g in table 3).

Response to Comments

Thank you for your kind words regarding the paper in the first paragraph of your comments. 

In the second paragraph you suggest a change to the paper – we agree with your comments.  To address this issue, the following changes have been made:

• The WFC equations are given more prominence in the text as advised
• The supervisory controller gain is now clearly stated in the text
• Numbered equations are used for the control equations

With these changes we feel that the control gains more prominence in the paper – a separate control section was not added as we felt this would break the flow of the paper that currently has the methodology sections followed by the case study.

We would like to thank you very much for the time you have taken to review the paper and hope that the edits detailed here meet your satisfaction.  Your review has helped improve the work and for that we are very thankful

Best regards,

Adam Stock, Matthew Cole, Mathieu Kervyn, Fulin Fan, James Ferguson, Anup Nambiar, Benjamin Pepper, Michael Smailes, and David Campos-Gaona

Reviewer 2 Report

This study delves into the Control of Wind Farm Power Output for Green Hydrogen Production in the absence of a grid connection.

In line 78, "were" should be replaced with "where."

Regarding line 121, could you kindly provide more details or further explanation on how energy is stored or released through pitching?

As you are addressing the scenario without a grid, it would be beneficial to include a few sentences explaining how the grid helps dampen fluctuations caused by wind power.

For sections 2.4-2.6, could you please specify the tools utilized for the electrolysers model, control model, and battery lifetime model?

In Fig. 4, it appears that the maximum energy extracted has already reached the top boundary of the plot. It might be helpful to extend the vertical range to provide a clearer representation.

For both Fig. 4 and Fig. 5, using thicker lines and more line types could enhance readability and make it easier to differentiate between elements.

Author Response

Many thanks for your review comments for our paper.  We have repeated the comments in the next section for clarity and provide our responses in the subsequent section. We have taken the liberty of numbering your comments for ease of reference.

Original Comments

This study delves into the Control of Wind Farm Power Output for Green Hydrogen Production in the absence of a grid connection.

1. In line 78, "were" should be replaced with "where."
2. Regarding line 121, could you kindly provide more details or further explanation on how energy is stored or released through pitching?
3. As you are addressing the scenario without a grid, it would be beneficial to include a few sentences explaining how the grid helps dampen fluctuations caused by wind power.
4. For sections 2.4-2.6, could you please specify the tools utilized for the electrolysers model, control model, and battery lifetime model?
5. In Fig. 4, it appears that the maximum energy extracted has already reached the top boundary of the plot. It might be helpful to extend the vertical range to provide a clearer representation.
6. For both Fig. 4 and Fig. 5, using thicker lines and more line types could enhance readability and make it easier to differentiate between elements.

Response to Comments

Using the numbering system of the prior section or responses to the comments are given below:

1. The typo on line 78 (of the original manuscript) has been corrected.
2. On line 121 (of the original manuscript) more clarity has been given regarding pitching and energy capture through the bolded additions repeated below:
   “However, unlike the previous case of extracting power, when injecting power to the rotor the wind turbine can pitch its blades to reduce the aerodynamic torque input and hence over-speed can be avoided [ 35, 36]. Note that if the wind turbine blades are pitched then energy that would have been captured is ‘lost’, as the aerodynamic efficiency of the rotor is reduced. Of course, in the case of power smoothing, where increases in electrical power can follow reductions in electrical power, it can be useful to allow the rotor to accelerate to some limited increase in rotor speed when reducing electrical power to provide additional energy reserves for future increases in electrical power. If the rotor speed is increased with no pitching then any loss in energy is small as the rotor still operates close to the optimum tip speed ratio and at the optimum blade pitch angle”
3. An addition has been made in the introduction addressing this point:
   “Battery systems can help to reduce the rate of electrolyser degradation in a system without a grid connection by reducing the rate of change in power experienced by the electrolyser and balancing power mismatch. A battery is required in this case as the grid cannot be used for this purpose as it would in a grid connected system. Additionally, batteries can help with controlling the DC link voltage between the wind farm and the electrolyser and with black start of the wind turbine or wind”
4. We have added the following text at the start of section 2:
   “All the models used run within the MATLAB Simulink simulation environment”
5. Figure 4 has had the scale adjusted
6. Figures 4 and 5 have been adjusted as suggested

We would like to thank you very much for the time you have taken to review the paper and hope that the edits detailed here meet your satisfaction.  Your review has helped improve the work and for that we are very thankful.

Best regards,

Adam Stock, Matthew Cole, Mathieu Kervyn, Fulin Fan, James Ferguson, Anup Nambiar, Benjamin Pepper, Michael Smailes, and David Campos-Gaona

Reviewer 3 Report

The manuscript entitled "Control of Wind Farm Power Output for Green Hydrogen Production without a Grid Connection" has been prepared by the authors. There are many concerns. It lacks innovation. it can be presented in workshops and conferences.

Dear Editor;

Minor editing of English language required.

Author Response

Many thanks for your review comments for our paper.  We have repeated the comments in the next section for clarity and provide our responses in the subsequent section.

Original Comments

The manuscript entitled "Control of Wind Farm Power Output for Green Hydrogen Production without a Grid Connection" has been prepared by the authors. There are many concerns. It lacks innovation. it can be presented in workshops and conferences.

Response to Comments

Regarding the stated concerns around innovation, whilst we appreciate the feedback, we respectfully disagree.  To make the innovation clearer we have made the following changes:

1. The abstract has been significantly updated, particularly with regards to highlighting the innovation and novelty of the work. The new abstract is included below with the new parts highlighted bold:
   
   “Green hydrogen is likely to play an important role in meeting the net zero targets of countries around the globe.  Hence, producing green hydrogen cheaply and effectively is an important area of research.  One potential option for green hydrogen production is to run electrolysers directly from offshore wind turbines, with no grid connection and hence no expensive cabling to shore. 
   The removal of the grid presents an unusual integration challenge. The variable nature of wind turbines and farms results in a power output that can fluctuate more quickly than the electrolyser's ability to respond without significantly stressing the electrolyser. Thus, the use of a battery, with the wind farm, becomes essential to even out some of the power variations that the electrolyser cannot deal with. 
   As yet, the authors are not aware of any study in the literature that considers the use of wind farm control to reduce power variation and hence increase battery life in such a system by reducing the magnitude and number of battery load cycles. Hence, in this work, an innovative proof of concept of a wind farm control methodology designed to reduce variability in wind farm active power output is presented. Smoothing the power supplied by the wind farm to the battery reduces the size and number of battery charge cycles and helps to increase battery lifetime. Considering off-grid wind farms which exclusively power an electrolyser, this work quantifies the impact of the wind farm control method on battery lifetime for wind farms of 1, 4, 9 and 16 wind turbines. This is achieved using suitable wind farm, battery and electrolyser models. As an example, for the largest wind farm studied, consisting of 16 x 5 MW wind turbines, batteries with a lifetime of 15 years have approximately a 30~\% reduction in required capacity (reduced from 14 MWh to 10 MWh) compared to operating without wind farm control. It is found that reducing the variability of the active power output of wind farms through the wind farm control methodology presented can have a significant impact on battery degradation and hence on battery lifetime.  Hence, wind farm control can reduce the required battery capacity for a given lifetime or it can increase the lifetime of a given battery capacity. The work presented shows, as a key innovation, that wind farm control for smoothing wind farm power output could play a critical role in reducing the levelised cost of green hydrogen produced from wind farms with no grid connection by reducing the damaging load cycles on batteries in the system. Hence this work paves the way for the design and testing of a full implementation of the wind farm control.”
   
   Further, in order to more accurately reflect the content, with the title of the paper has been changed to “Wind farm Control for Improved Battery Lifetime in Green Hydrogen Systems Without a Grid Connection”
   
   In addition to the aforementioned changes to the abstract, the introduction has also been updated in view of your comments. In particular, the following alterations (in bold) have been made towards the end of the introduction:
   “If used appropriately, a controller could be designed at either a wind turbine or a wind farm level to use the energy stored in the rotor to smooth the power output of a wind turbine and hence reduce the cost of the required battery storage when producing green hydrogen without a grid connection. Such a controller has not, to the authors' knowledge, been proposed before in the literature and is the focus of the work presented here.
   To summarise, this paper presents a proof of concept for a novel control method to smooth the active power of wind turbines and wind farms, connected in an off-grid fashion to a battery and an electrolyser. Prior to this work, the use of WFC for increased battery lifetime in off-grid wind farms has received little to no attention within the literature, and the investigation of WFC in the general context of electrolyser integration is a novel part of this work. The coupling of the electrolyser model, battery model and wind farm model presented in this work to investigate battery lifetimes in off-grid hydrogen systems is an innovative combination that has not previously been studied. The active power smoothing has the overall aim of reducing battery costs by either increasing battery lifetime for a given battery size or reducing battery size for a given lifetime.”

We believe that these changes better highlight the innovative aspects of the work, in particular the lack of prior study of wind farm control in the context of green hydrogen production without a grid connection.  Through an alteration to the title we believe that the key innovation points are clearer to readers.

We would like to thank you very much for the time you have taken to review the paper and hope that the edits detailed here meet your satisfaction. 

Best regards,

Adam Stock, Matthew Cole, Mathieu Kervyn, Fulin Fan, James Ferguson, Anup Nambiar, Benjamin Pepper, Michael Smailes, and David Campos-Gaona

Reviewer 4 Report

Investigation of the control of wind power output to produce green hydrogen with consideration of battery lifetime is a hot topic currently. This paper attempts to present a control approach and gives an actual example. The research is interesting for the readers. In order to improve the quality of the paper, some comments are given as below:

1. It advises to polish the abstract, and highlight the contents and innovation points.

2. What are the theoretical innovation points in this research? It advises to highlight the innovation points compared with the current research status in the Introduction part.

3. The specific control process of how to combine with the wind power and prolong the battery lifetime could be more analyzed.

4. In the part of Case Study, there is no description of hydrogen?

Author Response

Many thanks for your review comments for our paper.  We have Repeated the comments in the next section for clarity and provide our responses in the subsequent section.

Original Comments

Investigation of the control of wind power output to produce green hydrogen with consideration of battery lifetime is a hot topic currently. This paper attempts to present a control approach and gives an actual example. The research is interesting for the readers. In order to improve the quality of the paper, some comments are given as below:

1. It advises to polish the abstract, and highlight the contents and innovation points.
2. What are the theoretical innovation points in this research? It advises to highlight the innovation points compared with the current research status in the Introduction part.
3. The specific control process of how to combine with the wind power and prolong the battery lifetime could be more analyzed.
4. In the part of Case Study, there is no description of hydrogen?

Response to Comments

Using the numbering system of the prior section or responses to the comments are given below:

1. The abstract has been significantly updated, particularly with regards to highlighting the innovation and novelty of the work. The new abstract is included below with the new parts highlighted bold:
   
   “Green hydrogen is likely to play an important role in meeting the net zero targets of countries around the globe.  Hence, producing green hydrogen cheaply and effectively is an important area of research.  One potential option for green hydrogen production is to run electrolysers directly from offshore wind turbines, with no grid connection and hence no expensive cabling to shore. 
   The removal of the grid presents an unusual integration challenge. The variable nature of wind turbines and farms results in a power output that can fluctuate more quickly than the electrolyser's ability to respond without significantly stressing the electrolyser. Thus, the use of a battery, with the wind farm, becomes essential to even out some of the power variations that the electrolyser cannot deal with. 
   As yet, the authors are not aware of any study in the literature that considers the use of wind farm control to reduce power variation and hence increase battery life in such a system by reducing the magnitude and number of battery load cycles. Hence, in this work, an innovative proof of concept of a wind farm control methodology designed to reduce variability in wind farm active power output is presented. Smoothing the power supplied by the wind farm to the battery reduces the size and number of battery charge cycles and helps to increase battery lifetime. Considering off-grid wind farms which exclusively power an electrolyser, this work quantifies the impact of the wind farm control method on battery lifetime for wind farms of 1, 4, 9 and 16 wind turbines. This is achieved using suitable wind farm, battery and electrolyser models. As an example, for the largest wind farm studied, consisting of 16 x 5 MW wind turbines, batteries with a lifetime of 15 years have approximately a 30~\% reduction in required capacity (reduced from 14 MWh to 10 MWh) compared to operating without wind farm control. It is found that reducing the variability of the active power output of wind farms through the wind farm control methodology presented can have a significant impact on battery degradation and hence on battery lifetime.  Hence, wind farm control can reduce the required battery capacity for a given lifetime or it can increase the lifetime of a given battery capacity. The work presented shows, as a key innovation, that wind farm control for smoothing wind farm power output could play a critical role in reducing the levelised cost of green hydrogen produced from wind farms with no grid connection by reducing the damaging load cycles on batteries in the system. Hence this work paves the way for the design and testing of a full implementation of the wind farm control.”
   
   The title of the paper has also been adjusted to more accurately reflect the content, with the new title being “Wind farm Control for Improved Battery Lifetime in Green Hydrogen Systems Without a Grid Connection”
2. In addition to the aforementioned changes to the abstract, the introduction has also been updated in view of your comments. In particular, the following alterations (in bold) have been made towards the end of the introduction:
   “If used appropriately, a controller could be designed at either a wind turbine or a wind farm level to use the energy stored in the rotor to smooth the power output of a wind turbine and hence reduce the cost of the required battery storage when producing green hydrogen without a grid connection. Such a controller has not, to the authors' knowledge, been proposed before in the literature and is the focus of the work presented here.
   To summarise, this paper presents a proof of concept for a novel control method to smooth the active power of wind turbines and wind farms, connected in an off-grid fashion to a battery and an electrolyser. Prior to this work, the use of WFC for increased battery lifetime in off-grid wind farms has received little to no attention within the literature, and the investigation of WFC in the general context of electrolyser integration is a novel part of this work. The coupling of the electrolyser model, battery model and wind farm model presented in this work to investigate battery lifetimes in off-grid hydrogen systems is an innovative combination that has not previously been studied. The active power smoothing has the overall aim of reducing battery costs by either increasing battery lifetime for a given battery size or reducing battery size for a given lifetime.”
3. The following text has been added in section 2.3 to describe the motivating hypothesis behind the controller design:
   “The hypothesis for the controller design is that by reducing the variability of the power output from the wind farm (in terms of both the amplitude and number of sign changes), the battery will need to complete fewer cycles with smaller depth of discharge to match the wind farm power output to the electrolyser demand. Whilst the design of the battery may influence the impact of reduced cycling, it is expected that all batteries will benefit from smoother wind farm power, and so the specific battery design is not required in the controller design process.”
4. The volume of hydrogen generated is not mentioned in the work as it is not a key consideration. This has been made much clearer through the inclusion of the following line in section 2.4 (Electrolyser model):
   “Importantly, the primary role of the electrolyser is to provide a representative demand that the combined wind farm and battery must match - the quantity of hydrogen produced is not the focus of the work presented here.”
   The point is now also reiterated in the results section via the addition of the following line:
   “Note that, as remarked in section 2.4, the hydrogen output of the system is not a key consideration - the electrolyser model provides a representative demand that must be matched by the combination of wind farm power output and battery power output.”

We would like to thank you very much for the time you have taken to review the paper and hope that the edits detailed here meet your satisfaction.  Your review has helped improve the work and for that we are very thankful.

Best regards,

Adam Stock, Matthew Cole, Mathieu Kervyn, Fulin Fan, James Ferguson, Anup Nambiar, Benjamin Pepper, Michael Smailes, and David Campos-Gaona

Round 2

Reviewer 3 Report

The authors could not use the revision opportunity. The reviewer's concerns remain. However, the reviewer agrees to give another opportunity for improvement. Note the following;

1- The manuscript is very long. e.g. Abstract and Introduction are very long. (It bores the reader). The author's ability should be to summarize and simply express the subject for the reader.

2- Innovation isn't clear. It seems that it lacks innovation.

3- Results are very limited. It is nessacary to develop the results.

4- Recommended to use relevant and prestigious references. Use the references with the topic about Wind farm control methods such as the below reference;

-Saadatmand M, Gharehpetian GB, Siano P, Alhelou HH. PMU-based FOPID controller of large-scale wind-PV farms for LFO damping in smart grid. IEEE Access. 2021 Jul 2;9:94953-69. 

Minor editing of English language required

Author Response

Dear academic editor,

Please see attached for details.

Author Response File: Author Response.pdf