This article continues the exciting series of publications about the grand challenges faced by wind energy technology. The list of authors is the right one for such publication and I support the publication of this long article, which can become the backbone of many research proposals moving forward.

I do not have the expertise to add anything to some of the sections, but I do have a list of suggestions.

Overall structure: 

1) I would have liked to read a paragraph about what’s new in this article compared to  previous publications focusing on the grand challenges of wind energy, for example https://www.science.org/doi/10.1126/science.aau2027, https://www.nrel.gov/docs/fy19osti/72437.pdf, or https://doi.org/10.5194/wes-1-1-2016

2) Some topics are grouped together into the same section, and sometimes I miss the rationale. Why inflow and the design process together, but aerodynamics and aeroelasticity in a separate section? Why HFM and validation? And uncertainty quantification is described in a single paragraph in 9.6, although there is a large body of literature addressing it.

3) Some sections are built on references (sections #3, #4, #5, #6), which I think is a must for this type of articles. Other sections use very few (#7, #8, #9). I’d suggest consistency, possibly leaning toward the first approach. References are not present in section #2, and that might be ok, but a note pointing the reader to the following sections would help.

Abstract

Line 15: what about wind farm blockage?

Line 21: I see the lack of validation as a key obstacle for a wider adoption of HFM. BEM, with all its limitations, has shown to work and several publications show what matches and what not. HFM not yet, or at least far less.

Intro

Figure 2: I wouldn’t have the Emirates logo on a scientific paper that will receive thousands of downloads and citations. Please edit.

Section 2

Line 125: LCOE has clearly shown its limitations (https://doi.org/10.1016/j.tej.2021.106931, among many others). Such limitations will only grow with a higher share of wind energy in the mix and the growth of hybrid systems and storage. Should this be a challenge? 

Line 136: This sentence might sound a little unfair for the wind energy pioneers. Unknowns have always been out there. We’ve resolved some, some others are still there, and some new ones have come up

Line 257: It won't be possible to increase tip speeds without innovations on the leading edge erosion front. I'd suggest adding a note

Section 2.6: I miss a paragraph about the cost and complications of testing prototypes, which I understand to be a very real obstacle to innovation. What about allowing subcomponent testing? Or moving toward numerical prototyping in 2.7?

Line 341: I agree that validation is key for the HFM to succeed. But validation is easier said than done even when data are present. We lack rigorous data assimilation techniques merging multiple instruments and the inherent errors in their measurements into a single validation process. This is listed in section 9, but I’d suggest adding a note here as well.

Section 2.8: I found 2.8 somewhat blurry to me. I'd suggest to reformulate how the authors propose to move forward, which in my head is system design based on validated and therefore trustworthy numerical models

Section 3

As mentioned before, I am not sure whether I understand why inflow and design process come into the same section.

Line 418: I feel that a reader could get confused here as LCOE and value are used almost interchangeably. The references are very relevant and maybe this section could be expanded a little to better explain these trends without the need to accessing the references. Also, I would add a note about the arrival of storage solutions, which might change trends very rapidly.

Line 428: I personally like the words “numerical prototyping” to describe this innovation

Section 4

Line 780: I found this paragraph a little confusing. In my mind the biggest obstacle is the combination of a very complex (nonlinear) solution space generated by aero-servo-hydro-elastic models combined with high computational costs. This is probably what authors are saying, but I think it could be written more explicitly.

Line 812: This is possibly misleading. The 25% gain in LCOE was mostly generated by switching from 3 to 2 blades, not by the actual codesign optimization. That change was done upfront and it wasn’t driven by co-design approaches, but rather by a change in turbine configuration.

Section 5

Line 1050: I would add the lack of validation to the list of obstacles slowing HFM

Line 1080: you could add https://doi.org/10.1088/1742-6596/2265/4/042052 to the list of free form studies

Line 1100: I’d suggest to add references about successful scaled blade testing, for example https://doi.org/10.5194/wes-5-1411-2020, https://arc.aiaa.org/doi/abs/10.2514/6.2021-1718, and https://doi.org/10.1016/j.renene.2020.03.157

Line 1137: I see one big obstacle here. No funding agency / national lab / research institute has designed a multi-MW wind turbine, at least not in the last 20 years. How should these organizations convince an OEM to publish data? I’d add a note about this key obstacle

Line 1147: please reference https://doi.org/10.2172/1868328 

Section 7

The start of section 7 gives the impression that offshore wind is something of the future, whereas it is already reality. Also, the paragraph seems to imply that offshore wind is only present in the North Sea, whereas Asia also has offshore wind farms (maybe not yet fully commissioned?). The same consideration is valid for floating, which is certainly in a development stage (line 1342), but there are already the first commercial projects operating out there (Hywind in Scotland?)

Line 1467: an image would help describing DOFs and platform configurations

Section 8

I’ve struggled with this section, which I think needs some attention.

To start, showing a 40m long blade (figure 12) in a paper about the future of wind seems a little out of place

Also, the section alternates considerations about blades, tower, drivetrain. I found the resulting text very confusing and I would suggest splitting the section per component. 

The section also adopts very few references, which does not help. Focusing on the blades alone, some relevant recent studies in this broad field were led by Ennis, for example https://www.osti.gov/servlets/purl/1592956, or Murray, for example https://doi.org/10.1016/j.jclepro.2018.10.286

Going into the details:

Line 1672: I don’t think you should talk about downwind as related to manufacturing challenges. Also, if you do, there is a vast body of literature about downwind technology.

Line 1678 should specify whether this damping value is critical or logarithmic. A reference supporting this value would help, maybe https://www.osti.gov/pages/biblio/1726023 or http://www.measnet.com/wp-content/uploads/2020/04/16bt01-Report-IECRE.pdf. 

Line 1696: I don’t think we’re above 100 m/s yet due to aeroacoustic and erosion issues

Line 1709: I understood that the whole point of thermoplastics is about being recyclable, whereas the text seems to imply that recyclability is only a side advantage

Line 1710: CFRP is not necessarily linked to BTC, which can be realized with GFRP as well.

Line 1713: “Recent research” calls for references

Line 1719: I’m not so convinced that the price of CFRP has been steadily declining, especially recently. References or numerical values are needed

Line 1728: I wouldn’t link the EOL issues of blades with the immaturity of wind, which could be called mature since 2010 (at least). Isn’t it more like a lack of a good alternative? Research on thermoplastics have been going on for decades, see the body of literature from TU Delft for example with Prof Beukers and Prof Bersee

Section 9

The intro of section 9 is heavy on the progress funded by DOE, but there is more than that, see for example the work happening at DTU or at Uni Stuttgart (Prof Lutz)

Line 1859: I don’t understand the use of the word especially. I understood that was the actual issue.

I would have had section 9.5 at the top of the paper, since this affects every field, not just HFM. Just a suggestion, the authors can leave things as they are.

Similarly, section 9.6 looks weird. It’s only a tiny paragraph hidden inside a much wider discussion about HFM and validation, with no references.

Section 10

Line 2148: I think that a key improvement would be to move from single turbine design approaches to designs that account for wind farm effects.

Please see the attachment.