| 15 Aug 2025
Experimental investigation of wind turbine controllers for the Hybrid-Lambda Rotor
Abstract. The continuous growth in rotor diameter of offshore wind turbines must be accompanied by advanced control strategies that master the trade off between limiting extreme loads and maximizing power output, fostering a lightweight and cost-effective blade design. This is addressed by the Hybrid-Lambda Rotor design and control methodology which realizes two operating modes by following two different tip speed ratios (TSRs) below rated power with an overarching load constraint. Contrary to conventional wind turbine controllers, this leads to a wide range of wind speeds where the torque and pitch controllers are active simultaneously. The objective of this paper is to develop and apply such control strategies on the MoWiTO 1.8 model wind turbine and to experimentally validate them under turbulent reproducible inflow conditions in the wind tunnel using an active grid. The results are examined regarding extreme loads, power production, fatigue loads and pitch actuation. Further, we discuss the scaling of the controller characteristics and inflow test cases according to the model turbine scaling. Different versions of the pitch controller are introduced. First, a baseline controller with a model-based wind speed estimator which performed well in tracking the different TSRs. Second, a load feedback controller that overcame model uncertainties and performed well in setting the mean value of the loads. And third, an inflow feed-forward controller which was able to reduce load overshoots in gust events. With the results presented here, we make the next step in the experimental validation of the control methodology, which unlocks the full potential of aerodynamic efficiency and ensures the structural integrity of the Hybrid-Lambda Rotor.
### General comments
This works presents the result of an experimental wind tunnel study on the scaled hybrid-lambda rotor using four different control strategies. The amount of tests performed and the quality of the experiments is high and I would like to thank the authors for their efforts. I also think the scientific significance of this work is very good as the hybrid-lambda rotor concept and the different controller strategies here push the boundaries of what is currently available in academic literature and the authors make good recommendations for large-scale wind turbine controls from their work. Overall, I think this work is suitable for wind energy science and should be accepted after minor revisions.
I recommend the following main improvement for the manuscript:
1. The objective of the work is not aligned well between the abstract, introduction, and the conclusion, which becomes apparent when first reading the abstract, introduction, and then the conclusion before continuing with the rest of the paper. From reading the abstract the goal is to develop and apply control strategies on the MoWiTO and this objective is repeated in the introduction. However, the introduction doesn't include what type of controllers will be tested which makes it unclear to the reader what exactly the 3rd paragraph in the introduction is for (it doesn't connect that well to the rest of the introduction). Finally, in the conclusion emphasis is given how this experiment leads to recommendations for controller design for very large wind turbines. I think announcing this as an objective in the introduction would make it come less unexpectedly. I think most of this can be solved by rewriting the introduction. I recommend elaborating on the objective and connecting it to the earlier parts in the introduction.
A few smaller comments also apply more generally to the manuscript and are:
1. The resolution of the figures is quite small making it not pleasant to zoom in to see more detail as a reader. I would recommend exporting figures using a vector format.
2. The colour choice in figures is not always good. In black-and-white printing some colours are almost indistinguishable, such as baseline and FFLFB in figure 13. I also suspect that readers with a colour-vision deficiency would have difficulty distinguishing the different colours in most figures. I would recommend using scientific colour maps, as recommended by WES: https://zenodo.org/record/1243862
3. In the text, some lines in the figures are referred to by their colour instead of by their label. I think that this exacerbates the problem with the colour choice in the figures.
4. Not all lines in the figures are present in the legend and are sometimes only explained in the caption. I think that afraid this exacerbates the problem with the colour choice in the figures.
5. The captions of the figures currently have to explain more what is in the figures (also due to incomplete legends) so to understand the results I have to switch from the figure to the text and back a lot. It would improve readability if the captions would also include the main result to make the figures more interpretable on their own.
6. LUT1 and LUT2 are not very descriptive names for look-up tables, which made it slightly harder to follow some parts of the text. I think a more descriptive naming would improved the flow of the text.
### Specific comments
1. Abstract:
1. Line 2: "extreme loads" is usually associated with ultimate loads while it is also a trade-off with fatigue loads. I think either removing "extreme" or explicitly adding fatigue loads would be more complete.
2. Introduction:
1. Line 17: The first paragraph would benefit from one or two citations.
2. Line 18: "Rotors with a low-specific rating can capture more energy in light winds" is technically not correct. I think adding making it "relatively more energy" would read better.
3. Line 30: Reading this paragraph, I feel like I wouldn't know at which paper to start for a good general overview of the Hybrid-Lambda concept because the two citations used are introduced rather late as simulation studies of the concept (not as fundamental papers explaining the concept). Maybe it's good to cite one of your works earlier in the paragraph so that readers have a clear citation which they can learn about the concept of Hybrid-Lambda.
3. Methodology
1. The order of subsections and subsubsections is first an overview of the steady-state operating points, then the scaling considerations, then the torque controller, and then the pitch controller. After finishing the section on steady-state operating points I was expecting more information on the implementation of the controllers, as you already hint to some of the challenges that the hybrid-lambda concept poses for control. I would put the scaling considerations section somewhere else in the methodology so the control parts can be together.
2. Line 171-178: This is a good summary of the scaling considerations which makes this section possible to follow for people with less exposure to wind tunnel testing.
3. Line 201-216: I understand this scheme and think it's an interesting way to solve this problem but I had to reread this section a couple of times to convince myself that it works. I would recommend to rewrite this section and take the reader along in the process to make it more intuitive and understandable.
4. Line 222: "Additionally, we tested a combination of the latter two versions (FFLFB)." I would combine this with the previous sentence because as a reader I expected 4 controllers to be listed in one sentence.
5. Line 223-224: I think this is a creative and interesting approach. I would add a short explanation as to why this works. I guess the reasoning is that below the maximum rotor speed the PI controller always wants to speed up thus decrease the pitch but is then saturated by the minimum pitch angle.
4. Discussion
1. I'm familiar with the work of Lazzerini et al (2025) and it would be helpful to see a comparison to your method. I see their work as completely letting go of the concept of tip speed ratio, or going towards a kind of Hybrid-Lambda control concept with an infinite number of TSRs instead of 2. Do you think that their control approach would work well for your rotor design?
2. In extension to that, their torque controllers also use the wind speed estimate. Do you think that that would have worked in your case too and I'm curious why you made the decision to not use the wind speed estimate in your torque controller.
5. Conclusions
1. Line 558: "arranging" is too vague in this context and makes it hard to understand for a reader who has only read the abstract and introduction.
2. The first two paragraphs of the conclusion are clear. However, the third and fourth paragraph discuss the transfer from the presented control strategy to a full-scale system and make a concluding remark of the work. I thought that the full-scale discussion seemed to stand on its own rather than be explicitly related to this study. The last paragraph could be made clearer by defining a more specific point. I would revise these sections by defining a main point you want to make and making the connection to your work clearer.
### Technical corrections
I have no technical corrections to recommend.