The authors have performed a numerical study on the design of a Vertical-Axis Wind Turbine (VAWT) with inclined blades, with a specific focus on the X-Rotor concept. The effect of the different design parameters on several rotor performance indexes, including power production, weight and mechanical stresses, has been explored.  The reviewer believes that the topic and the activity are very interesting, innovative and worthy of investigation. The study is extensive and both methodology and results are presented in an exhaustive and clear way.

Notwithstanding this, the assumptions behind the adopted numerical methodology are, in the Reviewer experience, very strong for the chosen application and they need to be further justified to ensure the reliability of the presented results. More in detail:

1. Given the solidities and tip-speed ratios considered for the analysis, ignoring the flow curvature and unsteady aerodynamics/dynamic stall effects might bias the obtained trends, especially in terms of power coefficient and maximum blade stresses. Therefore, the limitations of the adopted methodology should be assessed against simulations performed with a higher-fidelity method, at least for a couple of points at the extremes of the chosen investigation domain;
2. The Prandtl tip loss factor has been conceived for HAWTs with straight blades. How do you justify its application to a VAWT with a largely "three-dimensional" geometry as the one considered here?
3. The use of a single set of polar data is reasonable, given the large blade Reynolds number involved. Have you verified, nonetheless, the accuracy of the obtained dataset, especially in the post-stall region?

Apart from this major aspect, minor modifications are required to improve the quality of the paper:

1. Abstract is too focused on the results and lacks the methodology part. Please re-write it to balance it out;
2. Section 2: it is not clear how a 2D tool is applied to the simulation of a 3D VAWT rotor, especially in presence of inclined blades. Please integrate this section to clarify this aspect;
3. Line 194: how is the λ' variable defined?
4. Line 236: how was the optimal H-rotor designed? Please clarify

REVIEW COMMENTS

Attached is the annotated review. I stopped at some part because you should ask for a major review and ask the editor to send the 2^nd review back to you since they have made certain assumption which is not only non-conventional but also very confusing and not intuitive. Here is the summary of review.

The authors have defined two dimensional variables as a basis for comparison of results. They refer to scaling these dimensional variables using the swept area of an H-rotor. It is not clear how these dimensional variables can give valuable insight as to performance improvement. This is not the traditional approach which makes use of non-dimensional groups (pi-groups) or parameters as a basis of comparison. Therefore, the results do not reflect superiority of one configuration over the other rather just reflect that the larger turbine will produce more power. For example, it does not reflect that how inclination angle effects two rotors with the same swept area or aspect ratio. In addition, the authors have not provided any comparison of their results with experimentally available data to substantiate their claims. In view of these findings, which have also been annotated on the manuscript, we recommend a major review of the manuscript to address all concerns for more clarity and conciseness.