An excellent review article covering "the state of play" of small wind turbines. Diffuser Augmented Wind Turbines (DAWTs) appear to offer significant perforrmance enhancements over bare wind turbines up to about 2 kW rated capacity. I suggest in this article a little more information on DAWTs and the likely impact this will have on their LCOE.

General comments

The paper provides a systematic review of the current SWT technology particularly in relation to their adoption, cost, technical challenges and challenges that currently persist in this sector of the industry. The paper also presents five grand challenges that this industry faces. Finally, key enablers are presented in the conclusion.

The paper abstract is well written and reflects accurately the content of this work. The introduction introduces the content of the paper to the reader. A number of shortcomings have been identified and given under “Specific comments”. Other more minor comments are presented under “Editorial comments”.

The work is an important contribution to this stuttering industry and promises to provide a foundation for future directions in policy making, research and a guideline to key industry players.

Specific comments

Section 2 - Does this data include also urban wind applications? Please specify given that the authors mentioned that they will “not include urban wind specifically…”. This question also applies to the contents of Table 1.

Table 1 – It is recommended to indicate whether FIT was in place for each of the countries, for the years shown. That would summarize the information conveyed in the text.

Table 2 – Is there a particular reason why this data is presented in the form of a table rather than a bar chart or line chart? Please consider a graphical representation.

Pg 9 – The authors should discuss in a bit more detail costs that might be associated with siting, resource assessment and turbine positioning. As discussed by the author extensively, especially in grand challenge 2, these considerations are sometimes ignored leading to lower capacity factors with an overall result of lower LCOE. Nevertheless, such assessments are done with associated costs. The authors are encouraged to be more critical on these aspects even though for instance in the sourced Table 4 this cost does not seem to feature (unless this is understood to fall under “Engineering”). Admittedly this cost might be difficult to quantify but its something that the authors may want to at least mention.

Section 4.3 Pg 22-25 – This section is somewhat too descriptive and summarises the IEC standard rather than critically assesses its implications on the SWT industry and state of the art. It could also be that this section may be redundant for the objectives that are being sought after in this work. The authors are invited to re-write critically this part of the text or if necessary remove this section altogether. The latter decision would need to be justified to ensure that it doesn’t break the flow of the paper.

Pg 29 Grand Challenge 5 – An interest action that took place some years ago is the RELY COST action http://cost-rely.eu which dealt with the social acceptability of renewable energy systems including small scale wind energy.

Pg 37 “Control” – For some references, no date is available. Please also see comment on references.

Line 1167, 1168 – Please double check whether the creation of a Europe wide incentive scheme is at all possible and within the operational lines of the EU. This is to avoid a proposal which may go against fundamental EU modus operandi.

Line 1171 and 1172 – This is a very interesting proposal which could provide guidance on a state by state basis to energy authorities.

Conclusions – It is recommended to put the key enablers into a separate section before the conclusion. The latter should be used as an overarching, bird’s eye view of the work rather than setting out new recommendations.

Line 1239 – The following paper could be an interesting addition to your reference list in relation to resource assessment:

D.R. Drew, J.F. Barlow, T.T. Cockerill, M.M. Vahdati, The importance of accurate wind resource assessment for evaluating the economic viability of small wind turbines, Renewable Energy, Volume 77, 2015, Pages 493-500, ISSN 0960-1481, https://doi.org/10.1016/j.renene.2014.12.032.

References – Please review all references as there are many of these that are missing basic information. For example Line 1321, line 1324, line 1325 and various others.

Editorial comments

Line 56 – “which constitute” not “which each…”

Line 414 – Cp - use subscript appropriately

Line 814 – “by installation in high altitude” should be “by installation at high altitude”