The Consequences of Air Density Variations over Northeastern Scotland for Offshore Wind Energy Potential

Round 1

Reviewer 1 Report

Review of 'The consequences of air density variations over Northeastern Scotland for offshore wind energy potential'

The paper discusses the effect of variation in air density on wind power production, a relevant topic that has indeed received little attention. I have one major problem with the paper and that is that the vertical variation of air density is ignored. The authors use the surface level pressure and temperature, but correcting these to hub height already yields a nearly 2% decrease in air density. This changes the results throughout the study.

line 86: This reads a bit strange, I would suggest: "ERA5 provides hourly values of a great number of atmospheric and oceanic variables at a spatial resolution of 31 km."

line 88: Temperature at which height?

line 102-104: it would be more clear to just refer to equation 2 instead of mentioning the software package.

line 120: has --> have

line 121: a hub height of 178 m (?)

line 123: How do you obtain the roughness length using a numerical solution?

Section 2.1.2: it should be mentioned here that this ignores the import aspect of stability on extrapolation of the wind.

Section 2.2.1: this section appears to ignore the effect of vertical variation of air density: using the value at sea level or at 178 m already gives a 1.7% difference according to the method in Floors et al. (2019). This effect should be accounted for.

line 186: referential --> reference

line 208: $rho$ --> $\rho$

Fig. 3-5: because all parameters are essentially constant over the area it would be more instructive to show plots of the air density/CF as a function of time. It appears the main message you want to convey here is that air density varies substantially in time, so then using a map is a bit confusing.

line 251: "The standard air density value is closer to the first quartile than to the mean value", again you should say something about the air density at hub height, because this will be quite different.

line 280: It is unrealistic to compare to a base case with a standard air density: It is very uncommon that a wind resource assessment engineer will not use a power curve that has been specifically given for the air density that is found at the site (see e.g. Mortensen (2015)). This should be mentioned here.

line 285: duplicate --> double?

References:

Floors R, Nielsen M. (2019) Estimating Air Density Using Observations and Re-Analysis Outputs for Wind Energy Purposes. Energies. 12(11):2038.

Mortensen, N. G. (2015). 46200 Planning and Development of Wind Farms: Wind resource assessment using the WAsP software. DTU Wind Energy. DTU Wind Energy E, No. 70(ed.2)

Author Response

Please, find attached the answers to the two reviewers.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is well written and structured. Some minor typos should be corrected. 

1) Some recent references seem to be missing from the paper: See for example: Jung, Schindler, 2019, Energy 171(C), pp. 385-392, Danook et al., 2019, Case Studies in Thermal Engineering, 14. I would suggest to make a more detailed review on the subject. 

2) Modify the list of abbreviations and add the corresponding measurement units 

3) In p. 7, lines 167-168 comment on the selection of the nearest grid point. Why you didn't apply a distance weighting scheme for the nearest four points?

4) In p. 5, line 121, correct the hub height. 

5) In p. 5, line 124, rephrase the sentence "is always around 0.0002 m". It is not always around this number.

Author Response

Please, find attached the answers to the two reviewers

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

All the questions and comments have been addressed satisfactorily.

Title: why is there the word 'two' here? Probably a typo?

Author Response

Thanks!! Yes, it is a huge typo. Removed.