The OWA was defined by a mean annual wind speed threshold
(Z6 m/s at 80 m above the sea level), bathymetry limit (r50 m
for fixed wind turbine technology), and sandy, soft-sediment
bottoms [25]. Wind speed data were obtained from the European
Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim
data set [6]. This dataset is the global atmospheric reanalysis
product of the ECMWF initiated in 2006, providing surface and
upper air as well as oceanic numerical data since 1979. According
to the US National Renewable Energy Laboratory, wind resource is
categorized into seven classes based on annual mean wind speed
(at 80 m above sea level) as follows: Class 1: 0–5.9 m/s, Class 2:
5.9–6.8 m/s, Class 3: 6.8–7.5 m/s, Class 4: 7.5–8.0 m/s, Class 5: 8.0–
8.6 m/s, Class 6: 8.6–9.4 m/s, and Class 7: 9.4–12.6 m/s. The specific
choice that was made in this work refers to potential areas for
offshore windfarm development characterized by wind speed
classes equal to or greater than Class 2, also considering that the
ERA-Interim data set underestimates (sometimes significantly)
wind speed in the Spanish seas [5]. A mean turbine array density
of 6 MW/km2 was used to estimate the theoretical installed energy
production capacity in both potential and optimal windfarm areas.
This is considered an adequate power average value obtained from
existing projects that depends on the turbine spacing (usually
ranging between 7–12 rotor diameters) and the specific wind
turbine type [23].