Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 332

wind direction. Depending on the specific site conditions, the respective distance
factor varies between 8 and 10, with regard to the main wind direction k A,x , and
between 4 and 5 crosswise to the main wind direction k A,y . Due to space restric-
tions as well as for economic purposes also lower distance factors k A,x of about 4
are possible. The minimum space required around a wind energy converter A WEC
is thus calculated by Equation (7.23), whereas d Rot refers to the rotor diameter of
the converter.

2

A = k k d

(7.23)

WEC A , x A , y Rot

If there is no preferred wind direction, as this may be the case for the inland,
and if there are no topographic restrictions impeding optimised converter ar-
rangement, shadowing must be optimised for all cardinal directions. Around any
wind energy converter an approximately circular area should thus be reserved,
which, for the sake of simplicity, is represented by an isosceles hexagon. The dis-
tance factor k A to be observed for this kind of converter arrangement varies be-
tween a range similar to that of converter arrangement with preferred wind direc-
tion (i.e. according to site and wind conditions between 6 and 15). The area to be
reserved to meet the above requirements is calculated by Equation (7.24).

3

( ) 2

A = k d (7.24)

WEC A Rot

4

If the site-specific optimum distances between the individual converters are ob-
served, shadowing is minimised while space consumption is optimised. The losses
incurred when compared to undisturbed operation are indicated by the wind park
efficiency; depending on the specific site conditions efficiencies vary between 90
and 98 %. In spite of the losses caused by shadowing, regardless whether opti-
mum distances are observed, installing converters in the form of wind parks is
usually more effective, since within the scope of an economic overall analysis the
described losses are usually overcompensated for by grid connection, access roads
and the generally low mean costs for maintenance, repair and monitoring of
losses.
With regard to offshore wind parks, located far off the shoreline, optimum
wind park sizes exceed by far the size of onshore wind parks. This is due to the
high grid connection costs. Costs are rather determined by the length of the re-
quired underwater cables than by the connected capacity /7-9/. For the actual di-
mensioning of offshore wind parks the generally higher wind speeds and lower
turbulence must be considered. However, experience has shown that the reduced
mean wind speed and the increased turbulence within the downwind flow of off-
shore wind energy converters are much more severe when compared to onshore
converters. When comparing offshore and onshore wind parks of identical dimen-
sions, offshore wind parks are characterised by lower efficiencies and more fa-