IV. VAWT ARRAY ROTATION-BASED EVOLUTION
Additional challenges are encountered when extracting large
amounts of wind energy since multiple turbines must be
arranged into a wind farm. As the turbines extract the energy
Fig. 11. Fittest evolved individuals with z-variability produced by the SGA
each generation. (a) 4th Gen. (b) 5th Gen.
Fig. 12. Example from Fig. 3 z-rotated (as in U.S. patent 7 371 135).
Fig. 13. Classic three blade Savonius.
from the wind, the energy content decreases and the amount
of turbulence increases downstream from each. See [71] for
photographs and explanation of the well-known wake effect
at the Horns Rev offshore wind farm in the North Sea. Due
to this, HAWTs must be spaced 3–5 turbine diameters apart
in the cross-wind direction and 6–10 diameters apart in the
downwind direction in order to maintain 90% of the performance
of isolated HAWTs [1]. The study of these wake
effects is therefore a very complex and important area of
research [72], as is turbine placement [73]. However, the spacing
constraints of HAWT often do not apply for VAWT, and
VAWT performance can even be increased by the exploitation
of interturbine flow effects [74]. Indeed, it has recently been
shown [1] that power densities an order of magnitude greater
can be potentially achieved by arranging VAWTs in layouts
utilizing counter-rotation that enable them to extract energy
from adjacent wakes and from above the wind farm.