Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 65

the rotor. Cables are usually sized and tensioned to a soft condition, producing a fundamental
system frequency greater than the rotor speed but less than the blade passing frequency ( i.e. ,
the number of blades times the rotor speed).
The height of the support stand is equal to the ground clearance. The minimum ground
clearance required for safety is usually much less for a VAWT than for a HAWT, because
the speed of a VAWT blade near the ground is relatively low. Any increase in support-
stand height above the minimum ground clearance again depends on a trade-off between the
marginal increase in energy capture and the marginal increase in system cost. The latter is
relatively high because many cable changes must be made in order to maintain the desired
fundamental system frequency as the upper bearing is raised. These include larger cables,
more cables, and higher cable tension, which in turn require more capacity in rotor bearings,
more buckling strength in the rotor column and support stand, and more weight in the cable
foundations.
All these cost factors combine to keep the elevation of the center of a typical Darrieus
VAWT rotor lower than that of a HAWT with the same swept area. Because average wind
speed generally increases with elevation, the annual average wind power density of a VAWT
rotor (in watts of wind power per square meter of swept area) is usually lower than that of a
comparable HAWT rotor at the same site.
The Foundation Subsystem
VAWT foundations include a central foundation under the support stand and a cable
foundation at the lower end of each set of support cables. Because the central foundation is
not subject to uplift or overturning loads, its weight is usually less than the combined weights
of the cable foundations, and it is not as wide or as heavily reinforced as a HAWT tower
foundation. VAWT cable foundations contain steel cable anchors, are heavily reinforced to
resist tensile stresses, and are sized to prevent uplift or shifting which would result in loss of
cable tension.
The Ground Equipment Station
The equipment on the ground that interfaces a VAWT with the electric utility system or
other user is essentially the same as that required for a HAWT of the same power rating. This
equipment may all be housed in an enclosure separate from the VAWT, or part of it may be
located on the central foundation.