Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 316

celle) and the tower head, as its components are incorporated into both system
components (Fig. 7.11).
The nacelle is usually adjusted to the respective wind direction by a gear wheel
mounted on top of the tower and operated by mechanical, hydraulic or electro-
mechanical adjustment mechanisms. Small wind energy converters, rarely built
nowadays, are provided with mechanical yaw mechanisms driven by wind vanes,
servomotors or small size windmills. Bigger converters are usually provided with
hydraulic, electromotive or electro-mechanical servo drives and are characterised
by lower costs, smaller size and bigger torque at comparable construction costs.
All converters are additionally equipped with a stopping brake to lock the re-
spective rotating mechanism. The brake compensates for low fluctuations in wind
direction that may exert strain on the rotating mechanism and thus reduce its tech-
nical service life. It also permits to lock the nacelle during prolonged downtimes
(e.g. during maintenance).
For bigger converters, the azimuth or tower head bearing is designed as anti-
friction bearing, whereas small converters are provided with friction bearings with
sliding (e.g. plastic) elements. The entire wind direction yaw mechanism is con-
trolled by a special control system that receives all relevant data from a wind di-
rection measuring device mounted at the nacelle shell.
Tower. The main function of the tower of a horizontal axis converter is to enable
wind energy utilisation at sufficient heights above ground, to absorb and securely
discharge static and dynamic stress exerted on the rotor, the power train and the
nacelle into the ground (Fig. 7.11). Another key factor regarding tower dimen-
sions and design is the natural vibration of the tower-nacelle-rotor overall system
in view of the prevention of dangerous resonance, particularly during rotor start-
up. Further influencing factors are dimensions and weight regarding transport re-
quirements and thus available roads, erection methods, cranes and accessibility of
the nacelle as well as long-term properties such as weathering resistance and ma-
terial fatigue.
Most towers are made of steel and/or concrete. As far as steel constructions are
concerned, besides the lattice towers usually observed for dated converters, there
are also anchored and self-supporting tubular steel towers in closed, commonly
conic design; the latter being the most common tower type applied nowadays.
The minimum tower height is determined by the rotor radius. Any additional
tower height is a(n economic) compromise between the increased costs at en-
hanced heights and the increased mean wind speeds and thus increased power
yield. Hence, the optimum between maximum energy yield and acceptable tower
costs needs to be determined. This is why currently tower heights vary considera-
bly with regard to site conditions; common tower heights vary between 40 and
80 m. On the mainland, due to generally lower wind speed increase at enhanced
heights, when compared to coastal sites, usually higher towers (e.g. of heights of
90 or even 100 m or above) are built.