These consist of propeller blades
turning generators located at the top of pole and tower supports.
Capacities of the generators, depending on wind conditions, may vary
from a fraction of a megawatt to several megawatts.
The outputs generally are varying with
the wind velocity. The electric output increasing as the cube with
the wind velocity, i.e. double wind velocity will produce 3 or 8
times as much output. As velocity tends to increase with height above
ground, the higher the mounting, the greater the velocity and
electric output.
Economics plays a great part. The
higher the structure, the greater its cost, and this must be balanced
against the greater costs of the generators, until a satisfactory
ratio of costs is reached. Also the length of the blades of the
propellers must be taken into account, the longer the blades or
blades (usually two or three) the greater the electric output,
although there is a practical mechanical limit as to their size.
As winds are variable in velocity and
direction, propeller blade angles or change in pitch may be varied
and the whole turret or nacelle mounted on the top of the structure
may be rotated to keep the propeller facing into the wind (known as
the yaw). The extent of these maneuvers again influenced by
economics.
As the electrical output sought may
require more than one such unit, care must be taken in situating
these units so that the air flow disturbed by one operating unit does
not affect its companion units. Electrically, the output must conform
to strict limits of voltage and frequency to interconnect with other
units or the utility system, if desired.
Hence the speed and pitch of the
propeller blades must also be controlled so that if the wind should
increase above or drop below certain limits, the unit is taken out of
service (and, of course, when there is no wind).
As winds tend to be greater in open
areas, such as off shore or level agricultural areas, costs of
associated transmission and distribution systems (if any) involving
transformer, switching and protective circuitry must be considered as
well as associated maintenance expenses.
On the other hand, areas where such
wind farms may be located may still be used for their original
purposes or other productive uses. Practically all wind power
installations employ propeller driven horizontal shafts. Air turbines
driving vertical shafts are sometimes used where appearance and bird
safety may be considered
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