Types of Modern Wind Turbine
Early machines - less than twenty years ago - were fairly
small (50-100 kW, 15-20 m diameter) but there has been
a steady growth in size and output power. Several commercial
types of wind turbine now have ratings over 1 MW and
machines for the offshore market have outputs up to
3 MW.
Machine sizes have increased for two reasons. They are
cheaper and they deliver more energy. The energy yield
is improved partly because the rotor is located higher
from the ground and so intercepts higher velocity winds,
and partly because they are slightly more efficient.
The higher yields are clearly shown in Figure 13.2,
which shows data from machines in Denmark; the productivity
of the 600 kW machines is around 50% higher than that
of the 55 kW machines. Reliability has improved steadily
and most wind turbine manufacturers now guarantee availabilities
of 95%.
Figure 13.2: Energy productivity and machine rating
The majority of the world's wind turbines have three
glass-reinforced plastic blades. The power train includes
a low speed shaft, a step-up gearbox and an induction
generator, either four or six-pole. There are numerous
other possibilities, however. Wood-epoxy is an alternative
blade material and some machines have two blades. Variable
speed machines are becoming more common and most generate
power using an AC/DC/AC system. Variable speed brings
several advantages - it means that the rotor turns more
slowly in low winds (which keeps noise levels down),
it reduces the loadings on the rotor and the power conversion
system is usually able to deliver current at any specified
power factor. A few manufacturers build direct-drive
machines, without a gearbox. These are usually of the
variable speed type, with power conditioning equipment.
Towers are usually made of steel and the great majority
are of the tubular type. Lattice towers, common in the
early days, are now rare, except for very small machines
in the range 100 kW and below.
As the power in the wind increases with the cube of
the wind speed, all wind turbines need to limit the
power output in very high winds. There are two principal
means of accomplishing this, with pitch control on the
blades or with fixed, stall-controlled blades. Pitch-controlled
blades are rotated as wind speeds increase so as to
limit the power output and, once the "rated power" is
reached, a reasonably steady output can be achieved,
subject to the control system response. Stall-controlled
rotors have fixed blades which gradually stall as the
wind speed increases, thus limiting the power by passive
means. These dispense with the necessity for a pitch
control mechanism, but it is rarely possible to achieve
constant power as wind speeds rise. Once peak output
is reached the power tends to fall off with increasing
wind speed, and so the energy capture may be less than
that of a pitch-controlled machine. The merits of the
two designs are finely balanced, which accounts for
the roughly equal numbers of machines.
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