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being the most popular. However, once the water depth exceeds around 30 m the
prospects for floating wind turbines become increasingly attractive. There are a few
experimental installations, and considerable research into the feasibility and costs
of possible designs is being undertaken.
This rapid growth of wind generation has been stimulated by the financial
support mechanisms and also by a very rapid maturing of the technology. Energy
outputs have improved, partly due to better reliability, partly due to the development
of larger machines. Economies of scale produce quite modest increases of efficiency,
but larger machines, on taller towers, intercept higher wind speeds. Technical
improvements have run parallel with cost reductions and the latter, in turn, have been
partly due to economies of scale, partly to better production techniques. Finally,
wind's success has also been due to the growing awareness that the resources are
substantial - especially offshore - and that energy costs are converging with those of
the conventional thermal sources of electricity generation. In some locations the
price of wind-generated electricity is lower than prices from the thermal sources.
1.3.2 Changes in size and output
Early machines, around 1980, were fairly small (50-100 kW, 15-20 m diameter),
but the size of commercial wind turbines has steadily increased. Figure 1.2 tracks
the average size of machine installed in Germany from 2000 to 2012; during that
period, the average rated output more than doubled - from 1,114 to 2,419 kW.
As machine ratings have increased, so have hub heights. As a rule of thumb, hub
height is roughly similar to rotor diameter but many manufacturers offer the option
of taller towers to achieve even higher output. Several machines with diameters
around 70 m have hub heights of 100 m. Another way of increasing energy yields is
to increase the rating of the generator. However, the higher the rating, the shorter the
time that maximum output is achieved. It does not make economic sense to install
generators with very high ratings, as the high wind speeds needed to reach maximum
output will only be encountered for a few hours in the year. As wind turbine man-
ufacturers are subject to similar economic pressures, most have settled for rated
Figure 1.2
Average machine ratings - Germany
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