Page 359 - Wind Energy Handbook

MACHINE RATING 333

6.3 Machine Rating

The machine rating determines the wind speed (known as rated wind speed) at
which rated power is reached. If the rating is too high, the rated power will only be
reached rarely, so the cost of the drive train and generator will not be justified by
the energy yield. On the other hand, if the rating is reduced below the optimum
then the cost of the rotor and its supporting structure will be excessive in relation to
energy yield.
The investigation of the optimum relationship between rotor diameter and rated
power can be carried out with the help of the cost modelling technique described in
the previous section.

6.3.1 Simplified cost model for optimizing machine rating in relation
to diameter

Assuming that the blade planform and twist distribution are fixed, the annual
energy yield can be calculated for a number of rated wind speeds, for a given
annual mean wind speed and Weibull shape factor. The turbine rotational speed is
assumed to vary in proportion to the rated wind speed for simplicity. The aim of
the optimization is to obtain the minimum cost of energy, which requires know-
ledge of how the costs of the various turbine components would be affected by the
rating change. Although, in theory, this could only be rigorously derived from
carrying out a series of detailed turbine designs, in practice it is possible to obtain a
useful indication of cost trends by identifying the parameters driving the design of
each component category and investigating their dependence on the rated wind
speed. If the cost split between various components is known for a baseline
machine, these cost trends can then be applied to it in order to determine the
optimum rating. In this case the cost shares given in Table 6.1 for a 60 m diameter,
1.5 MW machine are used once again. The machine is assumed to be pitch regulated.
The parameters determining the design of the major components are set out first.

(1) Blade weight: the following assumptions are made:
• the blade planform is constant;

• the blade design is governed by out-of-plane bending moments in fatigue;
• the out-of-plane bending moment fluctuations are proportional to the product
of the wind speed fluctuation and the rotational speed (see Equation (5.25) in
Section 5.7.5);
• the rotational speed is proportional to rated wind speed as already stated;
• the blade skin thickness is proportional to the out-of-plane bending moment
ranges.
Hence the blade skin thickness and therefore the blade weight are propor-
tional to the rated wind speed.