Page 367 - Wind Energy Handbook

NUMBER OF BLADES 341

2
ÙR 16ðR 1
Nc(ì) ¼
U 1 9C l ì
Hence it can be seen that, if the number of blades is reduced from three to two,
increasing the chord by 50 percent or the rotational speed by 22.5 percent are two of
the options for preserving optimized operation at the selected wind speed. (It is
assumed that the lift coefficient is maintained at a constant value by altering the
local blade pitch to maintain a constant angle of attack.)

6.5.3 Some performance and cost comparisons

Clear-cut comparisons between two- and three-bladed machines are notoriously
difficult because of the impossibility of establishing equivalent designs. Concep-
tually, the simplest option is to increase the chord by 50 percent at all radii and
leave everything else – including rotational speed – unchanged. In the absence of
tip loss, the induction factors, and hence the annual energy yield, remain the same,
but when tip loss is included, the annual energy yield drops by about 3 percent.
However, retention of the same rotor solidity largely negates one of the main
benefits of reducing the number of blades, namely reduction in rotor cost, and so
this option will not be pursued further. Instead it is proposed to take a realistic
blade design for a three-bladed machine and look at the performance and cost
implications of using the same blade on a two-bladed machine rotating at different
speeds.
Performance comparisons are affected both by the power rating in relation to
swept area (Section 6.3) and by the aerofoil data used. In this case a 40 m diameter
stall-regulated three-bladed turbine with TR blades (see Example 5.1 in Section
5.6.3) operating at 30 r.p.m. is adopted as the baseline machine, and a power rating
2
of 500 kW. is chosen, so that the specific power (398 W/m ) is close to the norm.
Empirical three-dimensional aerofoil data for a LM 19.0 blade is used (see Figure
5.9), with maximum lift coefficient increasing from blade tip to blade root, as this
results in more accurate power curve predictions. The data are taken from Petersen
et al. (1998). The blade twist distribution is set to give maximum energy yield at a
site where the annual mean wind speed is 7 m/s, while limiting the maximum
power to 500 kW. The design is thus somewhat different from the ideal design
considered in the preceding section, which was optimized for a particular wind
speed (see Figure 6.4 for the predicted power curve).
Two options for a corresponding 40 m diameter stall-regulated two-bladed de-
sign at a site with the same annual mean wind speed are examined and the notional
energy costs compared with that for the baseline three-bladed machine. The costs of
the two-bladed design options in relation to the baseline three-bladed machine are
considered with reference to changes in the cost of the components, using the cost
shares given in Table 6.1 and the methodology of Section 6.3.1.
As before, the blade weight is assumed to increase linearly with rotational speed,
but the cost element for the blades at the baseline rotational speed is reduced by
one third. The weights of the hub, shaft, nacelle and yaw system are also assumed