Blade Design, Manufacture, and Testing - Small Wind Turbines: Analysis, Design, and Application - page 126

The fitness array comprises the transposes of the arrays for C P and T s . The two
blade element functions for calculating power and starting time are very obvious
modifications of those presented previously. J is estimated as in Chap. 6 . Most of
the changes were made to the starting and power programs were aimed at reducing
the execution time. Using Matlab's profiler, it was found that nearly 80% of the
execution time was spent in the power calculation. Because of its iterative nature,
the calculations could not be vectorised. The main changes were in determining
the lift and drag. The experimental data for each Re was fitted using least squares
to generate a data file with equal increments in a, and the same minimum and
maximum a. Matlab interpolation routine interp1 in LandD.m in Chap. 5 was
replaced by a faster, special purpose linear interpolation in the blade element
program rather than a separate function. These changes resulted in significant time
savings but the execution time for the calculations in the next section was between
1 and 2 h on a moderately powerful laptop.
Further reductions in time could be achieved by parallelising the calculations as
the calculations for each blade are independent of all others, but this has not been
explored.

7.4 Example Blade Design: A 750 W Turbine

Figure 7.1 shows the characteristics of a typical small permanent magnet generator
(PMG): the 500A unit made by Ginlong Technologies. 2 This section describes the
use of the numerical optimisation routines of the previous section to design blades
for that generator.
Three blades are to be used. This is the common, but not universal number, as
seen from Chap. 1 . The main arguments in favour of three blades are:

• The aesthetic ''criteria'' that three blades are more visually appealing than, say,
two. However, this is more the case for large, slow turning turbines, than fast,
smaller ones, and
• The gyroscopic moments during yaw (as the blades rotate) are lower in mag-
nitude and more constant for three rather than two blades. This issue is revisited
in Chaps. 8 and 9 where it s seen that gyroscopic effects are significant for
turbine safety.

Against these arguments are the facts (1) that three blades are 50% more
expensive than two, and (2) it was shown in Chap. 5 that the choice of N does not
greatly influence the maximum C P but changes the value of k at which this
maximum occurs. Nevertheless, convention will be followed for this example.
The rated wind speed is set at 10 m/s. This is not an obvious choice and some
discussion is necessary, partly because there is a wide range of rated speeds quoted
for commercially-available small wind turbines. From Fig. 1.8 , it is clear that most

2

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