Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 109

Figure 18: RPM and force response to gust.

experts working in large companies and on the other hand real testing is possible
only for small wind turbines. An overview of small wind turbines is given fi rst.
According to IEC, small WTs have a swept area smaller than 200 m 2 . This gives
a diameter of less than 17 m. Sometimes a subclass of WT which have swept area
even less than 40 m 2 are also considered. These turbines have a diameter of 7 m or
less. The main problems for these turbines are the manufacturing costs and a com-
parable aerodynamic performance to their larger relatives. Due to Reynolds num-
ber effects, the

ma c of small wind turbines is approximately 0.3 instead of 0.5 for
the best standard (2-3 MW) commercial turbines. This is mainly due to the much
lower L2D ratio at Reynolds numbers below 10 5 . Thus several attempts have been
made to fi nd thicker (> 10%) profi les giving a better performance. See Figs 19 and
20 for examples.
Further information can be found in [ 63 ].

P

6.2 Main-stream wind turbines

It is obvious that most efforts have been made to optimize so-called main-stream
wind turbines with several MW rated power but below 3 MW. The author [50]
was able to compare measurements of three different blade sets (see Fig. 21) on
the same machine. Although the tip-speed ratio was 8.5 for one blade and only 7 for
the other blades the

ma c difference varies from 0.49 to 0.46 resulting in an
improvement of 6%. As a conclusion this shows that dedicated or tailored profi les
and blades can improve the performance signifi cantly.

P

ma c currently seems to have settled down at around 0.52 for the best turbines
as to be expected from the discussion in Section 2.
New developments therefore aim to reduce loads without losing power. This can
be achieved by reducing

P

max

c against c L ( L 2 D - max ). In contrast a safe distance

L

ma c also prevents early stalling. During the 1990s a lot of new aerodynamic
profi les were investigated in the US (Seri), in Delft (DU) and at Risø (Risø A,B

to

L