Page 404 - Wind Energy Handbook

378 COMPONENT DESIGN

regulated machines. Subsequent sections touch upon blade resonance, panel buck-
ling design and blade root fixings.

7.1.2 Aerodynamic design

The aerodynamic design encompasses the selection of aerofoil family and optimiza-
tion of the chord and twist distributions. The variation of thickness to chord ratio
along the blade also has to be considered, but this ratio is usually set at the
minimum value permitted by structural design considerations, as this minimizes
drag losses.
The process for optimizing the blade design of machines operating at a fixed tip
speed ratio is described in Section 3.7.2, where analytical expressions for the blade
geometry parameter,

Nc(ì)
ó r ºC l ¼ ºC l
2ðR

and the local inflow angle, ö, are derived as a function of the local tip speed ratio,
ºì ¼ ºr=R. (Equations (3.67a) and (3.68a)). If ºì 1, the expressions can be
approximated by

Nc(ì) 8 2
ó r ºC l ¼ ºC l ¼ and ö ¼ (7:1)
2ðR 9ºì 3ºì
If it is decided to maintain the angle of attack, Æ, and hence the lift coefficient, C l ,
constant along the blade, then these relations translate to

16ðR 1 2
c(ì) ¼ and â ¼ Æ (7:2)
9C l Nº ì 3ºì
2
so that both the chord and twist are inversely proportional to radius.
In the case of machines operating at constant rotational speed, and hence at
varying tip speed ratio, no parallel analytical solution for the optimum blade
geometry exists. Instead resort must be made to numerical methods based on blade
element – momentum theory, for example using Equations (3.51b) and (3.52a) in
Section 3.8.6.
For pitch-regulated machines, the annual energy capture attributed to the annular
ring swept out by each blade element is determined for the chosen wind speed
distribution, and the variation of energy capture with blade chord and twist at each
‘blade station’ computed. In this way the values of blade chord and twist at
each ‘blade station’ yielding maximum energy capture are identified.
For stall-regulated machines, the method is similar, but the total annual energy
capture has to be maximized within the constraint of limiting the maximum total
power output to the machine rating. The results of such an investigation are
reported by Fuglsang and Madsen (1995).