Design of passive, stall
controlled wind turbine blade
Background
There in an increased interest for offshore wind farms due
to their promising capability for large scale energy
production. However offshore wind turbines need to be
designed such that they require minimum amount of
maintenance due to the fact that the maintenance cost of
offshore wind turbines is significantly larger than their
onshore counterparts. One way of achieving this is looking
at the control systems, especially the pitch system and
finding a way of replacing it or at the very least minimizing
its operation. In addition, the removal of the pitch system
has snow ball effect on the total mass of the wind turbine
thus reducing further the initial capital cost. An alternative
approach to regulate the power is the stall control method.
The research goal is to design stall controlled wing turbine
using the twist coupling effect of the structure to induce
stall while minimizing the cost of energy.
For the stall control method, the wind turbine blade is
rotated(active or passive) to stall thus reducing the torque
and power. In order to regulate the power production of
wind turbine passively, the structural twist coupling is
optimized using ISO geometric design approach.
PhD Candidate: Etana Ferede
Department: AWEP
Section: Wind Energy & Structures
Supervisor: dr. M. M. Abdalla
Promoter: dr. G.J.W. van Bussel
Start date: 1-10-2011
Funding: FLOW
Cooperation: 2-B Energy
Iso-Geometric Analysis(IGA)
IGA unifies the numerical procedures used to design and
analyze a structure using the same framework for both
procedures. It offers the possibility to integrate FEA with
CAD by using the same shape functions that are used to
generate the geometry
1
Geometrical
Aerospace Engineering
CAD and IGD use the same basis functions
No approximation of the geometry during analysis
Circumvent difficulty during mesh generation
PDEs are solved using NURBS
Parameterization of wind turbine blade
The shape of a wind turbine blade is parameterized using 5
design variables at each control point. The design
parameters are: beam axis, twist, intersection of the beam
axis with a cross-section (Cg), and a scaling factor of the
airfoils in the design pool. The advantage of
parameterizing the airfoil shape in this manner is that a
wide range of airfoil shapes can be generated with limited
airfoil shapes in the pool. It also ensures that the airfoil
shape vary continuously along the beam axis.
• Beam axis:
 = non-uniform knot vector in [0,1]
The progress made so far
Cross-sectional modeller
•
•
•
•
Prismatic beam
Heterogeneous/anisotropic
Thin-walled
Open/closed/multi cell cross section
• Parametric model of wind turbine blade
NURBs based parametric geometry generator of slender
structures. The following FEM models are generated
•
Beam model
• Shell model
Structural
• Airfoil: r2 D
•
n  p 1
 
r3D   pi Ni  
i
•
 
1  i     i 1

0 otherwise
 
 i  p 1  

Bi 1, p 1
 i  p 1   i 1
Properties of B-splines
• Piecewise polynomials of degree p
• Non-negative partition of unity
• The function Bi , p is non zero in [ i ,  i  p 1 ]
2 bladed, 5MW stall controlled wind turbine
•
 
  i

Bi , p 1
 i p   i
 ,..., 
•
•
•
•
In general there are 3 methods to control the twist coupling.
Bi , p
Bi ,0
Difference between CAD-FEM and IGA
Method for twist coupling
Definition of B-splines(Bi)
Cij r j  s Ni  
 , s   
i j
• NURBs based non-linear Timoshenko beam
Material
• Twist:     i Ni  
i
With current optimization methods, it is time consuming to
perform shape optimization thus not cost effective. New
optimization platform needs to be set-up that can
investigate the design space that includes the size and
shape parameters.
With this method size and shape
optimization
can
be
performed
sequentially
or
simultaneously
• Cg:
•
cg     cgi Ni  
i
Bi   wi
N i   
 B j   w j
j
NURBS are weighted B-splines. They are commonly used in
CAD