This presentation describes how to design a furling system for a small wind turbine. It reviews three different approaches to furling system design from literature and then presents the design approach that was used for designing the furling system of the HOLI 300 small wind turbine.

1. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Furling for Small Wind Turbines
Dimensioning of a Cheap, Reliable and Multipurpose Solution
Florian Roscheck
Advanced Wind Turbine Systems Course, Kiel University of Applied Sciences
December 17, 2013
Florian Roscheck Furling for Small Wind Turbines 1/19

2. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Agenda
1 Introduction
What is Furling?
Horizontal Furling
2 The Dimensioning Challenge
Mission Statement
Balancing Many Design Parameters
3 Review: Furling Models
Monterrey Parameter Study
NREL Model
Do-it-yourself Dimensioning
4 The SWT Contest Approach
Requirements and Refined Mission Statement
Dimensioning Process
Getting the furl-in wind speed from NREL Equations
5 Conclusion
Florian Roscheck Furling for Small Wind Turbines 2/19

3. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
1 Introduction
What is Furling?
Horizontal Furling
2 The Dimensioning Challenge
3 Review: Furling Models
4 The SWT Contest Approach
5 Conclusion
Florian Roscheck Furling for Small Wind Turbines 3/19

4. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
What is Furling?
Purpose of furling:
Overspeed protection
Adjust rotor to wind direction via yaw system
0
60
120
180
0 10 20 30 40
RotorSpeedin1/min
Wind Speed in m/s
Example: Furling with const. TSR
Basic working principle:
Turning rotor out of the wind at
high wind speeds
Decreasing rotor speed
Decreasing generated power
Wood, David: Small Wind Turbines. London : Springer, 2011. – ISBN 9781849961745
Muljadi, J. ; Forsyth, T. ; Butterfield, C.P.: Soft-Stall Control versus Furling Control for Small
Wind Turbine Power Regulation (Windpower ’98). Bakersfield : NREL, 1998.
Florian Roscheck Furling for Small Wind Turbines 4/19

5. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Horizontal Furling
Figure: Vertical axis wind turbine with horizontal furling system in furled state
Sanchez, Orlando: Furling Turbine. https://www.youtube.com/watch?v=x6jiVABIAUk,
accessed Dec 11, 2013
Florian Roscheck Furling for Small Wind Turbines 5/19

6. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
1 Introduction
2 The Dimensioning Challenge
Mission Statement
Balancing Many Design Parameters
3 Review: Furling Models
4 The SWT Contest Approach
5 Conclusion
Florian Roscheck Furling for Small Wind Turbines 6/19

7. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Mission Statement
We want to find a furling system geometry
which fulfills certain requirements
by predicting
static and dynamic furling behavior.
Florian Roscheck Furling for Small Wind Turbines 7/19

8. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Balancing Many Design Parameters
Input: Ten design parameters
One weight
Two angles
Seven lengths
Output: Furling Behavior
Furl-In Speed
Furl-Out Speed
Time to Furl
System Oscillation
Challenge:
Balancing design parameters to
get required furling behavior
Audierene, Etienne ; Elizondo, Jorge ; Bergami, Leonardo ; Ibarra, Humberto ; Probst, Oliver:
Analysis of the furling behavior of small wind turbines.
In: Applied Energy 87 (2010), p. 2278–2292
Florian Roscheck Furling for Small Wind Turbines 8/19

9. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
1 Introduction
2 The Dimensioning Challenge
3 Review: Furling Models
Monterrey Parameter Study
NREL Model
Do-it-yourself Dimensioning
4 The SWT Contest Approach
5 Conclusion
Florian Roscheck Furling for Small Wind Turbines 9/19

10. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Monterrey Parameter Study
ð12Þ
he in-
hifted
ð13Þ
partial
ð14Þ
nd the
ð15Þ
refer-
pass-
vector
awing
first-order system with an inhomogeneity given both by the aero-
dynamic moments Qh and Q/ and the non-linear kinetic energy
terms. For a more realistic description additional terms accounting
for the friction of the bearings of both yawing and furling axes have
to be included. The equation of motion then reads:
1 0 0 0
0 1 0 0
0 0 J1 J3
0 0 J3 J2
0
B
B
B
@
1
C
C
C
A
_h1
_w1
_h2
_w2
0
B
B
B
@
1
C
C
C
A
¼
h2
w2
Qh À J0
1
_w2
_h2 À J0
3
_w2
2 À b1h2
Qw þ 1
2
J0
1
_h2
2 À @V
@w
À b2w2
0
B
B
B
B
@
1
C
C
C
C
A
¼
f1
f2
f3
f4
0
B
B
B
@
1
C
C
C
A
ð22Þ
Using the Gauss–Seidel method we can explicitly state the solution
of the system for a given time step t:
_h1
_w1
_h2
_w2
0
B
B
B
@
1
C
C
C
A
¼
f1
f2
f3
J1
À J3
J1
J1f4ÀJ3f3
J1J2ÀJ2
3
J1f4ÀJ3f3
J1J2ÀJ2
3
0
B
B
B
B
B
@
1
C
C
C
C
C
A
ð23Þ
The four angular variables appearing on the left-hand side of Eq.
(23) can now be calculated in an iterative manner.
2.3.2. Calculation of the aerodynamic moments
2.3.2.1. Yaw moments. First the torque created by the rotor thrust
_w1
_h2
_w2
B
B
B
@
C
C
C
A
¼
w2
Qh À J0
1
_w2
_h2 À J0
3
_w2
2
Qw þ 1
2
J0
1
_h2
2 À @V
@w
B
B
B
B
@
C
C
C
C
A
ð21Þ
tion the system can be viewed as a locally linear
with an inhomogeneity given both by the aero-
s Qh and Q/ and the non-linear kinetic energy
realistic description additional terms accounting
he bearings of both yawing and furling axes have
e equation of motion then reads:
_h1
_w1
_h2
_w2
0
B
B
B
@
1
C
C
C
A
¼
h2
w2
Qh À J0
1
_w2
_h2 À J0
3
_w2
2 À b1h2
Qw þ 1
2
J0
1
_h2
2 À @V
@w
À b2w2
0
B
B
B
B
@
1
C
C
C
C
A
¼
f1
f2
f3
f4
0
B
B
B
@
1
C
C
C
A
ð22Þ
eidel method we can explicitly state the solution
a given time step t:
f1
f2
3
1
J1f4ÀJ3f3
J1J2ÀJ2
3
1f4ÀJ3f3
J1J2ÀJ2
3
1
C
C
C
C
C
A
ð23Þ
Figure: Part of equation system
used in Monterrey Study
Characteristics:
Mainly steady wind conditions
Simple rotor thrust model
Wake model
Results: Yaw and furl angle over time
Problems:
Difficult to debug
Needs damping correction
Useful:
Geometry recommendations
Dynamic behaviour information
Audierene, Etienne ; Elizondo, Jorge ; Bergami, Leonardo ; Ibarra, Humberto ; Probst, Oliver:
Analysis of the furling behavior of small wind turbines.
In: Applied Energy 87 (2010), p. 2278–2292
Florian Roscheck Furling for Small Wind Turbines 10/19

11. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
NREL Model
4-15 M. Bikdash 23
Wind Energy Program Wind Energy Program
Relative wind
direction
θ
wθ−
θ∆
Electrical
Load
Yaw/Furling Dynamics
Wind Speed V
Mechanical
Torque
rpm
Moments,
Thrusts
Generator
Controller
General Wind Turbine Model
Aerodynamics
(YawDyn Fuzzy)⇒
Figure: NREL Model Flowchart
Characteristics:
Complex rotor aerodynamics
Generator and controller model
Results:
Yaw and furl angle over time
Steady-state equilibrium
solutions
Problems:
High calculation time
Wake not considered
Useful: Ready-to-use steady-state
equilibrium formulas
Bikdash, Marwan: Modeling and Control of a Bergey-Type furling Wind Turbine,
http://wind.nrel.gov/furling/bikdash.pdf, accessed May 5, 2009
Florian Roscheck Furling for Small Wind Turbines 11/19

12. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Do-it-yourself Dimensioning
Figure: Logo from Back Shed
Project Website
Characteristics:
Five static equlibrium equations
Non-scientific
Results: Furling Tail Geometry
Problems:
Assumes CP of 0.5
Very limited configuration
possibilities
Wake, controller etc. neglected
Useful: Estimate if on right track
Littleford, Glenn: TheBackShed.com – Furling,
http://www.thebackshed.com/Windmill/Docs/Furling.asp, accessed Dec 13, 2013
Florian Roscheck Furling for Small Wind Turbines 12/19

13. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
1 Introduction
2 The Dimensioning Challenge
3 Review: Furling Models
4 The SWT Contest Approach
Requirements and Refined Mission Statement
Dimensioning Process
Getting the furl-in wind speed from NREL Equations
5 Conclusion
Florian Roscheck Furling for Small Wind Turbines 13/19

14. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Requirements and Refined Mission Statement
Original mission statement:
“We want to find a furling system geometry which fulfills certain
requirements by predicting static and dynamic furling behavior.“
Requirements:
Well-defined furl-in wind speed vin
Low hysteresis ∆v = vin − vout
Adjustable vin in a given range ∆vin with adding weight ∆m to tail
Constraints: Only very limited time and limited software tools
Refined mission statement:
“We want to find a furling system geometry for a given vin with a low ∆v,
which is adjustable in the range ∆vin by adding ∆m to the tail
using a fast, manual iterative process.“
Florian Roscheck Furling for Small Wind Turbines 14/19

15. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Dimensioning Process
1 Estimate thrust
2 Estimate nacelle and rotor inertias,
choose L2, L1 := 0
3 Choose β, γ, L4 acc. to Monterrey
Study
4 Choose airfoil geometry acc. to
Wood
5 Choose iteration initial values acc.
to Monterrey, DIY
6 Iterate over NREL equations
changing L3, L5, m to acquire
requested vin
Florian Roscheck Furling for Small Wind Turbines 15/19

16. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Getting the furl-in wind speed from NREL Equations
∆θ: Yaw error resulting from thrust force on rotor and tail fin
vwind: Wind speed, vin: Furl-in wind speed
1 Solve yaw equilibrium condition for ∆θ, 0 vwind 1.5 · vcutout:
0 = Fthrust(vwind, ∆θ)L5 + Mtail(vwind, ∆θ)
Result: Curve f(vwind) = ∆θ
2 Calculate vin(∆θ), f(vwind)min ∆θ f(vwind)max:
vin(∆θ) = mgL3 sin γ cos γ
1
2
ρc·s
2
(L3+2
3
c) cos β(CL cos ∆θ+CD sin ∆θ)
Result: Curve f(∆θ) = vin
3 Plot f(vwind) = ∆θ, f(∆θ) = vin in a single diagram
4 Determine intersection point, read furl-in wind speed vin
Bikdash, Marwan: Modeling and Control of a Bergey-Type furling Wind Turbine,
http://wind.nrel.gov/furling/bikdash.pdf, accessed May 5, 2009
Florian Roscheck Furling for Small Wind Turbines 16/19

17. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
Getting the furl-in wind speed from NREL Equations
0
2
4
6
8
10
0 5 10 15 20 25
∆θindeg
vwind, vin in m/s
Example: Plot of solutions of NREL equations for specific geometry
f(vwind) = ∆θ
f(∆θ) = vin
m + ∆mm + 0.5∆mm
vin = 11.1 m/s
Florian Roscheck Furling for Small Wind Turbines 17/19

18. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
1 Introduction
2 The Dimensioning Challenge
3 Review: Furling Models
4 The SWT Contest Approach
5 Conclusion
Florian Roscheck Furling for Small Wind Turbines 18/19

19. Introduction The Dimensioning Challenge Review: Furling Models The SWT Contest Approach Conclusion
In case somebody ever asks you about furling for small wind turbines. . .
Many influence parameters
Limited literature around
You need to test your system!
Florian Roscheck Furling for Small Wind Turbines 19/19