WIND TURBINE AERODYNAMICS

1
The Kidwind Project www.kidwind.org
2
WIND TURBINE AERODYNAMICS

• BLADE BASICS

3
Number of Blades One

• Rotor must move more rapidly to capture same
  amount of wind
• Gearbox ratio reduced
• Added weight of counterbalance negates some
  benefits of lighter design
• Higher speed means more noise, visual, and
  wildlife impacts
• Blades easier to install because entire rotor can
  be assembled on ground
• Captures 10 less energy than two blade design
• Ultimately provide no cost savings

Slide by Joseph Rand
4
Number of Blades - Two

• Advantages disadvantages similar to one blade
• Need teetering hub and or shock absorbers because
  of gyroscopic imbalances
• Capture 5 less energy than three blade designs

Slide by Joseph Rand
5
Number of Blades - Three

• Balance of gyroscopic forces
• Slower rotation
• increases gearbox transmission costs
• More aesthetic, less noise, fewer bird strikes

Slide by Joseph Rand
6
Blade Composition Wood

• Wood
• Strong, light weight, cheap, abundant, flexible
• Popular on do-it yourself turbines
• Solid plank
• Laminates
• Veneers
• Composites

Slide by Joseph Rand
7
Blade CompositionMetal

• Steel
• Heavy expensive
• Aluminum
• Lighter-weight and easy to work with
• Expensive
• Subject to metal fatigue

Slide by Joseph Rand
8
Blade ConstructionFiberglass

• Lightweight, strong, inexpensive, good fatigue
  characteristics
• Variety of manufacturing processes
• Cloth over frame
• Pultrusion
• Filament winding to produce spars
• Most modern large turbines use fiberglass

Slide by Joseph Rand
9
NASA AMES 80X120 / 40X80 WIND TUNNEL
10
Wind Tunnel drive fans
11
Airfoil Nomenclaturewind turbines use the same
aerodynamic principals as aircraft
12
Airfoil Shape
Just like the wings of an airplane, wind turbine
blades use the airfoil shape to create lift and
maximize efficiency.
Slide by Joseph Rand
13
Lift Drag Forces

• The Lift Force is perpendicular to the direction
  of motion. We want to make this force BIG.
• The Drag Force is parallel to the direction of
  motion. We want to make this force small.

a low
a medium lt10 degrees
a High Stall!!
Slide by Joseph Rand
14
Lift Drag Forces
Equations for Lift Drag are as
follows Coefficient of Lift CL
L/(q8S) Coefficient of Drag CD D/(q8S) L
Lift force D Drag force S Airfoil frontage
area q8 Dynamic Pressure 1/2?V2 ? Air
density V Velocity
15
Airfoil in stall

• Stall arises due to separation of flow from
  airfoil
• Stall results in decreasing lift coefficient with
  increasing angle of attack
• Stall behavior complicated due to blade rotation

Slide by Joseph Rand
16
(No Transcript)
17
Lift and Drag vs. Angle of Attack
18
Pitch Control vs. Stall Control

• Pitch Control
• Blades rotate out of the wind when wind speed
  becomes too great
• Stall Control
• Blades are at a fixed pitch that starts to stall
  when wind speed is too great
• Pitch can be adjusted for particular locations
  wind regime
• Active Stall Control
• Many larger turbines today have active pitch
  control that turns the blades towards stall when
  wind speeds are too great

19
Twist Taper

• Speed through the air of a point on the blade
  changes with distance from hub
• Therefore, tip speed ratio varies as well
• To optimize angle of attack all along blade, it
  must twist from root to tip

Slide by Joseph Rand
20
(No Transcript)
21
Tip-Speed Ratio
Tip-speed ratio is the ratio of the speed of the
rotating blade tip to the speed of the free
stream wind. There is an optimum angle of attack
which creates the highest lift to drag
ratio. Because angle of attack is dependant on
wind speed, there is an optimum tip-speed ratio
OR
R
Where, O rotational speed in radians /sec R
Rotor Radius V Wind Free Stream Velocity
Slide by Joseph Rand
22
Tip-Speed Ratio
The speed and torque at which wind turbines
rotate must be controlled for several
reasons To optimize the aerodynamic efficiency
To keep mechanical equipment with in design
limits To keep the rotor hub within
centripetal force limits To keep the rotor
tower within their strength limits To enable
maintenance, bring turbine to full stop To
reduce noise levels, at higher speeds (gt60m/s)
vortices are shed from the tips causing the
swishing sound as the blades move through the air
23
Tip-Speed Ratio
GE 1.5 MW turbine Rotor Diameter (d) 77m Rated
wind speed (V) 14 m/s Revolutions / minute
(rpm) 20 Circumference of Circle (C) pd
(3.14) 77 Tip velocity Vt Rpm distance
traveled (3.14)7720/60 Vt 80 m/s (179
mph) TSR Vt/V 80/14 5.7 Modern Wind
turbines operate at tip speeds of 4 8
24
Performance Over Range of Tip Speed Ratios

• Power Coefficient Varies with Tip Speed Ratio
• Characterized by Cp vs. Tip Speed Ratio Curve

25
Power Coefficient
The power coefficient tells you how efficiently a
turbine converts the energy in the wind to
electricity.
Cp Electrical power output Wind
Energy input
Wind Speed
This turbine shows max power at 9 m/s
26
Power Coefficient
Lets assume the turbine above converts 70 of the
Betz Limit into electricity. Therefore, the Cp
of this wind turbine would be
0.7 x 0.59 0.41. So this wind turbine converts
41 of the available wind energy into
electricity. This is a pretty good coefficient
of power. Cp .41
27
Power Coefficient
Actual power generated for a Bergey XL1 (1kW
rating) At 28mph XL1 produces 1,200 watts
Theoretical amount is 6,000 watts What is the
Coefficient of power (Cp)?
28
Cp Energy Output / Energy Input
Cp 1200 / 6000 .2 or 20 at 28 mph winds
29
Capacity Factor
Tip Speed Ratio
30
Rotor Solidity
Solidity is the ratio of total rotor planform
area to total swept area Low solidity
(lt.10) Typical of electric generating wind
turbines .10 Cut in wind speed higher Lower
torque generated Higher efficiencies possible up
to 30 - 40 Lower material cost
R
a
A
Solidity 3a/A
31
Rotor Solidity
High solidity (gt0.80) low cut in speed high
torque Need higher torque to lift pumps piston
and raise water to surface, especially in light
winds. Material intensive At most only 15
efficient Wind flows around rotor not through
rotor, must retain enough kinetic energy so it
can keep moving to make way for air behind. Helps
prevent stall by reducing the difference in
pressure between upper lower surfaces. Uses
less blade pitch to move same amount of air.
32
Rotor Solidity
GE Turbine 1.5 MW Rotor Diameter (d)
77m Planform 140 m2 Swept Area 4657 m2
Solidity Planform / Swept Area Solidity 140 /
4657 .03 Solidity .03
33
Because climbing wind turbines is fun!
34
Blade Basics Study Guide
How many blades does the typical commercial wind
turbine have? ________ For what reasons is this
number of blades used? A_________________________
___________ B____________________________________
C____________________________________ What are
three common materials used to construct blades?
A_________________ B________________
C________________ The chord of an airfoil
is___________________________________________ The
mean camber line is_____________________________
_______________ __________________________________
______________________________ The two forces
that act on a blade are? ______________
_________________ Which force is the most
important? __________________________________
35
Blade Basics Study Guide
What is the other force use for?__________________
__________________ How is the force that turns
the blades created? ________________________
__________________________________________________
___________ What is stall? _______________________
___________________________ How is stall used in
wind turbines? _________________________________ W
hat are three ways blades are controlled in high
winds? A _____________________________________ B
_____________________________________ C
_____________________________________ Why is
there twist in turbine blades? ___________________
____________ _____________________________________
________________________ Tip-Speed Ratio is
_____________________________________________
36
Blade Basics Study Guide
Give three reasons why the tip-speed ration is
important? A ____________________________________
________________ B ______________________________
______________________ C ________________________
____________________________ Power Coefficient
tells me what? __________________________________
__________________________________________________
___________ Power Coefficient formula is?
____________________________________ Rotor
solidity is ______________________________________
_________ What are three characteristics of a low
solidity turbine blade? A ______________________
__________________________ B ____________________
____________________________ C
________________________________________________

37
Blade Basics Study Guide
What are three characteristics of a high solidity
turbine blade? A ________________________________
_____________________ B _________________________
____________________________ C
__________________________________________________
___ Why do we climb Wind Turbines?
_________________________________
38
QUESTIONS