By Mostafa Ghadamyari Winter 2012 Ferdowsi university of Mashhad

1. Wind
Turbine
Mostafa Ghadamyari
Ferdowsi university
1 of Mashhad Ken Youssefi / Hsu
Winter 2012

2. Wind Turbine
How wind is created ?
Sun -> Different absorption -> dT -> density difference -> WIND
Wind energy is created when the atmosphere is heated unevenly by
the Sun, some patches of air become warmer than others. These
warm patches of air rise, other air rushes in to replace them – thus,
wind blows :
A wind turbine extracts energy from moving air by slowing the wind
down, and transferring this energy into a spinning shaft, which
usually turns a generator to produce electricity. The power in the
wind that’s available for harvest depends on both the wind speed
and the area that’s swept by the turbine blades.

3. Wind Turbine Design
Two types of turbine design: Horizontal axis and Vertical axis.
Horizontal axis turbines: Can reach higher altitude wind but requires
a substantial tower structure. Used in most modern wind turbine
designs.
Vertical axis turbines: No need to turn into wind (yaw), easier
construction and maintenance (generator and gear box are on the
ground) level, lower efficiency.
Vertical Horizontal axis
axis Turbine
Turbine

4. Drag or Lift Design
Wind turbines are designed based on either aerodynamic
Drag or Lift force.
Drag Design
The wind literally pushes the blades out of the way.
Slower rotational speeds and high torque capabilities. Useful for
providing mechanical work (water pumping e.g.).
4 Engineering 10, SJSU Ken Youssefi / Hsu

5. Lift Design
•Blade is essentially an airfoil (like wings of
airplanes).
•When air flows past the blade, a wind speed
and pressure differential is created between Lift
the upper and lower blade surfaces. The
pressure at the lower surface is greater and
thus acts to "lift" the blade.
•The lift force is translated into rotational
motion.
•Lift design generally has higher efficiency
and is used in most modern turbines.
We focus our discussion on the Lift Design Horizontal Axis Turbine.

6. Main components of a Horizontal Axis
Wind Turbine
Blades and rotor: Converts the wind power to a rotational mechanical power.
Generator: Converts the rotational mechanical power to electrical power.
Gear box: Wind turbines rotate typically between 40 rpm and 400 rpm.
Generators typically rotates at 1,200 to 1,800 rpm. Most wind turbines require
a step-up gear-box for efficient generator operation (electricity production).

7. Power Generated by HWind Turbine
Power = ½ (ρ)(A)(V)3 (Cp)
ρ = Density of air = 1.2 kg/m3 at sea level, 20 oC and dry air
A = swept area = π(radius)2, m2
V = Wind Velocity, m/sec. A
Cp = Efficiency=0.35~0.45, typically
The power in the wind is Pwind = ½ (ρ)(A)(V)3. The amount of
power that can be captured by a turbine is only 35% to 45% of
that amount (i.e., Cp = 0.35 ~ 0.45).
The theoretical maximum for Cp is 0.593, i.e., the theoretical
maximum efficiency of a turbine is 59.3%. This maximum
efficiency is called Betz Limit.

9. Rotor Blade Variables
Blade Length
Blade Number
Blade Pitch
Blade Shape
Blade Materials
Blade Weight
What should be the blade profile?
What should be the angle of attack?
How many blades to use?

10. 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

11. 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

12. Number of Blades - Three
• Balance of gyroscopic forces
• Slower rotation
– increases gearbox &
transmission costs
– More aesthetic, less noise,
fewer bird strikes

13. Wind Turbine – Blade Design
Blade Angle
The angle between the chord line of the blade and the wind
direction (called angle of attack) has a large effect on the lift
force (see figure below). Typically, maximum lift force is
achieved with 1.0 to 15.0 degrees angle of attack.
ch
or
d
Angle of lin
e
Attack Lift
Wind

14. Wind Turbine – Blade Design
Relative Wind direction
Wind direction relative to blade depends on wind speed
and rotor speed.
Angle of attack
Blade wind
e
motion tiv n relative wind
direction e l a c ti o
R e due to blade
d ir
speed
wind
direction

15. Wind Turbine – Blade Design (Shape)
To see the wind moves relative to the rotor
blades, red ribbons are attached to the tip of
the rotor blades and yellow ribbons about 1/4
of distance from the hub.
If the tip of the rotor blade moves through the
air with a tip speed = 64 m/s, the speed at the
centre of the hub is zero. 1/4 out from the hub,
the speed will then be ~16 m/s.
The yellow ribbons close to the hub will be
blown more towards the back of the turbine
than the red ribbons at the tips of the blades.

16. Wind Turbine – Blade Design (Shape)
Recall that to reach the maximum lift, the angle of
attack must be at a specific value for a given blade
design.
Angle of attack depends on the relative wind speed
which varies along the blade (highest at the tip).
To achieve an optimal angle of attack throughout the
length of the blade, the blade must be “twisted” along
the blade length.
To maintain a uniform stress on the blade, the chord
length (blade width) is narrower near the tip.

17. Wind Turbine – Blade Design
Blade size and shape
Last profile next
5-station design as seen from the tip to the hub
First profile at
the tip

18. Typical Wind Turbine Operation
0 ~ 5 m/s --- Wind speed is too low for generating power. Turbine is not
operational. Rotor is locked.
5 ~ 15 m/s ---- 5 m/s is the minimum operational speed. It is called “Cut-in
speed”. In 10 ~ 25 mph wind, generated power increases
with the wind speed.
15 ~ 25 m/s ---- Typical wind turbines reach the rated power (maximum
operating power) at wind speed of 15 m/s (called Rated wind
speed). Further increase in wind speed will not result in
substantially higher generated power by design. This is
accomplished by, for example, pitching the blade angle to
reduce the turbine efficiency.
> 25 m/s ---- Turbine is shut down when wind speed is higher than 50mph
(called “Cut-out” speed) to prevent structure failure.

19. Worldwide Energy Sources
85%
1.24%
Engineering 10, SJSU

20. Questions?
Questions?

21. Acknowledgement
Special thanks to :
-Ken Youssefi / Hsu (San Jose State University)
-Jaime Carbonell (Carnegie Mellon University)
-Joseph Rand (The Kidwind Project)
For their powerpoints about wind power.
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