This presentation will describe the basics of wind power generation the technologies used in wind power. the energy conversion process used in wind power system are explained. This material was prepared for Debre Brihan Univesity 4th year power engineering students of 2017.

1. Chapter 7
Wind energy
By: MESFIN MEGRA.

2. WIND POWER – WHAT IS IT?
All renewable energy (except tidal and
geothermal power), ultimately comes from the
sun.
The earth receives 350 W/m2 from the sun.
About 2% of this energy (= 7 W/m2) is
converted to wind energy. 12/19/2017
2

3. WIND POWER – WHAT IS IT? Cont....
Differential heating of the earths surface and
atmosphere induces vertical and horizontal air
currents that are affected by the earth’s
rotation and contours of the land.
12/19/2017
3

4. Uneven heating:
Intensity of solar energy is varies due to the
angle of the sun (the equator vs the poles).
Land heats up faster than water does, but also
loses heat faster.
These difference in air temperature across the
globe can create wind. 12/19/2017
4

5. Uneven heating: Con….
35% of wind energy (2.45 W/m2) is dissipated
in the first kilometers above the Earth’s surface
and available for turbine.
12/19/2017
5

6. Available power:
Over a period of one year, the wind energy (E) is
approximately.
𝐸 = 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 ∗ 𝐸𝑎𝑟𝑡ℎ′
𝑠 𝑆𝐴 ∗ 𝑆𝑒𝑐𝑜𝑛𝑑𝑠 𝑝𝑒𝑟 𝑦𝑒𝑎𝑟
= 2.45 𝑊/𝑚2 5.1 𝑥 1014
𝑚2
3.2 𝑥 107
𝑠
= 4.0 𝑥 1022
𝐽
Which is 200 times larger than our energy
consumption (2.0 𝑥 1020
𝐽). 12/19/2017
6

7. Maximum Power Extracted
Calculate the power extracted from wind.
Calculate kinetic energy, 𝐾𝐸 =
1
2
𝑚𝑣2 of air
passing through the rotor of the wind turbine.
Measure mass of the air travelling through area
of circle swept out by rotor blades in time ∆𝑡.
12/19/2017
7

8. Con….
Time t = 0:
Time ∆𝑡:
𝑣. ∆𝑡
Figure 1: At time t=0, mass of air is just about to pass through the loop, but
∆𝑡 later, the mass of air is passed through the loop. The mass of this piece of
air is the product of its density 𝜌, area A, and length 𝑣. ∆𝑡.
A
12/19/2017
8

9. Con….
From this you can find the mass.
M𝑎𝑠𝑠 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 ∗ 𝑣𝑜𝑙𝑢𝑚𝑒
= 𝜌 ∗ 𝐴 ∗ 𝑣 ∗ ∆𝑡
𝑣 𝑖𝑠 𝑡ℎ𝑒 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟
𝜌 𝑖𝑠 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟 1.23 𝑓𝑜𝑟 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑇 𝑎𝑛𝑑 𝑃
∆𝑡 𝑖𝑠 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡𝑖𝑚𝑒 𝑓𝑜𝑟 𝑎 𝑢𝑛𝑖𝑡 𝑎𝑖𝑟 𝑡𝑜 𝑝𝑎𝑠𝑠 𝑡ℎ𝑟𝑜𝑢𝑔ℎ
𝐴 𝑖𝑠 𝑎𝑟𝑒𝑎 𝑠𝑤𝑒𝑝𝑡 𝑏𝑦 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒𝑠, 𝑛𝑜𝑡 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒 𝑎𝑟𝑒𝑎
12/19/2017
9

10. Con….
Therefore the kinetic energy, KE, is found to be.
𝐾𝐸 =
1
2
𝑚𝑣2 =
1
2
𝜌𝐴∆𝑡𝑣3
While the power of the wind passing through the
loop is.
𝑃 =
1
2
𝜌𝐴∆𝑡𝑣3
∆𝑡
=
𝟏
𝟐
𝝆𝑨𝒗 𝟑
12/19/2017
10

11. Con….
But turbines can’t extract all of the kinetic energy
of the wind.
Why not?
The above expression is true for a single wind
turbine in constant wind condition.
12/19/2017
11

12. Actual Power Extracted from the Wind
The actual power extracted by the rotor blades is
the difference between the upstream and the
downstream wind powers.
𝑃 𝑜
=
1
2
𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑 . (𝑉2
− 𝑉𝑜
2
)
𝑉 = 𝑢𝑝𝑠𝑡𝑟𝑒𝑎𝑚 𝑤𝑖𝑛𝑑 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝑉𝑜= 𝑑𝑜𝑤𝑛𝑠𝑡𝑟𝑒𝑎𝑚 𝑤𝑖𝑛𝑑 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝑃𝑜 = 𝑚𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑜𝑢𝑡𝑝𝑢𝑡 12/19/2017
12

13. Con….
The mass flow rate of air through the rotating
blades is, therefore, derived by multiplying the
density with the average velocity. That is
M𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 = 𝜌. 𝐴.
𝑉 + 𝑉𝑜
2
12/19/2017
13

14. Con….
The mechanical power extracted by the rotor,
which is driving the electrical generator is
therefore:
𝑃𝑜 =
1
2
𝜌. 𝐴.
𝑉 + 𝑉𝑜
2
. (𝑉2
− 𝑉𝑜
2
)
12/19/2017
14

15. Con….
The above expression can be algebraically
rearranged:
𝑃𝑜 =
1
2
𝜌. 𝐴. 𝑉3
.
1 +
𝑉𝑜
𝑉
1 −
𝑉𝑜
𝑉
2
2
12/19/2017
15

16. Con….
The power extracted by the blade is customarily
expressed as a fraction of the upstream wind power
as follows:
𝑃𝑜 =
1
2
𝜌. 𝐴. 𝑉3
. 𝐶 𝑝
where
𝐶 𝑝 =
1 +
𝑉𝑜
𝑉
1 −
𝑉𝑜
𝑉
2
2
12/19/2017
16

17. Con….
For a given upstream wind speed, the value of Cp
depends on the ratio of the downstream to the
upstream wind speeds that is
𝑉𝑜
𝑉
.
The plot of power coefficient versus (
𝑉𝑜
𝑉
) shows that Cp
is a single, maximum value function
12/19/2017
17

18. Con….
It has a maximum value of 0.59 (Betz limit) when the
( 𝑉𝑜
𝑉) is one-third (0.33).
The maximum power is extracted from the wind at
speed ratio, when the downstream wind speed equals
one-third of the upstream speed.
𝑃𝑚𝑎𝑥 =
1
2
𝜌. 𝐴. 𝑉3
. 0.59 12/19/2017
18

19. Con….
The theoretical maximum value of Cp is 0.59. in
practical designs, the maximum achievable Cp is below
0.5.
12/19/2017
19

20. Efficiency
Wind turbines are not 100% efficient.
Power = efficiency * max. power extracted
𝑃𝑜 = 𝜂
1
2
𝜌. 𝐴. 𝑉3
= 𝜂
1
2
𝜌. 𝑉3
. 𝜋
𝑑
2
2
=
1
8
𝜂. 𝜌. 𝑉3
. 𝜋𝑑2 12/19/2017
20

21. Factor affecting wind power harnessing
The power produced by a wind turbine depends
on:
Rotor area
Air density
Wind speed
Wind shear 12/19/2017
21

22. Con….
Wind shear is a difference in wind speed and direction
over a short distance and is caused by mountains,
coastline, and weather pattern.
Air density increases with colder temperatures,
decreased with altitude, and decreased humidity
Wind speed increases the farther you get away from
ground. 12/19/2017
22

23. Con….
As you get higher off the ground, the air speed
increases. The rotor are tilted slightly upwards so that
each part of the rotor is exposed to the same speed
12/19/2017
23

24. Con….
Wind turbines are most efficient when wind
moves uniformly in the same direction.
Turbulence is caused by buildings, trees, and land
formations (wind break).
The edge of a continental shelf, high ground and
tundra have low turbulence and are the best
locations to build a turbine
12/19/2017
24

25. Feasibility
Local wind speed is also an important factor
since power ∝ (𝑤𝑖𝑛𝑑 𝑠𝑝𝑒𝑒𝑑)3
.
The local wind speed should be, on average, at
least 7 m/s at 25 m above the earth’s surface in
order to make harnessing wind from it
worthwhile. 12/19/2017
25

26. Demand and Dependability
Wind is not locally predictable in the short term,
and so its use should be limited to only fulfill 5-
15% of the total energy demand of the area.
Setting up turbines in several locations makes
wind energy more reliable.
12/19/2017
26

27. Con….
The available power is averaged.
Globally there is always a relatively constant
amount of wind energy being harnessed at any
one moment.
12/19/2017
27

28. Configuration of Wind Turbine
12/19/2017
28

29. HAWT’s vs VAWT’s
HAWT’s
Advantage
Variable pitch
Tall tower- higher
wind speeds
Steady angle attack
Disadvantage
Tall tower/large blade-
difficult to install
High visibility
Yaw control is necessary
12/19/2017
29

30. HAWT’s vs VAWT’s cont.…
VAWT’s
Advantage
Smaller support
structure
Yaw control not needed
Generator components
located on the ground
Less noise compared to
HAWT’s
Disadvantage
Cyclic loading makes fatigue
failure more likely
Lower wind speed due to
shorter structure
12/19/2017
30

31. Con….
12/19/2017
31

32. Description of a double-fed WTG
12/19/2017
32

33. 1. Blade
2.Pitch Drive
3.Hub
4.Generator rotor
5.Generator stator
6.Yaw drive
7.Wind sensors
8.Service hoist
9.Top Box
10.Main frame
11.Nacelle cover
12.Tower
Nacelle
12/19/2017
33

34. 12/19/2017
34

35. Wind Farms
Cities and countries need huge wind farms to satisfy
their energy needs.
To optimize energy production in a wind farm,
Turbines are spread 5 – 9 rotor diameter apart in the
prevailing wind direction and,
3 – 5 rotor diameter apart in perpendicular direction
12/19/2017
35

36. Cont.….
12/19/2017
36

37. Wind Turbine Control
In general, wind turbines are designed to
operate when the incident wind is high enough
to generate electricity and to shut down when
the wind speed exceeds 25 to 30 m/s.
12/19/2017
37

38. Yaw Control
The yaw control continuously orients the rotor
in the direction of the wind.
It can be as simple as the tail vane or
More complex on modern towers, using wind
direction sensor and electric or hydraulic drive
motors, to orient the rotor 12/19/2017
38

39. speed Control
The speed control methods fall into the
following categories.
No speed control: here the wind systems are
designed to withstand the extreme wind speed.
Yaw and tilt control: here the rotor axis is shifted out
of the wind direction when wind speed exceeds the
design limit.
12/19/2017
39

40. Cont.….
Pitch control: it changes the pitch of the blade with
the changing wind speed to regulate the rotor
speed.
Stall control: here when the wind speed exceeds the
safe limit on the system, the blades are shifted into
a position such that they stall.
12/19/2017
40

41. Advantage vs dis advantage
Doesn’t produce hazardous wastes.
Use less space than traditional power stations.
Arguments against include fears of damages
From collapsing turbines
Noise
Unreliable power source
A less attractive skyline
Unnecessary high bird fatality
12/19/2017
41

42. Thank you!
12/19/2017
42
Thank you