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.
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
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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.
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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
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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.
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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
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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.
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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
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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
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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.
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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
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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
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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
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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
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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
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19. Con….
The theoretical maximum value of Cp is 0.59. in
practical designs, the maximum achievable Cp is below
0.5.
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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
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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
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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
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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
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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
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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
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27. Con….
The available power is averaged.
Globally there is always a relatively constant
amount of wind energy being harnessed at any
one moment.
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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
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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
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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
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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.
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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
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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
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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.
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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
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