Wind Energy
Basics
Power from the wind
o The
kinetic energy of wind is harvested using wind turbines
to generate electricty.
o Among various renewable energy sources, wind energy is
the second most technologically advanced renewable energy
source; hydropwer is the first.
o Wind blows in every cover of the earth; however, it does
not blow constantly.
o In addition, it must maintain a certain speed to be effective
for running a wind turbine and generating electricity.
Global Wind Power Cumulative Capacity
Figure 1
China Leads the World in Wind Capacity
Total Installed Generating Capacity (MW)
Figure 2
Regional suitability for onshore wind power generation
Figure 3 : Source: Eltamaly et al. 2012, Rehman et al. 2003, Apricum analysis.
Wind speeds measured at 10 m height.
Wind Turbine Components
Figure 4
The Mechanics of a Wind Turbine
Energy and power from wind
• The kinetic energy of wind is converted to mechanical or
electrical energy using wind turbines.
• The amount of energy captured by the rotor depends on the
density of the air, the rotor area and the wind speed.
• No other factor is more important to the amount of power
available in the wind than the speed of the wind.
• Power is a cubic function of wind speed
• 20% increase in wind speed means 73% more power.
• Doubling wind speed means 8 times more power.
Calculation of Wind Power
Wind Power
Pw = ½ρAV3
Swept Area: A = πR2 Area of the circle
swept by the rotor (m2).
R
Figure 6
Figure 5
•Example: V = 10m/s; A = (2 m)2 = 4 m2
= 1.2 kg/m3

1.2 kg
P
m 4m 10 m s 
2
kg  m 2
kg  m m
m
 2400
 2400 2   2400 N 
3
s
s
s
s
N m
Theoretical Maximum
P  2400
 2400 W
s
3
2
3
Betz Limit: 59.3% of the theoretical is the maximum amount extractable by a
wind energy conversion device (WEC).
PBetz  0.593 (2400W )  1423.2W
Practical Maximum
Calculation of Electrical power output
and capacity factor for a wind turbine
1.The electrical power output :
Pe  C p m g Pw
Where Cp is the efficiency coefficient of performance when the wind
is converted to mechanical power.
ηm is mechanical transmission efficiency and ηg is the electricity
transmission efficiency.
The optimistic values for these coefficients are: Cp = 0.45, ηm = 0.95
and ηg = 0.9, which give an overall efficiency of 38%.
For a given system, Pw and Pe will vary with wind speed.
2. The capacity Factor:
The capacity factor of a wind turbine is the actuel energy output for
the year divided by the energy ouput if the turbine operated at its
rated power output for the entire year.
Idealized wind turbine power curve
The power curve is an important item for a specific wind
turbine. The wind power curve also shows the relationship
between wind speed and generator electrical output.
Figure 7
Idealized wind turbine power curve
1. Cut-in wind speed
When the wind speed is below the cut-in wind speed (VC) shown in the Figure 7, the wind
turbines cannot start. Power in the low speed wind is not sufficient to overcome friction in
the drive train of the turbine. The generator is not able to generate any useful power below
cut in speed.
2. Rated wind speed
We can see from the Figure 7 that as the wind speed increases, the power delivered by the
generator will increase as the cube of wind speed. When the wind speed reached VR the
rated wind speed, the generator can deliver the rated power. If the wind speed exceeds VR,
there must be some methods to control the wind power or else the generator may be
damaged. Basically, there are three control approaches for large wind power machines:
active pitch-control, passive stall-control, and the combination of the two ways.
3. Cut-out or furling wind speed
Sometimes, the wind is too strong to damage the wind turbine. In the Figure 7 this wind
speed is called as cut-out or the furling wind speed. Above VF , the output power is zero.
In terms of active pitch-controlled and passive stall-controlled machines, the rotor can be
stopped by rotating the blades about their longitudinal axis to create a stall. However, for
the stall-controlled machines, there will be the spring-loaded on the large turbine and
rotating tips on the ends of the blades. When it is necessary, the hydraulic system will trip
the spring and blade tips rotate 90◦ out of the wind and stop the turbine.
Carnage!
Figure 8
Factors affecting wind power
1. Wind statistics
Wind resource is a highly variable power source, and there are several methods of
characterizing this variability. The most common method is the power duration curve.
Another method is to use a statistical representation, particularly a Weibull distribution
function. Long term wind records are used to select the rated wind speed for wind electric
generators. The wind is characterized by a Weibull density function.
2. Load factor
 There are two main objectives in wind turbine design. The first is to maximize the
average power output. The second one is to meet the necessary load factor requirement
of the load. The load factor is very important when the generator is pumping irrigation
water in asynchronous mode.
 Commonly assumed long-term average load factors may be anywhere from 25% to
30%.
3. Seasonal and diurnal variation of wind power
It is clear that the seasonal and diurnal variations have significant effects on wind.
The diurnal variation can be reduced by increasing the height of the wind power
generator tower. In the early morning, the average power is about 80% of the long term
annual average power. On the other hand, in early afternoon hours, the average power
can be 120% of the long term average power.
Factors affecting wind power
4. Impact of tower height:
Wind speed will increase with the height because the friction at earth surface is large. The
rate of the increase of wind speed that is often used to characterize the impact of the
roughness of the earth’s surface on wind speed is given as:
v  H 
   

 v0   H 0 

where v is the wind speed at height H, vo is the nominal wind speed at height Ho, and α is the
friction coefficient. This can be translated into a substantial increase in power at greater
heights.
Table 1.1 gives the typical
values of friction coefficient
for
various
terrain
characteristics.
Factors affecting wind power
5. Wind turbine sitting
The factors that should be considered while installing wind generator are
as follow:
(1) Availability of land.
(2) Availability of power grid (for a grid connected system).
(3) Accessibility of site.
(4) Terrain and soil.
(5) Frequency of lighting strokes
Once the wind resource at a particular site has been established, the
next factor that should be considered is the availability of land. The area
of the land required depends upon the size of wind farm. In order to
optimize the power output from a given site, some additional information
is needed, such as wind rose, wind speeds, vegetation, topography,
ground roughness, etc. In addition other information such as convenient
access to the wind farm site, load bearing capacity of the soil, frequency
of cyclones, earthquakes, etc., should also be considered.
Optimizing rotor diameter and generator rated power
• Figure 9 shows the trade-offs between rotor diameter and generator size as
methods to increase the energy delivered by a wind turbine. In terms of Fig. 9(a),
increasing the rotor diameter and keeping the same generator will shift the
power curve upward. In this situation, the turbine generator can get the rated
power at a lower wind speed. For Fig. 9(b), keeping the same rotor but increasing
the generator size will allow the power curve to continue upward to the new
rated power. Basically, for the lower speed winds, the generator rated power
need not change, but for the high wind speed area, increasing the rated power is
a good strategy.
Figure 9: (a) Increasing rotor diameter gives the rate power at lower wind speed, (b)
increasing the generator size increases rate power
Modern Wind Turbines
Turbines can be categorized into two classes based on the
orientation of the rotor. Most are horizontal axis wind turbines
(HAWT), but there are some with blades that spin around a vertical
axis (VAWT). Examples of the two types are shown in Figure 10.
Figure 10: Horizontal axis wind turbines (HAWT) are either upwind machines
(a) or downwind machines (b). Vertical axis wind turbines (VAWT) accept the
wind from any direction (c).
Modern Wind Turbines
• Vertical Axis Advantages
– Can place generator on ground
– You don’t need a yaw mechanism for wind angle
• Disadvantages
– Lower wind speeds at ground level
– Less efficiency
– Requires a “push” (generally, an external power source is
required to start the rotation)
• Horizontal Advantages
– Higher wind speeds
– Great efficiency
• Disadvantages
– Angle of turbine is relevant
– Difficult access to generator for repairs
Horizontal-Axis Wind Turbines
Small (10 kW)
• Homes
• Farms
• Remote Applications
(e.g. water pumping,
telecom sites,
icemaking)
Intermediate
(10-250 kW)
• Village Power
• Hybrid Systems
• Distributed Power
Large (250 kW - 2+MW)
• Central Station Wind Farms
• Distributed Power
Wind Turbine Size-Power Comparison
Figure 12
Various components of a wind turbine
Figure 13
Comparison Between Turbines
Wind turbines may be compared against each other by comparing their efficiency
coefficient of performance (CP) against tip-speed-ratio (TSR).
The efficiency coefficient of performance, also known as the power coefficients, is
defined by :
Cp
P

Pw
1
P  C p R 2 v 3
2
For a given windspeed, rotor efficiency is a function of the rate at which the rotor
turns. If the rotor turns too slowly, the efficiency drops off since the blades are letting
too much wind pass by unaffected. If the rotor turns too fast, efficiency is reduced as
the turbulence caused by one blade increasingly affects the blade that follows. The
usual way to illustrate rotor efficiency is to present it as a function of its tip-speed
ratio (TSR). The tip-speed-ratio is the speed at which the outer tip of the blade is
moving divided by the windspeed:
TSR 
speed of rotor tip
rpm *  * D

wind speed
60 * v
where rpm is the rotor speed, revolutions per minute; D is the rotor diameter
(m); and v is the wind speed (m/s) upwind of the turbine.
Typical efficiency for various rotor types
A plot of typical efficiency for various rotor types versus TSR is given in Figure 14.
The American multiblade spins relatively slowly, with an optimal TSR of less than 1
and maximum efficiency just over 30%. The two- and three-blade rotors spin much
faster, with optimum TSR in the 4–6 range and maximum efficiencies of roughly
40–50%. Also shown is a line corresponding to an “ideal efficiency,” which
approaches the Betz limit as the rotor speed increases. The curvature in the
maximum efficiency line reflects the fact that a slowly turning rotor does not
intercept all of the wind, which reduces the maximum possible efficiency to
something below the Betz limit.
Figure 14: Typical efficiency for
various rotor types