wind power

1. Wind Energy
GE Wind Turbine, The Netherlands
T. Ferguson,
University of

2. Wind Energy Basics
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Solar Driven
Immense Resource
High variability, poorly correlated to loads
Non-dispatchable
No economic storage of wind energy
Best resources require significant high
voltage transmission additions
• Power proportional to cube of wind speed
T. Ferguson,
University of

3. Wind Energy Basics
P(v)
A
Where
=
ρ 3
V
2
P(v) = power, in watts
A = area perpendicular to flow, in m2
ρ = density of fluid, in kg/m3
v = velocity of fluid, in m/s
The ratio of power of a 10 mph wind to a 5 mph wind is
substantial:
(v10/v5)3 = 8
Therefore, site selection is critical.
T. Ferguson,
University of

4. Wind Energy Basics
• Wind speed increases with height as ~ 1/7
power: v2/v1 = (h2/h1)1/7
• 50 m hub elevation compared to 30 m
height: about 7.6% higher speed
• Height enhanced by cube of velocity:
(1.076)3 = 1.245, or 24.5% more power
• Above about 1 km above earth’s surface,
wind not appreciably affected by surface
T. Ferguson,
University of

5. Wind Energy Basics
• ρair = 1.226 kg/m3 at 15 °C (288°K) and 1
atmosphere (29.92 inHg)
• Using PV = nRT (ideal gas law):
– ρ variation with Temperature extremes:
• 90°F = 32° = 305 °K: V305/V288 = 305/288 = 1.059
Therefore, ρ305 = 1/1.059 X ρ288 = 0.944 ρ288
Hot air (w.r.t. 15 °C) reduces available power by ~ 6%
• -40°F = 233°K: ρ233 = 1.236 ρ288
Cold air (w.r.t. 15 °C) increases available power by ~ 24%
• Therefore, wind power is relatively sensitive to air temp
T. Ferguson,
University of

6. Wind Energy Basics
• ρair = 1.226 kg/m3 at 15 °C (288°K) and 1
atmosphere (29.92 inHg, or 760 mmHg)
• Using PV = nRT (ideal gas law):
– ρ variation with pressure extremes:
• 30.15 in. = 0.8% increase in ρ (w.r.t. 29.92 in.)
• 29.00 in. = 3.1% decrease in ρ
• So, cold air consistent with high pressure
systems are advantageous
T. Ferguson,
University of

7. Wind Energy Systems
Now, place a wind turbine in the air flow:
• Maximum attainable wind power coefficient
= Betz ratio
= ηmax = 0.593
• Modern units capable of ~ 50% (40% after
gearbox and electrical losses)
T. Ferguson,
University of

8. Wind Energy Systems
Two or Three Blades: (Lift vs. drag design)
1. Electrical generation requires higher rotational speeds
possible with lift design
2. Lift design must avoid turbulence; hence, two or three
blades
3. Pocket of low pressure on downwind side
4. Pocket pulls blade toward it – lift
5. Direct force on blade pushes blade – drag
6. Force of lift is about 10 times that of drag
7. Both forces cause rotor to spin
8. Rotor typically at 40-400 rpm
9. AC generator requires 1200-1800 rpm
10. Hence, gearbox required
T. Ferguson,
University of

9. Wind Energy Systems
Angle of attack near
tip
Wind
Nacelle
Angle of attack near
Near hub
Hub end of
blade
1. Blade more prone to stall near hub
2. Twist is self protecting in high winds
3. Blades usually glass fiber-reinforced
plastic (steel and Al are heavy and
prone to metal fatigue)
4. Yaw control keeps perpendicular to
wind flow
5. Cable twist counter “yaws” the unit
occasionally to unravel cables
T. Ferguson,
University of
Photo from GE website;
2.5 MW Series Turbines

10. Wind Energy Systems
• Cut-in: Wind speed at which usable power
produced
• Rated: Minimum speed to produce rated power
• Cut-out: Wind speed at which unit brakes
• If interconnected to grid at speeds below cut-in,
unit would run as motor
• Once cut-in reached, generator needs load
attached to avoid overspeeding
• Units typically produce 660 v at 60 Hz;
transformer at each tower steps up to 10-35 kV
T. Ferguson,
University of

11. Wind Energy Systems
Note that, even though wind speed
(and available power) increases after
12 m/s, the turbine output remains flat
T. Ferguson,
University of
4 m/s = 9 mph
13 m/s = 29 mph
25 m/s = 55 mph
Source: GE website for 2.5 MW series wind turbines; GE claims to have
manufactured over 5000 megawatt-plus turbines

12. Wind Energy Systems
T. Ferguson,
University of
Source: GE website for 2.5 MW series wind turbine

13. Wind Energy Systems
• Adjacent turbines interfere to reduce energy
• Rules of thumb:
– Space towers at least 3 rotor diameters to get out of
windshade
– Usually 3 to 5 diameters perpendicular to prevailing
winds
– Usually 5 to 9 diameters in prevailing wind direction
– Mitigates the greater turbulence
T. Ferguson,
University of

14. Wind Energy Systems
A rough approximation of unit performance within a farm:
x
X
X
Efficiency of Nth row =
y
Prevailing Winds
F≈
X
Turbine/tower
location
T. Ferguson,
University of
X
Row
1
X
X
Row
2
-2N
e R2
Where R = x/D
D = rotor diameter
For D = 100 m and x = 1000 m,
R = 10.
Efficiency of second row is
F ≈ e(-4/100) = 96%
Efficiency of tenth row is
F ≈ e(-20/100) = 82%

15. Wind Energy Systems
“The fewer rows deep, the better”
T. Ferguson,
University of

16. Minnesota Wind Integration Study
• May, 2005: Legislature requires study of impacts
• The objectives of the study are to:
1. Evaluate the impacts on reliability and costs associated with
increasing wind capacity to 15%, 20%, and 25% of Minnesota
retail electric energy sales by 2020;
2. Identify and develop options to manage the impacts of the wind
resources;
3. Build upon prior wind integration studies and related technical
work;
4. Coordinate with recent and current regional power system study
work;
5. Produce meaningful, broadly supported results through a
technically rigorous, inclusive study process.
T. Ferguson,
University of

17. Minnesota Wind Integration Study
• The work reported here addresses two major
questions:
1. To what extent would wind generation contribute to
the electric supply
capacity needs for Minnesota electric utility companies?
2. What are the costs associated with scheduling and
operating conventional
generating resources to accommodate the variability
and uncertainty of wind
generation?
T. Ferguson,
University of

18. Meteorology
• Affects wind generation output across
wide areas
• Also affects electrical demand
• High winds during peak demand would be
ideal – integration costs would be low
• However, correlation is far from perfect
T. Ferguson,
University of

19. Meteorology
• Historical weather data from NWS used
• Wind penetration levels of 15, 20 and 25%
of projected 2020 retail electric loads
• Output at 152 grid points, at 40 MW each,
calculated every 5 minutes
• A 20% energy penetration in 2020 will
require 5000 MW of wind nameplate
T. Ferguson,
University of

20. Turbine Power Curve
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)

21. Operating Reliability Constraints
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•
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Objective: very high reliability; lowest cost
Continuously match generation to load
Voltage at all nodes within limits
Regulate frequency, maintain synchronism
System capable of withstanding major
outages or loss of major elements
T. Ferguson,
University of

22. Integration Study
Conclusions
• Increases costs to load ranging from
$2.11 (15% penetration) to $4.41/MWh
(25%)
• Combining balancing areas significantly
improves financial results
• Wider geographic area = more stable
output
• Effective load carrying capability (ELCC)
ranged from 5% to 20% of nameplate
• Ferguson,
Extensive transmission upgrades needed
T.
University of

23. Mean Wind Speed
@ 80m AGL
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)

24. Wind Net Capacity Factor
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)

25. Seasonal Output
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)

26. Seasonal Output
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)

27. Impact on Large Coal Units
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)

28. Environmental Benefits
T. Ferguson,
University of
Source: Final Report – 2006 Minnesota Wind Integration
Study – Volume 1 – November 30, 2006 (MN PUC)