r/ERA
United States
Environmental Protection
Agency
United States Environmental Protection Agency
August 2013
Renewable Energy Fact Sheet:
Wind Turbines
DESCRIPTION
Wind turbines can be used as Auxiliary and
Supplemental Power Sources (ASPSs) for
wastewater treatment plants (WWTPs). A wind
turbine is a machine, or windmill, that converts the
energy in wind into mechanical energy. A wind
generator then converts the mechanical energy to
electricity1. The generator is equipped with fan
blades and placed at the top of a tall tower. The
tower is tall so that high wind velocities can be
easily harnessed without being affected by
turbulence caused by obstacles on the ground,
such as trees, hills, and buildings. Individual wind
turbines are typically grouped together to give
rise to a wind farm (Figure 7). A single wind
turbine can range in size from a few kilowatts
(kW) for residential applications to more than 5
Megawatts (MW)2. Many wind farms are
producing energy on a megawatt (MW) scale,
ranging from a few MW to tens of MW.
Figure 1: Wind turbine farms.
There are primarily two types of wind turbines
which are based on the axis about which the
turbine rotates3 The more commonly used
horizontal axis wind turbine (HAWT), which
rotates around a horizontal axis, and the vertical-
axis turbine (VAWT), which is less frequently
used (Figure 2the two types of rotation).
HAWTs typically have three blades and are
operated with the blades facing the wind (upwind).
The wind rotates the blades which in turn spin a
shaft attached to a generator. A gear box connects
the low-speed turbine shaft to the high-speed
generator shaft. These gears increase the
rotational speeds from about 30 to 60 rotations
per minute (rpm) in the turbine shaft to about
1,200 to 1,500 rpm (the rotational speed required
by most generators) in the generator shaft. The
rotational energy produced by the shaft spins
copper coils within a magnet housed in the
generator. This magnet excites the electrons in
the wire, producing electricity. The quantity of
electricity depends on how fast the shaft can spin
in the magnetic field, the strength of the
magnetic field, and the quantity and arrangement
of the copper coils.
Ifctor
Ltomeler
Horizontal Axis
Vertical Axis
Figure 2: Two Wind Turbine Configurations.
To produce electricity at relatively low costs, the
shaft must rotate at high speeds. HAWTs also
include a computer operated yaw drive that
turns the rotor so that the turbines are always
facing the wind as wind direction changes.
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Vertical axis wind turbines (VAWTs) are not
widely used because they produce pulsating torque
during each revolution and provide a level of
difficulty when being mounted vertically on the
tower (Figure 2).
Commercially available wind turbines range
between 5 kW for small residential turbines and 5
MW for large scale utilities. Wind turbines are
20% to 40% efficient at converting wind into
energy. The typical life span of a wind turbine is
20 years, with routine maintenance required every
six months. Wind turbine power output is variable
due to the fluctuation in wind speed; however,
when coupled with an energy storage device,
wind power can provide a steady power output.
Wind turbines, called variable-speed turbines, can
be equipped with control features that regulate the
power at high wind velocities. These variable-
speed turbines can optimize power output without
exceeding the turbine's performance limits.
Common variable-speed wind turbines include
pitch-controlled, stall- controlled, and active stall-
controlled. An electronic controller checks the
power output several times per second. When
power output becomes too high, pitch-controlled
turbines turns the rotor blades slightly out of the
wind's path protecting the system from excessive
stress. The blades are then turned back into the
wind whenever the wind speed drops4.
During times of high wind speeds, stall-controlled
turbines create turbulence on the side of the rotor
blade which is not facing the wind. This stall
prevents the lifting force of the rotor blade from
acting on the rotor. About two thirds of the wind
turbines currently being installed in the world
today are stall-controlled turbine5. The active stall-
controlled turbines, which is more common
among larger wind turbines (1 MW and up), will
increase the angle of attack of the rotor blades
causing the blades go into a deeper stall (killing
the lift force of the blade), thus wasting the excess
energy in the wind6. Other power control methods
include ailerons (flaps) to control the power of the
rotor and to yaw (swing) the rotor partly out of the
wind to decrease power. Yaw control is used only
"7 o ^^
for tiny wind turbines (1 kW or less) ' . These
control mechanisms allow the turbine to operate
with the greatest aerodynamic efficiency, and
reduces excessive loads on the drive train,
providing reduced maintenance and longer turbine
life.
ADVANTAGES AND DISADVANTAGES
There are several advantages associated with the
use of wind power to generate electricity.
Depending on the size of the wind farm, energy
production can be inexpensive when compared to
conventional power production methods. The cost
to generate the electricity decreases as the size of
the farms increase. Wind turbine power is an
infinitely sustainable form of energy that does not
require any fuel for operation and generates no
harmful air or water pollution-produces no green
house gases and toxic or radioactive waste. In
addition, the land below each turbine can still be
used for animal grazing or farming.
Disadvantages of using wind turbines include the
need for more land space to support a wind farm
and the difficulty in having a location with enough
wind to produce maximum efficiency and power
(Figure 3). The placement of turbines in areas of
high population density can also result in aesthetic
problems. Other drawbacks include death of birds
and bats due to collision with spinning turbine
blades and turbine obstruction in their flight
paths9'10. Studies are being conducted to improve
turbine design so as to reduce wildlife contact and
mortality rates. In cold climates, ice and rime
formation on turbine blades can result in turbine
failure12
A heating system or a special coating of the
blade's surface can reduce the risk of failure.
However, the potential for ice to be thrown great
distances during windy conditions is a potential
health hazard. A recommended safety zone
area should be factored into the design
specification to reduce public access, potential
risks, and sound.
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Wind
Power
Class
1
2
10m (33 ft)"
Wind
Power
W/m'
1 1 °
J 100
U 150
LJ200
4
5
6
7
• 250
• 300
• 400
Jiooo
Speed"
ni/s mph
0 0
4.4 9.8
5.1 11.5
5.6 12.5
6.0 13.4
6.4 14.3
7.0 15.7
9.4 21.1
50m (164ft)11
Wind
Power
W/m2
0
200
300
400
500
600
800
2000
Speed"
m/s mph
0 0
5.6 12.5
6.4 14.3
7.0 15.7
7.5 16.8
8.0 17.9
8.8 19.7
11.9 24.4
Ridge Crest Estimates (Local relief >1000 ft).
a Vertical extrapolation of wind speed based on the 1/7 power law.
b Mean wind speed is based on Rayleigh speed distribution of
equivalent mean wind power density. Wind speed is for standard
sea- level conditions.
Figure 3: U.S. Annual Average Wind Power
Classes of Wind Power Density9
COST
The 2007 U.S. Department of Energy (DOE)
Annual Report on the development and trends of
wind power reports that the cost of wind power is
nearly very competitive with those of
conventional power technologies. And this does not
account for the environmental and health
benefits of using a non-polluting source of
energy. It is expected that over time, wind energy
cost will decrease as most conventional generation
technology costs continue to increase. Since 2002,
the cost of turbines has been on the rise because of
increase cost of input material, energy prices, and in
some cases, shortages in certain turbine
components13.
Large-scale wind farms can be installed for between
$1,000 and $2,000 per kilowatt. The cost of
electricity produced from wind farms can be
attributed to the annual capacity factor, location,
wind quality, and installation and maintenance
costs. The cost per kilowatt for small-scale wind
turbines is still relatively high, with costs up to
$3,000 per kilowatt. However, the cost per
kW decreases as the size of the turbine increases.
Wind availability at a site also influences cost.
Wind turbines installed in very windy locations
generates less expensive electricity than the same
unit installed in a less windy location. It is
therefore important to assess wind speeds at the
potential site during the planning stage
(See Figure 3).
APPLICATIONS OF WIND POWER AT
WASTEWATER TREATMENT PLANTS
Wind power use in the U.S. constitutes about
16% of the world's wind capacity. It is the
second largest new resource added to the U.S.
electrical grid (in terms of nameplate capacity)13.
In 2006, new wind plants contributed roughly
19% of new nameplate capacity, compared to
13% in 2005. Wind turbines have been installed
in 22 states, with Texas, California, and Iowa
leading the nation in annual capacity growth13.
The 40 MOD Atlantic County Utilities Authority
(ACUA) Wastewater Treatment Facility in
Atlantic City, New Jersey, supplements its
energy needs using wind turbines74 (Figure
4). When operating at design wind speeds of
over 12 mph, the five 1.5 MW wind turbines at
this facility are capable of producing up to 7.5
MW of electrical energy. Since this is much
more than the average 2.5 MW of power needed
each day by this facility, the remaining energy is
sold to the local power grid.
Power production occurs only when wind speed
is greater than 7 mph and shuts down at speeds
in excess of 45 mph to protect the machinery
inside. Therefore, on an annual basis, the ACUA
wind farm can supply more than 60% of the
electricity required by the plant. The remaining
electricity can be bought from the local power
grid when windmills are not at peak capacity
(during calm or gusty weather). The cost of wind
generated electricity is 7.90 per kWh delivered
for the next 20 years, while the current cost
delivered by the electrical grid is 120 per kWh
and rising. The estimated cost of the 7.5
MW wind farm was $12.5 million with an
expected cost saving of $350,000 per year15
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Figure 4: ACUA Wastewater Treatment Plant
wind farm in Atlantic City, New Jersey.
To encourage the use of renewable energy
resources, the town of Browning, Montana, and the
Blackfeet Indian Tribe have installed four Bergey
Excel 10 kW wind turbines adjacent to the town's
sewage treatment plant. The turbines provide about
one-quarter of the plant's electricity, displacing
energy bought from the grid. In the City of Fargo,
North Dakota, the installation of a 1.5 MW wind
turbine to provide 85% of the annual electricity
used by the city's wastewater treatment plant is
being considered. The Fargo wind turbine is
estimated to cost $2.4 million and could save
the plant about $203,000 in energy costs
annually
16
The Lynn wastewater treatment plant in
Massachusetts, that services the counties of Lynn,
Saugus, Swampscott, and Nahant, is considering the
installation of one or more wind turbine
generators to supply a substantial portion of the
plant's electricity. As of May 2007,
information is being collected on possible wind
turbine model options that comply with the Federal
Aviation Administration (FAA) height restrictions
of 254 feet (77 meters) above ground level; and
each model's estimated energy production, setback
requirements, and potential sound impacts17.
Several other WWTPs throughout the U.S. have
installed or are considering the installation of wind
turbines to temper the rising costs of electricity.
REFERENCES
l.U.S. Department of Energy (DOE).
Energy Efficiency and Renewable Energy. Wind
and Hydropower Technologies
Program, http://www.eere.energy.gov/win
dandhydro/windhow.html.
Retrieved July 9, 2007.
2. California Energy Commission.
California Distributed Energy Resource Guide.
http://www. energy, ca.gov/distgen/
equipment/wind/wind.html.
Retrieved August 21, 2007.
3. Wikipedia, the free encyclopedia. http://en.
wikipedia.org/wiki/Wind turbine.
Retrieved July 9, 2007.
4. Muljadi, E. and Butterfield, C.P. 2000.
Pitch-Controlled Variable-Speed Wind
Turbine Generation, NREL/CP-500-
27143. Conference paper, presented at the
1999 IEEE Industry Applications Society
Annual Meeting, Phoenix, Arizona October 3-7,
1999. http://www.nrel.gov
/docs/JyOOosti/2 7143.pdf.
Retrieved August 21, 2007.
5. Muljadi, E., Pierce, K., and Migliore, P.
1998. Control Strategy for Variable- Speed,
Stall-Regulated Wind Turbines.NREL/CP-500-
24311- UC Category: 1211. Conference paper,
presented at American Controls Conference,
Philadelphia, PA, June
une26,1998.. http://www.nrel.gov/docs/legosti
/fy98/24 311.pdf.
Retrieved August 21, 2007.
6. Polinder, H., Bang,D.,van Rooij, R.P.J.O.M.,
McDonald, A.S., and Mueller,M.A. 2007.
10-MW Wind Turbine Direct-Drive Generator
Design with Pitch or Active Speed Stall Control.
Electric Machines & Drives Conference, 2007.
IEMDC apos; 07. IEEE International, 2, 3-5, pp.
1390-1395.
7. Sathyajith.M. 2006. Wind Energy:
Fundamentals, Resource Analysis, and
Economics. Published 2006.Springer. ISBN
3540309055.
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8. Danish Wind Industry Association. Powe
Control of Wind Turbines. http://www.
windpower.org/en/tour/wtrb/powerreg.htm.
Retrieved August 21, 2007.
9. Wind Energy Resource Atlas of the United
§\&\.QS.http://rredc.nrel.gov/wind/puljs/atlas/ack
n owledge.html
10. Nicholls, B. and Racey, P.A. 2007. Bats
Avoid Radar Installations:Could Electromagnetic
Fields Deter Bats from Colliding with Wind
Turbines? PLoS ONE, 2, 3, e297.
11. Fielding, A.H., Whitfield, D.P. andMcLeod,
D.R. 2006. Spatial association as an indicator of
the potential for future interactions between wind
energy developments and golden eagles Aquila
chrysaetos in Scotland. Biological Conservation,
131,3,359-369.
12. Henry Seifert, 2004 Technical Requirements
for Rotor Blades Operating in Cold Climates,
DEWI Magazin Nr. 24.
13. U.S. Department of Energy (DOE). 2007.
Energy Efficiency and Renewable Energy.
Annual Report on U.S. Wind Power Installation,
Cost, and Performance Trends:, 2006.
DOE/GO-102007-243 3.
http://www. nrel.gov/docs/JyO 7osti/41435.pdf.
Retrieved July 10,2007.
14. Atlantic County Utilities Authority (AOUA),
Atlantic City Wind Farm
Project, http://www.acuc.com/alternative/ajtroj
ect dsply. cfm ?id=214
and http://www.acua.com/files/wmdfacts6o7.pdf
Retrieved July 9, 2007.
15. Wind Power for the Wastewater Treatment
Plant in Browning, Montana.
http://www.browningmontana.com/wind.html.
Retrieved July 9, 2007.
16. Fargo, North Dakota. http://www.prairie
public.org/features/riverwatch/news/foru
m/04 25 06.html and http://www.uswater
news.com/archives/arcquality/6fargmigh4
.html. Retrieved July 9, 2007.
17. Update to Wind Feasibility Study for Lynn,
Massachusetts, May 2007.
http://www. mtpc. org/Project%20Delivera
bles/Comm Wind/Lynn/TurbineSitingMe
moMay%202 2007.PDF.
Retrieved July 9, 2007.
Some of the information presented in
this fact sheet was provided by the
manufacturer or vendor and could not
be verified by the EPA.
The mention of trade names, specific
vendors, or products does not
represent an actual or presumed
endorsement, preference, or
acceptance by the EPA or federal
government.
Stated results, conclusions, usage, or
practices do not necessarily
represent the views or policies of
the EPA.
Environmental Protection Agency
Office of Wastewater Management
EPA 832-F-13-017
August 2013
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