SBft
United States
Environmental Protection
Agency
Auxiliary and Supplemental Power
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 1). A
single wind turbine can range in size from a
few kW for residential applications to more
than 5 MW2. 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 rotates . The more
commonly used horizontalaxis wind turbine
(HAWT), which rotates around a horizontal
axis, and the vertical-axis turbine (VAWT),
which is less frequently used (Figure 2).
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 in the turbine shaft
to about 1,200 to 1,500 revolutions per minute
(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.
Rotor
Ckarrelef
Ftotcr
Bade
Gearbox
Generator
Miccle
fetor
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•* Tower
Gearbox
Generator
Horizontal A»s
Vertical A»s
Figure 2. 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
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that turns the rotor so that the turbines are
always facing the wind as wind direction
changes.
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 percent 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 are stall-
controlled turbines5. The active stall-
controlled turbine, 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 for
tiny wind turbines (1 kW or less)7'8.
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
migratory flight paths9'10. Studies are being
conducted to improve turbine design so as to
reduce wildlife contact and mortality rates. In
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cold climates, ice and rime formation on
turbine blades can result in turbine failure
12
Wind
Power
Class
10 m (33 ft)"
Wind
Power
W/m2
0
100
150
200
250
300
400
1000
Speed"
m/s inph
0
4.4
5.1
5.6
6.0
6.4
7.0
9.4
0
9.8
11.5
12.5
13.4
14.3
15.7
21.1
50 m (164ft)a
Wind
Power
W/m2
0
200
300
400
500
600
800
2000
Speed"
m/s inph
0
5.6
6.4
7.0
7.5
8.0
8.8
0
12.5
14.3
15.7
16.8
17.9
19.7
11.9 24.4
Ridge Crest Estimates (Local relief > 1000 ft).
" 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'WindPower Densif.
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.
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
components
13
Large-scale wind farms can be installed for
between $1,000 and $2000 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 $3000 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 (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-MGD Atlantic County Utilities
Authority (ACUA) Wastewater Treatment
Facility in Atlantic City, New Jersey,
supplements its energy needs using wind
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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
percent 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.
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 annually16. 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 restriction of
254 ft (77.4 m) 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
1. U.S. Department of Energy (DOE).
Energy Efficiency and Renewable Energy.
Wind and Hydropower Technologies
Program, http://www.eere.energy.gov/win
dandhydro/wind how.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.
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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/fyOOosti/2714 3.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 24-26, 1998.
http://www.nrel.gov/docs/legosti/fy98/243
ll.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.
8. Danish Wind Industry Association. Power
Control of Wind Turbines. http://www.
w indpow er. org/en/tour/w trb/pow err eg. ht
m. Retrieved August 21, 2007.
9. Wind Energy Resource Atlas of the
United States.
http://rredc.nrel.gov/wind/pubs/atlas/ackn
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. and
McLeod, 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/fyO 7osti/41435.p
df. Retrieved July 10, 2007.
14. Atlantic County Utilities Authority
(ACUA). Atlantic City Wind Farm
Proj ect. http://www. acua. com/alternative/
a_projects dsply.cfm?id=214 and http:/
/www.acua. com/files/windfacts6o 7.pdf.
Retrieved July 3, 2007.
15. Wind Power for the Wastewater
Treatment Plant in Browning, Montana.
http://www. browningmontana. com/wind, h
tml. 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.
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The mention of trade names or commercial EPA 832-F-05-013
products does not constitute endorsement or Office of Water
Mention of trade recommendation for use by March 2006
the U.S. Environmental Protection Agency. Revised October 2007
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