United States
    Environmental Protection
                                          United States Environmental Protection Agency
                          	August 2013

                         Renewable Energy Fact Sheet:
                         Wind Turbines
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.
                                                           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.

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

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

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.



10m (33 ft)"
1 1 
J 100
U 150

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

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
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

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).

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

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
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.
l.U.S.  Department  of  Energy  (DOE).
Energy Efficiency and Renewable Energy. Wind
and Hydropower Technologies
Program, http://www.eere.energy.gov/win
Retrieved July 9, 2007.

2. California Energy Commission.
California Distributed Energy Resource Guide.
http://www. energy, ca.gov/distgen/
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.

7. Sathyajith.M. 2006. Wind Energy:
Fundamentals, Resource Analysis, and
Economics. Published 2006.Springer. ISBN

8. Danish Wind Industry Association. Powe
Control  of Wind  Turbines. http://www.
Retrieved August 21, 2007.

9. Wind Energy Resource Atlas of the United
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,

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.
Retrieved July 9, 2007.

16. Fargo, North Dakota.  http://www.prairie
m/04 25 06.html and  http://www.uswater
.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

    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