United States
     Environmental Protection
Auxiliary and Supplemental Power
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 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.
                                                        *  Tower
                                                    Horizontal As
                                     Vertical As
                    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

that  turns the rotor so that the turbines are
always  facing  the wind as wind direction

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.


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

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

cold  climates,  ice  and  rime  formation on
turbine blades can result in turbine failure
             10 m (33 ft)"
m/s  inph
                   50 m (164ft)a
m/s  inph
                                   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.


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 $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).


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

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.


1.  U.S.  Department  of  Energy   (DOE).
   Energy Efficiency and Renewable Energy.
   Wind   and  Hydropower   Technologies
   dandhydro/wind how.html. Retrieved July
   9, 2007.

2.  California     Energy     Commission.
   California Distributed  Energy  Resource
   Guide.  http://www.
   equipment/wind/wind.html.      Retrieved
   August 21,2007.

3.  Wikipedia, the free encyclopedia.  http://en. 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.
   /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.
   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.

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. 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
   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.

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