United States
  Environmental Protection
  Agency
Auxiliary and Supplemental Power
Fact Sheet: Solar Power
DESCRIPTION

Solar power is one of the most promising
renewable  energy  sources  today.  Solar
cells, also  known  as  photovoltaic  (PV)
cells, can  be  used  as  Auxiliary  and
Supplemental Power Sources (ASPSs) for
wastewater treatment  plants (WWTPs).
When  photons  in  sunlight  randomly
impact the surface  of solar cells,  free
electrons are generated, which flow to
produce electricity.
 Figure 1. Solar panels (or arrays) on the
           roofs of buildings.

Solar cells  are often assembled into flat
plate  systems  that can  be mounted on
rooftops  (Figure  1)  or  placed  at  other
sunny locations (Figure 2).  A solar cell is
composed of several layers  of different
materials. The top layer is a glass cover or
other encapsulating material  designed to
protect the cell from weather conditions.
               Figure 2. Solar "tree" placed out in the
               open where sunshine is abundant, Styria,
                             Austria.

              Beneath  the  glass layer  is  an  anti-
              reflective layer that prevents the cell from
              reflecting sunlight away. Below this layer
              are two  semiconductor  layers  that  are
              typically  made  from  n-  and p- silicon
              (Figure 3).

              A  set  of  metallic grids  or  electrical
              contacts    is   placed    around    the
              semiconductor material,  one above  the
              material and the other  below. The energy
              of the absorbed light is transferred to the
              semiconductor.   The   energy   knocks
              electrons loose  from the  semiconductor,
              allowing them to flow freely. An electric
              field within the solar cell forces the freed
              electrons to move in a certain  direction.
              The  top  grid,  or  contact, collects  the
              flowing electrons from the semiconductor.

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The bottom contact layer is connected to
the top  contact  layer to complete the
electrical circuit.
  Photons

Electrons


n- Silicon

p- Silicon
 Current
  Direction     Negative
             Contact
                                     Positive
                                     Contact
                                      H
                      Electron
                       Flow
  Figure 3. Electron and current flow in
               solar cell.

This flow of electrons is the current, and
by placing metal contacts on the top and
bottom of the solar  cell, that current can
be drawn off to be used externally. This
current, together with the  cell's voltage
(which is a result of the strength of its
built-in electric  field), defines  the power
(or  wattage)  that  the  solar  cell  can
produce.

Solar  cell   efficiency  varies  and   is
determined by the material from which it
is made and by the production technology
used to make it. Commercially available
solar modules are between 5 to 17 percent
efficient   at  converting   sunlight  into
electrical  energy,1 and in some cases can
be as  high as  40 percent.2 Research  is
always   underway  to  produce   cost-
effective  solar   panels  with  improved
efficiency  and  higher wattage.  In 2006,
SunPower  Corporation  announced  the
company's  newly   developed  SPR-315
solar panel which is  19.3 percent efficient
and  carries  a rated power  output  of 193
kilowatts. The new SPR-315 solar panel is
designed  to  generate  more power  with
fewer  panels,  thus  maximizing  energy
production  while  reducing  installation
cost. SunPower also claims that the new
SPR-315 solar panel performs better than
most other solar panels during cloudy or
hot weather.  The SPR-315 solar panel  is
now commercial availability.3'4

Solar  modules generally  can  produce
electric energy in the range from 1 to 160
kilowatts  (kW). An individual  solar cell
will  typically produce between  one and
two watts. To increase the power output,
several cells  can  be  interconnected to
form   a  module (Figure  4).  Similarly,
modules can  be connected  to form  an
array (Figures 1 and 2). A solar array with
a surface area  roughly  the  size  of two
football fields could produce 1000 kW of
peak  power.  Ideally,  a backup  storage
system should be included with the solar
system to store power  so that  it can be
used during  low light conditions or  at
night.
                                                    cell
                         module
                                                   array
                                                                        panel
                                              Figure 4. Solar module composed of
                                                     individual solar cells.
                                           ADVANTAGES & DISADVANTAGES

                                           There are  several advantages to using
                                           solar  cells.  Solar  cells   can  generate
                                           electricity with no moving parts, they can
                                           be  operated quietly  with no  emissions,
                                           they  require little  maintenance, and  are
                                           therefore ideal for remote locations.

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There  are  also  disadvantages associated
with the use of solar cells. Good weather
and location is essential since solar cells
require adequate sunlight to charge. Some
geographic   locations  do   not   receive
adequate levels of sunlight throughout the
year (Figure 5) and large areas are needed
to generate  power in regions that require
considerable amounts of power. Although
solar cells require very little maintenance,
they  can be difficult  to  repair when
maintenance is needed. Additionally, the
initial cost of solar cells is very high.
  Model  estimates  of  monthly  average
  daily total radiation using inputs derived
  from    satellite     and/or    surface
  observations  of cloud  cover,  aerosol
  optical  depth, precipitable water vapor,
  albedo,  atmospheric pressure, and ozone
  resamples to a 40 km resolution.
Produced  by  the  Electric   and  Hydrogen
Technologies and Systems Center, May 2004.5

Figure  5. Annual Solar Radiation in the
U.S. (Flat Face, Facing South,  Latitude
Tilt).
COST

Currently,  installed  solar  systems  cost
from $6,000/kW to $10,000/kW. The cost
of a solar system depends on the system's
size, equipment options,  and installation
labor costs.
The average factory price of a solar panel
is  about  $5/watt,  excluding  balance-of-
system (BOS) costs. BOS costs can result
in an additional 30 to 100 percent increase
to the factory costs. Major BOS cost items
include  control  equipment  (maximum
power point trackers,  inverters,  battery
charge  controllers),  solar array  support
structures, battery  storage (if present),
installation and associated fees, insurance,
and data acquisition system and sensors.

Solar Energy  prices  have declined  on
average 4% per annum over the past 15 to
20 years. In the early 1980's, system costs
were  more  than  $25/watt.  Costs   are
expected  to  decrease  40%   by  2010.
Improvements in conversion  efficiencies
and manufacturing  economies  of scale are
the underlying drivers.2
APPLICATIONS OF SOLAR POWER
AT  WASTEWATER   TREATMENT
PLANTS

Several wastewater treatment plants have
installed solar cells to generate electricity
for process controls.  Oroville, a town in
Northern California, operates a 6.5 MGD
WWTP which services 15,000 households
and many industrial users. In 2002, amidst
an  energy crisis that saw the price of
utilities  rise  41  percent,  the  Oroville
Sewage Commission (SC-OR) decided to
pursue solar power as a solution to reduce
costs and  increase  energy  self-reliance.
That same year, the utility installed a 520-
kW  ground-mounted solar  array capable
of being manually adjusted seasonally to
maximize the  solar harvest.  The  solar
array  consists  of  5,184  solar  panels
covering three acres of land adjacent to
the WWTP.   The total  cost of the solar
system, which is the  fifth-largest  solar
energy  system in the United States,  was

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$4.83 million, with a rebate to the utility
of $2.34 million from the Self-Generation
Incentive  Program of  Pacific  Gas  and
Electric (PG&E) and managed through the
California  Public Utilities Commission
(CPUC). SC-OR was able  to see an 80%
reduction in power costs.

The SC-OR solar  array  is  designed to
produce more power than the utility needs
during peak hours, and because the system
is  connected to the  local energy grid,  it
can feed all of the excess energy back to
the power  utility  so  that SC-OR  can
receive credit on their power bill.  This
credit goes toward paying for the off-peak
power that the treatment  plant uses at
night.  SC-OR saved $58,000 in the first
year and expects the solar array to pay for
itself in 9 years.

In addition to using wind power, the 40-
MGD Atlantic County Utilities Authority
(ACUA) Wastewater Treatment Facility
in Atlantic City, New Jersey, installed five
solar  array totaling 500-kilowatts for the
facility.6 The five solar arrays were place
at  different  locations  throughout  the
facility and include two "ground-mount"
arrays,  two "roof-mount"  arrays, and  a
"canopy"  array.  The roof-mount arrays
are  mounted   such  that they   could
withstand hurricane force winds.

The solar array can generate electricity at
rates lower than 5 0 per kWh for the next
20 years. This is the second largest  solar
array  in the state producing over 660,000
kWh  of electricity annually or about 3%
of the facility's 20 GWh annual electricity
needs.  This equivalent  amount of energy
displaces  388  barrels  of  oil  and  over
400,000 pounds of carbon dioxide. Energy
rebates of $1.9 M were  obtained from the
New Jersey Board of Public Utilities and
an additional anticipated savings of over
$35,000 is expected each year. The total
cost of the project was about $3.9 million.

In  2001,  SolarBee  Inc.  developed  the
SolarBeeŽ,  which is a  floating  solar-
powered  circulator  that  is capable  of
moving up to 10,000 gallons of water per
minute for long distances  (Figure 6). The
SolarBeeŽ possesses battery storage for
up  to  24-hour  operation,  which  is
beneficial  during low sunlight conditions.
A single SolarBeeŽ unit can effectively
aerate a 35-acre lake or treat a 25 million
gallon drinking water reservoir or tank.
 Figure 6.  SolarBeeŽ, manufactured by
             SolarBee Inc.

Since its creation, over 1,000 units  have
been   installed   in   many   treatment
applications    including    wastewater
lagoons. Use of the SolarBeeŽ circulator
can   effectively   improve   biochemical
oxygen demand  and sludge reductions,
control odor, and reduce total solids and
ammonia concentrations in the effluent.

In 2005, the  City of Myrtle  Beach  in
South  Carolina installed  five  SolarBees
into the first three cells of the  city's 50-
acre   wastewater   lagoon  (Figure  7).
Improvements in  dissolved  oxygen and
FkS levels  within  the lagoon after a few
months prompted  the city to budget for
five additional Solarbees for the following
year.  Once installed, electrical  savings is
expected to average $100,000 per year.7

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 Figure 7. SolarBeeŽ installed in City of
  Myrtle Beach, South Caolina, 50-acre
          wastewater lagoon.

In  southwest  Arizona,   the   City   of
Somerton   replaced   a  40-horsepower
wastewater lagoon  aeration motor with
four solar-power aerators. The project cost
was  about $100,000 with an  expected
electric  energy costs savings of $25,000.
Other   applications   of   solar-powered
aerators    include    the    Wastewater
Treatment Facility in Bennett, Colorado
and   the   Town  of  Discovery  Bay
Community     Services     (TDBCSD)
Wastewater Treatment Plant, California.

Benefits of using solar-power aerators not
only  include energy savings, but  also
reduces  odor, greenhouse gas emissions,
and biosolids volume at the bottom of a
pond  or basin that would otherwise have
to be dredged and disposed.
REFERENCES

    1.   Technical Information on
       Photovoltaics.
       http://www. ece.gatech. edu/researc
       h/UCEP/papers/solarfaqv2.pdf.
       Retrieved September 26, 2007.

    2.   Renewable Energy Access.Com.
       2006. Solar Cell Breaks the 40%
       Efficiency Barrier.
       http://www. renewableenergyacces
       s.com/rea/news/story?id=46765.
       Retrieved September 26, 2007.

    3.   SunPower Corporation. 2007.
       SunPower announces high power,
       higher efficiency solar panels.
       http:'//investors, sunpowercorp. com
       /releasedetail.cfm?ReleaseID=214
       653. Retrieved September 26,
       2007.

    4.   Solar Panel Efficiency and
       Performance Information.
       http://www. sunpowercorp. com/Pr
       oducts-and-Services/~/media/
       Downloads/for_products  services/
       SPWR315 DS.ashx. Retrieved
       September 26, 2007.

    5.   National Renewable Energy Lab.
       Solar Maps.
       http:'//www. nrel. gov/gis/solar. html
       #csp. Retrieved September 26,
       2007.

    6.   Atlantic County Utilities Authority
       (ACUA). Atlantic City Solar
       Array Project.
       http:'//www. acua. com/about/pressr
       eleasel.cfm?id=93. Retrieved July
       9, 2007.

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7.  Shelly, P. City of Myrtle Beach        The  mention   of   trade   names   or
   Cleaned Up by "Bees".               commercial products  does not constitute
   http://www.solarbee.com/news/SC     endorsement or recommendation for  use
   EnergvConnectionWinter2006.pdf     by the  U.S.  Environmental  Protection
   Retrieved September 26, 2007.        Agency or the Federal Government.

                                               EPA832-F-05-011
                                                 Office of Water
                                                  March 2005
                                              Revised October 2007

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