United States
    Environmental Protection
           United States Environmental Protection Agency
                                                      August 2013

Renewable Energy Fact Sheet:
Solar Cells
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) installed on
        the roof of an office building..

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-
                           reflecting LMayer that prevents the cell  from
                           reflecting sunlight away. Below this layer are
                           two semiconductor layers that are typically made
                           from n- and/?- 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.  The bottom contact  layer is
                           connected to the top contact layer to complete
                           the circuit.


                                p- layer (hole
n p Junction
(electric Fifld)
    Back contact solar cell (Courtesy: ECN, The Netherlands)

   Figure 3: Photovoltaic Solar Cell Diagram

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'* and in some cases
can be as high as  40%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% 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 commercially available3'4.

Solar modules generally can produce electric energy
in the  range of  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 1,000 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.
                                     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 they
                         are therefore ideal for remote locations.

                         There are also disadvantages associated with the use
                         of solar  cells.   Good weather and  location  are
                         essential since solar cells require adequate sunlight

                                              8.5 -9.0
                                              7.5 - 8.0
                                              7.0 - 7.5
                                              6.5 - 7.0

                                              4.5 - 5.0
                                              4.0 - 4.5
                                              3.5 - 4.0


   Model  estimates  of monthly  average daily total
   radiation using inputs derived from satellite and/or
   surfaceobservations of cloud cover, aerosol optical
   depth, precipitable water vapor, albedo, atmospheric
   pressure, and ozone resamples to a 40 km resolution.
Source: Electricand  HydrogenTechnologies and Systems Center, May

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


Currently, installed solar systems cost from $6.00
per kW to $10,000  per 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 per watt,
excluding balance-of-system (BOS)  costs.    BOS
costs  can result in  an additional 30%  to 100%
increase  to  the factory  costs. Major BOS cost
items include control equipment (maximum power
point  trackers,  inverters,  battery  charge,   and
controllers), solar array support structures,  battery
storage (if included),  installation and associated
fees, insurance,  and data acquisition system  and

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 per watt.
Costs  are expected  to  decrease 40% by  2010.
Improvements  in   conversion  efficiencies   and
manufacturing   economies    of  scale  are  the
underlying drivers5.
                                                      APPLICATIONS  OF  SOLAR  POWER
                                                      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 MOD 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  $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

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 arrays  totaling
500 kilowatts  for the facility6.  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
gigawatt hours (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 SolarBee®,
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 SolarBee®  possesses battery storage for  up
to 24-hour operation,  which  is beneficial during
low sunlight conditions. A single SolarBee® unit
can effectively aerate a 35 acre lake or treat a  25
million gallon drinking water reservoir or tank.
     Figure 6: SolarBeeR Surface Aerator
      manufactured by SolarBee, ® Inc.
Since its  creation,  over 1,000 units have  been
installed in many treatment applications including
wastewater  lagoons.  Use  of  the  SolarBee®
circulator  can  effectively  improve  biochemical
oxygen demand and sludge reductions,  control
odor,  and  reduce  total  solids   and  ammonia
concentrations in the effluent.

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.

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
H2S  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.
 Figure 7: SolarBee® Installed in City of Myrtle
  Beach, South Carolina, 50-Acre Wastewater

The  City of Somerton,  in southwest  Arizona
replaced   a   40-horsepower  wastewater  lagoon
aeration motor with four solar-power aerators.
The  project  cost was about  $100,000 with  an
annual expected electric energy cost 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  District
(TDBCSD)    Wastewater    Treatment    Plant,

1. Technical Information on Photovoltaics.
h/UCEP/paper s/solarfaqv2.pdf.
Retrieved September 26, 2007.

2. Renewable Energy Access.Com. 2006. Solar
Cell Breaks the 40% Efficiency Barrier.
s. com/rea/news/story ?id= 46 765.
Retrieved September 26, 2007.

3. SunPower Corporation. 2007. SunPower
announces high power, higher efficiency solar
panels. http://investors.sunpowercorp.com
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.
eleasel. cfm ?id=93.
Retrieved July 9, 2007.

7. Shelly, P.  City of Myrtle Beach Cleaned Up
by "Bees". http://www.solarbee.com/news/SC
Retrieved September 26, 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
       EPA 832-F-13-019
          August 2013