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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