vvEPA
United States
Environmental Protection
Agency
United States Environmental Protection Agency
August 2013
Renewable Energy Fact Sheet:
Solar Cells
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) 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.
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Front
Contact
Electric
Vottige
Contact
p- layer (hole
conductivity)
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
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kWh/m2/day
>9.0
8.5 -9.0
8.0-8.5
7.5 - 8.0
7.0 - 7.5
6.5 - 7.0
6.0-6.5
5.5-6.0
5.0-5.5
4.5 - 5.0
4.0 - 4.5
3.5 - 4.0
3.0-3.5
2.5-3.0
2.0-2.5
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
2004.5
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
high.
COST
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
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 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
AT
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
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.
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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
Lagoon
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,
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California.
REFERENCES
1. Technical Information on Photovoltaics.
http://www.ece.gatech.edu/researc
h/UCEP/paper s/solarfaqv2.pdf.
Retrieved September 26, 2007.
2. Renewable Energy Access.Com. 2006. Solar
Cell Breaks the 40% Efficiency Barrier.
http://www.renewableenergvacces
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
/releasedetail.cfm?ReleaseID=214653.
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.
7. Shelly, P. City of Myrtle Beach Cleaned Up
by "Bees". http://www.solarbee.com/news/SC
EnergyConnectionWinter2006.pdf
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
Management
EPA 832-F-13-019
August 2013
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