United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
EPA/625/5-87/021
Research and Development
Solar-Powered
Monitoring
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Research and Development EPA/625/5-87/021
Solar-Powered
Environmental Monitoring
November 1986
U.S. Environmental Protection Agencj
Region V, Li bivvy
230 South Dearborn Street
Chicago, Illinois 60604 -~-
This report was prepared jointly by:
Office of Environmental Engineering and Technology
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
and
Office of Renewable Energy
Office of Conservation and Renewable Energy
Department of Energy
Washington, D.C. 20585
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This document was prepared by Kevin Finneran, consultant, Washington,
D.C., and JACA Corp., Fort Washington, Pennsylvania, working under the
direction of David R. Berg, the program manager for the Federal Photovoltaic
Utilization Program projects within the U.S.Environmental Protection Agency.
Michael W. Pulscak is the overall manager of the program for the U.S.
Department of Energy. Photographs and technical reviews were also
provided by Jon Broadway and Jeffrey van Ee of the Office of Research and
Development, U.S. Environmental Protection Agency; by the U.S. Department
of Energy; and by the California Air Resources Board.
This document has been reviewed in accordance with U.S. Environmental
Protection Agency policy and approved for publication. Mention of trade
names or commercial products does not constitute endorsement or
recommendation for use.
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Contents
Illustrations
Page
1. Overview 1
2. Terminology and Principles of Operation 3
3. EPA Experience 5
Visibility Monitoring 5
Ozone Monitoring 5
High-Volume Air Sampling for Particulates 6
Water Quality Monitoring 6
Hazardous Waste Monitoring 6
Radiation Monitoring 6
Additional Projects 6
4. Lessons Learned 7
Best Uses 7
Sizing Systems and Storage 8
Reliability 8
Economics 8
5. System Design and Specification 9
Backpack Systems 9
Trailer-Mounted Systems 12
Stationary Systems 14
Appendix A. Types of Cells 17
Appendix B. Sources of Equipment and Further Information 19
Figures
1. The Photovoltaic Process 3
2. PV Systems Components 10
Tables
1. Technical Characteristics of PV Trailers 12
2. Technical Characteristics of Stationary Systems 14
MI
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1. Overview
In 1978 the Department of Energy
created the Federal Photovoltaic
Utilization Program (FPUP) to
commercialize the use of PV systems
within the Federal and private
sectors. The program also sought to
improve Federal agencies' technical
knowledge, stimulate their use of PV
power, and increase their application
of life cycle cost analysis for energy
investments. The U.S. Environ-
mental Protection Agency (EPA)
received funding under the program
to design and demonstrate PV
systems to power envionmental
monitoring equipment.
EPA completed the assembly,
deployment, and evaluation of 39 PV
systems in 1985. This document
summarizes what EPA learned in the
FPUP program. It includes a history
of EPA's experience, an analysis of
how best to use PV for
environmental applications, and
technical descriptions of EPA's FPUP
systems to help in the design of
future systems.
The systems described in this
document were built between 1980
and 1982. They are working, but
some of the equipment is no longer
commercially available because
manufacturers have introduced
improved models. In fact, PV power
has become even more
competitively priced and several PV •
manufacturers have begun offering
improved 10-year warranties on their
cells. Anyone building a system
should therefore look at the newest
technology, rather than building
exact copies of the equipment
described here.
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Since researchers produced the first
PV cell at Bell Laboratories in 1954,
the cells have steadily increased in
efficiency and decreased in price.
First used to power orbiting
satellites, PV cells have found their
way into applications ranging from
tiny cells that run watches to multi-
megawatt arrays that provide power
to electric utilities.
PV cells convert sunlight directly into
electricity. They are silent, emission-
free, long-lived, and virtually
maintenance-free, qualities which
make them an ideal power source for
certain environmental monitoring
activities.
The economics of PV cells also make
them attractive under many
conditions. The first PV cells used on
a satellite in 1958 cost about $600 per
Wp. They are now available for $6 to
$10 per Wp. As prices fell during the
1970s PV cells became a common
source of power for railroad
switches, navigation buoys, electric
fences, desalination devices,
microwave repeaters and other
communications equipment,
cathodic rust protection for bridges,
and scientific research stations.
Individuals began using PV power on
sailboats and remote vacation
homes. In developing countries PV
systems began replacing diesel
engines for pumping water. The
1980s saw the introduction of tiny PV
cells to power calculators, watches,
radios and small battery chargers,
and the construction of giant
centralized PV systems capable of
providing power to hundreds of
homes.
PV power is still too expensive to
compete with conventional coal or
nuclear plants for centralized
electricity generation, or to be used
for homes that are already
connected to the utility grid. But it is
practical and economical for many
specialized applications, particularly
those with small power needs in
remote locations away from the
power lines. PV power is then
competitive with batteries and diesel
generators. For these special uses,
PV power can offer advantages in
reliability, cost, and convenience. It
may also make feasible projects, that
could not have been done in the past,
such as certain remote monitoring
projects.
This document is designed for
several audiences: environmental
managers and decision makers who
might consider PV systems for
environmental monitoring, technical
professionals who manage or design
PV systems, and the scientific public
who may have future uses for PV
technology. Section 2 gives basic
terminology and principles
necessary for an understanding of
PV systems. Section 3 describes
EPA's experience with PV-powered
environmental monitoring. Section 4
analyzes the EPA experience and
should help managers decide if
solar-powered monitoring meets
their needs. Section 5 contains
detailed technical descriptions of the
systems used by EPA and will be of
interest primarily to technical
professionals. Appendix A discusses
several types of PV cells. Appendix B
has information about PV equipment
that can be borrowed from EPA, as
well as a list of contacts for further
information.
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2. Terminology and
Principles of
Operation
The photovoltaic (PV)ce// is the basic
unit of a photovoltaic system. The
typical cell measures about 4 in wide
and can be round or square. Each
cell is capable of generating about
1.25 watts of electricity. Cells are
mounted on a panel, wired together,
and covered with glass to produce a
module. A typical module contains
32 cells, produces 40 watts of elec-
tricity, and measures 1 ft by 4 ft.
Modules must be mounted on a
supporting structure that faces them
toward the sun at the optimum
angle. Each supporting structure
with its modules is called asubarray,
and the entire configuration of
subarrays is the array. The complete
photovoltaic system also includes
batteries to store the electricity,
electronic components to control the
flow of electricity and, in some cases,
components to convert direct current
to alternating current.
The size of a photovoltaic module or
system is expressed in waffs peak
(Wp), approximately equal to the
amount of power produced at noon
on a sunny day with the panel
directly facing the sun.
All these systems work on the same
general principle. As shown in Figure
1, the PV cell has two layers of
conductor silicon. The top layer is
treated to have a negative charge,
the bottom layer to have a positive
charge. The sun's rays excite and
dislodge the electrons in the
negative layer, causing them to
move toward the positive layer.
When they cross the barrier between
the layers to the positive layer, an
electric current is created. A grid of
metal contacts embedded in the PV
cell collects the electron flow and
delivers useable current from the
cell.
Figure!. The Photovoltaic Process
Sunlight (Photons)
Negative Layer
Positive Layer
Base Contact
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Close-up view of solar panels showing individual cells of single-crystal
silicon.
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3. EPA Experience
Under the Department of Energy's
FPUP program, EPA conducted 12
projects to design, construct, and
use 39 PV systems to power a
variety of environmental monitors.
The emphasis was on remote
applications, which required
portability and maintenance-free
operation. Some involved
emergencies requiring the quick
response possible with a mobile
system. Some applications entailed
hazards that limited human access.
The systems included backpack,
trailer-mounted, and large stationary
systems.
Visibility Monitoring
The National Park Service measures
visibility in remote areas of national
parks as an indicator of overall air
quality. The first solar-powered
visibility monitoring system,
installed at Lava Point in Zion
National Park in 1980, used a 1,400
Wp system. The PV system has since
been moved to provide electricity for
a ranger's cabin elsewhere in Zion.
The National Park Service now uses
a visibility monitoring system in Zion
that requires only 120 watts of peak
PV power.
EPA developed a prototype 120 Wp
visibility monitoring station for use
in the U.S. National Forest Flattops
Wilderness Area in Colorado. This
system, like the system in Zion,
measures visibility and transmits the
data via satellite to a computer in
Washington, D.C. After one year of
successful operation, the system
was removed because of changes in
the requirements of the national
visibility monitoring program. The
PV system served as a model for an
additional 10 systems built by the
Environmental Monitoring Systems
Laboratory-Las Vegas (EMSL-LV)
and used by the National Park
Service throughout the West in
places like Grand Canyon and Bryce
National Parks. Similar systems
could be used to develop baseline
environmental data before the
construction of large facilities such
as power plants or mines in rural
areas.
The visibility monitoring systems are
ideally suited for use in remote areas
of the national parks. They are small
enough to be unobtrusive and
reliable enough to require only
quarterly inspections to check the
monitoring equipment. Batteries
alone are not an alternative because
they can supply power for only a few
days before they must be replaced or
recharged. Without photovoltaic
power, visibility monitoring would
be feasible only near power lines.
Ozone Monitoring
Photochemical smog from California
cities is transported east and can be
trapped by the Sierra Nevada
Mountains. Researchers believe that
the smog may interfere with
reproduction in giant sequoias,
among the world's oldest and largest
trees. Therefore, officials at Sequoia
National Park and the California Air
Resources Board wanted to monitor
ozone levels at remote sites within
Sequoia National Park. EPA staff at
EMSL-LV adapted a commercial
alternating current (AC) ozone
monitor to run on direct current (DC)
power and built a 540 Wp PV system
to provide the power. Data from the
monitor are relayed via satellite and
telephone lines to the California Air
Resources Board for analysis. The
accuracy of the ozone monitor must
be checked every two weeks.
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To support the study of potential
adverse effects of smog on saguaro
cacti, the National Park Service is
borrowing a PV trailer from EMSL-LV
to power ozone monitoring
equipment in Saguaro National
Monument near Tucson, Arizona.
The National Park Service is also
using a PV-powered ozone monitor
in Great Smoky Mountain National
Park.
High-Volume Air Sampling
for Particulates
The Ute Indian Reservation at Fort
Duchesne, Utah, is surrounded by a
variety of energy development
projects that could degrade air
quality in that region. EPA provided a
480 Wp PV-powered high-volume air
sampler to monitor particulate
pollution at the reservation. The
system meets the specifications
established for conventional
samplers, and the Ute Indians
continue to use it.
Air samplers pump air through a
filter to trap particles or through an
absorbent to trap gases. The filter is
replaced by a new filter every week,
and taken to a lab for analysis of
particulates and gases in the air. The
high-volume air sampler is often
specified by EPA for routine
monitoring of particulates, but a
small sampler that can operate with
a 40 Wp PV power supply may be
adequate for many applications.
Water Quality Monitoring
EPA's lab in Montgomery, Alabama,
designed and built several trailer-
mounted mobile PV systems. The
320 Wp trailer powered a water
sampler at the Chattahoochee River,
and a 720 Wp trailer powered a water
sampler at the Alabama River.
Water samplers operate on the same
principle as air samplers. They pump
water through a filter, recording the
total volume of water. The filter is
removed periodically so that it can
be studied in a laboratory.
Hazardous Waste Monitoring
Monitoring air and water quality
near hazardous waste sites or at the
scene of accidents is essential to
public safety. EPA is understandably
concerned, however, about the
exposure of its personnel involved in
monitoring. Reliable PV-powered
monitoring equipment makes
monitoring feasible with a minimum
of human intervention. When the
Weldon Spring Chemical Plant in
Missouri was identified as a hazard,
EPA brought in one of the PV trailers
to power air sampling equipment in
the area.
Radiation Monitoring
Radiation is among the most potent
environmental hazards and is
impossible to detect without
specialized equipment. EPA has used
PV-powered systems to monitor
radiation in a variety of settings. In
1981 EPA installed a 4,000 watt
stationary PV system at the Farley
Nuclear Plant in Alabama to monitor
radiation levels at the fence that
surrounds the plant more than a mile
away. EPA found that using PV
power was less costly than
extending a power line from the
plant. The independent PV-powered
system would also operate no matter
what was happening in the plant,
potentially providing life-saving
information in the event of an
accident. EPA has a similar 6,000 Wp
PV system powering radiation and
other monitoring equipment in
Montgomery, Alabama.
EPA also installed three radiation
monitors powered by 37 Wp
backpack systems at the Nevada
Nuclear Weapons Test Site. The
monitors are linked to strip chart
recorders that can store 30 days of
monitoring data. These small
systems are operating reliably and
could be useful around nuclear
power plants as well.
Additional Projects
A number of institutions have
borrowed the EPA equipment for
other projects. Dartmouth College is
using a backpack system to monitor
acid rain in the Northeast; a
University of California researcher is
doing the same in Gothic, Colorado.
Carnegie-Mellon University has used
a backpack system to power an air
sampler in Nepal as part of the
United Nations Biosphere Reserve
Monitoring Program. The National
Park Service has monitored sulfur
dioxide levels at Volcanoes National
Park in Hawaii with five backpacks. It
also plans to use one of the trailers
for ozone or sulfur dioxide
monitoring in the Southwest.
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4. Lessons Learned
Best Uses
PV systems can be used successfully
to monitor radiation, visibility,
particulates, gases and water quality.
They can also be used to power
satellite telemetry equipment to
transmit data immediately.
For many remote environmental
monitoring applications, PV power is
not only the most practical and
economic power source, it is the only
feasible power source. Photovoltaic
power is usually the first choice in
remote areas where power lines are
not available, maintenance is
impractical, and monitoring is
needed for more than a few days.
Batteries alone must be replaced or
recharged frequently. Diesel
generators require frequent
refueling and produce exhaust
fumes that can distort
measurements. In national parks,
wilderness areas, and other areas
where environmental concerns are
particularly important, silent,
exhaust-free, long-lasting PV
systems are ideal.
PV systems can provide either
continuous or periodic power. Air
samplers, for example, do not
require continuous operation and so
a backpack system which has energy
storage capacity for only one day
may be adequate. (See Section 5 for
a description of EPA's backpack
system.) Ozone monitoring, on the
other hand, must be continuous. EPA
found that PV systems with 5 to 6
days of energy storage capacity—
such as the trailer-mounted systems
described in Section 5—could
reliably power an ozone monitor.
Finally, the PV backpacks and trailers
are well suited to emergencies.
Easily moved and completely self-
contained, they can be called into
action when the need arises. For
example, the radiation monitoring
equipment that measured the impact
of the Chernobyl nuclear accident
was powered by PV systems.
The appropriateness of using PV
power is specific to the site and
monitoring application. Sufficient
sun must reach the PV panels.
Shading by trees, mountains, or
heavy snow will obviously prevent
the system from producing power.
Storage capacity must also match
the application. If continuous
environmental monitoring is
needed, 5 to 6 days of energy
storage capacity is usually enough to
provide reliable operation. However
it may not be sufficient in areas with
prolonged periods of precipitation or
heavily overcast skies. Seasonal
variations in daylight also must be
considered. For example, PV power
will be of little use in winter in
northern Alaska.
The suitability of using PV power is
also a function of the monitoring
equipment specifications. Most
environmental monitoring
equipment runs on AC while PV cells
provide DC. PV trailers and the
stationary systems therefore include
DC to AC inverters, but backpack
systems have none. Using a DC to
AC inverter is possible, but it results
in as much as a 40 percent energy
loss. A better solution is either to find
DC monitoring equipment or to
convert AC equipment to DC
operation. The conversion requires a
one-time change in hardware
(wiring, transformer, etc.) but there
is no continuing cost or energy drain
on the system. The EMSL-LV staff
was able to adapt a commercially
available AC ozone analyzer to run
on DC power for a project in Sequoia
National Park.
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Sizing Systems and Storage
Sizing the PV system involves not
only considering site conditions and
the need for continuous or
interruptable power, but also
whether AC or DC power will be used
and the size of the storage batteries.
Personal computer software is now
commercially available for
calculating the specifications for PV
systems; this is helpful because the
specifications depend on so many
variables. A list of solar power
design software and where to obtain
them is available from the
Conservation and Renewable Energy
Information and Referral Service.
(See Appendix B, which also lists
other sources of technical
information.)
The amount of energy produced by a
PV system will depend on the power
rating of the PV array, the time of
year, and the location. For example,
in the United States only about half
as much sunlight is available in
December as in June, and Phoenix
receives about twice as much
sunlight as Cleveland in an average
year.
The orientation of the PV cells is also
a factor. In the northern hemisphere,
the modules should face due south.
Because the sun travels high in the
sky in summer and low in winter, the
tilt angle of the array from the
horizontal must be determined. For
maximum annual power production
in one position, a rule of thumb is
that the tilt angle of the array should
equal the latitude of the system. In
Washington, D.C., for example, the
panel should be tilted 39 degrees
from horizontal. If one wanted to
optimize annual output, the panel
could be raised an additional 15
degrees for midwinter operation and
lowered 15 degrees for midsummer
operation. However, seasonal
adjustment is not essential since a 15
degree shift increases annual power
production by only 5 percent. In
practice, optimizing power
production on short winter days is
often more important than achieving
the highest annual output. Systems
are therefore often set at a "winter"
angle, i.e., an angle equal to the
latitude plus 10 to 15 degrees.
The ratio of PV peak wattage to load
demand is also important in sizing a
system. A low ratio is possible for
the interrupted operation of a DC
load; a high ratio is needed for
continuous operation with an AC
load. A backpack system producing
18 Wp can power a 6 watt DC air
sampler (a 3:1 ratio) while providing
24 hours of storage. This will not
provide continuous operation, but
is usually acceptable for a small air
sampler. A 320 Wp trailer system
supplies 30 watts of continuous AC
power (a 10:1 ratio) with 5 to 6 days
of storage.
Reliability
EPA's PV systems have enjoyed
virtually problem-free operation. The
only significant problem was the
almost immediate failure of the DC
to AC inverter on one of the trailers
and on one of the large stationary
systems. The problem was with the
inverter itself, and the manufacturer
repaired it under warranty.
Nevertheless, it is worth shopping
around for a rugged inverter. With
the exception of one of the backpack
systems which was destroyed by
lava in Volcanoes National Park, the
systems have performed
consistently and provided power at
levels acceptably close to
specifications.
In fact, for virtually all applications,
the monitoring equipment itself will
be the limiting factor for durability
and maintenance. In general, the PV
systems are far more durable and
maintenance-free than the
monitoring equipment they power.
An ozone monitor, for example,
must be checked for accuracy every
2 weeks, while quarterly inspection
is adequate for the PV system.
Economics
Simple calculations of the value of
PV-powered environmental
monitoring are impossible. The price
of PV cells has dropped significantly
since 1980-82 when these systems
were built, and the price seems likely
to continue to drop. In addition, PV
power is most often used in
applications where other power
sources are not feasible so that one
has no standard for direct cost
comparison. In these cases one must
compare the cost of the PV system
with the value of the monitoring
information, which may be difficult
to quantify.
PV power can also be the most
economical option even when
interconnection with the grid is
possible, as at the Farley nuclear
power plant. Here, building a PV
system was less expensive than
extending the powerline a mile from
the plant to the fence where the
monitoring equipment was needed.
One particularly cost-effective option
when considering whether to use a
PV system is to borrow the
equipment that EPA already owns.
(See Appendix B for more
information.) With no equipment
cost, many remote monitoring
applications will be economically
attractive, and some monitoring
which has been done partially or not
at all may now become feasible.
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5. System Design and
Specification
Three PV systems have been
developed: backpack systems,
trailer-mounted systems, and
stationary systems. The basic
components of the systems are
shown in Figure 2.
Backpack Systems
Environmental monitoring is often
needed in very remote areas,
accessible only on foot. To make
monitoring possible in such areas,
EMSL-LV with assistance from
Lockheed Engineering Management
Services Company designed a PV
power system that could be carried
into a remote site on a backpack
frame. The 64 Ib solar backpack
includes a 37 watt PV panel, a nickel
cadmium aircraft battery, electrical
control equipment, a housing to
protect the battery and controls, an
adjustable frame for mounting the
PV panel at the proper angle to the
sun, and tools for setting up the
system at the site.
The PV panel can be wired in series
to produce 37 watts of power at 28
volts or in parallel to produce 14
volts. With its battery providing
backup power, this single backpack
system can provide 6 watts of
continuous power in most areas of
the country. However, the battery
stores only enough power to last
through a night and a cloudy day.
This small storage capacity limits the
use of a single backpack to
interruptable monitoring
applications. One can, however, wire
several backpacks together to
increase storage capacity and
provide continuous operation. This
flexibility makes it possible to use
the backpacks with various types of
monitoring equipment.
EPA chose a Solarex HE60 single
crystal silicon panel for use in the
backpack. The square cells make for
a high-density panel that provided
the most power for its surface area of
any panel on the market at the time.
The panel was also designed for
easy switching between 18 watt and
37 watt output. (The panel is no
longer commercially available.) The
battery was a General Electric 2-
AC02 nickel cadmium aircraft battery
chosen for its compactness, light
weight, and high-density storage
capacity.
Rather than designing a custom case
for protecting the battery and
controls, EPA used a commercially
available aluminum suitcase.
Rugged and waterproof, the case has
proven reliable in field tests. The PV
panel is mounted on a frame that is
permanently attached to the case.
The hinged frame makes it possible
to adjust the panel angle in 5 degree
increments between 15 and 60
degrees from the horizontal.
Adjustable legs on the underside of
the case make it possible to level the
case on south-facing slopes of up to
25 degrees. The individually
adjusting legs also allow for leveling
in the lateral direction.
A waterproof junction box mounted
on the back of the panel houses a
terminal strip for connecting the
panel to the battery. A Motorola
Schottky diode allows current to
charge the battery during the day
and prevents the battery from
discharging through the panel at
night.
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Figure 2. PV Systems Components
DC Output
Array
The backpack frame is a
commercially available model to
which EPA added an adjustable shelf
to locate the load in the best position
for the individual. Metal rings with
locking Allen screws position the
shelf on the vertical rods of the
backpack frame. Two pegs protrude
upward from the shelf and fit into
holes in the bottom of the PV panel
to secure its position. The case and
panel are mounted on the frame with
the panel facing toward the frame. A
foam pad protects the face of the
panel, and the load is secured with
two nylon webbing straps.
Enclosed in the case with the battery
is the electronic equipment. An LM-
117 voltage regulator insures that
power is delivered at a constant
voltage, typically 6 or 12 volts. A 5k
ohm potentiometer is used to select
the level of the output voltage. The
solar pack can provide up to 1.5
amps at regulated voltage for short
intervals. A 0.5 amp load is more
appropriate for long-term operation.
An LM-193 voltage comparator
protects the battery from complete
discharge. It controls a relay to
disconnect the load from the battery
when the battery's charge falls
below a preset level, usually 10.5
volts for a 12 volt battery, and
reconnects it when the voltage rises
to 12 volts. The addition of 100k
resistors and 50 microfarad
capacitors minimizes the transients
that occur as the load is picked up by
the solar pack circuitry. The circuitry
for the LM-193 includes two 0.68k
ohm resistors to provide 1 volt of
hysteresis to prevent premature
reconnection with the battery that
would result in rapid oscillations. A
Curtis CP3E Incachron elapsed time
meter records how long power is
flowing to the load.
Matching load to power output is
important. The PV panel produces
2.56 amps of current under standard
sunlight conditions. With a 500
milliamp load, the panel sends 2
amps to the battery, which could
lead to overcharging on consistently
sunny summer days. Moderate
overcharging will not damage the
battery, but will increase water loss
and require more frequent
replenishment. The angle of the
panel can be adjusted to decrease
power production and avoid
overcharging. Too large a load
causes the more serious problem of
insufficient operating time for the
monitoring equipment. As a rule of
thumb, the battery stores enough
power to feed a load through a night
and a cloudy day. Prolonged cloudy
periods or stormy weather cause the
system to disconnect itself from the
load. EPA found that during the
summer in the Pacific Northwest the
system could provide 6 watts of
continuous power.
10
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^r*» jppra>«*t* -^y
The solar backpack is used to power radiation monitoring equipment where small amounts of power are quickly
needed in remote locations.
11
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Trailer-Mounted Systems
The need for environmental
monitoring information seldom
requires that a site be monitored
perpetually. Data from a few years or
even a few weeks is often sufficient
for short-term monitoring projects
and for emergencies. Building a
permanent support structure for a PV
array makes little sense for these
short-term projects. A trailer-
mounted PV system fills a vital niche
for remote power supply.
EPA designed two trailer-mounted
PV systems that would contain
storage batteries and all necessary
electronics as well as the PV array.
One of the two systems is a 720 watt
trailer that supplies 80 watts of
continuous power, and the other is a
320 watt trailer that supplies 30 watts
of continuous power. Both trailers
can store enough power to get
through 5 or 6 cloudy days. Each also
has room for storing up to 1,000 Ib of
monitoring equipment.
As Table 1 illustrates, the trailers are
almost identical in design, differing
only in their power and storage
capacity. The PV modules are
attached to unistruts with telescopic
tubing for adjusting the angle of the
PV array. The small system has two
subarrays, and the large system has
three. Each subarray has gas springs
to facilitate raising the panels into
position and to protect the panels
from damage while lowering them.
Nickel cadmium batteries provide
storage.
Table 1.
Technical Characteristics of PV Trailers
Characteristic
320 Wp Unit
720 Wp Unit
PV cells
Configuration
Nominal output power
System voltage
Power demand capability
Energy storage
Storage capacity
Tilt range
Dimensions when packed
for transport
Weight
Additional storage capacity
for monitoring equipment
Eight 40 Wp modules
2 subarrays
30 watts AC continuous
120 volts
6 times nominal
20 McG raw-Edison
CED-120 nickel
cadmium batteries
300 amp hrs
0-65°
4'9"x8'3"x3'
2,500 Ib
1,000lb
Eighteen 40 Wp modules
3 subarrays
80 watts AC continuous
120 volts
4 times nominal
40 McGraw-Edison
CED-250 nickel
cadmium batteries
1,000 amp hrs
0-65°
T x12' x3'
4,000 Ib
1,000 Ib
Interior view of trailer-mounted power supply with dual 12 VDC power
outlets shown in upper left, gas springs supporting subassemblies shown in
upper left and right, and nickel-cadmium batteries shown in foreground.
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Each trailer includes instrumentation
for monitoring array current, load
current, and system voltage. The
array ammeter makes it possible to
measure current while the array is
being erected to optimize the tilt
angle of the array. All the control
mechanisms, instruments, and
terminal points are housed in
weathertight enclosures.
When the PV array is stored for
travel, the 720 Wp trailer measures
12 ft long, 7 ft wide, 3 ft high and
weighs 4,000 Ib. The smaller trailer is
8 ft 3 in long, 9 ft 3 in wide, 3 ft high,
and weighs 2,500 Ib. Each trailer
includes springs, shock absorbers,
and 10 in electric brakes operated by
a self-actuating electrical controller.
Each design includes three jacks to
level and stabilize the trailer: a
removable caster jack mounted on
the tongue, and screw-type folding
jacks under each corner.
Curbside view of mobile 720 Wp solar power supply which is used to provide
power to a variety of remote locations.
Roadside view of mobile 720 Wp solar power supply showing panel junction
boxes and support structures.
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Stationary Systems
Two stationary PV systems were
constructed by the EPA laboratory in
Montgomery, Alabama, to power
radiation monitoring equipment. A
4.08 kilowatt system is on the
grounds of the Farley nuclear power
plant in Alabama, and a 6.8 kilowatt
system is at the EPA Eastern
Environmental Radiation Facility in
Montgomery, Alabama. As shown in
Table 2, the systems are very similar
in design.
Both systems use Solarex
semicrystalline silicon PV modules.
Ten modules are connected in series
to produce a string that operates at
120 volts DC; the strings are then
connected in parallel. Each module
has a bypass diode installed in
parallel to prevent the batteries from
discharging through the panels at
night.
An anodized aluminum frame
supports the module. Adjustable
legs make it possible to achieve the
optimum panel tilt—in this case, 50
degrees from horizontal. The frame
is strapped to a concrete foundation
and anchored with ground augurs to
make it secure even in 100 mph
winds. Each subarray is 7 ft wide and
7 ft 3 in high. The rows are 12 ft apart
to prevent shadows on the back
rows.
Lead cadmium batteries from the
C&D Company provide storage so
that the system can run for five
sunless days. A Solarex controller
protects the batteries from
overcharging and complete
discharge. When the batteries near
capacity, the controller automatically
reduces the number of modules in
series in each string. The controller
allows for pump charging of the
batteries by adjusting the power
supply in small increments. This also
prevents electrolyte stratification
which can ruin the battery. The
controller panel indicates the status
of the system, and a voltmeter and
two ammeters monitor the
performance of the array, battery
bank, and load.
Table 2.
Technical Characteristics of Stationary Systems
Characteristic
4.08 Kilowatt
6.8 Kilowatt
PV cells
Configuration
Array area
Nominal continous
output power
System voltage
Storage capacity
Energy storage
Controller
Inverter
voltage
power
efficiency
sine wave output
Angle from horizontal
Storage building
dimensions
Support structure
Ground
120 Solarex HE51JG
single crystal
silicon 34 watt
modules
12 parallel strings
of 10 modules
connected in
series
40' wide x 40' deep
400 watts AC
120 volts
60 kilowatt hrs -
5 days without sun
60 C&D Company KCPSA-9
lead cadmium batteries
Solarex ACR-6
Abacus 413-3-120
120V DC to AC
1,000 W, 60 Hz
85%
less than 4% THD
50°
10'x 12'
Solarex A-1266 galvanized
aluminum frame
with telescoping legs
170 Solarex SX-120
semicrystalline
silicon 40 watt
modules
17 parallel strings
of 10 modules
connected in
series
40' wide x 60' deep
600 watts AC
120 volts
123 kilowatt hrs -
5 days without sun
60 C&D Company KCPSA-9
lead cadmium batteries
Solarex ACR-6
Nova 2KVA
120V DC to AC
2,000 W, 60 Hz
85%
less than 4% THD
50°
10'x 16'
Standard galvanized
aluminum frame (no
telescoping legs)
4' long x W diameter galvanized rod at center of each
row of modules
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Solar energy is used to power a high volume air sampler where measurements of particulates in the air are required
to assess pollution in remote areas.
When the batteries discharge to a
preset level, the controller
disconnects low-priority
components of the load. As the
battery charge continues to drop,
other loads are disconnected until
finally the most critical functions are
cut off. Loads are reconnected in
order as the batteries recharge. Class
R, 60 amp fuses protect the battery
bank from a direct short. The PV
array is protected from a direct short
by Class R, 20 amp fuses. Blocking
diodes, mounted in the bottom of the
controller enclosure, are connected
electronically in series between each
subarray of PV modules and the
array disconnect switch to prevent
the batteries from discharging
through the PV array at night. The
controller is grounded and has a
built-in lightning protection device to
protect against surges.
Because most monitoring
equipment runs on AC current, the
DC current from the PV system is
converted to AC by a 120 volt DC to
AC inverter. This will shut down
automatically to protect against
insufficient or excessive voltage and
restart when the voltage returns to
the proper level. The inverter is also
automatically protected from high
temperatures and short circuits.
Because the unit is convection
cooled, there are at least two inches
of clearance above and below the
unit.
An insulated storage building
houses the batteries, inverter, and
controller. Adjustable vents allow
airflow and temperature to be
regulated.
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Appendix A:
Types of Cells
Photovoltaic cells are made with a
variety of materials which differ in
their conversion efficiency,
durability, cost, and weight.
Understanding these differences is
essential for choosing the best type
of PV cell for a particular application.
The most common PV cells are made
from single crystal silicon, the same
material used in semiconductor
chips in computers. The wafers of
silicon are sliced from ingots of very
pure silicon. Single crystal silicon
cells were the first PV cells
developed and have proven their
durability and reliability since the
1950s. They are the standard against
which other cells are measured.
Polycrystalline silicon is similar to
single crystal silicon except that it is
not as pure and its crystal structure
not as regular. Ingots can be
produced less expensively, but
conversion efficiency is somewhat
lower. Although polycrystalline
silicon cells have been in use for only
a few years, they seem to be as
reliable as single crystal cells.
Polycrystalline silicon cells also can
be made by a process that produces
thin films of silicon that do not have
to be sliced from an ingot. These
cells are commercially available but
have not had much market success.
Their performance is acceptable, but
the manufacturers have not
succeeded in scaling up production
economically.
Amorphous silicon has an irregular
crystal pattern and is always
produced as a thin film. Amorphous
silicon cells are the least expensive
and are lighter than those sliced
from ingots. Amorphous silicon's
efficiency is lower, and the durability
of the cell in outdoor applications
has yet to be proven. Because the
cells are light and easily cut, they
have been used extensively in
calculators and other small
consumer products where efficiency
and durability are not critical.
Amorphous silicon cells are often
referred to as the cells of the future,
and several companies began
producing large amorphous silicon
panels for outdoor use in 1985.
Some photovoltaic modules come
equipped with concentrating lenses,
but these have not been used for
remote, untended applications. PV
cells made with other materials are
also appearing on the market, but
they are either unproven or
unsuitable for environmental
monitoring. Gallium arsenide cells,
for example, are used for space
applications because they are more
efficient than silicon cells. They are
too expensive, however, to be
competitive for nonspace
applications.
For remote environmental
monitoring there are only two
serious contenders—single crystal
and polycrystalline silicon. Both are
durable and reliable, and are
available from a number of
manufacturers. The choice between
the two depends on the cost and
efficiency requirements of the
particular panel. Efficiency is
improving and cost decreasing for
both, and neither has emerged as
clearly superior. Amorphous silicon
has the potential to significantly
lower costs in the next few years and
could become a viable option in the
late 1980s if it proves its durability in
outdoor applications.
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Appendix B:
Sources of Equipment
and Further
Information
The equipment developed for this
EPA program is still in use by EPA,
the National Park Service,
universities, and others. EPA is eager
to have these PV systems used. If
you have a need for one of the
portable systems described here,
contact Jon Broadway or Jeffrey van
Ee to find out if the equipment is
available.
Jon Broadway
U.S. Environmental Protection
Agency
Eastern Environmental Radiation
Facility
1890 Federal Drive
Montgomery, AL 36109
(205) 272-3402
Technical monitor for the EPA-FPUP
projects in Mongtomery, AL
Jeffrey van Ee
U.S. Environmental Protection
Agency
Environmental Monitoring System
Laboratory-Las Vegas
P.O. Box 15027
Las Vegas, NV 89114
(702) 798-2367
Technical monitor for the EPA-FPUP
projects in Las Vegas, NV
Other sources of information
include:
Michael W. Pulscak
U.S. Department of Energy
Photovoltaic Energy Technology
Division (CE 352)
1000 Independence Avenue, SW
Washington, DC 20585
(202) 252-6264
Manages the Department of Energy
Federal Photovaltaic Utilization
Program (FPUP)
David Berg
U.S. Environmental Protection
Agency
Office of Research and Development
(RD-681)
401 M Street, SW
Washington, DC 20460
(202) 382-5735
EPA program director for the EPA-
FPUP projects
Dr. G. J. Jones
Sandia National Laboratory
Division 6223
P.O. Box 5800
Albuquerque, NM 87185
(505) 844-2433
Conservation and Renewable Energy
Inquiry and Referral Service
(CAREIRS)
P.O. Box 8900
Silver Spring, MD 20907
(800) 523-2929
Federal program that provides
introductory information and
suggestions for finding more specific
or technical data
National Appropriate Technology
Assistance Service (NATAS)
U.S. Department of Energy
P.O. Box 2525
Butte, MT 59702
(800) 428-2525
(800) 428-1718; in Montana only,
Federal program that can provide
design assistance
Office of Scientific and Technical
Information (OSTI)
Department of Energy
P.O. Box 62
Oak Ridge, TN 37831
(615)576-6837
Technical information
Technical Inquiry Service
Solar Energy Research Institute
1617 Cole Boulevard
Golden, CO 80401
(303)231-7303
Leading Federal research facility for
photovoltaics
0 US GOVERNMENT PRINTING OFFICE 1987 - 748-12U406S5
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