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                        TECHNICAL PAPER # 69
                     UNDERSTANDING SOLAR CELLS
                    Dennis Elwell & Richard Komp
                         Technical Reviewers
                             Paul Dorvel
                            Robert Ethier
                             Joel Gordes
                             Published By
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virginia 22209 USA
                 Tel:  703276-1800 * Fax:   703243-1865
                       Understanding Solar Cells
                          ISBN:  0-86619-308-1
              (C) 1990, Volunteers in Technical Assistance
                      UNDERSTANDING SOLAR CELLS                      
          By VITA Volunteers Dennis Elwell and Richard Komp
Solar cells, also called photovoltaic (PV) cells, are a compact
source of small amounts of electricity.   They are rugged, dependable
devices for converting sunlight directly into electrical
energy.  They have no moving parts and a long working life.  System
maintenance costs are lower and reliability is much higher than
for other power sources.  They can be used on any scale, from
powering a digital watch to running a multi-megawatt generator
for a public utility.  Because they are usually arranged in modular
panels, it is possible to start with a small system and
expand it as necessary without making the early panels obsolete.
But because only small amounts of energy are converted by each
cell, large-scale electrical requirements require large and
costly arrays of PV cells.  Thus, the main applications of PV
cells have been to supply relatively low demands.   Planners who
may be considering long-term economics should also consider that
selecting PV power helps to achieve a pollution-free environment.
About 1 kilowatt (kW) of radiant energy falls on a square meter
(sq m) of the earth's tropics at midday.   If a solar panel has an
efficiency of 10%, then each square meter of cell array will
generate a peak of 100 W of electrical power.   A typical 10-W
panel, capable of keeping an automotive battery charged, measures
31 cm by 35 cm including the frame.
The idea of capturing solar energy in this way is not new.  The
copper oxide solar cell was discovered by Antoine Becquerel in
1839 and the amorphous-selenium cell came into use for photographic
light meters in the 1890s.  In the 1930s, selenium cells
were used for power on a small scale in remote locations in the
United States.  Serious development of photovoltaic technology
began, however, when silicon cells were developed and used in the
U.S. space program.  The first silicon solar cells were used in
the U.S. satellite Vanguard I in 1958.   Their cost was US$600 for
each watt of generating capacity.   It has now (1989) dropped to
less than $6/W for larger systems.
Solar cells are devices that absorb and convert radiant energy
from the sun directly into electrical energy.   They are made of
materials called semiconductors, which are crystalline solids
with an electrical conductivity between those of metals and
A thin wafer or sheet of the semiconductor is treated ("doped")
with chemicals to produce a negative charge (free electrons) on
one side and a positive charge (free protons) on the other.
(Virtually all commercial solar cells are made so that the front
or top surface is negative.)   The point at which the positive and
negative sides meet is an electronic barrier known as a p-n
The cells convert sunlight into electricity in three major processes:
1.  The semiconductor material absorbs the sunlight.
2.  Free positive and negative charges are generated and separated
    into the different regions of the cell.  The separation
    creates a voltage in the cell.
3.  The separated charges are transferred as electric current
    through electrical terminals to the intended application.
The processes work this way:   The energy of the incoming sunlight
causes electrons to cross the barrier and remain trapped on the
front, or negative, side.  When contacts are made to the front and
back sides of the solar cell, a current flows through wires and
devices connecting these contacts.   The current is proportional to
the intensity of the sunlight that falls on the cell.   The back,
or positive, electrical contact can be a continuous layer of
metal, but the front contact is made in the form of thin fingers,
to allow as much sunlight as possible to reach the back layers.
The cell is usually covered by an anti-reflection coating and a
protective cover to allow cleaning.   A more detailed explanation
of how photovoltaic cells work is given in references 8 and 9.
The structure of a solar cell is shown in Figure 1.

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Until recently most solar cells were made from single crystal
silicon wafers.  Crystals, usually 10 cm in diameter, are pulled
from ultra-pure molten silicon, then sliced and polished.  This
process is both costly and wasteful of this expensive, ultra-pure
material.  The p-n junction is made by diffusing phosphorus (which
produces n-type material) into the front surface of a wafer that
has been "doped" with boron to make it p-type.  Newer techniques
use technical-grade silicon cast into blocks, sawed into wafers,
and fabricated into cells using the same processes as used in
single crystal material.  This process is far less expensive and
uses considerably less energy to produce the finished cell; about
half of the today's large modules are made in this manner.  Another
approach, still in the pilot plant stage, involves pulling
a silicon thin ribbon that does not need cutting into slices.
Many other new ideas are being explored with the general aim of
producing an efficient, long-lived solar cell at lower cost.
Photovoltaic cells are also manufactured from thin films of
amorphous silicon, a glassy material with no regular crystal
structure.  While this material has proved eminently suitable for
small, low-power uses, like solar pocket calculators, amorphous
silicon cells cannot yet be used for power generation panels
because they become less efficient after a period of exposure to
sunlight.  In addition, their long-term stability is doubtful.
Solar cells should have a useful life of at least 10 years.
Solar cells have also been produced using combinations of different
compounds to form the p-n junction.   These are called
heterojunction solar cells.  Copper sulfide/cadmium sulfide cells
are inexpensive but their output also tends to degrade too
rapidly.  Such alternative materials as copper indium selenide
offer the promise that a so-called thin-film heterojunction solar
cell can be developed.  Very efficient but very expensive solar
cells can be made from gallium arsenide.   They may be marketed as
the active components of devices that focus the solar radiation
to reduce the size and number of cells needed.
The output characteristics of a typical photovoltaic cell are
plotted in Figure 2.  The highest voltage that can be produced by

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a cell is called the open-circuit voltage; this is about 0.55
volts (V) for silicon.  As more current is drawn from the cell by
the load, the voltage falls.   The maximum current that can be
drawn from a solar cell, the short-circuit current, is about 300
amperes per square meter in strong sun.   For maximum power, a
silicon cell should be operated at about 0.45 V (in full sun) and
90% of the short-circuit current.   As the intensity of solar
radiation falls, the open-circuit voltage falls slowly, but the
current falls roughly in proportion to the intensity.   Over a
daily cycle, the maximum power output is attained when the sun is
at its highest and, of course, falls to zero between dusk and
dawn.  Solar output is reduced on cloudy days, but diffuse sunlight
can still produce a useful fraction of full output.   Interestingly,
a solar cell or module can be shorted or left open
circuited indefinitely without being damaged.
The efficiency of a solar cell is defined as the ratio of the
electrical power output to the solar power input.   The typical
efficiency of a PV module is about 10%.   This means that when 750
W of sunlight is falling on a square meter of solar array (typical
sunlight intensity in most nondesert areas), the solar array
would produce 75 W/sq m  Solar-cell efficiency tends to fall as
the cell temperature rises.  This effect can be serious in hot
climates where the cell may operate at 50 [degrees] C or even higher.
Mounting the cell on an energy-absorbing support (heat sink) will
tend to keep the temperature down.
Commercial solar arrays or modules are about 35 by 150 cm and
are made with laminated tempered glass fronts and extruded aluminum
sides.  They can stand temperatures of up to 70 [degrees] C but the
plastic laminating material between the cells and the glass cover
will yellow with time if exposed to higher temperatures.  For
higher temperature use, silicon embedding compounds can be used.
Since photovoltaic cells give their highest output when pointed
directly at the sun, electrical performance can be optimized by
putting them on a moving mount that is always pointed toward the
sun.  Prototype scanning systems are relatively expensive and the
motor and sensor systems are more likely to fail than is the
solar-cell array.  Moreover, the scanning motors consume electricity.
One available scanner uses as sensors bulbs filled with
Freon, a gas now considered environmentally hazardous.   Under
present conditions, we recommend a simple, static support.  Manufacturers
provide advice on the best angle for mounting a solar
array in a chosen location but a good year-round guideline is to
point the array directly toward the equator, tilting it at an
angle equal to your latitude.   For example, if you are located at
10 [degrees] south latitude, lift the south edge of the panel until the
panel is tilted 10 [degrees] from horizontal.
Hybrid systems, which provide hot water in addition to electricity,
have also been investigated.   Although they work well
for remote homesteads in northern climates they do not seem
economically sound in tropical countries where the need for hot
water is less urgent.  Exceptions are remote clinics, hospitals,
or other operations that need a reliable supply of hot water.
Even low temperature steam can be made by a properly designed
hybrid array.  SunWatt Corporation and Alpha Solarco have developed
packaged hybrid modules.
Solar cells are usually sold in panels that vary in size but are
of standard voltage.  Connecting individual cells in series adds
the voltages of the individual cells,, while connecting cells in
parallel adds their current-carrying capacity.   Sixteen volts is
a popular choice for a solar panel, because that output voltage
is needed to charge a 12-V storage battery.
Storing and Converting the Energy
In some applications, such as the use of photovoltaic cells for
pumping water for irrigation, the change in output of the cells
through day and night is acceptable since the power is required
only for a few hours in each 24-h period.   For many applications,
however, the solar-cell array should be used together with a
battery storage system that can provide continuous power.  During
peak sunlight hours, the batteries are charged by the solar
cells, which produce more power than is required by the load.
During the night, the batteries discharge to operate lighting and
other loads.  Use of a diode is necessary to prevent the batteries
from passing reverse current into the solar cells at night, and a
voltage-regulating circuit is normally provided on larger systems
to keep the batteries from being overcharged by the PV array.
Some voltage regulators will also disconnect the load to prevent
damage if the battery charge gets too low.
Lead-acid batteries specially developed for photovoltaic-system
applications are generally used, but any deep-cycle lead-acid
battery may serve if necessary.   Automobile batteries are not
highly satisfactory for this application because daily charge and
discharge cycles greatly shorten their useful life.   For some
purposes, especially in remote locations, the more expensive
nickel-cadmium batteries are preferred since they require less
A solar-cell array with battery provides direct current (d.c.),
which has many uses.  A photovoltaic system for d.c. only is shown
in Figure 3.  For a simple arrangement of a few lights and a radio

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or TV set, this is the preferred system.   Incandescent lights for
12 V d.c. are available, and are almost twice as efficient as
their 220-V or 110-V counterparts.   Small 12-V TV's are very
efficient also, and a small, portable radio draws very little
power.  However, fluorescent lights, refrigerators, etc., designed
to operate on d.c. can be very much more expensive than their
counterparts that operate from the 220-V or 110-V alternating-current
(a.c.) mains in normal industrial and household use.   It
may therefore be desirable to include an inverter that converts
the d.c. supply to the 50 Hz or 60 Hz a.c. needed by these appliances.
Some loss of power results from the use of the inverter
(at least 10%), but this may be justified if it leads to big
savings in the cost of the appliances.   Alternatively, the inverter
can be used for only the a.c. appliances, while the rest
of the load is operated directly from d.c.
Basic Costs
Photovoltaic arrays can now be bought for about $6 to $10 per
peak watt.  This price has fallen slowly but steadily over the
last few years, and is expected to continue to fall.   Adding
battery storage (and regulator, if needed) adds 50% or more to
this cost.  The total cost is too high to compete with the local
utility rates in most places, but is far cheaper than the installation
and operating cost of a petrol or diesel generator.   As a
guideline, if a power line longer than one km must otherwise be
built, PV or PV plus wind-generating systems is a cheaper way to
get small to moderate amounts of electricity.
It is believed that photovoltaics will start to be used widely
when the price falls to about $2 per peak watt in 1989 prices.
At this level, and assuming that whole system costs fall at a
similar rate, solar electricity will be competitive with centralized,
fossil-fuel generating systems and will be used on a large
scale both by utility corporations and by individuals who own
rooftop arrays.  Even now, solar cells are probably cheaper than
diesel generators for most rural applications.   And if prices
fall as predicted, solar cells could be the most economical
electricity source for all applications in remote locations of
tropical countries, especially if combined with wind generators
(W.J. Bifano 1982).
In the next decade, applications of solar cells in developing
countries will probably be mainly in rural villages.   Many villages
do not have a power line fed by a central grid system;
the cost of extending a power grid to serve all villages would be
prohibitive in large countries.   However, pilot solar schemes are
now in progress in most developing countries (W.A. Brainard
1982).  See Table 1 for typical village power requirements for a
number of activities that can be powered by solar cells.
Solar powered water pumps are increasingly used for irrigation
and community water supplies.   The outstanding advantage of a
pumped system is the ease with which the water supply can be kept
free of contamination.  From the standpoint of community health,
a pump can be the most important investment a village makes.
As an example, Arco Solar Inc., described a portable photovoltaic
water supply for the village of Boera, Papua New Guinea (Arco
Solar Inc. 1982).  The village has a population of about 1,000,
and the system installed produces 440 peak watts, without battery
storage.  This system delivers about 5,500 liters an hour (L/h) in
full sunlight and about 3,300 L/h under overcast conditions.
Storage is provided by four tanks each of 5,500 L capacity that
are normally filled by midday.   The pump is then switched off by a
float valve.  The villagers pay about $0.01 per bucket of water.  A
portion of the funds is used by the community to maintain the
Assumptions:  500 people, 100 homes.  Sunlight equivalent of 5
              hours noonday sun.  Source:   ref. 3.
APPLICATION                                       ENERGY REQUIRED,
Water pumping (50 L/person-day)                           4.7
Lighting - indoor (2 lights/home)                        16.0
Lighting - outdoor (5 lights/village)                     2.4
Television (20 sets/village)                              1.6
Refrigerators (10/village)                               10.0
Grain Grinder (1 kg grain/person-day)                     6.0
Communications (1 two-way radio set/village)              0.4
Total kWh/day                                            41.1
Total kW Peak Required                                   10.7
Water for Drinking and Irrigation
Irrigation for agriculture is probably the greatest consumer of
energy in rural areas of developing countries.   Animal power and
diesel-fueled pumps are the main competing technologies.  The
quantity of water required for irrigation may range from 5,000 to
13,000 cubic meters per hectare (cu m/ha) over the growing period,
or 40 to 110 cu m/ha each day.   The required pumping capacity
is therefore about 4 to 10 L/second for each hectare, a typical
farm being 1 to 3 ha (W.A. Brainard 1982).
As in the case of drinking-water supply, the amount of power
required depends on the depth from which the water must be
pumped.  Usually this is less than 10 m, so the requirement is
for a few hundred W/ha.  If irrigation is to be economical, the
cost of obtaining the water must be less than the value of the
increase in crop production.   Wright estimated that irrigation is
not worthwhile unless the water costs less than about $0.05/cu m
(W.A. Brainard 1982).  He suggested that photovoltaic systems
were two to four times more expensive than their economic yield
for irrigation.  The break-even point in favorable cases (water
depth less than 5 m) probably already has been reached and the
number of photovoltaic-powered irrigation systems is likely to
expand in the near future.
Irrigation is important not only for food crops but also in the
early stages of reforestation.   Solar power may contribute to the
reversal of deforestation, which has been drastic in such countries
as India.  Another indirect economic benefit of irrigation
is that it may halt, or even reverse, the population shift from
the rural villages to the cities by improving the quality of
village life.  And, according to a recent review, irrigation must
increase by 250% over the next 25 years in order to support a
growing world population (J.L. Crutcher 1982).   Thus, the increased
food requirements of world population growth leads to a
prediction of increased use of solar cells.
Photovoltaic-powered desalination units to produce fresh water
from sea water have been installed in Saudi Arabia and Qatar
(J.L. Crutcher 1982).  They use reverse osmosis, in which the
dissolved salt is driven through a membrane.   Each liter of drinking
water requires 8 to 20 Wh of electricity, which compares
favorably with 2.4 kWh for a solar still and 200 kWh for a flash
evaporation unit.  The unit at Jeddah has been in operation since
January 1981 and supplies 2,000 L per day from an 8 kW (peak)
array and d.c.-powered pumps.   The system does not use a voltage
regulator; this raises efficiency but leads to fluctuating waterflow
rates and pressures.  The Jeddah unit produces water with a
salinity of less than 200 parts per million (= 200 mg/L).  In the
Qatar unit, the salinity is below 500 mg/L:   this relaxation in
standards permits 6,000 L/day to be achieved from an 11.2 kW
(peak) array.  Desalination is, in general, economically viable
only in relatively affluent communities that have a severe water
SunWatt Corporation has demonstrated a small PV/hybrid desalinator,
based on evaporation and condensation cycles, that produces
fresh water and electricity at the same time.   However,
production of such a machine on a commercial scale requires more
PV-powered refrigerators for medical supplies, have become a
regular component of pilot village schemes.   Refrigerators that
operate on d.c. are available, and it is also possible to buy a
refrigerator with its own independent photovoltaic panel.  The
reliability of solar-cell systems is vitally important when
storing vaccines and other medical supplies that would deteriorate
rapidly if not kept cool.  A typical refrigerator requires
about 300 peak watts and consumes about 1 kWh/day.   Experience
with 20 refrigerator systems in different countries has shown
that the units now available require very little maintenance
except of the power supply itself (G.F. Hein 1982).
Flour Milling
The performance of a solar-powered grain mill at Tangaye in
Burkina Faso has been well documented.   The mill began operation
in March 1979.  The 1.8 kW solar array was used to mill grain for
600 families, relieving the village women of a daily one-to
two-hour task.  The early modules were not very reliable, but by
1982 the original system worked well 98% of the time (D. Elwell
1981).  No problems of maintenance or operation were reported.  The
system was increased in size in May 1981 to 3.6 kW, and an improved
hammer mill was installed.  By 1982, the mill was grinding
1.2 tons of flour per week and the cooperative that runs the mill
demonstrated a small operating profit.
Lighting and Communications
Incandescent or the more efficient fluorescent lighting can
greatly improve communal village life by providing increased
opportunities for meetings and social events in the evenings.
Battery storage is essential if lighting is included in a scheme.
The price of the lights and the greater efficiency of d.c. should
be compared with cheaper ballasts for a.c. fluorescent lights
before deciding whether to buy an inverter; the inverter may be
the component with the greatest cost and lowest reliability.
Because they require comparatively little power, television
sets can be operated by solar cells.   The value of TV in rural
education is well documented in many locations, starting in 1976
with Cote d'Ivoire and India.
An emergency radio set is a useful addition to a village and has
been included in the development plans of some countries.  The
Mexican government has installed a solar-powered, rural telephone
station, and solar-powered telephones have also been used in
Saudi Arabia.  Solar power was preferred for a microwave communications
link in Papua New Guinea.  Telecommunications terminals
and data-processing microcomputers can also be operated by solar
cells.  VITA has installed solar-powered packet radio systems
where the computers communicate with each other via radio, in
remote areas of Sudan and the Philippines.   This paper was prepared,
in part, in a remote U.S. location on a solar-powered word
processor operating through a 2-kW inverter.   These examples
illustrate the variety of ways that solar cells can be used in
communications in remote locations.   As in other applications, the
reliability of solar cells is their main advantage.
Local Industries
Can PV arrays assist the development of small industries?  One
recent review specifically covered small, rural manufacturers, in
Mexico and the Philippines, employing fewer than 50 people and
producing simple consumer products.   Most industries were found
to require too large an investment in photovoltaics to be economically
viable at present.  However, viable possibilities do
exist in some industries that use small power tools.
Among small industries, an interesting possibility is the local
manufacture of photovoltaic modules themselves.   Small-scale,
labor-intensive plants can make modules from purchased cells.
They can even make the cells, from industrial grade silicon,
using recently developed fabrication techniques.   A VITA Volunteer
recently helped set up the first factory in Africa to produce PV
panels.  Using purchased cells, the Moroccan plant turns out 100
panels per week.  In plants like this, the economics of using a
few extra workers to replace a large capital investment in automated
equipment are very favorable.   A detailed analysis of a
500-kW PV plant now being planned for India showed how 11 extra
production workers can displace about $800,000 of capital investment.
Small solar-cell modules to charge batteries for portable
lights, radios, and other small electric appliances can be made
in even simpler shops; it can be done on a village level.
Three relatively small-scale plant models at different levels of
production are proposed below.   Cost equivalents are for illustration
and should not be used for planning.
o A small shop producing 5-W to 10-W solar battery chargers.
    Solar cells, plastic for cases, etc., are purchased.
    Output:   2,000 chargers per year, 8 per working day.
    Personnel:   1 to 2 persons.
    Capital:   $25,000 startup, $32,000 per year material cost.
o Labor-intensive factory making 40-W, laminated PV modules.
    Solar cells, glass, and other supplies are purchased.
    Output:   1/2 Megawatt (MW) in modules per year (12,500
    modules, 50 per day).
    Personnel:   18 production workers.
    Capital:   $250,000 startup, $2,000,000 per year materials
o Plant making solar cells from industrial grade silicon.
    Using cheaper grade silicon, the plant casts polysilicon
    shapes, cuts them into square wafers, dopes them, adds metal
    contacts, etc.
    Output:   1 MW per year (1,000,000 wafers, 4000 per day).
    Personnel:   20 workers (6 highly skilled).
    Capital:   $2,500,000 startup, $3,000,000 per year operating.
At present, photovoltaics cannot compete with centrally generated
electricity except when power lines must be installed over long
distances.  They are therefore most likely to be applied in rural
locations, especially in villages.   Their flexibility in use, in
large or small arrays, is a major advantage since a system can be
carefully tailored to the specific application and expanded as
needed.  In comparing the cost-effectiveness of solar and diesel
systems, particular or local economic factors may be decisive,
even when maintenance costs and reliability are taken into account.
Failure problems with the earliest modules appear to have
been solved; thus, wind power is the only serious competitor of
PV devices as a renewable source of electricity.   An alternative
that should also be seriously considered is solar thermal power.
Hot water or gas can be used to drive a Stirling engine, for
example in irrigation, and some engineers argue that this is
currently the most effective method.   Refrigerators and air conditioners
can also be driven by warm water, but need small electrically
powered pumps.  Here as elsewhere, one must choose from many
alternatives the one that offers the best combination of cost and
The choice of solar cells or wind generators for electricity
depends on the location.  However, it is likely that a combination
of these will become the major source of electricity in
areas that are not supplied with a central grid that distributes,
for example, hydroelectric or geothermal energy.   The cost of
solar cells is still high and there are few applications in which
a strong economic benefit can be demonstrated to justify their
introduction.  However, there is no doubt that solar arrays can
greatly improve the quality of rural village life.   The next
decade should see a great expansion in solar-cell utilization as
prices fall to the predicted $1 to $2 per peak watt.
Ideally, developing countries can follow the lead of India,
Morocco, and Mexico by starting to develop their own capacities
for solar-cell production.  Thus, a country can begin now to
develop technological capabilities in a field where future demand
seems certain.
1.   Arco Solar Inc.  Applications Bulletin A-18-82A (June 2,
     1982).   Woodland Hills, California:   Arco Solar Inc., 1982.
2.   Bifano, W.J., "Economic Viability of Photovoltaic Power for
     Development Assistance Applications."  Institute of Electrical
     and Electronics Engineers, Proceedings of the 16th Photovoltaics
     Specialists Conference (San Diego, California), vol.
     3, pp. 1183-1188, 1982.
3.   Brainard, W.A., "The Worldwide Market for Photovoltaics in the
     Rural Sector."  Institute of Electrical and Electronics Engineers,
     Proceedings of the 16th Photovoltaics Specialists Conference
     (San Diego, California), vol. 3, pp. 1308-1313, 1982.
4.   Chiles, James R., "Tomorrow's Energy Today."  AUDUBON, New
     York, New York, vol. 92, pp. 58-72, 1990.
5.   Crutcher, J.L.; Cummings, A.B.; Norbedo, A.J., "Photovoltaic-Powered
     Sea-Water Desalination Systems:  Experience in Two
     Installations."  Institute of Electrical and Electronics
     Engineers, Proceedings of the 16th Photovoltaics Specialists
     Conference (San Diego, California), vol. 3, pp. 1400-1404,
6.   Day, J. F., "An American View of Photovoltaics in Developing
     Countries."  Proceedings of the Third European Conference on
     Solar Energy, pp. 124-134.
7.   Elwell, D., "Solar Electricity Generation in Developing
     Countries."  Mazingira, vol. 5, no. 3, pp. 30-41. (1981)
8.   Hankins, Mark, Renewable Energy in Kenya, Nairobi, Kenya:
     PHEDA, 1987.
9.   Hein, G.F., "Design, Installation, and Operating Experiences
     of 20 Photovoltaic Medical Refrigerator Systems on Four
     Continents."   Institute of Electrical and Electronics Engineers,
     Proceedings of the 16th Photovoltaics Specialists
     Conference (San Diego, California), vol. 3, pp. 1394-1399,
10.  Komp, Richard J., Practical Photovoltaics:  Electricity from
     Solar Cells, 2nd ed. Ann Arbor, Michigan:  AATEC Publications,
11.  Maycock, Paul D.; Stirewalt, Edward, Photovoltaics:  Sunlight
     to Electricity in One Step.  Andover, Massachusetts:  Brick
     House. Publishing Co., 1981.
12.  Wright, D.E., "The Use of Photovoltaic Pumps for Small-Scale
     Irrigation in the Developing World:  a Progress Report on the
     UNDP/World Bank Project."  Proceedings of the Third European
     Conference on Solar Energy, pp. 117-123, 1981.
The main U.S. suppliers of photovoltaic modules and related
equipment are listed below:
Alpha Solarco, 11534 Gondola Drive, Cincinnati, Ohio 45241
Arco Solar Inc., P.O. Box 4400, Woodland Hills, California 91365
Photocomm Inc., 7861 East Gray Road, Scottsdale, Arizona 85260
Solarex Corp., 1335 Piccard Drive, Rockville, Maryland 20850
SunWatt Corporation, RFD Box 751, Addison, Maine 04606