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                        TECHNICAL PAPER # 23
                         A GENERAL OVERVIEW
                           Keith Giarman
                        Technical Reviewers
                           Kevin Rinneran
                         Christopher Flavin
                            Published By 
                    1600 Wilson Boulevard, Suite 500
                     Arlington, Virginia 22209 USA
                 Tel:  703/276-1800 * Fax:   703/243-1865                    
              Understanding Solar Energy:  a General Overview
                          ISBN:   0-86619-233-9                               
              [C] 1985, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Maria Giannuzzi
as editor, Julie Berman handling typesetting and layout, and
Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Keith Giarman, has a
strong background in conventional and alternative energy technologies,
particularly in policy issues relative to developing countries.
He is currently an editor in the communications division
of NUS Corporation in Gaithersburg, Maryland, an international
consulting firm that specializes in various energy and environmental
matters.  The reviewers of this paper are also VITA Volunteers
with experience in solar energy.   Kevin Finneran is the
research director of the Solar Energy Industries Association.  He
has worked as a consultant in the United States and in developing
countries for the International Institute for Environment and
Development and for the U.S. Agency for International Development.
Christopher Flavin is a senior researcher for the Worldwatch
Institute in Washington, D.C., where he researches and
writes papers and books on energy technologies and policies with
an international perspective.
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
                   by VITA Volunteer Keith Giarman
Developing countries are in a particularly good position to use
solar energy because so many receive an abundance of sunshine.
More important, the inhabitants of these countries are frequently
scattered over vast areas, making access to electricity or conventional
fossil fuels difficult as well as expensive.   Many
solar systems are easily built and operated, thus providing a
readily available source of energy at an affordable price.  People
in poorer regions of the globe, moreover, need energy primarily
for low-temperature applications--cooking food, drying crops,
and purifying water--to fulfill their most basic human needs.
Solar energy can satisfy these low-temperature needs and give
Third World inhabitants a welcome alternative to the burning of
traditional fuels:  wood, dung, and agricultural waste (biomass).
The poor throughout the world are caught in a vicious cycle.  As
they burn more and more wood to cook and stay warm, they gradually
undermine their ability to feed themselves in the future.
Uncontrolled and inefficient biomass combustion leads to malnourished
soil; nitrogen-rich organic matter is burned for fuel
instead of being added to the earth.   The cost of replacing these
lost nutrients with chemical fertilizers is prohibitively high in
much of the Third World.  Couple this problem with the deleterious
effects of erosion and desertification, as well as the adverse
health effects of indoor and outdoor air pollution caused
by the burning of biomass, and the need for alternative energy
forms in developing countries becomes clear.
Of course, the electrification of Third World villages is the
first step to their modernization and, eventually, better economic
conditions for the poor via new industry.   Electricity is
needed in many areas for water pumping, communications, refrigeration,
lighting, and other uses.  Photovoltaic (PV) cells, which
directly convert sunlight to electricity, are transportable,
environmentally clean, and easily operated; thus, they are particularly
well suited for providing electricity in rural villages.
At present, however, PV is only cost competitive with
fossil-fuel generators in the most remote sections of the world.
The effective application of solar systems, like other technologies,
can be problematic in developing countries.   Cultural and
climatic conditions.  Unique to a given region, along with the
materials, tools, and manpower available in that area, must be
considered before introducing solar technologies.   Too many efforts
to introduce worthwhile energy programs and technologies
have failed over the years, because local factors have been
While sunlight reaching Third World countries is generally quite
abundant compared with industrialized nations, a country's geographical
location and climate help determine the feasibility of
solar energy systems.  Compared to industrialized countries, many
developing countries are generally closer to the equator and
therefore receive stronger and more consistent solar energy supply.
It is unwise to implement solar technologies in any area,
however, without considering fluctuations in the availability of
Local climatological variables, like cloud cover, can interfere
with the receipt of solar radiation, limiting the applicability
of solar energy in even the warmest portions of the globe.  For
this reason, the common assumption that the tropics are a uniformly
desirable area for solar energy may be overstated.   Solar
crop drying devices, for instance, are useless in a normally
sunny tropical area where harvest time and a seasonal increase in
cloud cover coincide.
As already suggested, cultural factors can also have a profound
influence on the introduction of solar systems, even in areas
with ideal climate and abundant resources.   In Africa, for example,
women from some tribes have cooked with wood before sunrise
or after sunset for years.  How does one convince them that it is
better to use solar cookers during the day?   Changing social
habits can take time and many cultural practices have a practical
foundation not immediately visible to outside researchers.  As a
result, programs for introducing solar technologies must be flexible
enough to accommodate these cultural preferences.
What is important in this general overview of solar energy is
that a multitude of factors--social, climatological, technical,
and economic--will dictate the success or failure of a project in
any area.  The key to effective use of solar energy is identifying
the specific applications where it matches the needs, resources,
and social infrastructure of the people.   In the discussion that
follows, the potential obstacles to the introduction of solar
energy will be examined briefly in relation to specific solar
Except for photovoltaic cells, solar energy is harnessable in
either of two ways:  via active or passive systems. (*)  Passive systems
absorb or focus the sun's radiation without the aid of a
moving medium, such as circulating water.   Passive solar collectors
focus or collect and strategically trap heat, that is, allow
the heat to enter but not to escape.   A typical passive collector
will allow sunlight to pass through glass, onto a dark, heat-absorbing
backdrop.  The heat is trapped for a useful function,
perhaps to cook a chicken in a solar oven.   Or light can simply
be focused onto a certain area, say the bottom of a pot in a
solar cooker, to heat the contents to a desired temperature.  In
other passive systems, the natural thermosyphon effect (**) can be
used to circulate heated air to a home or barn for space heating.
Active solar systems are a bit more complicated, since a circulating
medium (usually water) must be heated to make these systems
function.  In a typical solar set-up, so called  flat plate'
collectors absorb heat from the sun via a large, flat, dark
surface area.  The heat is transferred to a liquid that circulates
through tubes or channels that are part of the absorber
surface.  The water can then be stored and tapped when necessary
to perform useful tasks that require hot water (washing utensils,
personal hygiene, pre-heating water for boiling, and so forth).
Some active solar systems use reflective surfaces to concentrate
the sun's rays on a small absorber surface such as a copper tube.
These concentrating collectors can produce higher temperatures
for commercial and industrial applications.
The sun's energy can also be converted into electricity by using
photovoltaic cells.  Photovoltaic (or solar) cells convert sunlight
directly into electricity, without mechanical generators.
The cells are usually composed of silicon, but other semi-conductor
materials are also used.  When sunlight strikes photovoltaic
cells, electrons are dislodged, creating an electrical current
which can then be drawn off.
(*) The division between active and passive systems is not clear.
Hybrid systems incorporate elements of both.   For example, a
passive solar structure can be built in such a way as to strategically
trap heat; a mechanical device, such as a fan, can be
used to move the heated air to other areas of the structure.
Thus, both passive and active elements are incorporated in the
same system.
(**) The tendency of heated liquids and gases to rise.   In a thermosyphon
system, a liquid or gas (air) circulates naturally without
means of a fan or pump.
Photovoltaic cells were first used in the 1950s to power space
satellites.  At that time they were quite expensive, costing more
than $1,000 per watt of capacity.   Although they are still too
costly for widespread use, their price has been brought down to
about $10 per watt.
Active solar systems require a higher capital and labor investment
than passive technologies, but they can sometimes provide a
quick return on that investment through their low maintenance and
zero fuel costs.  Moreover, the scarcity of traditional and
fossil fuels is so acute in some areas of the globe that active
solar water heaters may be the most practical source of substantial
quantities of hot water.
Hot water is imperative for modernizing rural areas, since it is
the key to improving sanitary conditions in public facilities
like health clinics, hospitals, and schools.   Of course, hot
water is important at the domestic level as well, particularly in
personal hygiene to combat disease.   Simple active systems made
from readily available and inexpensive materials are feasible in
areas where fuel and other resources are scarce.   A number of
simple solar water heaters have been developed which can be built
with locally available materials and tools (see Figure 1).

27p05.gif (600x600)

Solar-powered water pumps are also available.   Once set up, these
pumps are easily operated, but they are mechanically complex.
Water must be heated to 70 to 80 [degrees] C by a collector or concentrator
apparatus--similar to or the same as in solar heaters.   The water
then heats a liquid gas (such as Freon), which vaporizes and
expands, and drives an engine for pumping.   Unlike typical solar
water heating systems, such solar pumps cannot be built easily
from local materials and tools, and the principle behind their
operation is relatively complex.   More important, solar water
pumps are too expensive for the rural poor.   The capital cost
varies anywhere from U.S. $6,000 to $78,000 depending on the
pump's size, which, when compared to the cost of diesel generators
or photovoltaics, makes solar water pumping uneconomical.
Solar Cookers and Ovens
Because less complicated systems are more easily adapted in
developing countries, passive solar devices are preferred.  Simple
solar cookers and ovens are the most practical application of
solar energy in these countries (see Figure 2).   They can be built

27p06.gif (486x486)

quite easily by individuals using local materials or produced by
village industries.  High temperature ovens and extremely efficient
cookers have been developed.   However, cheap, easy-to-use
models of polished reflecting metal, or aluminum foil, stand a
better chance of acceptance in the poorer regions of the world.
For all their success, solar cookers are a frequently cited
example of an energy technology that failed because of cultural,
not economic, reasons.  For instance, in some areas of Africa
where meals are cooked before sunrise or after sunset in accordance
with cultural practice, solar cookers have been difficult to
introduce.  As suggested earlier, altering accepted cooking practices
rooted in social convention is a difficult process.   Technical
complexities and cost can compound these cultural barriers.
For example, some villagers have complained about having to
adjust the cooker's reflector to refocus the sun's rays on the
cooking utensil.  In China, the price of cookers is relatively
low, U.S. $10-30.  But a Chinese family could build a 10 cubic
meter biogas unit for U.S. $100.00, which could serve as an
essentially unlimited source of energy for a variety of household
and agricultural needs.  In many instances, it just does not make
sense for them to use solar cookers until they become extremely
Depending on the local area, the number of possible obstacles to
effective introduction can be great.   These barriers must be
identified as completely as possible before capital is devoted to
program implementation.  If properly defined, problems can be
circumvented or alleviated.  For instance, efforts by a Danish
church group to introduce solar cookers in Upper Volta succeeded
because villagers helped adapt the cooker to local needs and
Although there are a number of different designs, solar cookers
consist of three basic parts:   a reflector, a stand, and a pot
holder.  Reflectors are usually dish-shaped and have a shiny
reflective surface.  Aluminum and aluminized mylar have been used
successfully to focus the sun's radiation onto the cooking utensil.
The stand can be made of common materials, including wood, metal,
tubing, brass, or steel rods.   Whatever materials are used, they
must be strong enough to support the reflector and withstand
outdoor elements (wind in particular).   At the same time, the
stand must be light so the cooker as a unit is portable.
The pot holder must be constructed to maximize the efficiency of
the cooker, that is, the pot must sit near the focal point of the
reflected sun's rays, where heat will be spread out over the
bottom of the pot.  Other factors will contribute to the efficiency
of the solar cooking system as well:
     *     The bottom of the pot should be a dull black color to
          facilitate heat absorption.
     *     The pot should be covered.
     *     The cooker should be operated in bright sun.
     *     The position of the pot or reflectors should be adjusted
          every 10-30 minutes to accommodate changes in the
          sun's angle of incidence.
Solar ovens are different from cookers in that the sun's heat is
not simply focused--it is trapped in an enclosed area as well.
In a number of places where glass, wood, low-cost reflective
material, and some type of insulation are readily available, the
solar oven, like the cooker, can be easily built.   Cardboard can
even be sub-stituted for wood in some designs.
In the solar oven (see Figure 3), the black insulated interior

27p08.gif (600x600)

retains heat from the sunlight that is reflected off mirrors
extending from the oven's frame.   A double glass cover lets light
in, but does not allow heat to escape.   The let-heat-in-but-not-escape
operation is simple, achieving temperatures that may exceed
200 [degrees] C in a well-sealed, well-constructed oven.  The cook
merely drops whatever needs cooking inside the oven through the
back door, or a hinged cover, and the heat does the rest.
Like the solar cooker, the solar oven will function best in
strong sunlight.  But since the oven is utilizing a greater
surface area to absorb heat, it can operate under less than
ideal conditions.  Of course, the insulated box should be well
sealed to prevent unnecessary heat loss.
Solar Stills and Crop Dryers
Also based on the passive solar principle, the solar still is
useful for making salty or brackish water fresh.   Simple, inexpensive,
fuel-thrifty devices for purifying large and small quantities
of water are needed in developing countries where potable
water is in short supply.
Like solar cookers and ovens, solar stills are easy to assemble
and come in different models.   All stills consist of a heat-absorbing
container in which dirty water can be placed.   After
reaching a certain temperature, the dirty water in the closed
system vaporizes, leaving impurities in the container.   Fresh
water vapor collects on the surface of the still, condensing on
the glass or plastic cover, and slowly trickles into some sort of
collection system.
The simple solar still illustrated below (Figure 4) operates

27p09.gif (600x600)

like the solar oven previously described:   it absorbs and retains
heat.  A glass or plastic cover lets radiation in to heat the
impure water that sits in the black insulated pan.   The well-sealed
cover keeps enough heat in to achieve temperatures necessary
for distillation.
Solar heat can also be used to dry crops.   Indeed, farmers all
over the world have been using the sun's heat to dry crops for
centuries.  But simply hanging or spreading crops outside can
lead to substantial crop loss, due to exposure to dirt, animals,
insects, molds, and bad weather.
Gas-fired and electric dryers are expensive devices and, of
course, the cost of using them increases as fuel prices rise.
Small-scale solar dryers, which operate much like the solar oven
and still described earlier, can be made easily at low cost, but
simple and inexpensive large capacity models are available as
According to Daniel Deudney and Christopher Flavin of Worldwatch
Institute, a number of different types of dryers are being tested,
most with success.  For example, a simple solar dryer capable
of drying up to one ton of rice at a time is in use in Thailand
(see Figure 5).  The device consists of three connected parts:  a

27p10.gif (600x600)

solar collector, a container to hold the rice (known as a paddy
box in this model), and a chimney.   The collector floor is made of
a black substance to help absorb heat and the sides and cover are
clear.  As in other passive solar technologies, radiation enters
the system but cannot escape.   The rice or grain is placed in the
box, which sits above the collector.   Warm air from the collector
circulates through holes in the bottom of the box, drying the
foodstuff, and passes up and out of the system through a chimney.
According to the Renewable Energy Resources Information Center
of the Asian Institute of Technology, the one-ton dryer cost U.S.
$150 in Thailand in early 1982.   When one considers that drying
increases the marketing value of the food, the investment may pay
for itself quickly.  The Institute also notes that basic tools
and equipment can be used to build the dryer and maintenance is
simple if the bamboo used to support the system is treated to
prevent decay.
Passive Solar Structures
The passive solar principle can be applied on a greater scale in
developing countries.  Buildings can be designed to operate like
large solar collectors, that is, designed to absorb and trap
heat.  Passive space heating systems contain no mechanical parts
and are often cheaper than active space heating systems.  A passive
system uses the structural components of a building (walls,
windows, and floors) to collect and store solar energy.   Heat is
distributed by the natural processes of convection, conduction,
and radiation.
In a passive solar building located north of the equator, most of
the windows face south to let in as much sunlight as possible.
This principle is reversed in passive buildings south of the
equator--most windows face north.   Beat is stored in "thermal
mass"--thick masonry floors or walls, rock beds, water-filled
containers or any combination of these.   During the day, the thermal
mass absorbs a great deal of heat, particularly if it is in
direct sunlight.  At night, the stored heat gradually migrates to
the living area.  In a strictly passive house, this heat moves
naturally, without a mechanical boost.   However, hybrid systems
that incorporate fans or blowers for added circulation are common.
At night and during heavily overcast periods, movable
insulation in the form of heavy curtains or shades is pulled over
the windows to reduce heat loss.
In hotter climates, buildings can be constructed to stay cool.
Passive cooling is accomplished through the structure's design,
layout and components, as in passive heating.   Passive cooling
techniques control incoming sunlight and use a variety of methods
to encourage cooling air movement.
Sunlight can be kept out by shading windows with overhands,
trees, or awnings.  Movable insulation can be drawn over windows
during the day to reduce heat gain.
Natural ventilation is encouraged by opening the building to
summer breezes and providing a clear path for the air to move
along.  Induced ventilation depends upon use of the chimney
effect, where hot air that accumulates is allowed to rise and
exit rapidly through high vents.   At the same time cooler air from
another source (such as a well-shaded north yard) is drawn in.
A number of possible sun- and earth-tempered structures can reduce
the energy needed for space beating and cooling.   The most
appropriate of these structures are simple in design and easily
built with local materials.  Some forms of traditional housing
employ passive solar principles.   Northern China has thousands of
masonry buildings designed to trap the sun's heat in winter.
However, in some countries, this traditional architecture has
been replaced by inefficient modern designs.
Batch Water Heaters
Batch heaters are the simplest and most economical solar water
heaters.  One type of batch heater is simply a black plastic bag
of water placed in the sun.  Another type of batch heater consists
of a ditch that is lined with dark plastic.
Photovoltaic (PV) conversion has been touted for years as an
environmentally acceptable energy source for the future.  Initial
expectations that photovoltaics would become cost-competitive
with conventional energy sources by the mid-1980s were, however,
too optimistic.  Nevertheless, in many remote locations of the
world, where electricity is inaccessible and conventional or
traditional fuels are difficult to come by, PV can be cost-competitive.
In these isolated rural areas, diesel generators
are the prime source of electricity.   When one considers the
maintenance costs and potential fuel supply shortages of diesel
generators, PV is often a viable alternative in rural electricity
applications of three kilowatts or less.   Moreover, the cost of
fuels such as oil and wood is likely to rise while the cost of
solar cells should continue to fall.
The availability of electricity in developing countries can
greatly improve the quality of life.   PV is a clean, reliable
source of electricity, easy to use once installed, and transportable.
But the solar cell is just one part of a somewhat complicated
system needed to provide electricity at the village level.
Indeed, most sponsors of PV projects currently under way in
developing countries are evaluating the economics of total PV
electrical systems.  Many small-scale applications are cost-competitive
today, but large systems require site construction,
installation, some maintenance, batteries for storage, and control
circuits to regulate current and/or voltage.
On the other hand, PV comes in modular units, which means that an
expanding village could avoid the huge capital outlays necessary
to get conventional forms of electricity.   In any case, even small
amounts of electricity could lead to a substantial improvement in
living conditions in developing regions.   Photovoltaics have
practical applications as sources of power for water pumping,
communications, refrigeration, and lighting.   PV-powered water
pumps are not only useful for agricultural purposes, they also
supply safe drinking water in many villages.   Open wells can be
covered after a pump is installed, thus reducing the risk of
disease to drinkers.
In other areas, solar cells are powering microwave telephone
systems to link remote locations with industrial and urban areas.
As an energy source for television and lighting, PV also contributes
to educational programs and enables important village activities
and meetings to be held at night.   Refrigerators, essential
for storing and preserving food, drugs, and ice can also be
powered with PV.  Unfortunately, their price is relatively high--ranging
from about U.S. $2,000 to $5,000.
Progress in photovoltaics research has been impressive over the
last 10 years, and numerous research efforts are under way in
industrialized and developing nations alike.   China, Mexico,
India, and Pakistan have extensive research or pilot programs in
operation, and many other developing countries are participating
on a more modest scale.  As research breakthroughs occur and the
PV industry continues to mature, the cost of cells is sure to
drop.  As it does, the cost-effective applications of PV in developing
areas will multiply.
This overview has focused on the simplest solar technologies,
that is, those least likely to encounter economic and technical
barriers during their introduction.   Even these are likely to
experience cultural impediments that must be understood, confronted,
and resolved before total social acceptance is achieved
in Third World villages.
Solar energy has many uses in developing regions that are not
discussed in this paper, including solar-powered absorbative refrigeration,
active space cooling and heating systems, and combination
solar systems.  However, these are relatively complex and
expensive devices.  The most appropriate technologies are generally
those that follow the simple passive principle.
Experience over the years has proven that applying new energy
forms (no matter how simple) to developing areas will almost
always meet some sort of resistance or difficulty.   The poor in
the Third World desperately need cheap, clean, and simple energy
technologies to conserve traditional fuels, preserve the environment,
and satisfy fundamental human needs.   But it is only insightful
and sensitive energy planning--identifying problems
before they occur--that will make widespread use of solar energy
a reality in developing countries.
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Deudney, D., and Flavin, C. Renewable Energy:   The Power To Choose,
     New York, New York:  W.W. Norton and Company, 1983.
Giarman, R.K. "Chinese Energy:   Satisfying Needs at a Local
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Gregoire, Roger G., P.E.  Understanding Solar Food Dryers.  VITA
     Technical Paper #15.  Arlington, Virginia:   VITA, 1984.
National Research Council.  Supplement, Energy for Rural Development.
     Washington, D.C.:  National Academy Press, 1981.
Smil, V.  "Intermediate Energy Technology in China."   Bulletin of
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Swet, C.J.  Understanding Solar Water Pumps.  VITA Technical Paper
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VITA.  Solar Convection Grain Dryer.  VITA Technical Bulletin #63.
     Arlington, Virginia:   VITA, 1981.
VITA.  Solar Cooker Construction Manual.  Arlington, Virginia:  VITA,
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World Health Organization.  "Needing a Solar Rice Dryer?  Do It
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