TECHNICAL PAPER # 23
UNDERSTANDING SOLAR ENERGY:
A GENERAL OVERVIEW
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Understanding Solar Energy: a
1985, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
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
They are not intended to provide construction or
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
almost entirely by VITA Volunteer technical experts on a
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
strong background in conventional and alternative energy
particularly in policy issues relative to developing
He is currently an editor in the communications division
of NUS Corporation in Gaithersburg, Maryland, an
consulting firm that specializes in various energy and
reviewers of this paper are also VITA Volunteers
with experience in solar energy.
Kevin Finneran is the
research director of the Solar Energy Industries
has worked as a consultant in the United States and in
countries for the International Institute for Environment
Development and for the U.S. Agency for International
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
an international perspective.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING SOLAR ENERGY: A
VITA Volunteer Keith Giarman
Developing countries are in a particularly good position to
solar energy because so many receive an abundance of
More important, the inhabitants of these countries are
scattered over vast areas, making access to electricity or
fossil fuels difficult as well as expensive.
solar systems are easily built and operated, thus providing
readily available source of energy at an affordable
in poorer regions of the globe, moreover, need energy
for low-temperature applications--cooking food, drying
and purifying water--to fulfill their most basic human
Solar energy can satisfy these low-temperature needs and
Third World inhabitants a welcome alternative to the burning
wood, dung, and agricultural waste (biomass).
The poor throughout the world are caught in a vicious
they burn more and more wood to cook and stay warm, they
undermine their ability to feed themselves in the future.
Uncontrolled and inefficient biomass combustion leads to
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
much of the Third World.
Couple this problem with the deleterious
effects of erosion and desertification, as well as the
health effects of indoor and outdoor air pollution caused
by the burning of biomass, and the need for alternative
forms in developing countries becomes clear.
Of course, the electrification of Third World villages is
first step to their modernization and, eventually, better
conditions for the poor via new industry.
needed in many areas for water pumping, communications,
lighting, and other uses.
Photovoltaic (PV) cells, which
directly convert sunlight to electricity, are transportable,
environmentally clean, and easily operated; thus, they are
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
The effective application of solar systems, like other
can be problematic in developing countries.
Unique to a given region, along with the
materials, tools, and manpower available in that area, must
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
abundant compared with industrialized nations, a country's
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
It is unwise to implement solar technologies in any area,
however, without considering fluctuations in the
Local climatological variables, like cloud cover, can
with the receipt of solar radiation, limiting the
of solar energy in even the warmest portions of the
this reason, the common assumption that the tropics are a
desirable area for solar energy may be overstated.
crop drying devices, for instance, are useless in a normally
sunny tropical area where harvest time and a seasonal
cloud cover coincide.
As already suggested, cultural factors can also have a
influence on the introduction of solar systems, even in
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?
habits can take time and many cultural practices have a
foundation not immediately visible to outside
researchers. As a
result, programs for introducing solar technologies must be
enough to accommodate these cultural preferences.
What is important in this general overview of solar energy
that a multitude of factors--social, climatological,
and economic--will dictate the success or failure of a
any area. The key to
effective use of solar energy is identifying
the specific applications where it matches the needs,
and social infrastructure of the people.
In the discussion that
follows, the potential obstacles to the introduction of
energy will be examined briefly in relation to specific
Except for photovoltaic cells, solar energy is harnessable
either of two ways:
via active or 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,
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
other passive systems, the natural thermosyphon effect (**)
used to circulate heated air to a home or barn for space
Active solar systems are a bit more complicated, since a
medium (usually water) must be heated to make these systems
function. In a
typical solar set-up, so called flat
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
Some active solar systems use reflective surfaces to
the sun's rays on a small absorber surface such as a copper
These concentrating collectors can produce higher temperatures
for commercial and industrial applications.
The sun's energy can also be converted into electricity by
Photovoltaic (or solar) cells convert sunlight
directly into electricity, without mechanical generators.
The cells are usually composed of silicon, but other
materials are also used.
When sunlight strikes photovoltaic
cells, electrons are dislodged, creating an electrical
which can then be drawn off.
(*) The division between active and passive systems is not
Hybrid systems incorporate elements of both.
For example, a
passive solar structure can be built in such a way as to
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 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
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
about $10 per watt.
TWO ACTIVE SOLAR ENERGY SYSTEMS:
WATER HEATERS AND WATER PUMPS
Active solar systems require a higher capital and labor
than passive technologies, but they can sometimes provide a
quick return on that investment through their low
zero fuel costs.
Moreover, the scarcity of traditional and
fossil fuels is so acute in some areas of the globe that
solar water heaters may be the most practical source of
quantities of hot water.
Hot water is imperative for modernizing rural areas, since
the key to improving sanitary conditions in public
like health clinics, hospitals, and schools.
Of course, hot
water is important at the domestic level as well,
personal hygiene to combat disease.
Simple active systems made
from readily available and inexpensive materials are
areas where fuel and other resources are scarce.
A number of
simple solar water heaters have been developed which can be
with locally available materials and tools (see Figure 1).
Solar-powered water pumps are also available.
Once set up, these
pumps are easily operated, but they are mechanically
Water must be heated to 70 to 80 [degrees] C by a collector
apparatus--similar to or the same as in solar heaters.
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
from local materials and tools, and the principle behind
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
or photovoltaics, makes solar water pumping uneconomical.
PASSIVE SOLAR ENERGY SYSTEMS
Solar Cookers and Ovens
Because less complicated systems are more easily adapted in
developing countries, passive solar devices are
solar cookers and ovens are the most practical application
solar energy in these countries (see Figure 2).
They can be built
quite easily by individuals using local materials or
High temperature ovens and extremely efficient
cookers have been developed.
However, cheap, easy-to-use
models of polished reflecting metal, or aluminum foil, stand
better chance of acceptance in the poorer regions of the
For all their success, solar cookers are a frequently cited
example of an energy technology that failed because of
not economic, reasons.
For instance, in some areas of Africa
where meals are cooked before sunrise or after sunset in
with cultural practice, solar cookers have been difficult to
suggested earlier, altering accepted cooking practices
rooted in social convention is a difficult process.
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
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
and agricultural needs.
In many instances, it just does not make
sense for them to use solar cookers until they become
Depending on the local area, the number of possible
effective introduction can be great.
These barriers must be
identified as completely as possible before capital is
If properly defined, problems can be
circumvented or alleviated.
For instance, efforts by a Danish
church group to introduce solar cookers in Upper Volta
because villagers helped adapt the cooker to local needs and
Although there are a number of different designs, solar
consist of three basic parts:
a reflector, a stand, and a pot
are usually dish-shaped and have a shiny
Aluminum and aluminized mylar have been used
successfully to focus the sun's radiation onto the cooking
The stand can be made of common materials, including wood,
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
the cooker, that is, the pot must sit near the focal point
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
The pot should be covered.
The cooker should be operated in bright
The position of the pot or reflectors
should be adjusted
minutes to accommodate changes in the
Solar ovens are different from cookers in that the sun's
not simply focused--it is trapped in an enclosed area as
In a number of places where glass, wood, low-cost reflective
material, and some type of insulation are readily available,
solar oven, like the cooker, can be easily built.
even be sub-stituted for wood in some designs.
In the solar oven (see Figure 3), the black insulated
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.
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
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
useful for making salty or brackish water fresh.
fuel-thrifty devices for purifying large and small
of water are needed in developing countries where potable
water is in short supply.
Like solar cookers and ovens, solar stills are easy to
and come in different models.
All stills consist of a heat-absorbing
container in which dirty water can be placed.
reaching a certain temperature, the dirty water in the
system vaporizes, leaving impurities in the container.
water vapor collects on the surface of the still, condensing
the glass or plastic cover, and slowly trickles into some
The simple solar still illustrated below (Figure 4) operates
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.
cover keeps enough heat in to achieve temperatures necessary
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
simply hanging or spreading crops outside can
lead to substantial crop loss, due to exposure to dirt,
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
Small-scale solar dryers, which operate much like the solar
and still described earlier, can be made easily at low cost,
simple and inexpensive large capacity models are available as
According to Daniel Deudney and Christopher Flavin of
Institute, a number of different types of dryers are being
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
(see Figure 5). The
device consists of three connected parts:
solar collector, a container to hold the rice (known as a
box in this model), and a chimney.
The collector floor is made of
a black substance to help absorb heat and the sides and
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
foodstuff, and passes up and out of the system through a
According to the Renewable Energy Resources Information
of the Asian Institute of Technology, the one-ton dryer cost
$150 in Thailand in early 1982.
When one considers that drying
increases the marketing value of the food, the investment
for itself quickly.
The Institute also notes that basic tools
and equipment can be used to build the dryer and maintenance
simple if the bamboo used to support the system is treated
Passive Solar Structures
The passive solar principle can be applied on a greater
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.
distributed by the natural processes of convection,
In a passive solar building located north of the equator,
the windows face south to let in as much sunlight as
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,
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
At night and during heavily overcast periods, movable
insulation in the form of heavy curtains or shades is pulled
the windows to reduce heat loss.
In hotter climates, buildings can be constructed to stay
Passive cooling is accomplished through the structure's
layout and components, as in passive heating.
techniques control incoming sunlight and use a variety of
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
ventilation depends upon use of the chimney
effect, where hot air that accumulates is allowed to rise
exit rapidly through high vents.
At the same time cooler air from
another source (such as a well-shaded north yard) is drawn
A number of possible sun- and earth-tempered structures can
the energy needed for space beating and cooling.
appropriate of these structures are simple in design and
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
been replaced by inefficient modern designs.
Batch Water Heaters
Batch heaters are the simplest and most economical solar
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
expectations that photovoltaics would become
with conventional energy sources by the mid-1980s were,
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
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
generators, PV is often a viable alternative in rural
applications of three kilowatts or less.
Moreover, the cost of
fuels such as oil and wood is likely to rise while the cost
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
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
Many small-scale applications are cost-competitive
today, but large systems require site construction,
installation, some maintenance, batteries for storage, and
circuits to regulate current and/or voltage.
On the other hand, PV comes in modular units, which means
expanding village could avoid the huge capital outlays
to get conventional forms of electricity.
In any case, even small
amounts of electricity could lead to a substantial
living conditions in developing regions.
practical applications as sources of power for water
communications, refrigeration, and lighting.
pumps are not only useful for agricultural purposes, they
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
As an energy source for television and lighting, PV also
to educational programs and enables important village
and meetings to be held at night.
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
last 10 years, and numerous research efforts are under way
industrialized and developing nations alike.
India, and Pakistan have extensive research or pilot
operation, and many other developing countries are
on a more modest scale.
As research breakthroughs occur and the
PV industry continues to mature, the cost of cells is sure
drop. As it does,
the cost-effective applications of PV in developing
areas will multiply.
This overview has focused on the simplest solar
that is, those least likely to encounter economic and
barriers during their introduction.
Even these are likely to
experience cultural impediments that must be understood,
and resolved before total social acceptance is achieved
in Third World villages.
Solar energy has many uses in developing regions that are
discussed in this paper, including solar-powered absorbative
active space cooling and heating systems, and combination
However, these are relatively complex and
The most appropriate technologies are generally
those that follow the simple passive principle.
Experience over the years has proven that applying new
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
technologies to conserve traditional fuels, preserve the
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
a reality in developing countries.
BIBLIOGRAPHY/SUGGESTED READING LIST
Cecelski, E., et al.
Household Energy and the Poor in the Third
Resources for the Future, Inc.,
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
Gregoire, Roger G., P.E.
Understanding Solar Food Dryers.
#15. Arlington, Virginia:
National Research Council.
Supplement, Energy for Rural Development.
D.C.: National Academy Press, 1981.
"Intermediate Energy Technology in China."
Scientist (February 1977), p. 28.
Understanding Solar Water Pumps.
VITA Technical Paper
Energy. Appropriate Technology and
(Prepared by VITA for Action/Peace Corps).
Virginia: VITA, 1979.
Convection Grain Dryer. VITA Technical
VITA. Solar Cooker
Construction Manual. Arlington,
VITA. Solar Water
Heater. Arlington, Virginia:
World Health Organization.
"Needing a Solar Rice Dryer?
Technology For Health, Geneva, Switzerland:
Organization, Newsletter 11 (Autumn
1982), p. 10.
"Solar Electricity Generation in Developing
Technology for Health, Newsletter 11
Autumn 1982), p.