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TP# 37: 9/85
Jacques Le Nonmand
Darrell G. Phippen
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax:
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, Suzanne Brooks handling typesetting and layout,
Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Horace McCracken,
president of the McCracken Solar Company in Alturas,
The co-author, VITA Volunteer Joel Gordes, is currently the
design analyst for the State of Connecticut's Solar Mortgage
Subsidy Program. The
reviewers are also VITA volunteers.
Dunham has done consulting in solar and alternative sources
energy for VITA and AID.
He has lived and worked in India, Pakistan,
and Morocco. Mr.
Dunham has also prepared a state-of-the-art
survey on solar stills for AID.
Jacques Le Normand is Assistant
Director at the Brace Research Institute, Quebec, Canada,
which does research in renewable energy.
He has supervised work
with solar collectors and has written several publications
solar and wind energy, and conservation.
Darrell G. Phippen is a
mechanical engineer and development specialist who works
Food for the Hungry in Scottsdale, Arizona.
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.
For more information about VITA services in general, or the
technology presented in this paper, contact VITA at 1815
Lynn Street, Suite 200, Arlington, Virginia 22209 USA.
UNDERSTANDING SOLAR STILLS
Volunteers Horace McCracken and Joel Gordes
Ninety-seven percent of the earth's water mass lies in its
oceans. Of the
remaining 3 percent, 5/6 is brackish, leaving a
mere .5 percent as fresh water.
As a result, many people do not
have access to adequate and inexpensive supplies of potable
water. This leads to
population concentration around existing
water supplies, marginal health conditions, and a generally
standard of living.
Solar distillation uses the heat of the sun directly in a
piece of equipment to purify water.
The equipment, commonly
called a solar still, consists primarily of a shallow basin
a transparent glass cover.
The sun heats the water in the basin,
Moisture rises, condenses on the cover and
runs down into a collection trough, leaving behind the
minerals, and most other impurities, including germs.
Although it can be rather expensive to build a solar still
is both effective and long-lasting, it can produce purified
at a reasonable cost if it is built, operated, and
This paper focuses mainly on small-scale basin-type solar
as suppliers of potable water for families and other small
Of all the solar still designs developed thus far, the
continues to be the most economical.
HISTORY OF SOLAR DISTILLATION
Distillation has long been considered a way of making salt
drinkable and purifying water in remote locations.
As early as
the fourth century B.C., Aristotle described a method to
evaporate impure water and then condense it for potable use.
P.I. Cooper, in his efforts to document the development and
of solar stills, reports that Arabian alchemists were the
earliest known people to use solar distillation to produce
potable water in the sixteenth century.
But the first documented
reference for a device was made in 1742 by Nicolo Ghezzi of
Italy, although it is not known whether he went beyond the
conceptual stage and actually built it.
The first modern solar still was built in Las Salinas,
1872, by Charles Wilson.
It consisted of 64 water basins (a
total of 4,459 square meters) made of blackened wood with
glass covers. This
installation was used to supply water (20,000
liters per day) to animals working mining operations.
area was opened to the outside by railroad, the installation
allowed to deteriorate but was still in operation as late as
1912--40 years after its initial construction.
This design has
formed the basis for the majority of stills built since that
During the 1950s, interest in solar distillation was
in virtually all cases, the objective was to develop large
In California, the goal was to
develop plants capable of producing 1 million gallons, or
cubic meters of water per day.
However, after about 10 years,
researchers around the world concluded that large solar
plants were much too expensive to compete with fuel-fired
ones. So research
shifted to smaller solar distillation plants.
In the 1960s and 1970s, 38 plants were built in 14
with capacities ranging from a few hundred to around 30,000
liters of water per day.
Of these, about one third have since
been dismantled or abandoned due to materials failures.
this size range are reported to have been built in the last
Despite the growing discouragement over community-size
McCracken Solar Company in California continued its efforts
market solar stills for residential use.
Worldwide interest in
small residential-units is growing, and now that the price
is ten times what it was in the 1960s, interest in the
units may be revived.
Although solar distillation at present cannot compete with
desalination in large central plants, it will surely become
a viable technology within the next 100 years, when oil
will have approached exhaustion.
When that day arrives, the
primary question will be, "Which method of solar
Meanwhile, almost anyone hauling drinking water any
distance would be economically better off using a solar still.
NEEDS SERVED BY SOLAR DISTILLATION
Solar distillation could benefit developing countries in
Solar distillation can be a cost-effective
clean water for drinking, cooking, washing,
basic human needs.
It can improve health standards by removing
questionable water supplies.
It can help extend the usage of existing
fresh water in
where the quality or quantity of supply is
deteriorating. Where sea water
is available, it can
developing country's dependence on rainfall.
Solar stills, operating on sea or brackish
supplies of water during a time of drought.
Solar distillation generally uses less
energy to purify
It can foster cottage industries, animal
for food production in areas where such
are now limited by inadequate supplies of
water. Fishing could become important
where no drinking water is available for
Solar distillation will permit settlement in
thus relieving population
The energy from the sun used to distill water is free.
cost of building a still makes the cost of the distilled
rather high, at least for large-scale uses such as
and flushing away wastes in industry and homes.
the solar still is used principally to purify water for
and for some business, industry, laboratory, and green-house
also appears able to purify polluted water.
Solar Distilled Water for Irrigation
For field agriculture, the solar still is not very
takes about one meter depth of irrigation water per year to
produce crops in dry climates, whereas the solar still can
about two meters' depth.
Thus, one square meter of solar
still would irrigate two square meters of land.
the cost of building the still would make water more
than the crops being produced.
This may not be true, however,
for agriculture in controlled environments, i.e.,
well-designed hydroponically-operated greenhouse should be
to produce 8 to 10 times as much food, per unit volume of
consumed, as field crops.
Recovery of Salt from a Solar Still
Since salt is a very cheap industrial material, and a solar
cannot produce anymore than an open pond, combining the
of salt with the distilling of water is not attractive
a family is using a solar still to provide
water valued at $1 per day, the amount of salt they need
cost them half a cent.
Recovery of Potable Water from Sewage
Although it seems possible that potable water can be
from sewage, if contaminants such as odorous gases are
sewage water fed to the still, some portion of those gases
evaporate and condense with the distilled water.
probability they could be filtered out with activated
to date, however, no one has had any experience with this.
If the "contaminant" is alcohol, it can be
separated from the
water. But it would
take two or three passes through the still
to attain a high enough concentration of alcohol to be used
the current availability of fossil fuels,
producing alcohol in this way is not yet economical.
when fossil fuel supplies run low and the price rises, solar
distillation could play a significant role.
Recovery of Distilled Water From Polluted Water Bodies
Whether or not solar distillation can actually purify
water is not yet known.
Laboratory tests have shown, however,
that a solar still can eliminate bacteria.
If after additional
research, a quantity of clean water can be recovered from
polluted water, this capability may become economically more
important than the purification of sea water.
It may also be
used to remove toxic substances such as pesticides.
Preliminary laboratory tests show that a modified version of
still--now commercially available--can do a very good job of
removing such substances from feed water.
(TCE), for example, has been removed by a factor of 5,000 to
ethylene dibromide (EDB) by 100 to 1; nitrates by 50 to 1;
others within those ranges.
Of course, more work must be done to
quantify these numbers, not to mention the unending list of
chemicals that need to be tested.
Elimination of Algae.
While algae will grow in some deep basin
stills where the water temperature seldom gets very high, in
shallow basin still it is usually killed by the high
GENERAL THEORY OF SOLAR DISTILLATION
Distillation operates by the escape of moving molecules from
water surface into the gases above it.
Sensible heat--the kind
you can measure with a thermometer--is caused by the
molecules, zig-zagging about constantly, except that they
all moving at the same speed.
Add energy and they move faster,
and the fastest-moving ones may escape the surface to become
It takes a lot of energy for water to vaporize.
While a certain
amount of energy is needed to raise the temperature of a
of water from 0 [degrees] to 100 [degrees] Celsius (C), it
takes five and one-half
times that much to change it from water at 100 [degrees] C
to water vapor
at 100 [degrees] C.
Practically all this energy, however, is given back
when the water vapor condenses.
The salts and minerals do not evaporate along with the
Ordinary table salt, for example, does not turn into vapor
it gets over 1400 [degrees] C, so it remains in the brine
when the water
evaporates. This is
the way we get fresh water in the clouds
from the oceans, by solar distillation.
All the fresh water on
earth has been solar distilled.
It is not necessary for the water to actually boil to bring
Steaming it away gently does the same job as
boiling, except that in the solar still, it will usually
even more pure, because during boiling the breaking bubbles
contaminate the product water with tiny droplets of liquid
swept along with the vapor.
THE SOLAR DISTILLATION PROCESS
The solar distillation process is shown in Figure 1.
energy passing through a glass cover heats up the brine or
water in a pan; this causes the water to vaporize.
then rises and condenses on the underside of the cover and
down into distillate troughs.
Fresh Water from the Sun, by Daniel C.
Dunham, (Washington, D.C., August 1978),
A more technical description follows.:
The sun's energy in the form of short
through a clear glazing surface such as
Upon striking a darkened surface, this light
wavelength, becoming long waves of heat which
is added to the water in a shallow basin
glazing. As the water heats up,
it begins to evaporate.
The warmed vapor rises to a cooler
area. Almost all
are left behind in the basin.
The vapor condenses onto the underside of
accumulates into water droplets or sheets
The combination of gravity and the tilted
allows the water to run down the cover and into
a collection trough, where it is channeled
In most units, less than half the calories of radiant energy
falling on the still are used for the heat of vaporization
to produce the distilled water.
A commercial stills are
sold to date have had an efficiency range of 30 to 45
(The maximum efficiency is just over 60 percent.) Efficiency
calculated in the following manner:
Energy required for the vaporization
of the distillate that is recovered
= Energy in the sun's radiation
that falls on the still.
Providing the costs don't rise significantly, an efficiency
increase of a few percent is worth working for.
principally to be sought in materials and methods of
III. SOLAR STILL
Although there are many designs for solar stills, and four
general categories, (concentrating collector stills;
tray tilted stills; tilted wick solar stills; and basin
95 percent of all functioning stills are of the basin type.
CONCENTRATING COLLECTOR STILL
A concentrating collector still, as shown in Figure 2, uses
parabolic mirrors to focus sunlight onto an enclosed
concentrated sunlight provides extremely high
temperatures which are used to evaporate the contaminated
The vapor is transported to a separate chamber where it
condenses, and is transported to storage. This type of still
capable of producing from .5 to .6 gallons per day per
foot of reflector area.
This type of output far surpasses other
types of stills on a per square foot basis. Despite this
outstanding performance, it has many drawbacks; including
high cost of building and maintaining it, the need for
direct sunlight, and its fragile nature.
MULTIPLE TRAY TILTED STILL
A multiple tray tilted still (Figure 3), consists of a
shallow horizontal black trays enclosed in an insulated
with a transparent top glazing cover.
The vapor condenses onto
the cover and flows down to the collection channel for
This still can be used in higher latitudes because the whole
can be tilted to allow the sun's rays to strike perpendicular
the glazing surface.
The tilt feature, however, is less important
at and near the equator where there is less change in the
position over the still.
Even though efficiencies of up to 50
percent have been measured, the practicality of this design
remains doubtful due to:
the complicated nature of construction
increased cost for multiple trays and
TILTED WICK SOLAR STILL
A tilted wick solar still draws upon the capillary action of
fibers to distribute feed water over the entire surface of
wick in a thin layer.
The water is then exposed to sunlight.
(See Figure 4.)
A tilted wick solar still allows a higher temperature to
this thin layer than can be expected from a larger body of
This system is as efficient as the tilted tray design, but
use in the field remains questionable because of:
increased costs due to mounting requirements
the need to frequently clean the cloth wick
highlighting the need for an operable
the need to replace the black wick material
due to sun bleaching and physical
by ultra-violet radiation;
uneven wetting of the wick which will result
leading to reduced efficiency; and
the unnecessary aspect of the tilt feature
required higher latitudes.
A basin still (See Figure 5), is the most common type in
although not in current production.
While the basic design can take on many variations, the
shape and concept have not changed substantially from the
the Las Salinas, Chile stills built in 1872.
changes have involved the use of new building materials,
may have the potential to lower overall costs while
acceptably long useful life and better performance.
All basin stills have four major components:
a support structure;
a transparent glazing cover; and
a distillate trough (water channel).
In addition to these, ancillary components may include:
insulation (usually under the basin);
piping and valves;
facilities for storage;
an external cover to protect the other
and to make the still esthetically
a reflector to concentrate sunlight.
Physical Dimensions of the Basin Still
The actual dimensions of basin stills vary greatly,
the availability of materials, water requirements, ownership
patterns, and land location and availability.
If the only glazing available is one meter at its greatest
dimension, the still's maximum inner width will be just
meter. And the
length of the still will be set according to what
is needed to provide the amount of square meters to produce
required amount of water.
Likewise, if an entire village were to
own and use the still, the total installation would have to
It is generally best to design an installation with many
modular units to supply the water.
units to be added;
manageable components to be handled by
without expensive mechanical equipment;
maintenance can be carried out on some units
continue to operate.
Most community size stills 1/2 to 21/2 meters wide, with
ranging up to around 100 meters.
Their lengths usually run along
an eastwest axis to maximize the transmission of sunlight
the equatorialfacing sloped glass.
Residential, appliance type
units generally use glass about 0.65 to 0.9 meter wide with
lengths ranging from two to three meters.
A water depth of 1.5
to 2.5 cm is most common.
The usual argument for greater depths is that the stored
can be used at night to enhance production when the air
Unfortunately, no deep basin has ever attained
the 43 percent efficiency typical of a still of minimum
depth. The results
to date are clear: the shallower the
the better. Of
course, if the basin is too shallow, it will dry
out and salts will be deposited, which is not good.
solar heat can evaporate about 0.5 cm of water on a clear
summer. By setting
the initial charge at about 1.5 cm depth,
virtually all of the salts remain in the solution, and can
flushed out by the refilling operation.
MATERIAL REQUIREMENTS OF BASIN STILLS
The materials used for this type of still should have the
should have a long life under exposed
be inexpensive enough to be replaced upon
o They should be
sturdy enough to resist wind damage and
o They should be
nontoxic and not emit vapors or instill
taste to the water under elevated temperatures.
o They should be
able to resist corrosion from saline
o They should be
of a size and weight that can be
packaged, and carried by local
o They should be
easy to handle in the field.
materials should be used whenever possible to
lower initial costs
and to facilitate any necessary repairs, keep
in mind that solar
stills made with cheap, unsturdy materials
will not last as
long as those built with more costly, high-quality
With this in mind, you must decide whether
want to build an
inexpensive and thus short-lived still that
needs to be replaced
or repaired every few years, or build
durable and lasting in the hope that the distilled
water it produces
will be cheaper in the long run. Of the
stills that have
been built around the world, many have been
Building a more durable still that will last
or more seems to be
worth the additional investment.
Choosing materials for the components in contact with the
represents a serious problem.
Many plastics will give off a
substance which can be tasted or smelled in the product
for periods of anywhere from hours to years.
As a general guide,
if you are contemplating using any material other than glass
metal in contact with water, you may perform a useful
test by boiling a sample of the material in a cup of good
for half an hour, then let the water cool, and smell and
it. This is a
considerably accelerated test of what happens in
the still. If you
can tell any difference between the test water
and that you started with, the material is probably safe to
To get some experience, try this on polyethylene tubing, PVC
and fiberglass resin panel.
A basin still consists of the following basic
components: (1) a
basin, (2) support structures, (3) glazing, (4) a distillate
trough, and (5) insulation.
The section that follows describes
these components, the range of materials available for their
construction, and the advantages and disadvantages of some
The Basin. The basin
contains the saline (or brackish) water that
will undergo distillation.
As such, it must be waterproof and
dark (preferably black) so that it will better absorb the
sunlight and convert it to heat.
It should also have a
relatively smooth surface to make it easier to clean any
There are two general types of basins.
The first is made of a
material that maintains its own shape and provides the
containment by itself or with the aid of a surface material
applied directly to it.
The second type uses one set of
materials (such as wood or brick) to define the basin's
Into this is placed a second material that easily conforms
shape of the structural materials and serves as a waterproof
liner. No one
construction material is appropriate for all
circumstances or locations.
Table 1 lists the various materials
and rates them according to properties desirable for this
A Comparison of
Various Materials Used
in Solar Basin Construction
Type of Dura-
[a] = Unknown or depends upon local conditions.
Selecting a suitable material for basin construction is the
biggest problem in the solar still industry.
conditions at the water line can be so severe that basins
metal--even those coated with anti-corrosive materials--tend
corrode. Basins made
of copper, for example, are likely to be
eaten out in a few years.
Galvanized steel and anodized uncoated
aluminum are likely to corrode in a few months.
This is also
true of aluminum alloys used to make boats.
There are many
chemical reactions that double in rate with each 10
increase in temperature.
Whereas an aluminum boat might last 20
years in sea water at 25 [degrees] C if you increase that
50 [degrees], the durability of that aluminum may well be
only one or two
Porcelain-coated steel lasts only a few years before it is
out by corrosion.
The special glass used for porcelain is
slightly soluble in water, and inside a still it will
away. The typical
life of stills equipped with porcelain basins
is about five years, although several have been kept
much longer than that by repairing leaks with silicone
People have also tried to use concrete because it's
and simple to work with, but the failure rate has been high
because it often develops cracks if not during the first
then later on.
Concrete and abestoscement also absorb water.
water may not run right on through, but it does soak it up.
Everybody knows that satisfactory cisterns and reservoirs
built of concrete, but in a solar still the rules
part of it that is exposed to outside air will permit
it is salt water that is being evaporated,
salt crystals will form in the concrete near the surface and
break it up, turning it to powder.
What about plastic?
Every few years, someone decides that if we
could just mold the whole still--except for the glass and
seal--out of some plastic such as styrofoam, it would be so
and inexpensive. But
styrene foam melts at about 70 [degrees] Centigrade.
Urethane foam is a little more promising, but it tends to be
dimensionally unstable, and, if a still is constructed in
inclined-tray configuration, the efficiency suffers, because
non-wetted portions do not conduct heat to the wetted
What about fiberglass?
People have spent a lot of time trying to
build stills from fibreglass resin formulations.
Thus far, they
have found the material to be unusable for any part of the
(e.g., the basin or distillate trough) that comes in contact
water, either in liquid or vapor form.
Epoxy and polyester
resins can impart taste and odor to the distilled water, not
for weeks, but for years.
Researchers have found that this
problem cannot be eliminated by covering these materials
coat of acrylic br anything else.
The odors migrate right
through the coating and make the distilled water unsalable,
Moreover, using fiberglass resin is not a
particularly low-cost approach.
Finally, a fiberglass basin or
trough that is subjected to hot water for many years
researchers find a way to solve these problems,
fiberglass remains an unsuitable material.
One alternative is ordinary aluminum coated with silicone
The durability of basins made with this material increased
the 10- to 15-year range.
For the hundreds of stills one company
sold using this material, the coating was all done by
production roll coating equipment, the basin's durability
probably be increased even more.
Although stainless steel has been used, success has been
Support structures form the sides of the
still as well as the basin, and support the glazing
noted earlier, some materials used in forming the basin also
the still support structure while other still configurations
demand separate structures, especially to hold the glazing
The primary choices for support structures are wood, metal,
concrete, or plastics.
In most cases the choice of material is
based upon local availability.
Ideally, the frame for the
glazing cover should be built of small-sized members so they
not shade the basin excessively.
Wooden support structures are subject to warping, cracking,
and termite attack.
Choosing a high-quality wood, such as
Cypress, and letting it age may help to alleviate these
but, if high heat and high humidity prevail inside and
the still, the still will require frequent repair or
The main advantage of wood is that it can be easily worked
basic hand tools.
Metal may be used for the supports but is subject to
Since metals are not subject to warping, they can aid in
the integrity of the seals, although the expansion rate
of a metal must be taken into account to ensure its
with the glazing material and any sealants used.
Use of metal
for frame members is practically limited to aluminum and
steel. Both will
last almost indefinitely, if protected
Silicone rubber will not adhere well to galvanized steel,
does so very well to aluminum.
Concrete and other masonry materials may form the sides and
glazing support of a still as well as the membrane.
This is more
readily possible in a single-slope still (Figure 6) rather
in a double-slope still (Figure 7).
Glazing Cover. After
the pan, the glazing cover is the most
critical component of any solar still.
It is mounted above the
basin and must be able to transmit a lot of light in the
spectrum yet keep the heat generated by that light from
the basin. Exposure
to ultraviolet radiation requires a material
that can withstand the degradation effects or that is
enough to be replaced periodically.
Since it may encounter
temperatures approaching 95 [degrees] celsius (200 [degrees]
F), it must also be
able to support its weight at those temperatures and not
excessive expansion, which could destroy the airtight
film type cover, which must be supported by tension or air
pressure, seems like a very poor choice.
Ideally, the glazing material should also be strong enough
resist high winds, rain, hail, and small earth movements,
prevent the intrusion of insects and animals.
Moreover, it must
Wettability allows the condensing vapor to form
as sheets of water on the underside of a glazing cover
than as water droplets.
If the water does form as droplets, it
will reduce the performance of the still for the following
Water droplets restrict the amount of light
because they act as small mirrors and reflect
it back out.
A percentage of the distilled water that
the underside will fall back into the basin
flow down the glazing cover into the
trough. Except for temporary conditions
such a loss of water should not be tolerated.
Other factors determining the suitability of glazing
include the cost of the material, its weight, life
local availability, maximum temperature tolerance, and
resistance, as well as its ability to transmit solar energy
Table 2 compares various glazing materials based
on these factors.
Of the glazing materials listed in Table 2, tempered glass
best choice in terms of wettablity and its capability to
withstand high temperatures.
It is also three to five times
stronger than ordinary window glass and much safer to work
One disadvantage of tempered glass is its high cost.
tempered low-iron glass, in one series of tests, gave 6
additional production, it also added about 15 percent to the
of the still.
Moreover, glass cannot be cut after it has been
Nevertheless, it is a valid choice, certainly for a
top-quality, appliance type product.
A Comparison of Various Glazing Materials
Used in Building Solar Stills
Solar Infrared Light
400 [degrees]-600 [degrees] F
204 [degrees]-316 [degrees] C
91 less than 2
400 [degrees] F
225 [degrees] F
200 [degrees] F
260 [degrees] F
200 [degrees] F
(a) Costs are in
U.S. dollars, and were developed based on data published between 1981
Ordinary window glass is the next best choice, except that
an oily film when it comes from the factory, and must be
carefully with detergent and/or ammonia.
If you choose glass as
a glazing material, double-strength thickness (i.e.,
of an inch, or 32 millimeters) is satisfactory.
While some plastics are cheaper than either window glass or
tempered glass, they deteriorate under high temperatures and
Moreover, under temperature conditions typical
of solar stills, the chemicals in plastics are likely to
with the distilled water, possibly posing a health hazard.
What about the size of the glass?
Using a low slope of glass,
the goal is to make it as wide from north to south as
It doesn't take any more labor to install a 90 centimeter
of glass than it does to install one of 60 centimeters and
get more absorber area.
Also, loss of heat through the walls
will be the same whether the still is large or small.
pieces of glass wider than 90 centimeters (3 ft.) introduces
problems: (1) the
price per unit area of the glass goes up; and
(2) the labor costs and the danger of handling it
the basis of experience, one optimal size is about 86
(34"), a size that is commonly stocked and widely
especially in the solar collector industry.
The distillate trough is located at the base
of the tilted glazing.
It serves to collect the condensed water
and carry it to storage.
It should be as small as possible to
avoid shading the basin.
The materials used for the trough must satisfy the general
material requirements outlined previously.
Those most commonly
used include metal, formed materials used in basin
(with or without plastic liners), or treated materials.
Stainless steel is the material of choice, although it is
Common varieties, such as 316, are acceptable.
metals require protective coatings to prevent
is not supposed to corrode in distilled water, but it seems
preferable to rub a coating of silicone rubber over it
Galvanized iron probably will not last more than a few years
most, and copper and brass should not be used because they
create a health hazard.
Also, steel coated with porcelain is a
poor choice because the glass will dissolve slowly and allow
steel to rust.
Basins lined with butyl rubber or EPDM can have their liners
extend beyond the basin to form the trough.
This method is
inexpensive to implement and provides a corrosion-free
No version of polyethylene is acceptable because it breaks
emits an unpleasant odor and taste.
Some people have used
polyvinyl chloride (PVC) pipe, slit lengthwise.
However, it is
subject to significant distortion inside the still, can give
an undesirable gas, and is subject to becoming brittle when
exposed to sunlight and heat.
Butyl rubber should be okay, but
because it is black, the distillate trough becomes an
re-evaporates some of the distilled water (a minor problem).
Ancillary components include insulation, sealants, piping,
valves, fixtures, pumps, and water storage facilities.
general, it is best to use locally available materials,
Insulation, used to retard the flow of heat from a
solar still, increases the still's performance.
In most cases,
insulation is placed under the still basin since this is a
area susceptible to heat loss.
In stills where the depth of water in the basin is two
less, performance has been increased by as much as 14
but this gain decreases as the depth of the water in the
in performance resulting from the
installation of insulation materials are also less in those
locations where greater amounts of solar energy are
The least expensive insulation option is to build a solar
on land that has dry soil and good drainage.
The use of sand
helps to minimize solar heat losses, and may also serve as a
sink, which will return heat to the basin after the sun sets
prolong distillation process.
Insulation, which adds approximately 16 percent to
costs, may be extruded styrofoam or polyurethane (Note:
in contact with soil will absorb moisture and lose much of
its insulation value.)
the sealant is not a major component of a
solar still, it is important for efficient operation.
It is used
to secure the cover to the frame (support structure), take
difference in expansion and contraction between dissimilar
and keep the whole structure airtight.
Ideally, a good
sealant will meet all of the general material requirements
earlier in this paper.
Realistically, however, it might be
necessary to use a sealant that is of lesser quality and has
shorter lifespan but that may be locally available at prices
affordable to people in developing countries.
One major drawback
of applying low-cost sealants to stills is the frequent
input the stills require to keep them in serviceable
Sealing a solar still is more difficult than sealing a solar
water-heating panel on two counts:
(1) an imperfect seal could
cause a drop of rain water carrying micro-organisms to enter
still, which would contaminate the water; and (2) applying a
sealant that imparts a bad taste or odor to the distilled
will make it unpalatable.
Traditional sealants that are locally available include:
window putty (caulk and linseed oil);
asphalt caulking compound;
A wide variety of other caulks sealants is also
include latex, acrylic latex, butyl rubber and synthetic
polyethylene, polyurethane, silicone, and urethane
foam. Most of
these will be more costly than traditional varieties, but
may wear longer.
Of this group of sealants, molded silicone or EPDM, clamped
place, seems to be the most promising.
Silicone rubber sealant,
applied from a tube, is certainly a superior choice,
people have reported a few instances of degradation and seal
failure after 5 to 15 years when the seal was exposed to
Covering the sealant with a metal strip should extend its
Researchers are experimenting with an extruded
silicone seal, secured by compression.
One final note:
Remember a sealant that works well for windows
in a building does not assure that it will work in a solar
due to higher temperatures, presence of moisture, and the
that the water must be palatable and unpolluted.
Piping. Piping is
required to feed water into the still from the
supply source and from the still to the storage reservoir.
general material requirements cited earlier hold true for
While stainless steel is preferred, polybutylene is a
satisfactory pipe material.
Black polyethylene has held up well
for at least 15 years as drain tubing.
Nylon tubing breaks up if
exposed to sunlight for 5 to 10 years.
PVC (polyvinyl chloride)
pipe is tolerable, although during the first few weeks of
operation it usually emits a gas, making the distilled water
taste bad. Ordinary
clear vinyl tubing is unacceptable.
is a "food grade" clear vinyl tubing that is
supposed to be
satisfactory for drinking water, but the sun's rays are
degrade it if it's used in a solar still.
drinking water and milk in high-density polyethylene
have had satisfactory results.
But put the same plastic bottle
filled with water in the sun, and the plastic will degrade,
imparting a bad taste to the water.
Few plastics can withstand
heat and sunlight.
Brass, galvanized steel, or copper may be
used in the feed system, but not in the product system.
One final note:
Although a solar still repeatedly subjected to
freezing will remain unharmed, drain tubes so exposed may
shut unless you make them extra large.
Feed tubes can easily be
arranged with drain-back provision to prevent bursting.
are connection devices that hold pipe
If you put a solar still on the market with
instructions to consumers that connections be made
only", people could put a wrench on a connection,
loosen it, and
be faced with an expensive repair problem.
So, the options
include having tight control of installation personnel, or
a thorough training job, or making the equipment rugged
withstand ordinary plumbing practice.
A solar still is fed on a batch basis for an hour or two
day. It is necessary
to admit some extra water each day, to
flush out the brine.
There is very little pressure available to
get the water to drain, so drainage cannot proceed
prevent flooding, it's good practice to insure that the feed
does not exceed this maximum drainage rate.
If one uses needle
valves thus to restrict the flow, such valves have been
be unstable over the years, generally tending to plug up and
the flow. It has
proven to be a satisfactory solution to this
problem--when feeding from city water pressure of typically
p.s.i.--to use a length of small diameter copper tubing,
25 feet or more of 1/8 inch outside diameter, or 50 feet of
inch outside diameter tubing, to serve as a flow
needs to have a screen ahead of it, such as an ordinary hose
filter washer, with 50 mesh or finer stainless steel screen,
prevent the inlet end from plugging.
In selecting materials for the storage reservoir,
two precautions should be noted.
1) Distilled water
is chemically aggressive, wanting to dissolve
a little of
practically anything, until it gets "satisfied,"
and then the
rate of chemical attack is greatly slowed.
What this number
is, in terms of parts per million of
substances, is not well documented, but the
are that some things, such as steel,
steel, copper, brass, solder, and mortar, which
resulting in damage or destruction of the
and quite possibly in contamination of the
Stainless steel type 316) is a good
are okay but must not be exposed
sunlight. Butyl rubber lining of some
be okay. Galvanized steel would last
only a few
years, adding some zinc and iron to the water.
serve, again with the expectation that the
slowly crumble over many years' time.
tiny amount of
calcium carbonate that is leached out can be
used by the body
in the diet. In fact, one way to
attack is to introduce some limestone or
into the distilled water stream, or in the
itself, to pick up some calcium carbonate on
greatly slowing the attack on the tank itself.
precautions need to be taken to prevent entry of
airborne bacteria. Air must leave the
every time water
enters it, and must re-enter every time
water is drawn
off. Use a fine mesh--50 x 50 wires to
screen covering the vent, and turn the opening
vent/screen assembly downward, to prevent entry
water. If this is ignored for even one
insect can get
in, and you have germ soup from then on.
Storage capacity should be adequate to contain four to five
the average daily output of the still.
Factors to Consider in Selecting Materials for Basin Still
Let us review the functions of the basin:
It must contain water without leaking.
It must absorb solar energy.
It must be structurally supported to hold
It must be insulated against heat loss
from the bottom
An infinite number of combinations of materials will serve
membrane that holds water, for example, may be
stiff enough to support the water, but it doesn't have to
The basin may be rigid enough to support the glass, but it
doesn't have to be.
In short, a component need not satisfy two
functions at the same time.
Indeed, it is usually better to
select local material that will best do each job separately,
then put them together.
But if you can find a material that does
a couple of jobs well, so much the better.
In selecting materials for a solar still, there are almost
tradeoffs. You can
save money on materials, but you may lose so
much in productivity or durability that the
"saving" is poor
Summary of Materials Recommended for Basin Still
Where the objective is the lowest cost of water on a 20-year
cycle cost basis, the best materials for building a basin
silicone compound coating to blacken the
bottom of the
metal ribs spaced 40 centimeters (16
inches) apart to
underside of the basin;
about 25 to 38 millimeters of insulation
may be high-temperature urethane foam, or
a bottom covering of lightweight galvanized
sheet (note: if you plan to put the
and use an insulation that is impervious to
bottom sheet is needed);
metal siding, such as extruded aluminum, to
(note: extruded aluminum can be
it is expensive; thus, you may prefer a
material such as painted steel or aluminum;
a stainless steel trough;
tempered low-iron glass, or regular
glass. (If using patterned glass, put
extruded gaskets, compressed into final
type 316 stainless steel fittings
(note: brass is not
PVC is acceptable, but poor in very hot
a mirror behind the still for higher
Although these materials are representative of a high-cost
design, they are probably a good investment since none of
inexpensive designs has stayed on the market.
However, we must
also ask the question, "Expensive compared to
what?" Compared to
hauling purified water in bottles or tanks, solar distilled
would almost always be much less expensive.
Compared to hauling
vegetables by airplane to hot desert places, using a solar
to raise vegetables in a greenhouse should be less
Compared to the cost of boiling water to sterilize it, the
still should be competitive in many situations.
And although the
materials used in building a high-cost still will probably
be expensive, mass production could ultimately drive down
unit cost per still.
IV. OPERATION AND
MAINTENANCE OF SOLAR STILLS
OPERATING REQUIREMENTS OF BASIC STILLS
Protecting Distilled Water from Contamination
Protecting a solar still against the entry of insects and
rainwater is important.
After your still is installed, you
disinfect the interior of the still and
compounds (adding a few spoonfuls of laundry
bleach to a
few liters of water does the job nicely);
provide a vent(*) in the feed tube at the
stainless steel screen filter washer in a
fitting, turned downward to prevent entry of
rainwater. If these precautions are not
flying insects, attracted by the moisture, might
way in and die in the distillate trough.
Preventing contamination in a storage reservoir is a little
difficult, as the daily high temperature are not available
pasteurize the water.
Nevertheless, with diligent attention to
detail, the system can be used for decades without
Filling and Cleaning a Basin Still
Filling a basin still is a batch process (*), done once a
night or in the morning.
With a still of this design, about 5 to
7 percent of the day's total distilled water is produced
sundown, so it is important to wait until the still is cold.
Refilling it between three hours or more after sundown and
one or two hours after sunrise will cause little, if any,
(*) A vent allows air to enter and exit the still daily
operation and refilling.
It is not necessary to drain the still completely.
with at least twice as much as it produces will normally
and flush it adequately.
Three times as much would keep it a
little cleaner, and could be worth doing, provided the cost
feed water is nominal.
A rapid mechanical flushing is not
required; a gentle trickle does the job.
Feeding Hot Water to a Basin Still
If a basin still is fed water that is hotter than the ambient
air, the unit becomes a conventional distiller, except that
uses glass instead of copper as the condenser.
If the hot water
is virtually cost-free, as is geothermal or waste water, it
be well worth doing.
If the feed water is heated by fossil fuels
or by separate solar panels, the economics look doubtful,
feed line tends to plug up with scale.
FACTORS INFLUENCING SOLAR STILL OPERATING PERFORMANCE
In this section, we discuss some important factors that
the rate of production of distilled water.
climatic factors, thermal loss factors, and solar still
Effect on Efficiency. The amount of
a solar still receives is the single most important factor
affecting its performance.
The greater the amount of energy
received, the greater will be the quantity of water
Figure 8 shows the rate of production of a basin still on
basis of specific solar inputs.
Solar stills produce less distilled water in winter than in
summer, which is a problem.
To some extent, the demand for
drinking water also varies with the seasons, by as much as
2 to 1, summer over winter.
But the annual sunlight
variation affecting a still's solar distillation rate is
than that, at least in regions well outside the
tropics. In the
tropics, at latitudes of less than 20 [degrees], the annual
variation is probably well under 2 to 1, so it may not be a
problem there. The
farther away from the equator, the greater
the annual sunlight variation, to perhaps 7 to 1 at 40
latitudes. This is
unacceptable, making use of a solar still
difficult in winter at high latitudes.
(*) Note that there are other methods available for large
However, because they fall outside the
scope of this paper, they are not discussed here.
Many approaches have been tried to solve this problem.
the whole still up to more or less an equatorial mount
ratio down very nicely.
This is called the "inclined-tray"
still, and is accomplished by using many small pans in a
this arrangement, total sunlight
striking the aperture of the glass remains more constant,
light which glances off the water of one small tray warms
bottom of the one above it, improving performance.
While this is
a substantial advantage, it is the only advantage of this
and it must be weighed against the disadvantages of higher
associated with putting many small pans vs. only one in the
enclosure, and, most probably, higher installation costs due
holding the end of the pan higher off the supporting
protecting it against wind loads.
In latitudes perhaps 20 [degrees] on
up, it seems possible that the inclined-tray will find a
Using an inclined-tray still is only one solution to the
of annual variation in higher latitudes.
Some other steps that
can be taken include:
buying an extra large still that produces enough
water in winter, resulting in a likelihood
will have more water than you need in summer;
using less water in winter and/or using
some tap water;
buying supplemental water in winter; or
saving some of the excess distilled water
fall for use in winter;
installing a mirror behind the basin to
sunlight back into the still in winter.
back as much light as possible, use a
surface of about one-third to one-half of
aperture of the glass cover, tilted forward 10 [degrees]
vertical, mounted at the rear edge of the
In latitudes between 30 [degrees] and 40
[degrees], this gives
from 75 to
100 percent more yield in mid-winter.
Much work has been done to try
to obtain lower condensing temperatures, thereby increasing
temperature difference between the heated feed water and the
This approach undoubtedly derives from 100
years of steam power engineering, in which it is most
to get the steam temperature high and the condensing
low to gain efficiency.
But this principle does not hold true
for a solar still.
Steam for power is pure steam, whereas the
contents of a solar still are both air and water vapor.
been demonstrated repeatedly that the higher the operating
temperature of the still--insolation being equal--the higher
efficiency. For each
6 [degrees] celsius (10 [degrees] F) increase in ambient
temperature, the production of a still increases by 7 to 8
The practical effect of this is that a still operating in
a hot desert climate will produce typically as much as
more water than the same unit in a cooler climate.
(By the same token, cooling the glazing cover of a solar
spraying water on it or blowing air over it does not help
still produce more distillate.
In an experiment at the
University of California in the United States, two identical
stills were built.
The glazing cover of the first still was fan-cooled;
the cover of the second still was not.
Of the two
stills, the cooled unit produced significantly less
Consequently, it's better to put the still in a protected
rather than a windy area.)
Thermal Loss Factors
Production is also associated with the thermal efficiency of
still itself. This
efficiency may range from 30 to 60 percent,
depending on still construction, ambient temperatures, wind
velocity, and solar energy availability.
Thermal losses for a
typical still vary by season, as shown in Table 5.
Distribution of Incoming Solar Radiation
in the Distillation Process
Thermal Loss Factors
Reflection by Glass
Absorption by Glass
Radiative Loss from
Internal Air Circulation
Ground and Edge Loss
Re-Evaporation and Shading
[Remainder of Energy Used to Distill Water]
Direct Use of the Sun's Energy, Daniels, Farrington, 1964,
Ballantine Books, page 124.
Solar Still Design Factors
Slope of the Transparent Cover.
The angle at which the transparent
cover is set influences the amount of solar radiation
entering a solar still.
When sunlight strikes glass straight on,
at 90 [degrees] to the surface, about 90 percent of the
through. Tip the
glass a little, so that it strikes at a "grazing
angle" of 80 [degrees], and only a few percent is
lost. But tilt it
a few more tens of [degrees], and the curve goes over the
off to practically zero at 20 [degrees] grazing angle, where
direct light gets through.
In a greenhouse-type still, for a
large part of the year the half of the glass that is facing
from the equator is receiving sunlight at very low grazing
It is actually shadowing the back one-third of the still.
It is more efficient to make that half of the glass facing
equator as long as possible, and put a more or less
back wall to the rear.
This was one of the significant steps
that has increased the efficiency of basin stills from 31 to
about 43 percent, using a single slope of glass.
And it costs
less to build.
The slope of the glass cover does not affect the rate at
the distillate runs down its inner surface to the collection
trough. A common
misconception was that the glass cover must be
tilted to get the water to run off.
This may have arisen from
the fact that ordinary window glass, as it comes from the
factory, has a minute oily film on it.
But if the glass is
clean, the water itself will form filmwise condensation on
and will be able to run off at a slope as little as 1
There are three reasons why it is best to use as low a slope
possible: (1) the
higher the slope, the more glass and supporting
materials are needed to cover a given area of the basin; (2)
the higher slope increases the volume and weight [of the
and therefore shipping costs; and (3) setting the glass at a
slope increases the volume of air inside the still, which
the efficiency of the system.
A glass cover that is no more
than 5 to 7 centimeters from the water surface will allow
still to operate efficiently.
Conversely, as glass-to-water
distance increases, heat loss due to convection becomes
causing the still's efficiency to drop.
Some important stills have been built following the
design concept for the glass cover, yet using a short,
sloping piece of glass at the rear.
This requires either providing
an extra collection trough at the rear, or else making the
successive troughs touching heel and toe, so that it is
exceedingly difficult to get out in the middle of the array
service anything. It
also increases the condensing surface relative
to the absorber, which reduces operating temperatures in the
still, and is clearly disadvantageous.
A reflective and
insulated back may be preferable to glass.
Some years ago at the University of California, researchers
an experimental multiple tray tilted still with an average
distance of about 30 millimeters, showing an efficiency
of 62 percent, one of the highest ever recorded.
The loss of
efficiency is greatest the first centimeter, rather less the
second cm, and so on, tailing off to smaller rates of loss
distance as far as the test was carried.
This is one of the
principle reasons a high slope of glass is to be avoided.
In sum, it is clear that a solar still should be built in a
that will get the water as hot as possible, and keep it as
to the glass as possible.
This is achieved by keeping the glass
cover at a minimum distance from the water surface, which in
practical terms, falls between 5 and 7 cm., and by
depth of water in the pan, to about 1.5 cm.
Wicks and Related Techniques
Researchers have tried to improve the efficiency of a solar
by enhancing its surface evaporation area using wicks.
side-by-side test of two identical stills at the University
California, using a floating black synthetic fabric in one
and nothing in the other, the difference in production
the stills was indistinguishable, though similar tests have
reported some improvement.
It seems exceedingly difficult to
find a wick material that will last for 20 years in hot
water, and that will not get crusted up with salts over a
of time. As for
putting dye in the water, studies suggest that
the slight improvement in performance does not justify the
increased cost and maintenance and operating problems
with this technique.
Putting dark-colored rocks in the feedwater to store heat
after nightfall has
been reported by Zaki and his associates to
improve performance by 40 percent, but he does not give the
reference point from which this is measured.
If he was comparing
one still containing 4 cm. of water with another same water
but containing black stones, the productivity would increase
somewhat due to the decrease in thermal mass and resulting
in operating temperature.
Reducing the initial water
depth might have accomplished the same result.
For this reason,
placing dark-colored rocks in the feedwater does not appear
a promising technique for improvements in solar still
MAINTENANCE REQUIREMENTS OF BASIN STILLS
Ways of Handling the Buildup of Mineral Deposits
It is inevitable that some minerals are deposited on the
of the basin. In
most situations, including sea water and city
tap water, the amount deposited is so small that it creates
problem for decades.
One still in particular has been operated
for 20 years without ever having been opened or
cleaned. As long
as there is not an excessive buildup of deposits, indicated
formation of a dried-out island in the afternoon, they
mineral deposits become the normal absorber.
accumulation of these deposits changes the interior surface
basin from its original black color to a dark earth brown,
reflecting some sunlight, causing a 10 percent drop in still
offset this reduction, simply make the still 10
percent larger than it would need to be if it were cleaned
Some desert waters high in alkalis will deposit a whitish
scale on the bottom and sides of a basin.
In fact, almost any
feed water will do so, especially if the basin is allowed to
out. In some cases,
the alkaline water may form a crust of scale
which is held on the water's surface by air bubbles that are
discharged when the feed water is heated.
such as these may reduce production of the still by 50
more. Those that
settle to the bottom of the basin can be easily
coated black by mixing one tablespoon of black iron oxide
concrete coloring powder with about 10 or 15 liters of water
adding the solution to the still by means of a funnel
to the feed water pipe.
This blackening agent is inert, and
imparts no bad taste or odor to the distilled water.
solution reaches the basin through the feed water pipe, it
settles on the bottom of the basin and restores it to its
black color. Some
owners do this each fall, when production
begins to drop. Cost
is only pennies per application.
Deposits that float on the surface of the water in a basin
tougher problem and one that requires more research.
Australian solar still expert suggests agitating the
the still by recirculating, or stirring, the water in the
one hour each night, to minimize the buildup of floating
deposits. Adding a
pint or two of hydrochloric (swimming pool)
acid to the still whenever the bottom becomes
year or two, maybe oftener in some cases--is a satisfactory
way of removing practically all of the scale.
Accumulation of Dust on the Glazing Cover:
What to Do
In the vast majority of stills, dust accumulates on the
cover. But it does
not keep building up; it's held more or less
constant by the action of rain and dew.
accumulation causes production to drop from 5 to 15
offset this, simply make your still 10 percent larger than
would need to be if kept clean.
However, if the still is in an
unusually dusty area, or if it is large enough that a
is available at modest cost, cleaning the glazing cover is
percent of 10,000 liters per day may be enough
to justify cleaning the cover once a month in the dry
Repair and Replacement of Basin Still Components
As with all devices, the components of a basin still may
be repaired or replaced from time to time.
The frequency depends
on the type of material used to construct the still.
with premium materials will require almost no maintenance,
will entail a higher capital cost because many of the
must be imported materials.
Use of cheaper materials subject to
degradation will almost certainly lower the initial cost,
will increase the amount of maintenance.
Even so, if the long-term
cost of maintenance and the lower initial cost are less than
the higher initial cost for premium materials, this may
better option, especially if cost of capital is high.
called "life cycle cost analysis," and it is
SKILLS REQUIRED TO BUILD, OPERATE, AND MAINTAIN A BASIC
Craftmanship and attention to detail in construction are
important for an efficient, cost-effective still.
In addition, supervisory personnel must be on hand who know
to size stills to meet a community's water supply needs; who
how to orient stills; who are familiar with required
techniques; and who have the ability to train others in the
construction, operation and maintenance of stills.
Finally, it is important to ask local workers to participate
the planning and construction phases of a solar still
get the indigenous population to accept the technology.
of pride in the building of the project may well mean the
between long-term success or failure of the project.
The cost and economics of solar stills depend on many
cost of water produced or obtained by
availability of sunlight;
cost of locally-available materials;
cost of local labor;
cost of imported materials; and
loan availability and interest rates.
Table 6 shows the variation in costs for stills built in the
1970s in the Philippines, India, Pakistan, and Niger.
stills built in Niger in 1977 cost twice as much as those
in the Philippines in the same year, reflecting the wide
variation in local cost.
Variation in Costs for Stills Built in the
(Costs today are undoubtedly higher.)
WHY BUY A STILL?--It saves money.
A solar still must operate with extremely low costs for
maintenance arid operation.
Over a long period according to a
study by George Lof, it is valid to assume that 85 percent
cost of water from the still will be chargeable to the costs
buying it; the remainder to operation and maintenance.
It is easy to calculate the return on investment in a solar
still. Say you have
one that produces a daily amount of water
that would cost you $1 to buy in bottles:
then that still
returns you $365 per year.
If the still had cost you $365, then
it paid for itself in one year; if five times that much,
five years, etc.--not counting interest.
Cost of feeding water
into it is pretty small, but will increase the payout period
little also. In the
United States, the payout period tends to
run between two and five years, depending on the still's
SPECIAL DESIGN VARIATIONS
The majority of information presented thus far has centered
the basin-type solar still because it is the easiest to
and may use a wide range of materials, making it adaptable
But variations of the basin still are
possible, such as the double-slope and single-slope stills
depicted earlier in this paper.
In addition to these options,
there are other ways to design the still to increase its
efficiency or potential to produce potable water.
Some of these
are discussed below.
Basin Stills Equipped with Reflectors
Some stills have been equipped with reflective materials
have the potential to increase the amount of sunlight
the still without having to increase the area of the
latitudes in the thirties, performance increases in winter
100% have been achieved with a mirror of less than 1/2 the
of the glass. In the
tropics, of course, this function is not
required. A second
question arises about using mirrors to
enhance production year round.
This becomes a focusing collector,
which introduces substantial additional costs and
problems. If the
mirror assembly is cheaper than the pan
assembly, then it deserves to be looked at further, but it
attractive at this time.
Tentatively, reflective aluminum sheet
has the most advantages.
Basin Stills Equipped with Insulated Glazing Covers
Another innovation is the use of an insulated glazing cover
be put over the glazing at night or during extremely cold
weather. This cuts
heat losses, allowing distillation to
continue longer, and retains heat overnight, causing
to start earlier the next day.
Cost-benefit analysis of this
approach has not been made.
V. COMPARING THE
For a couple of gallons of purified water a day, there is no
method that can compete with solar distillation. For a
million gallons a day--AS LONG AS WE ARE WILLING TO BURN UP
INHERITANCE OF FOSSIL CHEMICAL BUILDING BLOCKS JUST TO
WATER--boiling distillation is the cheapest way to purify
In sum, solar stills have:
high initial costs;
the potential to use local materials;
the potential to use local labor for
low maintenance costs (ideally);
no energy costs (not subject to fuel supply
few environmental penalties; and
in residential sizes, no subsequent costs
water to the end user.
Most competing technologies are:
low in initial costs;
dependent on economy of scale;
high in operating and maintenance-costs;
high in energy input costs;
low in local job creation potential;
vulnerable to changes in energy supply and
VI. CHOOSING THE TECHNOLOGY RIGHT FOR YOU
FACTORS TO CONSIDER
Solar energy is an excellent choice for water distillation
those areas of the Third World that meet the following
expensive fresh water source (US) $1 or
more per 1,000
adequate solar energy; and
available low-quality water for
Other conditions suitability for solar stills are:
competing technologies that require
conventional wood, or petroleum fuels;
isolated communities that may not have
access to clean
limited technical manpower for operation
areas lacking a water distribution system;
the availability of low-cost construction
The greater the number of these conditions present, the more
solar stills are likely to be a viable alternative.
If the cost
of the water produced by a still over its useful life is
than by alternate methods, it is economical to pursue.
Other factors to consider are the availability and cost of
capital, as well as the local tax structure, which may allow
credits and depreciation allowances as a means to recover a
portion of the cost.
This has proved to be a major incentive in
the United States.
Finally, the acceptance of solar distillation will depend
on how well one understands and handles the many social
and cultural constraints that can hamper the introduction of
of the more important issues that may affect
the acceptance of solar distillation are outlined below.
Stills built for village use require
that may be foreign to some cultural
groups. If the distilled water
distributed, causing a family unit not to receive its
of water, this could become a source of
conflict. For this reason, a
family-sized solar still
a household has complete control over, may
practical than a unit that serves an entire
Potential users who think they will find
tasteless or not in keeping with what they are
to may become disappointed and possibly
altogether the thought of drinking the water.
of taste must be dealt with early on so as
not to give
people a reason to respond negatively to
technology as a whole.
In some societies, conflicts may arise
over whether it
responsibility of the man or the woman of the
household to operate the solar still.
Not dealing with
early on could result in the household's
rejection of the technology.
If solar distillation is perceived to be a
threat to a
traditional lifestyle, the community may
technology. Such concerns can be headed
technology is designed appropriately from the
introduced at the proper time.
is more likely to accept the technology if it
the importance of clean water and considers
priority to the degree that it is willing to
certain aspects of its lifestyle.
Three potential markets exist for solar stills.
First, a solar
still can be economically attractive almost any place in the
world where water is hauled and where a source of water is
available to feed the still.
Second, many people who boil their water to kill germs could
a solar still for the same purpose.
It will take more work to
demonstrate this function adequately, but early tests have
it seem highly promising.
A third market is in arid regions, whose untapped water
may be sufficient to economically provide a population with
Worldwide experience in researching and marketing solar
over three decades has provided an ample foundation for a
still industry. No
inherent technical or economic barriers have
been identified. A
solar still is suited to village
[manufacturing] techniques and to mass production.
world, concerns over water quality are increasing, and in
situations a solar still can provide a water supply more
economically than any other method.
Commercial activities are
picking up after a lull during the late 1970s.
It is now
possible to predict a rapid increase in the manufacture and
marketing of solar stills.
AND MANUFACTURERS OF SOLAR STILLS
P.O. Box 981
Laguna Beach, California 92652-0981
CEDEX No. 3
F. 92080 Paris La Defense
Cornell Energy, Inc.
4175 South Fremont
Cooper, P.I., "Solar Distillation--State of the Art and
Prospects." Solar Energy
and the Arab World (1983): 311-30.
Direct Use of the Sun's Energy.
Ballantine Books, 1975.
El-Rafaie, M.E.; El-Riedy, M.K.; and El-Wady, M.A.
of Fin Effect in
Predicting the Performance of Cascaded
Stills." Solar Energy and the Arab
World (1983): 336-40.
"Shedding Light on Solar Collector Glazing."
Engineering 90 (September 1979): 55-58.
Langa, Fred; Flower, Bob; and Sellers, Dave. "Solar
Review." New Shelter (January
Leckie, Jim; Master, Gil; Whitehouse, Harry; and Young,
More Other Homes
and Garbage. San Francisco, California:
"Solar Distillation Using Appropriate Technology."
Solar Energy and
the Arab World (1983): 341-45.
Talbert, S.G.; Eibling, J.A.; and Lof, George.
Manual on Solar
Saline Water. Springfield, Virginia:
Technical Information Service, April 1970.
Dunham, Daniel C.
Fresh Water From the Sun.
U.S. Agency for
Internation Development, August 1978.
Zaki, G.M.; El-Dali, T.; and El-Shafiey, M.
"Improved Performance of
Stills." Solar Energy and the Arab
Only a small amount of McCracken's work has been
published, but the data are
will be welcomed:
McCracken Solar Co.
P.O. Box 1008
Alturas, California 96101