by W. R. BRESLIN
illustrated by GEORGE R. CLARK
1600 Wilson Boulevard, Suite
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax:
Volunteers in Technical Assistance
WHAT IT IS AND HOW IT IS USEFUL
MAKING THE DECISION AND FOLLOWING THROUGH
of Plastic Films
CONSTRUCTION OF THE STILL
OPERATION AND MAINTENANCE
DICTIONARY OF TERMS
FURTHER INFORMATION RESOURCES
APPENDIX I. DECISION MAKING WORKSHEET
APPENDIX II. RECORD KEEPING WORKSHEET
I. WHAT IT IS AND
HOW IT IS USEFUL
A solar still is a device that uses energy from the sun to
purify salt- or brackish water.
Solar stills (as shown in
Figure 1) can be easy to construct and maintain.
their size, they can provide water for many uses.
And in desert
areas where sunshine is plentiful and water is not, a solar
still can be very important.
A solar still is little more than a shallow, watertight box
with a clear glass or plastic top.
The bottom of the box is
usually painted black to absorb the sun's heat.
The base of the
still is filled with nonpotable water, for example, brackish
water. The sun's
heat evaporates the water, which then condenses
on the inner surface of the cover.
The condensed water
runs into troughs from which it can be collected in storage
still's cover is tilted to collect the greatest
amount of solar energy.
Glass-covered solar stills are much
more rugged and trouble-free and are able to withstand
and environmental conditions much better than plastic.
a long period of time, the increased cost of glass will pay
Since the water is pure and free of harmful bacteria, there
no fear of water-borne diseases commonly associated with
supplies in many developing countries.
In some parts of the
world where the major supply of water is the sea or ocean,
solar distillation of saltwater has proved to be
feasible when compared to mechanical conversion of
The portable still described here produces 3 liters (.8
of water per day.
[While the basic design can be enlarged
to produce up to 758 liters (200 gallons) per day, the
still would be 185 sq m (2000 sq ft) and would be very
expensive to build.]
Once built, the only maintenance required
is to keep the outside of the glass clean and to flush out
interior occasionally to remove the salt buildup.
II. DECISION FACTORS
* Purifying salt- and brackish
Clean water supply for family needs, hospital
or dispensary, etc.
* No fuel costs
Still can produce up to 3 liters (.8 gallons
of water per day
Easy to build and operate
Portable design -- ideal for
Designed to catch rainwater run-off
* Limited output
Has to be filled manually
Operable only during daylight hours
Must be cleaned periodically
What is the water to be used for and how much is needed?
Consider these questions carefully before beginning.
clean water processed from the still is small in comparison
normal water usage which in a developing country runs from
24-40 liters (6-11 gallons) per day.
This limits the still's
value to those needs it can meet.
In many areas, the primary
use for a solar still has been to provide potable water from
seawater or brackish water which is unfit to drink in its
natural state. This
still could provide enough water to meet an
individual's drinking needs.
Consider also care and maintenance of the solar still.
has to fill and clean the still in the design presented
$15 to $30 (U.S., 1979) including material and labor.
(*)Cost estimates serve only as a guide and will vary from
country to country.
III. MAKING THE
DECISION AND FOLLOWING THROUGH
When determining whether a project is worth the time,
and expense involved, consider social, cultural, and
factors as well as economic ones.
What is the purpose of
the effort? Who will benefit most? What will the
be if the effort is successful? And if it fails?
Having made an informed technology choice, it is important
keep good records.
It is helpful from the beginning to keep
data on needs, site selection, resource availability,
progress, labor and materials costs, test findings, etc.
The information may prove an important reference if existing
plans and methods need to be altered.
It can be helpful in pin-pointing
"what went wrong?" And, of course, it is important
share data with other people.
The technologies presented in this series have been tested
carefully, and are actually used in many parts of the world.
However, extensive and controlled field tests have not been
conducted for many of them, even some of the most common
Even though we know that these technologies work well in
situations, it is important to gather specific information
why they perform better in one place than in another.
Well documented models of field activities provide important
information for the development worker.
It is obviously important
for a development worker in Colombia to have the technical
design for a still built and used in Senegal.
But it is even
more important to have a full narrative about the still that
provides details on materials, labor, design changes, and so
forth. This model
can provide a useful frame of reference.
A reliable bank of such field information is now
exists to help spread the word about these and other
lessening the dependence of the developing world on
expensive and finite energy resources.
A practical record keeping format can be found in Appendix
The relationship between the size of a solar still and its
capacity depends upon its design and efficiency.
capacity rate is approximately 10 to 1 if the unit is glass
covered and well insulated.
For example, a 114-liter- (30-gallon-)
per-day still will require 300 sq ft under optimum
cloudy or rainy days, production stops so it is
necessary to build a solar device to anticipate this
Therefore, it is best to provide for a good storage facility
hold the water produced.
Because this still is quite small, it is designed so that
collected can be drained into bottles.
The water could also be
collected in 208-liter (55-gallon) drums that have been
and rustproofed or in ferroconcrete water storage tanks--any
good catchment setup can be used.
The still requires unobstructed sunshine from early morning
late afternoon. It
should be placed so that the length of the
still runs from east to west.
The south-facing glass should
face due south as much as possible.
The still should be kept
The quality of the water produced can be greatly affected by
the storage facility and the collection method just to name
factors. Many prefer
to boil water which sits in a catchment of
some kind before using it as drinking water.
On the other hand,
if the still is kept clean and the distillate is drained
clean bottles for storage [20-30 liter (5-8 gallon) bottles
a good size], the water will remain clean.
DISADVANTAGES OF PLASTIC FILMS
Because of the following problems, glass-covered stills
to be more reliable:
* Plastic films
become brittle and deteriorate from the sun's
radiation. As a result, depending upon
the plastic, they
may have to be replaced every three to six
* Condensing water
usually forms droplets on the surface of the
These droplets reflect a portion of the
energy back to the
sky and they often drip back into the
* Plastic film is
easily damaged by heavy rains, winds, and
* Plastic collects
dust which can only be removed by using
fresh water from
* Wood saw
Wood chisel or router
* Metal saw
Drill with bits
1 Galvanized steel sheet, 58cm X 128cm X 0.3mm thick (water
1 Hardboard sheet, 60cm X 124cm X 3mm thick (insulation
2 Glass panes, 27.5cm X 122cm X 6mm thick (transparent
4 Lumber,(*) finished, 5cm X 5cm X 25cm, (legs)
4 Lumber,(*) finished, 2cm X 8cm X 128cm (base frame, long)
5 Lumber,(*) finished, 2cm X 8cm X 60cm (base frame, short)
2 Lumber,(*) finished, 5cm X 10cm X 120cm (side members)
3 Lumber,(*) finished, 4cm X 5cm X 50cm (tray ribs)
2 Lumber,(*) finished, 17.5cm X 60cm X 2cm thick, cut
shown or equivalent
1 Lumber,(*) finished, 4cm X 4cm X 124cm (glass support)
1 Copper tubing/galvanized steel pipe, 3/8" X 11cm
2 Copper tubing/galvanized steel pipe, 3/8" X 6cm long
1 Plastic tubing, length variable depending on collection
fit snugly over copper tubing
caulking, similar to that used for steel windows
(*)Preferably a white wood or equivalent (tulip, a
* Wood shavings, to
fill volume 0.3 cubic meters (insulation)
* Primer for
galvanized steel surfaces, preferably one coat
wash primer and
then one coat zinc chromate
* Aluminum paint
* Wood primer
* Flat black plastic
* White plastic
V. CONSTRUCTION OF THE STILL
1. On one end of the galvanized steel sheet, drill a 3cm
hole for the
drainpipe as shown in Figure 2.
2. Using tin snips or a metal saw, cut the galvanized steel
sheet 4cm from the
end on each long side, cutting 4cm deep
(as indicated in
3. Bend the long sides as
shown in Figure 3.
4. Bend the ends into corners. Solder all four corners at
top and bottom,
inside and out, as indicated in Figure 4.
5. Using clean water, test for leaks. If any leaks appear,
corner, inside and out.
6. Using a metal saw, cut the copper
drainpipe as shown
Figure 5. The
extend at least
5cm below the
bottom of the
installation of the
7. Bend the sections very
carefully as shown
Figure 6 and
with a hammer.
8. Turn the tray upside down and line up the hole in the
with the hole in
the bottom of the tray as shown in
Figure 7. Solder
all four tabs securely. Check for leaks.
9. Paint the tray with a suitable primer and then with a good
flat black plastic
paint. The paint should be able to withstand
immersion and temperatures of 65-70[degrees]C
and should not fade or discolor under the influence
of the sun's rays.
1. Grooves can be cut into the side members or built up for
rainwater troughs and the glass rest. Two
options are shown
below in Figure 8.
If Option 1 is used, holes should be drilled for the
after assembly, and edges sealed with caulking. The side
members should be primed and painted with good white plastic
paint. Be sure that the upper face containing the grooves is
thoroughly painted to prevent leakage.
2. Cut and prepare the end sections, cutting a door in one
piece, as shown in
Figure 9. Painting probably should be
assembly of still section as shown on the following
3. As shown in Figure 10, nail the end sections to the side
members. Nail the
tray ribs in place using nails at least
4. Nail the hardboard or plywood insulation retaining sheet
place beneath the
tray ribs (see Figure 10). (If hardboard
is used, it should
be soaked in water for at least 24 hours,
removed from water
and allowed to dry thoroughly; then
nailed in place.)
Nail edges closely to prevent bulging at
5. Place the tray in the still to get the drainpipe
Remove the tray
from the still and drill a hole for the
drainpipe in the
retaining sheet. Be sure it is in the end
where the door is
6. Paint the outside bottom face of the hardboard or plywood
7. Drill two holes for the distillate and the rainwater
in the door end
only. See Options 1 and 2 in Figure 11
Make a sturdy base for the still, using available materials.
The dimensions in Figure 12 should be used as a guide.
ASSEMBLE THE STILL
1. Place the insulation in the
still under the
between the ribs,
with the top of
(see Figure 13).
pack too firmly
evenly and fully.
2. Install the tray in its
3. Nail the tray into the framework at about 4cm intervals,
the top edge only.
Do not nail the tray into the rib supports
but only into the
side members as shown in Figure 14.
4. Install the glass support into the framework as shown in
5. Clean the glass panes extremely well and put them in
Care must be taken
to avoid fingerprints, putty, or paint
marks on the
glass. Caulk the glass well with non-hardening
rubber or similar caulking is good).
6. Secure the glass panes with several metal or wooden
(see Options 1 and
2 in Figure 15). They should prevent a
strong wind from
lifting and possibly breaking the glass.
7. Install the plastic tubing to the trough pipes and be
sufficient tubing to enter several centimeters into
VI. OPERATION AND MAINTENANCE
For proper operation and maintenance of your solar still,
follow the guidelines listed below:
* For the first use, fill the still with water to a depth of
(1"). From then on, early each morning, at
about 7 or 8
o'clock, drain the water remaining from the previous
day. Add fresh
water, again to a depth of about 2cm.
Be careful not to
touch the underside of the glass.
* Do not use the distillate produced by the still for the
few days; this
* Always wash out collecting bottles in fresh water and then
The collecting bottles must be large enough
to hold 1-3 liters
(1 gallon). Use only thin-necked collection
bottles with tops
loosely stoppered around the tubing
contamination of the distilled water.
* Keep the distillation unit and surrounding area clean at
times to maintain
quality distilled water.
* Keep the distillation water in a 20-30 liter (5-8 gallon)
container so that
there will always be extra water available.
The areas around
storage bottles must also be kept
* Clean the glass every
few days with
or clean cloth.
* Clean the outside glass before rainstorms during the rainy
season; the clean
rainwater can be collected and added to the
VII. DICTIONARY OF TERMS
BACTERIA--Any of numerous one-celled micro-organisms of the
Schizomycetes, having a wide range of biochemical,
BRACKISH WATER--Water containing some brine or salt.
BRITTLE--Likely to break, fragile.
BULGING--Swollen; grown larger or rounder.
CATCHMENT--A structure or vessel, such as a basin,
or barrel, for collecting water.
CAULK--To make watertight or airtight by filling in cracks.
CAULKING COMPOUND--Substance used to fill in cracks to keep
watertight or airtight.
CONDENSE--To reduce a gas or vapor to a liquid or solid.
CONTAMINATION--To make impure or unsuitable by contact or
DETERIORATE--To lower in quality, character, or value. To
DISTILLATE--The liquid condensed from vapor in distillation.
EVAPORATE--To convert from liquid to vapor.
FERROCONCRETE--Concrete containing steel bars or metal
to increase its
HANDICAP--Disadvantage or disability.
IMMERSE--TO cover completely in a liquid.
NONPOTABLE WATER--Contaminated water not fit for human
POLYETHYLENE--A plastic compound of ethylene used for
of containers, etc.
PORTABLE--Mobile, easily moved.
POTABLE WATER--Uncontaminated water fit for human consumption.
PUTTY--A doughlike cement made by mixing whiting and linseed
oil, used to
seal joints in pipes, fill holes in woodwork,
and secure panes
ROUTER--A tool or machine used to cut furrows or hollows in
TROUGH--A long, narrow, generally shallow receptacle,
VIII. CONVERSION TABLES
UNITS OF LENGTH
= 1760 Yards =
= 1000 Meters =
= 1.607 Kilometers
= 0.3048 Meter
= 3.2808 Feet =
= 2.54 Centimeters
= 0.3937 Inches
UNITS OF AREA
1 Square Mile
= 640 Acres
= 2.5899 Square Kilometers
Kilometer = 1,000,000 Square
Meters = 0.3861 Square Mile
= 43,560 Square Feet
1 Square Foot
= 144 Square Inches =
0.0929 Square Meter
1 Square Inch
= 6.452 Square Centimeters
1 Square Meter
= 10.764 Square Feet
1 Square Centimeter
= 0.155 Square Inch
UNITS OF VOLUME
1.0 Cubic Foot
= 1728 Cubic Inches =
7.48 US Gallons
1.0 British Imperial
= 1.2 US Gallons
1.0 Cubic Meter
= 35.314 Cubic Feet =
264.2 US Gallons
= 1000 Cubic Centimeters =
0.2642 US Gallons
1.0 Metric Ton
= 1000 Kilograms =
= 1000 Grams
= 2.2046 Pounds
1.0 Short Ton
= 2000 Pounds
UNITS OF PRESSURE
1.0 Pound per square inch
= 144 Pound per square foot
1.0 Pound per square inch
= 27.7 Inches of water(*)
1.0 Pound per square inch
= 2.31 Feet of water(*)
1.0 Pound per square inch
= 2.042 Inches of mercury(*)
= 14.7 Pounds per square inch (PSI)
= 33.95 Feet of water(*)
1.0 Foot of water = 0.433 PSI
= 62.355 Pounds per square foot
1.0 Kilogram per square centimeter
= 14.223 Pounds per square inch
1.0 Pound per square inch
= 0.0703 Kilogram per square
UNITS OF POWER
1.0 Horsepower (English)
= 746 Watt
= 0.746 Kilowatt (KW)
1.0 Horsepower (English)
= 550 Foot pounds per second
1.0 Horsepower (English)
= 33,000 Foot pounds per minute
1.0 Kilowatt (KW) =
1000 Watt = 1.34 Horsepoer (HP)
1.0 Horsepower (English)
= 1.0139 Metric horsepower
1.0 Metric horsepower
Meter X Kilogram/Second
1.0 Metric horsepower
= 0.736 Kilowatt
= 736 Watt
(*)At 62 degrees Fahrenheit (16.6 degrees Celsius).
IX. FURTHER INFORMATION RESOURCES
Brace Research. "How to Make a Solar Still (Plastic
#1 (Do It Yourself #1), January 1965.
McDonald College of McGill University,
Ste. Anne de
Bellevue, Quebec, Canada. Probably the most
of these three. Contains plans for a large,
solar still, especially designed for
areas. Plans include materials list, clear
drawings, and easily followed instructions.
Design given has
been used extensively in Barbados.
Edson, Lee and Weldy, James. "Glass-covered Solar
1967. Plans for a solar still very similar to
the one below,
only larger (6 X 8 ft), putting out 5 gallons
optimum conditions. Includes list of
drawings, and instructions. Available
University of California, 1301 S 46th Street,
Edson, Lee and Weldy, James. "How to Build a Solar
revised by B.W.
Tliemat, June 1966, 13 pp. Plans for
building a small "roof-type"
still of glass and wood, big
enough to supply
drinking water for one person under optimum
Includes list of materials, schematic
instructions. May be a bit too technical for
from Sea Water Conversion Lab, Richmond
University of California, 1301 S 46th
Richmond, California USA.
Department of Agriculture. Survival in the Desert (Solar
Available from VITA.
Dunham, Daniel C. Fresh Water from the Sun -- Family Sized
Technology: A Review and Analysis. 1978, 176
pp. Office of
Health, United States Agency for International
United States Department of State,
Gomkale, S.D. and Datta, R.L. "Some Aspects of Solar
Purification," Solar Energy, Vol. 14, 1973,
Papoulias, Nicholas G. Solar Stills. June 1975. Church World
Greece. Available from VITA.
Porteous, Andrew. "The Design of a Prefabricated Solar
for the Island
of Aldabra," Desalination. January 1969.
Publishing Company, Amsterdam, The Netherlands.
Read, W.R. "A Solar Still for Water Desalination
and Installation)," Report E.D. 9. September
1963. CSIRO, PO
Box 26, Highett, Victoria, Australia 3190.
VITA. "Solar Desalination." List of enclosures for
DECISION MAKING WORKSHEET
If you are using this as a guideline for using a solar still
a development effort, collect as much information as
and if you need assistance with the project, write VITA. A
report on your experiences and the uses of this manual will
help VITA both improve the book and aid other similar
VOLUNTEERS IN TECHNICAL ASSISTANCE
North Lynn Street, Suite 200
Arlington, Virginia 22209-8438 USA
CURRENT USE AND AVAILABILITY
* Note current domestic and agricultural practices which
have potential for
* Document days of sunshine, seasonal changes, haze, cloud
cover. Check to see
if such information has already been
collected for the
local area. Another way of finding the
information is to
search out annual rainfall figures and work
* Have solar technologies been introduced previously? If so,
with what results?
* Have solar technologies been introduced in nearby areas?
so, with what
* Are there other current practices which might be enhanced
improved use of
solar energy--for example, salt production?
IDENTIFY APPROPRIATENESS OF THIS TECHNOLOGY
* Is there a choice to be made between a solar technology
energy technology? Or, is it important to
do both on a
* Under what conditions would it be useful to introduce a
* If solar units are feasible for local manufacture, would
be used? Assuming
no "funding," could local people afford
them? Are there
ways to make the solar technologies "pay for
* Could this technology provide a basis for a small business
NEEDS AND RESOURCES
* What are the characteristics of the problem? How is the
sees it as a problem?
* Has any local person, particularly someone in a position
expressed the need or showed interest in solar
technology? If so,
can someone be found to help the technology
process? Are there local officials who
could be involved
and tapped as resources?
* How will you get the community involved with the decision
which technology is
appropriate for them.
* Based on descriptions of current practices and upon this
information, identify needs which solar technologies
appear able to meet.
* Are materials and tools available locally for construction
* Are there other projects already underway to which a solar
component might be
added so that the ongoing project acts as
a technical and
even financial resource for the new effort?
For example, if
there is a post-harvest grain loss project
improved solar drying techniques be introduced
in conjunction with
the other effort?
* What kinds of skills are available locally to assist with
maintenance? How much skill is necessary for
maintenance? Do you need to train people?
Can you meet the
* Some aspects of
this project require someone with experience
and/or welding. Estimated labor time
8 hours skilled labor
8 hours unskilled labor
* Do a cost estimate of the labor, parts, and materials
* How will the project be funded? Will outside funding be
required? Are local
funding sources available to sponsor the
* How much time do you have for the project? Are you aware
planting or harvesting seasons which may affect
* How will you arrange to spread knowledge and use of the
* How was the final decision reached, either to go ahead or
to go ahead, with
RECORD KEEPING WORKSHEET
Photographs of the construction process, as well as the
result, are helpful. They add interest and detail that
might be overlooked in the narrative.
A report on the construction process should include very
information. This kind of detail can often be monitored
most easily in charts (such as the one below). <see
Some other things to record include:
* Specification of materials used in construction.
* Adaptations or changes made in design to fit local
* Equipment costs.
* Time spent in construction--include volunteer time as well
paid labor, full-
* Problems--labor shortage, work stoppage, training
Keep log of operations for at least the first six weeks,
periodically for several days every few months. This log
vary with the technology, but should include full
outputs, duration of operation, training of operators, etc.
Include special problems that may come up--a damper that
close, gear that won't catch, procedures that don't seem to
make sense to workers, etc.
Maintenance records enable keeping track of where breakdowns
occur most frequently and may suggest areas for improvement
strengthening weakness in the design. Furthermore, these
records will give a good idea of how well the project is
working out by accurately recording how much of the time it
working and how often it breaks down. Routine maintenance
records should be kept for a minimum of six months to one
after the project goes into operation. <see report 2>
This category includes damage caused by weather, natural
disasters, vandalism, etc. Pattern the records after the
routine maintenance records. Describe for each separate
* Cause and extent of damage.
* Labor costs of repair (like maintenance account).
* Material costs of repair (like maintenance account).
* Measures taken to prevent recurrence.
MANUALS IN THE ENERGY SERIES
This book is one of a series of manuals on renewable energy
technologies. It is primarily intended for use by people in
international development projects. However, the
techniques and ideas presented here are useful to anyone
seeking to become more energy self-sufficient. The titles in
the series are:
Helical Sail Windmill
Making Charcoal: The Retort Method
Overshot Water-Wheel: Design
Small Michell (Banki) Turbine:
A Construction Manual
Solar Water Heater
Three Cubic Meter Bio-Gas Plant:
A Construction Manual
For a free catalogue of these and other VITA publications,
P. O. Box
Virginia 22209 USA
Volunteers in Technical Assistance (VITA) is a private,
international development organization.
available to individuals and groups in developing countries
variety of information and technical resources aimed at
self sufficiency--needs assessment and program development
support; by-mail and on-site consulting services;
information systems training; and management of long-term
field projects. VITA
promotes the application of simple,
inexpensive technologies to solve problems and create
in developing countries.
VITA places special emphasis on the areas of agriculture and
food processing, renewable energy applications, water supply
and sanitation, housing and construction, and small business
activities are facilitated by the active
involvement of VITA Volunteer technical experts from around
the world and by its documentation center containing
technical material of interest to people in developing