How to Build a Solar Still

This solar still used no fuel or power except that from the
sun’s rays and was thus able to provide pure water at a
cost unmatched by any other means of distillation. Oddly,
the method was forgotten in the intervening years and
fuel operated stills were used whenever it was necessary to
convert salty–or otherwise undrinkable water–to
fresh.

Not until World War II were solar stills used again except
by experimenters. Fliers forced down at sea needed a source
of supply of drinking water until they could be rescued.
Dr. Maria Telkes developed an inexpensive, lightweight
plastic still that could be included in even one-man life
rafts and that would produce a quart of fresh water a day.

Since that time Dr. Telkes and other scientists have worked
with solar stills of various sizes. Our government’s
Department of the Interior is interested in the idea, and
plans have been made for large seacoast installations to
purify salt water for drinking and irrigation. In some
designs no pumps would re needed because the sea itself
would fill the condensing tanks at high tide.

At present it is felt that the cost of such a system would
be too high, even considering that cost of operation would
be less than that of a fuel-run still. Engineers are
hopeful, however, that improved methods and materials will
make the plan feasible.

The principle of the solar still is a simple one, and is
observed on a grand scale in nature. Clouds are droplets of
water evaporated from the surface of the sea or from damp
ground and then condensed high in the air. In the process
of evaporation, solids such as salt are left behind. Many
readers will be familiar with the commercial harvesting of
salt in shallow ponds, for this is one of the oldest of
man’s uses of solar energy.

The still described here uses the same principle, but is a
little more complex than simply letting sea water run into
a pond to evaporate. By means of a glass-plate collector we
will increase the temperature in the still to speed
evaporation. The design is borrowed from Dr. Telkes, and
has been proved by careful testing over a long period of
time. Researchers have shown that it’s possible to produce
almost a quart and a half of water a day for each square
foot of the collector’s surface. Thus the unit should have
a maximum output of more than one gallon a day. Of course,
this figure represents ideal conditions, but it will be
interesting to compare your results with the calculated
output. The still described here was tested in mid-winter,
and produced more than one quart in six hours.

Still Materials and Assembly

1 by 4-inch redwood board (8 linear feet)
1/2-inch plywood (one piece, 24 by 24 inches)
Single-weight window glass (one piece, cut to
measure)
1/2-inch O.D. copper tubing (4 1/2 feet)
1/2-inch, 90-degree copper elbow (one)
1/2-inch copper pipe cap (one)
Galvanized iron (one piece, 3 by 24 inches)
1/4-inch I.D. copper tube (six inches)
Large tin can (one)
1/4-by-1 1/2-inch wood screws (approximately 30)
Black terry cloth (approximately two yards; i.e.: one large
bath towel)

To begin construction, first assemble all the needed
materials (except the glass). Redwood is preferred where
possible because it resists rotting, while other woods will
deteriorate with constant exposure to water.

At the lumberyard where you buy the 8-foot board (1 by 4
inches) have a groove cut 1/8 inch wide to a depth of 3/8
inch. Locate the cut 1/2 inch from one edge. This is the
slot for the glass window. Next cut the 1 by 4 redwood
side pieces to the proper length.

Drill two holes at each corner of the side pieces and
assemble them with wood screws. The 1/2-inch plywood back
may now be put in place and holes drilled for screws.
Notice that no screws are put in at the corners, to prevent
interference with those which hold the sides together.

Mark the locations for the large holes that will receive
the 1/2-inch tubing. Drill a 1/8-inch hole as a guide, and
then drill three 5/8inch holes and one 3/4-inch hole. Be sure the large hole is the proper size for the
cap, which we’ll solder to the end of the top tube.

The two drain holes–both 1/4 inch in
diameter–can now be drilled. One is located in the
center of the bottom 1 by 4; the other is in the side
1 by 4, positioned at the “v” formed by the bottom and
plywood back. The side drain can now be inserted.

This is a good time to caulk the joints at the bottom and
sides and apply several coats of sealer to the inside of
the collector box to make it watertight, and so that
the distilled water will run out the drain tube and not
seep through the bottom of the box. Allow to dry thoroughly
and check for leaks.

We know that any surface receiving heat will re-radiate part
of that warmth. To prevent as much heat loss as possible,
and also to present a smooth surface for condensation of
water vapor, we’ll line the inside of the box with aluminum
foil. Running the foil in one piece across the plywood back
and the bottom 1 by 4 will make an additional waterproof
layer to help proper drainage. Notice that the foil extends
into the bottom glass slot also.

A single piece of foil 24 inches wide will do the job. If
you find it necessary to use narrower foil, apply the
section toward the drain tube first and lap the other piece
over it. Use glue, rubber cement or airplane dope to apply
the covering. Start at the top, carefully unrolling the
foil for a smooth job. Let its edge extend a quarter inch
past the edge of the glass slot. This excess will be forced
into the slot later when the glass is slid into place. Next
line the inside surface of the other three 1 by 4’s to
complete the job.

Since some salty or otherwise undesirable water may not be
evaporated before it reaches the bottom of the black towel
wick, we provide a V-shaped trough to catch this waste
and prevent it from mixing with the distilled water at the
bottom of the still. Made from galvanized iron, this
24-inch trough is a 90-degree angle with legs 1 1/2 inches
wide. This can best be bent at the sheet metal shop.

Drill a hole in the very center of the angle to receive the center 1/4-inch copper drain tube.
Put the short length of tubing in place and solder
securely. To keep the waste water from spilling out the
ends of the trough, curve it slightly by “crimping” its
edges in several places.

Once the drain trough is completed it may be put in place
by inserting the copper tube in the hole drilled for it.
This should be a snug fit. Carefully punch through the
aluminum foil first with a pencil, then press the trough
down until it just touches the foil at the center.

We’re now ready to start on the still’s “wick.” The
1/2-inch tubing used at the top and bottom to support the
toweling is cut to size with a tubing cutter or fine
hacksaw. Or give your hardware dealer accurate dimensions
and ask him to do this for you.

In addition to holding the toweling in place, the top tube
is the distributor for the water supply. To accomplish this
we drill two rows of No. 50 holes, at right angles to each
other. The holes do not go through both walls of the tubing
and are spaced approximately 2 inches apart.

Our still uses a quart can as a reservoir. Cut the top from
a large juice container and remove the paper. Next, cut a
4-inch length of 1/2-inch copper tubing and flatten one end
in a vise or with a hammer. This tube will serve to meter
the water into the long tube so that too much isn’t fed to
the toweling.

With a chisel, carefully cut a slit in the center of the
reservoir’s bottom. Allow the metal to bend inward
slightly, and check the size of the slit until the
flattened tube fits snugly with about 1/2 inch extending
inside the can. Make sure the tube remains lined up while
you solder it in place.

Now solder the cap onto the other end of the drilled top
tube. Push the tube through the holes in the box, taking
care not to tear the aluminum foil lining. Prop up the box
so that it’s tilted back about 45 degrees from the vertical
and rotate the tube so that the two rows of holes are
properly positioned to feed water into the toweling when
it’s looped around the tube. The tilt of our still will
vary with the position of the sun, so we’re striking a
happy medium in locating these holes.

With the tube in the right position, force the end cap into
the 3/4-inch hole. This should be a snug fit and hold the
assembly in the proper place. Now solder the 1/2-inch elbow
to the open end of the tube so that it points straight up.
The free end of the tube soldered to the can is inserted in
the elbow and soldered in place. Some water in the bottom
of the can will prevent the joint at the can from melting
while you work on the elbow.

Next attach a hinged 1 by 4 prop to the back of the box,
using small wood screws as required. This supports the
still and also gives us a means of adjusting the tilt to
best face the sun. At this time also nail on the two legs,
making sure to leave the proper one about 1/4-inch long so
that water will run toward the fresh water drain.

Smooth the ends of the bottom tube that will support the
toweling and insert it in place. You’re now ready to
sew the toweling together. The still in our plans uses a
towel 24 inches wide, with the ends lapped and sewed in two
places (along both edges of the lap) to make a loop. In
measuring for this, make the loop slightly smaller than the
distance between the tubes, because the towel will stretch
somewhat when wet. Too much sag would cause it to touch the
back of the box and thus contaminate the distilled water.

With the towel sewed together and dampened, slip the
1/2-inch tubes halfway out and start the towel loop onto
the supports inside the box. The bottom tube
is slid back into place first, then the top one is
carefully raised into position and forced back into the
3/4-inch hole. Adjust the towel so that it covers the
entire length of the tubes and is smooth. Do not let it
touch the sides of the box.

At this point, remove the top 1 by 4 piece so that the
dimensions for the box’s glass front can be taken. Slide a
heavy piece of cardboard into the grooves and trim it to
fit. Then have your supplier cut a pane of glass to the
size of the pattern. The grade of window glass known as
“water-white” allows more of the sun’s rays to pass than an
ordinary pane does and is therefore more efficient for our
purpose. However, if it’s not available, standard
single-strength glass will do.

Slide the glass into position. Make sure it’s as clean as
it can be, particularly on the inner surface, which
will not be easy to reach when the still is assembled.
Detergents may interfere with proper forming of droplets on
the glass, so use plain water for cleansing.

Handle the glass very carefully to avoid cutting yourself
on its sharp edges. With the still at a 45-degree angle,
start the glass into the slots and ease it downward. When
it contacts the aluminum foil at the bottom groove the
glass should force the foil neatly into place. Replace the
top piece of wood, tighten the screws and the still is
ready for operation.

Distillation is easy with our solar plant. First, orient
the unit so that the sun’s rays strike it as close to a
right angle as possible. Mount the still on a level surface
so that the uneven legs will give it the proper slant for
draining. If the reservoir can is not vertical, carefully
twist the container until it is. The towel should be snug
enough to hold it, and the force fit of the end cap in the
3/4-inch hole also helps in this respect.

Now fill the reservoir, and to make sure that the still
really works, add some salt to the water! After a few
minutes the towel will begin to receive the liquid from the
distribution tube. If it doesn’t, or if the flow is too
slow, very carefully open up the flattened tube in the
bottom of the can with the point of an ice pick. Be
cautious about this, however, as we don’t want the water to
flow into the towel faster than it can be evaporated.

Heat from the sun is trapped in the box, and the black
towel absorbs this. Water is therefore evaporated from the
towel much faster than it would be normally. The vapor,
free of all solids, recondenses on the smooth surface of
the glass and the foil on the back and sides. You’ll see
this a few minutes after you put in the water; first the
glass steams up, then droplets form and run down to the
bottom.

Don’t expect the water to gush from the still like a
Niagara, but put a can or bottle under the fresh water drain to catch
the distilled fluid. On a sunny day the still will begin
producing soon after you set it in operation and will drip
water steadily into the container. To guard against
evaporation of this distilled water, some experimenters run
a tube from the drain to a corked bottle.

At first you’ll probably reposition the still every half
hour or so for greatest efficiency. When the novelty wears
off, however, you’ll likely decide on a compromise
tilt–your latitude plus 10 degrees or so–and
aim the collector south (unless you live below the Equator,
of course). 

©1959 by D.S. Halacy, Jr. Originally published by
the Macmillan Company as a chapter of the book,
Fun With The Sun. Reprinted by permission of the author.