Subject: Solar Stills 1 of 2

From: [email-masked] (Don Kulha)
Date: 7 Jul 94 03:24:22 GMT

Clean Water from the Sun
Laurie Stone

+1993 Laurie Stone
Many of us turn on the tap and take the stream of pure water for 
granted. Or we go down to the corner store and buy distilled water 
for our car or renewable energy system+s battery.  Many people throughout 
the world do not have these options. Of the 2.4 billion people in 
developing countries, less than 500 million have access to safe drinking 
water, let alone distilled water. In this country, many people who 
live in remote areas don't have running water, and are far from any 
store selling distilled water. A solar still is the answer to all 
these problems.

A solar still is a simple device that can convert saline, brackish, 
or polluted water into distilled water. Principles of solar distillation 
have been around for centuries. In the fourth century B.C., Aristotle 
suggested a method of evaporating sea water to produce potable water. 
However, the first solar still recorded was not until 1874, when 
J. Harding and C. Wilson built a still in Chile to provide fresh 
water to a nitrate mining community. This 4700 m2 still produced 
6000 gallons of water per day. Currently there are large installations 
in Australia, Greece, Spain and Tunisia, and on Petit St. Vincent 
Island in the Caribbean. Smaller stills are commonly used in other 
countries.

Solar Still Basics
 The most common still in use is the single basin solar still. The 
still consists of an air tight basin that holds the polluted water, 
covered by a sloped sheet of glass or plastic. The bottom of the 
basin is black to help absorb the solar radiation. The cover allows 
the radiation to enter the still and evaporate the water. The water 
then condenses on the under side of the cover, and runs down the 
sloped cover into a trough or tube. The tube is also inclined so 
that the collected water flows out of the still. When the water evaporates, 
the salt, dirt, and bacteria are left in the still. Thus you have 
perfectly clean water.  

Still Construction
While working at the solar department of the Engineering University 
of Nicaragua, we decided to experiment with distilling water by the 
sun. Although some commercial stills are available, we decided to 
construct our own. We built two different types of solar stills. 
The first one had a glass cover sloped to one side, and the second 
one had a plastic cover which was sloped on both sides. Both stills 
were made of wood. We lined the bottom and sides of the interior 
with black plastic. There is an inlet hole near the top for the dirty 
water to enter, and another hole at the end of a condensate trough 
for the clean water to leave.  We used some sawed off plastic tubing 
for the condensate trough.

For the glass still we used 1/4 inch thick glass. The thinner the 
glass the better, because thin glass stays cool on the inner surface 
which helps the water condense faster. The plastic we used for the 
second still is a 0.5 millimeter thick clear mylar. Both glazings 
seemed to work well although the water could more easily run down 
the glass.  The glass still has approximately 0.5 square meters of 
glazing and produced about 0.7 liters of water per day, or 1.36 liters 
per square meter of still. The plastic still has about 0.75 square 
meters of glazing and only produced 0.5 liters of water a day on 
the average. This corresponds to approximately 0.7 liters per square 
meter per day. Of course, the output of the stills depended greatly 
on how much sun there was. On very sunny days we could get over a 
liter of water out of the glass still.

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