Solar Water Distillation.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
Many people throughout
the world do not have access to clean water. Of the 2.4 billion people in
developing countries, less than 500 million have access to safe drinking water,
let alone distilled water. The answer to these problems is a solar still. A
solar still is a simple device that can convert saline, brackish, or polluted
water into distilled water. The 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 was not produced 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 24000 litres of water per day. Currently there are large still
installations in Australia, Greece, Spain and Tunisia, and on Petit St. Vincent
Island in the Caribbean. Smaller stills are commonly used in other
countries.
Practically any seacoast and many desert areas can be made
inhabitable by using sunshine to pump and purify water. Solar energy does the
pumping (see chapter on photovoltaics), purification, and controls
seawater feed to the stills.
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 or salt 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 (which is cooled by the outside air),
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.
The process is
exactly Mother Nature’s method of getting fresh water into the clouds from
oceans, lakes, swamps, etc. All the water we have ever consumed has already been
solar distilled a several thousand times around the hydrologic cycle.
SOLAR STILL PERFORMANCE
Operation of the still requires no routine
maintenance and has no routine operating costs. The rated production of the
still is an estimated annual average and is not exact, as the amount of sunshine
can vary widely. Stills produce more in hot climates than in cold ones, more at
low latitudes than high, and more in summer than in winter. At the 23° North
latitude of the central Bahamas, the estimated average production of the
installation was 12 times higher in June than in mid-winter. In higher
latitudes, addition of a mirror to the rear of each still increases winter
production. Some stills also functions in freezing climates. In general solar
still can produce 1 litre of distilled of water a day per square meter of still.
On very sunny days over one litre of water can be gained. The still is usually
filled once daily, at night or in the morning.
STILL COSTS
The
cost of a solar distillation system will vary widely, due to size and
site-specific circumstances. The stills are usually inexpensive to build. Some
small models designed in the USA cost 25 USD with glass or 18 USD with plastic
(the amount of water produced is smaller). If the stills are used for one year,
they will produce water at approximately 10 cents per litre.
WATER QUALITY
The distilled water produced is of very high quality, normally better
than that sold in bottles as distilled water. It routinely tests lower than one
part per million total dissolved solids. It is also aerated, as it condenses in
the presence of air inside the still. The water may taste a little strange at
first because distilled water does not have any of the minerals which most
people are accustomed to drinking. Tests have shown that the stills eliminated
all bacteria, and that the incidence of pesticides, fertilizers and solvents is
reduced by 75–99,5%. This is of great importance for many countries where
cholera and other water borne diseases are killing people daily.
DESIGNING SOLAR STILL
There are a few things to keep in mind when designing the solar
still:
 The tank can be made of cement, adobe, plastic, tile, or any other
water resistant material.
If plastic is used to line the bottom
of the still or for the condensate trough, make sure the tank never remains dry.
This could melt the plastic.
Insulation should be used if possible.
Even a small amount will greatly increase the efficiency of the still.
The
container holding the distilled water should be protected from solar radiation
to avoid re-evaporation.
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