The basic principles of solar water distillation are simple yet
effective, as distillation replicates the way nature makes rain.
The sun's energy heats water to the point of evaporation. As the
water evaporates, water vapour rises, condensing on the glass
surface for collection. This process removes impurities such as
salts and heavy metals as well as eliminates microbiological
organisms. The end result is water cleaner than the purest
rainwater. The SolAqua still is a passive solar distiller that
only needs sunshine to operate. There are no moving parts to
wear out.
The distilled water from a SolAqua still does not acquire the
"flat" taste of commercially distilled water since the water is
not boiled (which lowers pH). Solar stills use natural
evaporation and condensation, which is the rainwater process.
This allows for natural pH buffering that produces excellent
taste as compared to steam distillation. Solar stills can easily
provide enough water for family drinking and cooking needs.
Solar distillers can be used to effectively remove many
impurities ranging from salts to microorganisms and are even
used to make drinking water from seawater. SolAqua stills have
been well received by many users, both rural and urban, from
around the globe. SolAqua solar distillers can be successfully
used anywhere the sun shines.
The SolAqua solar stills are simple and have no moving parts.
They are made of quality materials designed to stand-up to the
harsh conditions produced by water and sunlight. Operation is
simple: water should be added (either manually or automatically)
once a day through the still's supply fill port. Excess water
will drain out of the overflow port and this will keep salts
from building up in the basin. Purified drinking water is
collected from the output collection port.
Supply Fill Port: Water should be added to the still via
this port. Water can be added either manually or automatically.
Normally, water is added once a day (in the summer it's normally
best to fill in the late evening and in the winter, in the early
morning). Care should be taken to add the water at a slow
enough flow rate to prevent splashing onto the interior of the
still glazing or overflowing into the collection trough.
Overflow Port: Once the still basin has filled, excess
water will flow out of this port. SolAqua recommends three times
daily distilled water production to be allowed to overflow from
the still on a daily basis to prevent salt build-up in the
basin. If your still produced 2 gallons of product water then
you should add 6 gallons of fresh feedwater through the fill
port. If flushed like this on a daily basis, the overflow water
can be used for other uses as appropriate for your feedwater
(for example, landscape watering).
Distilled Output Collection Port: Purified drinking water
is collected from this port, typically with a glass collection
container. Stills that are mounted on the roof can have the
distillate output piped directly to an interior collection
container. For a newly installed still, allow the collection
trough to be self-cleaned by producing water for a couple of
days before using the distillate output.
Solar Still Background
Solar distillation is a tried and true technology. The first
known use of stills dates back to 1551 when it was used by Arab
alchemists. Other scientists and naturalists used stills over
the coming centuries including Della Porta (1589), Lavoisier
(1862), and Mauchot (1869).
The first "conventional" solar still plant was built in 1872 by
the Swedish engineer Charles Wilson in the mining community of
Las Salinas in what is now northern Chile (Region II). This
still was a large basin-type still used for supplying fresh
water using brackish feedwater to a nitrate mining community.
The plant used wooden bays which had blackened bottoms using
logwood dye and alum. The total area of the distillation plant
was 4,700 square meters. On a typical summer day this plant
produced 4.9 kg of distilled water per square meter of still
surface, or more than 23,000 liters per day. This first stills
plant was in operation for 40 years!
Over the past century, literally hundreds of solar still plants
and thousands of individual stills have been built around the
world. SolAqua stills have built upon years of still research
and development, use NSF and FDA approved materials, and are the
state of the art for commercial solar still distillation.
Still Operation
A solar still operates on the same principle as rainwater:
evaporation and condensation. The water from the oceans
evaporates, only to cool, condense, and return to earth as rain.
When the water evaporates, it removes only pure water and leaves
all contaminants behind. Solar stills mimic this natural
process.
A SolAqua single basin solar still has a top cover made of
glass, with an interior surface made of a waterproof membrane.
This interior surface uses a blackened material to improve
absorption of the sun's rays. Water to be cleaned is poured into
the still to partially fill the basin. The glass cover allows
the solar radiation (short-wave) to pass into the still, which
is mostly absorbed by the blackened base. The water begins to
heat up and the moisture content of the air trapped between the
water surface and the glass cover increases. The base also
radiates energy in the infra-red region (long-wave) which is
reflected back into the still by the glass cover, trapping the
solar energy inside the still (the "greenhouse" effect). The
heated water vapour evaporates from the basin and condenses on
the inside of the glass cover. In this process, the salts and
microbes that were in the original water are left behind.
Condensed water trickles down the inclined glass cover to an
interior collection trough and out to a storage bottle.
The still is filled each morning or evening, and the total water
production for the day is collected at that time. The still will
continue to produce distillate after sundown until the water
temperature cools down. Feedwater should be added each day that
roughly exceeds the distillate production to provide proper
flushing of the basin water and to clean out excess salts left
behind during the evaporation process.
The intensity of solar energy falling on the still is the single
most important parameter affecting production. The daily
distilled water output (M e in kg/m2 day) is the amount of
energy utilized in vaporizing water in the still (Q e in J/m2
day) over the latent heat of vaporization of water (L in J/kg).
Solar still efficiency (n) is the amount of energy utilized in
vaporizing water in the still over the amount of incident solar
energy on the still (Q t in J/m2 day). These can be expressed
as:
Solar still production: M e = Q e / L
Solar still efficiency: n = Q e / Q t
Typical efficiencies for single basin solar stills approach 60
percent. General operation is simple and requires facing the
still towards solar noon, putting water in the still every
morning to fill and flush the basin, and recovering distillate
from the collection reservoir (for example, glass bottles).
Stills are modular and for greater water production
requirements, several stills can be connected together in series
and parallel as desired.
As water evaporates from the solar still basin, salts and
other contaminants are left behind. Over time, these salts can
build to the point of saturation if the still is not properly
maintained and flushed on a regular basis. Properly operating a
still requires about three times as much make-up water as the
distillate produced each day. If the still produced 3 gallons of
water, 9 gallons of make-up water should be added, of which 6
gallons leaves the still as excess. The excess water flushes the
still basin through the overflow to prevent salt buildup. If
this is done on a daily basis, the flushed water is of
approximately the same quality as the original feedwater that
was added to the still. The excess water is of suitable quality
that it can be used to water landscaping, wash pots and pans,
etc. No sediment or sludge will buildup if the still is properly
operated and flushed daily.
Still Water Production
Solar still production is a function of solar energy (insolation)
and ambient temperature. Production rates in the Southwest U.S.
can average about 2 liters per day in the winter to over 6
liters per day during the summer, per square meter.
Distillation Purification
Capabilities
Solar stills have proven to be highly effective in cleaning up
water supplies to provide safe drinking water. The effectiveness
of distillation for producing safe drinking water is well
established and recognized. Most commercial stills and water
purification systems require electrical or other fossil-fueled
power sources. Solar distillation technology produces the same
safe quality drinking water as other distillation technologies;
only the energy source is different: the sun.
Distillation is the only stand alone point-of-use (POU)
technology with National Sanitation Foundation (NSF)
international certification for arsenic removal, under Standard
62. Solar distillation removes all salts as well as biological
contaminants (for example,cryptosporidium, E. coli,
etc.). There are many studies in the literature, such as tests
conducted on solar stills at New Mexico State University and
Sandia National Laboratories, that clearly verify solar stills'
effectiveness in eliminating microbial contamination and salts.
If you have any questions about what a solar still can
effectively clean, please contact SolAqua for additional
information.
