S M A L L S C A L E W A T E R D I S T I L L A T I O N
U S I N G S O L A R W A R M I N G
SOLAR STILL FOR PURE WATER
SOLAR STILL - WHAT’S IT
• A solar still uses the heat of the sun directly to
purify water by vaporization - condensation.
• A solar still is simply a shallow basin with a
transparent glass cover. The sun heats the water in
the basin, causing evaporation. Vapour rises,
condenses on the cover and runs down into a
collection trough for pure water.
• Left behind is a fraction of input water with salts,
minerals, and other impurities, like germs. 2
This presentation focuses mainly on small-
scale basin-type solar stills as suppliers of
potable water for families and other small
Of all the solar still designs developed thus
far, the basin-type continues to be the most
simple and economical.
How’s -its operation: 1. The sun's energy - short
electromagnetic waves - passes through a clear
glazing surface such as glass. Upon striking a
darkened surface, this light changes wavelength,
becomes long waves of heat- added to the water in a
shallow basin below the glazing. As the water heats
up, it begins to evaporate.
2. The warmed vapor rises to a cooler area. Almost
all impurities are left behind in the basin.
How’s -its operation: … continued
3. The vapor condenses onto the underside of
the cooler glazing and accumulates into water
droplets or sheets of water.
4. The combination of gravity and the tilted
glazing surface allows the water to run down
the cover and into a collection trough, where it
is channeled into storage.
SOLAR DISTILLATION: ENERGY
• In this process, water is evaporated, thus
separating water vapor from dissolved matter,
the vapor is then condensed as pure water.
• At least 2260 kJ/kg is required to evaporate
• To pump a kg of water through 20m head
requires only 0.2 kJ/kg.
• Only where there is no local source of fresh
water that can be easily pumped or lifted,
distillation is therefore normally considered.
WHEN USE SOLAR DISTILLATION ?
• Solar stills should normally only be considered for
removal of dissolved salts from water.
• If there is no fresh water then the main alternatives
are desalination, transportation and rainwater
• Unlike other techniques of desalination, solar stills
are more attractive, the smaller the required output.
WHY USE A SOLAR STILL?
• Solar distillation can be a cost-effective means
of providing clean water -- on a small scale, for
drinking, cooking, washing and bathing--four
basic human needs.
• It can improve health standards by removing
low concentration inorganic impurities from
questionable water supplies.
•The solar still is also used to purify water for
some business, industry, laboratory, and
• It also appears able to purify polluted water.
WHY USE A SOLAR STILL ?
Design objectives for an efficient solar still
• For high efficiency the solar still should
• a high feed (un-distilled) water temperature
• a large temperature difference between
feed water and condensing surface
• low vapor leakage.
In most units, less than half the calories of radiant
energy falling on the still are used for the heat of
vaporization necessary to produce the distilled
All commercial stills sold to date have had an
efficiency range of 30 to 45 percent. (The maximum
efficiency is just over 60 percent.)
Efficiency = (Energy required for the
vaporization of the distillate that is
(Energy in the sun's radiation
that falls on the still.)
Efficiency is calculated in the following manner:
TO ACHIEVE HIGH EFFICIENCY-1:
A high feed water temperature can be achieved if:
• A high proportion of incoming radiation is absorbed by
the feed water as heat.
• Hence low absorption glazing and a good radiation
absorbing surface are required
• heat losses from the floor and walls are kept low
• the water is shallow so there is not so much to heat.
TO ACHIEVE HIGH EFFICIENCY-2:
A large temperature difference can be achieved if:
• the condensing surface absorbs little or none of
the incoming radiation
• condensing water dissipates heat which must be
removed rapidly from the condensing surface
• by, for example, a second flow of water or air, or
by condensing at night.
EFFICIENCY VS. COST OF STILL
• Provided the costs don't rise significantly, an
efficiency increase of a few percent is worth
• Improvements are principally to be sought in
materials and methods of construction.
DESIGN TYPES AND THEIR
• Single-basin stills have been much studied and their
behavior is well understood. Efficiencies of 25% are
• Daily output is a function of solar irradiation and is
greatest in the early evening when the feed water is
still hot but when outside temperatures are falling.
DESIGN TYPES AND THEIR
• Material selection is very important. The
cover can be either glass or plastic. Glass
is considered to be best for most long-term
applications, whereas a plastic (such as
polyethylene) can be used for short-term
DESIGN TYPES AND THEIR
• Sand concrete or waterproofed concrete are
considered best for the basin of a long-life
still if it is to be manufactured on-site,
• but for factory-manufactured stills,
prefabricated ferro-concrete is a suitable
OUTPUT OF A SOLAR STILL
• Q = [E x G x A] / 2.3
• Q = daily output of distilled water (litres /day)
• E = overall efficiency
• G = daily global solar irradiation (MJ/m²)
• A = aperture area of the still i. e, the plan areas
for a simple basin still
OUTPUT PER SQUARE METRE
OF AREA IS:
• The average, daily, global solar irradiation is typically
18.0 MJ/m² (5 kWh/m²).
• A simple basin still operates at an overall efficiency of
• daily output = [0.30 x 18.0 x 1] / 2.3
= 2.3 litres (per square metre)
• concentrating collector stills
• multiple tray tilted stills
• tilted wick solar stills
• and basin stills
• 95 percent of all functioning stills are of the
FOUR MAJOR COMPONENTS -
1. a basin;
2. a support structure;
3. a transparent glazing cover; and
4. a distillate trough (water channel)
1. insulation (usually under the basin);
3. piping and valves;
4. facilities for storage;
5. an external cover to protect the other
components from the weather and to
make the still esthetically pleasing; and
6. a reflector to concentrate sunlight.
• If the only glazing available is one meter at its
greatest dimension, the still's maximum inner
width will be just under one
meter. And the length of the still will be set
according to what is needed to provide the
amount of square meters to produce the
required amount of water. It is generally best to
design an installation with many small modular
units to supply the water.
COMMUNITY AND RESIDENTIAL
• Most community size stills are 1/2 to 2 1/2 meters wide,
with lengths ranging up to around 100 meters.
• Their lengths usually run along an east-west axis to
maximize the transmission of sunlight through the
equatorial facing sloped glass.
• Residential, appliance type units generally use glass about
0.65 to 0.9 meter wide with lengths ranging from two to
• A water depth of 1.5 to 2.5 cm is most common.
DEPTH OF WATER: THE SHALLOWER
THE DEPTH, THE BETTER.
• Note that solar heat can evaporate about 0.5 cm
of water on a clear day in summer.
• By setting the initial charge at about 1.5 cm
depth, virtually all of the salts remain in the
solution, and can be flushed out by the refilling
• Of course, if the basin is too shallow, it will dry
out and salts will be deposited, which is not
TWO GENERAL TYPES OF BASINS
• material that maintains its own shape and provides
the waterproof containment by itself / with the aid of a
surface material applied directly to it
• uses one set of materials (such as wood or brick) to
define the basin's shape; Into this is placed a second
material that easily conforms to the shape of the
structural materials and serves as a waterproof liner.
One alternative is ordinary aluminum coated
with silicone rubber. The durability of basins
made with this material increased into
the 10- to 15-year range. For the hundreds of
stills one company sold using this material,
the coating was all done by hand. With
production roll coating equipment, the basin's
durability could probably be increased even
Glazing Cover is a critical component of a solar
still. It is mounted above the basin and must be
able to transmit light in the visible spectrum yet
keep the heat generated by that light from escaping
Exposure to ultraviolet radiation requires a material
that can withstand the degradation effects or that is
inexpensive enough to be replaced periodically.
Since it may encounter temperatures
approaching 95 [degrees] Celsius , it must
also be able to support its weight at those
temperatures and not undergo excessive
expansion, which could destroy the
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