1.1
History
The earliest on set of solar energy use
to desalinate water is widely accredited to Aristotle during the fourth century
B.C.E.Earlier attributions reference the Bible & Moses’ use of a piece of
wood to remove the “bitterness” from water (Exodus 15:25, English Standard
Version). The first documented account of solar distillation use for
desalination was by Giovani Batista Della Porta in 1958.However, no solar
distillation publication of any repute leaves out the Father of solar
distillation, Carlos Wilson, the creator of the first modern sun-powered
desalination plant, built in Las Salinas (The Salts), Chile in 1872.
This
desalination plant, "can be considered to be the first industrial
installation for exploitation of solar energy." The Las Salinas plant was
envisioned to take advantage of the nearby saltpeter mining effluent to supply
the miners and their families freshwater. The facility was quite large for its
time and now.
The
plant was constructed of wood and timber framework covered with one sheet of
glass. It consisted of 64 bays having a total surface area of 4450 m2 and a
total land surface area of 7896 m2. It produced 22.70 m3 of fresh water per
day. The plant was in operation for about 40 years until the mines were
exhausted."
The
inner surface of the base known as basin liner is blackened to efficiently
absorb the solar radiation incident on it. There is a provision to collect
distillate output at lower ends of top cover. The brackish or saline water is
fed inside the basin for purification using solar energy. The performance of a
conventional solar distillation system can be predicted by various methods such
as, computer simulation (Cooper, 1969), thermic circuit and the diagrams
(Frick, 1970), periodic and transient analysis (Cooper, 1970; Nayak et al.,
1980; Sodhaet al., 1980; Tiwari and BapeswarRao, 1983; El-Sayed,1983; Tiwari
and Madhuri, 1987), iteration methods(Toure and Meukam, 1997), numerical
methods (Lofetal., 1961; Sartori, 1987; Sharma and Mullick, 1991).
In
most of the above-mentioned methods, the basic internal heat and mass transfer
relations, given by Dunkle (1961), has been used.
1.2
Defination
A device used for
converting brackish water into potable water using solar energy is called solar
still.
1.3
Why
required solar still
Water is one of the prime elements
responsible for life on earth. It covers three-fourths of the surface of the earth. However, most of the earth’s water is found in oceans as salt water,
contains too much of salt, cannot be used for drinking, growing crops or most
industrial uses. The remaining earth’s water supply is fresh water. Most of
this is locked up in glaciers and ice caps, mainly at the north and south
poles. If the polar ice caps were to melt, the sea level would rise and would
flood much of the present land surfaces in the world. The rest of the world’s
supply of fresh water is found in water bodies such as rivers, streams, lakes,
ponds and in the underground. So there is an important need for clean and pure
drinking water. In many coastal areas where sea water is abundant but potable
water is not available, solar water distillation is one of the many processes
that can be used for purification as well as desalination. Solar still is the
widely used solar desalination device.
1.4
How
it's work
The basin of the
still is filled with brackish or sea water, the incident solar radiation is
transmitted through the glass cover and is absorbed as heat by a black surface
(basin) which contains the brackish water.Thus the water is heated and gives off water vapor. The vapor condenses
on the glass cover, which is at a lower temperature because it is in contact
with the ambient air.
If the glass cover is tilted, the formed
condensation drops will start running down the cover by gravitational forces,
and may then be collected in a channel to go out the side of the still to a
storage tank.
Fig 1: Solar Still
1.5
Natural and Distillation
In this method
brackish or salient water is evaporated using thermal energy and the resulting
steam is collected and condensed as final product.
Distillation process
is considered to be one of the simplest and widely adopted techniques for converting sea water in to fresh water.
More than 90% of the worldwide installed sea water desalination capacity is
based on distillation process. One of the main advantages of the distillation process is that it required heat only up to120
C which can be supplied from solar energy or other cheap fuels, while in
reverse osmosis, vapour compression, and electro dialysis processes, some mechanical or electrical energy in used.
Solar energy is
abounded is, never lasting, available on site and population free energy.
however the cost of its collection
and utilization become high because it is diffuses, of low intensity, and
intermitted and therefore, required some kind thermal energy storage but for
application like distillation of brackish water, the intermittent nature of
solar energy will not limit its use and distilled water will be produced, as
and when solar energy available. Because of the simplicity of apparatus design,
requirement of fresh water, and free availability of thermal energy, work in
the field of solar distillation is in progress for more than one hundred year.
1.6
Basic
requirements of a good bottom reflector type solar still
·
Be easily assembled in the field.
·
Be constructed with locally available materials.
·
Be light weight for ease of handling and transportation.
·
Have an effective life of 10 to 20 Yrs.
·
No requirement of any external power sources.
·
Can also serve as a rainfall catchment surface.
·
Material with good reflectivity.
·
Materials used should not contaminate the distillate.
·
Meet standard civil and structural engineering standards, and,
·
Should be low in cost.
1.7
Materials for solar stills
Glazing: Should
have high transmittance for solar radiation, opaque to thermal radiation,
resistance to abrasion, long life, low cost, high wet ability for water,
lightweight, easy to handle and apply, and universal availability. Materials
used are: glass or treated plastic.
Liner:
Should absorb more solar
radiation, should be durable, should be water tight, easily cleanable, low
cost, and should be able to withstand temperature around 100 Deg C. Materials
used are: asphalt matt, black butyl rubber, black polyethylene etc.
Sealant: Should remain resilient at very low
temperatures, low cost, durable and easily applicable. Materials used are:
putty, tars, tapes silicon, and sealant.
Basin
tray: Should have long
life, high resistance to corrosion and low cost. Materials used are: wood,
galvanized iron, steel, aluminum, asbestos cement, masonry bricks, concrete,
etc.
Condensate
channel: Materials used
are: aluminum galvanized iron, concrete, plastic material, etc.
Reflector:
Aluminum foil, Mirror, High grade PVC sun reflector
fabric, Grey polyester, Customized Polycarbonate etc.
1.8
Components
of external bottom reflector type solar still
·
Basin
·
Black
Liner
·
Transparent
Cover
·
Condensate
Channel
·
Sealant
·
Reflector
·
Insulation
·
Supply
and Delivery System
1.9 Solar still output depends on many
parameters
A. Climatic Parameters
·
Solar
Radiation
·
Ambient
Temperature
·
Wind
Speed
·
Outside
Humidity
·
Sky
Conditions
B. Design Parameters
·
Single
Slope or double Slope
·
Glazing
material
·
Water
depth in basin
·
Bottom
insulation
·
Orientation
of still
·
Inclination
of glazing
·
Spacing
between water and glazing
·
Type of
solar still
C. Operational Parameters
·
Water
Depth
·
Preheating
of water
·
Coloring
of water
·
Salinity
of water
·
Rate of
alge growth
·
Input
water supply arrangement (continuously or in batches)
1.10
Classification of Solar Distillation Systems
v Passive
solar still :
The passive solar still systems are conventional solar
still systems which use solar energy as the only source of thermal energy.Passive solar still operates in low
temperature and the daily productivity is comparatively low.
v Active
solar still :
The active solar stills, extra thermal energy is given
to the passive solar still for faster evaporation.to increase the evaporation rate in an active mode the extra
thermal energy is fed into the basin. To increase the productivity of solar
still, the various active methods are being carried out by many researchers.
Most of the works were based on the flat plate collector and concentrating
collector.
1.11
Method of Purification of Water
1.11.1
Basin type solar still
Such solar stills have been operated for farm and
community use in several countries. It consists of a blackened basin containing
saline water at a shallow depth, over which is a transparent air tight cover
that encloses completely the space above the basin. It has a roof-like shape.
The cover which is usually glass, may be
of plastic is sloped towards a collection trough. Solar radiation passes through the cover and is absorbed and
converted into heat in the black surface. Impure water in the basin or tray is
heated and the vapor produced is condensed to purified water on the cooler
interior of the roof. The transparent roof material, (mainly glass) transmits
nearly all radiation falling on it and absorbs very little; hence it remains
cool enough to condense the water vapor. The condensed water flows down the
sloping roof and is collected in troughs at the bottom.
v Types of the basin type solar still
systems
A.
single-slope
B.
double-basin solar still
C. single-slop
triple-basin solar still
D.
pyramid-shaped solar still
E.
conventional solar still
F.
single-basin solar still
1.12 Vertical solar stills with a
flat plate solar collector :
The distillation unit consists of parallel vertical
plates. The first plate is insulated on its front side and the last plate is
exposed to ambient. Each plate in the enclosure is covered with wetted cloth on
one side. The cloth is extended into a feed through along the upper edge of each
plate. Feed water in the through is then drawn onto the plate surface by
capillary. Excess water moves down the plate and is conducted out of the still.
The last plate is cooled by air or water. The authors found that, the distillation output increases slightly when
the plate number is over 5, and it increased by about 34% and 15% when the
evaporating plate numbers are 1 and 6, respectively.
1.13 Solar still integrated with
mini solar pond :
Solar pond is an artificially constructed pond in
which significant temperature rises are caused to occur in the lower regions by
preventing convection. Solar ponds are used for collection and storage of solar
energy and it is used for various thermal applications like green-house
heating, process heat in dairy plants, power production and desalination. The
results show that, average increase in productivity, when a pond is integrated
with a still is 27.6% and when pond and sponge are integrated with a still is 57.8%.
Industrial effluent was used as feed for fin type single basin solar still and
stepped solar still. A mini solar pond connected to the stills to enhance the
productivity and tested individually. The results show that, maximum
productivity 100% was obtained when the fin solar still was integrated with
pebble and sponge. The productivity increases with increase in solar intensity
and water glass temperature difference and decreases with increase in wind
velocity. Pebbles, baffle plates, fins and sponges are used in the stepped solar
still for productivity augmentation. Their finding shows that, maximum
productivity of 78% occurred when fins and sponges were used in the stepped
solar still and also found that the productivity during night also improved
when pebbles were used in the solar stills.
There are three main types of solar stills: the box
solar still, the cone solar still and the pit solar still.
A.
Box
Solar Still
Fig
2: Box Solar Stills
The box shaped solar still is fairly complex
compared to the other solar stills made for obtaining pure emergency drinking
water. This type of still is usually created with a box that has a slanted
glass or plastic top with an insulated bottom. A set of tubes allow in impure
water and let out over flow and pure distilled water. Maintaining a tight seal
on a box shaped solar still is essential. Clearly, this is more of a home
project for a green minded person, but having a solid understanding of how all
solar stills work might come in handy when trying to construct one from scratch
for emergency drinking water.
B.
Cone
Solar Still
Fig 3: Cone Solar
Still
A cone shaped solar
still is a method of treating impure emergency drinking water rather than
gathering it from the atmosphere. A cone is constructed out of plastic designed
so that impure water in the bottom evaporates and is captured in a makeshift
reservoir as it runs down the side, relieved of impurities. The Water cone is
commercial solar still that allows you to distill water naturally at home or on
the go with having to construct a solar still of your own. Designed as a water
purification system that can generate potable water out of even brackish sea
water, the Water cone is also a candidate for providing clean drinking water in
developing and failed nations.
C.
Pit
Solar Still
Pit shaped solar
stills are the simplest solar still to make and require little equipment or
technical know-how. Simply dig a pit in the earth approximately two feet deep
and place a Number 10 sized can at the bottom. If available, set a drinking
tube in the bottom of the can leading out of the hole. Then secure a sheet of
plastic over the hole a place a stone in the sheet so that it indents the sheet
to just above the can's lid. So long as this solar still receives plenty of
sun, it will provide a pint or more of emergency drinking water.
Fig 4: Pit Solar
Still
