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Solar Stills
Use of solar energy for
water purifying system
The current boom in population
in urban areas and the increasing intensive irrigation requirements
for agriculture in rural regions have already resulted in considerable
drying up of wells and aquifers alike. The decreasing availability of
water has necessitated a concerted search for fresh sources of
drinking water for the population. The available water in most areas
in the country is brackish, saline or impure in other ways. Though the
State Governments are making efforts to meet the requirements for
water through public distribution systems there are inherent
limitations to this programme. This is where the MCRC principle of
augmentation of water availability restricted to individual/groups of
households becomes pertinent. This is easily applicable everywhere in
rural and urban India alike as the energy requirement for the
distillation units is met from solar radiation. India is gifted with
rich sunshine and therefore solar energy is not limiting here.
However, its diffused nature could affect the efficiency of
conventional solar stills, but MCRC has already come up with new
design solar stills through its R&D processes to yield 8 to 10 litres
of potable water/m2/day which is sufficient for a family.
MCRC’s involvement in
fabrication of solar stills started with the basic conical type named
“Thoyam” in 1979. The working mechanism of this type is that a pan of
impure water enclosed in a transparent cone-shaped enclosure traps
solar heat causing part of the heated water to evaporate, rise to the
top and condense on the cooler transparent cover. The pure water thus
formed drips through a channel to a reservoir. The yield from such a
simple set up is 3 to 4 litres/m2/day in bright sun light. This still
is inexpensive and can be fabricated using locally available
materials. Field trials in the eighties in villages along the East
Coast Road in the outskirts of Chennai where the need for pure and
hygienic water is acute have found these stills very useful. A change
in the transparent covering material from Mylar sheet to glass and
acrylic and the shape to pyramidal did not alter the yield. However,
two modifications of this type, designed to utilise maximum solar heat
energy, yield double. The quantity of water over that obtained from
the basic type.
Multiple effect still
This is specially designed to
exploit the enormous latent heat of vapourisation of water released
during condensation of water vapour which under normal conditions is
dissipated into the atmosphere in the conventional solar stills. This
is accomplished in a multi stage system which re-uses this heat for
distilling more water. But successive stages yield lesser quantities
of distilled water. However, re-use of energy several times results in
higher yield of distilled water.
Reflector still
Here the basal pan having a
black outer surface and impure water inside heated by focussing the
reflected light on it using a concave or semilunar reflector. This
results in greater evaporation of the water inside. The yield from
this basic type with a good reflector system is again 8 to 10 litres/m2/day.
A combination of these two
types i.e. a multiple effect still with an appropriate reflector
arrangement is currently being considered as a very effective water
purification system with wide applications in both urban and rural
areas. Steam-Cured Mud Block (SMB)
MCRC has adopted and developed
the SMB technology for further development and extension activities.
Extensive experimentation at Indian Institute of Sciences, Bangalore
has revealed that lime-clay reaction can be accelerated by the process
of steam curing at 80-90°C (atmosphere pressure). This property is the
basis for the technology of producing steam cured mud blocks from
inexpensive soils like black cotton soil and high clay soil. Major raw
materials are fly-ash, sand, lime, clay/soil with little cement.
The training Programme was
conducted for 25 participants consist of NGO’s, Entrepreneurs and
local masons who are in the civil related areas. For demonstration
units nearly 35,000 steam mud blocks were produced and as a model
building, a training centre with dormentry and auditorium were
constructed.
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