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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.