In spite of advancements on various frontiers, many parts of the world are still suffering from drinking water shortage. It is one of the most pressing problems today, and research is being conducted all over the world on reliable ways to supply clean water to dry areas.
Desalination is an effective way to get clean water; it involves removing salts and other contaminants from saline water to produce potable water. Conventional methods of desalination involves a large set-up, consumes a lot of energy that is derived from fossil fuels, and is expensive. Hence it is viable only for large-scale production. It is not suitable for isolated areas with small pockets of people, as is characteristic of India, particularly rural areas.
An alternative method is to use a 'solar still' for desalination. Solar stills are small and simple, have no moving parts, have low operating costs, and can be made using materials that are locally available. An added advantage is that solar energy is free, clean and renewable.
Dr Monto Mani, Associate Professor, Centre for Sustainable Technologies (CST) IISc, says, "Given impending scenarios of climate change and possible dry spells, solar-stills can go a long way in alleviating drinking water requirements, particularly in coastal stretches where ground water can be saline."
A solar still works on the same principle as rainfall. Evaporation is induced by solar energy. The vapours are made to condense, and the resulting droplets are collected. This collected water is safe. The drawback of a solar still is that the yield is low, and solar energy is not always regularly available.
Solar stills are currently just 30 pc efficient. Research is on to improve productivity and efficiency of solar stills. But modifications to improve efficiency leads to the system becoming very complex, impractical, bulky and expensive for domestic applications.
Several researchers all over the world have experimented with different construction materials, changes in the design, alternate shapes, and have tried hybrid solar stills. They have also studied the effects of various weather parameters, like wind velocity, temperature and humidity, etc on solar still performance.
But not much research has gone into the effects of a sealed solar still as opposed to an unsealed one, which is a crucial condition. To study this effect, researchers at CST designed and commissioned a Stepped Solar Still, with a low internal volume, to observe its operation under Sealed and Unsealed Conditions.
In a stepped solar still, there are a number of semi-circular pipe sections, attached to each other, in such a way that they form a slope. It is like a flight of stairs, except that each stair tread is a semi circular cross-section of a pipe. The untreated water is let into the top most pipe section, and the treated water is collected from the bottom most cross-section. The whole setup is covered by glass.
In the Sealed condition, the inlet valve was sealed tightly using an air-tight cap. In the Unsealed condition, the inlet was just covered with a filter cloth to prevent contamination.
Experiments showed that the productivity and efficiency under unsealed conditions was better than when it was sealed. This could be due to higher humidity, more convective currents inside the sealed system, and lower pressure, leading to faster condensation. But further research is necessary to confirm these findings.
According to Prof Mani, solar still technology is one of the most effective technologies yielding safe-water, for diverse feed water (ranging from urban grey water to sea water). He says, "The beauty of the technology is that, we can be reasonably sure that the yield water is safe for consumption, without actually testing the level of contamination of the inlet (feed) water."
About the paper:
International Journal of Low-Carbon Technologies, Volume 10, Issue 4, Pp. 354-364.
"Study into solar-still performance under sealed and unsealed conditions"
About the authors:
Rohit Pillai, A. T. Libin and M. Mani
Centre for Sustainable Technologies, Indian Institute of Science, Bangalore, Karnataka