A team of Indian engineers has designed a prototype low-cost solar-heated water desalination unit that can produce about five litres of drinking water each day and is intended for use by rural households.
The desalination unit may be used to turn brackish groundwater fit for drinking at any place with abundant solar energy, the team of engineers, who are from the National Institute of Technology in Kurukshetra and an engineering college in Bangalore, have said.
The laboratory-scale desalination unit they have built and tested in Bangalore produces five litres of drinking water on a sunny day and costs less than Rs 7,000, the engineers said.
More drinking water may be extracted in regions with greater sunshine and if the glass surface collecting solar energy is increased. The engineers described their design in the journal Current Science, published by the Indian Academy of Sciences, last week.
“This is a start — we wanted to see whether this idea works,” said Praveen Hunashikatti, a Bangalore-based team member who had worked on the project while doing his MTech at NIT Kurukshetra. “It looks promising but needs to be improved.”
Over 70 per cent of India’s estimated 600,000 villages use groundwater as their main source of drinking water, drawing it through pumps or wells. But much of this groundwater is brackish and contaminated with metallic ion impurities, from fluorides and nitrates to arsenic.
A report from the Central Ground Water Board, released in 2010, had documented that salt levels in groundwater from over 60 per cent of India’s landmass were beyond human taste limits.
Desalination units based on the reverse osmosis technology have already been installed in some rural areas, but reverse osmosis requires a steady supply of electricity and also generates waste water.
There are also solar-heated desalination units that use parabolic dishes that automatically change their orientation as the sun’s position changes through the day.
“Parabolic dishes that track the sun and change their orientation are expensive and unlikely to be affordable by average rural households,” said Kambalipura R. Suresh, professor of civil engineering at the BMS College of Engineering, Bangalore, and another team member.
The prototype from the Kurukshetra-Bangalore team will need to be scaled up to match the efficacy of the solar-heated desalination units that use parabolic dishes.
Hunashikatti calls the prototype a “coupled system” — a glass collector and a set of long tubes with air evacuated from them to avoid loss of heat. The slope of the glass collector and the orientation of the evacuated tubes are tailored to the latitude of the location.
The desalination is based on conventional distillation —evaporation and condensation. The solar energy, Hunashikatti explained, heats the water, which causes layers of hot water to move upward, evaporate and condense on the glass from where it can be extracted.
“This coupled system is our alternative to automatic tracking — instead of chasing the sun, we orient the slope and the tubes to retain maximum heat,” Suresh told The Telegraph.
A larger glass slope will mean more solar radiation and increase the amount of water generated.
In laboratory tests, the prototype was able to reduce the levels of fluorides, chlorides, nitrates, calcium carbonate, magnesium carbonate, and calcium in samples of water to levels below the acceptable limits for drinking water.
“A scaled-up version will need to be tested at multiple locations for different groundwater and sunshine conditions,” said Basavaraju Prathima, an environmental engineer at the BMS College and a team member.
“This will be important to establish its economic viability.”