Saturday, April 21, 2007

NIOT's OTEC-based Desalination Plant

Chennai-based National Institute of Ocean Technology (NIOT) has achieved a world's first in sustainable technology by building a floating water desalination plant. But what's so great about putting a desalination plant on a barge? The uniqueness is in the detail of the technology used.

The Technology

"The plant is mounted on a 65-metre-long by 16-metre-thick barge. The ocean's surface water is boiled inside a vacuum container. The vapour created in the flash boil process is condensed through a refrigeration process with the help of deep-sea water collected from nearly 600 metres below the surface of the sea." >>Link

Thus this plant benefits from NIOT's cutting edge research and plans on OTEC, a fledgling clean energy technology which has huge potential. OTEC (Ocean Thermal Energy Conversion) is a method to use the energy difference between the surface of the ocean, which is exposed to the sun, and the water at lower levels, which not being exposed to the sun is much cooler.

The temperature of the water 600m below the surface was one third of the temperature at the surface, but bringing it to the surface presented the biggest challenge of the project. The water was brought up using one meter thick HDPE pipes, which come in 12m lengths, and when assembled the 600m stretch weighed 100 tonnes. Further the salinity of the seawater would make the pipes float, necessitating the attachment of heavy weights. The salinity however is useful in another way - the clean water is filled into water bags each capable of holding 200,000 liters, which were then easily towed to the shore since clean water floats on saline water.

Cost Effective Technology?

The water is essentially distilled which is the best way to clean water, though normally not the most cost efficient. Thus, "
The total dissolvable solid proportion in this water is only 10 particles per million (PPM) as against a national standard of 2,000 PPM". But the production cost works out to about 6 paise (0.14 cents) per liter. This might look reasonable when considering the price of bottled water, but not if it is to be considered a reliable supply for industrial and municipal supply. Reverse osmosis plants in other parts of the world provide water at just half of that cost.

Looking ahead

Firstly this is a very small plant by international standards. For example the world's biggest desalination plant at Jebel Ali in the UAE produces over 800 million liters a day, whereas the NIOT plant produces just 1 million. As they work on increasing capacity, the cost of production should decrease too.

Secondly, NIOT has proposed a tie-up with the Tamil Nadu Electricity Board to explore a rather unique application for its Low Temperature Thermal Desalination - by using the heat from onshore thermal power plants.
If this works, the cost of production would work out to between 3-4 paise.

Water from TNEB's thermal power plants has to be cooled down before it can be released into the sea. NIOT thinks that the already warm water would be a good starting point for it to continue into the desalination process. That would be a double punch for sustainable technologies - a cost-efficient way to handle thermal pollution and produce clean water.

If NIOT can polish off this technology soon, then apart from current and upcoming thermal power plants, it could see big application in the soon to come nuclear power plants too.