Water desalination without emissions!

There are plenty of suggestions for how to beat the drought, and desalination is one that is frequently mentioned.

The problem with desalination has always been that it's expensive - plants cost a lot to build, and they're not cheap to run, either. But RMIT scientists say they've developed a solar-thermal desalination plant that turns salty groundwater into the good stuff, and produces no greenhouse emissions in the process. It'll be on display at Pyramid Hill.

Dr John Andrews, from the School of Aerospace, Mechanical and Manufacturing Engineering, is the Project Manager, and he says it's for real.

"It's certainly technically possible to use solar heat to desalinate salty groundwater or seawater. The groundwater at Pyramid Salt, where we're doing this demonstration, has a salinity quite near that of seawater.

"The technology is proven; we'll be showing that it can be done today."

So how does it work?

"The basic process is using solar heat to heat up the salty water. We then introduce it into a chamber where the air has been taken out, where it will boil at a temperature less than 100 degrees centigrade. We're using solar heat - at its hottest, around 80 degrees centigrade.

"The salty water boils, and just the water comes off as vapour. That's then condensed by passing over cooling coils, and then we collect the fresh water that comes out of that condensing process."

That leaves a concentrated salty solution behind, which a salt company can then use to produce salt.

Other processes have been used for producing fresh water out of salty; Perth's plant uses reverse osmosis, and other thermal processes include multi-stage flash or multiple-effect evaporation.

"In Perth, they're using reverse osmosis and that needs electricity," Dr Andrews says.

"They're overcoming the greenhouse problem - which is always the basic problem you've got to overcome - by having a wind farm that inputs electricity into the grid to an equivalent amount to what's used by the plant that's set up in the Kwinana complex.

"We're using solar heat, not electricity, to see if we can get a cheaper option that has no greenhouse emissions."

The salt producer hosting the demonstration has a history of collaboration with RMIT for solar power.

"We built a solar pond there in early 2000," Dr Andrews says.

"That hasn't produced electricity as yet. It's produced heat that was used in their commercial salt production process. The heat saves electricity that would otherwise have come from the grid, but didn't produce electricity directly."

So how competitive is solar-thermal desalination?

"We're paying, typically, a dollar per 1000 litres fresh water that we consume in our houses or factories currently. It's interesting to compare that with bottled water, which is about a dollar a litre, not 1000 litres.

"The indications are that we're in a factor of two or three. If this sort of technology is scaled up, we can probably get down to as low as two to three dollars per thousand litres. But that will need a lot of work to take the relatively small-scale unit that we're demonstrating today to a larger-scale one and a commercial proposition.

"We have been getting very cheap water historically. Inevitably, as water gets more scarce - or as fresh water gets scarce, because salty water isn't scarce, 97.5 per cent of the water on Earth is salty water - but as fresh water gets scarcer, then inevitably the price is going to go up, which will make alternatives more viable.

"I should say, the first thing to do is water conservation - eliminating waste, using water as efficiently as possible. Then we turn to the new supply options, of which desalination is one possibility."

A commercially scaled plant such as Perth's reverse osmosis plant generates up to 150 megalitres a day of fresh, potable water.

"The one we're demonstrating is a very much smaller scale system. We're looking at about 500 litres a day, but there's no reason at all why those sort of plants can't be scaled up. They're eminently scalable, once you've got the basic principles there.

"In the Middle East, there are [reverse osmosis] plants there that are working with around 450 megalitres a day of production of fresh water from seawater. But those plants are using the waste heat in the form of steam from oil-fired power stations. The real challenge in desalination...we've got to get the energy from renewable sources that have zero emissions. Otherwise we make the global warming worse in order to solve the water crisis, and there's not much point [to that]."

Wednesday, 6 December 2006

Reporter: (online) Terri-Anne Kingsley

Presenter: Jonathan Ridnell

Researcher: Jo Printz

http://www.abc.net.au/centralvic/stories/s1805531.htm