Salt Shakers

Historically, salt has been removed from water by boiling it to leave salt deposits, then condensing the resulting steam (vapour) back to water (liquid). However, this method is very energy-intensive.

Over the past 70 years, salt has been removed from water by filtration, which is a lot simpler and more energy-efficient. The salt water is filtered through membranes to remove the salt ions via a reverse osmosis process.

Now, an alternative water desalination and irrigation system is under development.

“We are researching a sustainable and economical freshwater management system based on clean thermal energy,” says Dr Abhijit Date from the School of Aerospace, Mechanical and Manufacturing Engineering at RMIT.

The system uses a special thermal water pump – developed at RMIT and the University of Pune, India – which is driven by low-temperature thermal energy rather than grid electricity.

The freshwater management system could be used to greatly benefit coastal areas of India and salt-affected farming land in Australia.

“There are many poor coastal communities in India where access to fresh water is an issue, but they cannot afford to use standard power-hungry desalination and irrigation systems,” Date says.

“The desalination and irrigation system we are developing is both cheap to run and sustainable, producing no greenhouse emissions.

“Not only could this system help many coastal communities, it could also enable saline groundwater to be turned into fresh water and used for agricultural irrigation – helping farmers in Australia and globally.”

The system runs on clean power sources – such as solar thermal, geothermal or waste heat – and generates both fresh water and water pumping power, using thermal energy at temperatures below 100C.

It works by boiling a refrigerant at constant temperature and using the pressurised refrigerant vapour to power a piston and pump water out. To suck water in, the vapour is cooled down, reducing the volume, pushing the piston in the opposite direction.

Date and his team have already built a lab-scale prototype of the thermal water pump system, with early tests showing the system can produce 1000 litres of fresh water from 2000 litres of saline feed water with a salt concentration between 5000 and 15,000 grams per cubic metre.

“We have a good working prototype, so the next phase is to optimise the technology for use in the field,” he says.

Story: Deborah Sippitts

Photo: Carla Gottgens

This story was first published in RMIT's Making Connections magazine.