'Thermal desalination' to bolster global water security
To help combat unprecedented global water shortages, researchers at The Australian National University (ANU) have developed what they are calling the world’s first thermal desalination method, where water remains in the liquid phase throughout the entire process.
Current desalination technologies — where salt is filtered through a membrane — require large amounts of electric power and expensive materials that need to be serviced and maintained. The ANU method, which is described in the journal Nature Communications, is by contrast triggered by moderate heat generated directly from sunlight, or waste heat from machines like air conditioners or industrial processes.
The concept of thermodiffusive desalination was proposed by Dr Juan Felipe Torres, lead chief investigator on the new project, who said the phenomenon behind the technology was discovered in the 19th century yet has remained underutilised.
“We’re going back to the thermal desalination method but applying a principle that has never been used before, where the driving force and energy behind the process is heat,” said Torres, a world-leading mechanical and aerospace engineer.
“Thermodiffusion was a phenomenon first reported in detail in the 1850s by Swiss scientist Charles Soret, who experimented with a 30 cm water tube where one part of the water was colder and the other hotter.
“He discovered that the salt ions move slowly to the cold side.”
To test whether this effect can be used for water desalination, the researchers pushed sea water through a narrow channel heated from above to 60°C and cooled from below to 20°C.
“Diffusion was taking 53 days to reach a steady state with a 30 cm tube, which is much too long for our purposes and isn’t scalable,” Torres said.
“Our mission became to find a way to fast-track the diffusion process.”
The researchers found that adjusting the conditions for separation could significantly increase the speed of the diffusion process to just a couple of minutes. According to Torres, “The key was reducing the channel height from 30 cm to 1 mm and adding multiple channels.”
PhD student and first author Shuqi Xu said that once the salt had migrated to the cooler water, the device reprocessed the warmer, purified water through the channel while the cooler, saltier water was removed. “Each time the water passed through the channel,” she said, “its salinity was reduced by 3%.
“Our research shows that after repeated cycles, seawater salinity can be reduced from 30,000 parts per million to less than 500.”
The project has already seen a state-of-the-art, solar-driven desalination unit deployed to Tonga, to pilot its application for agriculture and drought mitigation strategies. With further testing, the researchers hope to produce the first commercial unit within eight years.
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