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Simple desalination tech needs just a dash of heat

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Simple desalination tech needs just a dash of heat

New way to get fresh water from the oceans doesn’t rely on electricity, high temperatures or costly materials. Just some sunlight will do.
May 23, 2024

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Fueled by climate change and a growing population, water shortages are a growing crisis around the world. About 300 million people in over 150 countries rely on desalination, the removal of salt from ocean water for fresh water, according to the World Bank. But current desalination technologies consume a lot of energy and are prohibitively expensive for poorer communities most facing water scarcity.

Researchers in Australia have now devised a simple new desalination method that does not need high pressure, high temperature or complex materials such as membranes. And it only uses only a fifth of the energy of conventional methods.

Modern desalination plants, most of them found in parched but wealthy Middle Eastern countries such as Saudi Arabia and the UAE, rely on two main technologies. One, called reverse osmosis, sieves salt out of seawater using membranes that tend to foul and are expensive. The other method, thermal desalination, involves heating water to evaporate it and then condense it in pure form.

People have used heat to purify water for thousands of years, says Juan F. Torres of the Australian National University, but that is energy intensive on large scale. So he and his colleagues invented a desalination technique they call thermodiffusive desalination (TDD) that does not require high heat or complex membranes.

 

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Thermodiffusion is a phenomenon in which salt moves to the colder side of a smooth temperature change from hot to cold. The water stays liquid the entire time. “TDD is the first thermal desalination method that does not need phase change, such as evaporation or freezing,” Torres says. “This has tremendous potential for energy savings, as changing the phase of water is an energy-intensive process. Another advantage of TDD is its simplicity.”

In the new method, reported in the journal Nature Communications, the researchers made a 0.5 meter-long, 1 mm-high channel. They placed it between a top plate that was heated to over 60°C and a bottom plate that is cooled to 20°C. When they pushed seawater through the channel, salt moved towards the cool bottom, so less salty water emerged from the top part of the channel.

The researchers repeatedly pass the low-salinity water through the channel. After repeated cycles, the salinity of seawater goes can be brought down from 30,000 parts per million (ppm) to between 1,000–5,000 ppm. This salinity is “above the recommended salinity for potable water, but well within the salinity tolerance of many types of crops,” Torres says. “Hence, TDD is suitable for agriculture, which consumes about 70% of our worldwide freshwater needs.”

The method does require a lot of heat, but it is low-grade heat that can come from sunlight or waste heat from industrial processes, he says. The method can, in principle, be scaled up, he adds. “The key is to improve our manufacturing techniques to build low-cost, large-scale multi-channel devices where TDD takes place. We are now building such devices in my research group. Cost could be reduced with 3D printing techniques, suitable for fabrication in remote areas.”

Source: Shuqi Xu et al. Thermodiffusive desalination, Nat. Commun. 2024

Image: Agastya drinks the-ocean by Ramanarayanadatta astri

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