A new method for desalinating water without electricity looks promising

Researchers from the Australian National University have proposed a new method for desalinating water that avoids many of the unwanted side effects of traditional desalination techniques and reduces the energy required by about 80%.

Fresh water has become a critical issue in many regions around the world, and some already rely on desalination, that is, extracting fresh water from ocean water.

Humans now use about a third of all freshwater discharges, and it is expected to reach half by mid-century, especially in areas such as Israel, Mexico City, India, Southern California and many others. Water availability will arguably be the most serious crisis facing humanity this century, and will get worse as climate change melts snow, increases evaporation from surface water, changes patterns and amounts of precipitation, and increases the amount of water in the atmosphere.

A 2018 World Bank report found that about 300 million people in more than 150 countries around the world depend on desalination at an energy cost of 3 kilowatt-hours per cubic meter (kWh/m).³), a 10-fold decrease since 1970. However, desalination accounts for a quarter of the energy used to supply water, requiring about 100 billion kilowatt-hours of energy in 2018.

Desalination technologies to date are either material-based methods, such as reverse osmosis, which uses high pressure to separate molecular species in water, or thermal methods, such as solar-based evaporation or cryogenic desalination.

But these systems can receive and kill smaller marine life through heat, stress or chemicals, or pin larger marine life to intake screens. They leave behind brine, which is water with a very high salinity of more than 30% (ocean water contains about 30 to 35 grams of salt per kilogram of water, mostly sodium chloride) that is drained back into the ocean, where it sinks to the bottom. . It harms ecosystems.

These conventional desalination facilities require expensive materials that must be cleaned and maintained, as they are susceptible to membrane fouling, corrosion and degradation, as well as energy requirements ranging from 3 to 7 kWh/m³ For reverse osmosis and up to 100 kWh/m³ for other methods. They are suitable for urban and industrial environments, but are too large and expensive for developing countries or rural and remote areas.

The new method published in Nature Communications They do not rely on electricity, but can use low-level heat from sunlight, or heat that is a byproduct of an industrial process. It uses thermal diffusion, a phenomenon in which salt moves to the cooler side of a smooth temperature change (temperature gradient) from hot to cold. Water remains in the liquid phase all the time.

Led by Ph.D. The research team, candidate Shoki Xu, forced seawater through a narrow channel beneath an upper plate that was heated to more than 60 degrees Celsius and above a lower plate that was cooled to 20 degrees Celsius. Both temperature values ​​for panels can be extracted from the environment, as discussed above. Low-salinity water emerges from the water in the upper part of the canal, and high-salinity water comes out of the lower part of the canal.

In the group’s experimental setup, the channel was half a meter long, one millimeter high, with flow rates between 1 and 16 milliliters per minute.

After one pass through the canal, the cold, salty water is removed, and the warmer, less salty water is directed back through the canal. Each pass saw a 3% reduction in salinity. After repeated cycles, the salinity of seawater can be reduced from 30,000 ppm to less than 500 ppm.

“Our dream is to enable a paradigm shift in desalination technology, based on methods that can be driven by the low temperatures in our surrounding environment,” said Juan Felipe Torres, a professor at the Australian National University and lead researcher on this research.

“To our knowledge, thermal diffusion desalination is the first thermal water desalination method that does not require phase change. It operates entirely in the liquid phase and, most importantly, does not require membranes or other types of ion absorbing materials. Materials for water purification.”

“Thermal diffusion desalination is dirt-free,” Torres continued, “which could be a game-changer in the way we do large-scale desalination.” Agriculture, which uses about 69% of the world’s fresh water, needs desalinated water. About 95% of its basic value.

Since proving the concept, the group is now building a larger multi-channel seawater desalination device for use on the severely drought-stricken southwestern Pacific island of Tonga. There, the device will be powered by a solar panel the size of a human face.

Torres believes that diffusion desalination is vital for developing countries, many of which are suffering the worst of climate change, by decentralizing the desalination process and achieving water security in these regions.

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