open refrigerator fridge

Cooling technique uses just salt and water

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A device capable of generating a cooling effect without using electricity could help to lower energy usage in hot climates, and provide a greener alternative to refrigeration.

The system exploits the evaporation of liquid, as most cooling techniques already do. However, the research group, which is based at the Politecnico di Torino in Italy, have been using simple water and salt instead of chemicals which could be environmentally destructive.

The environmental impact of the new device is also reduced because it is based on passive phenomena such as evaporation, instead of pumps and compressors which require energy and maintenance.

“Pure water is in contact with an impermeable membrane that keeps separated from a highly concentrated salty solution,” said Matteo Alberghini. “The membrane can be imagined as a porous sieve with pore size in the order of one millionth of a meter. Owing to its water-repellent properties, our membrane liquid water does not pass through the membrane, whereas its vapour does.”

“In this way, the fresh and salt water do not mix, while a constant water vapour flux occurs from one end of the membrane to the other. As a result, pure water gets cooled, with this effect being further amplified thanks to the presence of different evaporation stages.”

He added that the salty water concentration decreases over time, reducing the cooling effect, although this difference in salinity can be continuously restored using solar energy.

The device has a modular design consisting of cooling units, each several centimetres thick, which can be stacked in series to increase the cooling effect. It is possible to finely tune the cooling according to individual needs, with the team hoping they can reach the cooling capacity needed for domestic use.

Water and salt do not need pumps or other auxiliaries to be transported within the device as it “moves” spontaneously, thanks to capillary effects of some components which are capable of absorbing and transporting water against gravity.

“Our passive prototype, based instead on evaporative cooling between two aqueous solutions with different salinities, could [create] a useful effect independent of external humidity,” the researchers said. “Moreover, we could obtain an even higher cooling capacity in the future by increasing the concentration of the saline solution or by resorting to a more sophisticated modular design of the device.”

They said the device would be cheap to make, and ideal for use in areas where access to electricity may be limited.

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