SEAWATER ROTATES INTO FRESHWATER BY SOLAR ENERGY – A NEW LOW-COST TECHNOLOGY.

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FAO evaluate, by 2025 nearly 2 billion people may not have enough drinking water to satisfy their daily needs. One of the possible solutions to this situation is desalination, namely treating seawater to make it drinkable. Also, removing salt from seawater requires 10 to 1000 times more energy than traditional methods of freshwater supply, namely pumping water from rivers or wells.

Motivated by this challenge, a group of engineers from the Department of Energy of Politecnico di Torino has devised a new prototype to desalinate seawater in a sustainable and low-cost way, using solar energy more efficiently. Contract to preceding solutions, the developed technology is, in fact, able to dual the amount of water produced at given solar power, and it may be subject to further efficiency improvement in the close future. The team of young researchers who recently printed these results in the journal Nature Sustainability is composed of Eliodoro Chiavazzo, Matteo Morciano, Francesca Viglino, Matteo Fasano and Pietro Asinari (Multi-Scale Modeling Lab).

Matteo Fasano and Matteo Morciano explain. The working concept of the advanced technology is elementary: “Inspired by plants, which transport water from roots to leaves by capillarity and transpiration, our floating device can collect seawater using low-cost porous information, thus avoiding the use of expensive and cumbersome pumps. The cool seawater is also heated up by solar energy, which assists the separation of salt from the evaporating water. This process can be facilitated by layers inserted between contaminated and drinking water to avoid their mixing, closely to some plants able to outlive in marine environments (for instance the mangroves).”

Conventional ‘active’ desalination technologies need high-cost mechanical or electrical parts (like pumps and/or control systems) and require specialized technicians for installation and maintenance, the desalination approach proposed by the team at Politecnico di Torino is based on spontaneous processes occurring without the aid of ancillary machinery and can, So, be referred to as ‘passive’ technology. This makes the device inherently inexpensive and easy to install and repair. The latter characteristics are especially good-looking  in coastal around that are suffering from a chronic lack of drinking water and not yet reached by centralized infrastructures and investments.

Up till now, a well-known disadvantage of ‘passive’ technologies for desalination has been the low energy efficiency as compared to ‘active’ ones. Researchers at Politecnico di Torino have faced this difficult with creativity: “While previous studies focused on how to maximize the solar energy absorption, we have shifted the attention to new efficient management of the absorbed solar thermal energy. By the way, we have been able to reach record values of productivity up to 20 liters per day of drinking water per square meter exposed to the Sun. The reason behind the showing increase is the ‘recycling’ of solar heat in several cascade evaporation processes, in line with the philosophy of ‘doing extra with less.’ Technologies based on this process are typically also called ‘multi-effect,’ and we provide the first evidence that this method can be very effective for ‘passive’ desalination technologies as well.”

After building the prototype for more than 2years and testing it directly in the Ligurian sea (Varazze, Italy), the Politecnico’s engineers claim that this technology could have an impact in isolated coastal locations with little drinking water but much solar energy, especially in developing countries. Also, the technique is particularly suitable for providing safe and low-cost drinking water in emergency conditions, for instance in areas hit by floods or tsunamis and left isolated for days or per weeks from electricity network and aqueduct. M application envisioned for this technology are floating gardens for food production, an exciting choice mainly in overpopulated areas. The researcher, who continue to work on this matter within the Clean Water Center at Politecnico di Torino, is now looking for possible industrial partners to make the prototype additional durable, scalable and versatile. For instance, engineered versions of the device could be employed in coastal areas where over-exploitation of groundwater causes the intrusion of saline water into freshwater aquifers (a particularly severe situation in some regions of Southern Italy) or could treat waters polluted by industrial or mining plants.

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