Passive design uses solar power to convert seawater into fresh
The working principle of the proposed technology is relatively simple and can be made sustainably. While conventional ‘active’ desalination technologies need complex mechanical or electrical components such as pumps and/or control systems and require specialized technicians for installation and maintenance, the desalination approach proposed by the team is based on spontaneous processes occurring without the aid of ancillary machinery.
“Our floating device is able to collect seawater using a low-cost porous material, thus avoiding the use of expensive and cumbersome pumps. The collected seawater is then heated up by solar energy, which sustains the separation of salt from the evaporating water. This process can be facilitated by membranes inserted between contaminated and drinking water to avoid their mixing, similarly to some plants able to survive in marine environments such as mangroves,” said Matteo Fasano and Matteo Morciano (above, testing out the system).
Up to now, such passive technologies for desalination has been the low energy efficiency as compared to active ones. Each unit stage for complete distillation is made of two hydrophilic layers separated by a hydrophobic microporous membrane, with no other mechanical support. Under realistic conditions, the engineers showed a distillate flow rate of almost 3 l/m2 per hour from seawater at less than one sun (1 kW/m2). This is twice the yield of recent passive complete distillation systems, and theoretical models also suggest that the design has the potential to further double the current distillate rate
“While previous studies focused on how to maximize the solar energy absorption, we have shifted the attention to a more efficient management of the absorbed solar thermal energy. In this way, we have been able to reach record values of productivity up to 20 litres per day of drinking water per square meter exposed to the Sun. The reason behind the performance increase is the recycling of solar heat in several cascade evaporation processes.”
After developing the prototype for more than two years and testing it directly in the Ligurian sea off Varazze, Italy, the engineers say that the technology could have an impact in isolated coastal locations with little drinking water but abundant solar energy, especially in developing countries. Furthermore, the technology is particularly suitable for providing safe and low-cost drinking water in emergency conditions, for example in areas hit by floods or tsunamis and left isolated for days or weeks from electricity grid and aqueduct. A further application envisioned for this technology are floating gardens for food production, an interesting option especially in overpopulated areas.
The researchers are now looking for industrial partners to make the prototype more durable, scalable and versatile. Versions of the device could be employed in coastal areas where over-exploitation of groundwater causes the intrusion of saline water into freshwater aquifers or treat waters polluted by industrial or mining plants.
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