A cool solution for water collection

Frost forms on almost any cold surface exposed to moist air. But a refrigeration system’s efficiency can be greatly reduced when frost forms on the heat exchanger that draws excess energy out of the coolant to keep it cold. By some estimates, this can lead to a 20% increase in energy consumption—and since refrigeration accounts for roughly 15% of global electricity use, this waste adds up.

This problem also offers an opportunity, notes Ghaffour. “Frost represents both a significant challenge in terms of energy consumption and a valuable source of water,” he says. “In some countries like Saudi Arabia, every drop counts.” 

Many researchers have devised strategies for harvesting water from frost, but most require expenditure of additional energy, undermining the efficiency gains from defrosting.

Jin and Ghaffour developed a system called the salt-solution-infused thin-film condenser (SSTFC). This comprises a porous, water repellent material covering a series of microchannels filled with saline solution. As water vapor accumulates on the surface, it forms larger droplets that subsequently permeate the layer and mix with the salt solution, which prevents freezing. 

This can quickly dilute the saline to a point where it is again prone to freezing, but the SSFTC is also coupled to a solar-powered evaporator. This evaporates the newly captured water into a condenser, from which it can be harvested for re-use, while also restoring the saline solution to its original, effective salt concentration. The authors demonstrated that the SSTFC can keep surfaces frost free, and Ghaffour notes their approach could “potentially contribute more than 50% energy savings compared with conventional defrosting systems.” 

Virginia Tech engineer Jonathan Boreyko, who was not involved in the research, praises the technology as a clever solution to a tricky problem. “The amount of water harvested in an hour is about 0.5 liters per meter squared, which sounds fairly impressive,” he says. But Boreyko adds that “the scalability of the system is an open question,” and it remains to be seen how efficiently the system—only demonstrated here at the centimeter scale—operates when applied over larger surface areas.

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