Transcript
Engineers at MIT and China’s Shanghai Jiao Tong University are aiming to turn seawater into drinking water with a completely passive device that is inspired by the ocean and powered by the sun. New research, just published in the journal Joule, outlines the design for a new solar desalination system that takes in saltwater and heats it with natural sunlight. The configuration of the device allows water to circulate in swirling eddies, in a manner similar to the much larger “thermohaline” circulation of the ocean.
This circulation, combined with the sun’s heat, drives water to evaporate, leaving salt behind. The resulting water vapor can then be condensed and collected as pure, drinkable water. In the meantime, the leftover salt continues to circulate through and out of the device, rather than accumulating and clogging the system. The new system has a higher water-production rate and a higher salt-rejection rate than any other passive solar desalination concept currently being tested.
The researchers estimate that if the system is scaled up only to the size of a small suitcase, it could produce 4 to 6 liters of drinking water per hour and last several years before requiring replacement parts. At this scale and performance, the system could produce drinking water at a rate and price that is cheaper than tap water. The team envisions a scaled-up device which could passively produce enough drinking water to meet the daily requirements of a small family.
The system could also supply entire off-grid, coastal communities where seawater is easily accessible. In their latest iteration, the team believes it has landed on a design that achieves both a high water-production rate, and high salt rejection, meaning that the system can quickly and reliably produce drinking water for an extended period.
The key to their new design is a combination of two previous concepts: a multistage system of evaporators and condensers, that is also configured to boost the circulation of water — and salt — within each stage. The small-scale circulations generated in the team’s new system is similar to “thermohaline” convection in the ocean. That phenomenon drives the movement of water around the world, based on differences in sea temperature and salinity. When seawater is exposed to air, sunlight drives water to evaporate.
Once water leaves the surface, salt remains. And the higher the salt concentration, the denser the liquid, and that heavier water wants to flow downward. By mimicking this kilometer-wide ocean phenomena in a small box, engineers can take advantage of this feature to reject salt. The team built several prototypes, with one, three, and 10 stages, and tested their performance in water of varying salinity, including natural seawater and water that was seven times saltier.
From these tests, the researchers calculated that if each stage were scaled up to a square meter, it would produce up to 5 liters of drinking water per hour, and that the system could desalinate water without accumulating salt for several years. Given this extended lifetime, and the fact that the system is entirely passive, requiring no electricity to run, the team estimates that the overall cost of running the system would be cheaper than what it costs to produce tap water in the United States.
Since this device is capable of achieving a long lifetime, this opens up the possibility for solar desalination to address real-world problems, for the first time.
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