MIT has made a breakthrough that could solve the water shortage problem in over 85% of countries potentially facing water shortages.

Engineers at MIT and in China are aiming to turn seawater into drinking water with a completely passive device that is inspired by the ocean, and powered by the sun. The design for a new solar desalination system takes in saltwater and heats it with natural sunlight.

The configuration of the device allows water to circulate in swirling eddy currents, in a manner similar to the much larger thermohaline currents in the sea based on differences in sea temperature (“thermo”) and salinity (“haline”).

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 all other passive solar desalination concepts currently being tested. The researchers estimate that if the system is scaled up to the size of a small suitcase, it could produce about 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 makes it affordable both industrially and for households.

“For the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water (in the US),” says Lenan Zhang, a research scientist in MIT’s Device Research Laboratory.

The team envisions a scaled-up device could passively produce enough drinking water to meet the daily requirements of a small family. The system could also supply off-grid, coastal communities where seawater is easily accessible.

The team’s new system improves on their previous design — a similar concept of multiple stages of evaporators and contenders that use heat to passively turn salt water into vapor. That design, which the team tested on the roof of an MIT building, efficiently converted the sun’s energy to evaporate water, which was then condensed into drinkable water. But the salt that was left over quickly accumulated as crystals that clogged the system after a few days. In a real-world setting, a user would have to place stages on a frequent basis, which would significantly increase the system’s overall cost. This is a major problem faced by all current designs. In a follow-up effort, they devised a solution with a similar layered configuration, this time with an added feature that helped to circulate the incoming water as well as any leftover salt, inspired by the movement of the ocean’s currents to distribute heat worldwide. 

“We introduce now an even more powerful convection, that is similar to what we typically see in the ocean, at kilometer-long scales,” Xu says.

“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 this heavier water wants to flow downward,” Zhang explains. “By mimicking this kilometer-wide phenomena in a small box, we can take advantage of this feature to reject salt.”