...
The teamās new system improves on their previous design ā a similar concept of multiple layers, called stages. Each stage contained an evaporator and a condenser that used heat from the sun to passively separate salt from incoming water. That design, which the team tested on the roof of an MIT building, efficiently converted the sunās 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 systemās overall cost.
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. While this design prevented salt from settling and accumulating on the device, it desalinated water at a relatively low rate.
In the 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 their two previous concepts: a multistage system of evaporators and condensers, that is also configured to boost the circulation of water ā and salt ā within each stage.
āWe introduce now an even more powerful convection, that is similar to what we typically see in the ocean, at kilometer-long scales,ā Xu says.
The small circulations generated in the teamās new system is similar to the āthermohalineā convection in the ocean ā a phenomenon that drives the movement of water around the world, based on differences in sea temperature (āthermoā) and salinity (āhalineā).
āWhen 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,ā Zhang explains. āBy mimicking this kilometer-wide phenomena in small box, we can take advantage of this feature to reject salt.ā
...
a childhood favorite 
