Outdoor test of the prototype under natural sunlight. Credit: Jintong Gao and Zhenyuan Xu
Third time’s the charm, right? That certainly rings true for engineers at MIT who recently took the roller coaster ride of trial and error for a spin. After a couple times around, they disembarked on the third go-around with a blueprint in hand to achieve what they set out to do—design a system that makes it possible for drinking water produced by sunlight to be cheaper than tap water.
If at first you don’t succeed…
The new system successfully turns seawater into drinking water thanks to a passive, extremely low-cost device that is inspired by the ocean and powered by the sun.
The team’s first go at it, in 2020, was successful at the experimental stage, but not scale up. They initially designed a system with 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.
In tests on the roof of an MIT building, the system efficiently converted the sun’s energy to evaporate water, which was then condensed into drinkable water. However, 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 maintenance and cost.
In a follow-up effort, the team, led by Lenan Zhang, 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 too low of a rate to be considered effective.
Now, in the third iteration described in a new study in Joule, the scientists believe they have landed on the perfect marriage—and slight upgrade—of their previous versions.
…try, try again
The heart of the team’s new design is a single stage that resembles a thin box, topped with a dark material that efficiently absorbs the heat of the sun. Inside, the box is separated into a top and bottom section. Water can flow through the top half, where the ceiling is lined with an evaporator layer that uses the sun’s heat to warm up and evaporate any water in direct contact. The water vapor is then funneled to the bottom half of the box, where a condensing layer air-cools the vapor into salt-free, drinkable liquid. The researchers set the entire box at a tilt within a larger, empty vessel, then attached a tube from the top half of the box down through the bottom of the vessel, and floated the vessel in saltwater.
In this configuration, water can naturally push up through the tube and into the box, where the tilt of the box, combined with the thermal energy from the sun, induces the water to swirl as it flows through. The small eddies help to bring water in contact with the upper evaporating layer while keeping salt circulating, rather than settling and clogging.
The small circulations generated in the new system is similar to the “thermohaline” convection in the ocean—a phenomenon that 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. The higher the salt concentration, the denser the liquid, and this heavier water wants to flow downward,” said Zhang. “By mimicking this kilometer-wide phenomena in small box, we can take advantage of this feature to reject salt.”
Positive scale-up
In tests, Zhang and team 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, 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.
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.
“We show that this device is capable of achieving a long lifetime,” Zhong says. “That means that, for the first time, it is possible for drinking water produced by sunlight to be cheaper than tap water. This opens up the possibility for solar desalination to address real-world problems.”