In a paper appearing in the journal Joule, the
team 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
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 is cheaper than tap water.
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
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 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.
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 — and salt — within each stage.
The small circulations generated in the team’s 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 (“thermo”) and salinity (“haline”).
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.
By mimicking this kilometer-wide phenomena in small box, the researchers take advantage
of this feature to reject salt.
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 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.