MIT researchers have developed a solar-powered desalination system
that is more efficient and less expensive than previous methods.
An estimated two-thirds of humanity is affected by shortages of water,
and many such areas in the developing world also face a lack of dependable electricity.
Widespread research efforts have thus focused on ways to desalinate seawater or brackish water
using just solar heat. Many such efforts have run into problems with
fouling of equipment caused by salt buildup, however, which often adds complexity and expense.
Now, a team of researchers at MIT and in China has come up with a
solution to the problem of salt accumulation — and in the process developed a desalination system
that is both more efficient and less expensive than previous solar desalination methods. The
process could also be used to treat contaminated wastewater or to generate steam for sterilizing
medical instruments, all without requiring any power source other than sunlight itself.
The findings are described in the journal Nature Communications.
Many attempts at solar desalination systems rely on some kind of wick to draw the saline water
through the device, but these wicks are vulnerable to salt accumulation and relatively difficult to
clean. The team focused on developing a wick-free system instead. The result is a layered system,
with dark material at the top to absorb the sun’s heat, then a thin layer of water above a
perforated layer of material, sitting atop a deep reservoir of the salty water such as a tank or a
pond. After careful calculations and experiments, the researchers determined the optimal size for
the holes drilled through the perforated material, which in their tests was made of polyurethane.
At 2.5 millimeters across, these holes can be easily made using commonly available waterjets.
The holes are large enough to allow for a natural convective
circulation between the warmer upper layer of water and the colder reservoir below.
That circulation naturally draws the salt from the thin layer above down into the much larger
body of water below, where it becomes well-diluted and no longer a problem.
The researcher says that the advantages of this system are both the high performance
and the reliable operation, especially under extreme conditions, where we can actually work
with near-saturation saline water. And that means it’s also very useful for wastewater treatment.
He adds that much work on such solar-powered desalination has focused on novel materials.
But in their case, they use really low-cost, almost household materials. The key was
analyzing and understanding the convective flow that drives this entirely passive system.
This new approach provides a promising and efficient path for desalination of
high salinity solutions, and could be a game changer in solar water desalination.
Just as hot air rises and cold air falls, natural convection drives the desalination process in this
device. In the confined water layer near the top, the evaporation happens at the very top
interface. Because of the salt, the density of water at the very top interface is higher, and
the bottom water has lower density. So, this is an original driving force for this natural convection
because the higher density at the top drives the salty liquid to go down. The water evaporated from
the top of the system can then be collected on a condensing surface, providing pure fresh water.
The rejection of salt to the water below could also cause heat to be lost in the process,
so preventing that required careful engineering, including making the perforated layer out of
highly insulating material to keep the heat concentrated above. The
solar heating at the top is accomplished through a simple layer of black paint.
So far, the team has proven the concept using small benchtop devices, so the next step will be
starting to scale up to devices that could have practical applications.
Based on their calculations, a system with just 1 square meter of collecting
area should be sufficient to provide a family’s daily needs for drinking water. The necessary
materials for a 1-square-meter device would cost only about $4.
Their test apparatus operated for a week with no signs of any salt accumulation.
And the device is remarkably stable.