Desalination system could produce freshwater that is cheaper than tap water
by Jennifer Chu , Massachusetts Institute of Technology
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.
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 4–6 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.
"For the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water," 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.
Zhang's study co-authors include MIT graduate student Yang Zhong, and Evelyn Wang, the Ford Professor of Engineering, along with Jintong Gao, Jinfang You, Zhanyu Ye, Ruzhu Wang, and Zhenyuan Xu of Shanghai Jiao Tong University in China.
A powerful convection
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.
