MIT engineers develop breakthrough desalination system for cheaper freshwater

With the global population increasing every day, many individuals face the problem of a lack of drinking water. A vast majority of the Earth's water is in the form of saltwater, so, humans have had to rely on sources of freshwater for their water needs. If seawater could be safely converted into drinking water, it could make the commodity available to more people. Fortunately, engineers at MIT and China have been working on a project to build a passive device that converts seawater into drinking water powered by the sun.

The device is fundamentally a solar desalination machine that takes in salt water and heats it with natural sunlight. What makes the machine efficient is its ability to circulate water in swirling eddies similar to larger "thermohaline" circulation that happens in the ocean. This movement, coupled with the heat of the sun, results in water evaporating and leaving the salt behind. The water vapor can then be collected and condensed to be used as pure drinking water.

The leftover salt would continue to move in and out of the device so as not to clog it up. While there have been many other passive solar desalination projects, this machine stands out with its higher water-production rate and higher salt-rejection rate. Researchers estimate that if the present system could be scaled up to the size of a suitcase, it could easily produce 4 to 6 liters of drinking water in an hour and even last for years without needing to replace parts. 

If the scaling up proved to be successful, then the system could basically produce drinking water at a rate and price that would be considered cheaper than tap water across the world. Lenan Zhang, a research scientist in MIT's Device Research Laboratory, said, "For the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water." According to the team, the scaled-up device could provide drinking water every day to a small family.

The device would also be very useful to communities that lived off-grid or in coastal areas. The team improved on a previous iteration of the device, which also worked on the concept of multiple layers that they referred to as "stages." Each stage would have an evaporator and a condenser that utilized the heat coming from the sun to split up salt from seawater. The initial design was tested by the team on the roof of an MIT building.

It worked successfully for the first few days until the accumulated salt began to turn into crystals, which clogged the system in a few days. This meant that an individual would have to constantly replace the stages on the device, driving up the cost of its maintenance. This new iteration involves a similar configuration but with a way to circulate the incoming water along with residual salt. Even though the new design did not witness any clogging because of the salt, it did have a lower rate of production.

The team's latest design strikes a balance with a high water-production rate while ensuring high salt rejection for extended periods of time. Yang Zhong, one of the co-authors in the study, said, "We show that this device is capable of achieving a long lifetime. 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."