Large swaths of the American Southwest are in danger of dying of thirst.
The Colorado River, which several states have relied on for clean drinking water for more than a century, is going dry due the effects of climate change and water overuse. That’s why states such as Arizona have turned to an expensive gamble in order to get the water they need: seawater desalination.
That is the process of removing the salt from ocean water in order to produce fresh drinking water. However, the process is often expensive—with one proposed plan for a desalination plant in Mexico to provide water to Arizona costing up to $5 billion. It’s also energy intensive, often requiring a tremendous amount of heat to separate the water from the salt.
That’s why an international team of engineers at MIT and China have created a device that can turn seawater into clean drinking water using solar energy. The process, which is described in a study published Wednesday in the journal Joule, could make freshwater that’s even more affordable than the water coming from Americans’ kitchen faucets.
“For the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water,” co-author Lenan Zhang, a mechanical engineer at MIT’s Device Research Laboratory, said in a statement.
The device works using thermal desalination, a technique that leverages heat to evaporate water into vapor that can then be condensed into freshwater—leaving behind salt crystals as a result. The saltier the water is, the more heat (and therefore energy) is required to separate the water from the salt.
The MIT device utilizes natural sunlight to do the same job. The team also incorporated a water circulation system to process salt out of the device more efficiently while also requiring less energy to desalinate the water. The system is similar to a process known as “thermohaline,” which occurs when water circulates through the ocean in response to heat.
“When seawater is exposed to air, sunlight drives water to evaporate,” Zhang explained. “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 a small box, we can take advantage of this feature to reject salt.”
There are some caveats. The device, as of now, is very small. The authors note that if it’s scaled up to the size of a suitcase, though, it could produce roughly a gallon of clean water every hour. Of course, if it’s scaled up even more, it could produce even more water. Since the entire system is passive and would only require replacing parts every few years, it would be much cheaper than what it costs to create tap water.
“We show that this device is capable of achieving a long lifetime,” co-author Yang Zhong, an engineer at MIT’s Device Research Laboratory, said in a statement. “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.”