A ten-stage prototype is placed in a boat-like reservoir. (Image Credit: Jintong Gao and Zhenyuan Xu)
MIT and Shanghai Jiao Tong University engineers developed a solar-powered desalination system designed to convert saltwater into safe drinking water by using the sun’s heat. This device works similarly to the ocean’s thermohaline circulation --- it circulates water via swirling eddies. As a result, the solar heat and circulation causes water to evaporate, leaving salt behind. Afterward, that water vapor is condensed and collected in the form of drinking water. Meanwhile, the remaining salt circulates through and out of the system instead of accumulating inside. If they scaled up the device, the researchers believe it could produce sufficient drinking water for a small family’s daily requirements. It can also be installed in off-grid coastal communities with accessible seawater.
The team’s device has a single-stage box-like structure with a dark material set on top to absorb the sun’s heat. A top and bottom section splits the box internally. In the top part, where water passes through, the ceiling has an evaporation layer lining that uses the sun’s heat to warm and evaporate the water. Afterward, the water vapor funnels to the bottom section, which features a condensing layer that cools the vapor into safe, drinkable water. The team placed the box and tilted it inside an empty vessel, which contained saltwater, and connected a tube from the top part of the box through the bottom of the vessel.
With that design, water flows up the tube and inside the box. The box’s tilt and the sun’s heat swirl the water while it flows. These eddies allow the water to interact with the evaporating layer as the salt circulates. “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.
Salt endurance performance in the laboratory environment. (Image Credit: joule)
The researchers developed other prototypes featuring one, three, and ten stages. They then tested them out to see how they’d perform with natural seawater and water with seven times more salt. The tests show the system could produce five liters of drinking water per hour by up-scaling all the stages to one square meter. Even better, the team says it could desalinate water without salt build-up for several years until it needs replacement parts. With this improved system lifetime and electricity-free functionality, the team believes that operating the system is more affordable than producing tap water in the US.
“We show that this device is capable of achieving a long lifetime,” Zhong says. “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.”
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