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MIT Engineers Turn Seawater into Cheap Drinking Water

Engineers at the Massachusetts Institute of Technology (MIT) and researchers in China have developed a groundbreaking solar desalination system that promises to provide fresh, drinkable water at a cost lower than tap water. This innovative system takes inspiration from the natural processes of the ocean and harnesses the power of the sun to convert seawater into clean, potable water. The results of their research, published on September 27 in the journal Joule, unveil a design that not only enhances efficiency but also solves critical issues faced by traditional desalination methods.

The conventional process of desalination involves removing salt and impurities from seawater, making it safe for consumption. While this method has been in use for decades, it often comes with significant energy and environmental costs. However, the new solar desalination system developed by the MIT and Chinese research team offers a sustainable and cost-effective alternative.

The system’s operation begins by drawing in saltwater and exposing it to natural sunlight. Through a series of ingenious design elements, the researchers have managed to replicate the ocean’s thermohaline circulation, a large-scale natural process driven by temperature and salinity differences in seawater. In their compact device, water circulates in swirling eddies, much like the ocean’s currents. This circulation, combined with solar heating, induces water to evaporate, leaving salt behind. The resulting water vapor is then condensed and collected as pure, drinkable water. Crucially, the leftover salt continues to circulate through the system, preventing accumulation and clogs.

The system’s efficiency and performance surpass other passive solar desalination methods currently under development. When scaled to the size of a small suitcase, it can produce approximately 4 to 6 liters of clean water per hour and has the potential to operate for several years before requiring replacement parts. This remarkable capability could make freshwater production cheaper than tap water, as Lenan Zhang, a research scientist at MIT’s Device Research Laboratory, points out.

The team envisions that a scaled-up version of the device could meet the daily water requirements of a small family or provide off-grid, coastal communities with easy access to freshwater. MIT graduate student Yang Zhong and Evelyn Wang, the Ford Professor of Engineering, led the research alongside their counterparts at Shanghai Jiao Tong University in China.

The innovative design represents an evolution of the team’s earlier attempts. Previous designs, which featured multiple stages with evaporators and condensers powered by solar heat, efficiently converted sunlight into water vapor but faced issues with salt accumulation. This accumulation led to clogs that required frequent maintenance, significantly increasing operational costs. In response, the researchers devised a layered configuration that prevented salt from settling but sacrificed desalination efficiency.

The latest iteration of the system strikes a balance between high water production rates and efficient salt rejection. It combines a multistage system of evaporators and condensers with enhanced water and salt circulation within each stage. This design innovation mirrors the kilometer-scale thermohaline convection seen in the ocean, replicating the natural phenomenon responsible for global water movement based on differences in temperature and salinity.

The core of the new design is a single stage that resembles a thin box topped with a heat-absorbing material. Water flows through the top half, where an evaporator layer uses solar heat to induce evaporation. The resulting water vapor is funneled to the bottom half of the box, where a condensing layer cools it into salt-free, drinkable liquid. The entire box is tilted within a larger vessel, allowing water to flow in and swirl as it undergoes the desalination process. These small eddies help maintain contact with the evaporator layer while preventing salt accumulation.

The researchers conducted tests on prototypes with one, three, and ten stages, using water with varying salinity levels, including natural seawater and water seven times saltier. These tests revealed that when scaled to a square meter per stage, the system could produce up to 5 liters of drinking water per hour without accumulating salt for an extended period. Due to its longevity and energy-efficient, passive operation, the researchers estimate that the overall cost of using this system would be lower than producing tap water in the United States.

Guihua Yu, a specialist in sustainable water and energy storage systems at the University of Texas at Austin, praised the innovative approach. He noted that the system’s modular design makes it suitable for household water production, allowing for scalability and adaptability to meet individual needs. The potential of this technology to address real-world water scarcity problems is evident, offering a sustainable and cost-effective solution to one of humanity’s most pressing challenges. MIT and its collaborators in China have taken a significant step towards ensuring that access to clean, affordable water becomes a reality for more people around the world.

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