Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.
Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.
Their demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber. In most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials, Wang says. “We’ve been able to decouple these two.”
The most expensive component of the prototype is a layer of transparent aerogel used as an insulator at the top of the stack, but the team suggests other less expensive insulators could be used as an alternative. (The aerogel itself is made from dirt-cheap silica but requires specialized drying equipment for its manufacture.)
Wang emphasizes that the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call thermally localized multistage desalination. The formulas they developed could likely be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.
One possible configuration would be floating panels on a body of saltwater such as an impoundment pond. These could constantly and passively deliver fresh water through pipes to the shore, as long as the sun shines each day. Other systems could be designed to serve a single household, perhaps using a flat panel on a large shallow tank of seawater that is pumped or carried in. The team estimates that a system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person. In production, they think a system built to serve the needs of a family might be built for around $100.
The researchers plan further experiments to continue to optimize the choice of materials and configurations, and to test the durability of the system under realistic conditions. They also will work on translating the design of their lab-scale device into a something that would be suitable for use by consumers. The hope is that it could ultimately play a role in alleviating water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.
“This new approach is very significant,” says Ravi Prasher, an associate lab director at
Lawrence Berkeley National Laboratory and adjunct professor of mechanical engineering at the University of California at Berkeley, who was not involved in this work. “One of the challenges in solar still-based desalination has been low efficiency due to the loss of significant energy in condensation. By efficiently harvesting the condensation energy, the overall solar to vapor efficiency is dramatically improved. … This increased efficiency will have an overall impact on reducing the cost of produced water.”
The research team included Bangjun Li, Chenxi Wang and Ruzhu Wang at the Shanghai Jiao Tong University, and Bikram Bhatia, Kyle Wilke, Youngsup Song, Omar Labban, and John Lienhard, who is the Abdul Latif Jameel Professor of Water at MIT. The research was supported by the National Natural Science Foundation of China, the Singapore-MIT Alliance for Research and Technology, and the MIT Tata Center for Technology and Design.
