A team of researchers at MIT and in China has come up with a desalination system that is both more efficient and less expensive than previously developed solar desalination methods. The process could also be used to treat contaminated wastewater or to generate steam for sterilizing medical instruments, all without requiring any power source other than sunlight itself.
The findings were published in the journal Nature Communications, in a paper by MIT graduate student Lenan Zhang, postdoc Xiangyu Li, professor of mechanical engineering Evelyn Wang, and four others.
The team focused on developing a wick-free system since these wicks are vulnerable to salt accumulation and often difficult to clean. The system resulted in a layered system, with dark material at the top to absorb the sun’s heat, then a thin layer of water above a perforated layer of material, sitting atop a deep reservoir of the salty water such as a tank or a pond. The researchers determined the optimal size for the holes drilled through the perforated polyurethane at 2.5 millimeters across, meaning they can be easily made using commonly available waterjets.
The holes are large enough to allow for a natural convective circulation between the warmer upper layer of water and the colder reservoir below. That circulation naturally draws the salt from the thin layer above down into the much larger body of water below, where it becomes well-diluted and no longer a problem.
Li, one of the lead researchers, says that the advantages of this system are “both the high performance and the reliable operation, especially under extreme conditions, where we can actually work with near-saturation saline water. And that means it’s also very useful for wastewater treatment.”
Further work is required to assess the use of the system in large settings and in long runs, but it could be a game changer in terms of solar water desalination.
Just as hot air rises and cold air falls, Zhang explains, natural convection drives the desalination process in this device. The water evaporated from the top of the system can then be collected on a condensing surface, providing pure fresh water.
“The rejection of salt to the water below could also cause heat to be lost in the process, so preventing that required careful engineering, including making the perforated layer out of highly insulating material to keep the heat concentrated above. The solar heating at the top is accomplished through a simple layer of black paint.”
According to the team’s calculations, a system with just 1 square meter of collecting area could provide a family’s daily needs for drinking water; and the necessary materials for would cost only about $4.
The device is remarkably stable as it operated for a week with no signs of any salt accumulation. The necessary work to translate this lab-scale proof of concept into workable commercial devices, and to improve the overall water production rate, should be possible within a few years, Zhang says. It is likely that the first applications will be providing safe water in remote off-grid locations, or for disaster relief.
