Solar-powered method for inexpensive desalination

Friday, 18 February, 2022

There has been plenty of research into finding ways to desalinate seawater or brackish water with solar, but many have run into expensive problems with fouling of equipment caused by salt build-up.

Now, a team of researchers at MIT and in China has come up with a solution to the problem of salt accumulation — and in the process developed a desalination system that is claimed to be both more efficient and less expensive than previous solar desalination methods. The process could also be used to treat contaminated wastewater or to generate steam for sterilising medical instruments, all without requiring any power source other than sunlight itself.

The findings are described 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.

“There have been a lot of demonstrations of really high-performing, salt-rejecting, solar-based evaporation designs of various devices,” Wang said. “The challenge has been the salt fouling issue, that people haven’t really addressed. So, we see these very attractive performance numbers, but they’re often limited because of longevity. Over time, things will foul.”

Many attempts at solar desalination systems rely on a wick to draw the saline water through the device, but these wicks are vulnerable to salt accumulation and can be difficult to clean. Therefore, the researchers focused on developing a wick-free system instead.

The result is 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.

After careful calculations and experiments, the researchers determined the optimal size for the holes drilled through the perforated material, which in their tests was made of polyurethane. At 2.5 mm across, these holes 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.

“It allows us to achieve high performance and yet also prevent this salt accumulation,” said Wang, who is the Ford Professor of Engineering and head of the Department of Mechanical Engineering.

Li said 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.”

He adds that much work on such solar-powered desalination has focused on novel materials. “But in our case, we use really low-cost, almost household materials.” The key was analysing and understanding the convective flow that drives this entirely passive system, he said.

“People say you always need new materials, expensive ones, or complicated structures or wicking structures to do that. And this is, I believe, the first one that does this without wicking structures.”

This new approach “provides a promising and efficient path for desalination of high-salinity solutions, and could be a game changer in solar water desalination”, said Hadi Ghasemi, a professor of chemical and biomolecular engineering at the University of Houston, who was not associated with this work. “Further work is required for assessment of this concept in large settings and in long runs,” he added.

Researchers test two identical outdoor experimental set-ups placed next to each other. Image courtesy of Lenan Zhang, Xiangyu Li, Evelyn Wang, et al.

Just as hot air rises and cold air falls, Zhang explained, natural convection drives the desalination process in this device. In the confined water layer near the top, “the evaporation happens at the very top interface. Because of the salt, the density of water at the very top interface is higher, and the bottom water has lower density. So, this is an original driving force for this natural convection because the higher density at the top drives the salty liquid to go down.” 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.

So far, the team has proven the concept using small benchtop devices, so the next step will be starting to scale up to devices that could have practical applications.

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 concluded.

Image credit: ©stock.adobe.com/au/mg1708

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