The passive solar evaporation system could be used to clean sewage, provide drinking water or sterilize medical tools in off-grid areas. –ScienceDaily

It is estimated that two-thirds of humanity are affected by water shortages, and many parts of the developing world also face a lack of reliable electricity. Many research efforts have therefore focused on ways to desalinate seawater or brackish water using only solar heat. However, many such efforts have run into problems with equipment fouling caused by salt buildup, which often adds complexity and expense.

Now, a team of researchers from MIT and China have found a solution to the problem of salt buildup and, in doing so, have developed a desalination system that is both more efficient and less expensive than solar desalination methods. previous ones. The process could also be used to treat contaminated wastewater or to generate steam to sterilize medical instruments, all without requiring any source of energy other than sunlight itself.

The results are described today in the journal Nature Communicationin an article by MIT graduate student Lenan Zhang, postdoc Xiangyu Li, mechanical engineering professor Evelyn Wang and four others.

“There have been many demonstrations of high-performance, salt-rejecting solar evaporation designs of various devices,” Wang said. “The challenge has been the salt fouling issue, which people haven’t really addressed. So we’re seeing these very attractive performance numbers, but they’re often limited due to longevity. Over time , things are going to get messy.”

Many attempts at solar desalination systems rely on some sort of wick to draw salt water through the device, but these wicks are vulnerable to salt buildup and relatively difficult to clean. Instead, the team focused on developing a wickless system. 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 layer of perforated material, resting on a deep reservoir of salt water such as a reservoir or pond. After careful calculations and experiments, the researchers determined the optimal size of the holes drilled through the perforated material, which in their tests was polyurethane. At 2.5 millimeters in diameter, these holes can be easily made using commonly available water jets.

The holes are large enough to allow natural convective circulation between the warmer upper water layer and the cooler reservoir below. This circulation naturally draws salt from the thin layer above to the much larger body of water below, where it becomes well diluted and no longer a problem. “It allows us to achieve high performance while preventing this salt buildup,” says Wang, Ford professor of engineering and head of the mechanical engineering department.

Li says the benefits of this system are “both high performance and reliable operation, especially in extreme conditions, where we can actually work with nearly saturated saline water. And this means that it is also very useful for the treatment of waste water.

He adds that much work on this solar-powered desalination has focused on new materials. “But in our case, we’re using very inexpensive, almost household materials.” The key was to analyze and understand the convective flow that powers this entirely passive system, he says. “People say you always need new materials, expensive materials, or complicated structures or wicking structures to do it. And this is, I believe, the first one that does it without wicking structures. .”

This new approach “provides a promising and efficient route for desalination of high salinity solutions, and could be a game-changer in solar water desalination,” says Hadi Ghasemi, professor of chemical and biomolecular engineering at the University of Houston. , who was not associated with this work. “Further work is needed to evaluate this concept in large, long-term contexts,” he adds.

Just as hot air rises and cold air sinks, Zhang explains, natural convection drives the desalination process in this device. In the confined water layer near the top, “evaporation occurs at the top-most interface. Due to the salt, the water density at the top-most interface is higher, and the water bottom has a lower density, so it’s a unique driving force for this natural convection because the higher density at the top pushes the salty liquid down.” Water evaporated from the top of the system can then be collected on a condensing surface, providing pure soft water.

The release of salt to the water below could also lead to heat loss in the process, preventing it from requiring careful engineering, including making the perforated layer from a highly insulating material to retain heat. focused above. The solar heating at the top is achieved with a simple coat of black paint.

So far, the team has proven the concept using small benchtop devices, so the next step will start moving to devices that could have practical applications. According to their calculations, a system with just 1 square meter (about one square meter) of collection area should be enough to meet a family’s daily drinking water needs, they say. Zhang says they calculated that the materials needed for a one-square-meter device would only cost about $4.

Their test device ran for a week without any signs of salt buildup, Li says. And the device is remarkably stable. “Even if we apply an extreme disturbance, such as waves on seawater or the lake,” where such a device could be installed as a floating platform, “it can return to its equilibrium position very quickly. origin,” he says.

The work needed to translate this lab-scale proof-of-concept into working commercial devices and to improve the overall rate of water production should be possible within a few years, Zhang said. Early applications are likely to provide potable water in remote off-grid locations, or for disaster relief after hurricanes, earthquakes, or other disruptions to normal water supplies.

Zhang adds that “if we can concentrate sunlight a little, we could use this passive device to generate high-temperature steam to perform medical sterilization” for off-grid rural areas.

“I think the developing world is a real opportunity,” says Wang. “I think that’s where the most likely short-term impact is, because of the simplicity of the design.” But, she adds, “if we really want to get it out there, we have to work with the end users as well, to really be able to embrace the way we design it so they’re ready to use it.”

The team also included Yang Zhong, Arny Leroy and Lin Zhao from MIT, and Zhenyuan Xu from Shanghai Jiao Tong University in China. The work was supported by the Singapore-MIT Alliance for Research and Technology, the United States-Egypt Joint Fund for Science and Technology, and used facilities supported by the National Science Foundation.

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