Desalination technology created by researchers is more efficient and less costly than prior techniques. The procedure might also be used to remediate tainted wastewater or produce steam for sterilizing medical tools, all without needing any power source other than sunshine.
Water shortages impact an estimated two-thirds of mankind, and many of these locations in the developing world also lack reliable energy. As a result, a lot of study has gone into finding techniques to desalinate saltwater or brackish water using just solar heat. However, many of these projects have fallen into issues with equipment fouling caused by salt accumulation, which adds complexity and cost.
Now, a group of MIT and Chinese academics has devised a solution to the issue of salt buildup, as well as a desalination system that is both more efficient and less costly than prior solar desalination techniques. The procedure might potentially be used to remediate tainted wastewater or create steam for sterilizing medical tools, all while using just sunlight as a power source.
The results were published today in Nature Communications by MIT graduate student Lenan Zhang, postdoc Xiangyu Li, mechanical engineering professor Evelyn Wang, and four other researchers.
“A number of demonstrations of exceptionally high-performing, salt-rejecting, solar-based evaporation designs of different devices have been made,” Wang explains. “The problem has been the salt fouling problem, which has gone unaddressed. As a result, we see these really appealing performance figures, but they are often restricted due to lifespan. Things will deteriorate with time.”
Many efforts at solar desalination systems use a wick to pull saline water through the device, however these wicks are susceptible to salt buildup and are difficult to clean. Instead, the team concentrated on inventing a wick-free method. The result is a layered structure with black material at the top to absorb the sun’s heat, a thin layer of water above a perforated layer of material, and all of this lying over a deep reservoir of saline water, such as a tank or pond. The researchers identified the appropriate size for the holes drilled in the perforated material, which in their studies was composed of polyurethane, after meticulous calculations and trials. These holes, which are 2.5 millimeters in diameter, are simple to make using commercially accessible waterjets.
The perforations are big enough to enable natural convection between the warmer top layer of water and the cooler reservoir below. The salt from the tiny layer above is naturally drawn down into the much bigger body of water below, where it is diluted and no longer a concern. “It enables us to achieve great performance while also preventing salt buildup,” explains Wang, who is also the Ford Professor of Engineering and the head of the Mechanical Engineering Department.
The benefits of this technology, according to Li, are: “both high performance and dependable operation, particularly in harsh settings where we can deal with near-saturation saline water As a result, it’s also ideal for wastewater treatment.”
He goes on to say that most of the research on solar-powered desalination has centered on new materials. “However, we employ very low-cost, nearly home items in our situation.” He claims that the key was examining and comprehending the convective flow that powers this completely passive system. “People say you need new materials all the time, that they’re pricey, that you need sophisticated structures or wicking structures to achieve it. And I think this is the first one to accomplish it without the use of wicking structures.”
This new method “provides a promising and efficient path for desalination of high salinity solutions, and could be a game changer in solar water desalination,” according to Hadi Ghasemi, a professor of chemical and biomolecular engineering at the University of Houston who was not involved in the research. “More study is needed to examine this notion in broad contexts and over lengthy periods of time,” he says.
Natural convection drives the desalination process in this device, just as hot air rises and cold air descends, Zhang says. Near the top of the restricted water layer, “Evaporation occurs at the very top of the interface. The density of water at the very top contact is greater due to the salt, whereas the density of water at the bottom interface is lower. As a result, the increased density at the top forces the salty liquid down, which is an initial driving force for natural convection.” Water evaporated from the system’s top may be collected on a condensing surface, resulting in clean fresh water.
Because the rejection of salt to the water below may result in heat loss in the process, careful engineering was necessary to avoid this, which included creating the perforated layer out of a highly insulating material to keep the heat focused above. A basic coat of black paint is used to provide solar heating at the top.
The team has already demonstrated the idea with tiny benchtop devices, so the next step will be to scale up to devices with real-world applications. According to their estimates, a system with only 1 square meter (about a square yard) of collecting area should be enough to meet a family’s daily drinking water demands. According to Zhang, the materials required for a 1-square-meter gadget would cost just approximately $4.
According to Li, their test equipment ran for a week without accumulating any salt. The gadget is also quite stable. “Even if we apply some significant perturbations, such as waves on the seashore or in a lake,” where such a device may be deployed as a floating platform, “it can return to its original equilibrium position very quickly,” he adds.
According to Zhang, the work required to turn this lab-scale proof of concept into commercially viable devices and enhance the total water production rate should be completed within a few years. The initial uses are anticipated to be supplying clean water in off-grid places or for disaster assistance after hurricanes, earthquakes, or other interruptions of typical water sources.
“If we can focus the sunlight a little bit,” Zhang continues, “we could utilize this passive technology to create high-temperature steam to undertake medical sterilization” in off-grid rural regions.
“I believe the developing world offers a significant chance,” Wang adds. “Because of the simplicity of the design, I believe that is where there will be the greatest immediate effect.” “If we truly want to get it out there,” she continues, “we also need to engage with the end users to be able to actually accept the way we design it so that they’re eager to utilize it.”
Yang Zhong, Arny Leroy, and Lin Zhao of MIT, as well as Zhenyuan Xu of Shanghai Jiao Tong University in China, were part of the team. The study was funded by the Singapore-MIT Alliance for Research and Technology, as well as the US-Egypt Science and Technology Joint Fund and National Science Foundation facilities.
