Overcoming the problem of salt accumulation, this proof-of-concept could be used to provide cheap, safe drinking water and sterilise medical tools in remote locations or for disaster relief.
In 2016, it was reported that approximately 4 billion or two-thirds of the global population faced water scarcity at least one month each year. Half a billion people in the world face severe water scarcity all year round. Besides water scarcity, many of these affected areas also face a lack of dependable electricity. To overcome these problems, much research has been devoted to developing ways to desalinate seawater or brackish water using only solar heat. Unfortunately, many of these attempts have been unsuccessful due to equipment fouling caused by salt build-up, which reduces the reliability of the devices.
Now, researchers at the Massachusetts Institute of Technology (MIT) and in China have not only innovated a solution to solve the problem of salt accumulation, but also developed a novel desalination system that is more efficient and less expensive than previous soil desalination methods. Besides, their process, which depends entirely on solar power, can also be broadly applied to treat contaminated wastewater or produce steam for sterilising medical instruments.
“There have been a lot of demonstrations of really high-performing, salt-rejecting, solar-based evaporation designs of various devices,” said Evelyn Wang, the Ford Professor of Engineering and Head of the Department of Mechanical Engineering. “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 solar desalination systems rely on some type of wick to draw the saline water through the device. However, since these wicks are vulnerable to salt accumulation and are difficult to clean, the researchers sought to develop a wick-free system. The result is a layered system with a dark material at the top of the device to absorb solar energy and a thin layer of water above a perforated layer of material. Their self-floating prototype can be placed atop a deep reservoir of salty water such as a tank or pond.
The desalination process in the device is driven by the natural convection process of hot air rising and cold air falling. Postdoc Xiangyu Li at MIT, who is also an author of the study, explained that 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.”
To prevent the rejection of salt to the water below, which would lead to heat loss, the researchers carefully constructed the perforated layer out of highly insulating material to keep the heat concentrated above. In the study, they used polyurethane. The solar heating at the top of the device is achieved by layering it with black paint.
Through meticulous calculations and experiments, the researchers established the optimal size for the holes drilled through the polyurethane perforated material. At 2.5 millimetres across, these holes can be easily drilled using commonly available waterjets. The holes are also large enough to allow for natural convective circulation between the warmer upper layer of water and the colder reservoir below. This circulation can naturally draw salt from the thin layer above own into the larger body of water below. In doing so, the salt becomes well-diluted, no longer posing a challenge for the device.
As their test apparatus remained fully functional for a week, showing no signs of salt accumulation, the device is evidently stable. “Even if we apply some extreme perturbation, like waves on the seawater or the lake,” where such a device could be installed as a floating platform, “it can return to its original equilibrium position very fast,” said Li.
According to Postdoc Xiangyu Li at MIT, 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.”
Although most solar-power desalination systems have been developed using novel materials, their innovation is composed of only low-cost household materials. Postdoc Li further added that the key was analysing and understanding the convective flow that drives this entirely passive system. “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.”
Currently, the scientists have only demonstrated the concept using small benchtop devices. Therefore, they are now aiming to scale up to devices that could have practical applications. According to their calculations, a system with only 1-square-meter of collecting area would be sufficient to provide a family’s daily needs for drinking water. Zhang revealed the necessary materials for a 1-square-meter device would cost only about $4. Within several years, Zhang believes that this lab-scale proof-of-concept can be translated into workable commercial devices, and to improve the overall water production rate. The team is looking forward to apply their technology to provide safe water in remote off-grid locations, or for disaster relief after hurricanes, earthquakes, or other disruptions of normal water supplies.
“I think a real opportunity is the developing world,” commented Wang. “I think that is where there’s most probable impact near-term, because of the simplicity of the design.” But, she adds, “If we really want to get it out there, we also need to work with the end users, to really be able to adopt the way we design it so that they’re willing to use it.” [APBN]
Source: Zhang et al. (2022). Highly efficient and salt rejecting solar evaporation via a wick-free confined water layer. Nature Communications, 13, 849.