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    Solar-powered system offers a route to inexpensive desalination

    An estimated two-thirds of humanity is affected by shortages of water, and many such areas in the developing world also face a lack of dependable electricity. Widespread research efforts have thus focused on ways to desalinate seawater or brackish water using just solar heat. Many such efforts have run into problems with fouling of equipment caused by salt buildup, however, which often adds complexity and expense.

    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 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 sterilizing medical instruments, all without requiring any power source other than sunlight itself.

    The findings are described today 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 says. “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 some kind of wick to draw the saline water through the device, but these wicks are vulnerable to salt accumulation and relatively difficult to clean. The team 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 millimeters 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,” says Wang, who is the Ford Professor of Engineering and head of the Department of Mechanical Engineering.

    Li 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.”

    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 analyzing and understanding the convective flow that drives this entirely passive system, he says. “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,” says 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 adds.

    Just as hot air rises and cold air falls, Zhang explains, 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.

    This gif shows fluid flow visualized by food dye. The left-side shows the slow transport of colored de-ionized water from the top to the bottom bulk water. The right-side shows the fast transport of colored saline water from the top to the bottom bulk water driven by the natural convection effect.

    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. Based on their calculations, a system with just 1 square meter (about a square yard) of collecting area should be sufficient to provide a family’s daily needs for drinking water, they say. Zhang says they calculated that the necessary materials for a 1-square-meter device would cost only about $4.

    Their test apparatus operated for a week with no signs of any salt accumulation, Li says. And the device is remarkably 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,” he says.

    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. The first applications are likely to be providing safe water in remote off-grid locations, or for disaster relief after hurricanes, earthquakes, or other disruptions of normal water supplies.

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

    “I think a real opportunity is the developing world,” Wang says. “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.”

    “This is a new strategy toward solving the salt accumulation problem in solar evaporation,” says Peng Wang, a professor at King Abdullah University of Science and Technology in Saudi Arabia, who was not associated with this research. “This elegant design will inspire new innovations in the design of advanced solar evaporators. The strategy is very promising due to its high energy efficiency, operation durability, and low cost, which contributes to low-cost and passive water desalination to produce fresh water from various source water with high salinity, e.g., seawater, brine, or brackish groundwater.”

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

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    Overcoming a bottleneck in carbon dioxide conversion

    If researchers could find a way to chemically convert carbon dioxide into fuels or other products, they might make a major dent in greenhouse gas emissions. But many such processes that have seemed promising in the lab haven’t performed as expected in scaled-up formats that would be suitable for use with a power plant or other emissions sources.

    Now, researchers at MIT have identified, quantified, and modeled a major reason for poor performance in such conversion systems. The culprit turns out to be a local depletion of the carbon dioxide gas right next to the electrodes being used to catalyze the conversion. The problem can be alleviated, the team found, by simply pulsing the current off and on at specific intervals, allowing time for the gas to build back up to the needed levels next to the electrode.

    The findings, which could spur progress on developing a variety of materials and designs for electrochemical carbon dioxide conversion systems, were published today in the journal Langmuir, in a paper by MIT postdoc Álvaro Moreno Soto, graduate student Jack Lake, and professor of mechanical engineering Kripa Varanasi.

    “Carbon dioxide mitigation is, I think, one of the important challenges of our time,” Varanasi says. While much of the research in the area has focused on carbon capture and sequestration, in which the gas is pumped into some kind of deep underground reservoir or converted to an inert solid such as limestone, another promising avenue has been converting the gas into other carbon compounds such as methane or ethanol, to be used as fuel, or ethylene, which serves as a precursor to useful polymers.

    There are several ways to do such conversions, including electrochemical, thermocatalytic, photothermal, or photochemical processes. “Each of these has problems or challenges,” Varanasi says. The thermal processes require very high temperature, and they don’t produce very high-value chemical products, which is a challenge with the light-activated processes as well, he says. “Efficiency is always at play, always an issue.”

    The team has focused on the electrochemical approaches, with a goal of getting “higher-C products” — compounds that contain more carbon atoms and tend to be higher-value fuels because of their energy per weight or volume. In these reactions, the biggest challenge has been curbing competing reactions that can take place at the same time, especially the splitting of water molecules into oxygen and hydrogen.

    The reactions take place as a stream of liquid electrolyte with the carbon dioxide dissolved in it passes over a metal catalytic surface that is electrically charged. But as the carbon dioxide gets converted, it leaves behind a region in the electrolyte stream where it has essentially been used up, and so the reaction within this depleted zone turns toward water splitting instead. This unwanted reaction uses up energy and greatly reduces the overall efficiency of the conversion process, the researchers found.

    “There’s a number of groups working on this, and a number of catalysts that are out there,” Varanasi says. “In all of these, I think the hydrogen co-evolution becomes a bottleneck.”

    One way of counteracting this depletion, they found, can be achieved by a pulsed system — a cycle of simply turning off the voltage, stopping the reaction and giving the carbon dioxide time to spread back into the depleted zone and reach usable levels again, and then resuming the reaction.

    Often, the researchers say, groups have found promising catalyst materials but haven’t run their lab tests long enough to observe these depletion effects, and thus have been frustrated in trying to scale up their systems. Furthermore, the concentration of carbon dioxide next to the catalyst dictates the products that are made. Hence, depletion can also change the mix of products that are produced and can make the process unreliable. “If you want to be able to make a system that works at industrial scale, you need to be able to run things over a long period of time,” Varanasi says, “and you need to not have these kinds of effects that reduce the efficiency or reliability of the process.”

    The team studied three different catalyst materials, including copper, and “we really focused on making sure that we understood and can quantify the depletion effects,” Lake says. In the process they were able to develop a simple and reliable way of monitoring the efficiency of the conversion process as it happens, by measuring the changing pH levels, a measure of acidity, in the system’s electrolyte.

    In their tests, they used more sophisticated analytical tools to characterize reaction products, including gas chromatography for analysis of the gaseous products, and nuclear magnetic resonance characterization for the system’s liquid products. But their analysis showed that the simple pH measurement of the electrolyte next to the electrode during operation could provide a sufficient measure of the efficiency of the reaction as it progressed.

    This ability to easily monitor the reaction in real-time could ultimately lead to a system optimized by machine-learning methods, controlling the production rate of the desired compounds through continuous feedback, Moreno Soto says.

    Now that the process is understood and quantified, other approaches to mitigating the carbon dioxide depletion might be developed, the researchers say, and could easily be tested using their methods.

    This work shows, Lake says, that “no matter what your catalyst material is” in such an electrocatalytic system, “you’ll be affected by this problem.” And now, by using the model they developed, it’s possible to determine exactly what kind of time window needs to be evaluated to get an accurate sense of the material’s overall efficiency and what kind of system operations could maximize its effectiveness.

    The research was supported by Shell, through the MIT Energy Initiative. More

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    Why boiling droplets can race across hot oily surfaces

    When you’re frying something in a skillet and some droplets of water fall into the pan, you may have noticed those droplets skittering around on top of the film of hot oil. Now, that seemingly trivial phenomenon has been analyzed and understood for the first time by researchers at MIT — and may have important implications for microfluidic devices, heat transfer systems, and other useful functions.

    A droplet of boiling water on a hot surface will sometimes levitate on a thin vapor film, a well-studied phenomenon called the Leidenfrost effect. Because it is suspended on a cushion of vapor, the droplet can move across the surface with little friction. If the surface is coated with hot oil, which has much greater friction than the vapor film under a Leidenfrost droplet, the hot droplet should be expected to move much more slowly. But, counterintuitively, the series of experiments at MIT has showed that the opposite effect happens: The droplet on oil zooms away much more rapidly than on bare metal.

    This effect, which propels droplets across a heated oily surface 10 to 100 times faster than on bare metal, could potentially be used for self-cleaning or de-icing systems, or to propel tiny amounts of liquid through the tiny tubing of microfluidic devices used for biomedical and chemical research and testing. The findings are described today in a paper in the journal Physical Review Letters, written by graduate student Victor Julio Leon and professor of mechanical engineering Kripa Varanasi.

    In previous research, Varanasi and his team showed that it would be possible to harness this phenomenon for some of these potential applications, but the new work, producing such high velocities (approximately 50 times faster), could open up even more new uses, Varanasi says.

    After long and painstaking analysis, Leon and Varanasi were able to determine the reason for the rapid ejection of these droplets from the hot surface. Under the right conditions of high temperature, oil viscosity, and oil thickness, the oil will form a kind of thin cloak coating the outside of each water droplet. As the droplet heats up, tiny bubbles of vapor form along the interface between the droplet and the oil. Because these minuscule bubbles accumulate randomly along the droplet’s base, asymmetries develop, and the lowered friction under the bubble loosens the droplet’s attachment to the surface and propels it away.

    The oily film acts almost like the rubber of a balloon, and when the tiny vapor bubbles burst through, they impart a force and “the balloon just flies off because the air is going out one side, creating a momentum transfer,” Varanasi says. Without the oil cloak, the vapor bubbles would just flow out of the droplet in all directions, preventing self-propulsion, but the cloaking effect holds them in like the skin of the balloon.

    Researchers used extreme high-speed photography to reveal the details of the moving droplets. “You can actually see the fluctuations on the surface,” graduate student Victor Leon says.

    The phenomenon sounds simple, but it turns out to depend on a complex interplay between events happening at different timescales.

    This newly analyzed self-ejection phenomenon depends on a number of factors, including the droplet size, the thickness and viscosity of the oil film, the thermal conductivity of the surface, the surface tension of the different liquids in the system, the type of oil, and the texture of the surface.

    In their experiments, the lowest viscosity of the several oils they tested was about 100 times more viscous than the surrounding air. So, it would have been expected to make bubbles move much more slowly than on the air cushion of the Leidenfrost effect. “That gives an idea of how surprising it is that this droplet is moving faster,” Leon says.

    As boiling starts, bubbles will randomly form from some nucleation site that is not right at its center. Bubble formation will increase on that side, leading to the propulsion off in one direction. So far, the researchers have not been able to control the direction of that randomly induced propulsion, but they are now working on some possible ways to control the directionality in the future. “We have ideas of how to trigger the propulsion in controlled directions,” Leon says.

    Remarkably, the tests showed that even though the oil film of the surface, which was a silicon wafer, was only 10 to 100 microns thick — about the thickness of a human hair — its behavior didn’t match the equations for a thin film. Instead, because of the vaporization the film, it was actually behaving like an infinitely deep pool of oil. “We were kind of astounded” by that finding, Leon says. While a thin film should have caused it to stick, the virtually infinite pool gave the droplet much lower friction, allowing it to move more rapidly than expected, Leon says.

    The effect depends on the fact that the formation of the tiny bubbles is a much more rapid process than the transfer of heat through the oil film, about a thousand times faster, leaving plenty of time for the asymmetries within the droplet to accumulate. When the bubbles of vapor initially form at the oil-water interface, they are  much more insulating that the liquid of the droplet, leading to significant thermal disturbances in the oil film. These disturbances cause the droplet to vibrate, reducing friction and increasing vaporization rate.

    It took extreme high-speed photography to reveal the details of this rapid effect, Leon says, using a 100,000 frames per second video camera. “You can actually see the fluctuations on the surface,” Leon says.

    Initially, Varanasi says, “we were stumped at multiple levels as to what was going on, because the effect was so unexpected. … It’s a fairly complex answer to what may look seemingly simple, but it really creates this fast propulsion.”

    In practice, the effect means that in certain situations, a simple heating of a surface, by the right amount and with the right kind of oily coating, could cause corrosive scaling drops to be cleared from a surface. Further down the line, once the researchers have more control over directionality, the system could potentially substitute for some high-tech pumps in microfluidic devices to propel droplets through the right tubes at the right time. This might be especially useful in microgravity situations, where ordinary pumps don’t function as usual.

    It may also be possible to attach a payload to the droplets, creating a kind of microscale robotic delivery system, Varanasi says. And while their tests focused on water droplets, potentially it could apply to many different kinds of liquids and sublimating solids, he says.

    The work was supported by the National Science Foundation. More