Recovering fresh water from power plants Prototype technology captures water evaporating from UTILITY PLANT cooling towers.
A NEW SYSTEM devised by MIT engineers could provide a low-cost source of drinking water for parched cities around the world while also cutting power plant operating costs. About 39 percent of all the fresh water withdrawn from rivers, lakes, and reservoirs in the U.S. is earmarked for the cooling needs of electric power plants that use fossil fuels or nuclear power, and much of that water ends up floating away in clouds of vapor. But the newMIT system could potentially save a substantial fraction of that lost water — and could even become a significant source of clean, safe drinking water for coastal cities where seawater is used to cool local power plants. The principle behind the new concept is deceptively simple: When air that’s rich in fog is zapped with a beam of electrically charged par- ticles, known as ions, water droplets become electrically charged and thus can be drawn toward a mesh of wires, similar to a window screen, placed in their path. The droplets then collect on that mesh, drain down into a collecting pan, and can be reused in the power plant or sent to a city’s water supply system. The system, which is the basis for a startup company called Infinite Cooling that recently won MIT’s $100,000 Entrepreneurship Competi- tion, is described in a paper published in the journal Science Advances, co-authored by Maher Damak, Ph.D., and Kripa Varanasi, Ph.D., as- sociate professor of mechanical engineering. Damak and Varanasi are among the co-founders of the startup. Varanasi’s vision was to develop highly efficient water recovery sys- tems by capturing water droplets from both natural fog and plumes of industrial cooling towers. The project began as part of Damak’s doc- toral thesis, which aimed to improve the efficiency of fog-harvesting systems that are used in many water-scarce coastal regions as a source of potable water. Those systems, which generally consist of some kind of plastic or metal mesh hung vertically in the path of fogbanks that regularly roll in from the sea, are extremely inefficient, capturing only about 1 to 3 percent of the water droplets that pass through them. Vara- nasi and Damak wondered if there was a way to make the mesh catch more of the droplets — and found a very simple and effective way of doing so. The reason for the inefficiency of existing systems became apparent in the team’s detailed lab experiments: The problem is in the aerodynam- ics of the system. As a stream of air passes an obstacle, such as the
wires in these mesh fog-catching screens, the airflow naturally devi- ates around the obstacle, much as air flowing around an airplane wing separates into streams that pass above and below the wing structure. These deviating airstreams carry droplets that were heading toward the wire off to the side, unless they were headed bang-on toward the wire’s center. The result is that the fraction of droplets captured is far lower than the fraction of the collection area occupied by the wires, because droplets are being swept aside from wires that lie in front of them. Just making the wires bigger or the spaces in the mesh smaller tends to be coun- terproductive because it hampers the overall airflow, resulting in a net decrease in collection. But when the incoming fog gets zapped first with an ion beam, the op- posite effect happens. Not only do all of the droplets that are in the path of the wires land on them, even droplets that were aiming for the holes in the mesh get pulled toward the wires. This system can thus capture a much larger fraction of the droplets passing through. As such, it could dramatically improve the efficiency of fog-catching systems, and at a surprisingly low cost. The equipment is simple, and the amount of power required is minimal. Next, the team focused on capturing water from the plumes of power plant cooling towers. There, the stream of water vapor is much more concentrated than any naturally occurring fog, and that makes the system even more efficient. And since capturing evaporated water is in itself a distillation process, the water captured is pure, even if the cooling water is salty or contaminated. At this point, Karim Khalil, another graduate student from Varanasi’s lab joined the team. “It’s distilled water, which is of higher quality, that’s now just wasted,” says Varanasi. “That’s what we’re trying to capture.” The water could be piped to a city’s drinking water system or used in processes that re- quire pure water, such as in a power plant’s boilers, as opposed to being used in its cooling system where water quality doesn’t matter much.
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august 2018
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