WASHINGTON, June 9 (Xinhua) -- American engineers have devised a new system that may provide a low-cost source of drinking water for arid city while cut power plant operating costs and revolutionize sea water desalination.
The system can efficiently capture the droplets from both natural fog and plumes of industrial cooling towers, according to a study published on Friday in the journal Science Advances.
About 39 percent of all the fresh water withdrawn from rivers, lakes and reservoirs in the United States is used for the cooling needs of power plants that use fossil fuels or nuclear power and much of that water ends up floating away in clouds of vapor.
The new installation could potentially save a substantial fraction of that lost water and even become a significant source of clean, safe drinking water for coastal cities where seawater is used to cool local power plants.
When the air that is rich in fog is zapped with a beam of electrically charged particles, known as ions, water droplets in it 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, according to the study.
Initially, Maher Damak, a postdoctoral graduate from Massachusetts Institute of Technology (MIT) used some kind of plastic or metal mesh hung vertically in the path of fogbanks that regularly roll in from the sea, but it could capture only about one to three percent of the water droplets.
As a stream of air passes an obstacle, the airflow naturally deviates around the obstacle, and in this case, water droplets are being swept aside from wires that lie in front of them, according to the researchers.
But then they found that when the incoming fog gets zapped first with an ion beam, not only 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.
SEA WATER DESALINATION
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.
Since capturing evaporated water is in itself a distillation process, the water captured is pure, even if the cooling water is salty or contaminated.
"It's distilled water, which is of higher quality, that's now just wasted," said Kripa Varanasi, associate professor of mechanical engineering at MIT, who co-founded a startup with Damak.
A typical 600-megawatt power plant, according to Varanasi, could capture 150 million gallons of water a year, representing a value of millions of dollars.
This represents about 20 to 30 percent of the water lost from cooling towers. With further refinements, the system may be able to capture even more of the output, Varanasi said.
Additionally, since power plants are already in place along many arid coastlines in the United States and many of them are cooled with seawater, this provides a very simple way to provide water desalination services at a tiny fraction of the cost of building a standalone desalination plant.
They estimated that the installation cost of such a conversion would be about one-third that of building a new desalination plant, and its operating costs would be about 1/50.
The payback time for installing such a system would be about two years and it would have essentially no environmental footprint, adding nothing to that of the original plant, according to Varanasi.
"This can be a great solution to address the global water crisis," Varanasi said. "It could offset the need for about 70 percent of new desalination plant installations in the next decade."
