Every once in a while, a statistic crops up that takes us by surprise. According to study published in the journal Science Advances, researchers at MIT claim 39% of all the freshwater withdrawn from rivers, lakes, and reservoirs in the US is earmarked for the cooling needs of electric power plants that use fossil fuels or nuclear power. What? That’s crazy talk, isn’t it?
Apparently not. Those generating facilities create lots of heat which is then used to make steam to run the turbines that make the electricity. Too much heat and the whole thing goes boom, much to the consternation of utility industry investors. A cooling tower is nothing more than the radiator in your car — a device that uses water to keep everything in the system working at the proper temperature.
Unlike a radiator, however, a cooling tower is an open system. Much of the water used is turned to vapor, which escapes into the surrounding atmosphere. Researchers at MIT have devised a method of recapturing some of that water vapor with a process they say is cost effective and may even save utilities money.
Working together, PhD candidate Maher Damak and associate professor of mechanical engineering Kripa Varanasi have devised a simple mesh screen that can be installed inside cooling towers. When the escaping moisture is exposed to an ion beam, some of the molecules are attracted to the metal mesh where they form droplets that then fall by gravity to be collected below.
Previous systems had an efficiency of less than 3% largely because the screens disturbed the air flow in the cooling towers. Using the ionization method, the water molecules cling to the mesh without as much disruption to the air flow, resulting in a moisture recapture rate of about 30%. For a typical 600 MW power plant, the system could recover about 150 million gallons of fresh water each year. That water could be a new source of revenue for power plant operators.
Notice the word “fresh” in that last sentence. The process works much like distillation. Only water molecules are collected. Impurities continue upward and out of the cooling tower. Since many generating plants are located in coastal areas and use sea water in their cooling towers, the effect is the same as operating a desalinization plant but at only 2% of the cost, says Varanasi. “It’s distilled water, which is of higher quality, that’s now just wasted. That’s what we’re trying to capture.”
Damak and Varanasi estimate that the installation cost of their system would be about one third that of building a new desalination plant, according to Science Daily. With such low operating costs, the payback time for such a system would be about two years, Varanasi says, and it would have essentially no environmental footprint, adding nothing to that of the original plant. “This can be a great solution to address the global water crisis,” he says. “It could offset the need for about 70 percent of new desalination plant installations in the next decade.”
The pair have formed a new business venture called Infinite Cooling, which last month won the top prize of $100,000 in the MIT Entrepreneurship Competition. Utility companies are the only prospective customers for the process and Varanasi says they tend to be highly risk averse. They typically don’t want to try something unless it has already been tested and proven elsewhere. MIT has agreed to install a prototype system at its Central Utility Plant this summer. Once in place, the pair can experiment with different mesh materials and gather data to demonstrate the effectiveness of the technology.
Of course, the whole process depends on customers who operate nuclear or fossil fueled generating plants, whose time may be coming to an end as the era of renewables continues to advance. But realistically, those old technology facilities are going to be around for decades more and if they can help contribute fresh water supplies in areas where access to water is limited in the meantime, the new MIT process will be welcome news.
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