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.
In 2013, thermo power generation in Canada consumed 397 million m3 of water according to Statistics Canada. In the U.S., about 39% of all fresh water withdrawn is earmarked for the cooling needs of electric power plants that use fossil fuels or nuclear power, and much of that water ends up floating away in clouds of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.
The new MIT 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.
When air that’s rich in fog is zapped with a beam of electrically charged particles, 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 last month won MIT’s $100K Entrepreneurship Competition, is described in a paper published in the journal Science Advances. It is co-authored by Maher Damak and Kripa Varanasi, both among the co-founders of the startup.
The project began as part of Damak’s doctoral thesis, which aimed to improve the efficiency of fog-harvesting systems that are used in many water-scarce coastal regions. Those systems, which generally consist of a mesh hung vertically in the path of fogbanks that regularly roll in from the sea, are extremely inefficient, capturing only about 1% to 3% of the water droplets that pass through them.
The reason for this inefficiency is in the aerodynamics of the system. As a stream of air passes an obstacle, such as the wires in these mesh fog-catching screens, the airflow naturally deviates 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 straight towards the wire’s center.
But when the incoming fog gets hit first with an ion beam, the opposite 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.
Video: How the system works
Next, the team focused on capturing water from the plumes of power plant cooling towers, which are much more concentrated than naturally occurring fog. This also 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.
A typical 600-megawatt power plant, Varanasi says, could capture 568,000 m3 of water a year, representing a value of millions of dollars. This represents about 20% to 30% of the water lost from cooling towers. With further refinements, the system may be able to capture even more of the output, he says.
For the many power plants located on arid coastlines and that 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.
Damak and Varanasi estimate 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 1/50th. Varanasi says the payback time would be about two years, and it would have essentially no environmental footprint.
The team is currently building a full-scale test version of their system to be placed on the cooling tower of MIT’s Central Utility Plant, a natural-gas cogeneration power plant that provides most of the campus’ electricity, heating, and cooling. The setup is expected to be in place by the end of the summer and will undergo testing in the fall.
Testing should provide the needed evidence to enable power plant operators to adopt the system. Because power plants have decades-long operating lifetimes, their operators tend to “be very risk-averse” and want to know “has this been done somewhere else?” Varanasi said. The campus power plant tests will not only “de-risk” the technology, but will also help the MIT campus improve its water footprint.