Engineers from MIT have developed a new system that could provide an economical source of drinking water whilst also helping to reduce power plant operating costs.
Almost 39% of all the fresh water taken from lakes, rivers, and reservoirs in the U.S. is reserved for the cooling requirements of electric power plants that use nuclear power or fossil fuels, and a lot of that water ends up vaporizing.
The new system developed by MIT aims to save a considerable fraction of that lost water and could become an important source of clean, safe drinking water for cities situated near the sea where seawater is employed to cool local power plants.
When the air that is rich in fog is hit with a beam of electrically charged particles, called ions, water droplets become electrically charged and as a result can be drawn toward a mesh of wires, resembling a window screen, positioned in their path.
The droplets begin to collect on that mesh, and then drain down into a collecting pan. This water can be reused in the power plant or transported to the city’s water supply system.
The system, which is the foundation for a startup company called Infinite Cooling that last month won MIT’s $100K Entrepreneurship Competition, is illustrated in a paper published in the June issue of the journal Science Advances, co-authored by Maher Damak PhD ’18 and associate professor of mechanical engineering Kripa Varanasi. Damak and Varanasi are two of the startup’s co-founders.
Varanasi’s vision was to build efficient water recovery systems by capturing water droplets from natural fog as well as plumes of industrial cooling towers. The project started as part of Damak’s doctoral thesis, which aimed to enhance the efficiency of fog-harvesting systems that are used in numerous water-scarce coastal regions as a source of drinkable water.
Those systems, which commonly consist of some kind of metal or plastic mesh hung vertically in the path of fogbanks that commonly roll in from the sea, are very inefficient, capturing just about 1 to 3% of the water droplets that pass via them. Varanasi and Damak speculated if there was a way to make the mesh capture more of the droplets—and discovered a very simple and effective approach to doing so.
The reason for the inadequacy of current systems became obvious in the team’s thorough lab experiments: the system’s aerodynamics. As a stream of air reaches an obstacle, such as the wires in these mesh fog-catching screens, the airflow naturally swerves around the obstacle, quite like air flowing around an airplane wing divides into streams that pass over and under the wing structure.
These deviating airstreams transport droplets that were heading toward the wire off to the side, unless they were headed straight toward the wire’s center.
The result is that the fraction of droplets trapped is a lot lower than the fraction of the collection area occupied by the wires, as droplets are being pushed aside from wires that lie in front of them. Simply making the wires bigger or the spaces in the mesh smaller is likely to be counterproductive as it obstructs the overall airflow, causing a net decrease in the collection.
But when the incoming fog gets hit first with an ion beam, the opposite effect takes place. Not only do all of the droplets that are in the path of the wires fall on them, even droplets that were aiming for the holes in the mesh tend to get pulled toward the wires.
This system can trap a much larger fraction of the droplets passing through. As such, it could greatly enhance the efficiency of fog-catching systems, and at a remarkably low cost. The equipment is basic, and the amount of power needed is minimal.
Subsequently, the team concentrated on trapping water from the plumes of power plant cooling towers. Here, the stream of water vapor is a lot thicker than any naturally occurring fog, and that makes the system much more efficient.
Furthermore, since trapping evaporated water is in itself a distillation process, the water captured is pure, even if the cooling water is contaminated or salty. At this juncture, 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. That’s what we’re trying to capture.”
Kripa Varanasi. Damak, Co-Author
The water could be piped to a drinking water system in the city, or used in processes that require pure water, such as in a power plant’s boilers, in contrast with being used in its cooling system where water quality does not matter much.
A standard 600-megawatt power plant, Varanasi says, could capture 150 million gallons of water annually, signifying a value of millions of dollars. This signifies around 20 to 30% of the water lost from cooling towers. With additional refinements, the system may be able to trap even more of the output, he adds.
This can be a great solution to address the global water crisis. It could offset the need for about 70 percent of new desalination plant installations in the next decade.”
Kripa Varanasi. Damak, Co-Author
In a series of proof-of-concept experiments, Damak, Khalil, and Varanasi showed the concept by developing a small lab version of a stack releasing a plume of water droplets, akin to those observed on actual power plant cooling towers, and positioned their ion beam and mesh screen on it.
In the video of the experiment, a thick plume of fog droplets can be seen rising from the device and almost instantaneously disappears as soon as the system is turned on.
The team is presently developing a full-scale test version of their system to be positioned on the cooling tower of MIT’s Central Utility Plant, a natural-gas cogeneration power plant that provides most of the campus’ electricity, cooling, and heating.
The installation is expected to be ready by the end of the summer and will be tried out in the fall. The trial tests will include attempting various variations of the mesh and its supporting structure, Damak explains.