Kripa VARANASI , et al.


Related: NELSON: Goodbye, Cape Town  //  BLOMGREN: ES Cooling


New system recovers fresh water from power plants

Technology captures water evaporating from cooling towers; prototype to be installed on MIT’s Central Utility Plant.

David L. Chandler

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About 39 percent of all the fresh water withdrawn from rivers, lakes, and reservoirs in the U.S. is earmarked for the cooling needs of electric power plants that use fossil fuels or nuclear power, and much of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 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.

The principle behind the new concept is deceptively simple: When air that’s rich in fog is zapped with a beam of electrically charged particles, 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 last month won MIT’s $100K Entrepreneurship Competition, is described in a paper published today in the journal Science Advances, co-authored by Maher Damak PhD ’18 and associate professor of mechanical engineering Kripa Varanasi. Damak and Varanasi are among the co-founders of the startup, and their research is supported in part by the Tata Center for Technology and Design.

Varanasi’s vision was to develop highly efficient water recovery systems by capturing water droplets from both natural fog and plumes of industrial cooling towers. 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 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. Varanasi 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 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 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 counterproductive 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 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. 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 require 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.

A typical 600-megawatt power plant, Varanasi says, 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, he says.

What’s more, since power plants are already in place along many arid coastlines, 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. Damak and Varanasi estimate that the installation cost of such a conversion would be about one-third that of a 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, 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,” Varanasi says. “It could offset the need for about 70 percent of new desalination plant installations in the next decade.”

In a series of dramatic proof-of-concept experiments, Damak, Khalil, and Varanasi demonstrated the concept by building a small lab version of a stack emitting a plume of water droplets, similar to those seen on actual power plant cooling towers, and placed their ion beam and mesh screen on it. In video of the experiment, a thick plume of fog droplets is seen rising from the device — and almost instantly disappears as soon as the system is switched on.

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. The tests will include trying different variations of the mesh and its supporting structure, Damak says.

That should provide the needed evidence to enable power plant operators, who tend to be conservative in their technology choices, 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 says. The campus power plant tests will not only “de-risk” the technology, but will also help the MIT campus improve its water footprint, he says. “This can have a high impact on water use on campus.”

Science Advances  08 Jun 2018:Vol. 4, no. 6, eaao5323

DOI: 10.1126/sciadv.aao5323

Electrostatically driven fog collection using space charge injection
Maher Damak and Kripa K. Varanasi

Fog collection can be a sustainable solution to water scarcity in many regions around the world. Most proposed collectors are meshes that rely on inertial collision for droplet capture and are inherently limited by aerodynamics. We propose a new approach in which we introduce electrical forces that can overcome aerodynamic drag forces. Using an ion emitter, we introduce a space charge into the fog to impart a net charge to the incoming fog droplets and direct them toward a collector using an imposed electric field. We experimentally measure the collection efficiency on single wires, two-wire systems, and meshes and propose a physical model to quantify it. We identify the regimes of optimal collection and provide insights into designing effective fog harvesting systems...

There are more than 1.1 billion people who lack access to safe drinking water worldwide, according to the World Water Council (1). Many remote drought-prone coastal areas have little or no rain and prohibitively expensive water transportation costs but have dense fog that occurs on a regular basis (2). Fog harvesting is a promising solution to provide clean water in these regions. Areas prone to dense fog formation are usually close to oceans, where fog clouds form over the water and are then transported by the wind (3). Fog is composed of tiny droplets with a typical diameter of 10 μm. Researchers have designed various artificial fog harvesting systems, some of which mimic natural fog collection mechanisms in animals and plants (4–11), and have successfully implemented small-scale fog collectors (12–14).

The most common design for fog collectors is a mesh that stands perpendicular to the fog-laden wind (13, 15–17). Upon impact, droplets stick to the mesh and grow as they coalesce with other incoming droplets. When they reach a critical size, they shed by gravity into a container. Meshes are used because they cause a smaller deviation of the incoming flow streamlines by letting air pass through their openings. Nevertheless, field studies have shown that these meshes typically have very low efficiencies of around 1 to 2% (17).

The main mechanisms that limit the efficiency of mesh-based fog collectors are the shedding rate and the aerodynamic deviation of fog droplets. The shedding rate can decrease the efficiency by two mechanisms. Water can clog the mesh openings, making the mesh locally act as a plate. Re-entrainment of the droplets due to wind drag may also occur before these droplets are collected. Researchers have developed coatings to improve the shedding rate, but the overall efficiency of these meshes remained low, around 10% in laboratory setups, suggesting that the main limitation is not the shedding rate (7, 9, 17).

The main limitation is the aerodynamic deviation of fog droplets, which can occur on two scales: the mesh (aerodynamic efficiency ηa) and individual mesh wires (deposition efficiency ηd). The overall collection efficiency is η = ηaηd. ηa is the number of droplets directed toward the mesh wires divided by the number of droplets that were directed toward the collector far upstream (15). It depends on the shading coefficient SC (surface fraction of the wires), and it has been shown that an SC around 55% gives a maximum ηa (16, 17).

A more significant bottleneck in the fog collection process is the deviation of the droplets around the individual wires. ηd is defined as the ratio of captured droplets to those initially directed toward the wire. The flow through the mesh wires can be modeled as a flow past a cylinder (18). Close to the cylinder of radius Rc, in a region of characteristic size Rc, air streamlines start deviating and the flow separates, as schematically shown in Fig. 1A. The ability of droplets to follow streamlines is characterized by the Stokes number, which is the ratio of the droplet inertia to the drag force (18). Embedded Image, where Rd is the droplet radius, ρw is the water density, U is the air speed, and ηg is the air viscosity. At low Stokes number, the droplet trajectories follow the streamlines closely, and few droplets are collected. Figure 1B and movie S1 show the air streamlines and droplet trajectories around a cylindrical wire for St = 0.05. At high Stokes number, drag forces do not affect the trajectories, and droplets directed toward the cylinder continue along their trajectories and collide with the cylinder. An empirical formula has been established: Embedded Image (17). However, large Stokes numbers require very fine meshes, which are difficult to fabricate and lack structural integrity. Hence, low deposition efficiencies remain a significant challenge in fog collection.

Fig. 1 Trajectories of fog droplets around a cylinder with and without the application of corona discharge.
(A and B) Schematic of air streamlines and droplet trajectories and photograph of droplet trajectories in the absence of an electric field. The inset shows the velocities of the wind Embedded Image and the particle Embedded Image and the drag force acting on the droplet. The bright ring is the edge of the cylinder. (C and D) Schematic of air streamlines, electric field lines, and droplet trajectories and photograph under corona discharge. Droplets closely follow the electric field lines in this case. The inset shows the additional electric force acting on a droplet. The cylinders in (B) and (D) have a diameter of 1.88 mm.



Fig. 2 Mechanism of droplet collection on a cylindrical wire.
(A) Schematic of simplified experimental setup and droplet trajectories. (B) Schematic of the acceleration phase undergone by droplets. The electric field, the initial and terminal velocities, as well as the forces acting on a droplet are shown. (C) Added velocity as a function of V2. A linear fit of the data (R2 = 0.94) gives a slope of 0.006 m/s per kV2. The gray area is where the voltage is not high enough to induce corona discharge. The error bars reflect the SD over four measurements. (D) Schematic of the cross section of the collection phase near the cylinder. Streamlines, field lines, and trajectories of the droplets are shown. (E) Nondimensional collection area as a function of V2 for four different wind speeds. The gray area is where there is no corona discharge.

Fig. 3 Dependence of the deposition efficiency on the electrical number.
The data correspond to five values of the wind speed and five values of the voltage. The colors represent different wind speeds (red, 0.55 m/s; blue, 0.6 m/s; green, 1 m/s; yellow, 1.65 m/s; purple, 3.3 m/s). The solid line is a linear fit (R2 = 0.92), with a slope of 0.26.

Fig. 4 Mechanism of droplet collection on two parallel cylindrical wires.
(A) Schematic of droplet trajectories with two distant wires. The collection area of each single wire Asw and the distance D are shown. (B) Schematic of droplet trajectories in a two-wire system with spacing saturation. The parameters Ain, Aout, and D are shown. The white arrows show the single-wire areas of collection Asw. (C) Photograph of the droplet trajectories for two distant wires. The wire diameters are 1.88 mm. The distance between them is 10 mm. The applied voltage is 10 kV. (D) Photograph of the droplet trajectories in a spacing saturation case. The wire diameters are 1.88 mm. The distance between them is 6 mm. The applied voltage is 14 kV. (E) Embedded Image as a function of Ke for three different wire distances. The gray region covers theoretically inaccessible values. The vertical dashed lines represent the predicted spacing saturation values for D* = 1.7 and 4.2.

Fig. 5 Fog collection on meshes.
(A) Snapshots of meshes at different time intervals of fog exposure. In the first row, a 15-kV voltage was applied, while there was no electric field in the second row. Red dye was added to the dispersed fog for visualization purposes. (B) Photographs showing the collection mesh and the storage beaker for collected water after 30 min of exposure. The case with high voltage resulted in the collection of 30 ml of water, while only three droplets were collected without electric field. Complete video of collection is shown in movies S3 to S4. (C) Mass of the collected water as a function of Ke for different meshes. The vertical dashed lines represent the predicted onset of spacing saturation from the two-wire model for meshes 1 to 3. (D) Deposition efficiency of the five meshes as a function of Ke. The colors represent different meshes according to the color code of (C).F


Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/6/eaao5323/DC1

Tackling the global water crisis

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.


Water-Energy Nexus

​Infinite Cooling's mission is to provide novel technology to enable water-sustainable thermoelectric power. According to studies by the UN and the US State Department, we are on the path to an extreme freshwater shortage by 2030. Power plants are the US’s largest water consumer - power plants use 161 billion gallons of freshwater per day, 39% of total US freshwater withdrawals. Power plants typically use evaporative cooling, where a portion of the water is evaporated to cool the remaining water. The vapor is released into the atmosphere, where it forms a plume. New water needs to be frequently added to the cooling system to account for the lost water vapor. The system's remaining water becomes more concentrated in salts and pollutants and needs to be treated, at an additional cost. A single 600MW power plant will consume the same amount of water as 100,000 residential users and spend $5M/year on water alone. Infinite Cooling can help power plants produce reliable electricity while using significantly less water.

Novel water capture technology

Our patent-pending technology developed at MIT reduces water consumption in evaporative cooling tower systems by over 20% by capturing water from cooling tower plumes
Water Savings -- 20-30% reduction in total plant water consumption (make-up and blowdown)
Costs Savings -- $1M annual savings in water sourcing and water treatment costs for a 600 MW system
Plume Abatement -- 100% plume elimination on cooling towers retrofitted with Infinite Cooling technology

US2017135340 / AU2016342200
Systems and methods for surface retention of fluids


Systems and methods related to the formation of a reaction product on a surface are generally provided. The systems and methods described herein may allow for collection of the retention of a fluid by a surface in a relatively high amount. Such systems and methods may be useful in various applications including, for example, agriculture. In some embodiments, the systems and methods enhance water retention on hydrophobic surfaces of plants. Advantageously, the methods described herein may, in some cases, result in the formation of reaction products on a surface that serve to prevent fluids from being removed from the surface. Advantageously, the systems and methods described herein may suppress the adverse effects of natural conditions such as high surface energies and wind.

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