Water Catchers
At first, it sounds like a magic trick: Watch and be amazed as we pull water out of thin air. Yet for a team of Stanford mechanical engineers, the research is grounded firmly in reality.
Water vapor in the atmosphere is equivalent to about 15 percent of global rivers and lakes. This is why professors Juan Santiago, Kenneth Goodson, adjunct professor Mehdi Asheghi, and former postdoctoral scholar Alexandros Terzis (pictured l to r) see air as a viable resource for meeting humanity’s growing water needs.
Last year they demonstrated a more efficient method for extracting water from air, in preliminary research supported by the TomKat Center for Sustainable Energy. They see the technology as a complement to electrical power plants that are in need of water for cooling towers. It could become a virtuous loop, as those same towers go on to produce warm air that could drive the water harvesting process.
“We basically take low-grade heat and turn it into freshwater,” says Santiago.
“You can actually increase the energy efficiency of the power plant. In so many ways, it’s a beautiful match of these technologies,” adds Asheghi.
Super soakers
Atmospheric water harvesting is not a new idea. Today over 70 companies sell devices for this purpose. Some of these devices are similar to dehumidifiers, using a cooling coil to mimic how dew forms in nature. Others are built with desiccants, those same moisture-wicking crystals found in sachets in vitamin jars and snack packs.
The Stanford professors are exploring a newer kind of material called metal-organic frameworks, or MOFs. First invented in the late 1990s, MOFs are made up of metal ions and ligands linked together in a porous network. Typically, MOFs are prepared as fine powders – the largest particles are only a few hundred microns in diameter.
MOFs are famous for their exceptionally high surface areas. Depending on the choice of metal ion and ligand, some MOFs have the ability to take up an astonishing amount of water as air passes through their pores. The water can be released by increasing the temperature and collected by a condenser, providing a clever way to harvest liquid water from the air.
The catch is that MOFs are pricey, although costs have lowered significantly thanks to the efforts of many groups throughout the world.
The Stanford team focused their efforts on improving a particular metric: the water flow rate per mass of material. Whereas prototypes by past innovators could only run a few cycles of adsorption/desorption in a day, the Stanford proof-of-concept has topped 40 cycles in a day.
“It’s like we can bake a 40-minute cake in one minute,” says Santiago, explaining the significance of this order of magnitude improvement.
Bench-scale discoveries
The demonstration project is roughly the size of a pint glass of beer with tubes running in and out of it to control the humidity and temperature.
While past inventions often tamped down and even bound MOF particles together, the Stanford researchers took a different approach and kept the microscopic powder loose and in its raw form.
When cycled through what is called a fluidized bed—where solid particles are levitated by an air stream—the MOF works more quickly. This approach allows for easier mass and temperature exchange as the MOF particles jumble up and down, looking like a frothy geyser in a test tube. Then, by adding some heat, the super-absorbent material produces a high humidity air stream that can be cooled (e.g., at or near room temperature) to condense and recover the water.
Large-scale fluidized beds are commonly used in the petrochemical industry and sometimes in wastewater treatment. This is the first time the idea has been applied to atmospheric water harvesting.
Synergy in savings
The researchers are most excited about how the advance could synch up with power plants. What is now a bench-scale model could one day become a utility-scale solution.
In the United States, the vast majority of water is used to cool thermoelectric power plants and to irrigate farmland, far more than households consume for daily chores. For example, according to the U.S. Geological Survey, in 2015, thermoelectric power withdrew 133 billion gallons of water compared to 38 billion gallons for the public supply.
Because it takes electricity to purify and transport water, and because water is needed to generate electricity, this is sometimes called the energy-water nexus. Only instead of a dilemma, the Stanford researchers see the nexus as a chance for synergy.
Their water harvester depends on a flow of warm air. Meanwhile, power plants have an abundance of low-grade energy that can be used to heat the air. In fact, an estimated 20 to 50 percent of industrial energy is lost this way. Most power plants produce a steady stream of what is often called “waste heat” because it is too lukewarm to be useful—that is, perhaps, until now.
The 24/7 waste heat would allow the water harvester to operate overnight, when air humidity is the highest. That water would then be used to help cool the power plant in the hottest part of the day. Unlike desalination, which is energy intensive and produces a salty brine that is difficult to dispose of, this freshwater technology could become a clean, closed loop.
“Our waste products are heat and a little dry air—and those were headed into the environment anyway. We’re just getting in the middle,” says Santiago.
Funding a future
The TomKat Center grant allowed the team to complete important early research.
“Other agencies wouldn’t have touched it. The research was too high risk,” says Santiago, explaining how their innovation, if it works on a large scale, will be highly impactful—but the if was too much of a gamble for traditional research grants.
Asheghi mentions that since publishing their research in the journal Cell Reports Physical Science last May, that perspective has begun to shift. “From the point of view of external funders, now we have a proof of concept,” he says.
He envisions other applications for the same fluidized bed design, perhaps using MOFs to capture carbon emissions from the atmosphere or to build out devices for drinking water that could be useful for desert communities or the military. For now, he is pleased by promising starts, as the team filed a patent last fall.
“If we can pull this off, it’s a win-win,” says Santiago.
