Cape Town narrowly avoided Day Zero earlier this year, when the city's taps were projected to run dry, but the city is expected to face another critical shortage in 2019. The situation has become so dire that officials are seriously considering importing icebergs to augment the water supply. But why try to tow 70,000 tons of ice 1,200 miles up from Antartica when modern technology can suck the humidity we need out of thin air?

Pulling potable water from the atmosphere goes back to the days of the Incas, who captured and collected dew from "fog fences." And why wouldn't they. Earth's atmosphere contains an estimated 13 trillion liters of water vapor at any given time -- that's the equivalent to 10 percent of all of the planet's available surface freshwater. Of course, the process of collecting morning dew for a pre-industrial society exists on a completely different scale than what will be needed to quench the thirst of the 4 billion or so people who will be impacted by water shortages in the coming years.

But just because the challenge is daunting doesn't mean it's impossible. A number of researchers and private companies are developing the next generation of atmospheric water harvesting devices. The current state-of-the-art dew-harvesting refrigeration-based machines operate by lowering the temperature of the surrounding air below the dew point and then collecting the vapor. Unfortunately, as the relative humidity drops below 50 percent, the efficiency of these machines nosedives too, requiring untenable amounts of electrical power to operate the refrigeration units.

However a newly developed sorbent-based alternative has recently shown that it can harvest atmospheric moisture even when the relative humidity drops to around 10 percent. Under those conditions, that works out to around three liters of water for every million liters of air. If you were to try to harvest that using dewing technology, you'd have to drop the air temperature to below freezing. Once that happens, it "becomes technologically infeasible because you have to first unfreeze the water," Dr. Evelyn Wang, a Professor of Mechanical Engineering at MIT who helped construct the test device, told Engadget. "The advantage of the work that we're doing is the fact that you can in fact operate efficiently the system at low humidity conditions."

Created by a collaborative team from MIT and UC Berkeley, this proof of concept device uses a metal-organic framework (MOFs), a material invented in the 1990s by UC Berkeley chemistry professor Omar Yaghi. Depending on the metal and organic molecules used, chemists can control what gases will bind to the MOF and how strong those connections will be.

Yaghi's team has developed more than 20,000 types of MOF in the past two decades, one of which bound to methane and has been considered for use in the fuel tanks of natural gas-powered vehicles. In this case, however, the research team used a variant dubbed MOF-801 -- Zr6O4(OH)4(fumarate)6 -- which had previously shown itself to collect more water and need smaller temperature swings to work than conventional sorbents like zeolite or silica gel.

Rather than being powered by an electrical grid, the MOF device designed by Wang's team only requires a heat source to separate water molecules from the MOF's organic bits. "[The heat source] can be that of the sun, it could be other sources as well," Wang explained. "What we anticipate is pretty much a passive system."

The device itself is a simple design with two main parts: the absorption layer (aka the MOF) and an enclosed, an air-cooled condenser. The backside of the MOF is painted black, to operate as a solar absorber.

At night, the enclosure walls are opened to enable airflow past the MOF and it becomes saturated with vapor. Once morning rolls around, the enclosure is buttoned up and the black side of the MOF is covered with an "optically transparent thermal insulator" (OTTI). When the MOF is exposed to sunlight, it heats up, causing its organic molecules to release their hold on their water molecules, which condense at ambient temperature for collection.

Last year, the team set up the MOF device atop a roof on the University of Arizona campus, where the relative humidity during the day drops to as low as 10 percent but climbs to an average of 30 percent at night. Using just 3g of MOF, the team extracted about .75g of water -- not enough to quench your thirst but plenty to validate the team's modelling predictions.

"A lot of the tailoring of the MOFs is important because there are other types of material that can absorb water," Wang explained. "The key here is that it's not just the amount of water you absorb, but also the amount you can release via low-grade heat sources."

The MOF device is still in its early prototype stage, though Dr. Wang does expect to have a commercially viable iteration capable of providing the daily needs for a family of four available within 3 to 5 years. She envisions this technology being used as a primary source of drinking water in the developing world and remote locations where there is no water infrastructure but could also be employed in urban settings, much like home solar panels are used today.

But before that can happen, Dr. Wang cites a couple challenges that must be addressed, "MOFs are really compelling in terms of the performance but right now the scalability to larger volumes is limited for certain types and the cost is also pretty high."

Dr. Wang's team is far from the only outfit working to collect water from cloudless skies. The US EPA recently signed a Cooperative Research and Development Agreement with Israel-based Water-Gen. The company's "GENius" technology has been around since 2014 and works much like a standard air conditioner, except that it produces potable water. Unfortunately, for it to generate 2 liters per hour, each half-meter of structural material has to be exposed to air with a relative humidity of 60 percent.

Similarly, the Australian Renewable Energy Agency (ARENA) agreed earlier this week to purchase 150 solar-powered harvesters from Arizona's Zero Mass Water. These 300-pound devices rely on a 32 square foot solar panel to power a small fan which pushes ambient air past the company's proprietary hygroscopic material for capture. The water then filters down into a 30 liter reservoir. Depending on the relative humidity, these machines can collect between four and 10 liters of water daily. And unlike Water-Gen's technology, Zero Mass' devices operate with humidity levels as low as 10 percent.

It remains unlikely that atmospheric water generation will be the magic bullet to solving the Earth's growing potable water shortage issues. But combined with other evolving technologies such as oceanic desalinization, humanity may just be able to continue slaking the thirst of an increasingly populated and parched planet.

Images: UC Berkeley and Berkeley Lab (MOF structure); Nature Communications (harvest process)