Fresh water makes up about 2.5 percent of all the water on Earth. A small fraction of that, 0.04 percent, is held in the atmosphere in the form of clouds or water vapor. Mankind has dreamed of harnessing this water source for untold centuries. Ancient civilizations from the Atacama and Namib deserts to the South Downs of England took their cues from plants and rock formations to induce condensation using piles of stones and dew ponds. More recently, scientists have experimented with higher-tech means, strewing chemicals and even shooting laser beams to try to seed clouds and trigger rainfall. (The techniques, however, have so far proved only slightly more effective than traditional rain dances.)
Today, efforts to coax water from the air have taken on renewed importance as climate change models project significant shifts in rainfall patterns around the world. Adapting to these disruptions will be essential for the world's agricultural industries to preserve the conditions that various crops require to thrive. To that end, scientists are pushing the limits to take precipitation into their own hands. But what would it take to pull water from the atmosphere on an industrial scale?
The law of thermodynamics that governs a gas's transition to liquid form is simple enough: Increasing pressure or lowering temperature beyond a certain threshold results in condensation. The full physics involved in the formation of droplets in large, dusty air masses in motion, such as those in our planet's atmosphere, is far from completely elucidated, though. Turning theoretical insights into systems suitable for daily operation is largely a matter of trial and error. Methods to capture water from the air include passive operations — such as those used in ancient civilizations — and active techniques, which require the application of outside energy.
On the Fence About Fog Harvesting
One of the simplest passive methods uses fine-mesh nets hung vertically to trap the moisture from fog. As the net attracts water molecules in the air, droplets gather and drip off the net by force of gravity. Partial evaporation of the moisture collecting in the net, meanwhile, lowers its temperature, thereby encouraging further condensation. A gutter hanging below catches the falling drops from the underside of the net and routes the water collected where it is needed — for instance, to irrigate plants nearby. The setup mimics the way foliage catches dew, but using a much larger surface area, pushing agricultural cultivation beyond what local conditions naturally allow. It can even help reverse desertification by helping to initiate plant growth until vegetation is lush enough to catch the dew necessary to sustain itself. More important, it's an easy and low-cost solution that will work anywhere under the right conditions, at least in theory.
But in practice, fog nets have yielded lackluster results. Part of the problem is cultural: Poor farming communities often view the technique as too primitive to be of much use and instead prefer modern pipeline systems to bring water in from elsewhere. Protecting the nets against sustained exposure to sun and wind, as well as creeping degradation from fungal growth, is another challenge. Perhaps the biggest obstacle, however, is the inherently limited capacity of the cheap plastics used to coax moisture from the air.
Researchers at the Massachusetts Institute of Technology have found a breakthrough solution for the last problem at least. Inspired by the fogstand beetle — an insect famous for catching vapor droplets with its wings in southern Africa's Namib Desert — the team developed coatings made from water-attracting and -repelling nanofibers woven in an alternating pattern. NDB Nano, a company based in Boston that is working to commercialize the discovery, has persuaded a German firm to use the material to increase the efficiency of the condenser tubes it manufactures for large power plants. The technology has further applications in paints to protect plastic and metal structures against humidity and in coatings for fog-resistant windows. As the production scales up, the resulting drop in its price could eventually enable its use in lower-end plastic fabrics, such as the nets that make up fog fences.
The Condensed Version
Along with the passive technologies, an array of devices are available that could be used to actively condense water. Designs for appliances such as air conditioners and refrigerators could be adapted to maximize the condensation they produce, turning what is generally viewed as a nuisance into an asset. Dehumidifiers can also be reimagined as atmospheric water-makers. The same thermodynamic principles that govern the low-tech fog fence are still at play in these more complicated contraptions; the devil lies in the combination of technical details.
Simply modifying an air conditioner to condense water won't cut it at an industrial scale. Maximizing efficiency depends on several factors, including the source of energy used to power the device, the medium from which the water is extracted, the means of condensation and the requirements necessary to render and keep the water generated potable. Water-Gen LTD, a company based in Israel, is at the forefront of atmospheric water generator technology. The firm offers vapor extractors with capacities ranging from 6,000 liters (1,585 gallons) of pure water per day for industrial applications to about 25 liters per day for home use. No matter their size, though, atmospheric water generators guzzle energy. In average weather conditions, machines that produce about 4,000 liters of water per day, and can be mounted on trailers for military purposes or emergency relief, burn one unit of fuel for every five units of water they generate. A reverse-osmosis generator that desalinates brackish water requires just a fraction of that energy by comparison.
Recent technological developments promise to address this cost-benefit bottleneck. As solar panels become steadily more efficient for generating electricity, their off-grid applications are expanding to include possible uses in atmospheric water generation. And other breakthroughs — such as an innovation to make semi-conductor materials that heat up on one side and cool down on the other when an electrical current passes through (a phenomenon known as the Peltier effect) — could drastically simplify the design of these machines.
The systems available today to generate precipitation without a water source nearby still cover only niche applications. Narrow operating conditions limit the utility of passive technologies, while active methods have a long way to go before they are efficient enough for widespread use. Nonetheless, each system offers a meaningful step forward on humanity's quest to tame the increasing vagaries of the water cycle.