A More Efficient Way to Drink the Ocean
August 15, 2017 By Bob Silberg
It was unfortunate for Coleridge’s Ancient Mariner and his shipmates, becalmed in mid-ocean with no drinking water, that they did not serve on a modern U.S. Navy ship. Most Navy vessels carry desalination equipment, which extracts freshwater from seawater, so their crews never go thirsty.
Increasingly, landlubbers with better access to the ocean than to sources of freshwater are also employing technology that enables them to drink water from the sea. And a pair of students in Caltech’s PhD program, Jinglin Huang and Cong Wang, are developing a way to improve that technology.
There are two main approaches to desalination. The most popular is reverse osmosis (RO), in which seawater is pumped through membranes that allow freshwater to pass through but reject the salts.
The second technique is nature’s method, familiar to everyone who has encountered the water cycle in school. Heated water evaporates from seawater as a vapor, leaving the salts behind, then cools and condenses into liquid freshwater. It’s called “distillation” when people do it.
One major challenge of distillation is to avoid scaling, a buildup of those left-behind salts on the heated surfaces over which the seawater passes. If that happens, the operation has to be shut down periodically for cleaning. “That’s an expensive, tiresome process,” said Richard Stover, one of the directors of the International Desalination Association (IDA). “But any well-operated plant is not going to have regular cleaning requirements.”
How do modern plants avoid the need to clean scale? By keeping the water temperature below the limit at which scale forms.
In contrast to substances like sugar and table salt, some of the salts in seawater—like calcium sulfate and calcium carbonate—become less soluble as water gets hotter. At a certain temperature, which depends in part on how concentrated the saltwater solution is, they precipitate out, causing scaling.
So limiting water temperature prevents scaling, but at the expense of efficiency. That’s where Huang and Wang’s invention comes in.
Making Desalination Greener
The innovation under development at Caltech, if successful, will significantly reduce or even eliminate the risk of scaling. And that will enable higher temperatures, greater efficiency and lower cost.
According to Leon Awerbuch, who is dean of the IDA Desalination Academy in addition to serving on IDA’s board of directors, water temperature for a popular type of distillation process called multi-effect distillation (MED) is currently kept to about 65 to 75 degrees Celsius (149 to 167 degrees Fahrenheit). If the improvement under development by Huang and Wang is successful, he said, “we could increase the temperature to something like 85 degrees, hopefully. And that would improve the efficiency by 30 or 40 percent.”
Making the distillation process more efficient could also accelerate the move toward green energy sources for desalination.
Awerbuch reported that the IDA, the United Arab Emirates and the French government have created the Global Clean Water Desalination Alliance with the goal of making desalination sustainable. “By 2030, we hope to power about 80 percent of all desalination plants with renewable energy,” he said. “There are over 18,000 desalination plants around the world, so this is a challenge.”
Huang and Wang’s invention may expedite that effort. Some RO plants are powered by photovoltaic panels, those devices that convert sunlight to electricity. But they can operate only when the sun is shining. The technology to store electricity has not yet produced a battery that can keep such a plant running after dark.
Distillation plants need less electricity for pumping the water since they don’t have to push seawater through a membrane. They do need to heat the water, but they can do that with sunlight. And it’s currently much easier to store hot water than electricity.
“Thermal processes allow 24-hour operation,” Awerbuch said. “It’s my personal belief that in renewable energy, particularly solar energy, thermal processes would probably be better than photovoltaic.”
Further, Huang and Wang point out that water in the distillation process offers gradients in both temperature and salt concentration, and that such gradients can be used to generate electricity. “Maybe we don’t need any (external) power, just the power it generates, itself,” Cong said. “We really want to make the whole system work as a sustainable thing,” Huang added.
An Effervescent Invention
So what is this invention that’s underway at Caltech? It’s a coating material consisting of carbon nanotubes (CNTs), which are cylinders only one-billionth of a meter in diameter. A human hair is more than 10,000 times thicker. Each cylinder is made out of a sheet of carbon only one atom thick.
“There are millions of them packed together,” Huang said. “It’s basically like carpet fibers that are vertically aligned and packed right next to each other.”
Huang and Wang have devised a way to manipulate CNTs so that they become superhydrophobic—that is, extremely water-repellent. “We use the same kind of materials for drag reduction on Navy ships,” said Morteza Gharib, the Caltech professor in whose lab Huang and Wang are working. (Yet another advantage today’s Navy has over the sailing vessel of the Ancient Mariner.)
As the CNT layer repels the water, a cushion of nano-scale air bubbles forms between it and the liquid. “The salt does not deposit because salt water never touches the surface,” Gharib said.
Though part of the MED process takes place in vacuum, that critical air layer should survive because, just as the air shields the surface from the seawater, the seawater shields the air layer from the vacuum.
“Over a long period of time, a small amount of air from the bubbles will gradually dissolve into the water,” Wang said. “Vacuum will tend to pull the dissolved air out from the water. So it could be a challenge if the vacuum condition is strict.” However, he added, their system will be able to replenish any air that is lost.
Another advantage of CNTs is that they conduct heat extremely well. That further enhances efficiency and lowers the cost of the distillation process.
But how much would this CNT surface add to the cost of a desalination plant? “Carbon nanotubes are cheap to manufacture,” Gharib said. “It’s basically soot.”
Huang and Wang estimate that their innovation may be market-ready after another five years or so of development. “I think a lot of engineering needs to be done,” Gharib said. “But the concept is really novel.”
Illustration of a carbon nanotube (credit: Mstroeck). Caltech's Jinglin Huang (L) and Cong Wang (R)
Thirsty in a World of Water
Earth’s surface is mostly ocean, which accounts for some 97 percent of the planet’s water. Yet we rely almost exclusively on the less than one percent in lakes, rivers and aquifers. That may not be enough.
According to a 2012 report by the U.S. Director of National Intelligence, annual global water requirements are expected to rise to 40 percent above current sustainable water supplies by 2030 even as, in many places, climate change tightens the availability of freshwater.
The U.N.’s water agency notes that water stress is already high in many developing countries. Without improved management of water resources, it says, progress toward poverty reduction and sustainable development will be jeopardized.
The Union of Concerned Scientists says that overall, wet areas are expected to become wetter and dry areas drier, placing additional stress on overtaxed water systems in the United States.
According to the International Desalination Association (IDA), some 300 million people in 150 countries rely at least in part on desalinated water today. It also reports that the cost of desalination has been decreasing during the past 20 years and currently costs less than a half-cent per gallon.
“Seawater clearly is an unlimited source,” said the IDA’s Leon Awerbuch. “That’s why we believe that after conservation, after you do waste-water recovery, reclamation and recycling, ultimately you have to deal with seawater desalination.”
- Traditional sources are expected to be unable to provide enough freshwater worldwide, making seawater desalination an attractive option in many places.
- In modern distillation plants, water is kept below the temperature at which scale forms and requires periodic cleaning.
- Two Caltech grad students are developing a surface material that promises to reduce or eliminate scaling at hotter temperatures, enabling much greater efficiency.
- Making distillation more efficient could expedite efforts to make desalination more sustainable and less costly.