In an increasingly hot and crowded world, clean water is becoming a precious commodity. Two thirds of the global population will have problems accessing fresh water by 2025, and removing salt and contaminants from the oceans and groundwater is one way to slake humanity's thirst.
Today's large desalination plants, though, cost millions of dollars to build. Most use reverse osmosis, which forces seawater through salt-blocking membranes. The required electricity accounts for up to half of a plant's expenses, and the process leaves behind a supersalty, chemical-laced soup that can harm local ecosystems. Such facilities are typically powered by carbon-emitting fossil fuels; efforts have been made (especially in the Middle East, Asia and Africa) to use solar panels instead, but that also comes at a cost and does not address the toxic discharge.
So researchers are trying to use the sun's heat more directly to remove salt and other contaminants. The simplest option is to let water evaporate, leaving behind salts and chemicals, and then condense the vapor into clean water. Humans have used versions of this technique, called solar distillation, for hundreds of years. Today Saudi Arabian engineers plan to build a plant with giant mirrors that concentrate sunlight and superheat water within a steel-and-glass dome more than 50 meters across.
But by using novel materials and designs, researchers are trying to make the process cheaper, simpler and portable enough to make high-quality desalination far more accessible worldwide. “The needs for clean water in developing countries are enormous,” says Naomi Halas, an electrical and computer engineer at Rice University. “Solar-thermal technologies should allow you to lower the energy needs of desalination but also to do it in remote locations where you are completely off the grid.”
The U.S. Department of Energy will soon announce semifinalists for its Solar Desalination Prize. The goal: a system that produces 1,000 liters of usable water for $1.50. “No technology today can handle high-salinity water at these costs,” says Qilin Li, a civil and environmental engineer at Rice.
Such systems could surmount a big downside of reverse osmosis: it typically desalinates only half of the input saltwater, and the solution left behind eventually builds up enough salt to clog the membrane, says Craig Turchi of the DOE's National Renewable Energy Laboratory (NREL). This noxious by-product, called a brine, is typically dumped into the ocean or injected underground. Solar-thermal desalination systems can purify water with salt concentrations at least twice that of seawater. This would include brines from reverse osmosis plants and brackish groundwater from places such as the U.S. Southwest, as well as some industrial and agricultural wastes that reverse osmosis cannot handle, says NREL spokesperson Meghan Hughes: “Generally, only thermally driven technologies, like the ones we're working to develop through this program, can treat these highly concentrated brines.”
Li, Halas and their colleagues have built a solar desalination device with a porous plastic membrane that lets water vapor through but not liquid. One side of it is coated with tiny carbon particles that heat up in the sun, vaporizing the salty water as it contacts them. This vapor passes through and condenses as clean water on the membrane's other side. Halas's group recently boosted the system's efficiency by 50 percent by using plastic lenses to focus sunlight on the membrane, producing more heat.
The team's calculations show that meeting the DOE's cost target, with a square-meter-sized device that produces up to 20 liters of water an hour, should be possible in a few years. “We're at the Ford Model T stage—not the Mustang stage yet,” Halas says. “But it's good enough that we're starting to get commercial interest.”
Civil and environmental engineer David Jassby's group at the University of California, Los Angeles, integrated heat-conducting materials into the membrane in a similar setup. Underneath it, the researchers added a fine aluminum mesh that heats up in sunlight. “So you can roll the membrane into spiral modules because you don't have to have large surface areas directly exposed to the sun,” he says. In rooftop tests, the device produced eight liters of fresh water per square meter of membrane in an hour.
Such systems could lend themselves to compact units suitable for off-grid villages in Asia and Africa, communities with brackish groundwater, and emergency uses almost anywhere. But they will need to pick up the pace and convert more solar heat into vapor, says Lenan Zhang, a graduate student in mechanical engineer Evelyn Wang's laboratory at the Massachusetts Institute of Technology.
Wang's team boosts its device's efficiency by “reusing energy over and over,” Zhang says. It includes 10 stages, each a nylon frame holding a black sun-absorbing layer, a paper towel and an aluminum film. When heated, the black layer evaporates salty water as it wicks up into the paper towel, and the vapor condenses on the aluminum. Condensation releases heat, which rises to the next paper towel layer and aids evaporation instead of being lost. The $100 setup yields almost six liters an hour in the lab and about half of that outdoors; with more sophisticated materials and stages, Zhang says, the efficiency could be doubled.
Another intriguing approach takes advantage of humidification by passing air through a saltwater spray. “Air absorbs the water and leaves behind solid salts,” says Oregon State University mechanical engineer Bahman Abbasi. His system uses solar radiation to heat, compress and eject a mix of saltwater and air through nozzles at high speed, thereby creating a vortex that pushes salts and other solids to the device's walls as the humidified air rises for collection and condensation. Abbasi says the backpack-sized device can clean water with salinity up to three times higher than that of seawater and produce about 20 liters an hour.
All these relatively low-cost technologies could unlock new markets for portable water cleaners or off-grid uses—and beyond. They may eventually lead to large-scale solar-thermal systems to provide cities with drinking water, Turchi says. For now they “will complement reverse osmosis and be a key player in niche applications where reverse osmosis may not work.”