A simple, scalable technique boosts the efficiency of solar desalination

Over one-third of the world’s population lacks access to clean water. Sunlight-driven distillation, which can desalinate seawater and purify contaminated water, is a sustainable, cost-effective way to provide potable water. And now, researchers report a simple method to boost the technique’s efficiency by 50% (Proc. Natl. Acad. Sci. USA, 2019. DOI: 10.1073/pnas.1905311116). The work could lead to portable, solar, water-purification devices ideal for remote places that are not connected to an electrical grid.

Commercial desalination plants typically use reverse-osmosis, an energy-intensive technique in which seawater is pushed through salt-blocking membranes. Conventional distillation, which involves evaporating salty or dirty water and then condensing the steam, also takes a lot of energy.

Two years ago, Naomi Halas, Peter Nordlander, and their colleagues at Rice University developed a low-cost solar distillation technique. The system uses a polymer membrane that blocks water but lets water vapor pass through. One side of the membrane is covered by a layer of low-cost carbon black nanoparticles that absorb sunlight and heat up. The particles help boil seawater placed on the membrane, and the resulting water vapor passes through the material and gets collected on the other side.

The Rice team and others have tried to boost the efficiency of the system by using solar collectors, large parabolic mirrors, or dishes that focus more light on the membranes to heat the seawater quickly. Such collectors are commonly used in solar thermal power plants.

But the Rice team has now found that those expensive collectors might not be necessary. They can increase the rate at which their systems produce clean water simply by using an array of small, plastic lenses to focus light into hot spots on the membrane. The heat produced by the focused light exponentially increases the vapor pressure, forcing more water vapor through the membrane.

“We are significantly increasing the output of desalinated water without increasing the solar collection area or the size of the device,” Halas says. “This is very important for off-grid or portable solar thermal desalination.”

In their demonstration, the researchers used a 10.16 by 40.64 cm membrane covered with an array of eight 5.08 cm-wide lenses that focused sunlight into 5 mm-wide spots on the membrane. Over a period of 24 hours, the lens-covered membrane produced 30% more clean water compared with a setup without the lenses. The highest increase in production rate was about 50% at mid-day, when the sun was most intense.

“It’s a very simple technique that can be easily scaled up to square yards,” Nordlander says. He adds that the team is now working on boosting efficiencies further by using advanced, light-focusing technologies such as plasmonic nanoantennas and metasurfaces.

The ability to increase water-vapor pressure through intense heating was well known, “but it was not previously linked to solar optical intensity and thus recognized in the solar thermal desalination process,” says Peng Wang, an environmental scientist and engineer at the King Abdullah University of Science and Technology. “This work elucidates an important mechanism.” These findings could have broad implications, he says, since the idea could be used to enhance a wide range of technologies that use solar heating, such as photocatalysis.