Video: Using Only Sunlight to Turn Saltwater into Fresh Drinking Water

A scaled-up test bed of NEWT’s solar desalination technology that uses carbon black nanoparticles to convert as much as 80% of sunlight into energy. Source: Rice UniversityResearchers from the Center of Nanotechnology Enabled Water Treatment (NEWT), a multi-institutional engineering research center based at Rice University, have created a method for off-grid technology that uses sunlight alone to turn saltwater into fresh drinking water.

Called nanophotonics-enabled solar membrane distillation (NESMD), the system could potentially change how more than 150 countries get fresh water from more than 18,000 desalination plants.

"Direct solar desalination could be a game changer for some of the estimated 1 billion people who lack access to clean drinking water," says Qilin Li, a scientist at Rice. "This off-grid technology is capable of providing sufficient clean water for family use in a compact footprint, and it can be scaled up to provide water for larger communities."

Older methods of making freshwater require the saltwater to be boiled and the steam is captured after being run through a condensing coil. While this method has been used for many years, it is complex and energy inefficient due to the amount of heat required to boil water and produce steam.

In membrane distillation, hot salt water flows across one side of a porous membrane and cold freshwater is flows across the other. Water vapor is naturally drawn through the membrane from the hot to the cold side and because no boiling is required, energy requirements are less than traditional distillation. But even in this method, energy costs are still significant because heat is continuously lost from the hot side to the cold.

NEWT’s technology uses engineered nanoparticles that harvest as much as 80% of sunlight to generate steam. By adding commercially-available nanoparticles to a porous membrane, the center has turned the membrane itself into a one-sided heating element that alone heats the water to drive membrane distillation.

"The integration of photothermal heating capabilities within a water purification membrane for direct, solar-driven desalination opens new opportunities in water purification," says Menachem Elimelech, lead researcher from membrane processes at NEWT.

How It Works

The initial NESMD chamber is about the size of three postage stamps and just a few millimeters thick. The distillation membrane contains a top layer of carbon black nanoparticles infused into a porous polymer. The light-capturing nanoparticles heated the entire surface of the membrane when exposed to sunlight. A thin half-millimeter-thick layer of saltwater flowed on top the carbon-black layer and a cool freshwater stream followed below.

The team has already made a much larger system with a panel that is about 70 centimeters by 25 centimeters. NEWT hopes to produce a modular system where users could order as many panels as needed based on daily water demands.

"You could assemble these together, just as you would the panels in a solar farm," Li says. "Depending on the water production rate you need, you could calculate how much membrane area you would need. For example, if you need 20 liters per hour, and the panels produce 6 liters per hour per square meter, you would order a little over 3 square meters of panels."

The full research can be found in Proceedings of the National Academy of Sciences of the U.S.