Called nanophotonics-enabled solar membrane distillation technology, or NESMD for short, the development has come from the Center for Nanotechnology Enabled Water Treatment (NEWT), based at Rice University.
The system works whereby hot salt water is flowed across one side of a porous membrane and cold freshwater is flowed across the other.
Water vapor is naturally drawn through the membrane from the hot to the cold side, and because the seawater doesn’t need to be boiled, the energy requirements are less than they would be for traditional distillation, according to the researchers.
However, the energy costs are still significant because heat is continuously lost from the hot side of the membrane to the cold.
“Unlike traditional membrane distillation, NESMD benefits from increasing efficiency with scale,” said Rice’s Naomi Halas, a corresponding author on the paper and the leader of NEWT’s nanophotonics research efforts. “It requires minimal pumping energy for optimal distillate conversion, and there are a number of ways we can further optimise the technology to make it more productive and efficient.”
In the PNAS study, researchers offered proof-of-concept results based on tests with an NESMD chamber about the size of three postage stamps and just a few millimeters thick.
The distillation membrane in the chamber contained a specially designed 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 salt water flowed atop the carbon-black layer, and a cool freshwater stream flowed below.
Rice scientist and water treatment expert Qilin Li said the water production rate increased greatly by concentrating the sunlight: “The intensity got up 17.5 kilowatts per meter squared when a lens was used to concentrate sunlight by 25 times, and the water production increased to about 6 liters per meter squared per hour.”
Li said NEWT’s research team has already made a much larger system that contains a panel that is about 70 centimeters by 25 centimeters. Ultimately, she said, NEWT hopes to produce a modular system where users could order as many panels as they needed based on their daily water demands.
“You could assemble these together, just as you would the panels in a solar farm,” she said. “Depending on the water production rate you need, you could calculate how much membrane area you would need. For example, if you need 20 litres per hour, and the panels produce six litres per hour per square meter, you would order a little over three square meters of panels.”
Yale University‘s Menachem Elimelech, a co-author of the new study and NEWT’s lead researcher for membrane processes, added: “The integration of photothermal heating capabilities within a water purification membrane for direct, solar-driven desalination opens new opportunities in water purification.»
Below you can see a video from Rice University about the potential of the technology.