Direct solar desalination offers modular, off-grid water treatment
In conventional membrane distillation (top image), hot saltwater is flowed across one side of a porous membrane and cold freshwater is flowed across the other. Water vapour is naturally drawn through the membrane from the hot to the cold side. In NEWT's nanotechnology-enabled solar membrane distillation (lower image), a porous layer of sunlight-activated carbon black nanoparticles acts as the heating element for the process. (Image courtesy of P. Dongare/Rice University.)
Video: NEWT Centre will use nanotechnology to transform water treatment.
NEWT's 'nanophotonics-enabled solar membrane distillation' technology, or NESMD, is described in an article in the Proceedings of the National Academy of Sciences (PNAS).
Direct solar desalination
More than 18,000 desalination plants operate in 150 countries, but NEWT says its desalination technology is unlike any other used today.
"Direct solar desalination could be a game changer for some of the estimated 1 billion people who lack access to clean drinking water," says Rice scientist and water treatment expert Qilin Li, a corresponding author on the study. "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."
The oldest method for making freshwater from salt water is distillation, the research team explains. Salt water is boiled, and the steam is captured and run through a condensing coil. However, distillation requires complex infrastructure and is energy inefficient due to the amount of heat required to boil water and produce steam. More than half the cost of operating a water distillation plant is for energy.
Membrane distillation is an emerging technology for desalination. Hot salt water is flowed across one side of a porous membrane and cold freshwater is flowed across the other. Water vapour is naturally drawn through the membrane from the hot to the cold side. Because the seawater does not need to be boiled, the energy requirements are less than for traditional distillation, but 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," explains Rice's Naomi Halas, a corresponding author on the paper and the leader of NEWT's nanophotonics 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."
NEWT's new technology builds upon research in Halas' lab to create engineered nanoparticles that harvest as much as 80% of sunlight to generate steam. By adding low-cost, commercially available nanoparticles to a porous membrane, NEWT has essentially turned the membrane into a one-sided heating element that 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 Yale University 's Menachem 'Meny'Elimelech, a co-author of the new study and NEWT's lead researcher for membrane processes.
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 millimetres 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-millimetre-thick layer of salt water flowed atop the carbon-black layer, and a cool freshwater stream flowed below.
Li says the water production rate increased greatly by concentrating the sunlight. "The intensity got up 17.5 kW/m2 when a lens was used to concentrate sunlight by 25 times, and the water production increased to about 6 l/m2 per hour."
The NEWT team has already made a much larger system that contains a panel that is about 70 cm by 25 cm. Ultimately, they hope to produce a modular system where users could order as many panels as they 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 l/hr, and the panels produce 6 l/hr per m2, you would order a little over 3 m2 of panels."
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