Freshwater from salt water using only solar energy: Modular, off-grid desalination technology

Freshwater from salt water using only solar energy
In conventional membrane distillation (top), hot saltwater 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. In NEWT's "nanotechnology-enabled solar membrane distillation," or NESMD (bottom), a porous layer of sunlight-activated carbon black nanoparticles acts as the heating element for the process. Credit: P. Dongare/Rice University

A federally funded research effort to revolutionize water treatment has yielded an off-grid technology that uses energy from sunlight alone to turn salt water into fresh drinking water. The desalination system, which uses a combination of membrane distillation technology and light-harvesting nanophotonics, is the first major innovation from the Center for Nanotechnology Enabled Water Treatment (NEWT), a multi-institutional engineering research center based at Rice University.

NEWT's "nanophotonics-enabled solar membrane distillation" technology, or NESMD, combines tried-and-true water treatment methods with cutting-edge nanotechnology that converts sunlight to heat. The technology is described online this week in the Proceedings of the National Academy of Sciences.

More than 18,000 desalination plants operate in 150 countries, but NEWT's 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," said 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. Salt water is boiled, and the steam is captured and run through a condensing coil. Distillation has been used for centuries, but it 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.

Freshwater from salt water using only solar energy
This scaled-up test bed of NEWT's direct solar desalination technology uses carbon black nanoparticles that convert as much as 80 percent of sunlight energy into heat. Results from an earlier prototype showed the technology could produce as much as six liters of freshwater per hour per square meter of solar membrane. Credit: Jeff Fitlow/Rice University

An emerging technology for desalination is membrane distillation, where 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 need not be boiled, the energy requirements are less than they would be for traditional distillation. 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 optimize 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 percent of sunlight to generate steam. By adding low-cost, commercially available nanoparticles to a , NEWT has essentially turned the membrane itself into a one-sided heating element that alone heats the water to drive .

"The integration of photothermal heating capabilities within a water purification membrane for direct, solar-driven desalination opens new opportunities in water purification," said 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 millimeters thick. The 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 flowed atop the carbon-black layer, and a cool freshwater stream flowed below.

Freshwater from salt water using only solar energy
Rice University researchers (from left) Naomi Halas, Qilin Li, Peter Nordlander, Seth Pederson, Alessandro Alabastri and Pratiksha Dongare with a scaled-up test bed of the NEWT Center's direct solar desalination system. Credit: Jeff Fitlow/Rice University

Li, the leader of NEWT's advanced treatment test beds at Rice, 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 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."

Established by the National Science Foundation in 2015, NEWT aims to develop compact, mobile, off-grid water-treatment systems that can provide clean water to millions of people who lack it and make U.S. energy production more sustainable and cost-effective. NEWT, which is expected to leverage more than $40 million in federal and industrial support over the next decade, is the first NSF Engineering Research Center (ERC) in Houston and only the third in Texas since NSF began the ERC program in 1985. NEWT focuses on applications for humanitarian emergency response, rural systems and wastewater treatment and reuse at remote sites, including both onshore and offshore drilling platforms for oil and gas exploration.


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More information: Pratiksha D. Dongare el al., "Nanophotonics-enabled solar membrane distillation for off-grid water purification," PNAS (2017). www.pnas.org/cgi/doi/10.1073/pnas.1701835114
Provided by Rice University
Citation: Freshwater from salt water using only solar energy: Modular, off-grid desalination technology (2017, June 19) retrieved 11 December 2018 from https://techxplore.com/news/2017-06-freshwater-salt-solar-energy-modular.html
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Jun 19, 2017
How long does it last before the salt builds up and makes it stop working?

Dug
Jun 19, 2017
Beyond salt build up there is biofouling. Most semi-permeable membrane desalinization technologies are limited by the extensive chemical cleaning processes they have to cycle thru relatively frequently. This system doesn't seem to have a solution for membrane fouling or cleaning.

Jun 19, 2017
I am often amused when people describe new technology using "nanoparticles". In essence, the top layer is just a polymer infused with carbon black dust. You can produce this by simply mechanically grinding a bunch of carbon black particles together. This is hardly new technology.

This is a quite interesting application anyway. Like the other readers I would be very interested in knowing the cost of the membrane (use of noble metals would make it expensive), and the rate of fouling of all sorts. Some of these things are just a product of engineering (biofouling and salt accumulation can be compensated for and reduced with various methods) and the cost (if currently prohibitive) could be reduced using various known technologies.

Jun 19, 2017
Hopefully they are not selling the salt to the public as a byproduct and calling it "sea salt".

Jun 21, 2017
Hopefully they are not selling the salt to the public as a byproduct and calling it "sea salt".


Why not?

That's how sea salt is made.

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