Engineers boost output of solar desalination system by 50%

'Hot spots' increase efficiency of solar desalination
Concentrating the sunlight on tiny spots on the heat-generating membrane exploits an inherent and previously unrecognized nonlinear relationship between photothermal heating and vapor pressure. Credit: Pratiksha Dongare/Rice University

Rice University's solar-powered approach for purifying salt water with sunlight and nanoparticles is even more efficient than its creators first believed.

Researchers in Rice's Laboratory for Nanophotonics (LANP) this week showed they could boost the efficiency of their solar-powered desalination system by more than 50% simply by adding inexpensive plastic lenses to concentrate sunlight into "hot spots." The results are available online in the Proceedings of the National Academy of Sciences.

"The typical way to boost performance in solar-driven systems is to add solar concentrators and bring in more light," said Pratiksha Dongare, a graduate student in applied physics at Rice's Brown School of Engineering and co-lead author of the paper. "The big difference here is that we're using the same amount of light. We've shown it's possible to inexpensively redistribute that power and dramatically increase the rate of purified production."

In conventional membrane distillation, hot, salty water is flowed across one side of a sheetlike membrane while cool, filtered water flows across the other. The temperature difference creates a difference in that drives water vapor from the heated side through the membrane toward the cooler, lower-pressure side. Scaling up the technology is difficult because the across the membrane—and the resulting output of clean water—decreases as the size of the membrane increases. Rice's "nanophotonics-enabled solar membrane distillation" (NESMD) technology addresses this by using light-absorbing nanoparticles to turn the membrane itself into a solar-driven .

'Hot spots' increase efficiency of solar desalination
Rice University researchers (from left) Pratiksha Dongare, Alessandro Alabastri and Oara Neumann showed that Rice's 'nanophotonics-enabled solar membrane distillation' (NESMD) system was more efficient when the size of the device was scaled up and light was concentrated in 'hot spots.' Credit: Jeff Fitlow/Rice University

Dongare and colleagues, including study co-lead author Alessandro Alabastri, coat the top layer of their membranes with low-cost, commercially available nanoparticles that are designed to convert more than 80% of sunlight energy into heat. The solar-driven nanoparticle heating reduces production costs, and Rice engineers are working to scale up the technology for applications in that have no access to electricity.

The concept and particles used in NESMD were first demonstrated in 2012 by LANP director Naomi Halas and research scientist Oara Neumann, who are both co-authors on the new study. In this week's study, Halas, Dongare, Alabastri, Neumann and LANP physicist Peter Nordlander found they could exploit an inherent and previously unrecognized nonlinear relationship between incident light intensity and vapor pressure.

Alabastri, a physicist and Texas Instruments Research Assistant Professor in Rice's Department of Electrical and Computer Engineering, used a simple mathematical example to describe the difference between a linear and nonlinear relationship. "If you take any two numbers that equal 10—seven and three, five and five, six and four—you will always get 10 if you add them together. But if the process is nonlinear, you might square them or even cube them before adding. So if we have nine and one, that would be nine squared, or 81, plus one squared, which equals 82. That is far better than 10, which is the best you can do with a linear relationship."

In the case of NESMD, the nonlinear improvement comes from concentrating sunlight into tiny spots, much like a child might with a magnifying glass on a sunny day. Concentrating the light on a tiny spot on the membrane results in a linear increase in heat, but the heating, in turn, produces a nonlinear increase in vapor pressure. And the increased pressure forces more purified steam through the membrane in less time.

'Hot spots' increase efficiency of solar desalination
Researchers from Rice University’s Laboratory for Nanophotonics found they could boost the efficiency of their solar-powered desalination system by more than 50% by adding inexpensive plastic lenses to concentrate sunlight into “hot spots.” . Credit: Pratiksha Dongare/Rice University

"We showed that it's always better to have more photons in a smaller area than to have a homogeneous distribution of photons across the entire ," Alabastri said.

Halas, a chemist and engineer who's spent more than 25 years pioneering the use of light-activated nanomaterials, said, "The efficiencies provided by this nonlinear optical process are important because water scarcity is a daily reality for about half of the world's people, and efficient solar distillation could change that.

"Beyond water purification, this nonlinear optical effect also could improve technologies that use solar heating to drive chemical processes like photocatalysis," Halas said.

For example, LANP is developing a copper-based nanoparticle for converting ammonia into hydrogen fuel at ambient pressure.

Halas is the Stanley C. Moore Professor of Electrical and Computer Engineering, director of Rice's Smalley-Curl Institute and a professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering.

NESMD is in development at the Rice-based Center for Nanotechnology Enabled Water Treatment (NEWT) and won research and development funding from the Department of Energy's Solar Desalination program in 2018.

Explore further

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

More information: Pratiksha D. Dongare et al, Solar thermal desalination as a nonlinear optical process, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1905311116
Provided by Rice University
Citation: Engineers boost output of solar desalination system by 50% (2019, June 18) retrieved 24 July 2019 from
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User comments

Jun 18, 2019
Build 100 1000 megawatt fission plants and you'll have plenty of output for desalination.

If you don't support fission you don't support carbon free energy.

carry on.

Jun 18, 2019
I'm going with build a million of these for less than a tenth of the money.

Carry on.

Jun 18, 2019
I'd like to see an article about the ammonia to hydrogen process they mention...

Jun 18, 2019
Build 100 1000 megawatt fission plants and you'll have plenty of output for desalination.

Why go to the trouble and expense of building a fission solution to a desalination problem that fusion (our sun) has already solved? How eager are you to place those nuclear power plants in the places that need desalinated water most (e.g.: the Middle East, Africa)? Will you implement a fuel cycle that does not support nuclear weapons proliferation?

Jun 19, 2019
Shootist only ever posts the same handful of BS far-right talking points, there is no point in trying to reason with him. He won't respond anyway.

Jun 19, 2019
congratulations Rice University researchers.

Jun 19, 2019
Build 100 1000 megawatt fission plants are going to build those in these
remote areas that have no access to electricity.
...and staff them with...who exactly? And have them operate reliably? (And have them paid for by...who exactly?)

Here's a fun fact: nuclear powerplants need access to power from the grid. They aren't stand-alone for good reasons (e.g. the cooling must continue even during downtime/maintenance/emergencies...and this over time scales where diesel generators cannot cope.)
In such areas there is no grid and therefore nuclear powerplants are an absolute no-go.

Jun 19, 2019
What I find curious is multistage evaporation with flash, heat exchangers, etc is a very old technology. Apply pinch technology to the process and you reach the thermodynamic limit for a deta t of your choice. All this was mature in the 1980's. Compare it to sugar technology from beets or cane where removing water from a solution with multi-stage evaporation, boiling point rise and a lots of dissolved solids. Where is the innovation?

Jun 19, 2019
Having operated commercial projects on small scale membrane seawater RO units - I can say that biofouling and mineralization is a major problem and cost factor. The nanoparticle membrane seems to have no solution to either of these inherent desalination problems. While you could prefilter the water source to eliminate solids and heat to sterilize it to reduce biofouling - not only would this not eliminate mineralization of the carbon black (even if it was a replacable cartridge) over time, but the added costs would defeat economics and the reliability of the process. It will be interesting to see if the concept ever scales.

Jun 24, 2019
What happens to the membrane when the dirty water side becomes clogged up with silt? Can the flow of the dirty water automatically take away the residues?

Jun 24, 2019
The standard solution to this problem is to backflush the filters once a week or so.

Jun 24, 2019
As I suspected, the comments indicate this is not the revelation it is touted to be. WIKI on desal plants used to note how often they were down, 6 mo, a year, etc. Now the admission is the energy they demand. I recall reading that a western non-profit proposed using liter bottles filled with dirty water and exposing them to sunlight for purification. The problem would seem to be depleted aquifers and wells.

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