Boosting fresh water supplies with plasmonics

Scientists have found a new solar desalination process that relies on aluminium.
20 June, 2017
Researchers have long sought better solutions for desalinating sea water, an especially critical problem in an era of climate change that's placing pressure on fresh water supplies. What they've not been able to do is develop methods that are easy to use and scale, or that are affordable and worth the process.
That may have changed, according to engineers at Georgia Institute of Technology and Nanjing University who recently published their research findings in the journal Nature. What they report is the successful development of a solar thermal desalination process that relies on plasmonics, using nanoparticles of aluminium on an oxide membrane to absorb and focus light from the sun.

The membrane floats atop the seawater, transferring about 90 percent of the solar energy to the surface and vaporizing the water. The researchers found they were able to boost known properties of aluminium as a plasmonic material – meaning its ability to generate a wave of electron energy when incoming light hits the metal – by enhancing its bandwidth and light absorption. The scientists achieved that by taking nanopore perforated sheets of aluminium oxide, and coating them with aluminium nanoparticles. The particles then assemble themselves in and throughout the surface and pores of the membrane, dramatically improving the ability to capture the light energy needed for desalination.
Image: Wanhuajing
"Our plasmon-enhanced solar desalination device can significantly increase the energy transfer efficiency with not only enhanced light absorption, but also more localized heating," researchers said.

One of the potentially biggest advantages of this new method is that it is based on abundant and readily available aluminium, as opposed to desalination techniques that rely on less accessible components. That also means that the process is far cheaper, improving the accessibility and real-world application of the plasmonic process that until now wasn't able to achieve the necessary threshold for production.

The end result is an affordable and easily manufactured membrane that stretches across the surface of the water, absorbs the energy and generates enough heat to vaporize the water so that, when cooled, it has been transformed into fresh water. The aluminium membranes are also durable, and scientists were able to replicate the process in 25 cycles of various light conditions with stable performance results.
"Different from most existing desalination strategies, our plasmon-enhanced solar desalination device is highly portable, and thus ideal for personal or miniaturized applications," the authors conclude. "These aluminium-based plasmonic structures—with low-cost materials and scalable fabrication processes—could therefore provide a portable solution for solar desalination with a minimal carbon footprint."

That minimal carbon footprint is one of the most promising features of the new process. Desalination methods are energy intensive, and the American state of California serves as a good example of why the aluminium-based plasmonic process may be a game-changer. The state has had reverse osmosis facilities since the 1950s, and large-scale plants used for industrial cooling have been online since the 1960s. But the energy demands make traditional desalination expensive, with emissions and environmental discharge impacts that are far less "green" than many communities might like.
The efficiencies of existing desalination plants aren't likely to improve more than they already have, making the thermal aluminium membranes an attractive option for a sustainable future.
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