Making steam without water, thanks to nanoparticles

 

It is possible to create steam within seconds by focusing sunlight on nanoparticles mixed into water, according to new research.

That observation, reported Monday by scientists at Rice University in Texas, suggests myriad applications in places that lack electricity or burnable fuels. A sun-powered boiler could desalinate sea water, distill alcohol, sterilize medical equipment and perform other useful tasks.

"We can build a portable, compact steam generator that depends only on sunlight for input. It is something that could really be good in remote or resource-limited locations," said Naomi Halas, an engineer and physicist at Rice who ran the experiment.

Whether the rig she and her colleagues describe would work on an industrial scale is unknown. If it does, it could mark an advance for solar-powered energy more generally.

"We will see how far it can ultimately go. There are certainly places and situations where it would be valuable to generate steam," said Paul Weiss, editor of the American Chemical Society's journal ACS Nano, which published the paper online in advance of the journal's December print publication.

The experiment is more evidence that nanoscale devices — in this case, beads one-tenth the diameter of a human hair — behave in ways different from bigger objects.

In the apparatus designed by the Rice team, steam forms in a vessel of water long before the water becomes warm to the touch. It is, in effect, possible to turn a container of water into steam before it gets hot enough to boil.

"There is a disconnect between what happens when we heat a pot of water and what happens when we put nanoparticles in that water," said Weiss, who is a chemist and director of the California Nanosystems Institute at the University of California at Los Angeles.

"This is a novel proposed application of nanoparticles," said Paul Alivisatos, director of the Lawrence Berkeley National Laboratory and a nanotechnology expert. "I think it is very interesting and will stimulate a lot of others to think about the heating of water with sunlight."

In the Rice experiment, the researchers stirred a small amount of nanoparticles into water and put the mixture into a glass vessel. They then focused sunlight on the mixture with a lens.

The nanoparticles — either carbon or gold-coated silicon dioxide beads — have a diameter shorter than the wavelength of visible light. That allows them to absorb most of a wave of light's energy. If they had been larger, the particles would have scattered much of the light.

In the focused light, a nanoparticle rapidly becomes hot enough to vaporize the layer of water around it. It then becomes enveloped in a bubble of steam. That, in turn, insulates it from the mass of water that, an instant before the steam formed, was bathing and cooling it.

Insulated in that fashion, the particle heats up further and forms more steam. It eventually becomes buoyant enough to rise. As it floats toward the surface, it hits and merges with other bubbles.

At the surface, the nanoparticles-in-bubbles release their steam into the air. They then sink back toward the bottom of the vessel. When they encounter the focused light, the process begins again. All of this occurs within seconds.

In all, about 80 percent of the light energy a nanoparticle absorbs goes into making steam, and only 20 percent is "lost" in heating the water. This is far different from creating steam in a tea kettle. There, all the water must reach boiling temperature before an appreciable number of water molecules fly into the air as steam.

The phenomenon is such that it is possible to put the vessel containing the water-and-nanoparticle soup into an ice bath, focus light on it and make steam.

"It shows you could make steam in an arctic environment. There might be some interesting applications there," Halas said.

The apparatus can also separate mixtures of water and other substances into their components — the process known as distillation — more completely than is usually possible. For example, with normal distillation of a water-and-alcohol mixture, it isn't possible to get more than 95 percent pure alcohol. Using nanoparticles to create the steam, 99 percent alcohol can be collected.

Halas said the nanoparticles are not expensive to make and, because they act essentially as catalysts, are not used up. A nanoparticle steam generator could be used over and over. And, as James Watt and other 18th-century inventors showed, if you can generate steam easily, you can create an industrial revolution.

The research is being funded in part by the Bill & Melinda Gates Foundation in the hope it might prove useful in developing countries. Halas and her team recently spent three days in Seattle demonstrating the apparatus.

"Luckily," she said, "it was sunny."

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