Efficient Nano Solar Energy

Wednesday, November 21, 2012 @ 01:11 PM gHale


A new technology using nanoparticles to convert solar energy directly into steam is now in development.

This new “solar steam” method is so effective it can even produce steam from icy cold water, said researchers at Rice University’s Laboratory for Nanophotonics (LANP).

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The technology has an overall energy efficiency of 24 percent. Photovoltaic solar panels, by comparison, typically have an overall energy efficiency around 15 percent. However, the inventors of solar steam said they expect the first uses of the new technology will not be for electricity generation but rather for sanitation and water purification in developing countries.

“This is about a lot more than electricity,” said LANP Director Naomi Halas, the lead scientist on the project. “With this technology, we are beginning to think about solar thermal power in a completely different way.”

The efficiency of solar steam is due to the light-capturing nanoparticles that convert sunlight into heat. When submerged in water and exposed to sunlight, the particles heat up so quickly they instantly vaporize water and create steam. Halas said the solar steam’s overall energy efficiency can probably increase as they refine the technology.

“We’re going from heating water on the macro scale to heating it at the nanoscale,” Halas said. “Our particles are very small — even smaller than a wavelength of light — which means they have an extremely small surface area to dissipate heat. This intense heating allows us to generate steam locally, right at the surface of the particle, and the idea of generating steam locally is really counterintuitive.”

To show just how counterintuitive, Rice graduate student Oara Neumann videotaped a solar steam demonstration in which a test tube of water containing light-activated nanoparticles submerged into a bath of ice water. Using a lens to concentrate sunlight onto the near-freezing mixture in the tube, Neumann showed she could create steam from nearly frozen water.

Steam is one of the world’s most-used industrial fluids. About 90 percent of electricity comes from steam, and steam also sterilizes medical waste and surgical instruments, to prepare food and to purify water.

Most industrial steam ends up made in large boilers, and Halas said solar steam’s efficiency could allow steam to become economical on a much smaller scale.

“Solar steam is remarkable because of its efficiency,” said Neumann. “It does not require acres of mirrors or solar panels. In fact, the footprint can be very small.”

Another potential use could be in powering hybrid air-conditioning and heating systems that run off of sunlight during the day and electricity at night. Halas, Neumann and colleagues have also conducted distillation experiments and found solar steam is about two-and-a-half times more efficient than existing distillation columns.



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