Washington, Nov 20 (IANS): Scientists at an American university have unveiled a revolutionary new technology that uses nanoparticles to convert solar energy directly into steam that could be used for sanitation and water purification in developing countries.

The new "solar steam" method is so effective it can even produce steam from icy cold water, said inventors from the Laboratory for Nanophotonics (LANP) at Rice University in Houston, Texas.

The new technology has an overall energy efficiency of 24 percent. Photovoltaic solar panels, by comparison, typically only have an overall energy efficiency around 15 percent, said the university.

The inventors 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," said Halas.

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 be increased as the technology is refined.

"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."

Rice graduate student Oara Neumann videotaped a solar steam demonstration in which a test tube of water containing light-activated nanoparticles was submerged into a bath of ice water. Using 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 is produced from steam, and steam is also used to sterilize medical waste and surgical instruments, to prepare food and to purify water.

Most industrial steam is produced in large boilers, and Halas said solar steam's efficiency could allow steam to become economical on a much smaller scale.

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.