Scientists Claim Energy Breakthrough

Jan. 23, 2008 — -- Scientists are developing a new device that could have a profound impact on global energy supplies by converting wasted heat into electricity. It could potentially have an impact on everything from power plants to cell phones, and it came about because of a serendipitous discovery that had eluded scientists for half a century.

Researchers at Lawrence Berkeley Laboratory and the University of California, Berkeley, have found a way to use ordinary silicon to convert heat to electricity. The technique could mean that some day you will be able to recharge your cell phone with electricity produced by your own body heat, and enormous amounts of energy that is now wasted could be recycled.

"We feel that this is a breakthrough," said Arun Majumdar, a mechanical engineer and materials scientist with joint appointments at the Berkeley lab and UC Berkeley. "I'm very excited about this."

Astonishingly, Majumdar and his colleagues didn't set out to achieve what they have done.

"It was serendipitous," he said. "We never planned for it."

And perhaps even more surprising, they did it with a material that most scientists thought would never work for this purpose — ordinary silicon, a cheap, abundant material that is the foundation for the multibillion-dollar semiconductor industry.

Majumdar and his fellow researchers, including chemist Peidong Yang, a noted leader in the rapidly growing field of technology at the incredibly small "nano" scale, reported on their work in the Jan. 10 issue of the journal Nature. It's not clear yet why the device they have created works.

"We don't have all the answers at this point," Majumdar said. But laboratory experiments show that it does, indeed, work. At least on a small scale. The device, placed between a hot plate and a cold plate, produced enough electricity to power a light bulb, although they didn't do that demonstration. Instead, they measured the current flowing from the hot plate toward the cold plate, and it was sufficient to claim success, he said.

Although the results so far are very promising, there are still many potential roadblocks on the highway from the lab to the marketplace.

"The issues are cost and performance," Majumdar said.

But he is encouraged by the fact that silicon is intimately understood by the international semiconductor industry, and he said he has had inquiries from several electronics firms. The automotive industry is particularly interested because of the potential to reclaim the heat that is released through a car's exhaust system and use it to power electronic devices, especially in an age of hybrids.

The concept of converting heat to electricity is not new, of course. About 90 percent of all power plants, for example, use heat from fossil or nuclear fuels to produce mechanical energy that is then converted to electrical energy. The heat cools rapidly as it passes through the turbine, but much of it is lost into the environment.

"Nature requires that you have to dump some heat into the environment," Majumdar said. "There's no freebie out there."

But the wasted heat from a power plant is still very hot, "so there's still some juice left," as Majumdar put it. If just some of that wasted heat, say 4 percent to 7 percent, could be recycled the impact globally would be enormous.

That may sound like a fairly simple engineering challenge, but it isn't, despite the well-known fact that heat can be used directly to produce an electric current. If you heat one end of a wire, for example, electrons will flow toward the cold end of the wire, and that's electricity. But for that process to continue, one end of the wire must remain hot, and the other cold. And that's a problem because most materials that conduct electricity also conduct heat.

That's especially true for silicon, so most scientists had concluded that it was not a very good candidate for thermoelectric conversion. And that's not what Majumdar and Yang and their colleagues had in mind when they immersed a silicon wafer in a chemical solution. They were looking for an easier, and cheaper, way to grow tiny wires, called nanowires, on a silicon wafer. These microscopic wires have high potential in a wide range of uses, but it's not easy to produce them.