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.
The researchers thought they had hit pay dirt when a microscopic forest of nanowires grew on the surface of their submerged wafer. But when they looked closer, they found that the wires were substandard. Instead of the smooth surface they had expected, the surface of their nanowires was rough.
But thinking the flawed wires might have some purpose, they tried to use them in a photovoltaic device.
"It didn't work," Majumdar said.
Intrigued, they tried to use their wafer as a thermoelectric device. And that's when they found success. The rough nanowires did allow current to flow from a heat source toward a cold source, but astonishingly, the heat did not also flow from hot to cold. That seems contrary to all that is known about silicon, and it was a bit baffling to the researchers.
Although more work needs to be done before the physics is understood, Majumdar has a hunch about what's going on.
Heat travels through a material like sound waves travel through air. He compares the nanowires to a corrugated tube. If you were to shout into the tube, some of the sound waves would bounce off the corrugated ridges and valleys, and eventually nearly all of the sound would be lost. Similarly, the heat waves may simply be bouncing off the rough contours on the surface of the nanowires, allowing electricity to flow, but dissipating the heat.
So the roughness of the nanowires, which was not planned, turns out "to be the key," Majumdar said.
This "breakthrough," as Majumdar describes it, won't solve the world's energy problems, and it won't end global warming, but it could help. And since there is already an infrastructure in place, the $100 billion semiconductor industry, it may come sooner rather than later. All because of a timely bit of serendipity.
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.