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From heat to electricity and back again

Email|Print|Single Page| Text size + By Felicia Mello
Globe Correspondent / March 24, 2008

Would you like to have a jacket that charges your cellphone using your own body heat? How about your own mini-power plant that uses the heat from your apartment to light your lamps? A car that would use tailpipe exhaust to recharge its battery?

Such futuristic-sounding gadgets are theoretically possible, due to a scientific phenomenon called the thermoelectric effect: Some materials can transform heat into electricity and vice versa. But high costs and low efficiency have made thermoelectrics the forgotten stepchild of more mainstream clean energy sources like solar and wind - relegated to niche markets like temperature-controlled seats for luxury automobiles.

Now, researchers at Boston College and MIT say they have found a way to make thermoelectric devices commercially viable by dramatically improving their ability to conduct electricity without conducting too much heat.

The discovery could contribute to efforts to control global warming, allowing power plant operators and car manufacturers to improve fuel economy by converting waste heat into electric power.

"We've all expected this to work and they've somehow managed to get it right," said Jeff Snyder, a thermoelectrics specialist at the California Institute of Technology, who was not involved in the research.

In a thermoelectric circuit, heat applied to one end causes electrons to move to the other end, creating an electric current. The reverse is also true: Applying a current to the device will carry heat from one side to the other, where it is ejected.

Thermoelectric devices are already used to power NASA space shuttles, while companies have used their heat-transferring properties to create cooling systems without the polluting chemicals used in most air conditioners.

More widespread adoption has stalled, however, because most materials that conduct electricity also conduct heat and therefore can lose a lot of energy to leakage.

"What we see today is a very limited use of thermoelectrics due to the basic electrical inefficiency of the structures, systems and materials that exist," said Dan Coker, CEO of Amerigon, a Detroit-based company working on a Department of Energy project to design a car that could recharge its battery with its own exhaust. "An improved thermoelectric material opens up opportunities for new products to be developed."

In the experiment, published Thursday in the online version of the journal Science, scientists took a substance commonly used in thermoelectrics, bismuth antimony telluride, and crushed it into tiny nanoparticles about one-1,000th the width of a human hair. They then heated the powder and pressed it back together, creating a new structure that allowed electrons to move freely but impeded the flow of heat. The new material performed 40 percent more effectively than existing compounds, according to the study.

"By grinding the material down to a very small size, we created a cheap way to make environmentally friendly material in large quantities for existing cooling systems and for potentially breaking into the power generation market," said Zhifeng Ren, a physicist at Boston College and one of the leaders of the project.

The researchers have formed a Newton-based company, GMZ Energy, Inc., to develop the technology.

Scientists have been working to improve thermoelectric converters since the 1950s using everything from silicon wires to intricate - and expensive - lattices. This project surpasses previous efforts because the team was able to change the material while still maintaining its ability to conduct electricity, Snyder said.

Snyder and his colleagues in the relatively small field of thermoelectrics envision a future where thermoelectric coolers replace conventional refrigerators, and thermoelectric power generation becomes a standard part of the design of any green home.

That would require much greater improvements in efficiency than those achieved by the MIT-Boston College team. But with oil prices skyrocketing, said Snyder, "efficient" becomes a relative term.

"All of our ideas never come to fruition mostly because of the cost," he said. "But we're coming to a new era where energy might become significantly more expensive. These ideas that we've pushed aside as being not viable may now start to become useful as part of a sustainable future."

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