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Hot Advance for Thermoelectrics

Cheap organic molecules could more efficiently convert waste heat into electricity.
February 22, 2007

Inside fossil-fuel and nuclear-power plants, as well as in cars and trucks, the lion’s share of energy in fuel is wasted as heat rather than converted into electricity or mechanical power. But the search for a practical material that can convert at least some of this waste heat into electricity has been long and frustrating.

By trapping organic molecules between a gold surface and the ultrafine gold tip of a scanning tunneling microscope, researchers have shown that the molecules could be used to generate electricity.

Researchers have long known that some inorganic semiconductors can do this. Indeed, deep-space probes have been powered by using such materials. But these inorganic materials are costly and difficult to make, and have low efficiencies. Now, new research shows that certain organic molecules produce voltage when exposed to heat. Ultimately, they could be much cheaper and thus more practical to implement.

“This is the first demonstration that you can use organic molecules in this kind of energy generation,” says Rachel Segalman, professor of chemical engineering at the University of California, Berkeley, who with her colleagues reported new measurements last week in Science Express. “That’s really significant because they are so inexpensive and abundant,” she says.

Experts had previously theorized that some organic molecules could have the qualities necessary to generate electricity from heat. But until now, they lacked experimental proof, which the Berkeley researchers were able to provide by isolating and measuring the properties of just a few molecules of organic substances called benzene dithiols at a time.

These were “very difficult experiments,” says Brian Sales, a senior research scientist at the Oak Ridge National Laboratory, who was not involved with the work. The researchers trapped a few molecules between a sheet of gold and the ultrafine gold tip of a scanning tunneling microscope, which is so sharp it can end in a single atom. They heated up the gold surface and measured, via the microscope tip, the voltage that was created. “These are the type of difficult experiments that get nanotechnology past the ‘picture’ stage [and] into the realm of real science,” Sales says.

The experiments showed that the organic molecules have the three qualities that make for good thermoelectric materials. The first is the ability to create a voltage. But this works best when the materials have two other qualities: they do not conduct heat, but they do conduct electrons. That way, applying heat, rather than just raising the temperature of the material, actually drives electrons, creating a current.

The results confirmed that the organic molecules could indeed be used to generate electricity from heat. Before they can be put to use, however, it will be important, Sales says, to design the molecules so that they arrange themselves between metal layers to make large-scale thermoelectric materials. What’s more, so far the efficiency is very low, the researchers say. To improve this, they are creating and testing new versions of the molecules.

“These are very simple molecules that the group is looking at,” says J. Fraser Stoddart, professor of chemistry at the University of California, Los Angeles. He’s interested in the researchers’ plans to alter the molecules to improve their thermoelectric properties. “That’s where my heart starts to beat,” he says. “I hope they follow this research up.”

The research is only the first step, the researchers say, and, because much work remains, applications will be many years away.

If all goes well, though, so-called thermoelectric devices based on the molecules could prove to be an important source of power–and a way to reduce greenhouse-gas emissions by making far more efficient use of fossil fuel. “Ninety percent of the world’s electricity is generated by thermal-mechanical means,” says Arun Majumdar, professor of mechanical engineering at UC Berkeley and another researcher on the project. “And a lot of the heat is wasted. One and a half times the power that is generated is actually wasted.”

For example, a typical way to generate electricity is by heating up steam to drive a turbine. After the steam passes through the turbine, it still contains energy in the form of heat, although not enough to drive a turbine, Segalman says. That heat typically escapes into the atmosphere and is wasted. By wrapping thermoelectric materials around exhaust pipes, that heat could be put to work. In cars, thermoelectrics could replace the alternator and save hundreds of millions of gallons of gas a year, according to an estimate from a General Motors researcher. (See “Free Power for Cars.”)

Organic materials are appealing because they cost much less than thermoelectric inorganic materials: even if they are inefficient, they might still be economical. “These molecules are dirt cheap,” Majumdar says. “If the efficiency is low, that’s fine. You’re throwing that heat away anyway.”

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