Power from heat

The thermoelectric effect can produce small amounts of electricity from almost any source of heat, but its low efficiency has so far limited its uses. A team has now found a simple way to make one thermoelectric alloy more efficient.

When two ends of a stick of a thermoelectric material are exposed to different temperatures, a voltage appears. The electrons in the stick act like the molecules in a gas: Just as gas expands when heated, the heated electrons move from the hotter side to the cooler side. The resulting voltage can create current.

Since the 1950s, researchers have known that the alloy bismuth antimony telluride is a good thermoelectric material, says Gang Chen of the Massachusetts Institute of Technology. But Chen wondered if the effect could be made better. Chen and his colleagues ground up the alloy and recompressed it. The grinding reduced the size of the alloy’s crystalline grains by about a factor of a thousand. This change slightly improved the material’s ability to conduct electricity but, most crucially, made it a worse heat conductor. That was good because heat conduction tends to equalize temperatures, counteracting the whole thermoelectric principle.

The researchers made the alloy 15 to 30 percent more efficient without substantially increasing its cost, Chen says. The results appeared online March 20 in Science.

The material could have applications, including in a new kind of solar panel that harnesses the difference in temperature between the panel’s hot, sunny side and its cool, shaded side. Such panels, the researchers say, might turn 5 to 7 percent of solar energy into electricity—less efficient than traditional photovoltaics, but potentially at a lower cost per watt.