New ways of turning heat into sound waves - and then into electricity - may be the next step toward a practical new source of alternative energy.

Scientists have known for decades that they can turn heat into sound using simple devices called acoustic heat engines. But this week a team of University of Utah researchers plan to show they’ve succeeded in miniaturising and optimising the devices, which then turn the sound into usable electricity.

If true, the advance could open the door to super-efficient power plants, cars, and computers, as well as a new generation of solar cells.

Acoustic heat engines usually use a copper plate to conduct heat to a high-surface-area material like glass wool, which then heats the surrounding air. The movement of the hot air generates a single frequency sound wave, rather like a flute. And this in turns vibrates a piezoelectric electrode, producing voltage.

High efficiency

Most engines are large or inefficient, though, making them undesirable for interfacing with computers or other small applications.

To improve their prospects, Orest Symko and his team built smaller engines ranging from 11 to 18 centimeters long. At 40% efficient, the engines rival gasoline and diesel engines at energy conversion.

The team’s discoveries have also raised some eyebrows, however. "I realise anything to do with energy is really important these days," says Scott Backhaus, who studies thermoacoustics at the Los Alamos National Laboratory. "But we’re working on some applications for diesel engines, and I can tell you we’re not getting anywhere near 40% efficiency. I’m sceptical."

Tiny engines

The Utah researchers have also built the smallest known acoustic heat engines, which at 1.8 millimeters long could produce 1 Watt of electricity per cubic centimeter when clustered together. Symko speculates that the clusters could be used as the 'cells' in a new type of solar panel.

He plans to test the devices within a year to produce electricity from waste heat at a military radar facility.

“It looks very promising, but at this point there is still much work to be done. We’re still working on an array,” he says, adding that he hopes to begin mass-production of miniature engines within the next year.

If all goes well, they could be installed on natural gas and coal-fired power plants shortly thereafter. The team will present their research on Friday at the annual meeting of the Acoustical Society of America in Salt Lake City, Utah.