Studies Improve Efficiency of Acoustic Conversion of Heat to Electricity
Here are summaries of the studies by Symko's doctoral students:
-- Student Bonnie McLaughlin showed it was possible to double the efficiency of converting heat into sound by optimizing the geometry and insulation of the acoustic resonator and by injecting heat directly into the hot heat exchanger.
She built cylindrical devices 1.5 inches long and a half-inch wide, and worked to improve how much heat was converted to sound rather than escaping. As little as a 90-degree Fahrenheit temperature difference between hot and cold heat exchangers produced sound. Some devices produced sound at 135 decibels -- as loud as a jackhammer.
-- Student Nick Webb showed that by pressurizing the air in a similar-sized resonator, it was able to produce more sound, and thus more electricity.
He also showed that by increasing air pressure, a smaller temperature difference between heat exchangers is needed for heat to begin converting into sound. That makes it practical to use the acoustic devices to cool laptop computers and other electronics that emit relatively small amounts of waste heat, Symko says.
-- Numerous heat-to-sound-to-electricity devices will be needed to harness solar power or to cool large, industrial sources of waste heat. Student Brenna Gillman learned how to get the devices -- mounted together to form an array -- to work together.
For an array to efficiently convert heat to sound and electricity, its individual devices must be "coupled" to produce the same frequency of sound and vibrate in sync.
Gillman used various metals to build supports to hold five of the devices at once. She found the devices could be synchronized if a support was made of a less dense metal such as aluminum and, more important, if the ratio of the support's weight to the array's total weight fell within a specific range. The devices could be synchronized even better if they were "coupled" when their sound waves interacted in an air cavity in the support.
-- Student Ivan Rodriguez used a different approach in building an acoustic device to convert heat to electricity. Instead of a cylinder, he built a resonator from a quarter-inch-diameter hollow steel tube bent to form a ring about 1.3 inches across.
In cylinder-shaped resonators, sound waves bounce against the ends of the cylinder. But when heat is applied to Rodriguez's ring-shaped resonator, sound waves keep circling through the device with nothing to reflect them.
Symko says the ring-shaped device is twice as efficient as cylindrical devices in converting heat into sound and electricity. That is because the pressure and speed of air in the ring-shaped device are always in sync, unlike in cylinder-shaped devices.
-- Student Myra Flitcroft designed a cylinder-shaped heat engine one-third the size of the other devices. It is less than half as wide as a penny, producing a much higher pitch than the other resonators. When heated, the device generated sound at 120 decibels -- the level produced by a siren or a rock concert.
"It's an extremely small thermoacoustic device -- one of the smallest built -- and it opens the way for producing them in an array," Symko says.
Now if we can only find a way to raise the Earths temperature a couple degrees ... /s