How a loudspeaker in a cookstove can power appliances and cut pollution

This is the prototype of a thermoacoustic device that attaches to cookstoves to generate electricity. Photo courtesy of Paul Montgomery

A device rigged with a loudspeaker and other cheap components can clean up a dirty, woodburning cookstove and convert it into a power generator. It’s a small thermoacoustic generator that attaches to the stove to convert heat into sound waves and then into electricity. The concept is rooted in NASA space probe propulsion technology, but it is affordable enough for use in developing regions.

Paul Montgomery, a recent mechanical engineering graduate of Pennsylvania State University, developed a working prototype of the device last year. As he explains, it uses the stove’s leftover heat to produce a high-amplitude sound wave within a resonator. It then channels the wave through a loudspeaker operating in reverse to generate electricity. The advantages are its low cost—projected at about $25, it has no moving parts other than the loudspeaker, and it could be more efficient than a thermoelectric generator.

Taking a look inside, the device is made of a ceramic “stack” that’s heated at one end and cooled at the other. The stack is inside a resonator with a loudspeaker at one end. This is how it works.

This Celcor ceramic “stack” is inside the resonator. The stack’s matrix of cells runs length-wise down the resonator and allows the air to flow back and forth. Photo courtesy of Paul Montgomery

From heat to sound
Montgomery kept the materials costs down and built a resonator, an elongated chamber, from folded sheet metal. Inside is the ceramic stack, which is a matrix of hollow rectangular passages that run the length of the resonator. As the stove burns biomass, it heats up one side of the stack. The air inside the stack’s passages heats up and expands over to the other side. There, the air contracts as it dissipates its heat into an off-the-shelf heat sink, a flat piece of metal that bleeds the heat off into the atmosphere, like those found in laptops.

When the air cools, it contracts. That oscillating expansion and contraction within the resonator is, by definition, a soundwave. The greater the temperature gradient—the difference between the hot and cold ends—the more the air expands and contracts. That action boosts the wave’s amplitude, making it louder and more powerful. That also explains some of its efficiency. While a thermoelectric generator can take advantage of a temperature gradient of about 200 degrees C, this kind of generator could handle three or four times that, Montgomery says.

From sound to electricity
Normally, a speaker converts electricity into mechanical energy that vibrates the cone to produce soundwaves. An electromagnet inside the speaker, called the voice coil, is surrounded by a permanent magnet and moves back and forth within that magnetic field. The current flowing into the electromagnet switches directions rapidly, changing the electromagnet’s polarity back and forth. That switching polarity creates an identity crisis that alternately attracts the electromagnet to—and repels it from—either end of the permanent magnet. Its back-and-forth motion vibrates the cone. How Stuff Works has a more in-depth explanation of what goes on inside a speaker.

In Montgomery’s device, the oscillating air hits a loudspeaker beyond the cool end of the stack. Working in reverse, the sound waves vibrate the cone back and forth. The cone pushes and pulls the electromagnet within the magnetic field, generating electricity. Montgomery explains the design in detailin this paper published online by the Acoustical Society of America.