Cook your Batteries for Lights, Phones (and the Stove)

today..
From Mr. Lalloobhoy Battliwala

A thermo-acoustic generator, prototype in PennState lab. A blog intro below. After Lighting Africa, forget Lighting Asia. Let there be a Heat, Light, and Sound initiative (there will be speeches and music, meals free, darkness by choice).

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This is the prototype of a thermoacoustic device that attaches to cookstoves to generate electricity.

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 detail in this paper published online by the Acoustical Society of America.

Clean stoves save lives

Once the device performs its acoustical magic, it charges a battery with the converted heat of the stove in use. The battery can light up LEDs, charge phones or power other small devices. But—and here's the “clean” part—first, it powers a small fan.

We have reported on another fan-powering cookstove and the concept here is the same. The fan blows the sooty smoke back over the burning biomass so it can incinerate nearly all of the material. Black carbon and other greenhouse gases and carcinogens are destroyed without entering the kitchen, the cook's lungs, or the atmosphere. The fan also improves the stove's efficiency, so it requires less fuel and cooks faster.

Why a clean cookstove? Montgomery drew inspiration for his project from a speech that US Secretary of State Hillary Clinton gave last year at the annual meeting of the Clinton Global Initiative. She announced the launch of a new alliance of governments and private businesses that will push for cleaner cookstoves. Today, nearly half of all households worldwide still cook with dirty, smoke-spewing stoves and open fires. The Global Alliance for Clean Cookstoves aims to replace 100 million dirty stoves and open fire pits by 2020, which would be a good start towards solving this problem.

What's next
Montgomery graduated last year and left the stove prototype to an upcoming class. The prototype proved the technology, but the new class will have to increase the amount of electricity it d produces. “Future models will use a moving-magnet transducer instead of the speaker,” Montgomery says.

The new class may take up the project again in the fall. Steven Garrett, professor in the graduate department of acoustics, said they will welcome collaboration with experts in the field.