The Thermoacoustic Engine, Explained

The thermoacoustic engine is one of the weirdest forms of renewable energy I’ve heard of, and I had to have it explained to me several times before I started to get it. No description I read on the Internet made any sense. After consulting with John Barrie, an inventor who is designing a low-cost model for use in rural Guatemala, I created a description the rest of us could understand.

In short: The thermoacoustic engine uses heat to create sound, and sound to create electricity.

How’s that again? For the statement above to make sense, one has to understand the intimate relationship between sound and heat.

Imagine an impatient driver honking as you amble across the street. As the purple-faced motorist presses his horn, your innocent ears perceive the honk as a sound wave. What is a sound wave? Though we perceive it as sound, in reality it’s a wave of pressure. The crest of the wave compresses air molecules as it travels, while the trough of the wave is a little decompressed. That pressure wave enters your ear and strikes the tympanic membrane like a drumstick on a drum, making you turn and glance at the driver.

Now here’s where the heat comes in. The high and low-pressure parts of a sound wave actually have different temperatures, like the difference in mood between the angry driver and your cool self. The high-pressure part is hotter and the low pressure part is cooler. It’s this gap between hot and cold that makes a thermoacoustic engine work.

A typical thermoacoustic engine is a cylinder with a heat source warming up its middle. (Any of several heat sources will work: flame, an engine’s  waste heat, or solar energy.) As the middle gets hot, the ends stay cool. Observe the flame at mid-cylinder in this video:

Pressure waves of heat and cold begin to bounce back and forth between the center and the ends. If the pipe is the right length and if the heat source is adequate, these chaotic waves fall into a steady rhythm known as a standing wave. Crucial to the engine is the “stack,” a perforated stopper that stands between the hot and cold parts like a cork with tiny holes in it. The stack serves two purposes. It’s an accelerator, causing air molecules to speed up as they move through the small openings. It also serves as insulator, to keep the hot side hot and the cool side cool.

At one end of the cylinder, the pressure waves create motion. Many thermoacoustic engines, also known as lamina flow engines, use the pressure waves to move a piston. That’s the design in this video:

Barrie’s design instead employs a magnet moving on a spring (like a drumstick on a drum). The magnet moves next to a copper coil, and the magnetic field between them creates electricity.

But wait – where does the sound come in? The sound is part and parcel of those pressure waves, though it doesn’t serve a useful purpose. In the same way that heat is a waste product of an internal-combustion engine, sound is a waste product of the thermoacoustic engine. Controlling that sound is part of the design challenge.

An engine could issue a whine of 135 to 180 decibels, which is louder than cozying up to a jackhammer. But if encased in a steel tube, it presents as a low hum, like the sound of a refrigerator running.