Powerful sounds

Sound waves are the latest potential source for renewable energy as E&T discovers.

We've had wind, solar and even tidal energy, but the latest ideas turning heat into sound waves - and then into electricity - may be the next step toward a practical new source of alternative energy.

Turning heat into sound is nothing new, acoustic heat engines have been around for decades, but work from Professor Orest Symko and his team at University of Utah has succeeded in miniaturising and optimising the devices, which then turn the sound into usable electricity.

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

"We are converting waste heat to electricity in an efficient, simple way by using sound," says Symko, a University of Utah physics professor who leads the effort. "It is a new source of renewable energy from waste heat.

"We have now perfected them. We've been building a whole range of devices that operate at different frequencies. The frequencies determine the size and we have been building them for various applications."

Coupling devices

One project that Symko and his team are currently working on is to try to remove the heat from a heat stack at a power plant to raise the efficiency. "We're working on coupling the devices because it's not that simple to have individual engines. That's an interesting problem by itself.

"We have been trying to put them in parallel and run them together to synchronise them. To date we've managed to synchronise five of them so they are operating in phase to give us more power." A method of synchronising the engines is crucial to the success of the devices. When you take one engine it produces sound, which is an increase and decrease in pressure at a certain frequency.

"If you couple two of them together there is no guarantee that they will work together, one can go against the other," Symko says. "If you have them going together, pushing together at the same time pulling together at the same time, you get quite a bit of power out of that but if they're working against each other you get nothing. There's no guarantee that when you switch them on they will be so called in phase so what we've been doing is synchronising them to always make sure that when they do couple that they're coupled 'in phase'."

The team have been working on a variety of miniature versions of the engine. The smallest one is about 3mm long with a 1mm bore that operates at 22kHz, and to date they have managed to have two of them synchronised. "Our ultimate goal is to have a large array of them synchronised as well for focus of application," he explains. "Finally, we have been working on a device which uses what we call a travelling wave, using something like a wave guide similar to a ring laser.

"It's a ring where we generate sound and sound feeds back at the input and gives us much higher efficiency than the other devices because of the way it works. That has been working at frequencies ranging from 2-4kHz. The frequency determines the size of the device, so the higher the frequency the smaller the device."

Alternative energy

Symko plans to test the devices within a year to produce electricity from waste heat at a military radar facility and at the university's hot-water-generating plant.

The research is funded by the US Army, which is interested in "taking care of waste heat from radar, and also producing a portable source of electrical energy which you can use in the battlefield to run electronics", he says.

Symko expects the devices could be used within two years as an alternative to photovoltaic cells for converting sunlight into electricity. The heat engines also could be used to cool laptops and other computers that generate more heat as their electronics grow more complex. And Symko foresees using the devices to generate electricity from heat that now is released from nuclear power plant cooling towers.

Symko's work on converting heat into electricity via sound stems from his ongoing research to develop tiny thermoacoustic refrigerators for cooling electronics.

In 2005, he began a five-year heat-sound-electricity conversion research project called Thermal Acoustic Piezo Energy Conversion (TAPEC). Symko works with collaborators at Washington State University and the University of Mississippi.

The project has received $2m in funding during the past two years, and Symko hopes it will grow as small heat-sound-electricity devices shrink further so they can be incorporated in micromachines (known as microelectromechanical systems, or MEMS) for use in cooling computers and other electronic devices such as amplifiers.

Using sound to convert heat into electricity has two key steps. For the first step heat engines (technically called 'thermoacoustic prime movers') convert heat into sound. Then they convert the sound into electricity using existing technology: 'piezoelectric' devices that are squeezed in response to pressure, including sound waves, and change that pressure into electrical current. 'piezo' means pressure or squeezing.

Resonators

Most of the heat-to-electricity acoustic devices built in Symko's laboratory are housed in cylinder-shaped 'resonators' that fit in the palm of your hand. Each cylinder, or resonator, contains a 'stack' of material with a large surface area - such as metal or plastic plates, or fibres made of glass, cotton or steel wool - placed between a cold heat exchanger and a hot heat exchanger.

When heat is applied - with matches, a blowtorch or a heating element - the heat builds to a threshold. Then the hot moving air produces sound at a single frequency, similar to air blown into a flute.

"You have heat, which is so disorderly and chaotic, and all of a sudden you have sound coming out at one frequency," Symko says. Then the sound waves squeeze the piezoelectric device, producing an electrical voltage. Symko says it's similar to what happens if you hit a nerve in your elbow, producing a painful electrical nerve impulse. Longer resonator cylinders produce lower tones, while shorter tubes produce higher-pitched tones.

Devices that convert heat to sound and then to electricity lack moving parts, so such devices will require little maintenance and last a long time. They do not need to be built as precisely as, say, pistons in an engine, which lose efficiency as they wear.

Symko says the devices won't create noise pollution. First, as smaller devices are developed, they will convert heat to ultrasonic frequencies people cannot hear. Second, sound volume goes down as it is converted to electricity. Finally, "it's easy to contain the noise by putting a sound absorber around the device", he says.

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