Heat with sound: The dawn of thermoacoustics is here

Thermoacoustic cooling system (source)

Heat and cool with sound

Heating water with sound; how could this be? Let’s look at the basics.

Heat measures the movement of atoms and molecules. The more they move and giggle, the greater the heat energy.

Sound is a vibration that propagates through gases, liquids, or solids. Sound is a snooker game of molecules bouncing into each other, each new bounce moving forward an acoustic wave.

So if the sound is a wave of bouncing molecules and heat relates to jiggling molecules, can we use sound for heating?

We can.

The field is called thermoacoustics, and it’s heating up. Thermoacoustics discovery goes back to the XIX century. However, its nonlinear performance delayed its development which only started in the last decades.

Thermoacoustics applications

Thermoacoustics can recuperate energy from waste heat and convert it to power. Its performance is equivalent or superior to existing technologies but at a lower cost with higher reliability.

Medicine can also use thermoacoustics for imagining. Thermoacoustics delivers higher spatial resolution and deeper imagining than most optical imaging techniques.

It allows more niche applications, including generating electricity from a log fire burning to heat water. This application aims at rural areas (see figure 1)

Figure 1: Prototype of thermoacoustic water heater-generator (source)

Thermoacoustics also allows both the heating and cooling of gases and liquids. Furthermore, it can divert existing heat, allowing for thermoacoustic heat pumps.

Thermoacoustics heat pumps

Heat pumps are more efficient than direct heating. When you heat air or water directly, you convert one unit of another form of energy into heat. It can be chemical, burning natural gas, or radiant, using solar collectors to heat water.

Converting one form of energy into heat results in a one-to-one ratio: for each kWh we spend, we get, at best, a kWh of heat. And in practice, we do not even get 1:1; there are always losses in conversion.

However, when redirecting existing heat, efficiency increases. It allows one to get several kWh of heat from a single kWh of another form of energy.

Heat pumps are the backbone of our heating and cooling in the future. Air conditioning, refrigerators, freezers, and water heating benefit from heat pumps, powering the transition to renewable energy.

The dawn of thermoacoustics

The technology seems on the brink of commercialization. Prototypes are leaving research labs out into commercial applications.

Figure 2 shows the installation of a thermoacoustic cooling system on a rooftop at a school by the Dutch company SoundEnergy.

Figure 2: SoundEnergy’s thermoacoustic cooling system (source)

A French startup Equium plans to launch a thermoacoustic residential water heating heat pump at the end of this year (2023). They are currently in field tests. The heater capacity is scalable from 1 kW to 3 kW and has a COP between 3 and 4. (Figure 3).

Figure 3: Equium thermoacoustic heat pump water heater (source)

Equium claims better seasonal performance and a lower total ownership cost than gas boilers and current heat pumps. Although one expects such claims from a starting company, there are some underlying reasons to give them some credence.

They won’t be the first. Dutch company Blue Heart is also in field tests with thermoacoustic heat pumps.

Thermoacoustic equipment displays several advantages:

-No moving parts;

-Easy to build and install;

-Low maintenance;

-No need for (toxic) fluids;

-No use of exotic materials;

-And it’s silent, even working with sound.

All these reasons, coupled with the start of commercial productions, point to the beginning of the adoption of thermoacoustics systems.