Electricity-free air con: Thermoacoustic device turns waste heat into cold using no additional power
New Atlas, 25/1/2019, Loz Blain
Beginning with the principles of the Stirling engine, SoundEnergy’s THEAC thermal acoustic engine takes heat – either industrial waste heat or solar heat – and turns it into powerful cooling without requiring any other power source. This completely renewable technology could prove highly disruptive.
The THEAC system uses no mechanical moving parts, no refrigerants, no CO2, no precious metals or materials. Instead it uses Argon gas, which is plentiful and has zero global warming potential, and is totally sustainable, relying solely on the energy of incoming heat to produce cold. The technology is also claimed to make about as much noise as a running shower, and is scalable, way up from the company’s 25-kW demo unit, which can produce cooling temperatures as low as -25° C (-13° F).
How on Earth does it work, then? Through the principles of thermoacoustics, it turns out, which we can only explain up to a certain point. Thermoacoustic effects have been observed for centuries, particularly by glass-blowers, who noticed that occasionally when they were blowing a hot bulb at the end of a cold, narrow tube, a loud, monotone sound would be produced. Experiments in the 1850s figured out that the temperature differential was key, and that the volume and intensity of the sound would vary with the length of the tube and the size of the bulb.
Sound, of course, is merely an audible vibration of the air, consisting of pressure peaks and troughs. Gases expand and contract with heat, meaning that a temperature differential can create a pressure differential. This is the principle behind the operation of the Stirling engine, and it’s the source of the pressure waves that were causing sonic oscillations in the glass tubes.
1850s Physicist Pieter Rijke was able to demonstrate that adding a heated wire screen a quarter of the way up the tube would greatly magnify the sound – it was effectively giving extra energy to the air in the tube at its point of greatest pressure. Further experiments showed that taking away energy by cooling the air at its points of minimal pressure would have a similar amplifying effect on the thermoacoustic wave.
The SoundEnergy device uses heat differential to create an acoustic wave in an infinite loop tube, and amplifies that wave until it reaches a high intensity. Then, just as the heat differential was converted into a pressure differential, the pressure differential is converted back into another heat differential, this time in reverse.
In an interview with Forbes magazine, SoundEnergy CFO Roy Hamans says “this huge mechanical power will be transformed into a delta T [another temperature differential] down in the last two vessels by connecting them in reverse. The sound waves produce cold by distracting the heat from the particles like in a classical Stirling cycle.”
As odd as it sounds, what you get is a fixed system without moving parts that can accept heat and pump out cold. The heat can come from anywhere – excess industrial heat or that coming out of a cruise ship motor are prime candidates, but like with a Stirling engine, the source isn’t important and can just as easily be supplied by the Sun under the right conditions, using vacuum tube collectors. The cold can be used to cool whatever needs cooling, be it fresh produce, cold storage, or any number of industrial cooling purposes. Excess heat that can’t be converted to cold is sent away to a water/glycol heat sink to be dispersed.
Or, of course, air conditioning – a huge global energy drain that only stands to increase as global temperatures slowly rise and more households and offices start running air con more of the time. It’s easy to see how the SoundEnergy THEAC system could be a significant and disruptive technology, with a potential ability to reduce global energy usage while efficiently scavenging waste heat at an efficiency of 40-50 percent.
This kind of thermoacoustic cooling has been in development since the mid-70s and 80s. A similar thermoacoustic refrigeration device was used for cryogenic cooling on Space Shuttle Discovery all the way back in 1992 – in this case, with a moving loudspeaker diaphragm. But thus far, nobody seems to have made it into the kind of commercial success that could make it a world-changing device. So hopes are high for SoundEnergy’s efforts to get this gear out to market.
The company has already made its first couple of sales – one to tech-hungry Dubai – and says that large units will cost around US$50,000. Prices are expected to drop as production scales up, and the company says it’s possible to create residential/consumer products at much lower prices. Each system should have an expected lifespan of around 20-30 years, which is certainly better than a conventional air con unit.