Frequently Asked Questions about Thermoacoustics

1. Since the acoustic amplitudes are so large within these devices, won't it be loud outside too?

Not if it is designed properly! The pressure amplitudes within the thermoacoustic resonator are only a small fraction (5%) of the static internal pressures which are approximately 20 atmospheres. Given the relatively small acoustic pressure amplitudes, a pressure vessel which is strong enough to safely contain the static pressure cannot yield enough under the acoustic pressure variations to radiate much sound to the environment. If there is any perceptible acoustic radiation at all, it is usually due to some imbalance in the electroacoustic driver. In the SETAC fridge, there were two drivers which created a small oscillatory torsional moment that caused the large, flat insulation panels surrounding the cold portions of the refrigerator to radiate. In that device, the sound level due to the radiation of the insulation panels, though preceptible, was well below that of the noise produced by the pumps which circulated the transport fluids and the fan which removed the exhaust heat.

2. How efficient are thermoacoustic refrigerators?

At the present time, the efficiency of thermoacoustic refrigerators is 20-30% lower than their vapor compression counterparts. Part of that lower efficiency is due to the intrinsic irreversibilities of the thermoacoustic heat transport process. These intrinsic irreversibilities are also the favorable aspects of the cycle, since they make for mechanical simplicity, with few or no moving parts. A greater part of the inefficiency of current thermoacoustic refrigerators is simply due to technical immaturity. With time, improvements in heat exchangers and other sub-systems should narrow the gap. It is also likely that the efficiency in many applications will improve due only to the fact that thermoacoustic refrigerators are well suited to proportional control. One can easily and continuously control the cooling capacity of a thermoacoustic refrigerator so that its output can be adjusted accurately for varying load conditions. This could lead to higher efficiencies than conventional vapor compression chillers which have constant displacement compressors and are therefore only capable of binary (on/off) control. Proportional control avoids losses due to start-up surges in conventional compressors and reduces the inefficiencies in the heat exchangers, since the proportional systems can operate over smaller temperature gaps between the coolant fluid and the heat load.

2a. Could you say more about proportional control?

The second law of thermodynamics sets an absolute limit on the performance ("efficiency") of a refrigerator of any design. The larger the temperature difference which a refrigerator must produce, the less efficient it can be, even if it is perfectly designed and built. One feature of thermoacoustic devices which may allow them to overcome some of the inefficiency of the cycle is that they can use proportional control. Proportional control means that the output of the device may be turned up or down gradually depending on conditions. A dimmer switch on a lamp is an example of this kind of control. In contrast, an ordinary light switch is an example of binary control-it is either on or off, with no in-between. A vapor compression refrigerator uses binary control: it comes on for a while, then it goes off. If the conditions require more output, the unit comes on more frequently, but it is never partially on. A thermoacoustic cooler, on the other hand, can be partially on. The advantage to this is that the less hard a refrigerator is working, the more efficient it becomes. When producing maximum output, a vapor compression refrigerator is more efficient than a thermoacoustic fridge of the same capacity, but when less output is needed (which is most of the time), the thermoacoustic device increases in efficiency, but the vapor compression fridge does not. There are other advantages to proportional control. You can imagine that it would be nicer if your home air conditioner would keep the house at a constant cool temperature rather than cycling between somewhat too hot and somewhat too cold. Similarly, the performance and lifetime of some types of electronics could be increase by the steadier temperatures available through proportional control. Proportional control also eliminates the electronics-damaging "power surges" that occur throughout the electrical system when the compressor in a conventional chiller turns on or off.

3. How large/heavy are thermoacoustic refrigerators compared to their vapor compression counterparts?

For all thermodynamic devices, there will always be a trade-off between efficiency and power density. For the small power devices built thus far (less than 1,400 Btu/hr = 400 W thermal) and the larger devices currently under construction (36,000 Btu/hr = 10 kW thermal), the size and weight are similar to their vapor compression equivalents. The cooling capacity of vapor compression units depends upon operating pressure and the amount of phase-change fluid. The size of a thermoacoustic device is determined (roughly) by its operating frequency. If small size is important, higher frequency operation may be required.

4. What are the possible applications for thermoacoustic technology? Are they useful at all temperatures?

At this point, we do not see any cooling application which is not suited to thermoacoustics. Conventional, single-stage, electrically operated thermoacoustic refrigerators can reach cold-side temperatures which are two-thirds to three-quarters of ambient, so they are not well suited to cryogenic applications (T < -40 C = -40 F). Thermoacoustically driven pulse-tube style refrigerators can reach the cryogenic temperatures required to liquefy air or natural gas. In its early commercial stages, thermoacoustic refrigerators will probably be limited to niche applications such as in military systems which are required to operate in closed environments and food merchandising where toxicity is an important issue. As global environmental legislation, such as the Montreal Protocols on Substances which Deplete Stratospheric Ozone and the Berlin Mandate on Global Warming Gases become more restrictive, we expect the scope of thermoacoustic applications to expand both domestically and in emerging markets.

5. How soon will we be able to purchase commercial thermoacoustic refrigerators and air conditioners?

The answer depends upon the availability of funds for research and development and the severity of the restrictions which will be placed on conventional vapor compression chemical refrigerants. CFCs were banned internationally ten years after the signing of the Montreal Protocols in 1986. It is not yet clear what will be the fate of the CFC substitutes (e.g., HFCs and HCFCs) which have very large global warming potentials and unknown toxicity. Those regulations are currently being debated and are due to be ratified in Tokyo in December, 1997. In any case, we expect that it will be at least 3-5 more years before thermoacoustic refrigerators start appearing in "specialty" applications and probably 10 more years before they start to appear in appliance stores.

6. When thermoacoustic refrigerators and air conditioners become commercially available, will they cost more than their conventional vapor compression equivalents?

There are no intrinsically expensive components in thermoacoustic refrigerators. They operate at pressures which are similar to vapor compression refrigerators. Thermoacoustic refrigerators do not require any exotic materials and do not depend upon close tolerances nor do they require lubrication, since they have no sliding seals. Unlike vapor compression refrigeration, which use the "working fluid" as the heat transport fluid, all of the thermoacoustic refrigerators which provide more that 10 to 20 W thermal of cooling capacity have used secondary heat transport fluids. This could increase the cost, since additional heat exchangers and fluid pumps are required. On the other hand, the separation of the working fluid and the heat transport fluids allow each to be optimized independently. This could lead to more uniform thermal distribution and higher efficiency which might increase acquisition cost but reduce life-cycle costs. This is not a cost factor in many applications which currently use secondary heat transport fluids.

7. Although my home and car air conditioners require costly periodic maintenance, my home refrigerator seems to work for ten to fifteen years without any maintenance. Will there be maintenance problems with thermoacoustic refrigerators and air conditioners?

Thermoacoustic refrigerators will be at least as trouble-free as current home refrigerators. Thermoacoustic refrigerators and air conditioners use inert gases which will never be controlled substances and will always be readily available (The atmosphere is 1% argon. The atmospheric concentration of CO2 is only 0.03%). Since they have no sliding seals, they do not require lubrication. At the present time, we have not been able to identify what will be the possible failure modes for thermoacoustic refrigerators and air conditioners but the leading candidate is metal fatigue in the elastic suspension. It appears that proper design of these springs can lead to "infinite" lifetimes. The reason your home refrigerator is so trouble-free is that it uses CFCs and has a hermetically sealed compressor. CFCs are compatible with hydrocarbon lubricants (oil) and do not decompose when exposed to electrical discharge. The new substitute chemicals are less stable than CFCs, so that they won't travel up to the stratosphere and destroy the ozone. This decreased chemical stability makes them incompatible with hydrocarbon lubricants, so the compressors are far more difficult to lubricate. Don't expect your new refrigerators, which will use HFCs, such as R-134a, to be as robust. Both fixed and mobile air conditioners will also be experiencing more maintenance problems now that CFCs are very expensive and will eventually become unavailable at any cost.

8. Who is working on developing thermoacoustic refrigerators and air conditioners?

It is difficult to tell exactly how many groups are working on developing thermoacoustic technology, either domestically or internationally. Commercial refrigeration and air conditioning manufacturers do not "advertise" their new product development efforts. Ford Motor Company is the only industrial laboratory which has published their research in this area, although IBM, Macrosonix (an acoustic compressor manufacturer) and Modine Manufacturing (a heat exchanger manufacturer) have been issued patents on thermoacoustic technology and Cryenco has a working device which uses a thermoacoustically-driven pulse-tube refrigerator to liquefy natural gas. In Japan, there is a association of 100 researchers from industry and academia who are working on thermoacoustic refrigeration (including pulse-tube refrigerators). There are several academic institutions and government laboratories which are doing varying amounts of research on issues surrounding thermoacoustic heat transport (e.g., U. Mississippi, Johns Hopkins, Ohio U., U. Utah, NIST, U. Nevada - Desert Research Institute, etc.) but only Los Alamos National Laboratory, the Naval Postgraduate School, Penn State University and (soon) Purdue University have complete working thermoacoustic cooling systems. Outside the United States, there are academic and/or industrial efforts in at least France, England, Argentina, Bangladesh and South Africa.

9. What are the hurdles to commercialization of thermoacoustic technology?

The largest hurdle to commercialization is the "talent bottleneck." Due to the novelty of thermoacoustic technology, there are very few people who have the combination of expertise in acoustics, transduction, heat exchanger design, and instrumentation required to produce complete thermoacoustic cooling systems. There are not even people with that combination of expertise outside of the thermoacoustic community who can be called upon to provide an independent assessment of the current state of the technology or the prospects for future improvements. There is also no existing supplier base or commercial infrastructure which is currently producing components such as high-power, high-efficiency (narrow bandwidth) loudspeakers (linear motors) or heat exchangers optimized for high-frequency oscillatory flow of compressed gases. Until there are component suppliers, it will be difficult to create a group of systems assemblers who will market thermoacoustic devices.

10. What are the other alternative refrigeration techniques which are as environmentally benign as thermoacoustics?

Prior to the commercial introduction of CFCs in the 1940's, ammonia was used as a working fluid for vapor compression refrigeration in homes. It was abandoned due to its toxicity, but is still in widespread use for industrial and agricultural applications. In Europe, and particularly in Germany, hydrocarbons, such as propane and butane, are used in small quantities for domestic refrigerators. Due to their flammability and explosive potential, they may not be suitable for applications requiring larger cooling capacities, such as air conditioners. (The hydrocarbon issues are related to saftey, not global warming.) Stirling cycle refrigerators also work best with inert gases and have efficiencies which are equal to or better than thermoacoustics. Their drawback is that they are far more complicated and therefore much more expensive to produce and maintain. It is also possible to produce refrigerators which are based on solid-state thermoelectric materials. At the present time, they are far less efficient than thermoacoustic or Stirling cycle refrigerators and the best thermoelectric materials are both brittle and hydroscopic.

11. What are the outstanding research issues which should ba addressed to understand how to improve thermoacoustic refrigerator performance in the future?

The reason that thermoacoustic technology has progressed so rapidly during the past decade is that there has been an excellent theoretical understanding of the thermoacoustic heat pumping process which was developed by N. Rott in the late 1960's and early 1970's, and by J. Wheatley and G. Swift in the 1980's and G. Swift in the 1990's. Unfortunately, that understanding has been limited to a fairly small portion of the available "parameter space." In particular, existing models have been limited to fairly low acoustic Mach Numbers (Mac < 3% or p1/pm < 5%), due to the one-dimensional nature of the equations, the limitations of linear acoustics, the absence of mean flow, and the assumption of a stable laminar boundary layer.

Since the power density of thermoacoustic devices depends upon (p1/pm)^2, there is quite a strong motivation to understand thermoacoustics at higher amplitudes. Progress in this direction will require the construction of thermoacoustic refrigerators which can achieve higher acoustic Mach Numbers and theoretical advances which could require a solution to the full non-linear thermo-hydrodynamic equtions in two- or three-dimensions. It would also be useful to study new structures for components such as stacks, resonators, heat exchangers and electroacoustic driver mechanisms. At the present time, there are no models for the stack/heat exchanger interface. There are no models for heat transport between the thermoacoustically oscillating gas and the heat exchanger surfaces which could be used to suggest what geometries would optimize the useful transfer of heat on and off of the stack. All electrically-driven thermoacoustic refrigerators to date have employed electrodynamic drive mechanisms (moving coil or moving magnet). There are several "solid-state" materials, such as piezoelectric and magnetostrictive compounds, which have high energy densities and low losses, but which have not been adapted to thermoacoustic loads. Most of the world's machines are powered by rotary motors. What is the best way to incorporate such rotary drive mechanisms in thermoacoustic devices?

The above is only a small subset of the possibilities which could lead to a more compltete understanding and better devices. With an increase in the number of working devices and motivated investigators, the rate at which thermoacoustics will progress should increase steadily for many more years.