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Thermoacoustic engines, or acoustic heat engines, are energy-conversion devices that achieve simplicity and concomitant reliability by use of acoustic technology. Their efficiency can be a substantial fraction of Carnot’s efficiency. In thermoacoustic prime movers, heat flow from a high-temperature source to a low-temperature sink generates acoustic power (which may be converted to electric power using a transducer). In thermoacoustic heat pumps and refrigerators, acoustic power is used to pump heat from a low-temperature source to a high-temperature sink. This review teaches the fundamentals of thermoacoustic engines, by analysis, intuition, and example.

A new type of thermoacoustic engine based on traveling waves and ideally reversible heat transfer is described. Measurements and analysis of its performance are presented. This new engine outperforms previous thermoacoustic engines, which are based on standing waves and intrinsically irreversible heat transfer, by more than 50%. At its most efficient operating point, it delivers 710 W of acoustic power to its resonator with a thermal efficiency of 0.30, corresponding to 41% of the Carnot efficiency. At its most powerful operating point, it delivers 890 W to its resonator with a thermal efficiency of 0.22. The efficiency of this engine can be degraded by two types of acoustic streaming. These are suppressed by appropriate tapering of crucial surfaces in the engine and by using additional nonlinearity to induce an opposing time-averaged pressure difference. Data are presented which show the nearly complete elimination of the streaming convective heat loads. Analysis of these and other irreversibilities show which components of the engine require further research to achieve higher efficiency. Additionally, these data show that the dynamics and acoustic power flows are well understood, but the details of the streaming suppression and associated heat convection are only qualitatively understood.

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Measurements and analysis of a 13-cm-diam thermoacoustic engine are presented. At its most powerful operating point, using 13.8-bar helium, the engine delivered 630 W to an external acoustic load, converting heat to delivered acoustic power with an efficiency of 9%. At low acoustic amplitudes, where (linear) thermoacoustic theory is expected to apply, measurements of temperature difference and frequency agree with the predictions of theory to within 4%, over conditions spanning factors of 4 in mean pressure, 10 in pressure amplitude, 6 in frequency, and 3 in gas sound speeds. But measurements of the square of pressure amplitude versus heater power differ from the predictions of theory by 20%, twice the estimated uncertainty in the results. At higher pressure amplitudes (up to 16% of the mean pressure), even more significant deviation from existing thermoacoustic theory is observed. Several causes of this amplitude-dependent deviation are identified, including resonance-enhanced harmonic content in the acoustic wave, and a new, first-order temperature defect in thermoacoustic heat exchangers. These causes explain some, but not all, of the amplitude-dependent deviation of the high-amplitude measurements from existing (linear) theory.