Electrical and mechanical power, together with other forms of useful work, are generated worldwide at a rate of about 1012 watts, mostly using heat engines. The efficiency of such engines is limited by the laws of thermodynamics and by practical considerations such as the cost of building and operating them. Engines with high efficiency help to conserve fossil fuels and other natural resources, reducing global-warming emissions and pollutants. In practice, the highest efficiencies are obtained only in the most expensive, sophisticated engines, such as the turbines in central utility electrical plants. Here we demonstrate an inexpensive thermoacoustic engine that employs the inherently efficient Stirling cycle1. The design is based on a simple acoustic apparatus with no moving parts. Our first small laboratory prototype, constructed using inexpensive hardware (steel pipes), achieves an efficiency of 0.30, which exceeds the values of 0.10–0.25 attained in other heat engines5,6 with no moving parts. Moreover, the efficiency of our prototype is comparable to that of the common internal combustion engine2 (0.25–0.40) and piston-driven Stirling engines3,4 (0.20–0.38).

A thermoacoustic Stirling heat engine

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.

A thermoacoustic-Stirling heat engine: detailed study

Traveling-wave thermoacoustic heat engines have been demonstrated to convert high-temperature heat to acoustic power with high efficiency without using moving parts Electrodynamic linear alternators and compressors have demonstrated high acoustic-to-electric transduction efficiency as well as long maintenance-free lifetimes By optimizing a small-scale traveling-wave thermoacoustic engine for use with an electrodynamic linear alternator, we have created a traveling-wave thermoacoustic electric generator; a power conversion system suitable for demanding applications such as electricity generation aboard spacecraft

Traveling-wave thermoacoustic electric generator

We ordinarily think of a sound wave in a gas as consisting of coupled pressure and displacement oscillations. However, temperature oscillations always accompany the pressure changes. The combination of all these oscillations, and their interaction with solid boundaries, produces a rich variety of “thermoacoustic” effects. Although these effects as they occur in everyday life are too small to be noticed, one can harness extremely loud sound waves in acoustically sealed chambers to produce powerful heat engines, heat pumps and refrigerators. Whereas typical engines and refrigerators have crankshaft‐coupled pistons or rotating turbines, thermoacoustic engines and refrigerators have at most a single flexing moving part (as in a loudspeaker) with no sliding seals. Thermoacoustic devices may be of practical use where simplicity, reliability or low cost is more important than the highest efficiency (although one cannot say much more about their cost‐competitiveness at this early stage).

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Thermoacoustic Engines and Refrigerators

https://www.nevis.columbia.edu/~ju/Paper/Paper-thermoacoustic/Design%20thermo%20refrigerator.pdf

G. W. Swift

Papers

Design environment for low-amplitude thermoacoustic engines