Generalized thermoacoustic heat engines with unconventional working substances: A review

Thermoacoustic heat engines can convert thermal energy into acoustic power or vice versa with advantages of high reliability and low environmental impact. In conventional thermoacoustic heat engines, fluids with ideal-gas behaviors are used. In past decades, the thermoacoustic heat engines with unconventional working substances have also been explored, which not only expands the regime of thermoacoustics, but also provides alternatives to improvement of thermoacoustic conversion and may offer potential breakthrough to this area. In this work, progress on generalized thermoacoustic engines and refrigerators using unconventional working substances, including liquid, transcritical fluid, two-phase fluid, fluid with wall mass transfer and solid in recent decades have been critically reviewed. Their mechanisms, existing prototypes, characteristics of operation and potential applications are discussed comprehensively. These systems exhibit unique advantages over the conventional thermoacoustic systems (e.g., a better performance with low temperature difference operation, larger pressure amplitudes, or more compact heat exchangers), demonstrating potential in applications on low-grade heat recovery, household air conditioning, etc.