A review is presented for the research development of Stirling cycle engines for recovering low and moderate temperature heat. The Stirling cycle engines are categorized into four types, including kinetic, thermoacoustic, free-piston, and liquid piston types. The working characteristics, features, technological details, and performances of the related Stirling cycle engines are summarized. Upon comparing the available experimental results and the technology potentials, the research directions and the possible applications of different Stirling cycle engines are further discussed and identified. It is concluded that kinetic Stirling engines and thermoacoustic engines have the greatest application prospect in low and moderate temperature heat recoveries in terms of output power scale, conversion efficiency, and costs. In particular, kinetic Stirling engines should be oriented toward two directions for practical applications, including providing low-cost solutions for low temperatures, and moderate efficient solutions with moderate costs for medium temperatures. Thermoacoustic engines for low temperature applications are especially attractive due to their low costs, high efficiencies, superior reliabilities, and simplicities over the other mechanical Stirling engines. This work indicates that a cost effective Stirling cycle engine is practical for recovering small-scale distributed low-grade thermal energy from various sources.
A traveling-wave thermoacoustic electric generator, which is composed of a traveling wave thermoacoustic engine and linear alternators, is promising in solar power generation and energy recovery due to its high efficiency, high reliability, and capability of utilizing low-grade heat. An equivalent acoustic circuit of a linear alternator is first built and analyzed using electro-mechano-acoustical analogy. It is found that the acoustic coupling of the linear alternators to the traveling-wave thermoacoustic engine is crucial to the performance of the system. A traveling-wave thermoacoustic electric generator with a variable electric R-C load is then constructed and experimentally studied. Both the theoretical analysis and the experimental results show the importance of mechanical and electrical resonances to the overall performance of the system. Furthermore, the thermal-to-electric efficiency and the electric power are found to be proportional to the pressure amplitude and the square of it in front of the piston of the linear alternator, respectively. By optimizing the load impedance, the traveling-wave thermoacoustic electric generator has achieved a maximum electric power of 345.3 W with a thermal-to-electric efficiency of 9.34% and a maximum efficiency of 12.33% with an electric power of 321.8 W at around 65 Hz when helium of 3.0 MPa is used as the working gas.
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