Cascaded thermoacoustic devices

#The present invention relates generally to oscillating wave engines and refrigerators, and, more particularly, to thermoacoustic engines and refrigerators, including Stirling engines and refrigerators and their hybrids.

#According to thermodynamic principles, acoustic power in a gas—a nonzero time average product of oscillating pressure and oscillating volume flow rate—is as valuable as other forms of work such as electrical power, rotating shaft power, and hydraulic power. For example, acoustic power can be used to produce refrigeration, such as in orifice pulse tube refrigerators; it can be used to produce electricity, via linear alternators; and it can be used to generate rotating shaft power, e.g., with a Wells turbine. Furthermore, acoustic power can be created from heat in a variety of heat engines such as Stirling engines and thermoacoustic engines.

#Historically, Stirling's hot-air engine of the early 19th century was the first heat engine to use oscillating pressure and oscillating volume flow rate in a gas in a sealed system, although the time-averaged product thereof was not called acoustic power. Since then, a variety of related engines and refrigerators have been developed, including Stirling refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Combinations thereof, such as the Vuilleumier refrigerator and the thermoacoustically driven orifice pulse tube refrigerator, have provided heat-driven refrigeration.

#The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

#FIG. 1 schematically depicts a prior art free piston Stirling engine, a regenerator-based system.

#FIG. 2 schematically depicts a prior art thermoacoustic-Stirling hybrid engine, another regenerator-based system.

#In accordance with the present invention, various stack and regenerator units, with optional pistons, are placed in a cascading relationship. FIGS. 4A and 4B illustrate one example that has been investigated in detail using the computer implemented DeltaE simulation code. A cascade 72 provides, in series, (1st) a stack unit 82 functioning as an engine, (2nd) a first regenerator unit 84 functioning as an engine, and (3rd) a second regenerator unit 86 functioning as an engine, which finally delivers acoustic power to a load 74 (e.g., a refrigerator or a linear alternator) below.

#Stack unit 82 includes stack 88 with input ambient heat exchanger 92 on the top side and hot heat exchanger 94 on the bottom side. Regenerator unit 84 is separated from stack unit 82 by thermal buffer tube 104, as explained below, and includes regenerator 96 with ambient heat exchanger 98 on the top side and hot heat exchanger 102 on the bottom side. A second regenerator unit 86 is separated from regenerator unit 84 by thermal buffer tube 114. Regenerator 106 has ambient heat exchanger 108 on the top side and hot heat exchanger 112 on the bottom side. Hot heat exchanger 112 is connected to an output resonator through thermal buffer tube 116. As used herein, “top” and “bottom” sides are well defined, because thermal buffer tubes 104, 114, 116 have to be hot side up for gravity stability.

#The space inside of pressure housing 118 and outside of stack unit 82, regenerator units 84 and 86, and thermal buffer tubes 104, 114, and 116 may be filled with a thermally insulating material such as ceramic fiber and pressurized to the average thermoacoustic gas pressure. Thus, the stack, regenerator, and thermal buffer tube walls can be thin because they need only to support the oscillating part of the pressure, thereby reducing the heat leaks from hot to ambient. Bellows 120 accommodates axial thermal expansion of stack unit 82, regenerator units 84 and 86, and thermal buffer tubes 104, 114, and 116 within the fixed axial length of pressure housing 118.