Abstract:
A thermo-electro-acoustic engine comprises a sealed body having a regenerator, hot and cold heat exchangers, an acoustic source, and an acoustic energy converter. An acoustic pressure wave is generated in a gas in the region of the regenerator. The converter converts a portion of the acoustic pressure into electrical energy. A portion of the electrical energy is used to drive the acoustic source. The acoustic source is controllably driven to produce acoustic energy which constructively adds to that of the acoustic pressure wave in the region of the regenerator. The remaining electrical energy produced by the converter is used external to the engine, such as to drive a load or for storage. The resonant frequency of the engine and the frequency of the energy output can be controlled electronically or electromechanically, and is not limited solely by the physical structure of the engine body and its elements.

Description:
BACKGROUND 
     The present disclosure is related to thermoacoustic devices, and more specifically to a thermoacoustic device employing an acoustic energy converter and electrical impedance network in place of selected portions of an acoustic impedance network. 
     The Stirling cycle is a well-known 4-part thermodynamic process, typically operating on a gas, to produce work, or conversely to effect heating or refrigeration. The 4 parts are: isothermal expansion, isochoric heat extraction, isothermal compression, and isochoric heat addition. The process is closed, in that the gas remains within the system performing the cycle at all times during the cycle. 
     A number of devices employing the Stirling cycle are generically referred to as Stirling engines. A typical Stirling engine has a mechanical piston (often two pistons), which responds to the heating/expansion and cooling/contraction of a contained gas as part of the Stirling cycle. The motion of the piston(s) drives a crank from which work can be extracted. An element, typically called a regenerative heat exchanger or regenerator, increases the engine&#39;s thermal efficiency. Stirling engines often include a piston and other related moving parts. Devices of this type are often complex, involve seals and pistons, and require regular maintenance. 
     Thermoacoustic engines are another group of devices utilizing a Stirling thermodynamic cycle. These devices share some fundamental physical properties with Stirling engines, namely a contained gas which approximates a Stirling cycle. However, a thermoacoustic engine differs from a Stirling engine in that a temperature differential amplifies acoustic energy which is extracted for work. Often, there are no pistons or cranks such as typically found in a Stirling engine. 
       FIG. 5  is a cross-sectional representation of one example  30  of known thermoacoustic engine designs. One embodiment of a thermoacoustic engine known in the art is a hollow, looped, sealed body structure  32  having a regenerator  34  located therein. The regenerator is often simply a metal mesh or matrix. The regenerator is proximate a first heat exchanger  36 , generally a “cold” exchanger, at a first end thereof and a second heat exchanger  38 , generally a “hot” exchanger, at the opposite end thereof. A third heat exchanger, generally at ambient temperature, may optionally be present. A resonator  40 , often in the form of an extension of the hollow body structure is provided. The body structure is filled with a pressurized gas. The temperature differential across the regenerator, i.e., between the hot and cold heat exchangers, subjects the gas to localized heat transfer. Acoustic energy in the form of a pressure wave in the region of the regenerator subjects the gas to local periodic compression and expansion. Under favorable acoustic conditions, the gas effectively undergoes an approximate Stirling cycle in the regenerator. 
     If the acoustic impedance at the regenerator is low, the pressure oscillations associated with the sound waves are associated with large fluidic displacement velocities. This results in large fluidic resistance losses which degrade the efficiency of the device. Therefore, it is desirable to have a large acoustic impedance at the regenerator. Current thermoacoustic heat engines use an acoustic resonator and/or an acoustic feedback network to achieve this large impedance. 
     Thus, the body structure and resonator form a physical acoustic impedance network such that the pressure wave travels across the regenerator and resonantly feeds back within the body structure. Due to this feedback and the thermal gradient between heat exchangers, the working gas undergoes a Stirling cycle and does work external to the engine. For example, a transducer  42  may be disposed within the body structure or resonator, and a portion of the energy of the pressure wave may be converted to electrical energy by the transducer. 
     Unlike conventional Stirling engines, most thermoacoustic engines have few if any mechanical moving parts and are therefore very reliable. Furthermore, unlike conventional Stirling engines, the gas within a thermoacoustic engine does not travel significantly within the body structure. Rather, the pressure wave propagates through the gas and the Stirling cycle takes place locally inside the regenerator. Thermoacoustic engines may operate with either primarily traveling- or standing-wave phasing of the acoustic wave in the regenerator. Standing-wave devices are known to be less efficient than traveling-wave devices. 
     There are numerous other examples of thermoacoustic engines known in the art. U.S. Pat. No. 7,143,586 to Smith et al., U.S. Pat. No. 7,081,699 to Keolian et al., and U.S. Pat. No. 6,578,364 to Corey illustrate several examples, respectively. Each of these US Patents is incorporated herein by reference. However, each of these examples presents its own set of disadvantages. One disadvantage of certain prior art devices is their relatively large size, primarily due to the necessary length of the resonant acoustic impedance network. This size is a disadvantage for many applications, where a compact device is required. And, the topology and/or the relatively large volume of the body results in an increase in acoustic, thermal, and, in some cases, mass-streaming losses. Another disadvantage is that while pistons sealed within cylinders, and crank arms do not always form a part of known thermoacoustic engines, a number of other elements such as drivers for driving motion of portions of the engine body, bellows, and other moving parts (excluding transducers) are present in prior art engines, adding complexity, size, weight, and cost, increasing the number of elements susceptible to failure, and increasing acoustic loss within the system. A still further disadvantage is that the operating frequency of the engine is set by the physical dimensions of the body and resonator, the construction of the regenerator, and the gas used within the body. It cannot be chosen based, for example, on the desire for improved heat transfer at lower frequencies, matching the frequency of the load, etc. 
     SUMMARY 
     Accordingly, the present disclosure is directed to a simple, compact, and lightweight traveling-wave thermoacoustic engine. One characteristic of the engine disclosed herein is that the acoustic body structure forming the engine does not require a resonator. Another characteristic is that the engine&#39;s only moving parts are the mechanical elements of the acoustic source and converter chosen to form a part of the thermoacoustic engine disclosed herein. 
     The engine consists of a body housing a regenerator, two heat exchangers with one on each side of the regenerator, two electroacoustic transducers with one on each end of the body opposite one another relative to the regenerator, and an external electrical network which serves to control the motion of the two transducers, and to couple useful energy to an electrical load. The engine may also contain a third heat exchanger separated from the hot heat exchanger by a length of tube. 
     According to one aspect of the disclosure, acoustic energy produced by the thermoacoustic engine is converted by one of the electroacoustic transducers the “acoustic energy converter,” to electric energy. A portion of the electric energy is output for the purpose of performing work or alternatively is stored. The remaining electrical energy output is fed back to the other electroacoustic transducer, the “acoustic source”, to further drive the cycle of energy production in the engine. 
     According to this aspect, an electrical impedance network replaces the acoustic impedance network, effectively obviating the need for the resonator portion within the thermoacoustic engine. The electrical impedance network may take a variety of forms, and be comprised of a variety of passive and/or active elements. For this reason, the device disclosed herein is referred to as a thermo-electro-acoustic engine. 
     The acoustic source drives a pressure wave within a closed body structure containing a gas. The closed body structure further contains a regenerator, and first and second heat exchangers, through which the pressure wave may travel. A thermal gradient is established between the first and second heat exchangers. The thermal gradient amplifies the acoustic energy in the regenerator. Located opposite the acoustic source relative to the regenerator is an acoustic energy converter, which converts the amplified pressure wave to an electrical signal. The third heat exchanger, if present, serves to control the temperature of the gas at a distance from the hot heat exchanger. 
     Accordingly, a portion of the electrical energy provided by the converter is output from the engine, for example to drive a load or for storage. The balance of the electrical energy to be fed back to the acoustic source is subjected to an appropriate phase delay and impedance such that it is in phase and of a desired amplitude to result in constructive feedback. The electrical network, in combination with the electroacoustic transducers and acoustic elements, thus sets the acoustic impedance in the region of the regenerator, and the frequency of the electrical energy provided for useful work outside of the engine without need for inverters or other elements to perform frequency conversion. 
     The above is a summary of a number of the unique aspects, features, and advantages of the present disclosure. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present disclosure will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings: 
         FIG. 1  is a schematic illustration of a first embodiment of a thermo-electro-acoustic engine according to the present disclosure. 
         FIG. 2  is a schematic illustration of an impedance circuit for use in thermo-electro-acoustic engine of  FIG. 1 . 
         FIGS. 3A and 3B  are a schematic illustrations of a power splitter and power combiner, respectively, for use in thermo-electro-acoustic engine of  FIG. 1 . 
         FIG. 4  is a graph of pressure versus volume illustrating the Stirling cycle as approximated by the gas in the regenerator of the thermo-electro-acoustic engine of  FIG. 1 . 
         FIG. 5  is an illustration of a thermoacoustic engine of a type known in the art. 
         FIG. 6  is a flow chart illustrating method of operating a thermo-electro-acoustic refrigerator according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is shown therein a first embodiment  10  of a thermo-electro-acoustic engine according to the present disclosure. Engine  10  comprises a generally tubular body  12 . The material from which body  12  is constructed may vary depending upon the application of the present invention. However, body  12  should generally be thermally and acoustically insulative, and capable of withstanding pressurization to at least several atmospheres. Exemplary materials for body  12  include stainless steel or an iron-nickel-chromium alloy. 
     Disposed within body  12  is regenerator  14 . Regenerator  14  may be constructed of any of a wide variety of materials and structural arrangements which provide a relatively high thermal mass and high surface area of interaction with the gas but low acoustic attenuation. A wire mesh or screen, open-cell material, random fiber mesh or screen, or other material and arrangement as will be understood by one skilled in the art may be employed. The density of the material comprising regenerator  14  may be constant, or may vary along its longitudinal axis such that the area of interaction between the gas and wall, and the acoustic impedance, across the longitudinal dimension of regenerator  14  may be tailored for optimal efficiency. Details of regenerator design are otherwise known in the art and are therefore not further discussed herein. 
     Adjacent each lateral end of regenerator  14  are first and second heat exchangers  16 ,  18 , respectively. Heat exchangers  16 ,  18  may be constructed of any of a wide variety of materials and structural arrangements which provide a relatively high efficiency of heat transfer from within body  12  to a transfer medium. In one embodiment, heat exchangers  16 ,  18  may be one or more tubes (not shown) for carrying therein a fluid to be heated or cooled. The tubes are formed of a material and sized and positioned to efficiently transfer thermal energy (heating or cooling) between the fluid therein and the gas within body  12  during operation of the engine. To enhance heat transfer, the surface area of the tubes may be increased with fins or other structures as is well known in the art. Details of heat exchanger design are otherwise known in the art and are therefore not further discussed herein. 
     Optionally, a third heat exchanger  19  may be disposed within one end of body  12 , for example such that heat exchanger  18  is located between third heat exchanger  19  and regenerator  14 . Third heat exchanger  19  may be of a similar construction to first and second heat exchangers  16 ,  18  such as one or more tubes (not shown) formed of a material and sized and positioned to efficiently transfer thermal energy (heating or cooling) between a fluid therein and the gas within body  12  during operation of the engine. 
     An acoustic source  20  is disposed at a first longitudinal end of body  12 , and an acoustic converter  22  is disposed at a second longitudinal end of body  12  opposite to said acoustic source  20  relative to said regenerator  14 . Many different types of devices may serve the function of acoustic source  20 . A well-known moving coil, piezo-electric, electro-static, ribbon or other form of loud speaker may form acoustic source  20 . A very efficient, compact, low-moving-mass, frequency tunable, and frequency stable speaker design is preferred so that the energy output from the engine may be maximized. 
     Likewise, many different types of devices may serve the function of acoustic converter  22 . A well-known electrostatic, electromagnetic, piezo-electric or other form of microphone or pressure transducer may form acoustic converter  22 . In addition, gas-spring, compliance elements, inertance elements, or other acoustic elements, may also be employed to enhance the function of converter  22 . Again, efficiency is a preferred attribute of acoustic converter  22  so that the energy output from the engine may be maximized. 
     The input a, b to acoustic source  20  is from a impedance circuit Z 1 . Outputs c, d, from acoustic converter  22  are provided to a impedance circuit Z 2 . The output g, h of impedance circuit Z 2  is provided to a splitter  26  (one example shown in  FIG. 3A ). A portion o, p of the output of splitter  26  is provided to impedance circuit Z 3 . The output of impedance circuit Z 3  is input to a phase delay circuit φ(ω), employed to achieve desired phasing as is well understood in the art. The output of phase delay circuit φ(ω) is input to impedance circuit Z 4 , the output of which is input to a combiner  27  (shown in  FIG. 3B ), and ultimately fed back to impedance circuit Z 1  as inputs w, x. The roles of impedance circuits Z 1 , Z 2 , Z 3 , and Z 4  are to match the system impedances and drive acoustic source  20  at a desired frequency and phase. Each impedance circuit Z 1 , Z 2 , Z 3 , and Z 4  may for example be a circuit such as circuit  24  illustrated in  FIG. 2 . Frequency selection is discussed further below. In addition, combiner  27  may be provided with an external input e, f used for example for grid frequency- and phase-locking, to speed up start-up, etc. 
     With the basic physical elements and their interconnections described above, we now turn to the operation of engine  10 . Initially, a gas, such as helium, is sealed within body  12 . A temperature gradient is established in regenerator  14  by establishing first heat exchanger  16  as a “cold” heat exchanger and second heat exchanger  18  as a “hot” heat exchanger. With proper choice of the dimensions and material choices for body  12  and regenerator  14 , of the gas, and of the temperatures of the cold and hot heat exchangers, when the gas undergoes acoustic oscillations, an approximate Stirling cycle is initiated in the region of the regenerator. This cycle, illustrated in  FIG. 4 , comprises a constant-volume heating of the gas as it moves in the direction from the cold heat exchanger to the hot heat exchanger at stage  1 , isothermal expansion of the gas at stage  2 , constant-volume cooling of the gas as it moves in the direction from the hot heat exchanger to the cold heat exchanger at stage  3 , and consequent isothermal contraction of the gas at stage  4 , at which point the gas heats again and the process repeats itself. In this way the acoustic oscillations in the regenerator  14  are amplified. Regenerator  14  serves to store heat energy and greatly improves the efficiency of energy conversion. 
     The acoustic source  20  produces an acoustic wave which is amplified in the regenerator  14  in the manner described above. The amplified acoustic energy is incident on converter  22 , which converts a portion of that energy into electric energy. A portion of this electric energy is fed back to and drives acoustic source  20 . The values of the electrical components (e.g., R 1-4 , L 1-3 , and C 1-3 ) are chosen such that in conjunction with the mechanical and acoustic components, positive feedback is established to maintain the oscillations. As acoustic energy is amplified in this process, converter  22  is therefore able to produce an increased amount of electrical energy, a part of which goes to further driving acoustic source  20 , and the remainder of which goes to driving the load, storage, etc. 
     As mentioned, a large portion of the volume and length of prior art thermoacoustic engines forms the acoustic network. Thus, replacing the acoustic network with an electrical network permits the formation of a more compact engine, for which there are numerous advantages such as use in more confined and size- and weight-limited applications, increased efficiency due to reduced viscous and thermal relaxation losses, etc. Engines of the type disclosed herein exhibit fewer acoustic losses because of the reduced size and reduction of streaming as compared to the prior art. Furthermore, by replacing part of the physical acoustic network with a tunable electric network the operational frequency of the device is no longer defined by the engine structure, but may instead be independently chosen. This allows lower frequencies to be used, which, as they admit better heat transfer to and from the regenerator as compared to higher frequencies, provide a higher efficiency. Typically, a particular device is designed for a specific frequency. Electrical tuning may optimize the operation of the engine, enable frequency-locking to the grid, etc. However, a particular device can be designed for a large range of frequencies without requiring a change to the acoustical portion of the device. Still further, this allows electrical energy to be generated at the frequency of the load, such as an electrical grid, obviating the need for inverters and other elements used for frequency conversion. Further still, the electrical components can be more easily tuned than acoustic elements, facilitating optimization of the device. 
     With reference to  FIG. 6 , a method of operating a thermo-electro-acoustic refrigerator pursuant to the above description of an embodiment of the present disclosure is shown. 
     No limitation in the description of the present disclosure or its claims can or should be read as absolute. The limitations of the claims are intended to define the boundaries of the present disclosure, up to and including those limitations. To further highlight this, the term “substantially” may occasionally be used herein in association with a claim limitation (although consideration for variations and imperfections is not restricted to only those limitations used with that term). While as difficult to precisely define as the limitations of the present disclosure themselves, we intend that this term be interpreted as “to a large extent”, “as nearly as practicable”, “within technical limitations”, and the like. 
     Furthermore, while a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. For example, while the above description is in terms of a tubular structure with coaxially arranged elements, other physical arrangements may be advantageous for one application or another, such as a curved or folded body, locating either or both source and transceiver non-coaxially (e.g., on a side as opposed to end of the body), and are contemplated by the present description and claims, below. Thus, various of the above-disclosed and other features and functions, or alternative thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications variations, or improvements therein or thereon may be subsequently made by those skilled in the art which are also intended to be encompassed by the claims, below. 
     Therefore, the foregoing description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.