WO2010107308A1

WO2010107308A1 - Multistage traveling wave thermoacoustic engine with phase distributed power extraction

Info

Publication number: WO2010107308A1
Application number: PCT/NL2010/050057
Authority: WO
Inventors: Cornelis Maria De Blok
Original assignee: Cornelis Maria De Blok
Priority date: 2009-02-25
Filing date: 2010-02-08
Publication date: 2010-09-23

Abstract

The invention relates to lowering the onset and operation temperature of a thermoacoustic energy heat engine by using multiple thermoacoustic energy converters (TEAC's) and by enhancing the acoustic power transfer from engine to load which could be a heat pump as well as a acousto-electπc converter or linear alternator. According to the invention the acoustic power gain will be increased by a series connection of multiple TEAC's mutually coupled by acoustic wave guides. Pertain to the invention is that the acoustic impedance of each of these mutual wave guides is set to such a value that at the operating frequency the acoustic input impedance of one TEAC is equal or close to the output impedance of the previous TEAC while at the same time at the regenerator position of each TEAC a nearly real acoustic impedance is created. A preferred implementation of the invention is to use four identical TEAC with a identical mutual acoustic spacing of a quarter wavelength causes acoustic impedance in each TEAC to be matched "by default" avoiding the need for acoustic adjustment afterward and potentially reduce production cost because this particular configuration has identical components for each stage. Because due to the traveling wave propagation acoustic phase is distributed proportional along the total length of the feedback loop. Acoustic power therefore can be extracted with a three phase (0°-120°- 240°) of four phase ( 0o-90°-180o-270°) relation between pressure amplitude of the various loads e.g. alternators for generating multi-phase electricity.

Description

Multistage traveling wave thermoacoustic engine with phase distributed power extraction

BACKGROUND OF THE INVENTION The invention relates to lowering the onset and operation temperature of a thermoacoustic energy converter (TAEC) by reduction of the acoustic losses in an integrated system which consist of a TEAC configured as heat engine and a load like, for example, a second TEAC configured as heat pump or an alternator for generating electricity. Thermoacoustic energy conversion is the common appellation for thermodynamic cycles, converting heat in acoustic (mechanical) energy or vice verse, in which the function of pistons and displacers is taken over by the compression, expansion and displacement in a powerful acoustic wave.

A TEAC comprise of a fixed positioned regenerator clamped between two heat exchangers. This so called regenerator unit is inserted in a gas filled acoustic resonance and feedback circuit. A TAEC can be used as a heat pump or as an engine. In the former case acoustic energy is supplied, by which the gas is brought into oscillation by means of e.g. a membrane, bellows or a free piston construction; by means of the oscillating gas heat is then "pumped" from the one heat exchanger to the other. In the latter case, as an engine, heat is supplied to the one heat exchanger and heat is drained at the other, whereby oscillation of the gas column is kept up; the acoustic power from the oscillation can be coupled out as useful energy. Said heat pump can also be driven directly without intervention of a membrane and electro-mechanic converter by said engine, by which a heat pumping system driven by heat comes about without any moving parts at all. Acoustic energy of said engine can be also converted into electricity by a (linear) alternator

From literature [1,2] measures are know for obtaining the correct relation between gas and pressure amplitude which requires an high and nearly real impedance at the position of the regenerator. This is a strict condition for obtaining a high conversion efficiency. Conversion efficiency is herewith defined as the ratio between acoustic output power measured at the junction to the acoustic resonance circuit and the heat supplied to high temperature heat exchanger of the TEAC. In well known implementations a TEAC configured as engine is placed at one end of a standing wave type resonator while a second TEAC configured as heat pump is placed at the other end In common implementations the mid-section of the standing wave resonator is narrowed in order to reduce length and suppress harmonics which makes it appearance similar to a so called Helmholtz resonator.

A TEAC behaves as an acoustic power amplifier and therefore the net available acoustic output power of the engine equals the acoustic power stored in the resonance circuit times the acoustic power gain which equals the ratio between the absolute temperatures at the in-and output heat exchangers of the TEAC

The acoustic power in the resonance circuit depends on the pressure ratio and the acoustic impedance in the regenerator of the TEAC. As said this impedance should be set by measures described in [1,2] to a high and real values. This minimize velocity in the regenerator and with that viscous losses. Consequence of this increase in impedance is a deterioration of the ration acoustic power to be amplified and acoustic losses at the same pressure amplitude enhanced by the occurrence of local pressure and velocity maxima due to the interfering waves in the standing wave resonator. As a consequence, the commonly used configuration of TEAC configured as engine by a torus or bypass and combined with standing wave type resonator will exhibit an increasing reversion in efficiency at lower operating temperatures consequently impede or limiting the practical application in the low temperature range like waste and solar heat

Well known is that at low operating temperature acoustic power gain can be increased by placing multiple TEAC acoustically in series. In the commonly used configurations [1,2,3] however it is not possible to create by acoustical means the required high and real impedance in more than two TEAC ' s at the same time. This limitation holds for the loop type resonator [3] as well as for the usual torus or bypass configuration [1,2] combined with a standing wave resonator. For other known configurations [4] using multiple TEAC's the total wall surface and with that an increase in boundary layer losses counteracting the profit in gain.

Dependent of the available temperature to drive the thermoacoustic engine 20% to 50% of the net output power of the engine is lost in the acoustic resonance circuit seriously affecting the performance of an integral system. Related to the invention it is well known that the acoustic power could be converted to electricity by a so called linear alternator which is basically a moving coil in a static magnetic field or a moving magnet in a fixed positioned coil. The periodic pressure amplitude will be applied to a membrane of free moving piston which in its turn is used to move the coil or magnet and generate electricity. The moving mass of these alternators is quite high so in order to eliminate external vibration they commonly will be applied in pairs [5] .

SUMMARY OF THE INVENTION

The invention relates to lowering the onset and operation temperature of a thermoacoustic energy converter (TAEC) by reduction of the acoustic losses and increasing acoustic power gain in a thermoacoustic integrated system in such a way that the known limitations are terminated. According to the invention the acoustic power gain will be increased by a series connection of multiple TEAC ' s mutually coupled by acoustic wave guides. Pertain to the invention is that the acoustic impedance of each of the mutual wave guides is set (by diameter and length) to such a value that at the operating frequency the acoustic input impedance of one TEAC is set equal or close to the output impedance of the previous TEAC while at the same time at the position of each TEAC a nearly real acoustic impedance is created.

In this traveling wave configuration local acoustic velocity inside the heat exchangers and regenerator of the TEAC is reduced by an increase of the cross-sectional area of each TEAC with respect to the local cross-sectional area or diameter of the wave guide. Because volume flow rate is maintained acoustic power is not affected while viscous loss is reduced. Acoustic losses at the diameter transition between TEAC and wave guide are minimized by an appropriate rounding. A similar impedance matching is proposed for the feedback wave guides between the last TEAC and the acoustic load and between load and first TEAC. Because according to the invention acoustic reflection between all components is minimized the acoustic power transfer is maximal (traveling waves) at the smallest posible wave guide diameter. Because of the minimal wall surface and the absence of local high pressure spots, as they occur in standing wave resonators, the acoustic loss as compared with acoustic power transferred is minimal resulting in onset temperature differeneces less than 40 K. Contrary to existing implementations the number of TEAC ' s which can be applied is arbitrary with the only remark that all wave guides between TEAC 's and the TEAC itself should have different dimensions.

A preferred implementation of the invention is to use four identical TEAC with a mutual acoustic spacing of a quarter wavelength (1/4 λ) . Acoustic matching in this particular case is obtained by the fact that reflections with a mutual distance of 1/4 λ tend to cancel out each other. Due to this property acoustic impedance in each TEAC is matched "by default" avoiding the need for acoustic adjustment afterward and potentially reduce production cost because this particular configuration has identical components per stage.

The invention permits also to extract acoustic power on multiple positions and with different phase along the, one acoustic wavelength long, feedback circuit. This option could be used avoid high power spots and minimize losses but it also allows multi phase power extraction. Because due to the traveling wave propagation the phase is distributed proportional along the total length of the feedback loop acoustic power can be extracted with a three phase ( 0°-120°-240°) of four phase ( 0^o-90°-180^o-270°) relation between pressure amplitude at the various loads e.g. alternators for generating multi phase electricity.

According to the invention and in particular due to the option of multi-phase power extraction the traveling wave can be converted by means of mutual coupled pistons or membranes into a rotating force field having the same rotational speed as the acoustic oscillation frequency. This rotating force field can be converted to a rotation by an exentric of swivel plate for direct driving a classic rotating alternator or other rotating device like a pump.

REFERENCES

[1] Thermoacoustic device, Internationale publicatie WO99/20957

[2] S. Backhouse and G. W. Swift. "A thermoacoustic Stirling heat engine: Detailed study", J. Acoust. Soc. Am 107, 3148 (2000)

[3] Resonant travelling wave heat engine Ceperly, US patent 4,355,517 [4] Cascaded Thermoacoustic Devices, Swift at all, US patent 6,658,862

[5] Backhaus, S., G. W. Swift (2004) : Traveling-wave thermoacoustic electric generator. Applied Physics Letters, 85 [6], pp. 1085-1087, 2004.

EXEMPLARY EMBODIMENTS

Figure 1 shows a multistage thermoacoustic heat engine which according to the invention is build up from, in this example, four TEAC ' s (1,2,3,4) each comprising a regenerator ( 5 ) clamped between high temperature heat exchanger (6) and a low temperature heat exchanger (7) by which means heat is supplied at a high temperature and drained at a low temperature and a not further described acousto-mechanical or acousto-electrical converter (8) for converting acoustic energy in for example electricity. The TEAC ' s and converter are mutually coupled with acoustic wave guides (9,10,11,12) of which length and diameter is set in such a way that combined with the actual in- and output impedance of each TEAC and converter in each said wave guide reflection is minimized and a near traveling wave is created while at the same time at the position of the regenerator (5) of each TEAC (1,2,3,4) near zero phase difference between acoustic pressure and velocity amplitude is maintained. Viscous loss in the regenerator of each TEAC (1,2,3,4) is reduced by increasing the cross-sectional area by a factor of at least 5 with respect to the cross-sectional area of the connected wave guides (10,11,12,13) . Acoustic feedback needed to maintain oscillation is performed by a fifth wave guide (13) having a length equal to one or an integer multiple of the acoustic wavelength at the oscillation frequency minus the acoustic length of the TEAC ' s and converter and is in its turn build up from multiple here not further specified wave guide sections of various length and diameter in such a way, e.q. a l/4λ transformer, that the input impedance of the first TEAC (4), as seen in the propagation direction, equals the acoustic output impedance of the converter (8) and minimizing reflection .

Figure 2 shows an exemplary embodiment at which the converter (8) is removed and at which in one of the TEACs, in this case (1), the function of the heat exchangers is reversed and said TEAC acts here as a heat pump in which case a heat driven heat pump is created. By means of a here not detailed combination of wave guides (9) and (13) in the feedback loop the input impedance of the first TEAC as seen in the propagation direction is matched to the acoustic output impedance of the as heat pump configured TEAC (1) . Figure 3 shows a specific and preferred symmetric exemplary embodiment having a mutual acoustic distance of one quarter wavelength between the, in this case, identical TEACs. Acoustic matching in this particular case is obtained by the fact that reflections with a mutual distance of 1/4 λ tends to cancel out each other and as a result the low output impedance of each TEAC is transformed to a high impedance at the input of the next TEAC. Typical for this configuration is that each TEAC is matched "by default" avoiding the need for acoustic matching adjustments afterward. This symmetric build up is maintained by extracting the acoustic output power per TEAC by a not further described multi phase acousto- mechanical or acousto-electrical converter (18) . Power transfer from the feedback loop to the multiphase converter (18) takes place by multiple, in this case, four wave guides (14,15,16,17) of which length and diameter in conjunction with the impedance of the converter (18) is set to minimize reflection and optimise power transfer.

Figure 4 shows an exemplary embodiment of a multi-phase acousto mechanical converter in which part of the traveling wave power in the feedback loop is transferred by wave guides (14,15,16,17) to four mutual connected membranes or pistons (19,20,21,22), resulting in a rotating force field (23) having the same rotational speed as the oscillation frequency and which by means of an exentric (24), or similar device for converting a periodic movement in a rotation, is coupled to a rotating device (25) like an common alternator or pump

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Claims

1. A thermoacoustic engine comprising of multiple thermoacoustic energy converters (TEAC's) in which each TEAC build up from a regenerator clamped between two heat exchangers and in which the TEAC's are mutually acoustically coupled in such a way that acoustic impedances of in- and output of the adjacent TEAC's or load are acoustically matched in such a way that reflection in the coupling wave guides is minimized and mutual coupling between the TEAC's and load take place by means of near traveling waves.

2. A multi-stage thermoacoust engine according to claim 1, characterized by a symmetrical build up consisting of four identical TEAC's with a mutual acoustic distance of a quarter wavelength or by two identical TEAC's mutually coupled by wave guides with respectively a quarter and a three quarter wavelength.

3. A multi-stage thermoacoust engine according to claim 1 or claim 2, characterized by a cross-sectional area of the regenerator in each TEAC which is at least 4 times the cross-sectional area of the preceding wave guide.

4. A multi-stage thermoacoustic engine according to claim 1 or 2, in which no acousto-mechanical or acousto-electπc load is present and in which one of the TEAC's is configured as a heat pump yielding a heat driven heat pump in which mechanical moving components are absent at all.

5. A multi-stage thermoacoustic engine according to claim 1 or 2, characterized in that the acoustic load is distributed amongst multiple acousto-mechanical or acousto-electrical converters spaced along the traveling wave feedback loop and at which acoustic power is extracted with a three phase ( 0°-120°-240°) of four phase ( 0°-90°-180°- 270°) relation between pressure amplitude at the various loads e.g. alternators for generating multi phase electricity.

6. A multi-stage thermoacoustic engine according to claim 1,2 and 5, characterized by four mutually connected membranes or pistons (19,20,21,22) coupled to the traveling wave feedback loop, resulting in a rotating force field (23) having the same rotational speed as the oscillation frequency and by which means of an exentric (24), or similar device for converting a periodic movement in a rotation, is coupled to a standard rotating device (25) like an alternator or pump.