A regenerative TAEC comprises an acoustic or acoustic-mechanical resonance circuit, in which a gas is present, as well as two heat exchangers, on both sides of a"regenerator"of a porous material with good heat exchange properties.

A TAEC can be used as a heat pump or as an engine. In the former case mechanical energy is added, 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 gas movement can be coupled out as useful energy through the membrane.

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.

Experimental set-ups and prototypes, as for example described in [1], often are equipped with a electrical heater for convenient power measurement. For real applications this is not an option because electrical heating introduce additional superfluous energy conversion steps. Efficiency of an thermoacoustic heat engine can be utilised only when such an engine is powered directly by a primary energy source like gas, oil or solar energy or, in general by heat at an appropriate temperature.

Methods for heat supply at high temperature are known from [2] at which a bar like, cylindrical radiation burner with or without recuperation is used and maintained on a high temperature (e. g. 900- 1000 °C) by a primary energy source. In order to maximise absorption of the radiated heat from the burner surface the regenerator is build co- axially around this radiation burner.

Pertain to applied radiation burners and co-axially build regenerators is the relatively large longitudinal acoustic length along the burner and regenerator assembly as seen in the propagation direction of the

acoustic wave. Due to this long inherent burner length and the associated longitudinal length of regenerator and timing circuit, as described in [2], even at low acoustic frequencies, a substantial longitudinal phase difference exist along the regenerator. As a consequence, appropriate timing can be maintained only in a small part of the regenerator. Timing of the thermodynamic energy conversion process in a substantial part of the regenerator therefore is suboptimal, resulting in a severe efficiency penalty. This penalty occurs even for longitudinal burner or regenerator lengths less than 1% of the acoustic wavelength. For burner and regenerator of longitudinal lengths of 3% of the acoustic wavelength efficiency is halved in some configurations.

This is inevitable when using known acoustic measures for creating the required travelling wave condition of a near zero phase difference between pressure and velocity amplitude in longitudinal extended regenerator assemblies.

This limitation in longitudinal burner length also sets a limit to the maximum thermal power which can be supplied by radiation at given temperature and burner diameter and consequently this will limit the acoustic output power.

The same limitation in longitudinal length will occur when the radiation burner is replaced by a cylindrical heat exchanger or, in case of a heat pump, by a cylindrical heat exchanger at the cold side of the regenerator. The present invention aims at increasing the capacity of a TAEC in a way wherein the efficiency loss observed in said exemplary embodiments does not or hardly take place.

SUMMARY OF THE INVENTION According to the invention the regenerator and low temperature or ambient temperature heat exchanger will be build up, as short, longitudinal, separated ring shaped sections connected acoustic an thermal in parallel over the total length of the burner while each section is provided with a dedicated timing circuit implemented as a radial volume and space between the separate sections. As a result the acoustic impedance in the regenerator of each separate section can be set to the appropriate high absolute value and nearly zero phase difference between pressure and velocity amplitude as described in [3], by adjusting the volume per section or space between the individual sections making them nearly independent of each other and

of the longitudinal position (acoustic impedance) of the standing wave along the burner inside the resonator room.

As a result of the measures according to the invention the correct timing in the regenerator is hardly dependent of the position in the standing wave or Helmholz resonator and can be maintained in an arbitrary number of separate sections positioned over about 7% of the acoustic wavelength with hardly no penalty in efficiency as mentioned before. In addition the increased burner length allows for a much higher thermal and acoustic power level at given system dimensions.

When combined with a travelling wave resonator, multiple separate impedance matched sections can be located even on an arbitrary position of the acoustic wave length without loss of correct timing per section resulting in a further increase of the thermal and acoustic power level of a TAEC per given system dimensions.

The implementation according to the invention can also be used in case heat is supplied at lower operating temperatures at which radiation is inadequate as heat transfer mechanism or in case of a TAEC when configured as heat pump. In both cases the radiant burner is replaced by a cylindrical or tube like heat exchanger per section, having an outside diameter which equals the regenerator inside diameter resulting in an optimal acoustic and thermal contact and a longitudinal length as short as the regenerator.

REFERENCES [1] S. Backhouse and G. W. Swift."A thermoacoustic Stirling heat engine", j. Acoust. Soc. Am. 107,3148 (2000) [2] Patent application NL 1021412 [3] International publication W099/20957 EXEMPLARY EMBODIMENTS Figure 1 shows an exemplary embodiment of a regenerative thermoacoustic energy converter (TAEC) powered by a radiant burner located in an housing 1 coupled with a narrowed tube 2 to a second housing 3 together shaped as a Helmholtz or standing wave resonator, in which housing 3 could contain, a not drawn, second TAEC which is configured for example as a heat pump. In the first TAEC a regenerator 4 and low or ambient temperature heat exchanger 5 are implemented co- axial and cylindrical around a, not further described, recuperative radiation burner 6. This is further detailed in the cross-sectional view of figure 2.

The radiant burner is kept on a high temperature by burning a fossil fuel supplied by connection 7. Combustion products and heated gas are leaving the burner 6 by the outlet 8.

Acoustic timing is provided by the volume 9 and bypass 10. Such a device is known from the Dutch patent application 1021412 as a burner driven thermoacoustic heat pump.

Figure 2 shows an exemplary embodiment according to the invention comprising a first TAEC build from multiple sections each of them equipped with a ring shaped or cylindrical regenerator 4, a ring shaped or cylindrical low temperature or ambient heat exchanger 5 and a dedicated timing circuit composed as a radial implemented buffer volume 9 and an acoustic bypass formed by the intersectional space 10.

Longitudinal length of each separate section is less than 2% of the acoustic wavelength. The sections are coupled, thermal and acoustic, in parallel to the resonator 1,2 which resonator room also contains the radiant burner 6.

Figure 3 describes a second implementation of the invention characterised by non uniform dimensions of acoustic timing circuit 9,10 of the separate sections resulting in an further improved matching to the standing acoustic wave in the resonator room 1.

Figure 4 describes a third implementation of the invention at which regenerator 5 and low temperature or ambient temperature heat exchanger 5 are not uniform in diameter, resulting in an further improved matching to the standing acoustic wave in the resonator room 1.

Figure 5 describes a fourth implementation of the invention at which at which the burner 6 is not uniform in diameter, resulting in an further improved matching to the standing acoustic wave in the resonator room 1.

Figure 6 finally describes an implementation of the invention at which the sections are coupled by a travelling wave resonator 2 comprising an impedance matching and acoustic power extraction circuit 11. In a well matched travelling wave resonator pressure amplitude is nearly independent of the longitudinal position allowing a large number of

separate section to be coupled to a TAEC having a aggregate length equal or to a substantial part of the acoustic wave length In stead of a radiant burner 6, halogen lamps or concentrated solar energy can be used. For applications in which no high temperature is available as for example is the case in a heat pump, the burner 6 can be replaced by a cylindrical heat exchanger per section having an outside diameter which equals the inside diameter of the regenerator to establish an optimal acoustic and thermal contact.