EP2556236B1 - Stirling machine - Google Patents

Description

The present invention relates to a Stirling machine comprising a transfer piston and a movable member of a generator or an electric motor, the transfer piston being mounted in a cylinder, in which it periodically moves a working gas between a an expansion chamber and a compression chamber constituting the working volume of said Stirling machine, respectively associated with two working faces of said transfer piston by passing said gas through a heat exchanger, connected to a heat source, a regenerator and a cooling exchanger connected to a heat sink and resilient biasing means exerting a force on said transfer piston, the ratio of section a _C / a _E between the two working faces of said piston being ≥0.35 so that its displacement along an axis oriented towards the expansion volume generates a pressure component of said working gas in phase opposite said displacement of said udder ton, so as to transmit between the transfer piston and said movable member all of said mechanical energy produced.

One type of Stirling engine consists of a transfer piston that periodically moves the working gas between a hot and a cold volume and a motor piston that closes the working volume and ensures the transfer of mechanical energy produced to the moving part of an electric generator. In kinematic motors, the two pistons are connected by a mechanical system with a crankshaft, which imposes a repetitive periodic movement with a fixed offset.

In free piston engines, the two pistons are provided with elastic suspensions, dimensioned so as to give the two pistons a periodic movement at the frequency desired, with a prescribed phase shift. The absence of linkages simplifies the construction of these engines: by eliminating the joints the lubrication problems of these are removed. However, these engines often require complex control systems to ensure their startup and to stabilize the oscillating movement of the two pistons with amplitudes and phase angles determined.

A Stirling engine, developed by the American firm Sunpower Inc. Athens, Ohio is described in an article entitled "Development of a 3kW Free-piston Stirling Engine" by G. Chen and J. McEntee, Proceedings of the 26th Intersociety Energy Conversion Engineering Conference, vol. 5, pp.233-238 , where part of the motive power is induced by the forces of the gas on the transfer piston, then transmitted by a pneumatic spring to the engine piston. In this engine, the transfer piston therefore not only serves to transfer the gas between the hot and cold volumes located at both ends of the cylinder in which the piston moves, but also to generate a portion of the motive power.

The EP 1'165'955 discloses an engine where all the motive power is produced by means of the transfer piston, which is associated with the moving part of the electric generator. A resonance tube is coupled to this device, in which a pressure wave is established which is out of phase with the excitation wave produced by the transfer piston. The disadvantage of this solution lies mainly in the energy losses generated by the friction of the gas in the tube which limit the performance of these engines. Moreover, the bulk of the resonance tube has, in many applications a significant disadvantage.

The JP 2127758 U illustrates the figure 3 a Stirling machine in which the transfer piston is connected by a linkage to an electric motor. With this arrangement, the amplitude of the transfer piston is controlled mechanically, thus making the use of a flexible stop superfluous. This machine also comprises a working piston and a load. In this configuration, only a fraction of the energy produced can be transmitted to the electric motor associated with the transfer piston.

The object of the present invention is to remedy, at least in part, these drawbacks, to simplify the control cycle of the Stirling machine and increase its stability of operation, as well as improve its performance.

For this purpose, this invention relates to a Stirling machine as defined by claim 1.

The essential advantage of the invention over two-piston Stirling machines according to the state of the art lies in the fact that the resonant piston no longer needs to be controlled, to eliminate any active servocontrol requiring electronics complex.

Advantageously, the resonant piston of the machine object of the invention is a free piston, suspended by a mechanical spring and which delimits the working volume. This resonant piston thus fulfills a function similar to that of the resonance tube described in the patent EP 1'165'955 . The mechanical and thermal losses caused by friction and leakage through the piston seals are much smaller than those of a resonance tube. By its movement the pressure of the working gas varies. This resonant piston can be compactly incorporated into the volume of the Stirling machine.

With proper sizing, both pistons oscillate stably. The operation of the system can easily be controlled, both in the phase of starting in steady state, as will be explained in detail later.

• La figure 1 est une vue en coupe diamétrale d'une forme d'exécution;
• la figure 2 est une vue en coupe diamétrale partielle d'une variante de la machine;
• la figure 3 est une vue en coupe diamétrale d'une variante hybride;
• la figure 3A est une vue partielle d'une variante des figures 1 ou 3;
• la figure 4 est un diagramme vectoriel relatif au processus de fonctionnement;
• la figure 5 est un diagramme relatif au travail fourni par cycle en fonction de la température de l'échangeur chaud, pour un moteur selon l'invention, comparé à un moteur comportant un piston de transfert et un piston moteur;
• la figure 6 est un diagramme relatif au rendement thermique du moteur Stirling en fonction du travail fourni par cycle, pour un moteur selon l'invention comparé à un moteur comportant un piston de transfert et un piston moteur ;
• la figure 7 est une vue en coupe diamétrale d'une autre forme d'exécution de la machine, comportant deux pistons résonants oscillant en directions opposées;
• la figure 8 est une vue en coupe transversale d'une variante de la figure 7;
• la figure 9 est un schéma de principe illustrant une coupe transversale de la machine, au niveau des pistons résonants ;
• la figure 10 est un schéma de principe illustrant un dispositif servant à réduire les vibrations induites par le mouvement périodique du piston de transfert à l'aide d'une masse additionnelle ;
• la figure 11 est une vue en coupe diamétrale partielle d'une variante de la machine ;
• la figure 12 est une variante de la coupe diamétrale de la figure 11.

Other features and advantages of the machine which is the subject of the invention will become apparent on reading the description which follows, as well as the appended drawings, which illustrate, schematically and by way of example, two embodiments and various variants of this machine.

• The figure 1 is a diametrical sectional view of an embodiment;
• the figure 2 is a partial sectional view of a variant of the machine;
• the figure 3 is a diametrical sectional view of a hybrid variant;
• the figure 3A is a partial view of a variant of figures 1 or 3 ;
• the figure 4 is a vector diagram relating to the operating process;
• the figure 5 is a diagram relating to the work supplied per cycle as a function of the temperature of the hot exchanger, for an engine according to the invention, compared to a motor comprising a transfer piston and a motor piston;
• the figure 6 is a diagram relating to the thermal efficiency of the Stirling engine according to the work supplied per cycle, for an engine according to the invention compared to a motor comprising a transfer piston and a driving piston;
• the figure 7 is a diametrical sectional view of another embodiment of the machine, comprising two resonant pistons oscillating in opposite directions;
• the figure 8 is a cross-sectional view of a variant of the figure 7 ;
• the figure 9 is a block diagram illustrating a cross section of the machine, at the level of the resonant pistons;
• the figure 10 is a block diagram illustrating a device for reducing the vibrations induced by the periodic movement of the transfer piston by means of an additional mass;
• the figure 11 is a partial sectional view of a variant of the machine;
• the figure 12 is a variant of the diametrical section of the figure 11 .

The Stirling machine illustrated by the figure 1 comprises an elongate casing 1 formed of two cylindrical parts 2, 3, assembled by an element 4, acting as a frame. The interior of this casing 1 is filled with a pressurized working gas. The cylindrical housing 5 of the part 2, constitutes a working volume of a Stirling engine, in which a two-part transfer piston 6, 6a is mounted, free to move longitudinally. The volume located between the transfer piston 6, 6a and the outer end of the housing 5 communicates with a hot heat exchanger 7 connected to a hot source (not shown) and constitutes the hot chamber or expansion volume V _E of the Stirling engine, while the volume located at the other end of this cylindrical housing 5 communicates with a cold exchanger 8 connected to a cold source (not shown), which is the cold room or compression volume V _C Stirling engine. A regenerator 9 is disposed between the heat exchanger 7 and cold 8.

The tubular portion 6a of the transfer piston 6, 6a adjacent to the compression chamber V _C is engaged in the cylindrical opening of a second ring-shaped and axisymmetrical resonant piston 10 with respect to the piston 6, 6a. This second piston 10, secured to a support 11 is free to move along the longitudinal axis of the cylindrical housing 5.

An elastic suspension member 12 is fixed by its central portion to the support 11 and by its periphery to a support 13 secured to the frame 4. This elastic suspension member 12 is a flat member with a spiral-shaped arm. In the variant illustrated by the figure 3A , the resonant piston 10 is suspended from the frame 4 by helical springs 12a, arranged symmetrically about the axis and exerting an axial force on the piston, centered relative thereto.

Seals 25 disposed between the pistons 6a and 10 on the one hand and between these pistons and the cylindrical housing 5 on the other hand, serve to contain the gas leaks to tolerable levels.

The internal volume of the cylindrical portion 3 encloses a movable member 14 of an electric generator, here constituted by a cylindrical element carrying permanent magnets. This movable element 14 is integral with the periphery of an annular support 15, whose inner edge is integral with an annular elastic suspension member 16, similar to the member 12. The periphery of this member 12 is fixed to the frame 4 and its center is secured to a rod 17, one end of which is fixed to the transfer piston 6, 6a. The armature of the generator is formed of an assembly of plates 18 arranged radially and in which are housed one or more windings 19 of annular shape. The movable element 14 of the electric generator is surrounded by an armature 20, formed here of an assembly of plates arranged in radial planes.

The elastic suspension of the transfer piston 6, 6a can be reinforced by one or more helical springs 21, arranged between fixed supports 22, integral with the frame 4 and movable supports 23, integral with the rod 17.

A conduit having an adjustment valve 24 placed between the cold compression volume and the generator volume makes it possible to adjust the pressure amplitude of the working gas, thus the power of the engine. This valve also makes it possible to adjust the amplitude of the movement described by the resonant piston.

The figure 2 shows a partial diametrical section through the second resonant piston 10, illustrating an alternative solution of the cylindrical bearing surfaces of the two pistons 6a and 10. In place of the seals, it is advantageous to provide between the cylindrical surfaces of the pistons and their enclosures annular slots with games of the order of 20 to 50 microns, as a means of guiding and levitation. These sets are perfectly acceptable both from the point of view of manufacturing tolerances and the influence of working gas leaks on the energy efficiency of these devices. The mechanical friction of the pistons can be reduced with wear-resistant and self-lubricating surface coatings able to reduce static and dynamic friction. In a preferred embodiment, it is also intended to use static gas bearings as described in US 3'127'955.

For this purpose, the interior of the piston 10 is hollow, providing a housing 26 serving as a gas reservoir for supplying nozzles 27 opening into the annular slots between the two pistons 6a and 10, respectively between the pistons and the adjacent surfaces of the housing elongated 1, respectively of the wall of the piston 6a. The compartment 26 is fed through a non-return valve 28 from the working volume and maintained permanently at the maximum pressure prevailing in this volume. Compartment 26 can also placed in the transfer piston 6, 6a or in the frame 4, to feed the nozzles 27 of the static gas bearings.

The figure 3 represents a hybrid variant where the housing 5 of the part 2 with the pistons 6, 6a and 10 forming the driving part of the Stirling are similar to the embodiment described above. Part 2 is connected to a compartment 30, comprising a rotary electric generator 31. The transfer-motor piston 6, 6a is connected by a rod 17 to a linkage 32 which transmits the axial movements and forces of the piston 6, 6a to a crankshaft 33 secured to the movable portion of a rotary electric generator 31.

Different forms of execution of linkages can be envisaged. In the figure 3 a crankshaft type Ross is sketched, as described in detail p. ex. in the Proceedings of the 8th International Conference of Stirling Engines held May 27-30, 1997 in Ancona . On page 519ff is described the calculation of the linkage, allowing to minimize the lateral displacement of the rod with respect to its axis of movement. Other embodiments of the linkages are possible, such as the trapezoidal linkage used by Philips (eg, shown on page 60 of the Proceedings of the seminar "Stirling Cycle Prime Movers" of June 14-15, 1978).

The moving part of the electric generator may be provided with a flywheel 34, to balance the rotary movement and thus to smooth the superimposed waves to the generated voltage. Moreover, a mass 35 makes it possible to attenuate the vibrations due to the reciprocating movement of the pistons.

The operation of the Stirling machine described is as follows: The movement of the second resonant piston 10 is dictated by the forces communicated by the elastic elements and the gas pressure exerted on its axial surfaces. By its movement, the pressure of the working gas varies.

The transfer piston 6, 6a then plays the dual role of transferring the working gas between the expansion chamber V _E and the compression chamber V _C and producing all the motive power transmitted to the movable member 14 of the linear generator, provided that certain conditions, which we are going to talk about now, are fulfilled.

To achieve this objective, it is necessary to determine the ratio between the surface a _C of the transfer piston 6, 6a, delimiting the compression volume V _C and the surface a _E of the same transfer piston 6, 6a, delimiting the volume expansion V _E.

The analysis of the isothermal cycle shows that the pressure of the working gas in the working volume becomes independent of the position of the transfer piston 6, 6a if: $$\frac{{\mathrm{at}}_{\mathrm{VS}}}{{\mathrm{at}}_E}=\frac{T_{\mathrm{VS}}}{T_H}$$

Example:

Temperature T _H of the hot volume V _E , T _H = 923 ° K = 650 ° C Temperature T _C of the cold volume V _C , T _C = 323 ° K = 50 ° C

$${\mathrm{at}}_{\mathrm{VS}}/{\mathrm{at}}_{\mathrm{E}}\ge 0.35$$

The operation of the motor is possible only if the surface ratio at _C / a _E is greater than this limit, that is to say that the displacement of the transfer piston 6, 6a ( Figure 4 ) must induce a pressure component P _X which must be opposed to the displacement X of this piston 6, 6a. The displacement of the transfer piston 6, 6a is positive if it moves towards the volume V _E.

This transfer-motor piston can be designed as a free piston. Its elastic suspension must then be tuned so that the piston oscillates at the same frequency as the resonant piston. Its amplitude is controlled by the forces electric powered by the generator; it remains fixed if a constant electric charge is applied to the terminals of the electric generator.

In a hybrid machine, the piston 6, 6a is mechanically connected to the axis of the moving part of a rotary electrical generator by a linkage. The stroke of the piston 6, 6a is then fixed by the geometry of this linkage. Its rotation speed is electrically controlled by the electric generator and its frequency must correspond to that of the second resonant piston 10.

The figure 4 represents a vector diagram illustrating the most important characteristics of the system, the time t running in the direction of clockwise. The vector X represents the displacement of the engine-transfer piston 6, 6a, the vector Y that of the resonant piston 10. Under resonance conditions, Y is delayed with respect to X. By its displacement, the transfer-motor piston 6 , 6a creates a small pressure variation P _X , opposite to X. The displacement Y of the resonant piston 10 creates a pressure variation P _Y in the direction of Y, the pressure variation P of the working gas being the sum of the two components P _X and P _Y.

At each cycle, the resonant piston 10 receives a certain amount of energy proportional to the pressure component P _X which keeps the piston moving. Since P _X depends on the heating temperature T _H , the amplitude Y of the resonant piston 10 varies as a function of this temperature T _H. Since the pressure amplitude P _Y is proportional to Y, this and the mechanical power generated by the Stirling engine increase sharply with the heating temperature T _H.

The figure 5 compares the mechanical energy released by a Stirling engine comprising a transfer piston and a working piston, as a function of the temperature T _H of the heating tubes (curve 1) with that of an engine according to the invention (curve 2). In order to start the Stirling machine which is the subject of the invention, the hot exchanger must first be brought to a relatively high temperature T _H (for example 600 ° C.), which threshold depends on the ratio a _C / a _E chosen. The transfer-motor piston 6, 6a is then oscillated using the electric generator associated therewith. The resonant piston 10 first oscillates with a small amplitude, which gradually increases with the heating temperature T _H. The amplitude of the working gas pressure also increases, as well as the mechanical power supplied by this machine. The nominal power is reached when the heat exchanger is heated to about 700 ° C.

Stirling engines with a transfer piston and a piston engine start at significantly lower heating temperatures (around 300 to 400 ° C depending on their design). The power then increases gradually with the temperature T _H , to achieve, under comparable nominal conditions, a power similar to that of the machine object of the invention.

In the machine object of the invention, a small increase in the temperature of the hot exchanger causes a sharp increase in the power developed by this engine. By the relaxation of the gas in this hot part, the thermal power withdrawn also increases strongly with this temperature. The stability of the engine speed therefore depends precisely on the heat input to the hot heat exchanger and its adjustment can be carried out by simple means. The temperature T _H being precisely controlled by the power released by the engine, the risk of overheating of the hot part is minimal.

The figure 6 compares the thermal efficiency ETA of the conventional machine (curve 1) with that of the machine according to the invention (curve 2), plotted as a function of the energy produced per cycle (WRK). At rated speed, both machines have comparable performance. At partial load, the Stirling machine according to the invention operates at heating temperature T _{H levels} significantly higher than the conventional machine, so under conditions that promote the conversion of thermal energy into mechanical energy. Thus, the machine according to the invention achieves higher ETA thermal efficiencies in a wide range of partial loads.

In the machine according to the invention, the resonant piston 10 receives at each cycle a small amount of energy which serves to compensate for its friction losses and keep it in oscillating motion. The amplitude of its movement Y determines the pressure variation of the working gas and therefore the engine speed. Fine tuning is possible insofar as piston friction remains relatively constant over time as can be achieved using the aforementioned static gas bearings. Moreover, the control valve 24 makes it possible to adjust the pressure amplitude of the working gas, and therefore the amplitude of the resonant piston.

The use of a resonant piston allows the system to operate with a light working gas, such as pure helium, while a resonance tube works better with a heavier gas mixture. The losses in the heat exchange members of the Stirling machine (heating, regenerator, cooler) depend on the density of the gas and are lower in the case of the present invention.

The fact that the temperatures T _H of the heat exchanger vary only slightly with the engine load is particularly advantageous in units heated with fuel. In general, the operation of a burner depends strongly on the temperature conditions which settle there; complete combustion with a minimum of pollutants can only be achieved if the temperature conditions remain sufficiently stable.

• la température du mélange formé par l'apport d'air frais et les gaz recyclés doit se situer au-dessus de la température d'inflammation du combustible ; pour le gaz naturel dans une atmosphère diluée ce seuil se situe au-dessus de 720°C ;
• pour éviter la formation massive de NO_X, la température des gaz ne doit nulle part dépasser la limite des 1300 à 1400°C ;
• la température T_H des surfaces de l'échangeur chaud s'établit comme un équilibre entre l'énergie libérée lors de la combustion et celle soutirée à l'échangeur chaud par la détente du gaz de travail du Stirling. Les conditions de fonctionnement sous le régime du DE 102'17913 restent satisfaites dans une plage de puissance étendue, à condition que T_H ne varie que peu avec la puissance du moteur, comme c'est le cas avec le moteur Stirling objet de l'invention.

An in-depth study has highlighted these advantages for burners using internal recirculation of combustion gases, a technique applied in various forms for Stirling engines (see DE 102'17913 A1). By the dilution of the oxidizer, a flameless combustion settles in the combustion chamber, occupying a large part of this volume. Complete combustion can be achieved with very little excess air if several conditions are satisfied, in particular:

• the temperature of the mixture formed by the supply of fresh air and the recycled gases must be above the ignition temperature of the fuel; for natural gas in a dilute atmosphere this threshold is above 720 ° C;
• to avoid the massive formation of NO _X , the temperature of the gases must nowhere exceed the limit of 1300 to 1400 ° C;
• the temperature T _H of the surfaces of the hot exchanger is established as an equilibrium between the energy released during combustion and that drawn off at the hot exchanger by the expansion of the Stirling working gas. Operating conditions under the regime of DE 102'17913 remain satisfied in an extended power range, provided that T _H varies only slightly with the power of the engine, as is the case with the Stirling engine object of the invention.

Stirling machines with conventional free pistons require sophisticated adjustment means (for example US6'871'495 , or US2008 / 0122408 ) to keep the engine speed under control, both during the start-up phase of the machine and to stabilize the operation around the nominal conditions. In these machines, a deviation Optimum operating conditions can greatly reduce the performance of these engines.

The control of the Stirling machine object of the invention proves to be much simpler, essentially for the following reasons: The two pistons are above all coupled with the enclosure of the system and only incidentally between them. The beat between the two pistons of the machine object of the invention can thus be easily amortized, or even completely removed. In addition, the burner of this Stirling machine responds more quickly to a variation in power since its temperature changes little with the transferred thermal power. Any variation of T _H of the hot source modifies P _X and thus the power transferred to the resonant piston, causing a rapid change in its amplitude Y. The pressure amplitude is thus modified, which adjusts the power of the engine.

In free-piston Stirling engines designed according to the state of the art, the movement of the transfer piston depends on the pressure variations of the working gas. A small variation in its amplitude causes a change in the amount of energy exchanged between the regenerator and the gas that passes through it; this influences the instantaneous pressure of the working gas, which in turn influences the movement of the transfer piston. Instability can thus occur, which can only be controlled indirectly by the action of the electric generator on the engine piston.

In the present invention, the amplitude of the movement of the transfer piston is directly controlled by the electrical generator associated therewith. The variations of its amplitude are thus directly controlled by the load applied to the electric generator, thus preventing any significant disturbance compared to the nominal cycle of the motor. Thanks to this quality of control, these engines can work with large pressure amplitudes and thus achieve power densities higher than those that are controllable in known configurations.

The figure 7 shows in a diametral section a configuration of the Stirling machine comprising two resonant pistons 10a, 10b arranged in outer cylinders and connected to the compression volume V _C of the Stirling engine. The two resonant pistons are suspended with elastic means 40 in their respective cylinders. The mass of each piston and the mechanical and pneumatic elastic forces acting on it are adjusted to give these pistons a resonant frequency equal to the operating frequency of the machine. The two subsets formed by these pistons 10a, 10b and their cylinders are identical. The two pistons 10a, 10b are coaxial and arranged symmetrically with respect to the axis of the machine. Under the action of the variable pressure of the machine, the two resonant pistons oscillate in opposite directions and their inertial forces counterbalance each other.

In the variant of the figure 8 , the two pistons 10a and 10b are arranged coaxially in a common cylinder disposed laterally to the main axis of the machine. The two external volumes 45a and 45b of the common cylinder are connected to the compression volume V _C of the Stirling engine by ducts 29. The central volume 45c can be connected by a duct 44 to a volume 48 exposed to an almost constant mean pressure, through example that of the volume of the electric generator. When these two pistons 10a and 10b oscillate under the action of a variable pressure, their inertial forces cancel each other out. As an alternative, the central volume 45c can be connected to the cold room V _C and the external volumes 45a and 45c to the volume 48. By arranging several pairs of coaxial resonant pistons 10a, 10b symmetrically with respect to the main axis of the machine, lateral force exerted by all these resonant pistons 10a, 10b vanishes, provided that all these resonant pistons describe the same movement.

The figure 9 illustrates, by way of example, an arrangement of diamond resonant pistons 10a, 10b, 10c, 10d. This makes it possible to arrange them with their cylinders in an enclosure of reduced diameter. No lateral force is exerted by these resonant pistons on the whole machine as long as their movements are identical. More generally, the inertial forces of these resonant pistons 10a, 10b, 10c, 10d are canceled if these pistons are arranged in the form of a symmetrical arrangement with respect to the main axis of the machine.

A recurring problem with free-piston Stirling machines is caused by the large vibratory forces transmitted to the frame by the oscillating pistons. To reduce the noise pollution transmitted to the outside, these machines must be placed in acoustic speakers and isolated from the ground. In addition, the vibration of the frame can affect the speed of these machines and may disrupt their operation.

These vibrations can be compensated with 2 identical machines, arranged around a common combustion chamber and oriented in opposite directions with respect to each other. These arrangements in tandem sets have been proposed for example in the paper ICSC 95-26 by the company Sunmachine ( Proceedings of the 7th International Conference on Stirling Cycle Machines, November 1995, Tokyo ). These solutions are particularly suitable for machines developing relatively high powers.

The figure 7 illustrates a known means of attenuation of the vibrations of the frame, comprising an additional mass 41, suspended by elastic means 42 to the enclosure 3, integral with the frame 4 of the machine. By adjusting the natural frequency of this resonator on the frequency of operation of the machine, it is possible to reduce the vibrations thereof. However, if the agreement is not precise enough, beats can result which may create nuisance and disrupt the operation of the machine.

To remedy at least part of this drawback, the present invention proposes another system for attenuating the vibrations transmitted to the enclosure of the machine, illustrated by the figure 10 . According to this concept, the additional mass 41 is elastically connected to the transfer piston 6, 6a and to the frame 4 of the machine. The elastic suspensions 42a, b and c are adjusted so that at the operating frequency of the machine, these two masses oscillate in opposite directions with respect to each other, so that the vibratory forces transmitted to the enclosure or frame of the machine are canceled. The vibrations generated by the movement of the pistons are thus reduced at the source.

The elastic means 42a, b and c may consist of spiral or flat mechanical springs, electromagnets, pneumatic means or combinations of these various elastic supports. This vibration suppression system effectively compensates for the action of a single oscillator. It is therefore particularly suitable for Stirling machines with opposite resonant masses, since only vibrations generated by the transfer piston must be compensated.

The figure 11 illustrates, by way of example, the cylindrical compartment 3 of a Stirling machine. In this form of execution, the additional mass 41 forms a movable piston, placed inside an extension of the tubular element of the piston 6a, delimiting a volume 46 of a pneumatic spring 42b. This additional mass 41 in the form of a piston may be provided with sealing segments 25. In a variant, the sealing of the volume 46 may be ensured by the cylindrical surfaces of the piston formed by the additional mass 41 and by the wall of its tubular enclosure , by providing a very small annular space between the cylindrical wall of the piston and that of the tubular enclosure. This annular space can also be provided with a stationary gas bearing to stabilize the radial position between the additional mass 41 and the tubular extension of the piston 6a, thus reducing the friction between these two surfaces.

This additional mass 41 is centered and resiliently suspended by a mechanical spring, preferably by a flat spring with spiral arms 42c. An auxiliary mass 41a, associated with the additional mass 41 serves to adjust the oscillations of this additional mass, so that the transfer piston 6, 6a and the additional mass 41 oscillate in phase opposition; vibratory forces transmitted to the frame can thus be reduced to a minimum.

As indicated in this figure, the armature and the windings can surround the mobile part of the generator and the armature can be placed inside thereof.

The figure 12 illustrates a variant of the figure 11 wherein the additional mass 41 is housed in an auxiliary cylinder 49 attached to a support 47 rigidly connected to the frame 4 of the machine. The air spring 42b of the figure 10 then consists of a first variable volume 46a located in the extension of the transfer piston 6, 6a and delimited by a stationary piston 50. This volume 46a is connected by a tube 43 to a second volume 46b, located in the cylinder 49. The tube 43 is fixed rigidly to the support 47, integral with the frame 4 of the machine, and it passes through the stationary piston 50.

The two variable volumes 46a and 46b are sealingly closed by means of movable or fixed pistons provided with seals 25 or smooth surfaces with a very small radial clearance with respect to their respective cylinders. These can be equipped with stationary gas bearings to reduce friction losses.

In the form of execution according to figure 12 the oscillating masses 6, 6a and 41 are guided separately by respective supports. This solution ensures optimum lift of these moving elements, minimizing their radial movements as well as friction losses. The disadvantage of this solution lies in the relatively large size.

Many variants of the system with two oscillating masses are possible. For example, the variant according to Fig. 12 may comprise a cylindrical movable mass 41 which surrounds a stationary piston, integral with the support 47. Furthermore, in all these variants, additional mechanical springs may be used to enhance the action of the air spring 42b.

• pour le chauffage domestique, il peut fonctionner en mi-saison à charge partielle, avec un minimum d'arrêts/redémarrages de l'installation. On évite ainsi les pertes d'énergie liées à chaque démarrage et on réduit la fatigue des métaux soumis à de fréquents cycles thermiques. Par ailleurs, la flexibilité du système permet de mieux adapter le fonctionnement aux besoins en énergie électrique domestique et de mieux gérer le stockage d'eau chaude sanitaire.
• Lors de la combustion de biomasse, le dégagement de chaleur peut fluctuer en fonction de la qualité du combustible. Avec la machine objet de l'invention, la température des tubes de chauffage varie peu, de sorte qu'une combustion stable est maintenue sous des conditions optimales.
• La flexibilité du système et les bons rendements à charge partielle permettent de mieux convertir l'énergie solaire, par exemple le matin, le soir ou par temps couvert. En moyenne annuelle, la machine Stirling objet de l'invention permet donc un fonctionnement pendant une durée de temps plus longue que les systèmes conventionnels.

The absence of a complex and expensive servocontrol system, the reduction of the vibrations generated by these machines as well as the favorable operating conditions under partial loads have considerable advantages in many applications, for example:

• for domestic heating, it can operate in mid-season at partial load, with a minimum of stops / restarts of the installation. This avoids the energy losses associated with each start and reduces the fatigue of metals subjected to frequent thermal cycles. Moreover, the flexibility of the system makes it possible to better adapt the operation to the needs of domestic electrical energy and to better manage the hot water storage.
• When biomass is burned, the release of heat can fluctuate depending on the quality of the fuel. With the machine object of the invention, the temperature of the heating tubes varies little, so that a stable combustion is maintained under optimal conditions.
• The flexibility of the system and the good yields at partial load make it possible to convert solar energy better, for example in the morning, in the evening or on cloudy days. In annual average, the Stirling machine object of the invention therefore allows operation for a longer period of time than conventional systems.

The use of rotary generators makes it possible to generate tri-phase current which can easily be injected into an electrical network.

The hybrid engines described above are also distinguished by good yields at partial load. They can advantageously be used in all applications requiring great flexibility of operation.

When starting, the movement of the resonant mass and the pressure amplitude thus generated are low. The machine can then be started without balancing the pressures between the different volumes: the use of a short circuit valve which is generally used in conventional kinematic machines is no longer necessary.

Claims (17)

1. A Stirling machine comprising a displacer piston (6, 6a) and a moving member (14) of a generator or of an electric motor, the displacer piston (6, 6a) being mounted in a cylinder (2), in which it periodically displaces a working gas between an expansion chamber (V_E) and a compression chamber (V_C) which constitute the working volume of said Stirling machine, respectively associated with two working faces of said displacer piston (6, 6a) and causing said gas to pass through a hot side heat exchanger (7), linked to a heat source, a regenerator (9) and a cooling exchanger (8) linked to a heat sink, the cross-sectional area ratio (a_C/a_E) between the two working faces of said piston (6, 6a) being ≥ 0.35 so that its displacement along an axis X oriented toward the expansion volume V_E generates an in-phase pressure component P_x of said working gas opposing said displacement of said piston (6, 6a), so as to transmit all of said mechanical energy produced between this displacer piston (6, 6a) and said moving member (14), characterized in that the ratio of cross-sectional area a_C/a_E is less than 0.70 and in that it includes at least one resonant piston (10), coupled to said displacer piston (6, 6a), intended to receive a certain quantity of energy, proportional to said pressure component P_x, which is used to compensate its losses by friction and to maintain its oscillating movement.
2. The Stirling machine as claimed in claim 1, in which said resonant piston is a free piston guided via support means.
3. The Stirling machine as claimed in one of the preceding claims, in which said resonant piston has no active servocontrol.
4. The Stirling machine as claimed in one of the preceding claims, in which the displacer piston is suspended by elastic means, thus forming a free piston, said moving member exhibiting linear displacement.
5. The Stirling machine as claimed in one of claims 1 to 3, in which the displacer piston is linked to said rotary moving member by a mechanical linkage.
6. The Stirling machine as claimed in one of the preceding claims, in which the ratio of the working surfaces a_C/a_E of the displacer piston (6, 6a) is between 35 and 60%, preferably between 40 and 55%.
7. The Stirling machine as claimed in one of the preceding claims, in which each piston is guided in a radial direction by a dynamic seal formed by a radial gap of between 20 µm and 50 pm, at least one of the two surfaces of which being provided with a wear-resistant and self-lubricating coating capable of reducing the static and dynamic friction.
8. The Stirling machine as claimed in one of the preceding claims, in which the dynamic seals formed between the pistons and the cylinders which surround them are pressurized with the working gas contained in at least one volume of gas formed in the walls of the cylinder or in the pistons.
9. The Stirling machine as claimed in claim 8, in which said volume of gas is provided with at least one non-return valve placed in proximity to a volume exposed to pressures that are variable in time, and supplied with working gas when this volume is exposed to the highest cyclic pressures.
10. The Stirling machine as claimed in one of the preceding claims, in which each piston is a free piston suspended from the cylinder by a flat spring with spiral-shaped arms.
11. The Stirling machine as claimed in one of claims 1 to 9, in which the resonant piston (10) and/or the displacer piston are suspended from the frame (4) by helical springs, positioned symmetrically about the axis of said piston or pistons and exerting an axial force on said piston or pistons, centered in relation to this or these pistons.
12. The Stirling machine as claimed in one of the preceding claims, in which an adjustment valve is provided on a duct which links the cold working volume with the volume of the electrical generator.
13. The Stirling machine as claimed in one of the preceding claims, comprising at least one pair of similar coaxial resonant pistons, positioned symmetrically in relation to the axis of the machine and oscillating in opposite directions.
14. The Stirling machine as claimed in one of the preceding claims, comprising at least two pairs of similar resonant pistons (10a, 10b, 10c, 10d), positioned in the form of a symmetrical arrangement in relation to the main axis of said machine.
15. The Stirling machine as claimed in one of the preceding claims, in which an additional mass (41a) is suspended from the frame by elastic means (42c), so that its natural frequency is adjusted to that of the displacer piston (6, 6a) of the machine and that its oscillating movement compensates the vibrations of said displacer piston (6, 6a).
16. The Stirling machine as claimed in one of claims 1 to 14, in which the additional mass (41a) is suspended from the frame of the machine and from said displacer piston (6, 6a) by elastic means (42c) adjusted so that, at the operating frequency of said displacer piston (6, 6a) of the machine, this mass oscillates in direction opposite to that of the displacer piston.
17. The Stirling machine as claimed in claim 16, in which a pneumatic spring (46a) links the displacer piston (6, 6a) to the pneumatic spring (46b) of the additional mass (41) and is at least partly incorporated in a tubular element (6a) situated in an extension of the displacer piston (6, 6a).

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