Abstract:
A thermodynamic machine includes at least one chamber in which an isothermal expansion and/or compression is to be carried out, said chamber being longitudinally defined by first and second walls that are mobile relative to each other. The chamber is divided by partitions extending longitudinally from each of the first and second walls, the partitions being interleaved within each other, and the distance between the partitions extending from a same wall being such that the ratio between the distance squared and the cycle duration of the thermodynamic machine is lower than the average thermal diffusivity of the gas contained in the chamber.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a heat exchanger structure. The present invention also relates to a chamber in which isothermal compressions and/or expansions are performed. The present invention further relates to a high-efficiency reversible thermodynamic engine comprising such a chamber, for example, a Stirling engine. 
       DISCUSSION OF PRIOR ART 
       [0002]    Stirling engines are sometimes used for industrial refrigeration and in military or space applications. Such engines have the advantage of being usable as motors or to generate heat or cold without the use of refrigerants, which are generally polluting. Another advantage of a Stirling engine is that its hot source is external and that this source can thus be obtained by means of any known fuel type, or even solar radiation. 
         [0003]    In a Stirling cycle, a gas, for example, air, hydrogen, or helium, is submitted to a four-phase cycle: an isochoric heating, an isothermal expansion, an isochoric cooling, and an isothermal compression. 
         [0004]      FIG. 1  is a generic diagram of a Stirling engine. A first chamber  3  is connected to a second chamber  5  via a first heat exchanger  7 , a regenerator  9 , and a second heat exchanger  11 . The assembly comprised of the chambers, the exchangers, and the regenerator may be cylindrical. First and second exchangers  7  and  11  respectively are in contact with a hot source at a hot temperature T C  and with a cold source at a cold temperature T F . Chambers  3  and  5  are respectively closed by moving pistons  13  and  15  which define the variable volumes of chambers  3  and  5 . It should be understood that there are different ways for the different elements of the Stirling engine shown in  FIG. 1  to be mobile with respect to one another: for example, the two pistons  13  and  15  may be mobile and regenerator  9  and exchangers  7  and  11  may be fixed, in the case of a so-called alpha configuration. One of pistons  13  or  15  may also be fixed if the central portion of the engine is mobile. The assembly formed of regenerator  9  and of exchangers  7  and  11  may also be provided to be fixed and the variable volumes of chambers  3  and  5  may be formed of a single variable volume separated in two by a mobile wall, called a displacer. Such a configuration is called beta or gamma configuration. 
         [0005]      FIGS. 2A to 2D  illustrate steps of a Stirling engine cycle. 
         [0006]    In an initial arbitrary state A illustrated in  FIG. 2A , a gas volume is stored in first chamber  3 , second chamber  5  preferably having a zero or very low volume. 
         [0007]    The gas in first chamber  3  is heated by the hot source and its pressure increases. This displaces piston  13  to a state B in which the volume taken up by the gas in chamber  3  is greater than the volume of this same chamber at state A. During the isothermal expansion phase (step A and B), mechanical work is extracted. 
         [0008]    An isochoric cooling then enables to pass from state B to a state C in which the gas in hot chamber  3  is transferred into cold chamber  5 . During this transfer, the gas stored in chamber  3  passes through regenerator  9  and has cooled down as it reaches chamber  5 . The heat contained in the hot gas is “extracted” in the regenerator, as will be seen hereafter, and the gas cools down. 
         [0009]    An isothermal compression enables to pass from state C to a state D in which the volume taken up by the gas in chamber  5  is lower than the volume of this same chamber at state C. This compression is performed by actuating piston  15  to decrease the volume of chamber  5 . This step consumes power, but less than the power provided between states A and B. 
         [0010]    Finally, an isochoric transfer enables to pass from state D to initial state A in which the gas is stored in hot chamber  3 . During this step, the gas passes from cold chamber  5  to hot chamber  3  via regenerator  9 . In the regenerator, the heat extracted during the isochoric cooling (step B to C) is given back to the gas as it passes through the regenerator for the second time (step D to A). Thus, the gas heats up before coming in contact with exchanger  7 . It should be noted that, preferably, in known engines, chambers  3  and  5  alternately almost totally empty during the cycle. 
         [0011]    In engine cycle, the mechanical work extracted during the expansion between steps A and B is partly used for the isothermal compression (steps C to D). The regenerator enables for the heat extracted during the passing from state B to state C to be distributed to the gas during the passing from state D to state A and avoids heat losses. Indeed, the regenerator operates as a counterflow heat exchanger: when a hot gas passes in a cold regenerator, it cools down while heating up the regenerator and, conversely, a cold gas crossing the hot regenerator heats up while cooling down the regenerator. To perform its function, the regenerator must be made of materials which are poor heat conductors in the gas flow direction, for example, insulating materials. 
         [0012]    Engines which are desired to be reversible, that is, capable of being used in engine cycle or in heat pump cycle, are considered herein. It should be noted that this definition of reversibility differs from the current definition, for which a reversible engine is an engine with cold and hot sources which may be inverted. A problem linked to current Stirling engines is that, when they have a good engine cycle efficiency, they will have a low heat pump cycle efficiency, and conversely. 
         [0013]    Low efficiencies in the reversible use of such engines or in their use over a wide operating range originate from the different losses occurring therein and, especially, from temperature differences in heat exchanges. Another source of irreversible losses in Stirling engines and in any engine implementing theoretically isothermal compressions and expansions is that real systems are far from being capable of enabling such iso-thermal compressions and expansions. 
       SUMMARY OF THE INVENTION 
       [0014]    An object of an embodiment of the present invention is to provide a thermodynamic engine having a cycle involving almost ideally isothermal compressions and/or expansions. 
         [0015]    An object of an embodiment of the present invention is to provide a thermodynamic engine with low losses and a high efficiency over a wide operating range. 
         [0016]    Another object of an embodiment of the present invention is to provide a reversible thermodynamic engine. 
         [0017]    Another object of an embodiment of the present invention is to provide an optimized heat exchanger. 
         [0018]    Thus, an embodiment of the present invention provides a thermodynamic engine intended to operate with a minimum cycle time, comprising at least one compression/expansion and heat exchange chamber, this chamber being longitudinally delimited by first and second walls, mobile with respect to each other, characterized in that said chamber is divided by partitions extending longitudinally from each of the first and second walls, the partitions being interleaved, the distance between partitions extending from a same wall being such that the ratio between the square of this distance and the minimum cycle time is smaller than the average thermal diffusivity of the gas contained in the chamber. 
         [0019]    According to an embodiment of the present invention, the distance between partitions extending from a same wall is such that said ratio is smaller than half the average diffusivity of the gas contained in the chamber. 
         [0020]    According to an embodiment of the present invention, the first wall is gas-tight and is intended to be placed in contact with a heat source and the second wall is capable of letting gas flow to the outside of the compression/expansion chamber. 
         [0021]    According to an embodiment of the present invention, the distance between partitions extending from a same wall is shorter than 2 mm, the gas contained in the compression/expansion chamber being hydrogen or helium. 
         [0022]    According to an embodiment of the present invention, the distance between partitions extending from a same wall is shorter than 0.5 mm. 
         [0023]    According to an embodiment of the present invention, the chamber is cylindrical and the partitions have, in cross-section along a direction perpendicular to the chamber length, a spiral shape. 
         [0024]    According to an embodiment of the present invention, the assembly formed of a wall and of the associated partitions is formed of a winding of a wide strip and of at least one separation strip. 
         [0025]    According to an embodiment of the present invention, the separation strip is a wavy strip. 
         [0026]    According to an embodiment of the present invention, the separation strip is formed of two corrugated strips placed in opposition, with overlapping corrugations. 
         [0027]    According to an embodiment of the present invention, the chamber is cylindrical and the partitions form, in cross-section along a direction perpendicular to the chamber length, an assembly of parallel wavy portions. 
         [0028]    According to an embodiment of the present invention, the chamber is cylindrical and the partitions form, in cross-section along a direction perpendicular to the chamber length, an assembly of parallel planar portions. 
         [0029]    According to an embodiment of the present invention, at least one wall forms the end of a controllable piston. 
         [0030]    According to an embodiment of the present invention, the partitions are made of a thermally conductive ceramic, for example, silicon carbide or aluminum nitride, copper, aluminum, or steel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings: 
           [0032]      FIG. 1 , previously described, is a generic diagram of a Stirling engine; 
           [0033]      FIGS. 2A to 2D , previously described, illustrate steps of a Stirling engine cycle; 
           [0034]      FIGS. 3A to 3C  are cross-section views of a portion of an engine according to an embodiment of the present invention, in several configurations; 
           [0035]      FIGS. 4 and 5  are two perspective views of portions of engines according to an embodiment of the present invention; 
           [0036]      FIG. 6  illustrates a possible embodiment of a half-exchanger according to an embodiment of the present invention; 
           [0037]      FIG. 7  is a cross-section view of a Stirling engine according to an embodiment of the present invention; 
           [0038]      FIGS. 8A and 8B  illustrate another possible embodiment of a half-exchanger according to an embodiment of the present invention; and 
           [0039]      FIG. 9  is a curve illustrating an advantage of an engine according to an embodiment of the present invention. 
       
    
    
       [0040]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. 
       DETAILED DESCRIPTION 
       [0041]    An embodiment of the present invention first provides directly placing heat exchangers in compression and expansion chambers. It further provides forming compression and expansion chambers in which the exchangers comprise many portions forming partitions in the chambers. Such partitions extend from two opposite walls of the chambers and interleave when the chamber volume decreases. 
         [0042]      FIGS. 3A to 3C  illustrate, in longitudinal cross-section view, a compression or expansion chamber such as described hereabove forming, for example, a portion of a Stirling engine. These drawings illustrate different states in an isothermal expansion. 
         [0043]    In  FIG. 3A , a chamber  21  is formed in a cylinder and is delimited by two walls  23  and  25  mobile with respect to each other in the cylinder. The shown example considers a mobile wall  23  associated with a piston axis  27 , and a fixed wall  25 , fixed with respect to a regenerator  29  (not detailed). It should be understood that walls  23  and  25  may be mobile with respect to each other in another way. Wall  23  is tight and wall  25  is permeable to gases, for example, by being provided with many perforations. 
         [0044]    Partitions  31  extend in chamber  21  from wall  23  and partitions  33  extend in chamber  21  from wall  25 . Partitions  31  and  33  extend in the longitudinal direction of the cylinder and are arranged in alternation in cross-section view. Partitions  31  and  33  form two half-exchangers. 
         [0045]    In the state of  FIG. 3A , the ends of partitions  31  are close to wall  25  and the ends of partitions  33  are close to wall  23 . The volume of chamber  21  is thus minimum. A hot source (or a cold source in the opposite case of a compression) is connected to one of walls  23  or  25 , here wall  23 , by adapted means, not shown. Wall  23  may be in direct contact with the hot source or be in contact therewith via a hot or cold fluid flow. 
         [0046]      FIG. 3C  illustrates the device when the volume of chamber  21  is maximum, that is, piston  23 - 27  and partitions  31  are as distant from wall  25  as possible. In the drawing, the free ends of partitions  31  and  33  are shown opposite to one another in chamber  21 . It may also be provided for the ends of partitions  31  and  33  to be slightly distant from one another. 
         [0047]      FIG. 3B  illustrates the device in a position intermediary between the positions of  FIGS. 3A and 3C . 
         [0048]    The interleaved structure of the two half-exchangers enables, at any moment, for each molecule of the gas present in chamber  21  to be relatively close to a partition  31  or  33 . Thus, in the case of an expansion where partitions  31  and  33  are hot, all the gas molecules are close to a hot partition during the expansion, which enables to avoid the forming of gas pockets having a temperature lower than that of the hot source, and thus ensures an isothermal expansion. The structure discussed herein thus enables to improve the ability of the assembly to conduct heat from the heat source to the gas of chamber  21  and to attenuate losses due to temperatures differences between the heat source and the gas. 
         [0049]    To provide good exchanges between the heat source and the gas and avoid losses due to a dead volume in the chamber, the inventor provides for the partition to be arranged so that: 
         [0000]    
       
      
       d 
       2 
       /T&lt;D,  
      
     
         [0000]    d being the distance between two successive partitions  31  or  33  of a same half-exchanger;
 
T being the minimum cycle time of the thermodynamic engine (that is, the time of a minimum reciprocating motion in the case of the Stirling engine described in relation with  FIGS. 2A to 2D ); and
 
D being the average diffusivity over a cycle of the gas in the chamber.
 
         [0050]    Preferably, ratio d 2 /T will be smaller than half thermal diffusivity D of the gas. This enables to maintain a substantially uniform gas temperature in chamber  21 , substantially equal to the temperature of the heat source, and thus to perform almost ideally isothermal compressions and expansions. The application of the above inequation enables to use heat transfers by thermal diffusion from the partitions extending from the compression/expansion chamber to the gas. Thus, heat transfers are mainly performed by diffusion, possible turbulence phenomena having little or no influence on the transfers. 
         [0051]    Partitions  31  and  33  may be made of a thermally conductive material, for example, of a ceramic such as silicon carbide, aluminum nitride, or also copper or aluminum. In this case, it should be understood that, in the position of  FIG. 3A , partitions  33  are heated by partitions  31  via the gas. During the expansion, partitions  33  distribute the stored heat to the gas, and especially to the gas located close to wall  25 . For a proper operation, it should be understood that the engine cycle time must be sufficiently long for heat exchanges between partitions  31  and  33  and the gas to have time to occur. 
         [0052]    The inventor has noted that partitions  33  may also be made of poorly conductive materials, without for all this modifying the isothermal character of the expansions/compressions. Similarly, partitions  31  may be formed of poorly conductive materials, except at their ends connected to wall  23 . Indeed, in this case, in the state of  FIG. 3A , the heat is transmitted from wall  23  to the adjacent areas of partitions  31  and then, via the gas, to the free ends of partitions  33 . During the expansion, the free ends of partitions  33  are successively opposite to the different portions of partitions  31  and the heat thus passes from the end of partitions  33  to partitions  31 , and then again from partitions  31  to partitions  33 . When the volume of chamber  21  decreases, the heat also passes between partitions  31  and partitions  33  via the gas. Thus, during a cycle, partitions  31  and  33  are entirely hot and transmit their heat to the gas. 
         [0053]    In the case where a poorly conductive material is used for partitions  31  and  33 , the following relation must be satisfied: 
         [0000]    
       
         
           
             
               
                 
                   λ 
                   gas 
                 
                 · 
                 
                   a 
                   2 
                 
               
               
                 d 
                 ′ 
               
             
             &gt; 
             
               e 
               . 
               
                 λ 
                 partition 
               
             
           
         
       
     
         [0000]    λ gas  being the thermal conductivity of the gas;
 
λ partition  being the thermal conductivity of the material forming partitions  31  and  33 ;
 
a being the amplitude of the relative motion of partitions  31  and  33 ;
 
d′ being the distance separating two successive partitions  31  and  33  belonging to two different half-exchangers; and
 
e being the average thickness of partitions  31  and  33 .
 
         [0054]    The possible use of poorly conductive materials enables to form partitions  31  and  33  made of many materials, for example, of light materials, of low-cost materials, or other materials well adapted to the forming of such exchangers, for example, steel. 
         [0055]    It should be noted that the discussed structure comprising two reciprocating slidingly interleaved half-exchangers may be generalized to form any type of exchanger between a hot (or cold) source and a gas. Indeed, advantage may be taken of the improved heat propagation between half-exchangers, due to their relative motions and their interleaving, to form any type of exchanger, for example, radiators through which a gas flows. The gas may for example enter through one of the walls and come out through the opposite wall. 
         [0056]    As an example of numerical values, distance d between partitions  31  and  33  may range between 0.3 and 2 mm and the partitions may have a thickness ranging between 0.1 and 0.6 mm, if the gas in chamber  21  is hydrogen or helium. The engine cylinder may have a diameter ranging between 15 and 20 cm and wall  23  may move by approximately 3 cm within the cylinder. With such dimensions, a cycle time ranging between 0.02 and 0.5 second enables to comply with inequation d 2 /T&lt;D. 
         [0057]    It should also be noted that losses in the exchangers are further attenuated if partitions  31 ,  33  are slightly thinner at their free ends than at their holding ends (towards walls  23  and  25 ). 
         [0058]      FIG. 4  is a perspective view of portions of an engine capable of performing an isothermal compression or expansion according to an embodiment of the present invention. In this drawing, for simplification, the external cylinder in which the engine elements are moving is not shown. Further, partitions  31  and  33  have been shown as distant from one another to make the understanding easier. In practice, the partitions are interleaved. 
         [0059]    In this embodiment, partitions  31  and  33  have, in cross-section in a plane perpendicular to the chamber length, spiral shapes. A first spiral forms partitions  31  and a second spiral forms partitions  33 . Spirals  31  and  33  are provided to interleave as the volume of chamber  21  decreases. 
         [0060]      FIG. 5  is a perspective view of portions of an engine capable of performing an isothermal compression or expansion according to another embodiment of the present invention. 
         [0061]    In this embodiment, partitions  31  and  33  are formed, in cross-section along a direction perpendicular to the chamber length, of many parallel plates separated by a pitch. In the illustrated example, although the plates are wavy to improve their hold, it should be noted that these plates may also be planar. Wavy portions  31  are shifted from wavy portions  33 , for example, by a half-step, for these portions to interleave without touching as the volume of chamber  21  varies. 
         [0062]    Wall  23 , which is a wall external to the system, must be gas-tight. Thus, in wall  23 , portions  31 , whether they have a spiral shape or the shape of parallel plates, are separated by a material ensuring the gas tightness and/or are attached to a piston body. Conversely, the walls internal to the system, for example, wall  25  of  FIGS. 3A to 3C , play two roles: enabling the holding of portions  33  and letting through the gas, for example, towards a regenerator. Thus, they may for example be perforated. 
         [0063]      FIG. 6  illustrates a possible embodiment of a structure for holding a spiral-shaped partition. 
         [0064]    To hold a spiral-shaped partition  31  or  33  such as that in  FIG. 4 , spacing or mechanical hold means may be placed between the different coils, on the side where the spiral is attached to the wall. In the example of  FIG. 6 , such means are formed of a strip  41  which is wound at the same time as the strip of thermally conductive or insulating material and which is thus inside of the winding. In this example, strip  41  is wavy and the height of the waves sets the pitch between the coils of spiral  31 ,  33 . It should be noted that the structure of  FIG. 6  may also be used to form walls  25 , strip  41  being then used to hold wall  25  letting through the gas towards the regenerator. In the case where strip  41  is located between conductive spirals, this strip will preferably have a high conductivity, for example, by being made of an aluminum alloy. 
         [0065]      FIG. 7  is a detailed cross-section view of the body of a Stirling engine implementing an embodiment of the present invention. 
         [0066]    The engine is formed in a cylinder  51  and comprises a first chamber  53  and a second chamber  55  separated by a regenerator  57 . An exchanger, formed of two half-exchangers such as discussed hereabove, is formed in each of chambers  53  and  55 . A first half-exchanger  59 , respectively  61 , located in chamber  53 , respectively  55 , extends from a wall external to engine  63 , respectively  65 . A second half-exchanger  67 , respectively  69 , located in chamber  53 , respectively  55 , extends from a wall internal to engine  71 , respectively  73 , which delimits the position of the regenerator. 
         [0067]    In the shown example, regenerator  51  is formed of partitions  75 ,  77  which respectively extend from walls  71  and  73 . Partitions  75  and  77  are shown in interleaved configuration, for example, with a shape identical to that of portions  59  and  67  or  61  and  69 . Partitions  75  and  77  are preferably made of a material which is a poor thermal conductor but has good proper-ties of thermal exchange with the gas, that is, a sufficient thermal effusivity. For example, partitions  75  and  77  may be made of polycarbonate. Guides parallel to the gas flow may be added in the regenerator to ensure for the gas transiting therethrough to follow the same path in both displacement directions. It should be noted that the regenerator structure de-scribed herein is an example only and that any known regenerator type may be used with the exchangers of  FIGS. 3A to 3C . 
         [0068]    In the shown example, a central shaft  79  is located at the core of cylinder  51 . This shaft contains elements enabling to position the different elements of the thermodynamic engine with respect to one another. Partitions  59 ,  67 ,  61 , and  69 , or even partitions  75  and  77 , may be spiral-shaped around shaft  79 . Elements providing the tightness, the thermal insulation, the mechanical hold, and/or the displacement of the different walls  63 ,  65 ,  71 , and  73  in cylinder  51  are shown in  FIG. 7  by hatched portions. 
         [0069]      FIGS. 8A and 8B  illustrate another possible embodiment of a structure for holding a spiral-shaped partition. 
         [0070]    In  FIG. 8A , a layer  31 ,  33 , thermally conductive or not, is wound around an axis  81 . During the winding of layer  31 ,  33  on axis  81 , two strips  83  and  85  having a corrugated shape in top view are also wound between two spirals of layer  31 ,  33 . The two strips  83  and  85  are positioned on each other so that the corrugations are in opposition and slightly superposed to one another. It should be noted that the arrangement of strips  83  and  85  on material  31 ,  33  is shown at the end of the winding only in  FIG. 8A . 
         [0071]      FIG. 8B  further illustrates the arrangement of strips  83  and  85  with respect to each other and the superposition of portions of the corrugations of these strips. The superposition of strips  83  and  85  enables to hold material  31 - 33  while enabling the gas to flow between the different spirals of the structure of  FIG. 8A . Thus, the structure of  FIGS. 8A and 8B  may have the same function as that in  FIG. 6 . 
         [0072]      FIG. 9  is a curve illustrating the corrective effect (Eff) associated with the use of a device according to an embodiment with respect to the use of a conventional device (partition-less chamber), in proportion, in an expansion or a compression, according to ratio D.T/d 2 . Given that ratio d 2 /T must be lower than diffusivity D of the gas, this means that ratio D.T/d 2  must be greater than 1. In this curve, a 0% corrective effect means that losses due to temperature differences in the chamber during compressions and expansions are not attenuated, and a 100% corrective effect means that such losses are nonexistent. 
         [0073]    The use of the structures described herein provides a corrective effect of approximately 50% when ratio D.T/d 2  is on the order of 2 and of approximately 90% when this ratio is on the order of 10. Thus, the present invention enables to strongly decrease losses due to temperature differences during compressions and expansions and thus to perform isothermal transformations. 
         [0074]    The use of interleaved exchangers enables to obtain Stirling engines capable of having an efficiency of 85% of the maximum Carnot efficiency over significant operating ranges. It is also possible to manufacture engines of lower volume for an efficiency similar to that of current engines. Further, the efficiency is stable over a significant hot and cold temperature range, with no modification of the engine geometry. The efficiency also remains good over significant power ranges, by varying the cycle time. A good efficiency is also obtained in case of a reversible operation. 
         [0075]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, it should be noted that the different advantages of the present invention have been described with respect to its application to reversible Stirling engines. It should be noted that the forming of conductive partitions in compression or expansion chambers to make such compressions or expansions isothermal may be applied to any engine carrying out such transformations, for example, Ericsson engines. The present invention may also be applied to any type of compressor or air injection machine with a linear piston. 
         [0076]    It should also be noted that the present invention applies to cylindrical compression and/or expansion chambers having any shape, be it rotational or not.