TECHNICAL FIELD 
       [0001]    The present disclosure relates to arrangement of a heat exchanger in a thermally-driven heat pump. 
         [0002]    BACKGROUND ART 
         [0003]    A prior art Vuilleumier heat pump  5  is shown in  FIG. 1 . A hot displacer  1  and a cold displacer  2  are driven by a crank  4 . In  FIG. 2 , the upper end of cold displacer  2  and the lower end of hot displacer  1  are shown plotted in  FIG. 2 , the two are out of phase by 90°. The paths of the two displacers overlap in the middle region of the cylinder in which the displacers reciprocate. 
         [0004]    A thermally-driven heat pump in which the displacers are mechatronically actuated is disclosed in commonly-assigned patent application PCT/US13/36101 filed 11 Apr. 2013. The displacers are independently actuated such that one displacer may be held stationary while the other moves. In the heat pump disclosed in PCT/US13/36101, neither displacer reciprocates into the stroke of the other displacer, i.e., no overlap over in space of the two displacers. This is in contrast with the movement shown in  FIG. 2  for a crank-driven Vuilleumier heat pump in which the displacers overlap in space, but not in time to prevent the displacers from colliding. A heat pump with independently-actuated displacers, such as disclosed in the PCT/US13/36101 application, presents opportunities not available in heat pumps in which the displacer movement is constrained by crank geometry. 
       DISCLOSURE 
       [0005]    To exploit an opportunity provided by independent displacer operation over prior Vuilleumier heat pumps associated with crank movement, a heat pump is disclosed that includes a housing having: a hot cap, a cold cap, a hot cylinder portion proximate the hot cap, and a cold cylinder portion proximate the cold cap. The heat pump includes a cold displacer disposed within the cold cylinder portion, a hot displacer disposed within the hot cylinder portion, a post coupled to the cold cap and extending toward the hot cap along a centerline of the cold cylinder portion, and a substantially disk-shaped warm heat exchanger. The warm heat exchanger is located between the hot and cold displacers. An opening is defined in the warm heat exchanger to accommodate the post. A diameter of the opening in the warm heat exchanger is less than a diameter of the cold displacer. The warm heat exchanger is housed within a warm heat exchanger cylinder portion. In some embodiments, the hot cylinder portion, the cold cylinder portion, and the warm heat exchanger cylinder portion are of the same diameter. 
         [0006]    The warm heat exchanger has an inlet that pierces the warm heat exchanger cylinder portion and an outlet that pierces the warm heat exchanger cylinder portion. 
         [0007]    In one embodiment, the warm heat exchanger comprises at least one tube wrapped in a spiral with adjacent turns of the spiral separated by at most a predetermined distance. A working fluid within the hot and cold cylinder portions pass through the separations between adjacent turns of the spiral in response to movement of the displacers. Alternatively, the warm heat exchanger comprises a tube-and-shell heat exchanger with a working fluid within the hot and cold cylinder portions passing between the tubes in response to movement of the displacers. Alternatively, any suitable heat exchanger configuration may be provided for the warm heat exchanger. 
         [0008]    The heat pump may further include a hot heat exchanger located proximate the hot cap and fluidly coupled to a chamber within the hot cylinder portion, a hot regenerator having one end fluidly coupled to the hot heat exchanger and one end fluidly coupled to a chamber within the cold cylinder portion, a cold heat exchanger fluidly coupled to a cold chamber within the cold cylinder portion, and a cold regenerator having one end fluidly coupled to the cold heat exchanger and one end fluidly coupled to a hot warm chamber within the hot cylinder portion. 
         [0009]    The hot displacer reciprocation is limited by the hot cap and the warm heat exchanger. The cold displacer reciprocation is limited by the cold cap and the warm heat exchanger. 
         [0010]    The heat pump may further include a hot heat exchanger proximate the hot cap and fluidly coupled to a hot chamber within the hot cylinder portion; a cold heat exchanger fluidly coupled to a cold chamber within the cold cylinder portion; an annular-shaped hot regenerator arranged outside the hot cylinder portion; and an annular-shaped cold regenerator arranged outside the cold cylinder portion. A first end of the hot regenerator is fluidly coupled to the hot heat exchanger and a second end of the hot regenerator is fluidly coupled to a cold warm chamber within the cold cylinder portion. A first end of the cold regenerator is fluidly coupled to the cold heat exchanger and a second end of the cold regenerator is fluidly coupled to a hot warm chamber within the hot cylinder portion. 
         [0011]    The hot chamber is delimited by the hot cap, the hot cylinder portion, and the hot displacer. The cold chamber is delimited by the cold cap, the cold cylinder portion, and the cold displacer. The hot warm chamber is delimited by the warm heat exchanger, the hot cylinder portion, and the hot displacer. The cold warm chamber is delimited by the warm heat exchanger, the cold cylinder portion, and the cold displacer. 
         [0012]    Also disclosed is a heat pump that includes a heat pump housing with: a hot cap, a cold cap, a hot cylinder portion near the hot cap, and a cold cylinder portion near the cold cap. The heat pump has a cold displacer disposed within the cold cylinder portion, a hot displacer disposed within the hot cylinder portion, and a substantially disk-shaped warm heat exchanger between the hot cylinder portion and the cold cylinder portion. The cold displacer reciprocates between the warm heat exchanger and the cold cap; and the hot displacer reciprocates between the warm heat exchanger and the hot cap. 
         [0013]    The heat pump has a post coupled to the cold cap and extending toward the hot cap along a centerline of the cold cylinder portion. An opening is defined in the warm heat exchanger to accommodate the post. A diameter of the opening in the warm heat exchanger is less than a diameter of the cold displacer. 
         [0014]    The heat pump further includes: a hot heat exchanger located proximate the hot cap and fluidly coupled to the hot cylinder portion, a hot regenerator having one end fluidly coupled to the hot heat exchanger and one end fluidly coupled to the cold cylinder portion, a cold heat exchanger fluidly coupled to the cold cylinder portion, and a cold regenerator having one end fluidly coupled to the cold heat exchanger and one end fluidly coupled to the hot cylinder portion. 
         [0015]    The heat pump may include an annularly-shaped hot regenerator arranged outside the hot cylinder portion, an annularly-shaped cold regenerator arranged outside the cold cylinder portion, a hot chamber delimited by the hot cylinder portion, the hot displacer, and the hot cap, a hot warm chamber delimited by the hot cylinder portion, the hot displacer, and the warm heat exchanger, a cold warm chamber delimited by the cold cylinder portion, the cold displacer, and the warm heat exchanger, a cold chamber delimited by the cold cylinder portion, the cold displacer, and the cold cap, a hot heat exchanger fluidly coupled to the hot chamber and to the hot regenerator, and a cold heat exchanger fluidly coupled to the cold chamber and to the cold regenerator. The hot regenerator is fluidly coupled to the hot heat exchanger and the cold warm chamber. The cold regenerator is fluidly coupled to the cold heat exchanger and the hot warm chamber. 
         [0016]    In some embodiments, the heat pump also has a hot heat exchanger disposed in the hot cap and a cold heat exchanger annularly arranged around the cold cylinder portion. 
         [0017]    According to an embodiment of the present disclosure, a heat pump has a housing having a hot cap on one end of the housing and a cold cap on the other end of the housing, a cylinder within the housing, a substantially disk-shaped warm heat exchanger disposed within the housing and roughly centrally located between the hot cap and the cold cap, a hot displacer disposed in a portion of the cylinder between the warm heat exchanger and the hot cap, and a cold displacer disposed in a portion of the cylinder between the warm heat exchanger and the cold cap. The cylinder has a hot cylinder portion and a cold cylinder portion. A hot chamber is delimited by the hot cap, the hot cylinder portion, and the hot displacer. A cold chamber is delimited by the cold cap, the cold cylinder portion, and the cold displacer. A hot warm chamber is delimited by the warm heat exchanger, the hot cylinder portion, and the hot displacer. A cold warm chamber is delimited by the warm heat exchanger, the cold cylinder portion, and the cold displacer. The heat pump includes: a hot heat exchanger proximate fluidly coupled to the hot chamber, a cold heat exchanger fluidly coupled to the cold chamber, a hot regenerator, and a cold regenerator. A first end of the hot regenerator is fluidly coupled to the hot heat exchanger. A second end of the hot regenerator is fluidly coupled to a cold warm chamber. A first end of the cold regenerator is fluidly coupled to the cold heat exchanger. A second end of the cold regenerator is fluidly coupled to a hot warm chamber. 
         [0018]    In some embodiments, the hot regenerator is annularly arranged outside the cylinder near the hot cap and the cold regenerator is annularly arranged outside the cylinder near the cold cap. 
         [0019]    In heat pumps in which the displacers are driven by a crank arrangement, a warm heat exchanger cannot be placed within the cylinder unless the displacers were to be separated so that they do not overlap the same space. Such an arrangement would yield too much dead volume and would seriously impair thermal efficiency. The mechatronically-driven heat pump allows for the warm heat exchanger to be located within the cylinder without such a large dead volume. The gases in the cylinder readily flow over the warm heat exchanger compared with the prior-art configuration where the warm heat exchanger was in an annular volume outside the cylinder. 
         [0020]    An advantage of the disclosed configuration is that the warm heat exchanger is more easily manufactured compared to a heat exchanger that resides in an annular volume. Another advantage of embodiments in which recuperators are disposed in the passages is that the recuperators are more easily formed in a circular or other simple cross-sectional shape compared with an annulus. Yet another advantage in some alternatives is a reduction in dead volume by obviating passages to and from the warm heat exchanger. 
         [0021]    Vuilleumier heat pump  5  in  FIG. 1  has four heat exchangers: a hot heat exchanger  6 , a warm-hot heat exchanger  7 , a warm-cold heat exchanger  8 , and a cold heat exchanger  9 . When hot displacer  1  moves downward and cold displacer  2  is mostly stationary (moving at its slowest near reversal at its lower position), or moving minimally, gases between displacers  1  and  2  are pushed through warm-hot heat exchanger  7  into a hot chamber (volume in cylinder above hot displacer  1 ). When cold displacer  2  moves upward and hot displacer  1  is mostly stationary (moving at its slowest near reversal at its upper position) gases between displacers  1  and  2  are pushed through warm-cold heat exchanger  8  through a regenerator and a cold chamber (volume in cylinder below cold displacer  2 ). There are times in the cycle in which gases are pulled into the space between the displacers from warm-hot displacer  7  and other times from warm-cold displacer  8 . This present inefficiencies in there being more dead space than if a single heat exchanger could be used, simply the fact that there are two heat exchangers and the cost and complication, and the extra plumbing for the two heat exchangers. Yet other advantages according to embodiments of the present disclosure include: a single warm heat exchanger and the lack of plumbing from the space between the displacers to the warm heat exchanger. Instead, the gases flow directly from one side of the single warm heat exchanger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is an illustration of a prior art Vuilleumier heat pump in which displacers are crank driven; 
           [0023]      FIG. 2  is an illustration of displacer movement of the crank-driven displacers of  FIG. 1 ; 
           [0024]      FIG. 3  is a heat pump with mechatronically-actuated displacers; 
           [0025]      FIGS. 4 and 16  are illustrations of a heat pump according to embodiments of the present disclosure; 
           [0026]      FIGS. 5-15  are illustrations of the heat pump of  FIG. 4  in a range of displacer positions; 
           [0027]      FIGS. 17 and 18  are plan views of warm heat exchanger embodiments; and 
           [0028]      FIG. 19  is an illustration of a heat pump according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations whether or not explicitly described or illustrated. 
         [0030]    A thermally-driven heat pump  300  has a housing  302  which has a hot cap  304  on each end is shown in  FIG. 3 . Inside housing  302  is a cylinder  306 . In the annular space between housing  302  and cylinder  306 , a hot regenerator  332 , a warm heat exchanger  334 , a cold regenerator  336 , and a cold heat exchanger  338  are arranged. A hot displacer  312  and a cold displacer  314  are disposed within cylinder  306  and delimit hot chamber  320 , warm chamber  322 , and cold chamber  326 . A post  318  is coupled to cold cap  302 . Displacers  312  and  314  reciprocate within cylinder  306  under control by an ECU  340 . Displacers  312  and  314  have springs (not shown) that cause them to oscillate between upper and lower positions, respectively. Also not shown are the electromagnets and ferromagnetic plates associated with displacers  312  and  314 . ECU  340  provides a signal to the electromagnets to attract the ferromagnetic plates to grab the displacer in one of its extreme positions. The electromagnet can hold its associated displacer until ECU  340  commands the electromagnet to de-energize to allow the displacer to act under spring control to travel to its other extreme position. More detail of mechatronic control of the displacers is found in: PCT/US13/36101 filed 11 Apr. 2013. 
         [0031]    Continuing to refer to  FIG. 3 , an energy source (not shown) provides energy to a working fluid within housing  302  via a hot heat exchanger  330 . When hot displacer  312  moves upward, gases flow from hot chamber  320  into hot heat exchanger  330  into hot regenerator  332  into warm heat exchanger  334  through openings  344  in cylinder  306  into the into warm chamber  322  and in the reverse order upon hot displacer  312  moving downward. Upon cold displacer  314  moving downward, the working fluid in cold chamber  326  moves through openings  342  in cylinder  316  into cold heat exchanger  338 , into cold regenerator  336 , into hot heat exchanger  334  into warm chamber  332  via openings  344  in cylinder  306  and in reverse order upon cold displacer  314  moving upward. Heat exchangers  334  and  338  have two fluids exchanging energy: the working fluid and a second fluid, such as a liquid. In one embodiment, the heat pump is providing domestic heating and the second fluid is water that is heated within warm heat exchanger  334 . Energy is extracted from a fluid provided to cold heat exchanger  338  in such an embodiment. In another embodiment, a second fluid is provided to cold heat exchanger  338  for cooling purposes. In such embodiment, energy is exhausted via another fluid provided to warm heat exchanger  334 . Provisions for inlets and outlets for a fluid other than the working fluid to heat exchangers  330 ,  334 , and  338  are not shown in  FIG. 3 . 
         [0032]    One example of motion of the displacers of  FIG. 3  is illustrated in  FIG. 4 . Curve  240  is an illustration of the movement of the bottom edge of hot displacer ( 312  in  FIG. 3 ) and line  242  shows motion of the upper edge of the cold displacer ( 314  in  FIG. 3 ). At time  0 , the displacers are proximate each other. At time  0 , the electromagnet holding the hot displacer is de-energized to allow the hot displacer to travel toward its upper position. The motion is roughly sinusoidal. At time a, the upper electromagnet is energized to grab and hold the hot displacer. From time  0  to a, the cold displacer remains stationary at its upper position. At time a, the electromagnet grabbing the cold displacer is de-energized to allow the cold displacer to travel downward. At time  2   a , the electromagnet associated with the cold displacer grabs the cold displacer. Electromagnets are energized and de-energized to complete a cycle is complete from time  0  to time  4   a  and continuing on. The dwell periods of the hot and cold displacers can be lengthened to alter the cycle to meet heating or cooling demand. 
         [0033]    In  FIG. 5 , an interior of a heat pump  10  having a hot displacer  12  which reciprocates within a hot cylinder portion  16  and a cold displacer  14  which reciprocates within a cold cylinder portion  17  is illustrated. The electronic controls to manage the electromagnets (not shown) are provided through post  18 . Post  18  extends from cold cap  38  toward hot cap  28  along centerline  31 . A hot chamber  60  is above hot displacer  12  and a hot warm chamber  62  is below hot displacer  12 . A cold chamber  66  is below cold displacer  14  and a cold warm chamber  64  is above cold displacer  14 . A hot heat exchanger  20  is proximate a hot cap  28  of heat pump  10 . Hot heat exchanger  20  may, in some embodiments, be in contact with a burner. Or, in other embodiments, another energy source, such as a solar collector is used. A warm heat exchanger  40  is within a warm heat exchanger cylinder portion  15  which is located between hot cylinder portion  16  and cold cylinder portion  17 . Warm heat exchanger  40  is substantially in a disk shape, although with an opening in the center to accommodate post  18 . A fluid passes inside the tubes of warm heat exchanger  40  with an inlet  42  and an exit  44 . The working fluid within heat pump  10  passes by warm heat exchanger  40  upon movement of displacers  12  and  14 . Energy is exchanged with a fluid provided to warm heat exchanger through inlet  42 . A cold heat exchanger  30  is proximate a cold cap  38  of heat pump  10 . A fluid passes inside the tubes of cold heat exchanger  30  with an inlet  32  and an outlet  34 . The working fluid passes on the outside of the tubes of cold heat exchanger  30  with the gases flowing from cold chamber  66  to passage  36  into a cold displacer  54 , into a passage  56  and exits hot warm chamber  62 , when displacer  14  moves downward; and in reverse when displacer  14  moves upward. A hot regenerator  50  is located outside of cylinder  16  in passages  26  and  52 . When hot displacer  12  moves upward, gases in hot chamber  60  flows on the outer surface of heat exchanger  20  into passage  26  through hot regenerator  50  through passage  52  and exits into lower warm exchanger  64 . 
         [0034]    In the embodiment in  FIG. 5 , cylinders  15 ,  16 , and  17  are the same size. Alternatively, one of cylinders  16  and  17  has greater diameter than the other cylinder. 
         [0035]    Movement of the displacers of  FIG. 5  is shown in  FIG. 6 . Movement of hot displacer  12  is shown by curve  244  and movement of cold displacer is shown by curve  246 . The movement of the displacers, curves  240  and  242 , is the same as the movement of the displacers, curves  244  and  246 , as shown in  FIG. 4  except that the two, in  FIG. 5 , are separated from each other by the height of a warm heat exchanger  250 . 
         [0036]    In  FIGS. 7-15 , a cycle in heat pump  10  is shown. Starting in  FIG. 7 , hot displacer  12  is at its lower position and cold displacer  14  is at its upper position. 
         [0037]    In  FIG. 8 , hot displacer  12  is moving upward, as shown by arrow  98 . Movement of hot displacer  12  causes gas  102  in hot chamber  60  to pass through heat exchanger  20  into passage  26 , as shown by arrow  104 , through hot generator  50  into passage  52 , as shown by arrow  106 , and through warm exchanger  40  into hot warm chamber  62 , as shown by arrow  108 . 
         [0038]    In  FIG. 9 , hot displacer  12  has reached its upper position and cold displacer  14  is still in its upper position. 
         [0039]    In  FIG. 10 , cold displacer  14  is moving downwards, as indicated by arrow  120 . Gas is pushed out of cold chamber  66 , as shown by arrow  122 , into cold heat exchanger  30  into passage  36 , as shown by arrow  124 , into cold regenerator  54  into passages  56 , as shown by arrow  126 , into hot warm chamber  62 , through warm heat exchanger  40 , as illustrated by arrow  128 , and finally into cold warm chamber  64 . 
         [0040]    In  FIG. 11 , cold displacer has reached is lower position and hot displacer  12  is still in it upper position. 
         [0041]    In  FIG. 12 , cold displacer  12  moves downwardly, as indicated by arrow  140 . Gas is pushed out of hot warm chamber  62  through warm heat exchanger  40 , as indicated by arrow  142 , into passage  52 , as indicated by arrow  144 , into cold regenerator  50 , into passage  26 , as indicated by arrow  146 , into hot heat exchanger  20 , and into hot chamber  60 , as indicated by arrow  148 . In  FIG. 13 , hot displacer  14  reaches its lower position and cold displacer  12  remains at its upper position. 
         [0042]    In  FIG. 14 , cold displacer  14  moves upwardly as indicated by arrow  160 . Gas from upper warm chamber  64  is forced through warm heat exchanger  40 , as indicated by arrow  162 , into passage  56 , as indicated by arrow  164 , into cold regenerator  54  into passage  36 , as indicated by arrow  166  through cold heat exchanger  30  into cold chamber  66 , as indicated by arrow  168 . 
         [0043]    In  FIG. 15 , cold displacer  14  has achieved it upper position while hot displacer remains in its lower position. The cycle is completed as the position of the displacers in  FIG. 15  is the same as the start position shown in  FIG. 5 . 
         [0044]    The description of the gas movement implies that the gases make a complete loop. However, the gases move in the path described, but gases starting on one side of the displacer do not make the complete path to the other side of the displacer, but instead make travel through part of the loop. 
         [0045]    An alternative heat pump  100  configuration is illustrated in  FIG. 16  in which regenerators are integral with the housing. The configuration in  FIG. 16  shows hot regenerator  70  annularly arranged outside of cylinder  16  and cold regenerator  74  annularly arrangement outside of cylinder  17 . Also shown in heat pump  100  is a tube and shell heat exchanger  90 . An inlet  92  allows a liquid, as an example, to travel through the shell of heat exchanger  90 . The fluid exits at  94 . The working fluid within heat pump  100  passes within the tubes of heat exchanger  90 . In  FIG. 16 , a cold heat exchanger  31  is annularly arranged around cylinder  17 . Cold heat exchanger  31  is fluidly coupled to cold regenerator  74  and to cold chamber  66 . The other fluid provided to cold heat exchanger  31  has an inlet  33  and an outlet  35 . 
         [0046]    A view down the cylinder of the two warm heat exchanger alternatives previously illustrated is shown in  FIGS. 17 and 18 . In  FIG. 17 , a spiral heat exchanger  150  similar to that illustrated in  FIGS. 4-15  is shown within a cylinder portion  158 . Due to the reversal in the center of the spiral, both an inlet  152  and an outlet  154  are on the periphery of the spiral. An opening  156  is provided through spiral heat exchanger  150  to accommodate a post that carries electrical conductors (element  18  shown in  FIG. 4 ). 
         [0047]    A shell-and-tube heat exchanger  160  similar to that illustrated in  FIG. 16  is shown in  FIG. 18 . Heat exchanger  160  is contained within a cylinder  168 . An opening  166  is provided through spiral heat exchanger  150  to accommodate the post. 
         [0048]    In the embodiment of heat pump  200  in  FIG. 19 , regenerators  250  and  254  are placed along cylinder wall  220  in which displacers  12  and  14  reciprocate. A heat exchanger  240  is located between displacers  12  and  14 . An opening through cylinder wall  220  fluidly connects hot regenerator  250  with the chamber  62 , but on the lower side of heat exchanger  240 . In regards to cold regenerator  254 , there is an opening in cylinder wall  220  to fluidly connect cold regenerator  254  with chamber  62 , on the upper side of heat exchanger  240 . Water or other fluid to be heated is provided to heat exchanger  240  through inlet  192  that goes through hot regenerator  250  and exits through outlet  194 , which goes through cold regenerator  254 . In other embodiments, the lower portion of the space for hot regenerator  250  and the upper portion of the space for cold regenerator  254  is actually an extension of heat exchanger  240 . In such embodiment, part of heat exchanger  240  is within cylinder wall  220  and part of heat exchanger  240  is outside cylinder wall  220 . 
         [0049]    While the best mode has been described in detail with respect to particular embodiments, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are characterized as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.
A thermally driven heat pump is disclosed in which at least most of the warm heat exchanger is disposed within the cylinder between the hot and cold displacers. Such an arrangement is not suitable for a prior art heat pump in which movement of the displacers is based on a crank because it would lead to too much dead volume in the system. However, with mechatronically-controlled displacers in which the displacers are independently controlled, the displacers can reciprocate up to the heat exchanger. Such a configuration reduces dead volume compared to prior art Vuilleumier heat pumps in which the warm exchanger occupies a portion of an annular space between the cylinder in which the displacers move.