Abstract:
A Vuilleumier heat pump is disclosed in which hot and cold displacers are controlled by first and second electromagnetic actuators, respectively. The first actuator is capable of moving the hot displacer between the first and second ends of travel while the cold displacer remains stationary and the second actuator is capable of moving the cold displacer while the hot displacer remains stationary. Prior art crank arrangements are unable to provide dwell in one displacer while moving the other displacer. Actuation of the displacers according to embodiments of the present disclosure provides a higher coefficient of performance than crank arrangements.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a system and method for pumping fluid in a heat pump. 
     BACKGROUND 
     A Vuilleumier heat pump was disclosed in U.S. Pat. No. 1,275,507, filed 29 Jan. 1917. In a Vuilleumier heat pump  10 , two displacers (or pistons) are provided in a cylinder  20  and defining three chambers: a hot displacer  12  between a hot chamber  22  and a warm chamber  24  and a cold displacer  16  between the warm chamber  24  and a cold chamber  26 , as example of which is shown in  FIG. 1 . Displacers  12  and  16  reciprocate within cylinder  20  to change the volume of fluid contained in the three chambers. E.g., when hot displacer  12  is an extreme position towards hot chamber  22 , most of the fluid is pushed out of hot chamber  22 , through a hot heat exchanger  28 . Hot heat exchanger  28  is coupled to a burner  27  that is supplied fuel and air. The fluid travels next through a hot recuperator  30 , a warm heat exchanger  32 , a cold recuperator  34 , and a cold heat exchanger  36 . Elements  28 ,  30 ,  32 ,  34 , and  36  are fluidly coupled to cylinder  20  external to the cylinder and having a passage  38  between warm heat exchanger  32  and warm chamber  24 . Displacers  12  and  16  are caused to reciprocate by a crank arrangement  40 . 
     The movement of displacers  12  and  16  as driven by crank arrangement  40  is substantially sinusoidal, as illustrated in  FIG. 2 . The displacer height and their movement during reciprocation is illustrated as a function of crank angle degree in  FIG. 2  and identified as D_h and D_c. The volumes between the hot and cold displacers in the 3 chambers are also illustrates in  FIG. 2 : V_h, V_w, and V_c. Movement of cold displacer  16  is offset from that of hot displacer  12  by a phase angle, such as 90°. Chambers  22 ,  24 , and  26  are fluidly coupled to each other with little flow restriction. Thus, pressure in the three chambers is substantially the same, but varies as a function of time, as shown in  FIG. 3 . The pressure in the cylinder rises when flow through hot exchanger  28  raises the overall temperature of the gases within the closed system and the pressure within the cylinder falls when energy is extracted via warm heat exchanger  32 . 
     A Vuilleumier heat pump is a closed thermodynamic cycle in which the working fluid, a gas, remains in the cylinders. Energy is transferred to and from the heat pump through heat exchangers. In a heating mode, energy is transferred to the hot chamber via a burner or other high temperature energy source. Energy is also transferred to the fluid in the cold heat exchanger from the environment. The energy transferred for space heating or hot water heating, as examples, is extracted from the warm chamber via a heat exchanger. Because some of the energy is extracted from the environment, the coefficient of performance substantially exceeds 1 at many operating conditions. This is in comparison to a standard furnace in which the coefficient of performance can at best approach 1 and only in furnaces in which the water vapor in the exhaust is condensed. The heat pump may also be used for cooling by energy extracted in the cold heat exchanger. Vuilleumier heat pumps have been used to develop cryogenic temperatures. 
     Through modeling of the system, it has been found that coefficient of performance of the system could be improved if one of the displacers could dwell at its extreme position while the other displacer moves and vice versa, rather than having both of them be in continuous movement, i.e., separated by a fixed phase angle. 
     To obtain reasonable performance in the Vuilleumier heat pump, the working fluid is either hydrogen or helium, which is pressurized to about 100 bar, as a non-limiting example. Pressure is fairly constant throughout the cylinder, but varies as a function of crank angle degree, as shown in  FIG. 3 . 
     Preventing leakage of either of these gases is a challenge. In prior art Vuilleumier heat pumps, the rotating crank arrangement to which the displacers are coupled may be located outside the housing such that the moving connecting rods that attach to the displacers pass through the wall of the housing. Sealing around a moving and rocking connecting rod presents a sealing challenge. Alternatively, the rotating crank arrangement is within the sealed housing yielding a heavier, bulkier heat pump. 
     SUMMARY 
     To overcome at least one shortcoming in the prior art, a heat pump is disclosed that has: a housing having an outer wall and a cylinder liner within the housing, a hot displacer disposed within the cylinder liner, a hot displacer actuator coupled to at least one of the cylinder and the housing, and an electronic control unit (ECU) electronically coupled to the hot displacer actuator. The ECU may be electronically coupled by a physical connection such as a wire, via wireless communication, or anything suitable. The hot displacer reciprocates within the cylinder between a first end of travel associated with the hot displacer and a second end of travel associated with the hot displacer based on a signal from the ECU to the hot displacer actuator. The hot displacer has a generally cylindrical body, a first cap at a first end of the cylindrical body, and a second cap coupled to a second end of the cylindrical body. The first cap may be integrally formed with the body of the displacer or coupled to the body of the displacer by any suitable method, such as by the following non-limiting examples: friction welding, brazing, welding, gluing, bolting, and clamping. In addition to a substantially cylindrical cross section, the body of the hot displacer can have a cross section of any shape, e.g., oval or polygonal, as non-limiting examples. The heat pump may further include: a cold displacer disposed within the cylinder liner, and a cold displacer actuator that is coupled to at least one of the cylinder and the housing, and is electronically coupled to the ECU. The cold displacer reciprocates within the cylinder between a first end of travel associated with the cold displacer and a second end of travel associated with the cold displacer based on a signal from the ECU to the cold displacer actuator. The cold displacer has a generally cylindrical body, a third cap coupled to a first end of the cylindrical body of the cold displacer, and a fourth cap coupled to a second end of the cylindrical body of the cold displacer. 
     During operation of the heat pump, the hot displacer has selectable dwell periods at the first and second ends of travel associated with the hot displacer and the cold displacer has selectable dwell periods at first and second ends of travel associated with the cold displacer. 
     The housing has a hot end and a cold end. The hot displacer actuator is a hot displacer electromechanical device that has a first spring coupled between a first stationary element associated with the heat pump and the hot displacer, a second spring coupled between a second stationary element associated with the heat pump and the hot displacer, an electromagnet associated with the hot displacer that is coupled to a third stationary element associated with the heat pump and electronically coupled to the ECU, a first ferromagnetic element coupled to the hot displacer, and a second ferromagnetic element coupled to the hot displacer. The first spring exerts a force on the hot displacer in a direction toward the cold end of the housing and the second spring exerts a force on the hot displacer in a direction toward the hot end of the housing. The first and second ferromagnetic elements are located a predetermined distance apart as measured along a direction of travel of the hot displacer in the cylinder. 
     In one embodiment, the first and second stationary elements are coupled to the cylinder; the third stationary element is a centrally-located post rigidly affixed to a cold end of the housing and extending into the housing; the second cap of the hot displacer defines a centrally located opening to accommodate the post passing into the hot displacer; and the first and second springs are located between the cylindrical body of the hot displacer and the cylinder liner. 
     In another embodiment, the third stationary element comprises a centrally-located post rigidly affixed to a cold end of the housing and extending into the housing; the first and second stationary elements are coupled to the post at a location distal from the cold end of the housing; the second cap of the hot displacer defines a centrally located opening to accommodate the post passing into the hot displacer; and the first and second springs are located inside the hot displacer. Some embodiments also include: a cold displacer disposed within the cylinder liner. The cold displacer has a generally cylindrical body, a third cap coupled to a first end of the cylindrical body of the cold displacer, and a fourth cap coupled to a second end of the cylindrical body of the cold displacer. The heat pump also includes a cold displacer electromechanical device that includes a third spring coupled between a fourth stationary element associated with the heat pump and the cold displacer, a fourth spring coupled between a fifth stationary element associated with the heat pump and the cold displacer, an electromagnet associated with the cold displacer that is coupled to the post and electronically coupled to the ECU; and a third ferromagnetic element coupled to the cold displacer; and a fourth ferromagnetic element coupled to the cold displacer. The first and second caps of the cold displacer define a centrally located opening to accommodate the post passing through the cold displacer and the third and fourth springs are located inside the cold displacer. The third spring exerts a force on the cold displacer in a direction toward the cold end of the housing and the fourth spring exerts a force on the cold displacer in a direction toward the hot end of the housing 
     When the hot displacer is at the first end of travel, the ECU commands the electromagnet to energize with a holding current to act on the first ferromagnetic element to hold the hot displacer at the first end of travel for a first selectable dwell period. After the first dwell period, the ECU commands the electromagnet to de-energize so that the hot displacer moves toward the second end of travel due to unbalanced spring forces acting on the hot displacer. When the hot displacer nears the second end of travel, the ECU commands the electromagnet to energize with a grabbing current sufficient to pull the second ferromagnetic element of the hot displacer so that the hot displacer to move into the second end of travel. When the hot displacer is at the second end of travel, the ECU commands the electromagnet to energize with a holding current to act on the second ferromagnetic element to hold the hot displacer at the second end of travel for a second selectable dwell period. After the second dwell period, the ECU commands the electromagnet to de-energize so that the hot displacer moves toward the first end of travel due to unbalanced spring forces acting on the hot displacer. 
     In some embodiments, the hot displacer has a first groove in an outer surface of the first cap and a second groove in an outer surface of the second cap. A first sealing ring is provided in the first groove and a second sealing ring is provided in the second groove. The rings ride on a surface of the cylinder liner during reciprocation of the hot displacer. 
     The housing and the cylinder liner define an annular chamber located between an inner surface of the housing and an outer surface of the cylinder liner. The heat pump may further include a hot recuperator, a warm heat exchanger, a cold recuperator, and a cold heat exchanger disposed in the annular chamber. The warm heat exchanger, the cold recuperator and the cold heat exchanger are arranged in the annular chamber arranged in the listed order with the hot recuperator proximate the hot end of the housing and the cold heat exchanger proximate the cold end of the housing. A hot heat exchanger may also be arranged within the annular chamber proximate the hot end of the housing. A burner may be provided external to the housing with products of combustion from the burner fluidly coupled to the hot heat exchanger. 
     The hot and cold displacers define three chambers within the housing: a hot chamber proximate the hot end of the housing; a cold chamber proximate the cold end of the housing, and a warm chamber located between the hot and cold displacers. The cylinder liner defines: a first set of openings in the cylinder liner proximate the hot end of the housing to provide fluidic communication between the hot chamber and the annular volume; a second set of openings in a middle of the cylinder liner to provide fluidic communication between the warm chamber and the annular chamber; and a third set of openings in the cold end of the housing that provide fluidic communication between the cold chamber and the annular chamber. 
     According to other embodiments, a system for pumping fluid within a heat pump includes: a housing having a cylinder therein; a hot displacer disposed within the cylinder and having a body, a first cap coupled to a first end of the body, and a second cap coupled to a second end of the body; a hot actuator coupled to the hot displacer; and an electronic control unit (ECU) electronically coupled to the hot actuator. The hot displacer moves between a first end of travel and a second end of travel. The hot displacer dwells at the first end of travel for a first selectable period and the hot displacer dwells at the second end of travel for a second selectable period. The system may further include a cold displacer disposed within the cylinder and having a body, a third cap coupled to a first end of the body of the cold displacer and a fourth cap coupled to a second end of the body of cold displacer; and a cold actuator coupled to the cold displacer and electronically coupled to the ECU. The hot actuator allows dwell of the hot displacer while the cold displacer moves and the second actuator allows dwell of the cold displacer while the hot displacer moves. Dwell refers to maintaining one of the displacer in a fixed position, e.g., causing the hot displacer to stay at the first end of travel for a selectable period. 
     The hot actuator may include a centrally-located post rigidly affixed to a cold end of the housing and extending into the housing, first and second springs coupled between the post and the hot displacer, first and second ferromagnetic elements affixed to the hot displacer, and a first electromagnet coupled to the post at a location between the first and second ferromagnetic elements. The first ferromagnetic element being displaced from the second ferromagnetic element by a predetermined distance as measured along a central axis of the cylinder. The cold actuator includes: third and fourth springs coupled between the post and the cold displacer, third and fourth ferromagnetic elements affixed to the cold displacer; and a second electromagnet coupled to the post at a location between the third and fourth ferromagnetic elements. The third ferromagnetic element is displaced from the fourth ferromagnetic element by the predetermined distance as measured along a central axis of the cylinder. 
     A method to operate a heat pump is disclosed in which the cold actuator is commanded to move the cold displacer from its first end of travel to its second end of travel while commanding the hot actuator to maintain the hot displacer at its first end of travel. The method may further include commanding the hot actuator to move the hot displacer from its second end of travel to its first end of travel while commanding the cold actuator to maintain the cold displacer at its second end of travel. 
     The first spring exerts a force on the associated displacer in a direction toward the cold end; the second spring exerts a force on the associated displacer in a direction toward the hot end; and the electromagnet is adapted to attract the ferromagnetic block or element when the electromagnet is energized. 
     The hot actuator causes the hot displacer to move from a lower position to an upper position by de-energizing the electromagnet; energizing the electromagnet with a grabbing current when the upper ferromagnetic block approaches the electromagnet due to the hot displacer moving due to unbalanced spring forces; and energizing the electromagnet with a holding current when the upper ferromagnetic block approaches the electromagnet. 
     An advantage according to embodiments of the present disclosure is that a higher coefficient of performance, in both cooling and heating, is provided due to the more desirable movement of the displacers, for example, the ability to hold the hot displacer in place while moving the cold displacer. In contrast, prior art heat pumps have the two displacers moving continuously with a constant phase angle difference. 
     Yet another advantage of the present disclosure is that the actuators are enclosed within the housing of the heat pump. This greatly aids in keeping the helium, hydrogen, or other low molecular weight working fluid sealed within the housing. 
     Vuilleumier heat pumps, in which the displacers reciprocate substantially sinusoidally with a 90° phase shift between the two, use a warm heat exchanger, such as shown in  FIG. 1  as element  32 . The alternative system for actuating displacers per embodiments described herein in which one of the displacers dwells at an extreme position while the other displacer moves, does not rely on a second warm heat exchanger between the warm chamber  24  and the cold recuperator  34 . Obviating one warm heat exchanger is another advantage provided by disclosed embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a Vuilleumier heat pump; 
         FIG. 2  is a graph indicating the sinusoidal movement of displacers in a Vuilleumier heat pump and the volumes in the three chambers; 
         FIG. 3  is a graph illustrating pressure within the Vuilleumier heat pump as the displacers move according to  FIG. 2 ; 
         FIGS. 4-6  are illustrations of Vuilleumier heat pumps according to embodiments of the present disclosure; 
         FIGS. 7-10  are schematic illustrations of a Vuilleumier-type heat pump in which the displacers are shown in their ends of travel used to describe a cycle in which the heat pump may be operated; and 
         FIGS. 11A-C  illustrate thermodynamic cycles associated with the hot, cold, and warm chambers, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     In  FIG. 4 , one embodiment of a heat pump  250  has a housing  252 . A cylinder liner  254  is provided in housing  252 . Hot and cold displacers  262  and  266 , respectively, are shown in their neutral position, i.e., not at either end of travel. The displacers define three chambers: a hot chamber  272 , a warm chamber, and a cold chamber  276 . With the positions of displacers  262  and  266  as illustrated in  FIG. 4 , the warm chamber has no volume and is thus not provided a numeral. Housing  252  has a hot end  282  and a cold end  286 . 
     A post  288  is affixed to the cold end  286  of housing  252  and extends into housing  252  along a central axis of housing  252 . Post  288  extends through cold displacer  266  and extends into one end of hot displacer  262 . Post  288  has electromagnets  292   a  and  292   c  disposed within hot displacer  262  and electromagnets  296   a  and  296   c  disposed within cold displacer  266 . 
     Ferromagnetic elements or blocks  222   a ,  222   b , and  222   c  are affixed to hot displacer  262 . Blocks  222   a ,  222   b , and  222   c  are displaced from each other by predetermined distances as measured in a direction along the axis of housing  252 . The predetermined distances are related to the desired travel of hot displacer  262 . Ferromagnetic blocks  226   a ,  226   b , and  226   c  are affixed to cold displacer  266 . Blocks  226   a ,  226   b , and  226   c  are displaced from each other by predetermined distances as measured in a direction along the axis of housing  252 . 
     In the embodiment in  FIG. 4 , one end of a spring  242   a  is attached to a top end of hot displacer  262  and the other end of spring  242   a  to a tab  282 . A second spring  242   b  is attached to tab  282  on one end and to a bottom end of hot displacer  262 . Similarly, cold displacer has springs  246   a  and  246   b  that couple between a tab  286  and top and bottom ends of cold displacer  266 . 
     Another embodiment shown in  FIG. 5 , a heat pump  350  has a housing  352  in which a hot displacer  362  and a cold displacer  366  are disposed in a cylinder  354 . A hot chamber  372  is defined between a hot end  382  of housing  352  and hot displacer  362 . A cold chamber  376  is defined between a cold end  386  of housing  352  and cold displacer  366 . A hot actuator that can move hot displacer  362  includes: two blocks  402  and  412  which may be made of a ferromagnetic material and an electromagnet  392  that can be energized under control by an electronic control unit (ECU)  400  to grab one or the other of blocks  402  and  412  to cause hot displacer  362  to move. Hot displacer also has two springs  442 , one of which is coupled between a cap  422  and tab  443  that is part of cylinder  354  and the other of which coupled between a cap  432  and tab  443 . Tab  443  can be a cylindrical lip or multiple tabs provided on the circumference of cylinder  354  to provide an attachment for the springs. In some embodiments, caps  422  and  432  are provided with seals  455  that ride on cylinder  354  during reciprocation. Similarly, a cold actuator to move cold displacer  366  includes: two blocks  406  and  416  which can be attracted by electromagnet  396  controlled by ECU  400 . The cold actuator also has springs  446 , one of which is coupled between a tab  447  and a cap  426  of cold displacer  366  and the other of which is coupled between tab  447  and a cap  436  of cold displacer. 
     Electromagnets  392  and  396  are mounted on a centrally-located post  388  that is coupled to the cold end  386  of housing  352 . Post  388  extends through the end caps of cold displacer  366  and through cap  432  of hot displacer  362 . Electrical wires to energize the electromagnets travel through post  388 . 
     Springs  446  are in compression the upper of which exerts a downward force and the lower of which exerts and upward force. Cold displacer  366  is in equilibrium in  FIG. 5  with the spring forces counteracting each other. Electromagnet  396  can be actuated to cause cold displacer  366  to move from the equilibrium position. 
     When the displacers move, fluid in the various chambers is pushed out from the chamber into an annular chamber  378  that is between the inner surface of housing  352  and the outer surface of cylinder  354 . Openings are provided in cylinder  354  to allow flow between the chambers within cylinder  354  and annular chamber  378  outside of cylinder  354 . Openings  462  allow flow between hot chamber  372  and annular chamber  378 ; openings  464  allow flow between a warm chamber (has no volume in the equilibrium position shown in  FIG. 5 ) and annular chamber  378 ; and openings  466  allow flow between cold chamber  376  and annular chamber  378 . 
     Annular chamber  378  has a hot recuperator  452 , a warm heat exchanger  454 , a cold recuperator  456 , and a cold heat exchanger  458  are disposed in annular chamber  378 . When heat pump  350  is operated in a heating mode, water or other fluid is provided through warm heat exchanger  454  through inlet  474  and outlet  472  that pierce housing  352 . Alternatively, flow through heat exchanger  454  is a reverse direction to that shown in  FIG. 5 . In both the heat and cooling modes, a fluid is provided through cold heat exchanger  458  that has inlet  476  and outlet  478  that pierce housing  352 . 
     The thermodynamic cycle efficiency is improved by reducing dead volume in the heat pump. Volume in the annular chamber is part of the dead volume. Also, the volume in which the springs are located at the outside of the displacer is a dead volume. It is desirable to make the recuperators and heat exchangers as compact as possible to reduce the volume. In another embodiment in  FIG. 6 , the springs are provided inside the displacers. 
     Referring now to  FIG. 6 , a heat pump  50  has a housing  52  and a cylinder  54  into which hot displacer  62  and cold displacer  66  are disposed. Displacers  62  and  66  reciprocate within cylinder liner  54  moving along central axis  53 . An actuator for hot displacer  62  includes: ferromagnetic elements  102  and  112 , electromagnet  92 , springs  142  and  144 , and a support structure  143 . Support structure  143 , as shown in  FIG. 6  is attached to the electromagnet  92 , which is coupled to a central post  88  that is coupled to a cold end  86  of housing  52 . Post  88 , electromagnet  92 , and support structure  143  are stationary. When hot displacer  62  reciprocates upward from the position shown in  FIG. 6 , spring  142  is compressed to a greater degree than its equilibrium preload and  144  is under a lower compression. Electromagnet  92  is energized to pull ferromagnetic elements  102  or  112  toward it, against the spring forces of springs  142  and  144 . Analogously, cold displacer  66  has a cold actuator that includes: an electromagnet  96  coupled to post  88 , a support structure  147  coupled to electromagnet  96 , and springs  146  and  148 . Spring  146  is coupled between support structure  147  and a first cap  126  of cold displacer  66 . Spring  148  is coupled between support structure  147  and a second cap  136  of cold displacer  66 . Electromagnet  92  and  96  are controlled via an electronic control unit (ECU)  100 . 
     Ferromagnetic blocks  102 ,  112 ,  106 , and  116  are coupled to: a standoff associated with a first cap  122  of hot displacer  62 , a second cap  132  of hot displacer  62 , a standoff associated with first cap  126  of cold displacer  66 , and second cap  136  of cold displacer  66 , respectively. Openings are provided in second cap  132  of hot displacer  62 , and first and second caps  126  and  136  of cold displacer  66  to accommodate post  88  extending upwardly through cold displacer  66  and into hot displacer  62 . 
     An annular chamber is formed between a portion of the inner surface of housing  52  and the outer surface of cylinder  54 . A hot recuperator  152 , a warm heat exchanger  154 , a cold recuperator  156 , and a cold heat exchanger  158  are disposed within the annular chamber. Openings through cylinder  54  allow fluid to pass between the interior of cylinder  54  to the annular chamber. Openings  166  allow for flow between a cold chamber  76  and cold heat exchanger  158  in the annular chamber. Openings  164  allow flow between a warm chamber (which has substantially no volume when the displacers are in the position shown in  FIG. 6 ) and the annular chamber. Heat pump  50  also has a hot heat exchanger  165  that is provided near a hot end  82  of housing  52 . Openings  162  through cap  82  lead to heat exchanger  165  which has passages  163  that lead to the annular chamber. Hot heat exchanger  165  may be associated with a burner arrangement or other energy source. 
     Continuing to refer to  FIG. 6 , a fluid that is to be heated flows to warm heat exchanger  154  into opening  174  and out opening  172 , cross flow. Fluid that is to be cooled flows to cold heat exchanger  158  in at opening  176  and exits at opening  178 . The flow through the heat exchangers may be reversed, parallel flow. 
     Referring to illustrations in  FIGS. 7-10 , an example thermodynamic cycle is described. In  FIG. 7 , heat pump  50  is shown with both displacers at the upward end of their travel. Ferromagnetic element  112  is drawn to electromagnet  92 . Electromagnet  92  is energized with a holding current sufficient to hold hot displacer  62  against the unbalanced spring force exerting a downward force on hot displacer. Similarly, ferromagnetic member  116  is drawn to electromagnet  96  with sufficient holding current to hold cold displacer  66  at the upper extreme position against the unbalanced spring force. The working fluid within housing  52  is primarily contained within cold chamber  76  and the annular chamber with the recuperators and heat exchangers. There is very little fluid within the hot and warm chambers. 
     In  FIG. 8 , the cold displacer  66  has moved from the upper end of travel to the lower end of travel. In this configuration, almost no fluid is contained in either the cold chamber or the hot chamber. Instead, the working fluid if found in warm chamber  74  and some in the annular chamber. From the position of cold displacer  66  shown in  FIG. 7  to attain the position shown in  FIG. 8 , electromagnet  96  that had been holding ferromagnetic member  116  is de-energized. The unbalanced spring force causes cold displacer  66  to travel downward. As ferromagnetic block  106  approaches electromagnet  96 , a grabbing current is applied to the electromagnet  96  so that it draws ferromagnetic block  106  into electromagnet  96 . After ferromagnetic block  106  is in contact with electromagnet  96 , a lesser holding current is commanded to electromagnet to hold block  106  against the balanced spring forces. 
     Referring now to  FIG. 9 , both displacers  62  and  66  are shown in their lower extreme positions. The majority of the working fluid within cylinder  52  is within hot chamber  72 . Hot displacer  62  moves from the upper end of travel shown in  FIG. 8  to the lower end of travel shown in  FIG. 9  when electromagnet  92  is de-energized so that the unbalanced spring force acts on hot displacer  62  to cause it to move downwardly. When ferromagnetic block  102  of hot displacer  62  approaches electromagnet  92 , a grabbing current is commanded to electromagnet  92 . Once ferromagnetic block  102  is in contact with electromagnet  92 , electromagnet is commanded to hold block  102  with a holding current. 
     Between  FIGS. 9 and 10 , both displacers  62  and  66  move from their lower extreme positions closer to cold end  86  and their upper extreme positions closer to hot end  82 . Both electromagnets  92  and  96  are de-energized to allow the displacer to move under control of the springs, then energized with a grabbing current when the displacer approaches the other end of travel to pull the displacer in, and then energized with a holding current to retain the displacer in place. Note that the cycle is complete as the displacer positions in  FIGS. 7 and 10  are identical. 
     When cold displacer  66  moves between the positions shown in  FIGS. 7 and 8 , fluid is pushed out of cold chamber  76  and into warm chamber  74  via the annular chamber. When hot displacer  62  moves between the positions shown in  FIGS. 8 and 9 , fluid is pushed out of warm chamber  74  and into hot chamber  72  via the annular chamber. Finally, when both displacers  62  and  66  move upward to their ends of travel when moving from the position shown in  FIG. 9  to that in  FIG. 10 , fluid is pushed out of hot chamber  72  through the annular chamber into cold chamber  76 . 
     The embodiment in  FIG. 6-10  uses no seals between the displacer ( 62  and  66 ) and cylinder liner  54 . In some embodiments, the displacer may seal sufficiently well against the cylinder to obviate the need of seals which can increase friction. The pressure within the housing is substantially similar throughout. Of course, when the displacers move, a pressure difference is created that is sufficient to overcome the pressure drop in the annular space, i.e., to cause the fluid to flow among the hot, cold, and warm chambers through the elements in the annular space. However, if the pressure drop is low enough and depending on the speed of operation of the heat pump, seals may be omitted. 
     In prior art heat pumps with a crank arrangement to drive the displacers, the displacer is not allowed to dwell at any particular position, but is in continuous movement. According to embodiments of the present disclosure, not only can the displacer dwell at their extreme positions, but for a selectable period. For some operating conditions, it may be desirable for the displacer to dwell longer at one its ends of travel longer than at the other end of travel, which embodiments disclosed herein allow. The heating or cooling output can be adjusted by increasing or decreasing the dwell period, essentially changing the frequency of reciprocation, according to disclosed embodiments. 
     The thermodynamic processes that the working fluid undergoes in the hot (h), cold (c), and warm (w) chambers undergo are illustrated in  FIGS. 11A-C , respectively, in which the axes are P, for pressure, and V, for volume. The points 1h, 2h, and 3h correspond to the thermodynamic state associated with  FIG. 11A ; 1w, 2w, and 3w with  FIG. 11B ; and 1c, 2c, and 3c with  FIG. 11C . By computing area over a cycle, i.e., the integral of V-dp, heat transferred in undergoing that cycle can be determined. In the situation in  FIGS. 11A and 11B  the cycle is clockwise and indicates heat transfer to the system. In  FIG. 11C , the cycle is counter clockwise and indicates heat transfer out of the system. The temperatures in the chambers are maintained substantially constant at ˜600° C., ˜50° C., and ˜−5° C. for the hot, warm, and cold chambers, respectively. (The temperatures are provided by way of example and not intended to be limiting.) The processes in  FIGS. 11A-C  are shown as straight lines. In reality, the real processes deviate from straight lines, some of the processes being properly represented by curved lines. Thus,  FIGS. 11A-C  are illustrative only. In a heating mode, energy is transferred from the warm heat exchanger to the space to be heated (e.g., home) or hot water heater. In a cooling mode, energy is transferred to the cold heat exchanger from the space to be cooled. 
     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, 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 with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.