Abstract:
A four-process cycle is disclosed for a Vuilleumier heat pump that has mechatronically-controlled displacers. Vuilleumier heat pumps that use a crank to drive the displacers have been previously developed. However, mechatronic controls provides a greater degree of freedom to control the displacers. The four-process cycle provides a higher coefficient of performance than prior cycles in the crank-driven Vuilleumier heat pump and those previously disclosed for a mechatronically-driven Vuilleumier heat pump.

Description:
FIELD OF INVENTION 
       [0001]    The present disclosure relates to cycles in heat pumps, particularly Vuilleumier heat pumps. 
       BACKGROUND 
       [0002]    The displacers in most prior art Vuilleumier heat pumps are driven by a crank, such as shown in U.S. Pat. No. 1,275,507. A schematic of such a heat pump with crank driven displacers is shown in  FIG. 1 . In the &#39;507 patent, the displacers have a phase difference of 90 degrees as shown in  FIG. 2 . A mechatronically-driven Vuilleumier heat pump, which is commonly assigned to the assignee of the present disclosure, has been disclosed in WO 2013/155258. In such a heat pump, the displacers are independently actuated allowing one displacer to remain stationary while the other displacer moves, which provides many additional degrees of freedom in controlling displacer motion. In the WO 2013/155258 A1 publication, a three-process cycle is also disclosed. A cycle that provides a high coefficient of performance is desired. 
       SUMMARY 
       [0003]    A four-process cycle is disclosed that demonstrates a higher coefficient of performance than the previously disclosed three-process cycle based on modeling results. 
         [0004]    A method to operate a heat pump is disclosed. The heat pump has a hot displacer adapted to reciprocate within a hot cylinder and a cold displacer adapted to reciprocate within a cold cylinder. The hot displacer has a remote position and a central position and the cold displacer has a central position and a remote position. The method includes: actuating the hot displacer to move from its central position to its remote position, actuating the cold displacer to move from its central position to its remote position, actuating the hot displacer to move from its remote position to its central position, and actuating the cold displacer to move from its remote position to its central position wherein the actuations occur in the given order. 
         [0005]    At some operating conditions, the cold displacer remains stationary for at least a portion of the time during which the hot displacer moves between its central and remote positions and the hot displacer remains stationary for at least a portion of the time during which the cold displacer moves between it remote and central positions. 
         [0006]    The actuating the hot displacer to move from its central position to its remote position comprises process one. The actuating the cold displacer to move from its central position to its remote position comprises process two. The actuating the hot displacer to move from its remote position to its central position comprises process three. The actuating the cold displacer to move from its remote position to its central position comprises process four. A cycle is made up of process one followed by process two followed by process three followed by process four. 
         [0007]    The method may further include: commanding both displacers to remain stationary for a first predetermined time between process one and process two, commanding both displacers to remain stationary for a second predetermined time between process two and process three, commanding both displacers to remain stationary for a third predetermined time between process three and process four, and commanding both displacers to remain stationary for a fourth predetermined time between process four and process one. 
         [0008]    A hot chamber is defined within the hot displacer cylinder with volume within the hot chamber related to the position of the hot displacer within the hot displacer cylinder. A cold chamber is defined within the cold displacer cylinder with volume within the cold chamber related to the position of the cold displacer within the cold displacer cylinder. When the hot displacer is in its remote position, the volume in the hot chamber is less than when the hot displacer is in its central position. When the cold displacer is in its remote position, the volume in the cold chamber is less than when the cold displacer is in its central position. 
         [0009]    A heat pump is disclosed that has a hot displacer disposed in a hot displacer cylinder, a cold displacer disposed in a cold displacer cylinder, a hot displacer actuator which when actuated causes the hot displacer to reciprocate between remote and central positions within the hot displacer cylinder, a cold displacer actuator which when actuated causes the cold displacer to reciprocate between remote and central positions within the cold displacer cylinder, and an electronic control unit (ECU) coupled to the hot displacer actuator and the cold displacer actuator. The ECU commands the hot displacer and cold displacer to move through a series of arrangements: a first arrangement in which the hot displacer is at its central position within the hot displacer cylinder and the cold displacer is proximate its central position with the cold displacer cylinder, a second arrangement in which the hot displacer is at its remote position within the hot displacer cylinder and the cold displacer is proximate its central position with the cold displacer cylinder, a third arrangement in which the hot displacer within the hot displacer cylinder is at its remote position and the cold displacer is proximate its remote position within the cold displacer cylinder, and a fourth arrangement in which the hot displacer is at its central position within the hot displacer cylinder and the cold displacer is proximate its remote position within the cold displacer cylinder. 
         [0010]    A cycle comprises moving from the first arrangement to the second arrangement to the third arrangement to the fourth arrangement to the first arrangement. 
         [0011]    The cold displacer remains stationary in its central position for at least a portion of the time that it takes for the hot displacer to move from its central position to its remote position. The hot displacer remains stationary in its remote position for at least a portion of the time that it takes the cold displacer to move from its central position to its remote position. The cold displacer remains stationary in its remote position for at least a portion of the time that it takes the hot displacer to move from its remote position to its central position. The hot displacer remains stationary in its central position for at least a portion of the time that it takes the cold displacer to move from its remote position to its central position. 
         [0012]    In some embodiments, the central axis of the cold displacer cylinder is collinear with a central axis of the hot displacer cylinder. In some embodiments, a diameter of the cold displacer cylinder is greater than a diameter of the hot displacer cylinder. In another embodiment, the diameter of the hot displacer cylinder is greater than a diameter of the cold displacer cylinder. In yet other embodiments, the heat pump of claim  6  wherein a diameter of the hot displacer cylinder is equal to a diameter of the cold displacer cylinder. In some embodiments, a distance that the hot displacer moves from its remote position to its central position is greater than a distance that the cold displacer moves from it remote position to its central position. In another embodiment, a distance that the hot displacer moves from its remote position to its central position is less than a distance that the cold displacer moves from it remote position to its central position. In some embodiments, a time that it takes for the hot displacer to move between its central and remote positions is different than a time that it takes for the cold displacer to move between its central and remote positions. In a heat pump in which the actuator includes springs, the springs acting on the displacers can be selected such that times for the displacers to move between their respective central and remote positions are unequal. 
         [0013]    A heat pump is disclosed in which a hot displacer disposed in a hot displacer cylinder is adapted to reciprocate within the hot displacer cylinder and a cold displacer is disposed in a cold displacer cylinder and adapted to reciprocate within the cold displacer cylinder. The heat pump has a hot displacer actuator coupled to the hot displacer, the hot displacer actuator is adapted to cause the hot displacer to move between a central position and a remote position within the hot displacer cylinder, a cold displacer actuator coupled to the cold displacer, the cold displacer actuator is adapted to cause the cold displacer to move between a central position and a remote position within the cold displacer cylinder, and an electronic control unit (ECU) coupled to the hot displacer actuator and the cold displacer actuator. A cycle includes the following processes in the following order: the hot displacer actuator commands the hot displacer to move from the central position to the remote position within the hot displacer cylinder, the cold displacer actuator commands the cold displacer to move from central position to the remote position within the cold displacer cylinder, the hot displacer actuator commands the hot displacer to move from the remote position to the central position within the hot displacer cylinder, and the cold displacer actuator commands the cold displacer to move from remote position to the central position within the cold displacer cylinder. 
         [0014]    The heat pump has a hot chamber at one end of the hot displacer cylinder, and a cold chamber at one end of the cold displacer cylinder. Volume in the hot chamber is greater when the hot displacer is in the central position than when the displacer is in the remote position. Volume in the cold chamber is greater when the cold displacer is in the central position than when the cold displacer is in the remote position. The heat pump includes a warm chamber which is a volume within the hot cylinder at the opposite end of the hot displacer from the hot chamber added to a volume within the cold cylinder at the opposite end of the cold displacer from the cold chamber. 
         [0015]    In some embodiments, a central axis of the hot displacer cylinder is collinear with a central axis of the cold displacer. In other embodiments, a central axis of the hot displacer cylinder is substantially parallel to and offset from a central axis of the cold displacer. In some embodiments, the diameter of the hot displacer cylinder is greater than the diameter of the cold displacer cylinder. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a schematic of a prior art Vuilleumier heat pump; 
           [0017]      FIG. 2  is a graph of displacer movement in the Vuilleumier heat pump with crank-driven displacers; 
           [0018]      FIG. 3  is a schematic representation of a Vuilleumier heat pump with mechatronically-controlled displacers; 
           [0019]      FIG. 4  is a representation of a three-process cycle in the Vuilleumier heat pump; 
           [0020]      FIG. 5  is a representation of a four-process cycle in the Vuilleumier heat pump; 
           [0021]      FIG. 6  is a chart showing movement of the hot and cold displacers as a function of time for a three-process cycle; 
           [0022]      FIG. 7  is a chart showing movement of the hot and cold displacers as a function of time for a four-process cycle; 
           [0023]      FIG. 8  is a chart showing movement of the hot and cold displacers as a function of time for a four-process cycle in which movement of the displacers overlap; 
           [0024]      FIG. 9  is a chart showing movement of the hot and cold displacers in which there are periods in which both displacers remain stationary; 
           [0025]      FIG. 10  is a representation of a Vuilleumier heat pump in which the diameter of the hot displacer cylinder is greater than the diameter of the cold displacer cylinder; and 
           [0026]      FIG. 11  is a representation of a Vuilleumier heat pump in which the stroke of the hot displacer is less than the stroke of the cold displacer. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    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. 
         [0028]    Before describing cycles that are facilitated by a mechatronically-actuated Vuilleumier heat pump, a non-limiting example of such a heat pump  50  is shown in  FIG. 3 . 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 . 
         [0029]    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 . 
         [0030]    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 and the annular chamber. Heat pump  50  also has a hot heat exchanger  165  that is provided near a hot end of housing  52 . Openings  162  through cap  82  lead to heat exchanger  165  which has passages  163  which lead to the annular chamber. Hot heat exchanger  165  may be associated with a burner arrangement or other energy source. 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. 
         [0031]    The end positions of the displacers in a three-process cycle in the Vuilleumier heat pump are illustrated in  FIG. 4 . At state ‘a’, both a hot displacer  12  and a cold displacer  14  are at their upper positions within a cylinder  10 . In state ‘b’ in  FIG. 3 , cold displacer  14  moves to its lower position. A change from state ‘a’ to state ‘b’ is a first process. From state ‘b’ to state ‘c’, hot displacer  12  moves from its upper to its lower position, i.e., a second process. In moving from state ‘c’ back to state ‘a’, both hot displacer  12  and cold displacer  14  move upwards, which is a third process. 
         [0032]    In the cycle illustrated in  FIG. 4 , hot displacer  12  and cold displacer  14  are in a central space within cylinder  10  at different points in the cycle. That is, at state ‘a’, cold displacer  14  is in the central space in cylinder  10  and at state ‘c’, hot displacer  12  is in the central space in cylinder  10 . The heat pump in  FIG. 3  is suitable for a three-process cycle. A heat pump that would allow a four-process cycle is similar to that in  FIG. 3 , except that the cylinder is elongated, the reason for which will become clear from the discussion below. 
         [0033]    A four-process cycle for use in a Vuilleumier heat pump is shown in  FIG. 5  in which a hot displacer  22  reciprocates within a hot displacer cylinder  20  and a cold displacer  24  reciprocates with a cold displacer cylinder  21 . At state ‘d’, a hot displacer  22  is at its central position within cylinder  20  and a cold displacer  24  is at its central position within cylinder  21 . In going from state ‘d’ to state ‘e’, hot displacer  22  moves to its remote position within cylinder  20 . This is a first process or process one. In going from state ‘e’ to ‘f’, cold displacer  24  moves to its remote position within cylinder  21 . This is a second process or process two. From state T to ‘g’, hot displacer  22  moves to its central position within cylinder  20 ; a third process or process three. In moving from state ‘g’ to back to state ‘d’, cold displacer  24  moves to its central position within cylinder  21 , undergoing a fourth process or process four. 
         [0034]    As discussed above, in the three-process cycle in  FIG. 4 , hot displacer  12  and cold displacer  14  occupy the same space but, of course, at different times during the cycle. In the four-process cycle of  FIG. 5 , hot displacer  22  and cold displacer  24  do not cross a center line  26 . Cylinders  20  and  21  are collinear and of the same diameter and are denoted by cylinder  20  being above center line  26  and cylinder  21  being below center line  26 . 
         [0035]    The displacer movement end positions illustrated in  FIG. 4  are shown as a function of time in  FIG. 6 . The movement of the lower edge of the hot displacer is shown as curve  16 . The movement of the upper edge of the cold displacer is shown as curve  18 . The cold displacer moves downward in going from state ‘a’ to state ‘b’ while the hot displacer is stationary. From ‘b’ to ‘c’, the hot displacer moves downward while the cold displacer is stationary. And from ‘c’ to ‘a’, which completes the cycle, both displacers move upward. 
         [0036]    The displacer movement end positions illustrated in  FIG. 5  are shown as a function of time in  FIG. 7 . The lower edge of the hot displacer is plotted as curve  28  and the upper edge of the cold displacer is plotted as curve  30 . At state ‘d’, the displacers are both in their central positions and proximate each other. From state ‘d’ to state ‘e’, the cold displacer remains stationary and the hot displacer moves upward. From ‘e’ to T, the hot displacer remains stationary and the cold displacer moves downward. From T to ‘g’, the hot displacer moves downward and the cold displacer remains stationary. From ‘g’ to return to the starting position ‘d’, the hot displacer remains stationary and the cold displacer moves upward. The cycle in  FIG. 6  is completed in three processes and the cycle in  FIG. 7  is completed in four processes. Thus, if the displacers move at the same speed in the cycle in  FIG. 6  as in  FIG. 7 , the cycle in  FIG. 7  takes longer, about 1⅓ times longer to complete than the cycle in  FIG. 6  when the displacers have the same dynamics. 
         [0037]    An alternative to the cycle in  FIG. 7  is a cycle shown in  FIG. 8  in which the movements of the displacers overlap slightly. The upper edge of the hot displacer movement is illustrated by curve  32 ; the lower edge of the cold displacer is illustrated by curve  34 . At time  220  in  FIG. 8 , the cold displacer is finishing its upward movement and the hot displacer is starting its upward movement. At time  222 , the cold displacer has attained its upper position (its remote position) and remains there until time  224 . At time  224 , the hot displacer has not yet arrived at the upper position (its remote position), which happens at time  226 . Meanwhile, the cold displacer finishes the upward travel during time  224  to  226 . The hot displacer is stationary at its upper position from  226  to  228 . The cold displacer completes the downward travel at time  230  and then stays at the lower position until time  232 . Meanwhile, the hot displacer moves downwardly from time  228  through time  234 . At time  232 , the cold displacer moves upwardly through time  234 , time  220 ′, and time  222 ′. The hot displacer remains stationary from time  234  through time  220 ′. At time  220 ′, a complete cycle has been completed; the positions of the displacers are the same at time  220  as at time  220 ′. 
         [0038]    The rate at the displacers move is determined by the spring constants and other properties of the system. As the illustrations in  FIGS. 7 and 8  refer to the same configuration, the displacers move at the same rate in  FIGS. 7 and 8 . However, because movement in the hot displacer is initiated before the cold displacer attains its extreme position and vice versa in the cycle shown in  FIG. 8 , the  FIG. 8  cycle occurs in less time than that in  FIG. 7 . Such a cycle provides a higher output. 
         [0039]    The discussion of cycles in regards to  FIGS. 6-8  describe the highest output cycles that are possible. To obtain a downturn in output, both displacers remain stationary for a period between portions of the cycle. An example of such displacer movement is shown in  FIG. 9 . The hot displacer movement is shown as curve  260  and the cold displacer movement is shown as curve  262 . At time  240 , both displacers are in their central positions within their cylinders. The hot displacer moves upward between time  240  and time  242 . Both displacers are stationary between time  242  and time  244 . The duration can be shorter or longer than that shown in  FIG. 9 . Other intervals during which both displacers are stationary are between time  246  and time  248  and between time  250  and time  252 . Again, these can be shorter or longer to meet demanded output. Furthermore, the interval during which the displacers may be different in different parts of the cycle. E.g., the interval between time  242  and time  244  when the hot displacer is at its remote position and the cold displacer is at its central position can be of a different length than either of the other intervals: time  246  to time  248  or time  250  to time  252 . 
         [0040]    A Vuilleumier heat pump in which the diameters of the cylinders are different is shown in  FIG. 10 . A hot displacer cylinder  28  has a greater diameter than cold displacer cylinder  30 . A hot displacer  32  that reciprocates within hot displacer cylinder  28  is also greater than cold displacer  34  that reciprocates within cold displacer cylinder  32 . A heat pump in which the strokes are different is shown in  FIG. 11 . A hot displacer cylinder  40  has a hot displacer  42 ; and a cold displacer cylinder  41  has a cold displacer  44 . The stroke of hot displacer  42  is less than the stroke of cold displacer  44 . 
         [0041]    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.