Patent Publication Number: US-9850895-B2

Title: Liquid pump and rankine cycle apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a liquid pump and a rankine cycle apparatus including the liquid pump. 
     2. Description of the Related Art 
     Energy systems that use natural energy sources such as sunlight or exhaust heat have attracted attention recently. One example of such energy systems is a rankine cycle system. In a typical rankine cycle system, an expander is activated by a high-temperature and high-pressure working fluid to generate electricity. The high-temperature and high-pressure working fluid is generated by a pump and a heat source (solar heat, geothermal heat, and exhaust heat from automobiles, for example). Thus, a liquid pump is used in the rankine cycle system. 
     As illustrated in  FIG. 7 , Japanese Patent No. 2977228 describes a canned refrigerant pump  300 . The canned refrigerant pump  300  includes a scroll pump  320  as a positive displacement pump mechanism. The scroll pump  320  includes a fixed scroll  321  and an orbiting scroll  322 . Rotational movement of the orbiting scroll  322  allows a refrigerant to be drawn through a suction pipe  333  and ejected into an ejection chamber  329 . Some of the refrigerant in the ejection chamber  329  flows through a first groove  348  or a second groove  349  as a lubricating refrigerant. As a result, a thrust bearing  330   a  and a surface of a bearing  309   a  are lubricated. Then, the refrigerant further flows into a space  343   a . A major part of the refrigerant in the ejection chamber  329  flows into the space  343   a , which is defined by a sealed case  306 , through a through hole  338 , a back pressure chamber  337 , and a case communication hole  344 . Then, the refrigerant in the space  343   a  flows into a space  343   b  through a passage  345  or a communication groove  350 . The refrigerant in the space  343   b  is expelled through a discharge pipe  347 . 
     As illustrated in  FIG. 8 , Japanese Unexamined Patent Application Publication No. 2001-41175 describes a liquid refrigerant pump  500 . The liquid refrigerant pump  500  includes a sealed container  501 , an electrical motor  502 , and a positive displacement pump mechanism  503 . The electrical motor  502  and the positive displacement pump mechanism  503  are disposed in the sealed container  501 . The positive displacement pump mechanism  503  includes a crankshaft  504 , a rolling piston  506 , and a cylinder block  570  fixed to the sealed container  501 . Rotary drive of the crankshaft  504  by the electrical motor  502  allows a liquid refrigerant to be drawn to the positive displacement pump mechanism  503  through a suction pipe  520  and an inlet  521  and allows the liquid refrigerant in a compressor  514  in the positive displacement pump mechanism  503  to be expelled through an outlet  523  and a discharge pipe  522 . In the liquid refrigerant pump  500 , the liquid refrigerant in the compressor  514  in the cylinder block  570  leaks to outside the cylinder block  570  through a groove  551 . The leaked liquid refrigerant is mixed into a liquid refrigerant E stored in the sealed container  501  as a lubricant. 
     SUMMARY 
     An improvement in reliability is desired in the canned refrigerant pump  300  described in Japanese Patent No. 2977228 and in the liquid refrigerant pump  500  described in Japanese Unexamined Patent Application Publication No. 2001-41175. 
     One non-limiting and exemplary embodiment provides a highly reliable liquid pump. 
     In one general aspect, the techniques disclosed here feature a liquid pump including: a container; a shaft disposed in the container; a bearing supporting the shaft; a pump mechanism disposed in the container to pump a liquid by rotation of the shaft; a storage space defined in the container at a position outside the pump mechanism, the storage space storing the liquid to be taken into the pump mechanism or the liquid to be discharged to outside of the container after being expelled from the pump mechanism; and a liquid supply passage including an inlet open facing to the storage space and supplying at least some of the liquid stored in the storage space to the bearing. 
     The above-described liquid pump has high reliability. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view illustrating a liquid pump according to an embodiment of the present disclosure; 
         FIG. 2  is a transverse cross-sectional view illustrating the liquid pump taken along a line II-II in  FIG. 1 ; 
         FIG. 3  is a magnified vertical cross-sectional view illustrating a portion of the liquid pump illustrated in  FIG. 1 ; 
         FIG. 4  is a configuration diagram of a rankine cycle apparatus according to an embodiment of the present disclosure; 
         FIG. 5  is a vertical cross-sectional view illustrating a liquid pump according to a modification; 
         FIG. 6  is a vertical cross-sectional view illustrating a liquid pump according to another modification; 
         FIG. 7  is a cross-sectional view illustrating a conventional canned refrigerant pump; and 
         FIG. 8  is a cross-sectional view illustrating a conventional liquid refrigerant pump. 
     
    
    
     DETAILED DESCRIPTION 
     As a liquid pump used in a rankine cycle system, for example, a positive displacement pump such as a gear pump or a rotary pump or a velocity pump such as a centrifugal pump is used in some cases. In such a liquid pump, if cavitation occurs in a liquid for lubricating a bearing, damage to the bearing may be caused. This lowers reliability of the liquid pump, leading to a decrease in pump efficiency. 
     Cavitation is a phenomenon in which a working fluid in liquid state in a fluid machine boils to generate microbubbles when a local pressure on the working fluid reaches a saturated vapor pressure. An impact pressure caused by bubble collapse may cause erosion in a component of the fluid machine. If such a phenomenon occurs in a bearing, the surface pressure on the bearing varies locally, which lowers the permissible load on the bearing. This may cause component wear. 
     In the canned refrigerant pump  300  described in Japanese Patent No. 2977228, some of the refrigerant in the ejection chamber  329  flows through the first groove  348  or the second groove  349  as a lubricating refrigerant. In the canned refrigerant pump  300 , the bearing is lubricated by the refrigerant flowing in the positive displacement pump mechanism at a position upstream of the case communication hole  344  through which the refrigerant is ejected from the scroll pump  320 , which is the positive displacement pump mechanism, into the space  343   a  in the sealed case  306 . The first groove  348  or the second groove  349  is not exactly adjacent to a space having a sufficiently large capacity and being filled with a fluid for lubricating the bearing. In this configuration, variation in the rotation frequency of the scroll pump  320  may result in short supply of the refrigerant to the bearing. This may cause component wear. In addition, since the refrigerant in the ejection chamber  329  is in liquid state, the refrigerant to be supplied to the bearing has a large pressure pulsation. This results in variations in the permissible load on the bearing, which may cause component wear, and results in an increase in friction loss, which may lower the pump efficiency. 
     In the liquid refrigerant pump  500  described in Japanese Unexamined Patent Application Publication No. 2001-41175, the liquid refrigerant that has leaked to the outside of the cylinder block  570  through the groove  551  is mixed into the lubricating liquid refrigerant E. However, a major part of the liquid refrigerant in the positive displacement pump mechanism  503  is expelled through the outlet  523  and the discharge pipe  522 . The liquid refrigerant in the positive displacement pump mechanism  503  is not entirely stored as the lubricating liquid refrigerant E. In the configuration in which the liquid leaks to the outside of the cylinder block  570  through the groove  551 , variation in the rotation frequency of the crankshaft  504  may result in short supply of the lubricating liquid refrigerant to the bearing of the crankshaft  504 . This may cause component wear. 
     A first aspect of the present disclosure provides a liquid pump including:
         a container;   a shaft disposed in the container;   a bearing supporting the shaft;   a pump mechanism disposed in the container to pump a liquid by rotation of the shaft;   a storage space defined in the container at a position outside the pump mechanism, the storage space storing the liquid to be taken into the pump mechanism or the liquid to be discharged to outside of the container after being expelled from the pump mechanism; and   a liquid supply passage including an inlet open facing to the storage space and supplying at least some of the liquid stored in the storage space to the bearing.       

     In the first aspect, the storage space stores the liquid to be taken into the pump mechanism or the liquid to be discharged to the outside of the container after being expelled from the pump mechanism, and the inlet of the liquid supply passage is open to the storage space. In this configuration, a large amount of the liquid is supplied to the storage space. In addition, since the storage space has a predetermined capacity, the pressure pulsation of the liquid is reduced and cavitation is unlikely to occur in the liquid to be supplied to the bearing. This reduces the variation in the permissible load on the bearing and prevents damage to the bearing. As a result, the liquid pump according to the first aspect has high reliability. In addition, since the container does not need to have a storage space provided especially for a liquid lubricating the bearing, the liquid pump has a simple structure. This reduces the production cost of the liquid pump. 
     A second aspect of the present disclosure according to the first aspect provides the liquid pump in which the storage space includes an inlet storage space for storing the liquid to be taken into the pump mechanism and an outlet storage space for storing the liquid to be discharged to the outside of the container after being expelled from the pump mechanism. In the second aspect, the capacity of the storage space in the container is large, and thus the occurrence of cavitation in the liquid to be supplied to the bearing is advantageously reduced. In addition, the pressure pulsation of each of the liquid to be taken into the pump mechanism and the liquid to be discharged to the outside of the container after being expelled from the pump mechanism is reduced. This improves the reliability of the bearing, and eventually the reliability of the liquid pump. 
     A third aspect of the present disclosure according to the second aspect provides the liquid pump in which the bearing includes a first bearing and a second bearing supporting the shaft at different positions in an axial direction of the shaft, and the liquid supply passage has an inlet liquid supply passage supplying at least some of the liquid stored in the inlet storage space to the first bearing and an outlet liquid supply passage supplying at least some of the liquid stored in the outlet storage space to the second bearing. In the third aspect, the inlet liquid supply passage and the outlet liquid supply passage enable the liquid to be supplied from the corresponding storage spaces to the first bearing and the second bearing. In addition, since the liquid supply passage has a simple structure, the production cost of the liquid pump is reduced. 
     A fourth aspect of the present disclosure according to any one of the first to third aspects provides the liquid pump in which the shaft has the liquid supply passage inside of the shaft. In the fourth aspect, the liquid supply passage is positioned close to the bearing, and thus the length of the liquid supply passage is short. This reduces pressure loss of the liquid flowing through the liquid supply passage. As a result, cavitation is unlikely to occur in the liquid supplied to the bearing. 
     A fifth aspect of the present disclosure according to any one of the first to fourth aspects provides the liquid pump further including a pressure boost mechanism that increases a pressure of the liquid to be supplied to the bearing through the liquid supply passage. In the fifth aspect, the liquid to be supplied to the bearing is a high-pressure liquid and the pressure is sufficiently higher than the pressure at which cavitation occurs, and thus cavitation is more unlikely to occur in the liquid supplied to the bearing. 
     A sixth aspect of the present disclosure according to the fifth aspect provides the liquid pump in which the pressure boost mechanism includes a flow path extending in the shaft in a radial direction of the shaft. In the sixth aspect, centrifugal force generated by the rotation of the shaft increases the pressure of the liquid flowing through the flow path extending in the radial direction of the shaft. As a result, cavitation is unlikely to occur in the liquid supplied to the bearing. In addition, the pressure boost mechanism has a simple configuration. 
     A seventh aspect of the present disclosure according to any one of the first to sixth aspects provides the liquid pump in which the shaft has at least one end open facing to the storage space. In the seventh aspect, the liquid that has lubricated the bearing returns to the storage space in a shorter time, because the bearing is typically positioned close to the end of the shaft. This configuration allows the liquid that has lubricated the bearing to be readily expelled from the bearing. Thus, if the liquid supplied to the bearing contains a foreign substance, the foreign substance can be readily eliminated. As a result, damage to the bearing is prevented. 
     An eighth aspect of the present disclosure according to any one of the first to seventh aspect provides the liquid pump further including a motor disposed in the storage space and fixed to the shaft. In the eighth aspect, loss due to the connection between the motor and the shaft is reduced, and thus pump efficiency is improved. In addition, a gap between the motor and the shaft due to the connection between the motor and the shaft is reduced, and eccentric rotation of the shaft due to misalignment between the rotation axis of the motor and the axis of the shaft is reduced. This improves the reliability of the bearing, and eventually the reliability of the liquid pump. 
     A ninth aspect of the present disclosure according to any one of the first to eighth aspects provides a rankine cycle apparatus including:
         the liquid pump according to any one of the first to eight aspects;   a heater that heats a working fluid;   an expander that expands the working fluid heated by the heater; and   a radiator that releases heat of the working fluid expanded by the expander, wherein   the liquid pump takes in as the liquid the working fluid flowing from the radiator in liquid state by using the pump mechanism and pumps out the liquid to the heater.       

     In the rankine cycle, the working fluid flowing from the radiator is preferably a supercooled liquid or a saturated liquid having the lowest degree of supercooling to improve efficiency in the rankine cycle. In such a case, the state of the working fluid changes to a gas-liquid two-phase state when the pressure of the working fluid slightly decreases or the working fluid is slightly heated. However, in the ninth aspect, cavitation does not occur in the liquid supplied to the bearing even if such a working fluid is supplied to the liquid pump. Thus, the liquid pump has high reliability even when the rankine cycle apparatus is in high-efficiency operation. 
     Hereinafter, an embodiment of the present disclosure is described with reference to the drawings. The following is a description of an example of the present disclosure, and the present disclosure is not limited by the description. 
     Liquid Pump 
     As illustrated in  FIG. 1 , a liquid pump  1   a  includes a container  10 , a shaft  30 , a bearing  40 , a pump mechanism  20 , a storage space  50 , and a liquid supply passage  60 . The container  10  is a pressure-resistant sealed container, for example. The shaft  30  is disposed in the container  10 . The shaft  30  extends in a vertical direction when the liquid pump  1   a  is mounted on a horizontal surface, for example. The liquid pump  1   a  may be configured so as to extend in a horizontal direction when the liquid pump  1   a  is mounted on the horizontal surface. The bearing  40  supports the shaft  30 . The bearing  40  is a plain bearing. The pump mechanism  20  is disposed in the container  10  so as to pump the liquid by rotation of the shaft  30 . The storage space  50  is defined in the container  10  at a position outside the pump mechanism  20  and stores the liquid to be taken into the pump mechanism  20  or the liquid to be discharged to the outside of the container  10  after being expelled from the pump mechanism  20 . The liquid supply passage  60  has an inlet open to the storage space  50  and allows at least some of the liquid stored in the storage space  50  to be supplied to the bearing  40  therethrough. 
     The storage space  50  is configured to store all the liquid passing through the liquid pump  1   a  for a predetermined time. This configuration enables an adequate amount of the liquid to be continuously supplied to the storage space  50  while the liquid pump  1   a  is in operation. 
     The storage space  50  may have any capacity larger than that of an internal space of the pump mechanism  20 , and may be forty times, preferably one-hundred times larger than that of the internal space of the pump mechanism  20 , for example. The average time the liquid takes, during the operation of the liquid pump  1   a , to pass through the pump mechanism  20  is defined as tp, and the average time the liquid takes to pass through the storage space  50  is defined as ts. The storage space  50  preferably satisfies ts&gt;5tp. The storage space  50  having the predetermined capacity is likely to reduce pressure pulsation caused by the liquid flowing into and out of the storage space  50 . In addition, since the inlet of the liquid supply passage  60  is open to the storage space  50 , the liquid having reduced pressure variation is supplied to the bearing  40 . Thus, the liquid is unlikely to vary in pressure at the bearing  40  and cavitation is unlikely to occur. 
     The pump mechanism  20  has an inlet hole  21   a  and an outlet hole  22   a . The inlet hole  21   a  allows the liquid to be supplied to the internal space of the pump mechanism  20  and is open to the outside of the pump mechanism  20 . The outlet hole  22   a  allows the liquid to be expelled to the outside of the pump mechanism  20  and is open to the outside of the pump mechanism  20 . The liquid pump  1   a  further includes a supply pipe  11  and a discharge pipe  13 , for example. The supply pipe  11  and the discharge pipe  13  are each attached to the container  10  so as to extend through the wall of the container  10 . The liquid pump  1   a  is a sealed pump. The internal space of the container  10  is allowed to be in communication with an external space of the container  10  only through the supply pipe  11  and the discharge pipe  13 . The liquid to be taken into the pump mechanism  20  is supplied to the internal space of the container  10  through the supply pipe  11 . The liquid to be discharged to the outside of the container  10  after being expelled from the pump mechanism  20  is discharged to the outside of the container  1  through the discharge pipe  13 . 
     As illustrated in  FIG. 1 , the storage space  50  includes an inlet storage space  51  and an outlet storage space  53 , for example. The inlet storage space  51  stores the liquid to be taken into the pump mechanism  20 . The inlet hole  21   a  of the pump mechanism  20  is open to the inlet storage space  51  and the supply pipe  11  has an end open to the inlet storage space  51 . The outlet storage space  53  stores the liquid to be discharged to the outside of the container  10  after being expelled from the pump mechanism  20 . The outlet hole  22   a  of the pump mechanism  20  is open to the outlet storage space  53  and the discharge pipe  13  has an end open to the outlet storage space  53 . Thus, the pressure of the liquid in the outlet storage space  53  is higher than that of the liquid in the inlet storage space  51 . 
     Each of the inlet storage space  51  and the outlet storage space  53  may have any capacity larger than that of the internal space of the pump mechanism  20 , and may be twenty times, preferably fifty times larger than that of the internal space of the pump mechanism  20 , for example. The average time the liquid takes, during the operation of the liquid pump  1   a , to pass through the pump mechanism  20  is defined as tp, and the average time the liquid takes to pass through each of the inlet storage space  51  and the outlet storage space  53  is defined as ts1 and ts2, respectively. The inlet storage space  51  and the outlet storage space  53  preferably satisfy ts1&gt;2tp and ts2&gt;2tp, respectively. The inlet storage space  51  and the outlet storage space  53  each having the predetermined capacity are likely to reduce the pressure pulsation caused by the liquid flowing into and out of the inlet storage space  51  and the outlet storage space  53 . In addition, most of the internal space of the pump mechanism  20  can be used as the storage space  50 . 
     As illustrated in  FIG. 1 , the bearing  40  includes a first bearing  41  and a second bearing  43 . The first bearing  41  and the second bearing  43  support the shaft  30  at different axial positions of the shaft  30 . The first bearing  41  and the second bearing  43  are disposed adjacent to the inlet storage space  51  and the outlet storage space  53 , respectively, for example. In such a case, the liquid supply passage  60  includes an inlet liquid supply passage  61  and an outlet liquid supply passage  63 . The inlet liquid supply passage  61  is a flow path through which at least some of the liquid stored in the inlet storage space  51  is supplied to the first bearing  41  and has an inlet open to the inlet storage space  51 . The outlet liquid supply passage  63  is a flow path through which at least some of the liquid stored in the outlet storage space  53  is supplied to the second bearing  43  and has an inlet open to the outlet storage space  53 . This configuration enables the liquid to be supplied from the corresponding storage spaces to the first bearing  41  and the second bearing  43 . In addition, the configuration of the liquid supply channel  60  is simple. 
     The pump mechanism  20  is an internal gear pump, for example. The pump mechanism  20  may be any gear pump other than the internal gear pump, and may be a piston pump, a vane pump, a rotary pump, a positive displacement pump such as a scroll pump, a velocity pump such as a centrifugal pump, a mixed flow pump, or an axial flow pump, or a screw pump. As illustrated in  FIG. 1 , the pump mechanism  20  includes a lower bearing member  21 , an upper bearing member  22 , a pump case  23 , an outer gear  24 , and an inner gear  25 , for example. The lower bearing member  21  and the upper bearing member  22  are plate-shaped members. The lower bearing member  21  and the upper bearing member  22  support the shaft  30  in a rotatable manner. A portion of the lower bearing member  21  that faces the shaft  30  functions as the first bearing  41  and a portion of the upper bearing member  22  that faces the shaft  30  functions as the second bearing  43 , for example. The shaft  30  extends through the center of each of the lower bearing member  21  and the upper bearing member  22 . The inlet hole  21   a  and the outlet hole  22   a  extend through the lower bearing member  21  and the upper bearing member  22 , respectively, in the thickness direction thereof. 
     The pump case  23 , the outer gear  24 , and the inner gear  25  are sandwiched between the lower bearing member  21  and the upper bearing member  22 . As illustrated in  FIG. 2 , the outer gear  24  and the inner gear  25  are disposed in the pump case  23 . The outer gear  24  surrounds the inner gear  25 . Teeth of the outer gear  24  are meshed with teeth of the outer gear  25 . The inner gear  25  is fixed to the shaft  30 . Thus, the rotation of the shaft  30  rotates the inner gear  25 . The rotation axis of the inner gear  25  is coincident with the rotation axis of the shaft  30 . The rotation axis of the outer gear  24  is displaced from the rotation axis of the shaft  30 . When the inner gear  25  rotates together with the shaft  30 , the teeth of the inner gear  25  push the outer gear  24  so that the outer gear  24  rotates together with the inner gear  25 . 
     In the pump mechanism  20 , the lower bearing member  21 , the upper bearing member  22 , the outer gear  24 , and the inner gear  25  define an operation chamber  26 . The rotation of the outer gear  24  and the inner gear  25  with the shaft  30  allows the pump mechanism  20  to repeatedly perform an inlet process and an output process. In other words, the rotation of the outer gear  24  and the inner gear  25  shifts a state of the operation chamber  26  from an inlet chamber  26   a  to an outlet chamber  26   c  or from the outlet chamber  26   c  to the inlet chamber  26   a . The inlet chamber  26   a  is a space of the operation chamber  26  and is in communication with the inlet hole  21   a . The outlet chamber  26   c  is a space of the operation chamber  26  and is in communication with the outlet hole  22   a . The capacity of the inlet chamber  26   a  increases as the shaft  30  rotates in the inlet process, and the inlet process terminates at the end of the communication between the inlet chamber  26   a  and the inlet hole  21   a . Further rotation of the shaft  30  allows the operation chamber  26  after the inlet process to be in communication with the outlet hole  22   a , which shifts the state of the operation chamber  26  to the outlet chamber  26   c . The capacity of the outlet chamber  26   c  decreases as the shaft  30  rotates. The outlet process terminates at the end of the communication between the outlet chamber  26   c  and the outlet hole  22   a . Due to the rotation of the shaft  30 , the liquid is taken into the pump mechanism  20  through the inlet hole  21   a  and expelled from the pump mechanism  20  through the outlet hole  22   a.    
     The pump mechanism  20  is fixed to the container  10  by an outer end portion of the upper bearing member  22  welded to an inner surface of the container  10 , for example. The upper bearing member  22  divides the internal space of the container  10  into the inlet storage space  51  and the outlet storage space  53 . The supply pipe  11  is attached to the container  10  at a position below the upper bearing member  22 , which is a side adjacent to the inlet hole  21   a , and the discharge pipe  13  is attached to the container  10  at a position above the upper bearing member  22 . The pump mechanism  20  may be fixed to the container  10  by an outer end portion of the lower bearing member  21  or an outer end portion of the pump case  23  welded to the inner surface of the container  10 . In such a case, the internal space of the container  10  is divided into the inlet storage space  51  and the outlet storage space  53  by the lower bearing member  21  or the pump case  23 . The inner surface of the container  10  defines only the storage space  50 . Specifically, the inner surface of the container  10  defines only the inlet storage space  51  and the outlet storage space  53 , for example. 
     As illustrated in  FIG. 1 , the liquid supply passage  60  extends in the shaft  30 , for example. The inlet liquid supply passage  61  includes a main channel  61   a  and an auxiliary channel  61   b , for example. The main channel  61   a  extends in the shaft  30  from the end of the shaft  30 , which is open to the inlet storage space  51 , in the axial direction of the shaft  30 . The auxiliary channel  61   b  extends from the main channel  61   a  in a radial direction of the shaft  30  so as to be in communication with a space between the shaft  30  and the first bearing  41 . The outlet liquid supply passage  63  includes a main channel  63   a  and an auxiliary channel  63   b , for example. The main channel  63   a  extends in the shaft  30  from the end of the shaft  30 , which is open to the outlet storage space  53 , in the axial direction of the shaft  30 . The auxiliary channel  63   b  extends from the main channel  63   a  in the radial direction of the shaft  30  so as to be in communication with a space between the shaft  30  and the second bearing  43 . This configuration enables the liquid stored in the inlet storage space  51  to be supplied to the first bearing  41  through the internal space of the shaft  30  and the liquid stored in the outlet storage space  53  to be supplied to the second bearing  43  through the internal space of the shaft  30 . As a result, the first bearing  41  and the second bearing  43  are lubricated by the liquid. 
     Since the liquid supply passage  60  extends in the shaft  30 , the liquid supply passage  60  is positioned close to the bearing  40 , and thus the length of the liquid supply passage  60  is short. This reduces pressure loss of the liquid flowing in the liquid supply passage  60 . As a result, cavitation is unlikely to occur in the liquid supplied to the bearing  40 . This advantage is more likely to be obtained when the bearing  40  supports the shaft  30  at a portion close to the end of the shaft  30 . In addition, the shaft  30  is efficiently cooled by the liquid flowing through the liquid supply passage  60 . The liquid supply passage  60  is not particularly limited and may be any flow path for supplying the liquid stored in the storage space  50  to the bearing  40 . The liquid supply passage  60  may be a spiral groove on an outer surface of the shaft  30  or a groove on a bearing surface of the bearing  40 . 
     The liquid pump  1   a  further includes a pressure boost mechanism  70 , for example. The pressure boost mechanism  70  boosts the pressure of the liquid to be supplied to the bearing  40  through the liquid supply passage  60 . The pressure boost mechanism  70  includes a flow path extending in the shaft  30  in the radial direction of the shaft  30 , for example. As illustrated in  FIG. 1 , the pressure boost mechanism  70  is constituted by the auxiliary channel  61   b  of the inlet liquid supply channel  61  or the auxiliary channel  63   b  of the outlet liquid supply channel  63 , for example. As illustrated in  FIG. 3 , the liquid is supplied to the bearing  40 , for example. The rotation of the shaft  30  generates centrifugal force. The centrifugal force acts on the liquid flowing through the auxiliary channel  61   b  or the auxiliary channel  63   b  such that the liquid at the increased pressure is supplied to the first bearing  41  or the second bearing  43 . The liquid to be supplied to the first bearing  41  or the second bearing  43  is a high-pressure liquid and the pressure is sufficiently higher that the pressure at which cavitation may occur. As a result, cavitation is unlikely to occur in the liquid supplied to the first bearing  41  or the second bearing  43  even if the pressure of the liquid is varied in the first bearing  41  or the second bearing  43 . As a result, damage to the bearing  40  is prevented. As illustrated in  FIG. 3 , the liquid supplied to the first bearing  41  is expelled to the inlet storage space  51  through the space between the first bearing  41  and the shaft  30 , and the liquid supplied to the second bearing  43  is expelled to the outlet storage space  53  through the space between the second bearing  43  and the shaft  30 . 
     The pressure boost mechanism  70  is not particularly limited, and may be any mechanism that can boost the pressure of the liquid to be supplied to the bearing  40  through the liquid supply passage  60 . The pressure boost mechanism  70  may be a gear pump disposed adjacent to the end of the shaft  30 , for example. 
     As illustrated in  FIG. 1 , at least one of the ends of the shaft  30  is open to the storage space  50 , for example. One of the ends of the shaft  30  is open to the inlet storage space  51 , for example. The first bearing  41  is disposed adjacent to the end of the shaft  30 . In this configuration, the liquid that has lubricated the first bearing  41  returns to the inlet storage space  51  through the short passage. This configuration allows the liquid that has lubricated the first bearing  41  to be readily expelled from the first bearing  41 . Thus, if the liquid supplied to the first bearing  41  contains a foreign substance, the foreign substance can be readily eliminated. As a result, damage to the bearing is prevented. 
     As illustrated in  FIG. 1  the liquid pump  1   a  includes a motor  80 . The motor  80  is connected to the pump mechanism  20  through the shaft  30  so as to activate the pump mechanism  20 . The motor  80  is disposed in the storage space  50  and is fixed to the shaft  30 , for example. Specifically, the motor  80  includes a rotor  81  and a stator  83 . The shaft  30  is fixed to the motor  80  with the shaft  30  being in contact with the rotor  81 . In other words, the shaft  30  is directly connected to the motor  80  without a connecting member. With this configuration, the rotation axis of the motor  80  is minimally displaced with respect to the axis of the shaft  30 . This reduces sliding loss between the shaft  30  and the first bearing  41  or the second bearing  43 , and thus wear of each of the shaft  30 , the first bearing  41 , and the second bearing  43  is reduced. As a result, the liquid pump  1   a  has high reliability. The stator  83  is fixed to the inner surface of the container  10 . The motor  80  is disposed in the outlet storage space  53 . The liquid pump  1   a  further includes a terminal  15  for supplying electricity to the motor  80 . The terminal  15  is attached to an upper portion of the container  10 . When electricity is supplied to the motor  80 , the shaft  30  rotates together with the rotor  81 , and the pump mechanism  20  operates as described above. 
     Rankine Cycle Apparatus 
     A rankine cycle apparatus  100  including the liquid pump  1   a  is described. As illustrated in  FIG. 4 , the rankine cycle apparatus  100  includes the liquid pump  1   a , a heater  2 , an expander  3 , and a radiator  4 . The rankine cycle apparatus  100  has flow paths  6   a ,  6   b ,  6   c , and  6   d  through which the liquid pump  1   a , the heater  2 , the expander  3 , and the radiator  4  are connected in this order in a ring shape. The flow path  6   a  extends between an outlet of the liquid pump  1   a  and an inlet of the heater  2 . The discharge pipe  13  is at least a portion of the flow path  6   a . The flow path  6   b  extends between an outlet of the heater  2  and an inlet of the expander  3 . The flow path  6   c  extends between an outlet of the expander  3  and an inlet of the radiator  4 . The flow path  6   d  extends between an outlet of the radiator  4  and an inlet of the liquid pump  1   a . The supply pipe  11  is at least a portion of the flow path  6   d.    
     An organic working fluid is preferably used as the working fluid of the rankine cycle apparatus  100 , for example, but the working fluid is not limited to an organic working fluid. The organic working fluid may be an organic compound such as a hydrogen halide, a carbon hydride, or an alcohol. Examples of a hydrogen halide include R-123, R365mfc, and R-245fa. Examples of a carbon hydride include propane, butane, pentane, and isopentane, which are alkanes. Examples of an alcohol include ethanol. The organic working fluid may be used alone, or two or more of the organic working fluids may be used in combination. Alternatively, the working fluid may be an inorganic working fluid such as water, carbon dioxide, or ammonia. 
     The heater  2  heats the working fluid in the rankine cycle. The heater  2  absorbs thermal energy from a heat medium such as geothermally heated water, combustion gas, or exhaust gas from a boiler or a furnace, and heats and evaporates the working fluid with the thermal energy. A flow path  2   a  for the heat medium is connected to the heater  2 . In the case where the heat medium is a liquid such as heated water, a plate heat exchanger or a double pipe heat exchanger is preferably used as the heater  2 . In the case where the heat medium is a gas such as a combustion gas or exhaust gas, a fin tube heat exchanger is preferably used as the heater  2 . In  FIG. 4 , solid arrows each indicate a flow direction of the working fluid, and dashed arrows each indicate a flow direction of the heat medium. 
     The expander  3  is a fluid machine that expands the working fluid heated by the heater  2 . The rankine cycle apparatus  100  further includes an electric generator  5 . The electric generator  5  is connected to the expander  3 . The working fluid expanded by the expander  3  provides rotational force to the expander  3 . The electric generator  5  converts the rotational force to electricity. The expander  3  may be a positive displacement expander or a velocity expander. Examples of positive displacement expanders include rotary, screw, reciprocating, and scroll expanders. Examples of velocity expanders include centrifugal and axial flow expanders. The expander  3  is typically a positive displacement expander. 
     The radiator  4  releases heat of the working fluid expanded by the expander  3 . Specifically, the heat of the working fluid is transferred to a cooling medium in the radiator  4 . A flow path  4   a  for the cooling medium is connected to the radiator  4 . In  FIG. 4 , one-dotted chain arrows each indicate a flow direction of the cooling medium. The radiator  4  may be a conventional heat exchanger, such as a plate heat exchanger, a double pipe heat exchanger, or a fin tube heat exchanger. The type of the radiator  4  is suitably determined depending on the kind of the cooling medium. In the case where the cooling medium is a liquid such as water, a plate heat exchanger or a double pipe heat exchanger is preferably used. In the case where the cooling medium is a gas such as air, a fin tube heat exchanger is preferably used. 
     The working fluid flowing from the radiator  4  is in liquid state. The working fluid in liquid state is expelled from the radiator  4  and introduced to the internal space of the container  10  through the supply pipe  11 . The liquid pump  1   a  takes in the working fluid in liquid state, which has passed through the radiator  4 , as the above-described liquid and pumps the liquid to the heater  2  by the pump mechanism  20 . The working fluid is pressurized by the liquid pump  1   a , and the pressurized working fluid is supplied to the heater  2  through the flow path  6   a . The working fluid flowing into the liquid pump  1   a  from the radiator  4  is preferably a supercooled liquid or a saturated liquid having the lowest degree of supercooling to improve the efficiency of the rankine cycle. However, the working fluid in such a state may become a two-phase liquid due to a slight reduction in pressure or slight heating. Thus, cavitation may occur in the liquid in the bearing  40  of the liquid pump  1   a  when the pressure of the liquid in the bearing  40  is reduced or the liquid is heated. However, in the liquid pump  1   a  having the above-described configuration, cavitation is unlikely to occur in the liquid in the first bearing  41  and the second bearing  43 , and thus damage to the first bearing  41  and the second bearing  43  is prevented. 
     In addition, since the outlet storage space  53  recovers the heat generated at the motor  80 , the liquid pump  1   a  has high efficiency. As a result, the rankine cycle apparatus  100  has high efficiency. 
     A pressure condition and a temperature condition of the working fluid in the rankine cycle are varied depending on operation conditions of the rankine cycle apparatus. The operation conditions include a temperature of a heat medium flowing into the heater  2 , the amount of heat exchanged between the working fluid and the heat medium in the heater  2 , a temperature of the cooling medium flowing into the radiator  4 , the amount of heat exchanged between the working fluid and the cooling medium in the radiator  4 , and a rotation frequency of the expander  3 . An optimum amount of the working fluid in the rankine cycle apparatus  100  is varied depending on the variation of the operation conditions of the rankine cycle apparatus  100 . Since the liquid pump  1   a  can store a predetermined amount of the working fluid in the liquid state in the inlet storage space  51 , for example, the liquid pump  1   a  can respond to the variation in the optimum amount of the working fluid caused by the variation in the operation conditions. Thus, the rankine cycle apparatus  100  operates with a high cycle efficiency. 
     Modifications 
     Various modifications may be added to the liquid pump  1   a . The liquid pump  1   a  may be modified as a liquid pump  1   b  illustrated in  FIG. 5 , for example. The liquid pump  1   b  has the same configuration as the liquid pump  1   a  unless otherwise specified. Components of the liquid pump  1   b  that are the same as those of the liquid pump  1   a  are assigned reference numerals the same as those of the liquid pump  1   a  and detailed description thereof is omitted in some cases. The description regarding the liquid pump  1   a  is applicable to the liquid pump  1   b  if no technical contradiction occurs. The same is applicable to a liquid pump  1   c , which is described later. 
     As illustrated in  FIG. 5 , the liquid pump  1   b  includes a supply pipe  11   a  instead of the supply pipe  11 . The supply pipe  11   a  is attached to the wall of the container  10 . An end of the supply pipe  11   a  is directly connected to the pump mechanism  20 . In other words, an internal space of the supply pipe  11   a  is in direct communication with the internal space of the inlet hole  21   a . This configuration enables the liquid to flow into the pump mechanism  20  through the supply pipe  11   a  without being stored in a space having a predetermined capacity. 
     The upper bearing member  22  has a communication hole  22   b  positioned radially outward from the pump case  23 . The communication hole  22   b  extends through the upper bearing member  22 . The space above the upper bearing member  22  and the space below the upper bearing member  22  are in communication with each other through the communication hole  22   b  and form the outlet storage space  53 . In such a case, the inner surface of the container  10 , for example, defines only the outlet storage space  53 . The liquid to be discharged to the outside of the container  10  after being expelled from the pump mechanism  20  is stored not only in the space of the outlet storage space  53  positioned above the upper bearing member  22  but also in the space of the outlet storage space  53  positioned below the upper bearing member  22 . Since the outlet storage space  53  has the predetermined capacity, the pressure pulsation, which may be caused by the liquid flowing from and into the outlet storage space  53 , is reduced. In addition, since the inlet of the liquid supply passage  60  is open to the outlet storage space  53 , the liquid having reduced pressure variation is supplied to the bearing  40 . As a result, the pressure variation in the liquid is reduced in the bearing  40 , and cavitation is unlikely to occur. 
     In the liquid pump  1   b , the liquid supply passage  60  includes two outlet liquid supply passages  63 . One of the outlet liquid supply passages  63  is a flow path through which the liquid stored in the space of the outlet storage space  53  positioned below the upper bearing member  22  is supplied to the first bearing  41 , and the other is a flow path through which the liquid stored in the space of the outlet storage space  53  positioned above the upper bearing member  22  is supplied to the second bearing  43 . 
     The liquid pump  1   a  may be modified as a liquid pump  1   c  illustrated in  FIG. 6 . As illustrated in  FIG. 6 , the liquid pump  1   c  includes a discharge pipe  13   a  instead of the discharge pipe  13 . The discharge pipe  13   a  is attached to the wall of the container  10 . An end of the discharge pipe  13   a  is directly connected to the pump mechanism  20 . In other words, an internal space of the discharge pipe  13   a  is in direct communication with the internal space of the outlet hole  22   a . This configuration enables the liquid that has expelled from the outlet hole  22   a  to be discharged to the outside of the liquid pump  1   c  through the discharge pipe  13   a  without being stored in a space having the predetermined capacity. 
     The upper bearing member  22  has a communication hole  22   b  positioned radially outward from the pump case  23 . The communication hole  22   b  extends through the upper bearing member  22 . The space positioned above the upper bearing member  22  and the space positioned below the upper bearing member  22  are in communication with each other through the communication hole  22   b  and form the inlet storage space  51 . In such a case, the inner surface of the container  10 , for example, defines only the inlet storage space  51 . The liquid to be taken into the pump mechanism  20  is stored not only in the space of the inlet storage space  51  positioned below the upper bearing member  22  but also in the space of the inlet storage space  51  positioned above the upper bearing member  22 . Since the inlet storage space  51  has the predetermined capacity, the pressure pulsation, which may be caused by the liquid flowing from and into the inlet storage space  51 , is reduced. In addition, since the inlet of the liquid supply passage  60  is open to the inlet storage space  51 , the liquid having reduced pressure variation is supplied to the bearing  40 . As a result, the pressure variation in the liquid is reduced in the bearing  40 , and cavitation is unlikely to occur. 
     In the liquid pump  1   c , the liquid supply passage  60  includes two inlet liquid supply passages  61 . One of the inlet liquid supply passages  61  is a flow path through which the liquid stored in the space of the inlet storage space  51  positioned below the upper bearing member  22  is supplied to the first bearing  41 , and the other is a flow path through which the liquid stored in the space of the inlet storage space  51  positioned above the upper bearing member  22  is supplied to the second bearing  43 .