Patent Publication Number: US-2012031141-A1

Title: Fluid Machine, and Refrigerant Circuit and Waste Heat Utilization Device Using the Fluid Machine

Description:
TECHNICAL FIELD 
     The present invention relates to a fluid machine, and a refrigerant circuit and a waste heat utilization device using the fluid machine. 
     BACKGROUND ART 
     A waste heat utilization device of this type is provided with a Rankine circuit including an expander which is inserted in a circulation path for circulating a working fluid to produce a drive force by expanding the working fluid whose pressure has been increased by the waste heat of a heat source. The Rankine circuit further includes a high-pressure path section which forms part of the circulation path and in which a high working fluid pressure on the inlet port side of the expander prevails, a low-pressure path section which forms part of the circulation path and in which a low working fluid pressure on the outlet port side of the expander prevails, and pressure holding means for holding the pressure in the high-pressure path section while the operation of the Rankine circuit is stopped. 
     Also, a technique has been disclosed in which three valves are inserted in the circulation path, the valves including an on-off valve (shutoff valve) for accumulating pressure in the high-pressure path section, a control valve (pressure regulating valve) for regulating the pressure in the high-pressure path section, and a relief valve (bypass valve) for releasing abnormally high pressure from the high-pressure path section. The valves are controlled so as to prevent abnormally high pressure from being developed in the high-pressure path section while the Rankine circuit is stopped, and also to improve the starting performance of the Rankine circuit (see Patent Document 1, for example). 
     An expander has also been known which is provided with high- and low-pressure sections for a working fluid, a drive section, a communication passage directly connecting the high- and low-pressure sections to each other while bypassing the drive section, and opening/closing means (bypass valve) for opening and closing the communication passage (see Patent Document 2, for example). 
     PRIOR ART LITERATURE 
     Patent Documents 
     
         
         Patent Document 1: Japanese Laid-open Patent Publication No. 2007-9897 
         Patent Document 2: Japanese Laid-open Patent Publication No. 2007-231855 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In Patent Document 1, the three valves (shutoff valve, pressure regulating valve, and bypass valve) are inserted in the circulation path, with the result that the Rankine circuit is complicated in construction. 
     The three valves may conceivably be arranged within the expander, like the opening/closing means (bypass valve) disclosed in Patent Document 2. This, however, leads to complicated structure of the expander, and a problem still arises in that the size and cost of the expander and the Rankine circuit and thus of the waste heat utilization device cannot be reduced. 
     The present invention was created in view of the above circumstances, and an object thereof is to provide a fluid machine, and a refrigerant circuit and a waste heat utilization device using the fluid machine, whereby the size and cost of a Rankine circuit and thus of the waste heat utilization device can be reduced while preventing an abnormally high pressure from being developed in the Rankine circuit and also improving starting performance of the Rankine circuit. 
     Means for Solving the Problems 
     To achieve the object, the present invention provides a fluid machine comprising, inside a housing thereof: a drive section configured to be driven by a working fluid introduced from an inlet port of the fluid machine, and to discharge the working fluid to an outlet port of the fluid machine; a communication passage allowing the working fluid to flow from the inlet port into the drive section; a bypass passage allowing the working fluid to flow from the inlet port to the outlet port while bypassing the drive section; and a valve mechanism configured to block inflow of the working fluid from the inlet port and to allow the inlet port to be connected selectively to the communication passage and the bypass passage (claim  1 ). 
     The present invention also provides a refrigerant circuit comprising the fluid machine of claim  1  inserted in a circulation path for circulating the working fluid (claim  2 ). 
     Further, the present invention provides a waste heat utilization device comprising, as the refrigerant circuit of claim  2 , a Rankine circuit including an expansion unit inserted in the circulation path, the expansion unit being contained in the housing of the fluid machine and configured to produce drive force by causing the drive section to expand the working fluid whose pressure has been increased by waste heat of a heat source, wherein the Rankine circuit further includes a high-pressure path section which is connected to the inlet port and in which high pressure of the working fluid circulated through the circulation path prevails, a low-pressure path section which is connected to the outlet port and in which low pressure of the working fluid circulated through the circulation path prevails, and pressure holding means for holding the pressure in the high-pressure path section, and wherein, when the pressure holding means is in operation, the valve mechanism blocks the inflow of the working fluid from the inlet port, and when the pressure in the high-pressure path section is higher than or equal to a first preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage or the communication passage (claim  3 ). 
     Preferably, when the Rankine circuit is in operation, the valve mechanism connects the inlet port to the communication passage, when the Rankine circuit is stopped, the valve mechanism operates the pressure holding means to block the inflow of the working fluid from the inlet port, when the pressure in the high-pressure path section is higher than or equal to the first preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage or the communication passage, and when the Rankine circuit is restarted after being stopped, the valve mechanism connects the inlet port to the communication passage (claim  4 ). 
     Preferably, when the Rankine circuit is in operation, the valve mechanism connects the inlet port to the communication passage, when the pressure in the high-pressure path section is lower than a second preset pressure during the operation of the Rankine circuit, the valve mechanism operates the pressure holding means to block the inflow of the working fluid from the inlet port, and when the pressure in the high-pressure path section is higher than or equal to a third preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the communication passage (claim  5 ). 
     Preferably, when the pressure in the high-pressure path section is higher than or equal to the first preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port only to the bypass passage (claim  6 ). 
     The valve mechanism preferably includes a valve element which allows the inlet port to selectively communicate with the communication passage and the bypass passage and on which the high pressure of the working fluid in the inlet port acts, and a spring mechanism supporting the valve element and pressing the valve element against the high pressure of the working fluid, to block and allow the inflow of the working fluid from the inlet port (claim  7 ). 
     Preferably, the spring mechanism includes a spring pushing the valve element, and an accommodation chamber accommodating the spring and sealed in a gastight fashion with respect to the inlet port (claim  8 ). 
     Preferably, the accommodation chamber communicates with the outlet port, and when a force exerted on the valve element by the high pressure of the working fluid in the inlet port is larger than a sum of a force exerted by the low pressure of the working fluid in the outlet port and a pushing force exerted by the spring, the spring mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage (claim  9 ). 
     Preferably, the accommodation chamber is open to atmosphere, and when a force exerted on the valve element by the high pressure of the working fluid in the inlet port is larger than a sum of a force exerted by an atmospheric pressure and a pushing force exerted by the spring, the spring mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage (claim  10 ). 
     Preferably, the valve element is a ball valve element or a rotary valve element and is rotated by a predetermined drive source (claim  11 ). 
     Advantageous Effects of the Invention 
     The fluid machine according to claim  1  includes, inside the housing thereof, the valve mechanism configured to block the inflow of the working fluid from the inlet port and to allow the inlet port to be connected selectively to the communication passage and the bypass passage. Since the fluid machine is equipped with the valve mechanism having multiple functions, the construction of the fluid machine can be simplified. 
     The refrigerant circuit according to claim  2  is provided with the fluid machine of claim  1  which is inserted in the circulation passage for circulating the working fluid. Since the refrigerant circuit need not be equipped with a valve for blocking and allowing the inflow of the working fluid or a valve for switching between the communication passage and the bypass passage, the construction of the refrigerant circuit can be simplified. 
     In the waste heat utilization device according to claim  3 , when the pressure holding means is in operation, the valve mechanism blocks the inflow of the working fluid from the inlet port, and when the pressure in the high-pressure path section is higher than or equal to the first preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage or the communication passage. Thus, the fluid machine is equipped with the valve mechanism which is single in number but is capable of functioning as three different valves, namely, a shutoff valve for accumulating pressure in the high-pressure path section, a pressure regulating valve for regulating the pressure in the high-pressure path section to prevent the pressure from becoming abnormally high, and a bypass valve for releasing abnormally high pressure of the working fluid when the pressure in the Rankine circuit is abnormally high. Accordingly, the size and cost of the Rankine circuit and thus of the waste heat utilization device can be reduced while preventing the pressure in the Rankine circuit from becoming abnormally high and also improving the starting performance of the Rankine circuit. 
     In the waste heat utilization device according to claim  4 , when the Rankine circuit is in operation, the valve mechanism connects the inlet port to the communication passage, when the Rankine circuit is stopped, the valve mechanism operates the pressure holding means to block the inflow of the working fluid from the inlet port, when the pressure in the high-pressure path section is higher than or equal to the first preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage or the communication passage, and when the Rankine circuit is restarted after being stopped, the valve mechanism connects the inlet port to the communication passage. Thus, while the Rankine circuit is stopped, the pressure is accumulated in the high-pressure path section and at the same time is prevented from becoming abnormally high, and when the Rankine circuit is restarted, the pressure of the working fluid accumulated in the high-pressure path section is used to start the Rankine circuit. 
     In the waste heat utilization device according to claim  5 , when the Rankine circuit is in operation, the valve mechanism connects the inlet port to the communication passage, when the pressure in the high-pressure path section is lower than the second preset pressure during the operation of the Rankine circuit, the valve mechanism operates the pressure holding means to block the inflow of the working fluid from the inlet port, and when the pressure in the high-pressure path section is higher than or equal to the third preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the communication passage. Thus, the pressure of the working fluid is accumulated in the high-pressure path section until an appropriate high pressure is reached, and when the working fluid pressure in the high-pressure path section becomes equal to the appropriate high pressure, the inlet port is connected to the communication passage. The working fluid pressure in the high-pressure path section of the Rankine circuit can therefore be kept at the appropriate high pressure. 
     In the waste heat utilization device according to claim  6 , when the pressure in the high-pressure path section is higher than or equal to the first preset pressure, the valve mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port only to the bypass passage. Thus, the working fluid of which the pressure is abnormally high is prevented from flowing into the communication passage and driving the drive section at high speed, whereby seizure of the drive section due to such high-speed operation can be avoided. The expansion unit, the Rankine circuit and thus the waste heat utilization device can be reliably protected, improving the reliability of the waste heat utilization device. 
     In the waste heat utilization device according to claim  7 , the valve mechanism includes the valve element which allows the inlet port to selectively communicate with the communication passage and the bypass passage and on which the high pressure of the working fluid in the inlet port acts, and the spring mechanism supporting the valve element and pressing the valve element against the high pressure of the working fluid, to block and allow the inflow of the working fluid from the inlet port. Thus, the valve mechanism has a simple construction. Also, since the inlet port is opened and closed by mechanical means, the abnormally high pressure can be quickly released, making it possible to further improve the reliability of the waste heat utilization device. 
     In the waste heat utilization device according to claim  8 , the spring mechanism includes the spring pushing the valve element, and the accommodation chamber accommodating the spring and sealed in a gastight fashion with respect to the inlet port. In this manner, the spring mechanism has a simple construction. 
     In the waste heat utilization device according to claim  9 , the accommodation chamber communicates with the outlet port, and when the force exerted on the valve element by the high pressure of the working fluid in the inlet port is larger than the sum of the force exerted by the low pressure of the working fluid in the outlet port and the pushing force exerted by the spring, the spring mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage. Thus, the abnormally high pressure in the high-pressure path section can be released in accordance with the pressure difference between the inlet and outlet ports, that is, between the high and low pressures of the working fluid (relative pressure control). Accordingly, not only variations in the high pressure of the working fluid but variations in the low pressure of the working fluid are quickly sensed, whereby the controllability in releasing the abnormally high pressure is enhanced, making it possible to further improve the reliability of the waste heat utilization device. 
     In the waste heat utilization device according to claim  10 , the accommodation chamber is open to the atmosphere, and when the force exerted on the valve element by the high pressure of the working fluid in the inlet port is larger than the sum of the force exerted by the atmospheric pressure and the pushing force exerted by the spring, the spring mechanism allows the inflow of the working fluid from the inlet port and connects the inlet port to the bypass passage. Thus, the abnormally high pressure in the high-pressure path section can be released only in accordance with the absolute pressure in the inlet port, namely, only in accordance with the high pressure of the working fluid (absolute pressure control). Thus, by determining the preset pressure for opening the inlet port to a value smaller than or equal to an allowable maximum design pressure of the fluid machine, the high-pressure path section and the Rankine circuit, it is possible to more reliably protect the expansion unit, the Rankine circuit and thus the waste heat utilization device, further enhancing the reliability of the waste heat utilization device. 
     In the waste heat utilization device according to claim  11 , the valve element is a ball valve element or a rotary valve element and is rotated by the predetermined drive source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates the configuration of a waste heat utilization device for a motor vehicle according to a first embodiment. 
         FIG. 2  is a schematic longitudinal sectional view of a fluid machine applied to the device of  FIG. 1 . 
         FIG. 3(   a ) is a longitudinal sectional view illustrating an operating state of a valve mechanism shown in  FIG. 2  during a high pressure holding operation of a Rankine circuit, and  FIG. 3(   b ) is a longitudinal sectional view illustrating an operating state of the valve mechanism in  FIG. 2  during an abnormally high pressure releasing operation of the Rankine circuit. 
         FIG. 4(   a ) is a longitudinal sectional view illustrating an operating state of a valve mechanism according to a second embodiment during the high pressure holding operation of the Rankine circuit, and  FIG. 4(   b ) is a longitudinal sectional view illustrating an operating state of the valve mechanism of the second embodiment during the abnormally high pressure releasing operation of the Rankine circuit. 
         FIG. 5(   a ) is a schematic longitudinal sectional view of a fluid machine according to a third embodiment, and  FIG. 5(   b ) schematically illustrates the fluid machine as viewed from the direction indicated by an arrow A in  FIG. 5(   a ). 
         FIG. 6(   a ) is a longitudinal sectional view illustrating an operating state of a valve mechanism shown in  FIG. 5  during a normal operation of the Rankine circuit, and  FIG. 6(   b ) is a schematic cross-sectional view of the valve mechanism taken along line B-B in  FIG. 6(   a ). 
         FIG. 7(   a ) is a longitudinal sectional view illustrating an operating state of the valve mechanism shown in  FIG. 5  during the high pressure holding operation of the Rankine circuit, and  FIG. 7(   b ) is a schematic cross-sectional view of the valve mechanism taken along line B-B in  FIG. 7(   a ). 
         FIG. 8(   a ) is a longitudinal sectional view illustrating an operating state of the valve mechanism shown in  FIG. 5  during the abnormally high pressure releasing operation of the Rankine circuit, and  FIG. 8(   b ) is a schematic cross-sectional view of the valve mechanism taken along line B-B in  FIG. 8(   a ). 
         FIG. 9(   a ) is a longitudinal sectional view illustrating an operating state of the valve mechanism shown in  FIG. 5  during a bypassing operation of the Rankine circuit, and  FIG. 9(   b ) is a schematic cross-sectional view of the valve mechanism taken along line B-B in  FIG. 9(   a ). 
     
    
    
     MODE OF CARRYING OUT THE INVENTION 
       FIG. 1  illustrates a waste heat utilization device  1  for a motor vehicle according to a first embodiment. The waste heat utilization device  1  recovers heat from, for example, the exhaust gas emitted from an engine (internal combustion engine)  2  of the vehicle. To this end, the waste heat utilization device  1  is equipped with a Rankine circuit  4  having a circulation path  5  for circulating a working fluid (heat medium) therethrough. The circulation path  5  is constituted by tubing or piping, for example. 
     A pump  6  is inserted in the circulation path  5  to move the working fluid. Further, a check valve  7 , a heater  8 , an expansion unit  12  of a fluid machine  10 , and a condenser  14  are sequentially arranged downstream of the pump  6  as viewed in the flowing direction of the working fluid. The pump  6  sucks in the working fluid from the circulation path extending from the condenser  14 , then pressurizes the working fluid to high pressure, and discharges the high-pressure working fluid to the heater  8 . The working fluid discharged from the pump  6  is in a low-temperature, high-pressure liquid state. 
     The heater  8 , which is a heat exchanger, has a low-temperature passage  8   a  forming part of the circulation path  5  and a high-temperature passage  8   b  capable of transferring heat therefrom to the passage  8   a . The passage  8   b  is inserted, for example, in an exhaust pipe  16  extending from the engine  2 . Thus, while passing through the heater  8 , the working fluid in the low-temperature, high-pressure state receives heat from the exhaust gas (heat source) emitted from the engine  2 . As a result, the working fluid is heated, turning into high-temperature, high-pressure superheated vapor. 
     The expansion unit  12  of the fluid machine  10  expands the working fluid in the superheated vapor state, so that the working fluid turns into high-temperature, low-pressure superheated vapor. 
     The condenser  14 , which is a heat radiator, condenses the working fluid flowing out of the expansion unit  12 , by allowing heat to transfer from the working fluid to outside air, and thus the working fluid turns into low-temperature, low-pressure liquid. More specifically, an electric fan (not shown) is arranged near the condenser  14  so that the working fluid may be cooled by the air currents produced by the electric fan or by the air flowing from the front of the vehicle. The working fluid thus cooled by the condenser  14  is again drawn into the pump  6  to be circulated through the circulation path  5 . 
     The aforementioned expansion unit  12  is capable of not only expanding the working fluid but converting the heat energy of the working fluid into torque (rotational force). An electric power generation unit  18  is coupled to the expansion unit  12  so as to make use of the torque output from the expansion unit  12 . The power generation unit  18  is connected with an electrical load  20  that uses or stores the electric power generated by the power generation unit  18 , for example, a battery. 
     As illustrated in  FIG. 2  in detail, the fluid machine  10  is constituted by the expansion unit  12  and the power generation unit  18  coupled in series with each other. 
     The expansion unit  12  is, for example, a scroll expander including a scroll unit  22  as a drive section. A cup-shaped casing  32  (expansion unit casing) of the expansion unit  12  has an opening almost covered with a partition wall  34 , and a through hole is formed in the center of the partition wall  34 . 
     A fixed scroll  36  is fixed inside the expansion unit casing  32 , and a high-pressure chamber  38  is defined on the rear side of the fixed scroll  36 . The high-pressure chamber  38  communicates with the heater  8  through an inlet port  33  formed through the expansion unit casing  32  and the part of the circulation path  5  connected to the inlet port  33 . 
     A movable scroll  40  is arranged on the front side of the fixed scroll  36  and engages with the fixed scroll  36 . An expansion chamber  42  for expanding the working fluid is defined between the fixed scroll  36  and the movable scroll  40 , and a low-pressure chamber  44  for receiving the expanded working fluid is defined around the movable scroll  40 . An admission hole  46  is formed through nearly the center of the base plate of the fixed scroll  36 , so that the expansion chamber  42  located at the radial center of the fixed and movable scrolls  36  and  40  and the high-pressure chamber  38  communicate with each other through the admission hole  46 . 
     The working fluid expands in the expansion chamber  42  located at the radial center of the fixed and movable scrolls  36  and  40 , whereupon the volume of the expansion chamber  42  increases and also the expansion chamber  42  moves radially outward along the spiral walls of the fixed and movable scrolls  36  and  40 . Finally, the expansion chamber  42  communicates with the low-pressure chamber  44 , allowing the expanded working fluid to flow into the low-pressure chamber  44 . The low-pressure chamber  44  communicates with the condenser  14  through an outlet port  45  and the part of the circulation path  5  connected to the outlet port  45 . 
     As the working fluid expands, the movable scroll  40  is caused to make orbiting motion relative to the fixed scroll  36 , and the orbiting motion is converted to rotary motion by a revolving mechanism. 
     Specifically, a boss integrally protrudes from the back of the base plate of the movable scroll  40 , and an eccentric bushing  50  is relatively rotatably fitted in the boss with a needle bearing  48  therebetween. A crankpin  52  projecting eccentrically from a disk  54  is inserted into the eccentric bushing  50 . A shaft portion  56  integrally and coaxially projects from a side of the disk  54  opposite the crankpin  52  and is rotatably supported by the partition wall  34  with a radial bearing  58  such as a ball bearing interposed therebetween. Thus, the orbiting motion of the movable scroll  40  is converted to rotary motion of the shaft portion  56 . 
     In order to receive thrust and at the same time prevent the orbiting movable scroll  40  from rotating on its axis, the revolving mechanism has a ball coupling  60 , for example. The ball coupling  60  is arranged between an outer peripheral portion of the base plate of the movable scroll  40  and a portion of the partition wall  34  facing the outer peripheral portion of the movable scroll  40 . 
     The power generation unit  18  has a cylindrical casing (power generation unit casing)  93  abutting against the partition wall  34 . The expansion unit casing  32 , the partition wall  34  and the power generation unit casing  93  are coupled together to constitute a housing for the fluid machine  10 . 
     A drive shaft  72  of the power generation unit  18  has one end formed integrally with the shaft portion  56 , extending up to the through hole in the partition wall  34  and rotatably supported by the partition wall  34  with a radial bearing  58  therebetween. The other end of the drive shaft  72  is rotatably supported by means of a radial bearing  74  fixed to a bottom of the expansion unit casing  32 . Thus, motive power can be transmitted from the shaft portion  56  to the drive shaft  72 , so that the shaft portion  56  and the drive shaft  72  rotate together with each other. 
     A rotor  96  is fixed on a portion of the drive shaft  72  extending within the power generation unit casing  93 . The rotor  96  comprises a permanent magnet, for example, and is disposed coaxially with the shaft portion  56 . 
     A stator is fixed on an inner peripheral surface of the power generation unit casing  93  so as to surround the rotor  96 . The stator includes a yoke  98  and three coil windings  100 , for example, wound around the yoke  98 . 
     The coils  100  are so wired as to generate a three-phase alternating current as the rotor  96  rotates, the generated alternating current being supplied to the external load  20  via a lead wire, not shown. 
     Since the power generation unit  18  is not expected to function as an electric motor, the shape of the yoke  98 , the number of turns of each coil  100 , and the like are suitably selected to achieve high power generation efficiency. 
     The fluid machine  10  configured in this manner is provided, in its housing, with a communication passage  102  allowing the inlet port  33  to communicate with the expansion unit  12  through the high-pressure chamber  38  and the admission hole  46 , a bypass passage  104  for guiding the working fluid from the inlet port  33  to the outlet port  45  while bypassing the expansion unit  12 , and a valve mechanism  106  allowing the inlet port  33  to selectively communicate with the communication passage  102  and the bypass passage  104 . 
     Specifically, the valve mechanism  106  is, for example, a ball valve having a ball  108  as a valve element. The ball  108  is rotatably supported in a gastight fashion between a supporting portion  110  supporting the fixed scroll  40  and an inner surface  32   a  of the expansion unit casing  32  in which the inlet port  33  opens, with a valve seat or the like interposed therebetween. 
     An L-shaped passage  108   a  for the working fluid extends through the ball  108 , and as the ball  108  is rotated by a drive shaft, not shown, of the valve mechanism  106 , the valve mechanism  106  allows the inlet port  33  to communicate selectively with the communication passage  102  or the bypass passage  104  via the passage  108   a . The drive shaft is driven, for example, by an electromagnetic valve (drive source) which is operated by an ECU (Electronic Control Unit), not shown, for controlling the entire device  1 . 
     In the following, the operation of the valve mechanism  106  will be explained along with the operation of the fluid machine  10  and of the Rankine circuit  4 . 
     &lt;Normal Operation&gt; 
     When the Rankine circuit  4  is started by the ECU, the ball  108  is rotated to a position such that the inlet port  33  communicates with the communication passage  102  only, as illustrated in  FIG. 2 . As a result, the valve mechanism  106  allows the working fluid to flow from the heater  8  into the high-pressure chamber  38  as indicated by an arrow, so that the expansion unit  12  operates, causing the shaft portion  56  to rotate together with the drive shaft  72 . 
     As the drive shaft  72  rotates, the rotor  96  of the power generation unit  18  also rotates, with the result that an alternating current is generated by the power generation unit  18 . The generated alternating current is supplied to the load  20  to be appropriately stored therein or consumed thereby. 
     &lt;High pressure Holding Operation&gt; 
     When the operation of the Rankine circuit  4  is stopped by the ECU, the ball  108  is rotated to a position where the inlet port  33  communicates with neither the communication passage  102  nor the bypass passage  104 , as illustrated in  FIG. 3(   a ). That is, the ball  108  blocks the inlet port  33 , so that the introduction of the working fluid from the heater  8  into the high-pressure chamber  38  is prevented by the valve mechanism  106 . At this time, the backflow of the working fluid from the heater  8  toward the pump  6  is prevented by the check valve  7 . Accordingly, a high-pressure path section  5   a  in which high pressure of the working fluid is accumulated is created (pressure holding means) within a range of the circulation path  5  ranging from the check valve  7  to the expansion unit  12  through the passage  8   a . On the other hand, a low-pressure path section  5   b  in which low pressure of the working fluid prevails is created within a range of the circulation path  5  ranging from the outlet port  45  to the check valve  7  through the condenser  14 . 
     While the Rankine circuit  4  is in operation, a pressure drop of the working fluid in the high-pressure path section  5   a  is detected by the ECU. If the detected pressure of the working fluid becomes lower than a predetermined appropriate pressure (second preset pressure), for example, the ball  108  blocks the inlet port  33 , as shown in  FIG. 3(   a ), so that the working fluid pressure in the high-pressure path section  5   a  is accumulated until the appropriate pressure is reached. On the other hand, if the pressure of the working fluid in the high-pressure path section  5   a  becomes higher than or equal to another predetermined appropriate pressure (third preset pressure), the ball  108  is rotated to the position illustrated in  FIG. 2 , so that the inlet port  33  communicate with the communication passage  102  only. The working fluid pressure at and above which the inlet port  33  is connected to the communication passage  102  is set to a value equal to or higher than that at and above which the working fluid pressure is accumulated in the high-pressure path section  5   a . Thus, since a hysteresis, or a dead zone, is provided between the preset pressure at and above which the working fluid pressure is accumulated and the preset pressure at and above which the inlet port  33  is connected to the communication passage  102 , hunting of the valve mechanism  106  can be satisfactorily prevented. 
     &lt;Abnormally High Pressure Releasing Operation&gt; 
     If, while the operation of the Rankine circuit  4  is stopped or during the high pressure holding operation of the Rankine circuit  4 , an abnormally high pressure in the high-pressure path section  5   a  higher than or equal to a preset pressure (first preset pressure) is detected by the ECU, the ball  108  is rotated to a position where the inlet port  33  is connected to the bypass passage  104 , as illustrated in  FIG. 3(   b ). Consequently, the inlet port  33  is no longer blocked by the ball  108 , and the valve mechanism  106  allows the working fluid in the high-pressure path section  5   a  to flow into the bypass passage  104 , as indicated by an arrow, so that the pressure in the high-pressure path section  5   a  is released to the low-pressure path section  5   b  through the outlet port  45 . The first preset pressure is set to be higher than the aforementioned appropriate pressures mentioned with reference to the high pressure holding operation. 
     &lt;Restarting Operation&gt; 
     If, while the Rankine circuit  4  is stopped with the high pressure stored in the high-pressure path section  5   a , the Rankine circuit  4  is restarted by the ECU, the ball  108  is rotated from the position where the inlet port  33  is blocked by the ball  108 , as illustrated in  FIG. 3(   a ), to the position where the inlet port  33  is connected to the communication passage  102 , as illustrated in  FIG. 2 , whereby the pressure in the high-pressure path section  5   a  is released to the communication passage  102 . The released pressure of the working fluid drives the expansion unit  12  to rotate the drive shaft  72 . As the drive shaft  72  is rotated, the rotor  96  of the power generation unit  18  rotates, so that the power generation unit  18  generates an alternating current. The alternating current is supplied to the load  20  to be appropriately stored therein or consumed thereby. 
     As described above, in the waste heat utilization device  1  of the first embodiment, the fluid machine  10  is equipped with the single valve mechanism  106  which functions as three different valves, namely, a shutoff valve for accumulating the working fluid pressure in the high-pressure path section  5   a  while the Rankine circuit  4  is stopped, a pressure regulating valve for regulating the working fluid pressure in the high-pressure path section  5   a  so that the pressure may not become abnormally high, and a bypass valve for releasing the abnormally high pressure developed in the high-pressure path section  5   a . This permits the fluid machine  10 , the Rankine circuit  4  and thus the waste heat utilization device  1  to be reduced in size and cost while at the same time preventing abnormally high pressure from being developed in the Rankine circuit  4  and also improving the starting performance of the Rankine circuit  4 . 
     While the Rankine circuit  4  is stopped, the working fluid pressure is accumulated in the high-pressure path section  5   a  while at the same time is prevented from becoming abnormally high. When the Rankine circuit  4  is restarted thereafter, the working fluid pressure accumulated in the high-pressure path section  5   a  is utilized to start the Rankine circuit  4 , whereby the starting performance of the Rankine circuit can be improved. 
     Further, the working fluid pressure is accumulated in the high-pressure path section  5   a  until the appropriate pressure is reached, and when the working fluid pressure becomes equal to the appropriate pressure, the high-pressure path section  5   a  is connected to the communication passage  102 . Accordingly, the pressure of the working fluid circulated through the Rankine circuit  4  can be kept at an appropriate high level in the high-pressure path section  5   a.    
     Furthermore, the valve mechanism  106  allows the inlet port  33  to communicate with the bypass passage  104  to release the abnormally high pressure of the working fluid. Thus, the abnormally high pressure of the working fluid is prevented from flowing into the communication passage  102  and driving the scroll unit  22  at high speed as a result, whereby seizure of the scroll unit  22  due to such high-speed operation can be avoided. It is therefore possible to reliably protect the expansion unit  12 , the Rankine circuit  4  and thus the waste heat utilization device  1 , improving the reliability of the waste heat utilization device  1 . 
       FIGS. 4(   a ) and  4 ( b ) schematically illustrate the construction of a valve mechanism  112  according to a second embodiment. In the figures, identical reference numerals are used to denote elements identical with those of the valve mechanism  106  of the first embodiment, and description of such elements is omitted. 
     The valve mechanism  112  further includes a spring mechanism  114  supporting the ball  108 . The spring mechanism  114  pushes the ball  108  against the high pressure of the working fluid in the high-pressure path section  5   a , to block and allow the inflow of the working fluid from the inlet port  33 . 
     The spring mechanism  114  has a spring  116  that exerts a pushing force on the ball  108 , and the spring  116  transmits its pushing force to the ball  108  through a cylindrical seat  117  with which the ball  108  is disposed in contact. 
     Also, an accommodation chamber  118  is formed inside the supporting portion  110  in a manner such that the chamber  118  is sealed in a gastight fashion with respect to the inlet port  33  and the high-pressure chamber  38 . The spring  116  and the seat  117  are accommodated in the accommodation chamber  118 . The accommodation chamber  118  has a pressure introducing hole  119  opening into the bypass passage  104  so that the low pressure of the working fluid may prevail in the interior of the accommodation chamber  118 . Namely, the accommodation chamber  118  communicates with the outlet port  45 . 
     In the following, only the abnormally high pressure releasing operation of the valve mechanism  112 , in which the pressure in the high-pressure path section  5   a  is released, will be explained along with the operation of the fluid machine  10  and of the Rankine circuit  4 . 
     &lt;Abnormally High Pressure Releasing Operation&gt; 
     When a force Fhp exerted on the ball  108  by the high-pressure working fluid in the inlet port  33  becomes larger than the sum of a force Flp exerted by the low-pressure working fluid on the side of the outlet port  45  and the pushing force Fs of the spring  116  due to abnormally high pressure developed in the high-pressure path section  5   a , the ball  108  is pushed away from the inner surface  32   a , as illustrated in  FIG. 4(   b ), with the result that the inlet port  33  opens. Consequently, the valve mechanism  112  allows the working fluid in the high-pressure path section  5   a  to flow into the communication passage  102  and the bypass passage  104 , as indicated by arrows, whereby the pressure in the high-pressure path section  5   a  is released to the low-pressure path section  5   b  through the outlet port  45 . 
     As described above, in the waste heat utilization device  1  of the second embodiment, the valve mechanism  112  includes the spring mechanism  114 , and the accommodation chamber  118  of the spring mechanism  114  communicates with the outlet port  45 . Thus, the valve mechanism  112  can be made to function as both of a selector valve for selectively connecting the inlet port to the communication passage  102  and the bypass passage  104 , and a safety valve which operates mechanically without the need for input signals from the ECU. 
     Also, abnormally high pressure in the high-pressure path section  5   a  can be released in accordance with the pressure difference between the pressure in the inlet port  33  and the pressure in the outlet port  45 , that is, the pressure difference between the high and low pressures of the working fluid (relative pressure control). Thus, in the second embodiment, the valve mechanism  112  can be operated mechanically without the need for input signals from the ECU, and also the control response to abnormally high pressure can be enhanced by quickly sensing not only the variations in the high pressure of the working fluid but the variations in the low pressure of the working fluid. The reliability of the waste heat utilization device  1  can therefore be further improved. 
       FIGS. 5(   a ) and  5 ( b ) illustrate a fluid machine  10  according to a third embodiment. In the figures, identical reference numerals are used to denote elements identical with those of the fluid machines  10  of the first and second embodiments, and description of such elements is omitted. 
     A valve unit  120  is attached to an outer surface  32   b  of the expansion unit casing  32  of the fluid machine  10 . The valve unit  120  is constituted by two blocks  120 A and  120 B. 
     The block  120 A, to which the circulation path  5  is connected, has a first passage  122  formed therethrough in communication with the circulation path  5 . The block  120 A is fastened to the block  120 B by screws. 
     On the other hand, the block  120 B has an accommodation hole  126  formed therein for accommodating a valve mechanism  124  of, this embodiment. A second passage  128  communicates with the accommodation hole  126 , and when the blocks  120 A and  120 B are fastened together, the first and second passages  122  and  128  are connected to each other in a gastight fashion. In other words, the passages  122  and  128  form the inlet port  33 . 
     A pressure introducing passage  130  branches off from the second passage  128  and opens in an innermost portion  133  of the accommodation hole  126  in the block  120 B located opposite a main opening  132  into which the accommodation hole  126  opens. The accommodation hole  126  communicates further with the communication passage  102  and the bypass passage  104 . 
     An electromagnetic valve (drive source)  134  is fastened to the block  120 B by screws in a gastight fashion so as to cover the main opening  132  with a housing thereof. 
     More specifically, as illustrated in  FIGS. 6(   a ) and  6 ( b ), the valve mechanism  124  is constituted by a valve element  136  and a spring  138 . 
     The valve element  136  is a piston-shaped rotary valve element having a hollow cylindrical portion  136   a  and a head portion  136   b . Between the cylindrical portion  136   a  and the head portion  136   b , an L-shaped radial passage  124   a  is formed through the valve element  136 , and an annular groove  136   c  is formed in the outer peripheral surface of the valve element  136  so as to be located closer to the head portion  136   b  than the passage  124   a.    
     The housing  135  of the electromagnetic valve  134  accommodates a drive shaft  140 , a rotor  142  fixed on the drive shaft  140  and constituted by a permanent magnet, for example, and a stator  144  fixed to the inner peripheral surface of the housing  135  so as to surround the rotor  142 . The stator  144  has a coil wound around a yoke, and the coil winding is connected to the ECU by a lead wire to be supplied with electric current as needed. 
     A boss  146 , which forms part of the drive shaft  140 , projects from the housing  135  of the electromagnetic valve  134 . The boss  146  is received in the main opening  132  of the accommodation hole  126  and rotatably supported by the inner peripheral surface of the accommodation hole  126 . 
     The spring  138  has one end pressed against the bottom surface of the boss  146  and the other end pressed against the bottom surface of the cylindrical portion  136   a , and pushes the valve element  136  toward the innermost portion  133  of the accommodation hole  126 . 
     The cylindrical portion  136  is fitted into the boss  146  with the outer peripheral surface of the cylindrical portion  136  engaged with the inner peripheral surface of the boss  146  by means of engaging portions  148  which are constituted, for example, by serrations extending in the axial direction of the drive shaft  140 . Thus, an accommodation chamber  139  for accommodating the spring  138  is defined between the boss  146  and the cylindrical portion  136   a  in a manner such that the accommodation chamber  139  is sealed in a gastight fashion with respect to the accommodation hole  126  and the inlet port  33 . The valve element  136  is movable in the direction in which the valve element  136  is pushed by the spring  138  accommodated in the accommodation chamber  139 , and also can be rotated together with the boss  146 , that is, the drive shaft  140 . 
     In this embodiment, a low-pressure passage  141  communicating with the low-pressure chamber  44  opens into the accommodation chamber  139 , so that the low-pressure working fluid flows into the accommodation chamber  139 . 
     In the valve mechanism  124  constructed as described above, the valve element  136  is rotated clockwise or counterclockwise by the drive shaft  140  over an angular range of 90 degrees, to connect the inlet port to the communication passage  102  or the bypass passage  104  through the passage  124   a . The drive shaft  140  is driven by the electromagnetic valve  134 , which in turn is operated by the ECU. 
     The following describes the operation of the valve mechanism  124 , along with the operation of the fluid machine  10  and of the Rankine circuit  4 . 
     &lt;Normal Operation&gt; 
     When the Rankine circuit  4  is started by the ECU, the valve element  136  is rotated to a position such that the inlet port is connected to the communication passage  102  only, as illustrated in  FIGS. 6(   a ) and  6 ( b ). As a result, the valve mechanism  124  allows the working fluid to flow from the heater  8  into the high-pressure chamber  38  as indicated by arrows, so that the expansion unit  12  operates. As the drive shaft  72  is rotated, the rotor  96  of the power generation unit  18  rotates, with the result that an alternating current is generated by the power generation unit  18 . The alternating current is supplied to the load  20  to be stored therein or consumed thereby as the case may be. 
     &lt;High pressure Holding Operation&gt; 
     When the operation of the Rankine circuit  4  is stopped by the ECU, the valve element  136  is rotated to a position where the inlet port communicates with neither the communication passage  102  nor the bypass passage  104 , as illustrated in  FIGS. 7(   a ) and  7 ( b ). That is, the valve element  136  blocks the second passage  128 , namely, the inlet port  33 , so that the introduction of the working fluid from the heater  8  into the high-pressure chamber  38  is blocked by the valve mechanism  124 . At this time, the backflow of the working fluid from the heater  8  toward the pump  6  is prevented by the check valve  7 , and accordingly, the high-pressure path section  5   a  and the low-pressure path section  5   b  are created. 
     While the Rankine circuit  4  is operating, a pressure drop of the working fluid in the high-pressure path section  5   a  is detected by the ECU. If the detected pressure of the working fluid decreases below the predetermined appropriate pressure (second preset pressure), for example, the valve element  136  blocks the inlet port  33 , as illustrated in  FIGS. 7(   a ) and  7 ( b ), so that the working fluid pressure is accumulated in the high-pressure path section  5   a  until the appropriate pressure is reached. On the other hand, if the working fluid pressure in the high-pressure path section  5   a  becomes higher than or equal to the other predetermined appropriate pressure (third preset pressure&gt;second preset pressure), the valve element  136  is rotated to the position illustrated in  FIGS. 6(   a ) and  6 ( b ), whereby the inlet port is connected only to the communication passage  102 . 
     &lt;Abnormally High Pressure Releasing Operation&gt; 
     When the force Fhp exerted on the head portion  136   a  of the valve element  136  by the high-pressure working fluid introduced from the pressure introducing passage  130  becomes larger than the sum of the force Flp exerted by the low-pressure working fluid on the side of the outlet port  45  and the pushing force Fs of the spring  138  due to abnormally high pressure developed in the high-pressure path section  5   a , the valve element  136  is moved away from the innermost portion  133  of the accommodation hole  126  toward the spring  138 , as illustrated in  FIGS. 8(   a ) and  8 ( b ), with the result that the annular groove  136   c  is located in communication with the second passage  128 . Thus, the second passage  128 , that is, the inlet port  33  is no longer blocked and communicates with the annular groove  136   c  instead. Consequently, the valve mechanism  124  allows the working fluid in the high-pressure path section  5   a  to flow into the communication passage  102  and the bypass passage  104 , as indicated by arrows, whereby the pressure in the high-pressure path section  5   a  is released to the low-pressure path section  5   b  through the outlet port  45 . Instead of the annular groove  136   c  which is formed in the outer peripheral surface of the valve element  136  along its entire circumference, a groove may be cut in the outer peripheral surface of the valve element  136  only over a predetermined circumferential range so that when the inlet port  33  is opened to release the high-pressure working fluid, the working fluid in the high-pressure path section  5   a  may be guided only to the bypass passage  104 . 
     &lt;Restarting Operation&gt; 
     When the Rankine circuit  4  is restarted by the ECU, 2.0 the valve element  136  is rotated from the position where the inlet port  33  is blocked by the valve element  136 , as illustrated in  FIGS. 7(   a ) and  7 ( b ), to the position where the inlet port  33  is connected to the communication passage  102 , as illustrated in  FIGS. 6(   a ) and  6 ( b ), whereby the pressure in the high-pressure path section  5   a  is released to the communication passage  102 . The released pressure of the working fluid drives the expansion unit  12  to rotate the drive shaft  72 . As the drive shaft  72  is rotated, the rotor  96  of the power generation unit  18  rotates, so that the power generation unit  18  generates an alternating current. The generated alternating current is supplied to the load  20  to be stored therein or consumed thereby as needed. 
     &lt;Bypassing Operation&gt; 
     When the pump  6  alone is started by the ECU during the initial starting operation of the Rankine circuit  4 , the valve element  136  is rotated to a position where the second passage  128  is connected to the bypass passage  104 , as illustrated in  FIGS. 9(   a ) and  9 ( b ). Accordingly, the resistance to the flow of the working fluid through the circulation path  5  is reduced, whereby the circulation amount of the working fluid can be smoothly controlled. 
     Also when the Rankine circuit  4  is brought to a complete stop, the valve element  136  is rotated to the position where the second passage  128  is connected to the bypass passage  104 . Since, in this case, the working fluid in the high-pressure path section  5   a  can be guided to the low-pressure path section  5   b , the working fluid pressure in the circulation path  5  as a whole can be made uniform, whereby the expansion unit  12  can be smoothly brought to a complete stop. 
     With the above waste heat utilization device  1  of the third embodiment, abnormally high pressure in the high-pressure path section  5   a  can be released mechanically according to the relative pressure control based on the pressure difference between the high and low pressures of the working fluid, like the second embodiment. It is therefore possible to enhance the control responsiveness in releasing abnormally high pressure. 
     In the third embodiment, the low working fluid pressure is introduced into the accommodation chamber  139  through the low-pressure passage  141 . Alternatively, an atmospheric pressure passage  148  opening to the atmosphere, in place of the low-pressure passage  141 , may be connected to the accommodation chamber  139  so that the accommodation chamber  139  may be open to the atmosphere. 
     In the following, how the valve mechanism  124  provided with the atmospheric pressure passage  148  operates to release abnormally high pressure from the high-pressure path section  5   a  will be described, along with the operation of the fluid machine  10  and of the Rankine circuit  4 . 
     &lt;Abnormally High Pressure Releasing Operation&gt; 
     When the force Fhp exerted on the head portion  136   a  of the valve element  136  by the high-pressure working fluid introduced from the pressure introducing passage  130  becomes larger than the sum of a force Fa exerted by the atmospheric pressure and the pushing force Fs of the spring  138  due to abnormally high pressure developed in the high-pressure path section  5   a , the valve element  136  is moved away from the innermost portion  133  of the accommodation hole  126 , as illustrated in  FIGS. 8(   a ) and  8 ( b ), with the result that the inlet port  33 , which has been blocked until then, communicates with the annular groove  136   c . Consequently, the valve mechanism  124  allows the working fluid in the high-pressure path section  5   a  to flow into both the communication passage  102  and the bypass passage  104  as indicated by the arrows, or only into the bypass passage  104 , whereby the pressure in the high-pressure path section  5   a  is released to the low-pressure path section  5   b  through the outlet port  45 . 
     In this case, the abnormally high pressure in the high-pressure path section  5   a  can be released only in response to the absolute pressure in the inlet port  33 , namely, the working fluid pressure in the high-pressure path section (absolute pressure control). Thus, by determining the preset pressure for opening the inlet port  33  to a value smaller than or equal to an allowable maximum design pressure of the high-pressure path section  5   a  and the expansion unit  12 , or more generally, the fluid machine  10  and the Rankine circuit  4 , it is possible to more reliably protect the Rankine circuit  4  and thus the waste heat utilization device  1 , further enhancing the reliability of the waste heat utilization device  1 . 
     When the pump  6  alone is started by the ECU during the initial starting operation of the Rankine circuit  4 , the valve element  136  is preferably rotated to the position illustrated in  FIGS. 9(   a ) and  9 ( b ), where the inlet port communicates with the bypass passage  104 , regardless of whether the pressure in the high-pressure path section is abnormally high or not. This serves to lessen the resistance to the flow of the working fluid through the circulation path  5 , making it possible to smoothly control the circulation amount of the working fluid. 
     Also, when the Rankine circuit  4  is brought to a complete stop, the valve element  136  is rotated to the position where the inlet port is connected to the bypass passage  104 . Thus, the working fluid in the high-pressure path section  5   a  is guided to the low-pressure path section  5   b , and since the working fluid pressure in the circulation path  5  as a whole can be made uniform, the expansion unit  12  can be smoothly brought to a complete stop. 
     The present invention is not limited to the foregoing embodiments and may be modified in various ways. 
     For example, the waste heat utilization device  1  described above converts the heat of the exhaust gas into electric power, but may be configured to convert the heat of the cooling water (heat source) of the engine  2  into electric power. Also, the waste heat utilization device  1  is applicable not only to motor vehicles but other devices. 
     Further, instead of the aforementioned fluid machine  10  constituted by the expansion unit  12  and the power generation unit  18 , a fluid machine may be used which is provided with a motor generator unit, in place of the power generation unit  18 , and with a pump unit serving as the pump  16 , and in which the motor generator unit and the pump unit share the drive shaft  72  with the expansion unit  12 . 
     In this case, if the pressure in the high-pressure path section  5   a  is not sufficiently high, electric power may be supplied from a suitable external unit when the Rankine circuit  4  is restarted, so that the motor generator unit may be operated as a motor by using the externally supplied power, to start the pump unit. After the working fluid pressure in the circulation path  5  becomes sufficiently high, part of the externally supplied power may be used to operate the pump unit while the remaining part may be used for rotating the rotor  96  of the power generation unit  18  to generate electricity. 
     On the other hand, if the pressure in the high-pressure path section  5   a  is sufficiently high, the pump unit can be started by the energy of the high-pressure working fluid. In this case, electric power need not be supplied externally to start the pump unit, saving the energy consumed by the fluid machine. 
     Further, instead of the aforementioned fluid machine  10 , an expander not including the power generation unit  18 , a fluid machine in which the output of the expander is transmitted to an internal combustion engine via a power transmission mechanism, and fluid machines with various other configurations may be used. 
     In the foregoing, moreover, the valve element of the valve mechanism is driven by the electromagnetic valve. The valve element may alternatively be driven by using the pressure of the working fluid or the output of the expansion unit. 
     INDUSTRIAL APPLICABILITY 
     The present invention pertains to a cooling system which is suited for use in a room air conditioner, a refrigerator freezer, a showcase refrigerator and the like and which is capable of ensuring satisfactory lubrication and sealing performance of the compressor and effectively improving the cooling efficiency, regardless of the outside air temperature or the temperature of the refrigerant discharged from the compressor. 
     EXPLANATION OF REFERENCE SIGNS 
     
         
           1 : waste heat utilization device 
           4 : Rankine circuit (refrigerant circuit) 
           5 : circulation path 
           5   a : high-pressure path section 
           5   b : low-pressure path section 
           10 : fluid machine 
           12 : expansion unit 
           22 : scroll unit (drive section) 
           33 : inlet port 
           45 : outlet port 
           102 : communication passage 
           104 : bypass passage 
           106 ,  112 ,  124 : valve mechanism 
           108 : ball (valve element) 
           114 : spring mechanism 
           116 ,  138 : spring 
           118 ,  139 : accommodation chamber 
           134 : electromagnetic valve (drive source) 
           136 : valve element