Patent Publication Number: US-2007101735-A1

Title: Heat pump apparatus using expander

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a heat pump apparatus using an expander.  
      2. Related Art  
      A conventional heat pump apparatus that is applied to air conditioners and water heaters decompresses and expands refrigerant with the use of an expansion valve. In recent years, there has been an attempt to recover the mechanical power with the use of an expander when the refrigerant expands from high pressure to low pressure, and at the same time to improve the energy efficiency of the vapor compression cycle utilizing that mechanical power. With the mechanical power obtained by the expander, a generator may be driven to generate electric power, and the electric power can then be supplied to a motor that drives the compressor, so that power consumption can be reduced.  
      The heat pump apparatus that uses an expansion valve and the heat pump apparatus that uses an expander require different operation stopping procedures. The operation stopping procedure for the heat pump apparatus using an expansion valve is quite simple, and it usually requires only stopping the driving of the compressor. In other words, it requires no control process. The state in which the refrigerant circuit is divided into a high-pressure side and a low-pressure side remains for a certain time, but this does not develop into a serious problem. Moreover, by fully opening the expansion valve when stopping the operation, the apparatus can make the pressure in the entire refrigerant circuit uniform quickly.  
      In contrast, the heat pump apparatus using an expander is incapable of making the pressure in the entire refrigerant circuit uniform instantly when stopping the operation. Since the high-pressure refrigerant remains at an intake side of the expander and the low-pressure refrigerant remains at a discharge side of the expander, the expander is kept under a large pressure difference between the high-pressure refrigerant and the low-pressure refrigerant. As a result, in the case of a rotary type expander, for example, its piston may undergo a free rotation in the cylinder at a high speed due to the remaining pressure difference, which may consequently cause the expander to break.  
      In the field of compressor, the problem associated with the remaining pressure difference when stopping the operation is known to a certain extent. The case of a compressor, however, is different from the case of an expander in that the compressor undergoes a force that induces a reverse rotation because the discharge side becomes a high pressure while the intake side becomes a low pressure. A proposal has been made to prevent the reverse rotation by, for example, providing a resistance element at a lower end of a shaft (JP 9-158851A).  FIG. 8  shows a cross-sectional view of the conventional scroll-type fluid machine disclosed in JP 9-158851A.  
      The scroll-type fluid machine  100  shown in  FIG. 8  includes a closed shell  1 , a compressor unit  20 , and a motor unit  7 . The bottom part of the closed shell  1  stores lubricating oil  37  for lubricating sliding parts of the compressor unit  20 . A rotating disk  40  is attached to a shaft  10 , and the rotating disk  40  is provided with slanted blades  41  serving as the resistance element in such a manner that they are submerged in the lubricating oil  37 .  
      Here, when the scroll-type fluid machine  100  stops its operation, even if the shaft  10  attempts to rotate in the reverse direction because of the pressure difference remaining in the compressor unit  20 , the slanted blades  41  of the rotating disk  40  receive resistance from the lubricating oil  37 , preventing the reverse rotation.  
      However, provision of the resistance element such as the slanted blades  41  necessitates extra driving power for the operation because the slanted blades  41  rotate in the lubricating oil  37  during the normal operation, causing resistance. First of all, in the case of the expander, the rotation in the normal direction is accelerated by the remaining pressure difference, so it is impossible to directly apply the techniques for preventing the reverse rotation of a compressor to the expander.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide a highly reliable heat pump apparatus using an expander, which is capable of preventing the expander from breakage when stopping the operation and requires no extra driving power.  
      The present invention provides a heat pump apparatus comprising:  
      a compressor for compressing a working fluid;  
      a motor for driving the compressor;  
      an expander for expanding the working fluid;  
      a generator connected to the expander, for converting mechanical power into electric power, the mechanical power being recovered by the expander when the working fluid expands; and  
      a variable-speed converter connected to the generator, for converting alternating current generated by the generator into direct current, wherein the variable-speed converter continues to control driving of the generator after an operation stop trigger occurs for lowering rotational speed of the motor and stopping the operation of the heat pump apparatus at least until a value of current flowing through the generator becomes equal to or less than a predetermined value, and stops the working of the generator after the value of current flowing through the generator becomes equal to or less than the predetermined value.  
      According to the invention as described above, the variable-speed converter continues to control the driving (to control the speed) of the generator after the operation stop trigger occurs at least until the current value generator becomes equal to or less than the predetermined value. Then, it waits until the value of current flowing through the generator becomes equal to or less than the predetermined value, in other words, until the high pressure/low pressure difference of the working fluid in the heat pump cycle reduces sufficiently, and then stops the working of the generator. That is, it does not allow the expander to freely rotate during the period in which the value of current flowing through the generator is large. Meanwhile, after the operation stop trigger occurs, the speed of the motor is reduced; therefore, the high pressure/low pressure difference of the working fluid in the heat pump cycle reduces gradually, and accordingly the value of current flowing through the generator decreases gradually. Then, after the value of current flowing through the generator becomes sufficiently small, the working of the generator is stopped. In this way, the expander, which revolves in synchronization with the generator, does not revolve at such a high speed that the constituting components may break. Moreover, since the control is performed by an electric circuit and no extra resistance element such as the slanted blades is needed, no additional driving power is required during the normal operation. Thus, a highly reliable heat pump apparatus is made available.  
      In another aspect, the present invention provides a heat pump apparatus comprising:  
      an expander for expanding a working fluid;  
      a generator connected to the expander, for converting mechanical power into electric power, the mechanical power being recovered by the expander when the working fluid expands;  
      a variable-speed converter connected to the generator, for converting alternating current generated by the generator into direct current; and  
      a protection circuit connected to the variable-speed converter, wherein the protection circuit is supplied with a control signal while the expander is being driven, and consumes or stores regenerative power generated by the generator when the supply of the control signal stops.  
      According to the present invention as described above, even if the driving of the motor stops abnormally such as in the event of power failure, the protection circuit consumes or stores the regenerative power in place of the motor. Therefore, the voltage of the power line does not excessively rise, and the electric circuits do not break. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram illustrating a heat pump apparatus using an expander, according to Embodiment 1 of the present invention;  
       FIG. 2  is a detailed view illustrating a portion of  FIG. 1 ;  
       FIG. 3  is a detailed circuit diagram illustrating a variable-speed converter;  
       FIG. 4A  is a transition diagram illustrating speed of the expander, generator&#39;s current, and speed of the compressor when the heat pump apparatus stops the operation;  
       FIG. 4B  is an another transition diagram illustrating speed of the expander, generator&#39;s current, and speed of the compressor when the heat pump apparatus stops the operation;  
       FIG. 5  is a general flowchart illustrating a stop process of the heat pump apparatus;  
       FIG. 6  is a detailed flowchart illustrating the stop process of the heat pump apparatus;  
       FIG. 7  is a partial block diagram illustrating a heat pump apparatus according to Embodiment 2 of the present invention; and  
       FIG. 8  is a cross-sectional view illustrating a conventional scroll-type fluid machine. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinbelow, preferred embodiments of the present invention will be described with reference to the drawings.  
     Embodiment 1  
       FIG. 1  is a block diagram of a heat pump apparatus using an expander, according to Embodiment 1 of the present invention. A heat pump apparatus  600  includes an expander  501  for expanding refrigerant serving as a working fluid, an evaporator  601  for evaporating the refrigerant expanded by the expander  501 , a compressor  602  for compressing the refrigerant that has evaporated, a radiator  603  at which the refrigerant compressed by the compressor  602  radiates its heat, and a refrigerant pipes  604  for connecting these components in that order. The expander  501 , the evaporator  601 , the compressor  602 , the radiator  603 , and the refrigerant pipes  604  together form a closed circuit in which the refrigerant circulates. Application examples of the heat pump apparatus  600  include air conditioners and water heaters. In the case of the air conditioner, one of the evaporator  601  and the radiator  603  serves as an indoor heat exchanger while the other serves as an outdoor heat exchanger. In the case of the water heater, the evaporator  601  serves as an outdoor heat exchanger while the radiator  603  serves as a water heat exchanger.  
      Examples of the type of the refrigerant include, but are not particularly limited to, carbon dioxide and hydrofluorocarbon (HFC). Carbon dioxide allows the high pressure/low pressure difference in the refrigeration cycle to be greater than other refrigerants, so application of the present invention is particularly effective for the heat pump apparatus that uses carbon dioxide as the refrigerant.  
      The heat pump apparatus  600  is further provided with a main control unit  650 . The main control unit  650  contains electrical components such as a microcomputer, an input/output circuit, and an A/D converter, and it manages the overall operations of the heat pump apparatus  600 .  
      The generator  503  is mechanically connected to the expander  501 , and a variable-speed converter  505  (a variable-frequency converter) is electrically connected to the generator  503 . The expander  501 , the generator  503 , and the variable-speed converter  505  together form a mechanical power recovery device  500  that recovers the expansion force of the refrigerant and converts it into electric power. The variable-speed converter  505  is communicatively connected to the main control unit  650 .  
      As illustrated in  FIG. 2 , which shows a detailed partial view, the mechanical power recovery device  500  further includes a shaft  502  for connecting the expander  501  and the generator  503 , a three-phase power line  504  for connecting the generator  503  and the variable-speed converter  505 , and a pair of DC power lines  506  and  507  connected to the variable-speed converter  505  and used for electric power regeneration. The expander  501  and the generator  503  rotate in synchronization with each other with the shaft  502 . Normally, the speed (rotational frequency) of the expander  501  and the speed (rotational frequency) of the generator  503  are the same because no speed changing mechanism is provided. Hereinbelow, the DC power lines  506  and  507  are also referred to as “regenerative power lines”.  
      Examples of the type of the expander  501  include positive displacement type expanders such as a scroll type expander in which two spiral blades rotate, a rotary type expander in which a piston is rotated in a cylinder, and a reciprocating type expander in which a piston is reciprocated in a cylinder. The generator  503  is, for example, a permanent magnet synchronous generator furnished with three-phase windings  508 , and converts the mechanical power that is recovered by the expander  501  when the refrigerant expands, into electric power. The variable-speed converter  505  includes a switching device group  509 . The switching device group  509  has the function as a converter unit that converts alternating current generated by the generator  503  into direct current. The variable-speed converter  505  is provided with a control unit  510 . The control unit  510  has the function to acquire the value of electric current of the three-phase power line  504 , which is the value of current flowing through the generator  503 , via an electric current value measuring line  511 , also the function to output a control signal to a control line  512  connected to the switching device group  509 , and the function to control the driving of the generator  503 . It should be noted that an induction generator may be used as the generator  503  in place of the permanent magnet synchronous generator.  
      As illustrated in  FIG. 1 , the compressor  602  is mechanically connected to a motor  605 , and the motor  605  is electrically connected to a motor controller  606 . The motor controller  606  is connected to main DC power lines  610  and  611 , which extend from an AC power supply  607  via a rectifier circuit  608  and a smoothing capacitor  609 . The main DC power lines  610  and  611  are also connected with the regenerative power lines  506  and  507 . One end of each of the regenerative power lines  506  and  507  is connected to the variable-speed converter  505 , and the other end thereof is connected to the main DC power lines  610  and  611 . As with the variable-speed converter  505 , the motor controller  606  is also communicatively connected to the main control unit  650 .  
      The motor  605  drives the compressor  602 . The motor controller  606  is an inverter that converts DC voltage into three-phase alternating voltage and controls the driving of the motor  605 . The rectifier circuit  608  rectifies the AC voltage from the AC power supply  607  to DC voltage, and the smoothing capacitor  609  smoothes the DC voltage. Examples of the type of the compressor  602  include a scroll type, a rotary type, and a reciprocating type, as in the case of the expander  501 .  
      The direct current recovered by the mechanical power recovery device  500  having the expander  501  is supplied through the regenerative power lines  506  and  507  to the main DC power lines  610  and  611 , and is consumed by the motor  605  for driving the compressor  602 .  
      As illustrated in  FIG. 2 , the mechanical power obtained when the refrigerant expands in the expander  501  is given to the generator  503  via the shaft  502  and is converted into AC power by the generator  503 . The alternating current originating from the AC power is converted into direct current by the variable-speed converter  505 , which is connected to the generator  503  by the three-phase power line  504 . The variable-speed converter  505  allows the generator  503  to revolve at a given target speed by switching the switching device group  509  using a PWM (Pulse Width Modulation) technique. The function to control the speed of the generator  503  makes it possible to control the speed of the expander  501 , which is connected to the generator  503  by the shaft  502 . In other words, the switching control of the variable-speed converter  505  makes it possible to control the speed of the generator  503  and the expander  501  over a wide range. The expansion power from the expander  501  that is converted into direct current by the variable-speed converter  505  is used for driving the motor  605  (cf.  FIG. 1 ).  
       FIG. 3  is a detailed circuit diagram of the variable-speed converter. Voltage dividers  813   a  and  813   b  are provided between the pair of the regenerative power lines  506  and  507 . The variable-speed converter  505  is furnished with: (a) two current sensors  805   a  and  805   b ; (b) a converter circuit composed of switching devices  803   a ,  803   b ,  803   c ,  803   d ,  803   e , and  803   f , each of which is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor), and free wheel diodes  804   a ,  804   b ,  804   c ,  804   d ,  804   e , and  804   f , which are provided to form pairs with the switching devices; and (c) a control circuit containing a biaxial current converting means  806 , a rotor-position and speed estimating means  807 , the control unit  510 , a base driver  808 , a sine-wave-voltage outputting means  809 , an electric-current controlling means  810 , an electric-current-reference creating means  811 , and a speed controlling means  812 . The converter circuit (b) is a part that corresponds to the switching device group  509  shown in  FIG. 2 . The control circuit (c) can be constructed by a general-purpose microcomputer, which incorporates and executes a program for achieving the functions that the foregoing respective means should take on, an input/output circuit, and so forth. The foregoing respective means may be composed of logic circuits that are designed to be dedicated to achieving these functions.  
      The three-phase alternating current output of the generator  503  is supplied to, for example, a DC power supply  801  and a smoothing capacitor  802  via the variable-speed converter  505 . The DC power supply  801  corresponds to the main DC power lines  611  and  610  shown in  FIG. 1 . The three-phase alternating current output is further converted into direct current by the variable-speed converter  505 . At that time, the variable-speed converter  505  performs a control process based on the information of a target speed supplied from outside (the main control unit  650  of the heat pump apparatus  600  in the present embodiment) so that the speed of the generator  503  will reach the target speed.  
      Specifically, a switching pattern of the switching devices  803   a  to  803   f  of the variable-speed converter  505  is determined based on the information about the speed of the generator  503 , the information about the target speed supplied from outside, and the magnetic pole position information of the generator  503  that is estimated from the electric current information of the generator  503 , which is obtained from the current sensors  805   a  and  805   b . This switching pattern signal is converted by the base driver  808  into drive signals for electrically driving the switching devices  803   a  to  803   f , and according to the drive signals, the switching devices  803   a  to  803   f  are operated.  
      Next, the behavior of the variable-speed converter  505  will be described. It should be noted that the behavior of the inverter  506  is generally common to the behavior of the variable-speed converter  505 , and therefore the behavior of the inverter  506  will not be elaborated upon in the present specification.  
      First, in order to attain a target speed ω* given from outside, an electric current reference I* is computed by the speed controlling means  812  based on the deviation from the actual speed ω according to the following (Eq. 1). The computing is carried out according to the common PI control technique.
 
 I*=G   pω ×(ω*−ω)+ G   iω ×Σ(ω*−ω)  [Eq. 1]
 
      In the equation, G pω and G iω are speed-control proportional gain and speed-control integral gain, respectively, ω is actual speed, ω* is target speed, and I* is electric current reference.  
      Next, using the computed electric current reference I*, the electric current reference creating means  811  computes d-axis current reference I d * and q-axis current reference I q * according to the following (Eq. 2) and (Eq. 3).
 
 I   d   *=I* ×sin(β)  [Eq. 2]
 
 I   q   *=I* ×cos(β)  [Eq. 3]
 
      In the equations, β represents current phase angle.  
      Meanwhile, the biaxial current converting means  806  converts phase currents I u  and I v  of the generator  503 , which have been detected by the current sensors  805   a  and  805   b , into biaxial currents, q-axis current I q  that contributes to the magnet torque of the generator  503  and d-axis current I d  that is orthogonal to the q-axis current I q , according to the following (Eq. 4).  
                 I   a     =         3   2       ×     I   u         ⁢     
     ⁢       I   b     =           1   2       ×       (       I   u     +     2   ×     I   v         )     ⁢     
     [           I   d               I   q           ]       =       [           cos   ⁡     (   θ   )             sin   ⁡     (   θ   )                 -     sin   ⁡     (   θ   )               cos   ⁡     (   θ   )             ]     ⁢           [           I   a               I   b           ]                 [     Eq   .           ⁢   4     ]             
 
      Here, θ represents rotor position (magnetic pole position of the generator).  
      Then, using the electric current references I d * and I q * as well as the electric current values I d  and I q , the electric current controlling means  810  executes a control operation according to the following (Eq. 5) and (Eq. 6) so as to accomplish the electric current references, and thus obtains output voltages V d  and V q .
 
 V   d   =G   pd ×( I   d   *−I   d )+ G   id ×Σ(I d   *−I   d )  [Eq. 5]
 
 V   q   =G   pq ×( I   q   *−I   q )+ G   iq ×Σ( I   q   *−I   q )  [Eq. 6]
 
      Here, V d  and V q  are d-axis voltage and q-axis voltage, respectively, G pd  and G id  are d-axis current control proportional gain and d-axis integral gain, respectively, and G pq  and G iq  are q-axis current control proportional gain and q-axis integral gain; respectively.  
      Next, from the output voltages V d  and V q  thus obtained, three-phase output voltages V u , V v , and V w  are obtained using the rotor position θ by a common two-phase/three-phase conversion as represented by the following (Eq. 7) so that the output waveform will be a sine wave.  
               [           V   a               V   b           ]     =           [           cos   ⁡     (   θ   )             -     sin   ⁡     (   θ   )                   sin   ⁡     (   θ   )             cos   ⁡     (   θ   )             ]     ⁢           [           V   d               V   q           ]     ⁢     
     [           V   u               V   v               V   w           ]     =       [             2   3           0             -       1   6                 1   2                 -       1   6               -       1   2               ]     ⁢           [           V   a               V   b           ]               [     Eq   .           ⁢   7     ]             
 
      Here, V u , V v , and V w  represent U-phase voltage, V-phase voltage, and W-phase voltage, respectively, and θ represents rotor position.  
      Further, the sine-wave-voltage outputting means  809  outputs a drive signal for driving the generator  503  to the base driver  808 , based on the output voltages V d , V q  and the information about the rotor position θ that is estimated by the rotor-position and speed estimating means  807 . The base driver  808  outputs a signal (PWM signal) for driving the switching devices  803   a  to  803   f  according to the drive signal. Thus, the generator  503  is driven at the target speed (target speed).  
      Next, the following describes the behavior of the heat pump apparatus  600  when stopping its operation.  
       FIGS. 4A and 4B  are transition diagrams illustrating speed of the expander  501 , electric current (root-mean-square value) flowing through the generator  503 , and speed of the compressor  602  when the heat pump apparatus  600  stops the operation.  FIG. 5  is a general flowchart illustrating a stopping procedure of the heat pump apparatus.  FIG. 6  is a flowchart illustrating the control process executed by the main control unit  650  when the heat pump apparatus  600  stops the operation.  
      The operation stopping sequence shown in  FIG. 5  is started in response to the occurrence of a predetermined operation stop trigger. In the case that the heat pump apparatus  600  is applied to an air conditioner, examples of the operation stop trigger may include turning off of the operation switch by the user, the elapse of a predetermined time counted by an automatic shut-off timer, and a room temperature that has reached a target value. In the case that the heat pump apparatus  600  is applied to a water heater, examples of the operation stop trigger include an amount of the stored hot water or a temperature of the stored hot water that has reached a target value.  
      Step S 1  shown in  FIG. 5  corresponds to t 1 -t 2  in  FIG. 4A , step S 2  corresponds to t 2 -t 3 , and step S 3  corresponds to t 3 -t 4 . Step S 1  is a step for lowering the target speed of the motor  605  and the generator  503  to lower the speed of the compressor  602  and the expander  501 . Thereby, the high pressure/low pressure difference of the refrigerant is reduced. Step S 2  is for retaining the speed of the compressor  602  and the expander  501  until the value of current flowing through the generator  503  becomes a predetermined value. However, if step S 1  is executed for a sufficiently long time, in other words, if the speed of the compressor  602  and the expander  501  are gradually lowered, the pressure difference of the refrigerant accordingly reduces sufficiently, and the value of current flowing through the generator  503  lowers also; therefore, if that is the case, step S 2  is not necessarily required. Lastly, in step S 3 , the working of the motor  605  and the generator  503  are stopped.  
      Thus, in response to the occurrence of the operation stop trigger, an inverter  606  lowers the speed of the motor  605 . After the operation stop trigger occurs, the variable-speed converter  505  continues to control the driving (to control the speed) of the generator  503  at least until the electric current flowing through the generator  503  becomes equal to or less than a predetermined value, and on the condition that the value of current flowing through the generator  503  becomes equal to or less than the predetermined value, it stops the working of the generator  503 . This prevents the expander  501  from rotating at a high speed due to the remaining pressure difference of the refrigerant.  
      Further details are given with reference to the flowchart of  FIG. 6 . After acquiring the operation stop trigger, the main control unit  650  of the heat pump apparatus  600  calls and executes the program shown in the flowchart of  FIG. 6 .  
      First, in step ST 1 , the main control unit  650  reads the value of current flowing through the generator  503 . The value of current through the generator  503  may be acquired from the control unit  510  of the variable-speed converter  505  or may be acquired directly from the current sensors  805   a  and  805   b  (cf.  FIG. 3 ).  
      Next, in step ST 2 , it is assessed whether or not the electric current value that has been read is equal to or less than a predetermined value (IE 1 ) (electric current value determining means (A)). Since this predetermined value (IE 1 ) is set at a sufficiently small value, the value of current flowing through the generator  503  is usually greater than the predetermined value (IE 1 ) immediately after the start of the operation stopping sequence. Next, in step ST 3 , it is assessed whether or not the speed of the motor  605  is equal to or less than a predetermined speed (C 1 ). If the speed of the motor  605  is greater than the predetermined speed (C 1 ), the process proceeds to step ST 4 , in which the target speed of the motor  605  is lowered. The speed of the motor  605  can be identified from the target speed of the motor that is stored in the main control unit  650  at all times. The newly set target speed is supplied from the main control unit  650  to the inverter  606 . It should be noted that the rate of lowering of the speed is not particularly limited, so it may be lowered gradually in a stepwise manner to the predetermined speed (C 1 ) or may be abruptly lowered to the predetermined speed (C 1 ) in one step, although the former is assumed in the present embodiment.  
      More specifically, the inverter  606  lowers the speed of the motor  605  in response to the occurrence of an operation stop trigger so that a high pressure/low pressure difference of the refrigerant reduces, and it continues to control the driving of the motor  605  in order to consume regenerative power until the value of current flowing through the generator  503  becomes equal to or less than the predetermined value (IE 1 ). If the motor  605  is stopped immediately after the operation stop trigger occurs, the electric power generated by the generator  503  will not be consumed and consequently the electric circuits may be destructed. According to the present embodiment, however, the regenerative power can be consumed reliably because the controlling of driving of the motor  605  is also continued, so the destruction of the electric circuits can be prevented reliably. Moreover, in the present embodiment, as will be seen from  FIG. 4A , the speed of the motor  605  and the speed of the generator  503  are lowered in synchronization with each other, and they are monotonously decreased at a constant rate. In particular, gradually lowering the speed of the motor  605  is preferable because it eliminates the possibility that the motor  605  may perform a regeneration operation.  
      Next, in step ST 5 , it is assessed whether or not the speed of the generator  503  is equal to or less than a predetermined speed (E 1 ). If the speed of the generator  503  is greater than the predetermined speed (E 1 ), the process proceeds to step ST 6 , in which the target speed of the generator  503  is lowered. The newly set target speed is supplied to the variable-speed converter  505 . As a consequence, the generator  503  is driven at a lower speed. The predetermined speed (E 1 ) of the generator  503  and the predetermined speed (C 1 ) of the motor  605  may be set at a low speed, for example, at 15 rps. to 20 rps., although they may depend on the volumes of the expander  501  and the compressor  602 . It should be noted here that in the present embodiment, the speed of the generator  503  always matches that of the expander  501 , and likewise, the speed of the motor  605  always matches that of the compressor  602 .  
      Steps ST 1  to ST 6  in the flowchart of  FIG. 6  correspond to t 1  to t 3  in the transition diagram of  FIG. 4A . As shown in  FIG. 4A , when the main control unit  650  starts the operation stopping sequence at time t 1 , the variable-speed converter  505  receives a lower target speed ω* of the generator  503  and accordingly lowers the speed of the expander  501 . Subsequently, when the speed reaches E 1  at time t 2 , the expander  501  is kept at the speed E 1  by the variable-speed converter  505 . The expander  501  is kept at the speed E 1  in this way in order to wait until the pressure difference in the expander  501  reduces sufficiently to prevent the expander  501  from revolving at a high speed due to the pressure difference. Of course, as already mentioned above, when the rate of lowering of the speed is sufficiently small, the step of keeping the speed is not necessarily required.  
      Further, the variable-speed converter  505  continues to control the driving of the generator  503  until the value of current flowing through the generator  503  becomes equal to or less than the predetermined value (IE 1 ) so as to attain a lower speed than that at the time when the operation stop trigger has occurred. In the present embodiment, as illustrated in the transition diagram of  FIG. 4A , the speed of the motor  605  and that of the generator  503  are synchronized with each other and decreased monotonously from the time at which the operation stop trigger occurs. This is advantageous to reduce the noise during stopping the operation.  
      It should be noted, however, that it is not necessary to reduce the speed of the generator  503  immediately after the operation stop trigger occurs, and the lowering of the speed does not need to start at the same time for both the motor  605  and the generator  503 . For example, as illustrated in the transition diagram of  FIG. 4B , immediately after the operation stop trigger occurs, the variable-speed converter  505  continues to control the driving of the generator  503  so as to attain the speed at the time when the operation stop trigger occurs (time t 1 ). The decelerating of the motor  605  is necessary in order to reduce the high pressure/low pressure difference of the refrigerant. After the high pressure/low pressure difference of the refrigerant has reduced to an appropriate level, the speed of the generator  503  is gradually lowered. This makes it possible to more quickly reduce the high pressure/low pressure difference of the refrigerant and is therefore advantageous in shortening the time required for completely stopping the operation. The timing for starting to lower the speed of the generator  503  or the rate of the lowering are not particularly limited, but they should be adjusted so as to avoid increasing the level of the noise or causing undue stress to the mechanical components.  
      As seen in step ST 2  shown in the flowchart of  FIG. 6 , the heat pump apparatus  600  is furnished with the means (A) for determining whether or not the value of current flowing through the generator  503  is equal to or less than the predetermined value (IE 1 ) in response to the occurrence of an operation stop trigger. Of course, the means (A) may be included in the variable-speed converter  505 . The variable-speed converter  505  continues to control the driving of the generator  503  until the value of current flowing through the generator  503  becomes equal to or less than the predetermined value (IE 1 ) if the value of current flowing through the generator  503  exceeds the predetermined value (IE 1 ), and further, it stops the working of the generator  503  if the value of current flowing through the generator  503  becomes equal to or less than the predetermined value (IE 1 ). Thus, performing the controlling of the generator  503  while monitoring the electric current value makes it possible to accurately identify the timing at which the value of current flowing through the generator  503  reaches the predetermined value (IE 1 ) and therefore serves to quickly stop the operation of the heat pump apparatus  600 .  
      Referring back to the flowchart of  FIG. 6 , the description will proceed further. When the speed of the motor  605  and the generator  503  are lowered and the value of current flowing through the generator  503  is reduced to a level equal to or less than the predetermined value (IE 1 ), it is assessed in step ST 7  whether or not the generator  503  is performing a powering operation. If the generator  503  is in the powering operation, the target speed is set to be zero in step ST 9 . That is, the inverter  606  is supplied with an instruction for immediately stopping the working of the motor  605  and the variable-speed converter  505  is supplied with an instruction for immediately stopping the working of the generator  503 .  
      There may be a very rare case in which the generator  503  is operated temporarily in a powering operation, in other words, driven as a motor. For example, when the high pressure/low pressure difference of the refrigerant is small immediately after starting the operation of the heat pump apparatus  600 , the generator  503  may temporarily be operated in a powering operation. If an operation stop trigger occurs in this situation, the controlling of the speed of the motor  605  and the generator  503  may be stopped immediately. Even if the expander  501  is permitted to freely rotate, it will not revolve at a high speed such as to cause destruction of the components. Rather, by immediately stopping the motor  605 , unnecessary electric power consumption can be avoided.  
      On the other hand, if the value of current flowing through the generator  503  is equal to or less than the predetermined value (IE 1 ) and the generator  503  is not in a powering operation, a DC excitation to the phase windings  508  of the generator  503  is performed in step ST 8 , and the rotation of the generator  503  is stopped. The target speed is set to be zero for both the motor  605  and the generator  503  (ST 9 ). The control unit  510  of the variable-speed converter  505  gives an instruction to the base driver  808  such that switching will be effected in the DC excitation pattern. Thereby, the field is fixed in the generator  503 , and a brake force according to the magnetic force is applied to the generator  503 .  
      As illustrated in  FIG. 4A , the current flowing through the generator  503  flows corresponding to the pressure difference of the refrigerant in the expander  501 , so the electric current value gradually decreases from time t 1 . When the current reaches the predetermined value (IE 1 ) at time t 3 , the DC excitation is performed for the phase windings  508  of the generator  503  by the control unit  510  of the variable-speed converter  505 . That is, a brake is applied to the generator  503  by passing a direct current (IE 2 ) therethrough until time t 4 , whereby the rotation of the expander  501  is stopped.  
      The predetermined value (IE 1 ) can be a value determined such that the electric current flowing through the generator  503  will not exceed the capacity of the switching device group  509  of the variable-speed converter  505  when a DC excitation is performed for the phase windings  508  of the generator  503 , in other words, it can be an electric current value that guarantees the switching devices not to be destructed. In addition, the predetermined value (IE 1 ) may be a value of current flowing through the generator  503  when the pressure difference in the expander  501  has become sufficiently small, and it may be, for example, about 10 amperes, at which the speed of the expander  501  does not increase again with the pressure difference at the time when the rotation of the expander  501  is stopped by the DC excitation. In other words, the predetermined value (IE 1 ) can be a value that is determined so that the speed of the generator  503  can reduce when the DC excitation is performed for the phase windings  508 . The electric current value (IE 2 ) when the DC excitation is conducted is not particularly limited, and it may be set at a value equal to, or slightly greater or less than, the predetermined value (IE 1 ).  
      Moreover, it is preferable that the generator  503  be a permanent magnet synchronous generator, which is highly efficient. In this case, the predetermined value (IE 1 ) can be a value that is determined to be such that the electric current flowing though the permanent magnet synchronous generator does not exceed the demagnetization current of the permanent magnet synchronous generator when the DC excitation is performed for the phase windings  508 .  
      Determining the predetermined value (IE 1 ) as described above makes it possible to stop the generator  503  and consequently stop the expander  501  reliably without destructing the switching devices or demagnetizing the magnets used in the generator  503 . Of course, the DC excitation is not necessarily required for the present invention, but when the apparatus needs to be stopped quickly, it is preferable that the DC excitation be performed.  
      It should be noted that, in the present embodiment, the value of current flowing through the generator  503  is successively monitored to adjust the stop timing, but the monitoring of the electric current value is not necessarily required from the viewpoint, according to the invention, that the apparatus should be stopped after the high pressure/low pressure difference of the refrigerant has reduced sufficiently. Specifically, it is possible to execute a predetermined operation stopping sequence without monitoring the electric current value successively, for example, an operation stopping sequence specified by the transition diagrams of  FIGS. 4A and 4B  of the present embodiment.  
      Furthermore, as will be discussed below, because it is possible to estimate the pressure difference between the high-pressure refrigerant and the low-pressure refrigerant in the rotary type expander  501  from the value of current flowing through the generator  503 , the stop timing may be adjusted by monitoring the pressure of the refrigerant, instead of monitoring the electric current value. For example, adjusting the stop timing based on the refrigerant pressure measured with a pressure sensor yields exactly the same result as that attained by the present embodiment, which adjusts the stop timing by monitoring the value of current flowing through the generator  503 . It should be noted, however, that pressure sensors are generally costly, so the technique that uses the electric current value as in the present embodiment is preferable.  
      Expander torque T exp , applied to the expander  501 , can be expressed as follows, with current I exp  flowing through the generator  503  and generator torque constant Kt inherent to the generator  503 .
 
 T   exp   =K   t   ×I   exp   [Eq. 8]
 
      In addition, the following (Eq. 9) holds where the pressure of the high-pressure refrigerant taken into the expander  501  is P d , the pressure of the low-pressure refrigerant discharged therefrom is P s , the designed intake volume of the expander  501  is V exp , the thermal insulation coefficient is k, and the speed of the expander  501  is f.
 
 T   exp   =k/ ( k− 1)× P   s   ×V   exp ×{( P   d   /P   s ) (k−1)/k −1}/2 nf   [Eq. 9]
 
      From (Eq. 8) and (Eq. 9), the pressure difference between the high-pressure refrigerant and the low-pressure refrigerant in the rotary type the expander  501  can be estimated.  
      According to the present embodiment, the rotation of the expander  501  is stopped based on the value of current flowing through the generator  503 , which relates to the pressure difference of the refrigerant in the expander  501 . This can prevent the speed of the expander  501  from increasing excessively due to the remaining pressure difference of the refrigerant. Moreover, since a brake is applied to the expander  501  when stopping the operation, the expander  501  can be stopped quickly. Furthermore, the variable-speed converter  505  continues to control the driving of the motor  605  even after the operation stop trigger occurs, so no electrical components will suffer from destruction. Thus, the present embodiment can provide a highly reliable heat pump apparatus using an expander.  
      When the inverter  606  stops controlling of the speed of the motor  605 , the speed of the compressor  602  rapidly drops to zero. It is important to gradually lower the speed of the motor  605  as in the present embodiment (preferably in synchronization with the generator  503 ) from the viewpoint of ensuring where the regenerative power is consumed. However, it is possible to finish controlling the speed of the motor  605  before the value of current flowing through the generator  503  becomes equal to or less than a predetermined value and to stop the compressor  602  if the regenerative power can be consumed even without using the motor  605 .  
      For example, it is possible to provide a protection circuit as illustrated in the second embodiment, which will be discussed below, specifically, a protection circuit that can consume the regenerative power of the generator  503  in place of the motor  605 .  
      As will be explained in Embodiment 2 (cf.  FIG. 7 ), such a protection circuit may include a first switch provided on the DC power line  506  connected to the variable-speed converter  505 , a second switch connected to the DC power lines  506  and  507  between the first switch and the variable-speed converter  505  and in parallel with the variable-speed converter  505 , and a load or an electricity storage unit connected to the DC power lines  506  and  507  in series with the second switch. During the normal operation, the first switch is on while the second switch is off. By turning the first switch off and the second switch on in response to the occurrence of an operation stop trigger, the regenerative power is consumed by the load or is stored in the electricity storage unit. As will be explained in Embodiment 2, it is preferable to turn the first switch off and turn the second switch on also in the event of power failure or abnormal shutdown of the power supply.  
      It should be noted that although the heat pump apparatus  600  according to the present embodiment has been described to have three separate control units, namely, the main control unit  650 , the control unit  510  for the variable-speed converter  505 , and a control unit (not shown) for the inverter  606 , it is also possible to put the functions of all the control units into one control unit.  
     Embodiment 2  
       FIG. 7  is a detailed view of a portion of a heat pump apparatus using an expander according to Embodiment 2 of the present invention, illustrating its expander side. In Embodiment 2 of the present invention, the components that serve the same functions as those in Embodiment 1 are denoted by same reference numerals, and the descriptions of their operations will be omitted.  
      A heat pump apparatus  700  is furnished with an expander  501 , a generator  503  connected to the expander  501  by a shaft  502 , a variable-speed converter  701  connected to the generator  503  by a three-phase power line  504 , and an expander protection circuit  702  connected to the variable-speed converter  701  by a pair of DC power lines  506  and  507 . The pair of DC power lines  506  and  507  extends from the expander protection circuit  702 .  
      The variable-speed converter  701  has a switching device group  509 . The switching device group  509  converts alternating current generated by the generator  503  into direct current. The expander protection circuit  702  is furnished with a first relay  703  provided on the DC power line  506 , and with a load resistance element  704  and a second relay  705 , which are connected in series between the pair of DC power lines  506  and  507  extending from the first relay  703  toward the variable-speed converter  701 . Furthermore, the expander protection circuit  702  has a relay controlling unit  706  for controlling opening/closing of the first relay  703  and the second relay  705  in response to a control current (control signal) that flows while the expander  501  is being driven. The relay controlling unit  706  has the function to control the first relay  703  and the second relay  705  with respective relay control signal lines  708  and  709 , for example, by receiving an input signal via a control current line  707  that passes the direct current obtained by converting the alternating current from an AC power supply.  
      During the normal operation of the heat pump apparatus  700 , the variable-speed converter  701  controls the first relay  703  and the second relay  705  so that the relay controlling unit  706  closes the first relay  703  and opens the second relay  705 . Consequently, the direct current converted by the variable-speed converter  701  is passed through the pair of DC power lines  506  and  507 . On the other hand, when the supply of the control current via the control current line  707  stops because of power failure or the like, the variable-speed converter  701  controls the first relay  703  and the second relay  705  so that the relay controlling unit  706  opens the first relay  703  and closes the second relay  705 , and thus, the direct current converted by the variable-speed converter  701  is consumed by the load resistance element  704 .  
      Therefore, even if direct current is generated due to the pressure difference of the refrigerant remaining in the expander  501  when the operation of the apparatus abnormally stops because of power failure or the like, the direct current is consumed by the load resistance element  704 , and the electric circuits are prevented from being destructed.  
      In addition, although this Embodiment 2 has illustrated an example in which the direct current converted by the variable-speed converter  701  is consumed by the load resistance element  704 , it is also possible to provide an electricity storage unit such as a capacitor in place of the load resistance element  704  to store the electricity.  
      Furthermore, although  FIG. 7  illustrates an example in which the first relay  703  is provided on the DC power line  506 , it is possible to provide the first relay  703  on the DC power line  507 . Moreover, non-contact type switches such as transistors may be employed in places of the relays  703  and  705 .  
      The heat pump apparatus according to the present invention is a highly reliable apparatus capable of preventing the expander from the damages originating from high-speed revolutions and free from destruction of the electric circuits when stopping the operation. Therefore, the invention is useful for such apparatus as air conditioners and water heaters containing this apparatus. Furthermore, the mechanical power recovery device contained in the heat pump apparatus of the present invention is also applicable to other heat pump cycles such as Rankine cycle, as a device for recovering the energy of expansion of working fluid.