Patent Publication Number: US-2011062062-A1

Title: Power recovery apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2009-212189, filed Sep. 14, 2009; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a power recovery apparatus. 
     BACKGROUND 
     A desalination apparatus supplies a reverse osmosis membrane (hereinafter referred to as a RO membrane) with seawater having a higher pressure than a reverse osmosis pressure. The desalination apparatus allows seawater to permeate the reverse osmosis membrane and thereby extracts fresh water from the seawater by filtering out salt. Further, the desalination apparatus ejects remaining seawater as highly-concentrated salt water (brine). At this time, the highly-concentrated salt water is ejected maintained at a high pressure, and therefore has high pressure energy. In recent years, aiming for energy saving, power recovery apparatuses are mounted on desalination apparatuses (see, Jpn. Pat. Appln. KOKAI Publication No. 2004-81913 and No. 2001-46842, for example). Power recovery apparatuses collect highly-concentrated salt water at a high pressure, and utilize pressure energy of the highly-concentrated salt water to press seawater. 
     Conventional power recovery apparatuses require a boost pump to further boost a pressure of seawater which has been pressed by using pressure energy. This is because the pressure of the seawater which has been pressed by using the pressure energy need be further boosted to a pressure of seawater to be supplied to the RO membrane. However, the boost pump is a factor which causes various problems. 
     Firstly, since the boost pump boosts up the pressure of seawater to a very high pressure, the boost pump need be constituted by a thick member so that the pump may not break down due to its own internal pressure. A problem therefore occurs in that pump efficiency extremely decreases and power consumption of the boost pump increases accordingly. 
     Further, the boost pump has a high internal pressure, which often causes leakages of inner fluids. Therefore, the working ratio of the apparatus decreases and causes a problem that clear water cannot stably supplied. 
     Further, a large number of pumps, such as water pumps, high pressure pumps, and boost pumps are installed in desalination plants. Since pumps require periodical maintenance, a large number of pumps installed in a plant cause increase in costs and labor for maintenance services. 
     Further, the boost pumps each are constituted by a thick member as described above, and are therefore relatively expensive components in plants. The boost pumps are therefore factors which increase construction costs of plants. 
     A power recovery apparatus described in one of the foregoing publications includes two RO membranes. Proposed herein is a technique to exclude installation of a boost pump, e.g., highly-concentrated salt water ejected from a first RO membranes is filtered by a second RO membranes. However, the RO membranes are expensive components, and the configuration described above is therefore a factor which may increase plant construction costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram representing a configuration of a desalination plant including a power recovery apparatus according to the first embodiment; 
         FIG. 2  represents a configuration of the power recovery apparatus in  FIG. 1  in a first state during operation of the power recovery apparatus; 
         FIG. 3  represents a configuration of the power recovery apparatus in  FIG. 1  in a second state during operation of the power recovery apparatus; 
         FIG. 4  illustrates a configuration of converters in  FIG. 2  and  FIG. 3 ; 
         FIG. 5  illustrates a configuration of a conventional power recovery apparatus; 
         FIG. 6  is a table listing specifications of a desalination apparatus used in numerical simulations; 
         FIG. 7  is a table listing results of a numerical simulation of a desalination apparatus including no power recovery apparatus; 
         FIG. 8  is a table listing results of a numerical simulation of a desalination apparatus including the power recovery apparatus in  FIG. 1 ; 
         FIG. 9  is a table listing results of a numerical simulation of a desalination apparatus including the power recovery apparatus in  FIG. 5 ; 
         FIG. 10  represents the first modification to the power recovery apparatus in  FIG. 2 ; 
         FIG. 11  represents the second modification to the power recovery apparatus in  FIG. 2 ; 
         FIG. 12  is a block diagram representing a configuration of a power recovery apparatus according to the second embodiment; 
         FIG. 13  is a block diagram representing a configuration of a power recovery apparatus according to the third embodiment; 
         FIG. 14  illustrates a crankshaft in  FIG. 13 ; 
         FIG. 15  is a block diagram representing a configuration of a power recovery apparatus according to the fourth embodiment; and 
         FIG. 16  illustrates a structure of rotary actuators in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a power recovery apparatus is used in a desalination apparatus. The desalination apparatus boosts a first pressure of seawater to a second pressure by a high-pressure pump, and extracts fresh water from the seawater at the second pressure and ejects concentrated water at a third pressure by a reverse osmosis membrane. The concentrated water at the third pressure is supplied to the power recovery apparatus. The power recovery apparatus collects energy of the concentrated water at the third pressure. The power recovery apparatus includes a pressure conversion section and a seawater supply section. The pressure conversion section includes a movable part dividing inside of the conversion section into first and second spaces, moves the movable part by causing the first space to receive the concentrated water at the third pressure from the reverse osmosis membrane, and pushes out seawater filled in the second space, in accordance with movement of the movable part, to output the seawater at the second pressure. The pressure conversion section further includes a drive mechanism which drives the movable part so as to output the seawater at the second pressure from the second space. The seawater supply section merges the seawater from the pressure conversion section with the seawater from the high-pressure pump. 
     First Embodiment 
       FIG. 1  is a block diagram representing a configuration of a desalination plant including a power recovery apparatus  60  according to the first embodiment. In the desalination plant in  FIG. 1 , seawater which is drawn up is subjected to a chemical treatment by a preprocessing system  10  and is fed to a safety filter  30  by a water pump  20 . Seawater which has passed through the safety filter  30  is supplied, on one end, to a high pressure pump  40  and, on another end, supplied to the power recovery apparatus  60 . At this time, a pressure P 3  of seawater output from the safety filter  30  is about 0.2 MPa. 
     The high pressure pump  40  boosts a pressure of the supplied seawater and outputs the boosted seawater to a high-pressure RO membrane  50 . At this time, a pressure P 4  after the boost is representatively 6.0 MPa although the pressure P 4  after the boost varies depending on the type of the high-pressure RO membrane  50 . 
     The high-pressure RO membrane  50  filters the seawater from the high pressure pump  40 . When the high-pressure RO membrane  50  has a recovery rate of 40%, 40% of seawater flowing into the high-pressure RO membrane  50  is extracted as fresh water and 60% of seawater is ejected as highly-concentrated salt water. The fresh water from the high-pressure RO membrane  50  is supplied to a low-pressure pump  80 , and the highly-concentrated salt water is supplied to the power recovery apparatus  60 . At this time, the pressure of the fresh water decreases to about 0.2 MPa (=P 3 ). However, a pressure P 6  of the highly-concentrated salt water is about 5.8 MPa. 
     The fresh water from the high-pressure RO membrane  50  is pressed again by the low-pressure pump  80 , and permeates the low-pressure RO membrane  90 , thereby filtering out contained boron. Further, the fresh water which has permeated the low-pressure RO membrane  90  is subjected to a chemical treatment in a clear water reservoir  100 , and is then supplied as clear water from a supply pump  110  to homes, etc. 
     The power recovery apparatus  60  boosts and outputs seawater from the safety filter  30  by using pressure energy which the highly-concentrated salt water has internally. Seawater from the power recovery apparatus  60  is merged with seawater from the high pressure pump  40  and is supplied together to the high-pressure RO membrane  50 . 
     An end of a valve  70  is open to air. An ejection flow rate of highly-concentrated salt water from which pressure energy has been collected by the power recovery apparatus  60  is controlled by the valve  70 . 
       FIG. 2  and  FIG. 3  are schematic diagrams each representing a configuration of the power recovery apparatus  60  according to the first embodiment, in operating states of the power recovery apparatus  60 . 
     At first, the configuration of the power recovery apparatus  60  will be described with reference to  FIG. 2 . The power recovery apparatus  60  in  FIG. 2  includes a pressure meter  61 , a 4-port switch valve  62 , a pressure conversion section  63 , a seawater supply section  64 , rod position detection sections  65 - 1  to  65 - 4 , a switch control section  66 , and a motor control section  67 . 
     The pressure meter  61  measures a pressure of highly-concentrated salt water supplied from the high-pressure RO membrane  50 , and notifies a measurement result thereof to the motor control section  67 . The 4-port switch valve  62  switches directions of flow of highly-concentrated salt water into a pressure conversion section  63  and ejection from the pressure conversion section  63 . The 4-port switch valve  62  switches the directions of flow-in and ejection of highly-concentrated salt water in accordance with a switch instruction from the switch control section  66 . A method for switching the 4-port switch valve may be of a pneumatic type, a hydraulic water type, a hydraulic oil type, and a solenoid coil type. Available as a water pressure source is highly-concentrated salt water, seawater from the water pump  20 , or high-pressure salt water from the high pressure pump  40 . 
     The pressure conversion section  63  includes converters  631 - 1  and  631 - 2 .  FIG. 4  is a schematic view illustrating a configuration of the converters  631 - 1  and  631 - 2 . The converters  631 - 1  and  631 - 2  have the same structures as each other. Therefore, only the converter  631 - 1  will be described with reference to  FIG. 4 . The converter  631 - 1  in  FIG. 4  includes a cylinder  6311 - 1 , a piston  6312 - 1 , and a shaft motor which consists of a movable member  6313 - 1  and a stationary member  6314 - 1 . 
     The cylinder  6311 - 1  includes three holes and forms a sealed space. 
     The piston  6312 - 1  is positioned inside the cylinder  6311 - 1 , and divides the sealed space into first and second spaces, with a seal material provided between the piston  6312 - 1  and the cylinder  6311 - 1 . The first space is supplied with highly-concentrated salt water, and the second space is supplied with seawater. 
     The movable member  6313 - 1  is constituted by a large number of magnets arrayed in a pipe. For example, the movable member  6313 - 1  is constituted by a coil. The movable member  6313 - 1  is driven in a lengthwise direction thereof when the stationary member  6314 - 1  is supplied with an electric current. The movable member  6313 - 1  and the stationary member  6314 - 1  make neither contact nor friction between each other. 
     Further, the movable member  6313 - 1  has an end bonded to the piston  6312 - 1  from the side of the second space, and another end protruding outside through a hole in the cylinder  6311 - 1 . A seal material is attached to an edge of the hole. Since the movable member  6313 - 1  is bonded to the piston  6312 - 1  from the side of the second space, an area A 1  where the piston  6312 - 1  faces the first space differs from an area A 2  where the piston  6312 - 1  faces the second space. Here, a relationship between the areas A 1  and A 2  is preset based on the pressure P 6  of highly-concentrated salt water from the high-pressure RO membrane  50 , the pressure P 4  of seawater from the high pressure pump  40 , friction between the cylinder  6311 - 1  and the piston  6312 - 1 , and friction between the cylinder  6311 - 1  and the movable member  6313 - 1 . 
     An electric current supplied to the stationary member  6314 - 1  is controlled by the motor control section  67 . 
     The seawater supply section  64  includes check valves  641 - 1  to  641 - 4 . The check valves  641 - 1  to  641 - 4  each independently open/close in accordance with environmental pressure differences. In this manner, seawater is supplied from the power recovery apparatus  60  to outside or to the pressure conversion section  63 . 
     The detection sections  65 - 1  and  65 - 2  are to detect positions of the movable member  6313 - 1  protruding from the converter  631 - 1 . The detection section  65 - 1  is located at a position where the movable member  6313 - 1  can be detected when the piston  6312 - 1  comes close to the left end of the cylinder  6311 - 1 . The detection section  65 - 2  is located at a position where the movable member  6313 - 1  is not detected when the piston  6312 - 1  comes close to the right end of the cylinder  6311 - 1 . The detection sections  65 - 1  and  65 - 2  output detection signals to the switch control section  66  when the movable member  6313 - 1  is detected and when the movable member  6313 - 1  is not detected, respectively. In this manner, the positions of the piston  6312 - 1  in the cylinder  6311 - 1  can be grasped. Detection sections  65 - 3  and  65 - 4  have the same configurations as the detection sections  65 - 1  and  65 - 2 , and detect positions of a movable member  6313 - 2  protruding from the converter  631 - 2 . The detection sections  65 - 3  and  65 - 4  output detection signals to the switch control section  66  when the movable member  6313 - 2  is detected and is not detected, respectively. In this manner, the positions of the piston  6312 - 2  in the cylinder  6311 - 2  can be grasped. A detection method for the detection sections  65 - 1  to  65 - 4  may be of a mechanical, electric, or optical type. Although the present embodiment is configured to output the detection signals to the switch control section  66 , movement of the movable members may alternatively be mechanically transmitted to the 4-port switch valve  62 . 
     The switch control section  66  outputs a switch instruction to the 4-port switch valve  62  in accordance with detection signals from the detection sections  65 - 1  to  65 - 4 . That is, when the switch control section  66  receives detection signals from the detection sections  65 - 1  and  65 - 4 , the switch control section  66  determines that the piston  6312 - 1  is positioned close to the left end of the cylinder  6311 - 1  and that the piston  6312 - 2  is positioned close to the right end of the cylinder  6311 - 2 . Further, the switch control section  66  outputs a switch instruction to make the converter  631 - 1  eject highly-concentrated salt water and to make the converter  631 - 2  be supplied with highly-concentrated salt water. Otherwise, when the switch control section  66  receives detection signals from the detection sections  65 - 2  and  65 - 3 , the switch control section  66  determines that the piston  6312 - 1  is positioned close to the right end of the cylinder  6311 - 1  and that the piston  6312 - 2  is positioned close to the left end of the cylinder  6311 - 2 . Further, the switch control section  66  outputs a switch instruction to the 4-port switch valve  62  to make the converter  631 - 1  be supplied with highly-concentrated salt water and to make the converter  631 - 2  eject highly-concentrated salt water. 
     The motor control section  67  controls an electric current supplied to the stationary members  6314 - 1  and  6314 - 2 , based on a measurement result from the pressure meter  61 . The stationary members  6314 - 1  and  6314 - 2  apply a leftward or rightward force to the movable members  6313 - 1  and  6313 - 2  based on the electric current supplied by the motor control section  67 . The pistons  6312 - 1  and  6312 - 2  are applied the leftward or rightward force by the movable members  6313 - 1  and  6313 - 2 . For example, when a measurement result from the pressure meter  61  decreases to be smaller than a value which has been expected beforehand, the motor control section  67  controls the electric current supplied to the stationary member  6314 - 1  in a manner that the movable member  6313 - 1  is driven to move in the same direction as the moving direction in which the piston  6312 - 1  is moving, in the state of  FIG. 2 . In the state of  FIG. 3 , the electric current supplied to the stationary member  6314 - 2  is controlled in a manner that the movable member  6313 - 2  is driven to move in the same direction as a moving direction in which the piston  6312 - 2  is moving. The shaft motors are driven when the piston  6312 - 1  and  6312 - 2  push the seawater filled in the second space while the shaft motors are not driven when the piston  6312 - 1  and  6312 - 2  eject highly-concentrated salt water from the first space. 
     Next, operation of the power recovery apparatus  60  configured as described above will be described. 
     The power recovery apparatus  60  in  FIG. 2  is in a state in which the converter  631 - 1  is supplied with highly-concentrated salt water while highly-concentrated salt water is ejected from the converter  631 - 2 . 
     Seawater from the safety filter  30  is supplied to a high-pressure pump  40  at 0.2 MPa (=P 3 ), and is supplied to the second space of the converter  631 - 2  through the check valve  641 - 4 . 
     Seawater boosted to 6.0 MPa (=P 4 ) by the high pressure pump  40  is merged with seawater from the power recovery apparatus  60 , and is supplied together to the high-pressure RO membrane  50 . At this time, the seawater from the power recovery apparatus  60  has been ejected from the second space of the converter  631 - 1  and passed through the check valve  641 - 2 . The high-pressure RO membrane  50  outputs fresh water and highly-concentrated salt water. 
     The highly-concentrated salt water ejected from the high-pressure RO membrane  50  passes through the pressure meter  61  and 4-port switch valve  62  and flows into the first space of the converter  631 - 1 . At this time, the second space of the converter  631 - 1  is filled with seawater. The highly-concentrated salt water moves the piston  6312 - 1  in the cylinder  6311 - 1  in a direction toward the second space, and ejects seawater in the second space while pressing the seawater. 
     A force N 1  acting in a leftward direction is now assumed to be applied to the piston  6312 - 1  from the movable member  6313 - 1 . An area where the piston  6312 - 1  faces the first space is A 1 , and an area where the piston  6312 - 1  faces the second space is A 2 . Hence, a pressure P 8  of the seawater which is ejected from the second space of the cylinder  6311 - 1  is expressed as P 8 =(P 7 *A 1 +N 1 )/A 2 , using a pressure P 7  of highly-concentrated salt water from the 4-port switch valve  62 . Accordingly, the pressure P 8  is substantially equal to or slightly higher than the pressure P 4 . The force N 1  can be either a positive or negative value, depending on differences between directions of motor thrusts. 
     States of the check valves  641 - 1  to  641 - 4  in  FIG. 2  will now be described below. 
     Since pressure P 8 &gt;pressure P 3 , the check valve  641 - 1  is closed. Since pressure P 8 &gt;pressure P 14 , the check valve  641 - 2  is opened. A pressure difference between the pressure P 8  and pressure P 14  can be considered to be a pressure loss when seawater passes through the check valve  641 - 2 . 
     Further, since pressure P 14 &gt;pressure P 13 , the check valve  641 - 3  is closed. Further, as an end of the valve  70  is open to air, a gauge pressure of the second space of the cylinder  6311 - 2  is therefore substantially zero. That is, P 13  is a small pressure. Therefore, P 3 &gt;P 13  is given, and the check valve  641 - 4  is opened. 
     Seawater from the safety filter  30  passes through the check valve  641 - 4  and flows into the second space of the converter  631 - 2 . At this time, the first space of the converter  631 - 2  is filled with highly-concentrated salt water. Since an end of the valve  70  is open to air, a gauge pressure of the first space of the converter  631 - 2  is substantially zero. Seawater which has passed through the check valve  641 - 4  has a pressure of 0.2 MPa, and moves the piston  6312 - 2  in the cylinder  6311 - 2  toward the first space. The piston  6312 - 2  moves toward the first space, thereby ejecting highly-concentrated salt water in the first space out through the 4-port switch valve  62  and the valve  70 . 
     When the operation as described above is continued, the piston  6312 - 1  moves close to the left end inside the cylinder  6311 - 1  and the piston  6312 - 2  moves close to the right end inside the cylinder  6311 - 2 . Then, the detection sections  65 - 1  detects the movable member  6313 - 1  to come into contact, and the detection sections  65 - 4  detects the movable member  6313 - 2  to go out of contact. Accordingly, detection signals are output from the detection sections  65 - 1  and  65 - 4  to the switch control section  66 . The switch control section  66  receives the detection signals from the detection sections  65 - 1  and  65 - 4 , and then issues a switch instruction to the 4-port switch valve  62  so as to switch directions of flow-in and ejection of highly-concentrated salt water. When flow-in and ejection of highly-concentrated salt water are switched over, the power recovery apparatus  60  enters into the state represented in  FIG. 3 . 
     In the power recovery apparatus  60  in  FIG. 3 , highly-concentrated salt water is supplied to the converter  631 - 2  and is ejected from the converter  631 - 1 . 
     The highly-concentrated salt water ejected from the high-pressure RO membrane  50  passes through the pressure meter  61  and 4-port switch valve  62  and flows into the first space of the converter  631 - 2 . At this time, the second space of the converter  631 - 2  is filled with seawater. Highly-concentrated salt water moves the piston  6312 - 2  in the cylinder  6311 - 2  toward the second space, and presses and ejects seawater in the second space. 
     A force N 2  acting in a leftward direction is now supposed to be applied to the piston  6312 - 2  from the movable member  6313 - 2 . An area where the piston  6312 - 2  faces the first space is A 1 , and an area where the piston  6312 - 2  faces the second space is A 2 . Accordingly, a pressure P 13  of the seawater which is ejected from the second space of the cylinder  6311 - 2  is expressed as P 13 =(P 7 *A 1 +N 2 )/A 2 , using a pressure P 7  of highly-concentrated salt water from the 4-port switch valve  62 . Accordingly, the pressure P 13  is substantially equal to or slightly higher than the pressure P 4 . The force N 2  can be either a positive or negative value, depending on differences between directions of motor thrusts. 
     States of the check valves  641 - 1  to  641 - 4  in  FIG. 3  will now be described below. 
     Since pressure P 13 &gt;pressure P 3 , the check valve  641 - 1  is closed. Since pressure P 13 &gt;pressure P 14 , the check valve  641 - 2  is opened. A pressure difference between the pressure P 13  and pressure P 14  can be considered to be a pressure loss when seawater passes through the check valve  641 - 3 . 
     Further, since pressure P 14 &gt;pressure P 8 , the check valve  641 - 2  is closed. Further, as an end of the valve  70  is open to air, a gauge pressure of the second space of the cylinder  6311 - 1  is therefore substantially zero. That is, P 8  is a small pressure. Therefore, P 3 &gt;P 8  is given, and the check valve  641 - 1  is opened. 
     Seawater from the safety filter  30  passes through the check valve  641 - 1  and flows into the second space of the converter  631 - 1 . At this time, the first space of the converter  631 - 1  is filled with highly-concentrated salt water. Since an end of the valve  70  is open to air, a gauge pressure of the first space of the converter  631 - 1  is substantially zero. Seawater which has passed through the check valve  641 - 1  has a pressure of 0.2 MPa, and moves the piston  6312 - 1  in the cylinder  6311 - 1  toward the first space. The piston  6312 - 1  moves toward the first space, and thereby ejects highly-concentrated salt water in the first space through the 4-port switch valve  62  and the valve  70 . 
     When the operation as described above is continued, the piston  6312 - 1  moves close to the right end inside the cylinder  6311 - 1 . Then, the detection section  65 - 3  detects that the movable member  6313 - 2  comes into contact, and the detection section  65 - 2  detects that the movable member  6313 - 1  goes out of contact. Therefore, detection signals are output from the detection sections  65 - 2  and  65 - 3  to the switch control section  66 . The switch control section  66  receives the detection signals from the detection sections  65 - 2  and  65 - 3 , and then issues a switch instruction to the 4-port switch valve  62  so as to switch directions of flow-in and ejection of highly-concentrated salt water. When flow-in and ejection of highly-concentrated salt water are switched over, the power recovery apparatus  60  enters again into the state represented in  FIG. 2 . 
     In the present embodiment, moving speeds of the piston  6312 - 1  and piston  6312 - 2  are made equal to each other by adjusting an opening rate of the valve  70 . In this manner, a flow rate of the water pump  20  does not chronographically vary, and stable operation is achieved. 
     Next, power consumption or, namely, desalination costs when fresh water of 1 m 3  is produced will be calculated and compared through numerical simulations between desalination apparatuses in three cases described below. The desalination apparatuses in the three cases are a desalination apparatus including no power recovery apparatus, a desalination apparatus including a conventional power recovery apparatus  120 , and a desalination apparatus including a power recovery apparatus  60  according to the present embodiment.  FIG. 5  is a schematic diagram representing a configuration of the conventional power recovery apparatus  120 . 
       FIG. 6  lists specifications of the desalination apparatuses used in the numerical simulations. Parameters for the desalination apparatuses are common to the numerical simulations. Further, pump efficiency of the boost pump  121  is set at a low value in consideration of the structure of the boost pump. 
       FIG. 7  lists results of a numerical simulation of the desalination apparatus including no power recovery apparatus. According to  FIG. 7 , a desalination cost is 5.08 kWh/m 3 . 
     Further,  FIG. 8  lists results of a numerical simulation of the desalination apparatus including the power recovery apparatus  60  according to the present embodiment.  FIG. 9  lists results of a numerical simulation of the desalination apparatus including a conventional power recovery apparatus  120 . 
     Descriptions below will be made with reference to  FIG. 8  and  FIG. 9 . 
     The valve  70  requires fluid resistance to some extent from reasons described above. Supposing that a pressure loss occurring in the valve  70  is proportional to a square of a flow rate (m 3 /s), resistance coefficients as represented in  FIG. 8  and  FIG. 9  are required. Further, when a piston moves inside a cylinder, frictional resistance is generated. Such frictional resistance was taken into consideration in the numerical simulations. In  FIG. 8  and  FIG. 9 , frictional resistance between a piston and a cylinder was set to 16333 N. Further, frictional resistance between a rod and a cylinder was set to 1776 N in  FIG. 8 . Also in  FIG. 8 , the power recovery apparatus  60  operated as intended, when cylinders were manufactured where an area ratio (A 2 /A 1 ) between pistons was set to 0.9882. 
     Results of numerical simulations, i.e., pressures and flow rates at respective sections in  FIG. 2  and  FIG. 5  were numerical values as listed in  FIG. 8  and  FIG. 9 . 
     A power W which a pump applies to a fluid is obtained by multiplying a flow rate Q by a pressure P. That is, the power of the water pump  20  in  FIG. 8  is calculated by 2.894*10 4  W, and power of the high-pressure pump  40  is calculated by 3.416*10 5  W. Further, a power of the water pump  20  in  FIG. 9  is calculated by 2.894*10 4  W, a power of the high-pressure pump  40  is calculated by 3.356*10 5  W, and a power of the boost pump  121  is calculated by 5.978*10 3  W. 
     Further, a required power W power recovery is obtained by an expression below. 
         W power recovery=ΣΔ PiQi/ηi   (1)
 
     In the above expression, ΔP is a pump head (Pa), Q is a flow rate (m3/s), and η is a pump efficiency. From the expression (1), the required power in  FIG. 8  is 452 kW. Further, the required power in  FIG. 9  is 460 kW. 
     Further, a power recovery rate ξ is calculated by an expression below. 
       ξ=100( W−W power recovery)/ W   (2)
 
     In the expression above, W is a required power (W) when no power recovery apparatus is included. From the expression (2), a power recovery rate in  FIG. 8  is 57.3%, and a power recovery rate in  FIG. 9  is 56.6%. 
     Further, a simple desalination cost γ is calculated by an expression below. 
       γ= W power recovery/ Q   (3)
 
     In the expression above, Q is a flow rate of fresh water per hour (m 3 /h). From the expression (3), a simple desalination cost in  FIG. 8  is 2.17 kWh/m 3 , and a simple desalination cost in  FIG. 9  is 2.21 kWh/m3. 
     In this manner, from comparison between  FIG. 7 ,  FIG. 8 , and  FIG. 9 , the desalination apparatus including the power recovery apparatus  60  or  120  is found to achieve a far higher power saving effect than the desalination apparatus which includes neither. 
     Further, the desalination apparatus including the power recovery apparatus  60  according to the present embodiment requires a lower desalination cost than the desalination apparatus including the conventional power recovery apparatus  120 . In this manner, the power recovery apparatus  60  according to the present embodiment is found to be capable of effectively collecting pressure energy from highly-concentrated salt water without using the boost pump  121 . Further, the lower desalination cost achieved by the desalination apparatus including the power recovery apparatus  60  owes to low pump efficiency of the boost pump  121 . 
     As has been described above, in the first embodiment, the movable members  6313 - 1  and  6313 - 2  are provided so as to penetrate the second spaces of the cylinders  6311 - 1  and  6311 - 2  to outside. The penetration to outside causes ends of the movable members  6313 - 1  and  6313 - 2  to receive a pressure equal to an atmospheric pressure. Therefore, each of areas where the pistons  6312 - 1  and  6312 - 2  respectively make contact with the second spaces is smaller than each of areas where the pistons  6312 - 1  and  6312 - 2  respectively make contact with the first spaces by each of cross-sectional areas of the movable members  6313 - 1  and  6313 - 2  vertical to their own lengthwise directions. That is, area A 1 &gt;area A 2 . In this manner, the power recovery apparatus  60  is capable of outputting seawater from the second spaces at a pressure equal to a pressure of seawater output from the high-pressure pump  40 , by using a pressure of highly-concentrated salt water supplied to the first spaces. 
     Also in the first embodiment, the positions of the movable members  6313 - 1  and  6313 - 2  protruding from the cylinders  6311 - 1  and  6311 - 2  are detected. Based on detection results thereof, the 4-port switch valve  62  is switched over. In this manner, the positions of the pistons  6312 - 1  and  6312 - 2  inside the cylinders  6311 - 1  and  6311 - 2  can be correctly and easily recognized. 
     Also in the first embodiment, the shaft motors are constituted by the movable members  6313 - 1  and  6313 - 2  and the stationary members  6314 - 1  and  6314 - 2 . Further, a leftward or rightward force is applied to the pistons  6312 - 1  and  6312 - 2  by controlling a supplied electric current by using the motor control section  67 . When the high-pressure RO membrane  50  is used for a long time, the RO membrane clogs and consequently decreases the pressure P 6  of highly-concentrated salt water from the high-pressure RO membrane  50 . The motor control section  67  maintains constantly a pressure of seawater ejected from the second spaces by controlling sizes and directions of forces generated by the shaft motors, even when a pressure of highly-concentrated salt water decreases. In this manner, the motor control section  67  is capable of constantly equalizing the pressure P 14  of seawater output from the power recovery apparatus  60  to the pressure P 4  of seawater output from the high-pressure pump  40 . 
     Therefore, the power recovery apparatus  60  according to the first embodiment can collect pressure energy existing in highly-concentrated salt water, without a boost pump. 
     Thus, the power recovery apparatus  60  according to the first embodiment requires no boost pump, and can therefore decrease power consumption for desalination. Further, a total number of pumps installed in a plant decreases, and accordingly, maintenance costs and plant construction costs can be reduced. 
     In addition, the power recovery apparatus  60  can achieve effects as described above by providing the movable members in the second spaces of the converters  631 - 1  and  631 - 2 . Therefore, plant construction costs can be more reduced. 
     In the first embodiment described above, the power recovery apparatus  60  may have a structure as represented in  FIG. 10 . The power recovery apparatus  60  in  FIG. 10  includes a 4-port switch valve  68  instead of the seawater supply section  64 . The switch control section  66  switches the 4-port switch valve  68  at the same time point when the 4-port switch valve  62  is switched. 
     Although the first embodiment has been described with reference to an example in which the power recovery apparatus  60  includes the 4-port switch valve  62 , a 5-port switch valve  69  may be used in place of the 4-port switch valve  62 , as represented in  FIG. 11 . 
     Also, the first embodiment has been described with reference to an example in which two converters  631 - 1  and  631 - 2  are mounted on the power recovery apparatus  60 . However, 2n converters (where n is an natural number) may be mounted. 
     Second Embodiment 
       FIG. 12  is a block diagram representing a configuration of a power recovery apparatus  130  according to the second embodiment of the present invention. Parts in  FIG. 12  which are common to  FIG. 2  will be denoted at common reference symbols, respectively, and only different parts will be described herein. 
     A pressure conversion section  131  in the power recovery apparatus  130  includes converters  1311 - 1  and  1311 - 2 . The converters  1311 - 1  and  1311 - 2  have the same structures as each other, and therefore, only the converter  1311 - 1  will be described herein. 
     The converter  1311 - 1  includes cylinders  13111 - 1  and  13112 - 1 , pistons  13113 - 1  and  13114 - 1 , and a shaft motor which consists of a movable member  13115 - 1  and a stationary member  13116 - 1 . 
     The cylinder  13111 - 1  has an open surface, and another surface where a hole is provided. Further, an inside area of a cross-section vertical to a lengthwise direction of the cylinder  13111 - 1  is A 1 . Further, the cylinder  13112 - 1  has an open surface, and another surface where a hole is provided. Further, an inside area of a cross-section vertical to a lengthwise direction of the cylinder  13112 - 1  is A 2 . Open surfaces of the cylinders  13111 - 1  and  13112 - 1  are opposed to each other. 
     The piston  13113 - 1  is positioned inside the cylinder  13111 - 1 , and forms a first space, with a seal material provided between the piston  13113 - 1  and the cylinder  13111 - 1 . The piston  13113 - 1  has an area A 1 . Further, the piston  13114 - 1  is positioned inside the cylinder  13112 - 1 , and forms a second space, with a seal material provided between the piston  13114 - 1  and the cylinder  13112 - 1 . The piston  13114 - 1  has an area A 2 . The first space is supplied with highly-concentrated salt water, and the second space is supplied with seawater. Here, a relationship between the areas A 1  and A 2  is preset on the basis of a pressure of highly-concentrated salt water from a high-pressure RO membrane  50 , a pressure of seawater from a high pressure pump  40 , friction between the cylinder  13111 - 1  and the piston  13113 - 1 , and friction between the cylinder  13112 - 1  and the piston  13114 - 1 . 
     The movable member  13115 - 1  is constituted by a large number of magnets arrayed in a pipe. The movable member  13115 - 1  is driven in a lengthwise direction thereof when the stationary member  13116 - 1  is supplied with an electric current. The electric current supplied to the stationary member  13116 - 1  is controlled by a motor control section  67 . The movable member  13115 - 1  and the stationary member  13116 - 1  make neither contact nor friction between each other. Further, the movable member  13115 - 1  connects the pistons  13113 - 1  and  13114 - 1 . A dog is formed at a predetermined position on the movable member  13115 - 1 . 
     Detection sections  132 - 1  and  132 - 2  are to detect positions of the dog. The detection section  132 - 1  is located at a position where contact with the dog can be detected when the piston  13114 - 1  comes close to the left end of the cylinder  13112 - 1 . The detection section  132 - 2  is located at a position where contact with the dog can be detected when the piston  13113 - 1  comes close to the right end of the cylinder  13111 - 1 . The detection sections  132 - 1  and  132 - 2  output detection signals to a switch control section  133  when the dog is detected. In this manner, the positions of the pistons  13113 - 1  and  13114 - 1  in the converter  1311 - 1  can be recognized. Further, detection sections  132 - 3  and  132 - 4  have the same configuration as the detection sections  132 - 1  and  132 - 2 , and are to detect positions of the dog on the movable member  13115 - 2 . When the detection sections  132 - 3  and  132 - 4  detect the dog, the detection sections  132 - 3  and  132 - 4  output detection signals to the switch control section  133 . In this manner, positions of the pistons  13113 - 2  and  13114 - 2  in the converter  1311 - 2  can be grasped. 
     The switch control section  133  outputs a switch instruction to a 4-port switch valve  62  in accordance with detection signals from the detection sections  132 - 1  to  132 - 4 . That is, when the control section  133  receives detection signals from the detection sections  132 - 1  and  132 - 4 , the switch control section  133  determines that the piston  13114 - 1  is positioned close to the left end of the cylinder  13112 - 1  and that the piston  13113 - 2  is positioned close to the right end of the cylinder  13111 - 2 . Further, the switch control section  133  outputs a switch instruction to the 4-port switch valve  62  to make the converter  1311 - 1  eject highly-concentrated salt water and to make the converter  1311 - 2  be supplied with highly-concentrated salt water. 
     Otherwise, when the switch control section  133  receives detection signals from the detection sections  132 - 2  and  132 - 3 , the switch control section  133  determines that the piston  13113 - 1  is positioned close to the right end of the cylinder  13111 - 1  and that the piston  13114 - 2  is positioned close to the left end of the cylinder  13112 - 2 . Further, the switch control section  133  outputs a switch instruction to the 4-port switch valve  62  to make the converter  1311 - 1  be supplied with highly-concentrated salt water and to make the converter  1311 - 2  eject highly-concentrated salt water. 
     With the configuration as described above, the power recovery apparatus  130  according to the above second embodiment can achieve the same operation and effects as the power recovery apparatus  60  according to the first embodiment. 
     Also, the above second embodiment has been described with reference to an example in which two converters  1311 - 1  and  1311 - 2  are mounted on the power recovery apparatus  130 . However, 2n converters (where n is an natural number) may be mounted. 
     Third Embodiment 
       FIG. 13  is a block diagram representing a configuration of a power recovery apparatus  140  according to the third embodiment of the present invention. Parts in  FIG. 13  which are common to  FIG. 2  will be denoted at common reference symbols, respectively, and only different parts will be described herein. 
     A pressure conversion section  141  in the power recovery apparatus  140  includes converters  1411 - 1 ,  1411 - 2 , and  1411 - 3 , a crankshaft  1412 , and a motor  1413 . The converters  1411 - 1 ,  1411 - 2 , and  1411 - 3  each are connected to the crankshaft  1412 . Arms of the crankshaft  1412  are designed to be arranged at angular intervals of 120 degrees between each other, as illustrated in  FIG. 14 . Further, the crankshaft  1412  is connected to the motor  1413  through an angle detection section  142 . An electric current supplied to the motor  1413  is controlled by a motor control section  144 . 
     The converters  1411 - 1 ,  1411 - 2 , and  1411 - 3  have the same structures as each other, and therefore, only the converter  1411 - 1  will be described herein. The converter  1411 - 1  includes cylinders  14111 - 1  and  14112 - 1 , pistons  14113 - 1  and  14114 - 1 , and connection rods  14115 - 1  and  14116 - 1 . 
     The cylinder  14111 - 1  has an open surface, and another surface where a hole is provided. Further, an inside area of a cross-section vertical to a lengthwise direction of the cylinder  14111 - 1  is A 1 . Further, the cylinder  14112 - 1  has an open surface and another surface where a hole is provided. Further, an inside area of a cross-section vertical to a lengthwise direction of the cylinder  14112 - 1  is A 2 . Open surfaces of the cylinders  14111 - 1  and  14112 - 1  are opposed to each other. 
     The piston  14113 - 1  is positioned inside the cylinder  14111 - 1  and forms a first space, with a seal material provided between the piston  14113 - 1  and the cylinder  14111 - 1 . The piston  14113 - 1  has an area A 1 . Further, the piston  14114 - 1  is positioned inside the cylinder  14112 - 1  and forms a second space, with a seal material provided between the piston  14114 - 1  and the cylinder  14112 - 1 . The piston  13114 - 1  has an area A 2 . The first space is supplied with highly-concentrated salt water, and the second space is supplied with seawater. Here, a relationship between the areas A 1  and A 2  is preset on the basis of a pressure of highly-concentrated salt water from a high-pressure RO membrane  50 , a pressure of seawater from a high pressure pump  40 , friction between the cylinder  14111 - 1  and the piston  14113 - 1 , and friction between the cylinder  14112 - 1  and the piston  14114 - 1 . 
     The connection rod  14115 - 1  connects the piston  14113 - 1  and a pin of the crankshaft  1412 . The connection rod  14116 - 1  connects the piston  14114 - 1  and a pin of the crankshaft  1412 . 
     In a state of  FIG. 13 , highly-concentrated salt water is made flow into the first space of the converter  1411 - 1 , and the piston  14113 - 1  is moved in a leftward direction by the highly-concentrated salt water. Further, seawater is made flow into the second spaces of the converters  1411 - 2  and  1411 - 3 , and the pistons  14113 - 2  and  14113 - 3  are moved in a rightward direction by the seawater. Accordingly, the crankshaft  1412  rotates in an arrow direction in  FIG. 13 . 
     The angle detection section  142  is to detect a rotation angle of the crankshaft  1412 . When the rotation angle reaches a predetermined angle, the angle detection section  142  then outputs a detection signal to a switch control section  143 . For example, total six angles are registered in advance in the angle detection section  142  as the predetermined angle. The six angles correspond to angles at which the pistons  14113 - 1  to  14113 - 3  come close to the right ends of the cylinders  14111 - 1  to  14111 - 3 , and angles at which the pistons  14114 - 1  to  14114 - 3  come close to the left ends of the cylinders  14112 - 1  to  14112 - 3 . When the rotation angle reaches any of the angles, the angle detection section  142  outputs a detection signal to the switch control section  143 . In this manner, the switch control section  143  can grasp positions of the pistons in the converters. 
     When the switch control section  143  receives the detection signal from the angle detection section  142 , the switch control section  143  issues a switch instruction to a 3-port valve among switch valves  62 - 1  to  62 - 3 , which is connected to one of the converters corresponding to the detection signal. 
     The motor control section  144  controls an electric current supplied to the motor  1413 , based on a measurement result from a pressure meter  61 . The motor  1413  applies torque to the crankshaft  1412  in a clockwise or anticlockwise direction based on the electric current supplied by the motor control section  144 . For example, when a measurement result from the pressure meter  61  decreases to be smaller than a value which has been expected beforehand, the motor control section  144  controls the electric current supplied to the motor  1413  so as to apply a load to the crankshaft  1412  in an anticlockwise direction. 
     Next, operation of the power recovery apparatus  140  configured as described above will be described. 
     The power recovery apparatus  140  in  FIG. 13  is in a state in which the converter  1431 - 1  is supplied with highly-concentrated salt water while highly-concentrated salt water is ejected from the converters  1411 - 2  and  1411 - 3 . 
     Seawater from a safety filter  30  is supplied to a high-pressure pump  40  at 0.2 MPa and is also supplied to the second spaces of the converters  1411 - 2  and  1411 - 3  through check valves  641 - 4  and  641 - 6 . 
     Seawater which has been boosted to 6.0 MPa by the high-pressure pump  40  is merged with seawater from the power recovery apparatus  140 , and is introduced into the high-pressure RO membrane  50 . At this time, the seawater from the power recovery apparatus  140  has been ejected from the second space of the converter  1411 - 1  and passed through the check valve  641 - 2 . The high-pressure RO membrane  50  outputs fresh water and highly-concentrated salt water. 
     The highly-concentrated salt water ejected from the high-pressure RO membrane  50  passes through the pressure meter  61  and 3-port switch valve  62 - 1 , and flows into the first space of the converter  1411 - 1 . At this time, the second space of the converter  1411 - 1  is filled with seawater. Highly-concentrated salt water moves the piston  14113 - 1  in the cylinder  14111 - 1  in a leftward direction, and the piston  14114 - 1  in the cylinder  14112 - 1  in a leftward direction. In this manner, seawater in the second space of the converter  1411 - 1  is pressed and ejected. At this time, the piston  14113 - 1  moves in the leftward direction, thereby applying torque to the crankshaft  1412  in a direction denoted in  FIG. 13 . 
     As torque in the clockwise direction is applied by the motor  1413 , the pistons  14113 - 1  and  14114 - 1  are applied with a force which will be hereinafter referred to as N 1 . The piston  14113 - 1  has an area A 1 , and the piston  14114 - 1  has an area A 2 . Thus, a pressure of seawater which is ejected from the second space of the converter  1411 - 1  is expressed as (P*A 1 +N 1 )/A 2 , using a pressure P of highly-concentrated salt water from the 3-port switch valve  62 - 1 . Accordingly, the pressure of seawater ejected from the second space of the converter  1411 - 1  is equal to or slightly higher than a pressure of seawater supplied to the high-pressure RO membrane  50 . The force N 1  can be either a positive or negative value, depending on differences between directions of motor thrusts. 
     When the crankshaft  1412  rotates in the arrow direction in  FIG. 13 , the pistons  14113 - 2 ,  14113 - 3 ,  14114 - 2 , and  14114 - 3  of the converters  1411 - 2  and  1411 - 3  connected to the crankshaft  1412  move in rightward directions. Accordingly, seawater is made flow from the check valves  641 - 4  and  641 - 6  to each of the second spaces in the converters  1411 - 2  and  1411 - 3 , and highly-concentrated salt water is ejected from each of the first spaces of the converters  1411 - 2  and  1411 - 3  through the 3-port switch valves  62 - 2  and  62 - 3  and the valve  70 . 
     When the operation as described above is continued, a detection signal is output from the angle detection section  142  to the switch control section  143  each time when the rotation angle of the crankshaft  1412  reaches the predetermined angle. The switch control section  143  receives the detection signal from the angle detection section  142 , and then switches the 3-port switch valves  62 - 1  to  62 - 3  successively so as to switch directions of flow-in and ejection of highly-concentrated salt water. 
     With the configuration as described above, the power recovery apparatus  140  according to the above third embodiment can achieve the same operation and effects as the power recovery apparatus  60  according to the first embodiment. 
     Further, in the third embodiment, the pistons are connected to the crankshaft  1412 . Therefore, displacements in lengthwise directions of the pistons transit like a sine curve. Further, the 3-port switch valves  62 - 1  to  62 - 3  switch directions of flow-in and ejection of highly-concentrated salt water corresponding to positions of the pistons in the cylinders. In this manner, pulsation which takes place when the 3-port switch valves  62 - 1  to  62 - 3  switch directions of flow-in and ejection is reduced. 
     The above third embodiment has been described with reference to an example in which three converters  14111 - 1  to  14111 - 3  are mounted on the power recovery apparatus  140 . However, 3n converters (where n is an natural number) may be mounted. 
     The areas A 1  and A 2  may be equal to each other. 
     Fourth Embodiment 
       FIG. 15  is a block diagram representing a configuration of a power recovery apparatus  150  according to the fourth embodiment of the present invention. Parts in  FIG. 15  which are common to  FIG. 2  will be denoted at common reference symbols, respectively, and only different parts will be described herein. 
     A pressure conversion section  151  in the power recovery apparatus  150  includes vane-type rotary actuators  1511 - 1  and  1511 - 2 , a rotary shaft  1512 , and a motor  1513 . The rotary actuators  1511 - 1  and  1511 - 2  are connected by the rotary shaft  1512 . Further, the rotary shaft  1512  is connected to the motor  1513  through an angle detector  152 . An electric current supplied to the motor  1513  is controlled by a motor control section  154 . 
       FIG. 16  is a schematic view illustrating a structure of the rotary actuators  1511 - 1  and  1511 - 2  according to the fourth embodiment of the present invention. In  FIG. 16 , the rotary actuator  1511 - 1  includes a housing  15111 - 1  and a vane  15112 - 1 . 
     The housing  15111 - 1  forms a sealed space and has a cylindrical shape having a radius r 1 . The rotary shaft  1512  is located so as to penetrate the housing  15111 - 1  along a center axis thereof. A screen part  15113 - 1  is formed to extend from an inner wall surface of the housing  15111 - 1  to the rotary shaft  1512 . The screen part  15113 - 1  is fixed inside the housing  15111 - 1 . 
     The vane  15112 - 1  is formed to be connected with the rotary shaft  1512 , and makes contact with the inner wall surface of the housing  15111 - 1  through a sealing agent. The vane  15112 - 1  has an area A 1 . 
     A sealed space formed by the housing  15111 - 1  is divided into first and third spaces by the vane  15112 - 1  and the screen part  15113 - 1 . When highly-concentrated salt water is made flow into the first space, the vane  15112 - 1  rotates in an arrow direction illustrated in  FIG. 16 , and pushes and ejects highly-concentrated salt water filled in the third space. Inversely, when highly-concentrated salt water is made flow into the third space, the vane  15112 - 1  rotates in a direction opposite to the arrow direction in  FIG. 16 , and pushes and ejects highly-concentrated salt water filled in the first space. 
     The rotary actuator  1511 - 2  includes a housing  15111 - 2  and a vane  15112 - 2 . The housing  15111 - 2  forms a sealed space and has a cylindrical shape having a radius r 2 . A relationship of radius r 1 &gt;radius r 2  is given. The rotary shaft  1512  is located so as to penetrate the housing  15111 - 2  along a center axis thereof. A screen part  15113 - 2  is formed to extend from an inner wall surface of the housing  15111 - 2  to the rotary shaft  1512 . The screen part  15113 - 2  is fixed inside the housing  15111 - 2 . 
     The vane  15112 - 2  is formed to be connected with the rotary shaft  1512 , and makes contact with the inner wall surface of the housing  15111 - 2  through a sealing agent. The vanes  15112 - 1  and  15112 - 2  maintain a same angle each other. 
     The vane  15112 - 2  has an area A 2 . Here, a relationship between the areas A 1  and A 2  is preset on the basis of a pressure of highly-concentrated salt water from a high-pressure RO membrane  50 , a pressure of seawater from a high pressure pump  40 , friction between the housings  15111 - 1  and  15111 - 2  and the vanes  15112 - 1  and  15112 - 2 . 
     A sealed space formed by the housing  15111 - 2  is divided into second and fourth spaces by the vane  15112 - 2  and the screen part  15113 - 2 . When seawater is made flow into the fourth space, the vane  15112 - 2  rotates in an arrow direction illustrated in  FIG. 16 , and pushes and ejects seawater filled in the second space. Inversely, when seawater is made flow into the second space, the vane  15112 - 2  rotates in a direction opposite to the arrow direction in  FIG. 16 , and pushes and ejects seawater filled in the fourth space. 
     The angle detection section  152  is to detect a rotation angle of the rotary shaft  1512 . When the rotation angle reaches a predetermined angle, the angle detection section  152  outputs a detection signal to a control section  153 . For example, two angles are registered in advance in the angle detection section  152  as the predetermined angle. One is an angle at which the vane  15112 - 1  and  15112 - 2  respectively come close to the screen part  15113 - 1  and  15113 - 2  from left sides. Another one is an angle at which the vanes  15112 - 1  and  15112 - 2  respectively come close to the screen parts  15113 - 1  and  15113 - 2  from right sides. When the rotation angle reaches any of the angles, the angle detection section  152  outputs detection signals to the control section  153 . In this manner, the positions of the vanes  15112 - 1  and  15112 - 2  in the rotary actuators  1511 - 1  and  1511 - 2  can be recognized. 
     When the control section  153  receives the detection signal from the angle detection section  152 , the control section  153  issues a switch instruction to a 4-port switch valve  62  so as to switch over the spaces into and from which highly-concentrated salt water is made flow and eject, respectively. 
     The motor control section  154  controls an electric current supplied to the motor  1513 , based on a measurement result from a pressure meter  61 . The motor  1513  applies left-handed or right-handed torque to the rotary shaft  1512  based on the electric current supplied by the motor control section  154 . For example, when a measurement result from a pressure meter  61  decreases to be smaller than a value which has been expected beforehand, the motor control section  154  controls the electric current supplied to the motor  1513  so as to apply torque to the rotary shaft  1512  in a same direction with a rotating direction thereof. 
     Next, operation of the power recovery apparatus  150  configured as described above will be described. 
     The power recovery apparatus  150  in  FIG. 15  is in a state in which highly-concentrated salt water is supplied to the first space in the rotary actuator  1511 - 1  and highly-concentrated salt water is ejected from the third space of the rotary actuator  1511 - 1 . 
     Seawater from a safety filter  30  is supplied to a high-pressure pump  40  at 0.2 MPa and is also supplied to the fourth space of the rotary actuator  1511 - 2  through a check valve  641 - 4 . 
     Seawater which has been boosted to 6.0 MPa by the high-pressure pump  40  is merged with seawater from the power recovery apparatus  150 , and is supplied to the high-pressure RO membrane  50 . At this time, the seawater from the power recovery apparatus  150  has been ejected from the second space of the rotary actuator  1511 - 2  and passed through the check valve  641 - 2 . The high-pressure RO membrane  50  outputs fresh water and highly-concentrated salt water. 
     The highly-concentrated salt water ejected from the high-pressure RO membrane  50  passes through the pressure meter  61  and 4-port switch valve  62  and flows into the first space of the rotary actuator  1511 - 1 . At this time, the third space of the rotary actuator  1511 - 1  is filled with highly-concentrated salt water. Highly-concentrated salt water rotates the vane  15112 - 1  in the rotary actuator  1511 - 1  in a direction toward the third space, and ejects highly-concentrated salt water in the third space through the 4-port switch valve  62  and valve  70 . 
     When the vane  15112 - 1  of the rotary actuator  1511 - 1  rotates, the vane  15112 - 2  of the rotary actuator  1511 - 2  connected by the rotary shaft  1512  rotates accordingly. Therefore, seawater is ejected from the second space of the rotary actuator  1511 - 2  through the check valve  641 - 2 , and seawater is made flow into the fourth space of the rotary actuator  1511 - 2  through the check valve  641 - 4 . 
     Here, the vane  15112 - 1  has an area A 1 , and the vane  15112 - 2  has an area A 2 . Thus, a pressure of seawater ejected from the second space of the rotary actuator  1511 - 2  is higher than that of highly-concentrated salt water from the 4-port switch valve  62 . 
     Operation of the motor will now be described. As positive or negative torque is applied by the motor  1513 , rotation torque of the vanes  15112 - 1  and  15112 - 2  increases or decreases. If a pressure measured by the pressure meter  61  is lower than a preset pressure, the motor generates torque in the presently rotating direction. Otherwise, if higher than the preset pressure, the motor generates torque in a direction opposite to the presently rotating direction. From the operation as described above, the pressure of seawater ejected from the second space of the rotary actuator  1511 - 2  is equal to or slightly higher than a pressure of seawater supplied to the high-pressure RO membrane  50 . 
     When the operation as described above is continued, the vanes  15112 - 1  and  15112 - 2  respectively come close to the screen parts  15113 - 1  and  15113 - 2  from left sides. Then, the angle detection section  152  detects the predetermined angle to be reached, and outputs the detection signal to the control section  153 . The control section  153  receives the detection signal from the angle detection section  152 , and then issues a switch instruction to the 4-port switch valve  62  so as to switch directions of flow-in and ejection of highly-concentrated salt water. 
     With the configuration as described above, the power recovery apparatus  150  according to the above fourth embodiment can achieve the same operation and effects as the power recovery apparatus  60  according to the first embodiment. 
     The above fourth embodiment has been described with reference to an example in which the pressure converter  151  includes the vane-type rotary actuators  1511 - 1  and  1511 - 2 . However, the present embodiment is not limited to this example. For example, the fourth embodiment is practicable even when a gear motor, an axial piston motor, a plunger pump, a radial piston motor, and a trochoid motor is included in place of the vane-type rotary actuators. 
     The areas A 1  and A 2  may be equal to each other. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.