Patent Publication Number: US-10787907-B2

Title: Turbine and turbine system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2016/086931, filed Dec. 12, 2016. The contents of this application are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     Embodiments of the present invention relate to a turbine and a turbine system. 
     BACKGROUND 
     For example, in a turbine used for a power generation plant or the like, when the turbine is expanded in one direction, a thrust force is generated in a turbine shaft by a pressure difference occurring between an entrance side and an exit side of the turbine. 
     The thrust force generated in the turbine shaft is supported by a thrust bearing. In a case of a large thrust force, the thrust bearing is to be formed large, leading to a problem of a cost increase and so on. Further, forming the thrust bearing large is limited in view of circumferential speed, resulting also in a problem that design cannot be done. 
     As a method for making a thrust force small, there is suggested a method of generating a counter thrust force by providing a balance piston using a shaft seal structure of a turbine. 
     The aforementioned turbine in which the thrust force is made small by providing the balance piston might have a following problem. 
     For example, when a gland seal which performs sealing between the inside and the outside (atmosphere) of a casing is constituted by a plurality of labyrinth seals, in order to prevent leakage of CO 2  or the like, suction from a space between these labyrinth seals is sometimes performed by a gland pump to control the space to have a negative pressure. Besides, a structure is considered in which an exhaust connection piping that connects a low pressure side of a balance piston and a turbine exhaust line is provided and also a balance piston extraction hole is provided in the middle of the balance piston, to thereby allow CO 2  (at low temperature and high pressure) for cooling or sealing to be extracted and converged in a middle stage of the turbine. In a case of the above configuration, the following problem occurs. 
     That is, in the turbine at the time of low load, pressure drawdown occurs in the front stage, and because of lowness of an original pressure, the pressure drawdown in the front stage reduces the pressure, so that pressure drawdown hardly occurs in the rear stage. In other words, a degree of pressure drawdown in the rear stage becomes smaller compared with that in the front stage, resulting in a smaller pressure difference in the rear stage. Therefore, at the time of low load, in the turbine of the above configuration, the pressure at a portion of the balance piston extraction hole communicating to the middle stage of the turbine is reduced, resulting in the smaller difference in relation to a low pressure side pressure of the balance piston. Thereby, a flow of CO 2  from the high pressure side to the low pressure side of the balance piston is decreased. Meanwhile, a suction force by the gland pump acts on a space on the low pressure side of the balance piston, so that a backflow of exhaust gas at high temperature might occur from the turbine exhaust line into the exhaust connection piping. 
     The problem to be solved by the present invention is to provide a turbine and a turbine system which can prevent occurrence of a backflow of exhaust gas at high temperature into an exhaust connection piping at the time of low load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram of a thermal power generation system provided with a turbine of an embodiment; 
         FIG. 2  is a diagram schematically illustrating a configuration of a first embodiment; 
         FIG. 3  is a diagram schematically illustrating a configuration of a modification example of the first embodiment; 
         FIG. 4  is a diagram schematically illustrating a configuration of a second embodiment; and 
         FIG. 5  is a diagram schematically illustrating a configuration of a modification example of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A turbine of an embodiment has: a casing; a turbine rotor disposed to penetrate the casing; a plurality of turbine stages disposed in the casing and provided along a shaft direction of the turbine rotor; a working fluid injection pipe allowing a working medium to be injected into the casing and to be distributed from the front stage toward the rear stage of the turbine stages, thereby rotating the turbine rotor; a balance piston disposed in the turbine rotor; a plurality of balance piston seals disposed on a casing side in a manner to face the balance piston; a balance piston extraction hole allowing extraction from between the plurality of balance piston seals to the middle stage of the turbine stages; an exhaust connection piping connecting a low pressure side of the balance piston to a turbine exhaust system; an exhaust connection piping valve mechanism provided in the exhaust connection piping; a plurality of seal mechanisms provided between the low pressure side of the balance piston and the atmosphere; and an exhaust piping allowing exhaust from between the plurality of seal mechanisms. 
     Hereinafter, embodiments will be described with reference to the drawings.  FIG. 1  is a system diagram of a thermal power generation system provided with a turbine of an embodiment. 
     As illustrated in  FIG. 1 , the thermal power generation system of this embodiment has a CO 2  pump  1 , a regenerative heat exchanger  2 , an oxygen producer  3 , a combustor  4 , a CO 2  turbine  5 , a power generator  6 , a cooler  7 , a humidity separator  8  and so on. CO 2  indicates carbon dioxide. 
     The CO 2  pump  1  compresses highly-pure CO 2  made by separating water from combustion gas (CO 2  and vapor) by the humidity separator  8 , and supplies the CO 2  at high pressure to the combustor  4  and the CO 2  turbine  5  in a branching manner through the regenerative heat exchanger  2 . 
     Note that the highly-pure CO 2  at high pressure generated in the CO 2  pump  1  may be stored or utilized for enhanced oil recovery. The one CO 2  pump  1  doubles as supply sources for CO 2  for working (hereinafter, referred to as “working CO 2 ”) and CO 2  for cooling (hereinafter, referred to as “cooling CO 2 ). The working CO 2  may be called working gas or working fluid, and the cooling CO 2  may be called cooling gas or cooling fluid. 
     The regenerative heat exchanger  2  supplies CO 2  increased in temperature by heat exchange to the combustor  4  and the CO 2  turbine  5 . CO 2  supplied to the combustor  4  is for working. CO 2  supplied to the CO 2  turbine  5  is for cooling or sealing. Further, the regenerative heat exchanger  2  cools through heat exchange the combustion gas (CO 2  and vapor) discharged from the CO 2  turbine  5 . 
     The oxygen producer  3  produces oxygen and supplies the produced oxygen to the combustor  4 . The combustor  4  combusts injected natural gas such as methane gas, CO 2  and oxygen to generate combustion gas (CO 2  and vapor) at high temperature and high pressure, and supplies the combustion gas to the CO 2  turbine  5  as the working CO 2 . 
     The CO 2  turbine  5  rotates rotor blades  13  (see  FIG. 2 ) in the turbine and a turbine rotor  11  supporting the rotor blades  13  by the working CO 2  at high temperature and high pressure, and transmits their rotation force to the power generator  6 . 
     In other words, the CO 2  turbine  5  is a turbine which uses CO 2  supplied from the one CO 2  pump  1  mainly as the working medium (working fluid) for rotating the turbine rotor  11  and a medium for cooling (cooling gas). 
     The power generator  6  generates power by a rotation force of an axle of the CO 2  turbine  5 . A combination of the CO 2  turbine  5  and the power generator  6  may be sometimes called a CO 2  turbine power generator. The cooler  7  further cools the combustion gas (CO 2  and vapor) having passed through the regenerative heat exchanger  2 , and the cooled combustion gas (CO 2  and vapor) is sent to the humidity separator  8 . 
     The humidity separator  8  separates water from the combustion gas (CO 2  and vapor) at low temperature sent from the cooler  7 , and returns highly-pure CO 2  back to the CO 2  pump  1 . 
     The thermal power generation system is constituted by a circulation system of oxygen combustion using CO 2  at supercritical pressure and is a zero emission power generation system which is capable of effectively utilizing CO 2  without discharging NOR. Use of this system makes it possible to recover and recycle the highly-pure CO 2  at high pressure without separately installing facilities for separating and recovering CO 2 . 
     In the case of this thermal power generation system, power is generated by rotating (the rotor blades of) the CO 2  turbine  5  by the CO 2  at high temperature (working CO 2 ) generated by injecting and combusting CO 2 , natural gas and oxygen. 
     Then, the combustion gas (CO 2  and vapor) discharged from the CO 2  turbine  5  is cooled through the regenerative heat exchanger  2  and the cooler  7  and has water therein separated in the humidity separator  8 , and thereafter, the CO 2  gas is circulated back to the CO 2  pump  1  and compressed. In this system, most of CO 2  being circulated to the combustor  4 , CO 2  generated by combustion can be recovered as it is. 
     First Embodiment 
     Next, a turbine and a turbine system according to a first embodiment will be described with reference to  FIG. 2 . 
     As illustrated in  FIG. 2 , a CO 2  turbine  5  of the first embodiment has a bearing  10 , a turbine rotor  11 , a balance piston  11   a , a flange portion  11   b , labyrinth seals  12   a ,  12   b ,  12   c  (seal mechanisms), a rotor blade  13 , an outer casing  14 , inner casings  15   a ,  15   b , a stationary blade  16 , partition walls  18   a ,  18   b , a partition wall hole  19 , a balance piston seal  23 , a labyrinth seal  24 , a hole  27 , wheel space seals  28   a ,  28   b , a balance piston extraction hole  29 , a working CO 2  injection pipe  31  (a working fluid injection pipe), a CO 2  discharge pipe  32  (a turbine exhaust system), a CO 2  injection pipe for cooling or sealing  33  (hereinafter, referred to as a “cooling CO 2  injection pipe  33 ”), a thrust bearing  34 , an exhaust connection piping  35 , an exhaust piping  37 , a gland pump  38 , a regulating valve  39  (an exhaust connection piping valve mechanism), a control unit  50 , pressure sensors  51 ,  53 , a load cell  52  (a thrust load detection sensor) and so on. Note that “high” in the drawing indicates a high pressure while “low” indicates a low pressure. 
     The bearing  10  rotatably supports shaft ends on both sides of the turbine rotor  11 . Further, the thrust bearing  34  rotatably supports the shaft end on one side of the turbine rotor  11 , and receives a thrust force by supporting the flange portion  11   b  provided in the turbine rotor  11 . The turbine rotor  11  has, in almost a center thereof, a plurality of moving blades  13  implanted in a circumferential direction. Besides, the turbine rotor  11  is provided with the balance piston  11   a.    
     In an inner periphery of the inner casing  15   a  which faces the balance piston  11   a , the balance piston seal  23  of a labyrinth structure is provided. The balance piston seal  23  suppresses a flow of CO 2  by a plurality of fins to thereby decrease a pressure. A pressure difference occurs between a right side and a left side of a clearance where the balance piston seal  23  is disposed in  FIG. 2 . In this example, the pressure on the right side of the balance piston seal is high and indicated as “high” while the pressure on the left side is low and indicated as “low”. 
     The balance piston seal  23  generates a pressure difference between a space divided by the balance piston seal  23  (a clearance portion continuing into a cooling chamber A and a cooling chamber B), to thereby generate a counter thrust force acting from the right side to the left side in  FIG. 2 . The counter thrust force decreases a thrust load in a shaft direction of the turbine rotor  11 . 
     In an inner periphery of the inner casing  15   a  which is inner side (right side in  FIG. 2 ) than a position of the balance piston  11   a , the labyrinth seal  24  of a labyrinth structure is provided. The labyrinth seal  24  adjusts CO 2  for cooling or sealing to have a proper pressure and supplies it to the wheel space seal  28   a , thereby performing sealing so as not to let the working CO 2  leak to a casing side at a minimum flow amount. 
     The outer casing  14  forms a shell of a turbine main body and has through holes  14   a ,  14   b  in both ends in the shaft direction. In a gap between the through hole  14   a  and the turbine rotor  11 , the labyrinth seals  12   a ,  12   c  are disposed. In a gap between the through hole  14   b  and the turbine rotor  11 , the labyrinth seal  12   b  is disposed. 
     The labyrinth seals  12   a ,  12   b ,  12   c  constitute the gland seal which seals a clearance between the turbine rotor  11  penetrating the through holes  14   a ,  14   b  of the outer casing  14  and openings of the through holes  14   a ,  14   b  in a manner that the turbine rotor  11  can rotate. Further, the exhaust piping  37  is connected between the labyrinth seal  12   a  and the labyrinth seal  12   c . The exhaust piping  37  is provided with the gland pump  38 . 
     The labyrinth seals  12   a ,  12   b ,  12   c  perform sealing in a manner that the turbine rotor  11  can rotate and expose end portions of the turbine rotor  11  to the outside of the outer casing  14 . This reduces leakage of the cooling CO 2  to the outside between the outer casing  14  and the turbine rotor  11 . Further, suction by the exhaust piping  37  from between the labyrinth seal  12   a  and the labyrinth seal  12   c  makes a space therebetween have a negative pressure, thereby further reducing leakage of the cooling CO 2  to the outside. 
     The inner casings  15   a ,  15   b  are provided in a bent form so as to form the cooling chamber A and an exhaust chamber E between the turbine rotor  11  and the inner casings  15   a ,  15   b.    
     The inner casings  15   a ,  15   b  and the outer casing  14  provided outside them constitute a double casing structure. Here, the double casing structure is exemplified, but the casing may be a single casing with one layer. 
     The inner casings  15   a ,  15   b  are provided with the stationary blades  16  in a manner to nest with the rotor blades  13  on the turbine rotor  11  side. One set of the rotor blade  13  and the stationary blade  16  is called a turbine stage, the one closest to the working CO 2  injection pipe  31  being called the first stage, the one second closest being called the second stage, and so on. The turbine stage close to the working CO 2  injection pipe  31  is the front stage, the turbine stage far from it is the rear stage, and the turbine stage in the middle thereof is the middle stage. 
     Further, the partition walls  18   a ,  18   b  are provided between the inner casings  15   a ,  15   b  and the outer casing  14 , and these partition walls  18   a ,  18   b  form cooling chambers B, C, D between the inner casings  15   a ,  15   b  and the outer casing  14 . 
     The casing structure provided with the outer casing  14  and the inner casings  15   a ,  15   b  has the cooling chamber A into which CO 2  for cooling the turbine or for sealing is injected at a predetermined temperature and a predetermined pressure, and the cooling chambers B, C, D into which the cooling CO 2  is injected at a pressure reduced from the pressure of the cooling chamber A. 
     The cooling CO 2  at high pressure injected into the cooling CO 2  injection pipe  33  flows in the cooling chambers A, B, C, D. Hereinafter, a flow, a temperature and a pressure of the cooling CO 2  at the time of rated output will be described. The sequence of dotted-line arrows  60  to  70  is the flow of the cooling CO 2  which cools the casing portion. The cooling CO 2  made to have a lower pressure at the portion of the balance piston seal  23  flows branching out into the cooling chamber B and the cooling chamber C. The pressure of the cooling CO 2  gradually decreases in the above flow. 
     Other than the above, as flow paths of the cooling CO 2 , there are flows to cool or seal the turbine which are indicated by dotted-line arrows  71 ,  72 . For example, the flow path of the arrow  72  is a cylindrical flow path provided inside the inner casings  15   a ,  15   b  and is to cool the stationary blade  16 . 
     In the cooling chamber B on the low pressure side of the balance piston  11   a , there is disposed the exhaust connection piping  35  connecting the cooling chamber B and the CO 2  discharge pipe  32  being a turbine discharge system. The exhaust connection piping  35  is provided with the regulating valve  39  as a exhaust connection piping valve mechanism. The exhaust connection piping  35  allows part of the cooling CO 2  which flows from the high pressure side of the balance piston  11   a  into the cooling chamber B being the low pressure side to be discharged to the CO 2  discharge pipe  32  at the time of rated output and so on. 
     Here, each of the cooling chambers A to D and the exhaust chamber E will be described. Into the cooling chamber A is injected the cooling CO 2  from the cooling CO 2  injection pipe  33 . The cooling CO 2  injected into the cooling chamber A is set at a temperature to properly cool turbine components which becomes to have a high temperature. 
     The pressure of the cooling chamber A is kept slightly higher than the pressure inside the working CO 2  injection pipe  31  for the purpose of preventing a backflow of the working CO 2  at high temperature. 
     Into the cooling chamber B, the cooling CO 2  subjected to pressure reduction at the balance piston seal  23  is injected from the cooling chamber A through the hole  27 . The cooling chamber B is a space for decreasing influence of the temperature and pressure which the labyrinth seals  12   a ,  12   c  receive. 
     The temperature of the cooling chamber B is almost the same as the temperature of the cooling chamber A. The pressure of the cooling chamber B is substantially reduced from that of the cooling chamber A by the balance piston seal  23 , to become almost the same pressure as that of the cooling chamber D (low pressure of about 1/10 of the pressure inside the working CO 2  injection pipe  31 ). 
     Into the cooling chamber C is injected the cooling CO 2  having branched into the balance piston extraction hole  29  positioned in the middle of two balance piston seals  23 . The cooling CO 2  injected into the cooling chamber C flows between the inner casings  15   a ,  15   b  and the outer casing  14  in directions of dotted-line arrows  63  to  65 . The pressure inside the cooling chamber C is lower than the pressure inside the cooling chamber A and higher than the pressure inside the cooling chamber B. 
     The partition wall  18   b  dividing the cooling chamber C and the cooling chamber D is provided with the partition wall hole  19  being the through hole, so that the cooling CO 2  from the cooling chamber C is injected into the cooling chamber D through the partition wall hole  19  (dotted-line arrow  66 ). The pressure inside the cooling chamber D becomes lower than the pressure inside the cooling chamber C. 
     The cooling chamber D is a space for cooling the inner casing  15   b  forming the exhaust chamber E, and a part of the labyrinth seal  12   b  is disposed inside the cooling chamber D. In the cooling chamber D, the cooling CO 2  at low temperature and at low pressure flows in directions of dotted-line arrows  67  to  70 . 
     The pressure inside the cooling chamber D is kept slightly higher (about 1/10 of pressure inside working CO 2  injection pipe  31 +ΔP) than the pressure of the exhaust CO 2  inside the exhaust chamber E, in order to prevent the exhaust CO 2  of the exhaust chamber E from flowing (leaking) from the portion of the wheel space seal  28   b  into the cooling chamber D. Hence, CO 2  for cooling or sealing, though in a small amount, flows from the cooling chamber D side into the exhaust chamber E through the wheel space seal  28   b.    
     Into the exhaust chamber E flows the exhaust CO 2 , that is, the working CO 2  having been injected from the working CO 2  injection pipe  31  and passed through the stationary blades  16  and the rotor blades  13 , and the exhaust CO 2  is discharged from the CO 2  discharge pipe  32 . The temperature of the exhaust CO 2  of the exhaust chamber E is about slightly more than half (for example, from 500° C. to 1000° C.) of the temperature of the working CO 2  injected from the working CO 2  injection pipe  31 , at the time of rated output. The pressure inside the exhaust chamber E is about 1/10 of the pressure inside the working CO 2  injection pipe  31  at the time of rated output. That is, the temperature is medium and the pressure is low inside the exhaust chamber E. 
     On the other hand, at the time of low load, the pressure inside the working CO 2  injection pipe  31  becomes low compared with the pressure at the time of rated output. More specifically, for example, the pressure becomes almost ⅕ of the output at the time of rated output. In such a case, a pressure drop occurs in the front stage of the turbine, but because of lowness of an original pressure, when the pressure is reduced by the pressure drop in the front stage, a pressure drop hardly occurs in the rear stage. In other words, a degree of pressure drop in the rear stage is small compared with that in the front stage. Accordingly, the pressure of the chamber C connected to the middle stage of the turbine becomes low compared with that at the time of rated discharge, making the pressure of the chamber C be almost the same as those of the chamber D and the chamber E on a downstream side in relation to the chamber C. Therefore, the pressure of the chamber C also becomes almost the same as the pressure of the chamber B. 
     Thus, the pressure difference between the pressure inside the balance piston extraction hole  29  of the balance piston  11   a  and the pressure on the low pressure side (chamber B side) becomes small, to thereby increase the flow of the cooling CO 2  flowing from the high pressure side of the balance piston  11   a  into the chamber C through the balance piston extraction hole  29 . Meanwhile, the flow amount of the cooling CO 2  flowing from the high pressure side (chamber A side) of the balance piston  11   a  to the low pressure side (chamber B side) decreases. 
     Meanwhile, from between the labyrinth seal  12   a  and the labyrinth seal  12   c  being the gland seals, suction is performed by the gland pump  38 . Alternatively, even in a case where the gland pump  38  is in a halt state, because of a pressure difference between the chamber B and the atmosphere, an outflow of the cooling CO 2  from the chamber B causes a pressure decrease in the chamber B and the exhaust CO 2  might flow back from the CO 2  discharge pipe  32  into the exhaust connection piping  35 . Because of the high temperature of the exhaust CO 2  compared with the cooling CO 2 , the exhaust connection piping  35  is subjected to be damaged by heat when the exhaust CO 2  flows back into the exhaust connection piping  35 . 
     In this embodiment, closing the regulating valve  39  disposed in the exhaust connection piping  35  can prevent a backflow of the exhaust CO 2  from the CO 2  discharge pipe  32  into the exhaust connection piping  35 . In other words, the aforementioned backflow can be prevented by closing the regulating valve  39  at the time of low load and opening the regulating valve when the load comes to have a certain value or more. 
     Opening and closing of the regulating valve  39  described above is controlled by the control unit  50 . The control unit  50  is constituted by a computer and so on, and constitutes the turbine system with the CO 2  turbine  5 . To the control unit  50 , there is inputted a detection signal from the pressure sensor  51  or the like detecting the pressure inside the working CO 2  injection pipe  31 . Based on the detection signal from the pressure sensor  51 , the control unit  50  closes the regulating valve  39  at the time of low load when the pressure is low, and opens the regulating valve  39  when the pressure becomes a high load of a certain value or more. This can prevent the aforementioned backflow of the exhaust CO 2  from the CO 2  discharge pipe  32  into the exhaust connection piping  35 . 
     Further, it is also possible to regulate a counter thrust force by regulating an opening degree of the regulating valve  39 , with the opening degree of the regulating valve  39  at the time of rated output being a medium opening degree. In other words, the counter thrust force can be made larger by raising (opening) the opening degree of the regulating valve  39  from the medium opening degree to thereby lower the pressure on the low pressure side of the balance piston  11   a . On the other hand, the counter thrust force can be made smaller by lowering (closing) the opening degree of the regulating valve  39  from the medium opening degree to thereby raise the pressure on the low pressure side of the balance piston  11   a.    
     The thrust force applied to the thrust bearing can be measured by a thrust load detection sensor disposed in the thrust bearing, for example, the load cell  52 . By inputting a detection signal of the load cell  52  to the control unit  50 , a largeness of the counter thrust force can be controlled so as to obtain a desired thrust force. In order to balance the thrust force and the counter thrust force stably, preferably
 
thrust force=counter thrust force+α
 
so that the thrust force may be slightly larger than the counter thrust force.
 
     Further, the pressure on the high pressure side of the balance piston  11   a  is detected by the pressure sensor  53  (high pressure side pressure detection sensor) and the pressure on the low pressure side is detected by the pressure sensor  53  (low pressure side pressure detection sensor), so that the counter thrust force can be found from a difference between detection values thereof. In other words, when the pressure on the high pressure side is indicated as P 1 , a pressure receiving area on the high pressure side is indicated as A 1 , the pressure on the low pressure side is indicated as P 2  and a pressure receiving area on the low pressure side is indicated as A 2 , the counter thrust force can be found by
 
counter thrust force= P 1× A 1− P 2× A 2.
 
Therefore, by inputting the detection signals from two pressure sensors  53  to the control unit  50 , the largeness of the counter thrust force can be controlled so as to obtain a desired thrust force.
 
     In order to increase a shut-off property in the exhaust connection piping  35 , it is preferable to dispose an opening/closing valve in addition to the regulating valve  39  in the exhaust connection piping  35 , as illustrated in  FIG. 3 . In this case, by closing the opening/closing valve  40  (exhaust connection piping valve mechanism), the exhaust CO 2  can be surely prevented from flowing back into the exhaust connection piping  35 . Besides, at the time of rated output or the like, by regulating the opening degree of the regulating valve  39  in a state where the opening/closing valve  40  is opened, the largeness of the counter thrust force can be controlled as stated above. Note that in a case where control of the counter thrust force is unnecessary, it is possible to provide only the opening/closing valve  40  without providing the regulating valve  39 . 
     Second Embodiment 
     Next, a second embodiment will be described with reference to  FIG. 4 . In the second embodiment, there is disposed a cooling CO 2  supply piping  80  (a cooling gas supply piping) to supply cooling CO 2  at high pressure from a cooling supply system to a low pressure side (chamber B) of a balance piston  11   a , and a regulating valve  81  (a cooling gas supply piping valve mechanism) is disposed in the cooling CO 2  supply piping  80  as a valve mechanism. The regulating valve  81  is controlled to open and close by a control unit  50 . Other parts are constituted similarly to those in the first embodiment illustrated in  FIG. 2 . 
     According to the second embodiment of the above configuration, when a regulating valve  39  or an opening/closing valve  40  is closed in order to prevent a backflow of exhaust CO 2  into an exhaust connection piping  35  at the time of low load or the like, it is possible to open the regulating valve  81  to thereby supply the cooling CO 2  at high pressure from the cooling CO 2  supply piping  80  to the low pressure side of the balance piston  11   a . This enables regulation of a counter thrust force. In this case, when the regulating valve  81  is opened and the cooling CO 2  at high pressure is supplied, the pressure on the low pressure side (chamber B) of the balance piston  11   a  rises, decreasing the counter thrust force. 
     In order to increase a shut-off property in the cooling CO 2  supply piping  80 , it is preferable to dispose an opening/closing valve  82  (a cooling gas supply piping valve mechanism) in addition to the regulating valve  81  in the cooling CO 2  supply piping  80 , as illustrated in  FIG. 5 . In this case, by closing the opening/closing valve  82 , the cooling CO 2  supply piping  80  can be surely shut off at the time of rated output or the like. Besides, at the time of low load or the like, by regulating an opening degree of the regulating valve  81  in a state where the opening/closing valve  82  is opened, a largeness of the counter thrust force can be controlled as stated above. The opening/closing valve  82  is controlled to open and close by the control unit  50 . 
     While certain embodiments of the present invention 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. 
     EXPLANATION OF REFERENCE NUMERALS 
       1  . . . CO 2  pump,  2  . . . regenerative heat exchanger,  3  . . . oxygen producer,  4  . . . combustor,  5  . . . CO 2  turbine,  6  . . . power generator,  7  . . . cooler,  8  . . . humidity separator,  10  . . . bearing,  11  . . . turbine rotor,  11   a  . . . balance piston,  12   a ,  12   b  . . . mechanical seal (seal mechanism),  13  . . . rotor blade,  14  . . . outer casing,  14   a ,  14   b  . . . through hole,  15   a ,  15   b  . . . inner casing,  16  . . . stationary blade,  18   a ,  18   b  . . . partition wall,  19  . . . partition wall hole,  23  . . . balance piston seal,  24  . . . labyrinth seal,  29  . . . balance piston extraction hole,  31  . . . working CO 2  injection pipe (working fluid injection pipe),  32  . . . CO 2  discharge pipe (turbine exhaust system),  33  . . . CO 2  injection pipe for cooling or sealing (cooling CO 2  injection pipe),  35  . . . exhaust connection piping,  39  . . . regulating valve (exhaust connection piping valve mechanism),  40  . . . opening/closing valve (exhaust connection piping valve mechanism),  50  . . . control unit,  51  . . . pressure sensor,  52  . . . load cell (thrust load detection sensor),  53  . . . pressure sensor (high pressure side pressure detection sensor, low pressure side pressure detection sensor),  54  . . . pressure sensor, A to D . . . cooling chamber, E . . . exhaust chamber,  80  . . . cooling CO 2  supply piping (cooling gas supply piping),  81  . . . regulating valve (cooling gas supply piping valve mechanism),  82  . . . opening/closing valve (cooling gas supply piping valve mechanism)