Abstract:
Provided are: a power generation system that can generate electric power efficiently with a fuel cell; and a method for operating said power generation system. This power generation system comprises: a fuel cell including a plurality of unit fuel cell modules; a gas turbine; various lines for circulating fuel gas, air, discharged fuel gas, and discharged air between the fuel cell and the gas turbine; and a control device. The control device determines the number of said unit fuel cell modules to be operated on the basis of the required power generation amount, and operates the determined number of said unit fuel cell modules.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power generation system combining a solid oxide fuel cell, a gas turbine, and a steam turbine, and to a method for operating a power generation system. 
       BACKGROUND ART 
       [0002]    A solid oxide fuel cell (hereinafter, referred to as SOFC) is known as a highly efficient fuel cell having a wide range of applications. Since the operating temperature of an SOFC is set to be high in order to increase ionic conductivity, it is possible to use compressed air ejected from a compressor of a gas turbine as air (an oxidant) to be supplied to an air electrode side. In addition, it is possible to use the high-temperature exhaust fuel gas exhausted from the SOFC as the fuel of the combustor of the gas turbine. 
         [0003]    Thus, for example, as described in Patent Literature  1  listed below, various combinations of an SOFC, a gas turbine, and a steam turbine have been proposed as power generation systems that achieve high power generation efficiency. In the combined system disclosed in Patent Literature  1 , the gas turbine has a compressor compressing air and supplying the compressed air to the SOFC and a combustor generating combustion gas from exhaust fuel gas exhausted from the SOFC and the compressed air. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-205930A 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0005]    A power generation system may adjust a power generation amount by adjusting the supplied fuel or air volume on the basis of the required power generation amount which changes according to various conditions. Here, for fuel cells such as SOFCs, since the power generation efficiency changes on the basis of the flow rate of the supplied fuel or air, the efficiency during power generation may be decreased when the flow rate is changed on the basis of changes in the power generation amount. 
         [0006]    The present invention was created to solve the problems described above and an object of the present invention is to provide a power generation system capable of generating power efficiently with a fuel cell, and a method for operating a power generation system. 
       Solution to Problem 
       [0007]    A power generation system of the present invention for achieving the object described above includes a fuel cell including a plurality of unit fuel cell modules, a gas turbine having a compressor and a combustor, a first compressed air supply line supplying compressed air from the compressor to the combustor, a second compressed air supply line supplying compressed air from the compressor to the fuel cell, a compressed air circulation line supplying exhaust air from the fuel cell to the combustor, a fuel gas supply line supplying fuel gas to the fuel cell, an exhaust fuel supply line supplying exhaust fuel gas discharged from the fuel cell to the combustor, and a control device which determines a number of the unit fuel cell modules to be operated on the basis of a required power generation amount, and operates the determined number of the unit fuel cell modules. The unit fuel cell modules are provided with a unit fuel cell, a line supplying compressed air from the second compressed air supply line to the unit fuel cell, a line supplying fuel gas from the fuel gas supply line to the unit fuel cell, a line supplying exhaust air from the unit fuel cell to the compressed air circulation line, and a line supplying fuel gas from the unit fuel cell to the exhaust fuel supply line. 
         [0008]    Accordingly, by setting the fuel cell as a plurality of unit fuel cell modules and controlling the number of the unit fuel cell modules to be operated on the basis of the required power generation amount, it is possible to generate power efficiently with the unit fuel cells of each of the unit fuel cell modules. In other words, it is possible to adjust the power generation amount for the fuel cells as a whole while maintaining high efficiency in each single unit fuel cell. Due to this, the fuel cell generates power efficiently. 
         [0009]    In the power generation system of the present invention, the control device calculates the number of the unit fuel cell modules to be operated to be able to output the required power generation amount while being able to be operated at a reference efficiency or higher, and sets the calculated number as the number of the unit fuel cell modules to be operated. 
         [0010]    Accordingly, it is possible to maintain high efficiency during power generation for each single unit fuel cell, and the fuel cell generates power efficiently. 
         [0011]    In the power generation system of the present invention, the control device executes a start-up process to be executed before operation in at least one of the unit fuel cell modules which is stopped. 
         [0012]    Accordingly, in a case where the unit fuel cell modules to be operated increase, it is possible to increase the number of operating unit fuel cell modules in a short time. 
         [0013]    In the power generation system of the present invention, when there is a stopped unit fuel cell module, the control device switches the stopped unit fuel cell module. 
         [0014]    Accordingly, it is possible to suppress the operating unit fuel cell modules to be specific unit fuel cell modules only. In addition, it is possible to sequentially inspect the unit fuel cell modules. 
         [0015]    In the power generation system of the present invention, the control device stops unit fuel cell modules for which the operation time is relatively long, and starts up unit fuel cell modules for which the operation time is relatively short. 
         [0016]    Accordingly, it is possible to suppress bias in the consumption of the unit fuel cell modules and it is possible to extend the life of the device as a whole. 
         [0017]    In the power generation system of the present invention, the fuel cell has a line discharging exhaust air from the unit fuel cell to the outside and a line discharging exhaust fuel gas from the unit fuel cell to the outside. When stopping the unit fuel cell module, the control device stops the supply of the exhaust air and exhaust fuel gas from the unit fuel cell of the unit fuel cell module to the gas turbine, discharges the exhaust air and exhaust fuel gas to the outside, reduces the volume of air and fuel gas to be supplied to the unit fuel cell, and stops the discharging of the exhaust air and the exhaust fuel gas to the outside and the supply of air and fuel gas to the unit fuel cell after cooling of the unit fuel cell is completed. 
         [0018]    Accordingly, it is possible to separately stop each of the unit fuel cell modules and it is possible to suppress influence on the other unit fuel cell modules. 
         [0019]    In addition, in a method for operating a power generation system of the present invention, the power generation system has a fuel cell including a plurality of unit fuel cell modules, a gas turbine having a compressor and a combustor, a first compressed air supply line supplying compressed air from the compressor to the combustor, a second compressed air supply line supplying compressed air from the compressor to the fuel cell, a compressed air circulation line supplying exhaust air from the fuel cell to the combustor, a fuel gas supply line supplying fuel gas to the fuel cell, and an exhaust fuel supply line supplying exhaust fuel gas discharged from the fuel cell to the combustor. The method includes the steps of determining a number of the unit fuel cell modules to be operated on the basis of the required power generation amount, and operating the determined number of the unit fuel cell modules. 
         [0020]    Accordingly, by setting the fuel cell as a plurality of unit fuel cell modules and controlling the number of the unit fuel cell modules to be operated on the basis of the required power generation amount, it is possible to generate power efficiently with the unit fuel cells of each of the unit fuel cell modules. In other words, it is possible to adjust the power generation amount for the fuel cells as a whole while maintaining high efficiency in each single unit fuel cell. Due to this, the fuel cell generates power efficiently. 
       Advantageous Effect Of Invention 
       [0021]    According to the power generation system and the method for operating a power generation system of the present invention, it is possible to suppress changes in the pressure of compressed air to be supplied to a fuel cell by adjusting the balance of the flowability of compressed air in a first compressed air supply line and a second compressed air supply line based on the flowability of compressed air in a fuel cell. Due to this, it is possible to stabilize the pressure of the compressed air to be supplied to the fuel cell. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a schematic configuration diagram representing a power generation system of the present embodiment. 
           [0023]      FIG. 2  is a schematic diagram illustrating a gas turbine, an SOFC, and a piping system in the power generation system according to the embodiment of the present invention. 
           [0024]      FIG. 3  is a flow chart illustrating an example of a method for operating a power generation system of the present embodiment. 
           [0025]      FIG. 4  is a flow chart illustrating an example of a method for operating a power generation system of the present embodiment. 
           [0026]      FIG. 5  is a schematic diagram illustrating another example of a gas turbine, an SOFC, and a piping system. 
           [0027]      FIG. 6  is a flow chart illustrating an example of the method for operating a power generation system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    A preferred embodiment of a power generation system and a method for operating the power generation system according to the present invention are described in detail below, with reference to the accompanying drawings. Note that the invention is not limited by the embodiment, and when a plurality of embodiments is present, the invention is intended to also include a configuration combining these embodiments. 
       Embodiment 
       [0029]    A power generation system of the present embodiment is a Triple Combined Cycle (registered trademark) that combines a solid oxide fuel cell (hereinafter, referred to as SOFC), a gas turbine, and a steam turbine. This Triple Combined Cycle is able to generate power in the three stages of the SOFC, the gas turbine, and the steam turbine by disposing the SOFC upstream of gas turbine combined cycle (GTCC) power generation, and is thus able to realize extremely high power generation efficiency. Note that the following description is made with a solid oxide fuel cell employed as the fuel cell of the present invention; however, no limitation to this type of fuel cell is intended. 
         [0030]      FIG. 1  is a schematic configuration diagram representing the power generation system of the present embodiment. In the embodiment, as illustrated in  FIG. 1 , a power generation system  10  includes a gas turbine  11  and a power generator  12 , an SOFC  13 , a steam turbine  14 , and a power generator  15 . The power generation system  10  combines power generation by the gas turbine  11 , power generation by the SOFC  13 , and power generation by the steam turbine  14 , so as to be configured to achieve high power generation efficiency. The power generation system  10  is also provided with a control device  62 . The control device  62  controls the operation of each component of the power generation system  10  in accordance with input settings, input instructions, results detected by a detection unit, and the like. 
         [0031]    The gas turbine  11  includes a compressor  21 , a combustor  22 , and a turbine  23 . The compressor  21  and the turbine  23  are coupled in an integrally rotatable manner by a rotating shaft  24 . The compressor  21  compresses air A taken in through an air intake line  25 . The combustor  22  mixes and combusts compressed air A 1  supplied from the compressor  21  through a first compressed air supply line  26  and fuel gas L 1  supplied from a first fuel gas supply line  27 . The turbine  23  is rotated by combustion gas G 1  supplied from the combustor  22  through an exhaust gas supply line  28 . Although not illustrated, the turbine  23  is supplied with the compressed air A 1  compressed by the compressor  21  through a casing, and cools blades and the like by using this compressed air A 1  as cooling air. The power generator  12  is provided coaxially with the turbine  23  and is able to generate power as the turbine  23  rotates. Note that, for example, liquefied natural gas (LNG) is used as the fuel gas L 1  to be supplied to the combustor  22 . 
         [0032]    The SOFC  13  is supplied with a high-temperature fuel gas as a reductant and with high-temperature air (oxidizing gas) as an oxidant, which react at a predetermined operating temperature to generate power. This SOFC  13  is constituted of an air electrode, a solid electrolyte, and a fuel electrode that are housed in a pressure container. A portion of compressed air A 2 , which has been compressed by the compressor  21 , is supplied to the air electrode and fuel gas L 2  is supplied to the fuel electrode, so that power is generated. The fuel gas L 2  supplied to the SOFC  13  is, for example, hydrocarbon gas such as liquefied natural gas (LNG), hydrogen (H 2 ) and carbon monoxide (CO), or methane (CH 4 ), or gas produced at gas production facilities from carbonaceous materials such as coal. The oxidizing gas supplied to the SOFC  13  is a gas containing approximately 15% to 30% oxygen. Typically, air is suitable. However, in addition to air, mixed gas of combustion exhaust gas and air, mixed gas of oxygen and air, or the like can be used (hereinafter, the oxidizing gas supplied to the SOFC  13  is referred to as air). 
         [0033]    This SOFC  13  is connected to a second compressed air supply line  31  that branches off from the first compressed air supply line  26 , so as to be able to supply the portion of compressed air A 2  compressed by the compressor  21  to an introduction part of the air electrode. This second compressed air supply line  31  is provided with a control valve  32  that is capable of adjusting the volume of air to be supplied, and a blower (booster)  33  that is capable of boosting the pressure of the compressed air A 2 , along the air-flow direction of the compressed air A 2 . The control valve  32  is provided upstream in the flow direction of the compressed air A 2  in the second compressed air supply line  31  and the blower  33  is provided downstream of the control valve  32 . The SOFC  13  is connected to an exhaust air line  34  discharging compressed air A 3  (exhaust air) that has been used by the air electrode. This exhaust air line  34  branches into a discharge line  35  that discharges the compressed air A 3  used by the air electrode to the outside, and a compressed air circulation line  36  that is connected to the combustor  22 . The discharge line  35  is provided with a control valve  37  that is capable of adjusting the volume of air to be discharged. The compressed air circulation line  36  is provided with a control valve  38  that is capable of adjusting the volume of air to be circulated. 
         [0034]    The SOFC  13  is also provided with a second fuel gas supply line  41  that supplies the fuel gas L 2  to the introduction part of the fuel electrode. The second fuel gas supply line  41  is provided with a control valve  42  that is capable of adjusting the volume of fuel gas to be supplied. The SOFC  13  is connected to an exhaust fuel line  43  discharging exhaust fuel gas L 3  that has been used by the fuel electrode. The exhaust fuel line  43  branches into a waste line  44  that discharges the exhaust fuel gas to the outside, and an exhaust fuel gas supply line  45  that is connected to the combustor  22 . The waste line  44  is provided with a control valve  46  that is capable of adjusting the volume of exhaust fuel gas to be discharged. The exhaust fuel gas supply line  45  is provided with a control valve  47  that is capable of adjusting the volume of the exhaust fuel gas to be supplied, and a blower  48  that is capable of boosting the exhaust fuel gas L 3 , along the flow direction of the exhaust fuel gas L 3 . The control valve  47  is provided upstream in the flow direction of the exhaust fuel gas L 3  in the exhaust fuel gas supply line  45 . The blower  48  is provided downstream of the control valve  47 . 
         [0035]    The SOFC  13  is also provided with a fuel gas recirculation line  49  that connects the exhaust fuel line  43  and the second fuel gas supply line  41 . A recirculation blower  50  which recirculates the exhaust fuel gas L 3  of the exhaust fuel line  43  in the second fuel gas supply line  41  is provided in the fuel gas recirculation line  49 . 
         [0036]    The steam turbine  14  rotates a turbine  52  using steam generated by a heat recovery steam generator (HRSG)  51 . The steam turbine  14  (the turbine  52 ) is provided with a steam supply line  54  and a feed water line  55  between the turbine and the heat recovery steam generator  51 . The feed water line  55  is provided with a condenser  56  and a feed water pump  57 . This heat recovery steam generator  51  is connected to an exhaust gas line  53  from the gas turbine  11  (turbine  23 ), and generates steam S through heat exchange between high-temperature exhaust gas G 2  supplied from the exhaust gas line  53  and water supplied from the feed water line  55 . The power generator  15  is provided coaxially with the turbine  52  and is able to generate power as the turbine  52  rotates. Note that the exhaust gas G 2  whose heat has been recovered by the heat recovery steam generator  51  is released into the atmosphere after removal of any toxic materials. 
         [0037]    The operation of the power generation system  10  of the present embodiment will now be described. When the power generation system  10  starts up, the gas turbine  11 , the steam turbine  14 , and the SOFC  13  are started up in the stated order. 
         [0038]    First, in the gas turbine  11 , the compressor  21  compresses the air A, the combustor  22  mixes the compressed air A 1  with the fuel gas L 1  and combusts the mixed gas, and the turbine  23  is rotated by the combustion gas G 1 . Thus, the power generator  12  begins to generate power. Next, in the steam turbine  14 , the turbine  52  rotates due to the steam S produced by the heat recovery steam generator  51 . Thus, the power generator  15  begins to generate power. 
         [0039]    Subsequently, in order to start up the SOFC  13 , the compressed air A 2  is supplied from the compressor  21  to the SOFC  13 , so as to start pressurization and heating of the SOFC  13 . The control valve  32  is opened to a predetermined degree while the control valve  37  of the discharge line  35  and the control valve  38  of the compressed air circulation line  36  are closed and the blower  33  of the second compressed air supply line  31  is stopped. Then, a portion of the compressed air A 2  compressed by the compressor  21  is supplied from the second compressed air supply line  31  toward the SOFC  13 . Accordingly, the pressure is raised on the air electrode side of the SOFC  13  as the compressed air A 2  is supplied thereto. 
         [0040]    Meanwhile, on the fuel electrode side of the SOFC  13 , the fuel gas L 2  is supplied thereto to start raising the pressure. With the control valve  46  of the waste line  44  and the control valve  47  of the exhaust fuel gas supply line  45  being closed and with the blower  48  being stopped, the control valve  42  of the second fuel gas supply line  41  is opened and the recirculation blower  50  of the fuel gas recirculation line  49  is driven. Then, the fuel gas L 2  is supplied from the second fuel gas supply line  41  to the SOFC  13 , and exhaust fuel gas L 3  is re-circulated by the fuel gas recirculation line  49 . Accordingly, the pressure is raised on the fuel electrode side of the SOFC  13  as the fuel gas L 2  is supplied thereto. 
         [0041]    Next, once the pressure on the air electrode side of the SOFC  13  reaches an outlet pressure of the compressor  21 , the control valve  32  is fully opened and the blower  33  is driven. The control valve  37  is simultaneously opened and the compressed air A 3  from the SOFC  13  is discharged from the discharge line  35 . Then, the compressed air A 2  is supplied toward the SOFC  13  by the blower  33 . The control valve  46  is simultaneously opened and the exhaust fuel gas L 3  from the SOFC  13  is discharged from the waste line  44 . Next, once the pressure on the air electrode side and the pressure on the fuel electrode side of the SOFC  13  reach a target pressure, the pressurization of the SOFC  13  is completed. 
         [0042]    Afterward, once the reaction (power generation) in the SOFC  13  stabilizes and the components of the compressed air A 3  and the exhaust fuel gas L 3  stabilize, the control valve  37  is closed while the control valve  38  is opened. Then, the compressed air A 3  from the SOFC  13  is supplied to the combustor  22  through the compressed air circulation line  36 . While the control valve  46  is closed, the control valve  47  is opened and the blower  48  is driven. Then, the exhaust fuel gas L 3  from the SOFC  13  is supplied to the combustor  22  through the exhaust fuel gas supply line  45 . At this point, the fuel gas L 1  supplied to the combustor  22  through the first fuel gas supply line  27  is reduced. 
         [0043]    Here, the power generation by the power generator  12  through the driving of the gas turbine  11 , the power generation by the SOFC  13 , and the power generation by the power generator  15  through the driving of the steam turbine  14  are all active, so that the power generation system  10  is in a steady operation state. 
         [0044]      FIG. 2  is a schematic diagram illustrating a gas turbine, an SOFC, and a piping system in the power generation system according to the embodiment of the present invention. In the power generation system  10  of the present embodiment, the SOFC  13  is provided with a plurality of unit SOFC modules (unit fuel cell modules)  120 . In addition,  FIG. 2  illustrates only the connection relationship between the control device  62  and one unit SOFC module  120 ; however, the control device  62  is connected to all of the unit SOFC modules  120  of the SOFC  13 . A plurality of the unit SOFC modules  120  is arranged in parallel. For each of the unit SOFC modules  120 , compressed air A 2  is supplied from the second compressed air supply line  31 , compressed air A 3  is discharged to the compressed air circulation line  36 , fuel gas (upgraded fuel gas) L 2  is supplied from the second fuel gas supply line (fuel gas supply line)  41 , and exhaust fuel gas L 3  is discharged to the exhaust fuel line  43 . In addition, compressed air discharged from the compressor  21  is supplied to the turbine  23  using a cooling air supply line  72  and is also used as air for cooling the turbine  23 . 
         [0045]    The unit SOFC modules  120  are provided with an air branch pipe  121 , a unit SOFC (unit fuel cell)  122 , an exhaust air branch pipe  124 , a control valve  126 , a control valve  128 , a fuel branch pipe  131 , an exhaust fuel branch pipe  134 , a control valve  136 , and a control valve  138 . 
         [0046]    First, the unit SOFC  122  has the same configuration as the SOFC  13  described above. The unit SOFC  122  is supplied with a high-temperature fuel gas as a reductant and with high-temperature air (oxidizing gas) as an oxidant, which react at a predetermined operating temperature to generate power. This unit SOFC  122  is constituted of an air electrode, a solid electrolyte, and a fuel electrode that are housed in a pressure container. 
         [0047]    One end section of the air branch pipe  121  is connected to the second compressed air supply line  31  and the other end section is connected to the unit SOFC  122 . One end section of the exhaust air branch pipe  124  is connected to the unit SOFC  122  and the other end section is connected to the compressed air circulation line  36 . The unit SOFC module  120  supplies compressed air A 2  to the unit SOFC  122  from the second compressed air supply line  31  through the air branch pipe  121 . In addition, the unit SOFC module  120  discharges the compressed air A 3  to the compressed air circulation line  36  from the unit SOFC  122  through the exhaust air branch pipe  124 . 
         [0048]    The control valve  126  is arranged in the air branch pipe  121 . Similarly to each of the control valves described above, the control valve  126  adjusts the compressed air A 2  flowing through the air branch pipe  121  by opening and closing and adjusting the degree of opening. The control valve  128  is arranged in the exhaust air branch pipe  124 . Similarly to the control valve described above, the control valve  128  adjusts the compressed air A 3  flowing through the exhaust air branch pipe  124  by opening and closing and adjusting the degree of opening. 
         [0049]    One end section of the fuel branch pipe  131  is connected to the second fuel gas supply line  41  and the other end section is connected to the unit SOFC  122 . One end section of the exhaust fuel branch pipe  134  is connected to the unit SOFC  122  and the other end section is connected to the exhaust fuel line  43 . The unit SOFC module  120  supplies fuel gas L 2  to the unit SOFC  122  from the second fuel gas supply line  41  through the fuel branch pipe  131 . In addition, the unit SOFC module  120  discharges the exhaust fuel gas L 3  to the exhaust fuel line  43  from the unit SOFC  122  through the exhaust fuel branch pipe  134 . 
         [0050]    The control valve  136  is arranged in the fuel branch pipe  131 . Similarly to each of the control valves described above, the control valve  136  adjusts the fuel gas L 2  flowing in the fuel branch pipe  131  by opening and closing and adjusting the degree of opening. The control valve  138  is arranged in the exhaust fuel branch pipe  134 . Similarly to the control valve described above, the control valve  138  adjusts the exhaust fuel gas L 3  flowing in the exhaust fuel branch pipe  134  by opening and closing and adjusting the degree of opening. 
         [0051]    The unit SOFC module  120  is constituted as above and it is possible to isolate the one unit SOFC module  120  from the paths in which the compressed air, the fuel gas, the exhaust air, and the exhaust fuel gas flow by closing the control valve  126 , the control valve  128 , the control valve  136 , and the control valve  138 . Due to this, it is possible for the SOFC  13  to switch between driving and stopping for each one of the unit SOFC modules  120 . The control device  62  realizes power generation with high efficiency with the SOFC  13  by controlling the unit SOFC modules  120  to be operated. In addition, by being able to switch the driving and stopping for each one of the unit SOFC modules  120 , it is possible to generate power with other unit SOFC modules  120  while carrying out maintenance on some unit SOFC modules  120 . In addition, even in a case where some unit SOFC modules  120  fail, since only such unit SOFC modules  120  are stopped, it is possible to continue the operation. 
         [0052]    Description will be given below of a method for operating the power generation system  10  of the present embodiment described above using  FIG. 3 .  FIG. 3  is a flow chart illustrating an example of a method for operating a power generation system of the present embodiment. It is possible to realize the operating method illustrated in  FIG. 3  by the control device (controller)  62  executing a calculation process based on the acquired power generation amount required for the SOFC  13 . Here, the control device  62  repeatedly executes the processes illustrated in  FIG. 3 . 
         [0053]    First, the control device  62  calculates the total load based on the power generation amount required for the SOFC  13  (step S 12 ). For example, the total load is set to 100% in a case where all of the unit SOFC modules of the SOFC  13  are operating with a utilization rate of 100% and the required power generation amount is a necessary load in order to generate power with the SOFC  13 . 
         [0054]    Once the control device  62  calculates the total load, the number (number of units) of unit SOFC modules to be operated is determined (step S 14 ). Once the control device  62  determines the number of the unit SOFC modules to be operated, it is determined whether or not there is a unit SOFC module to be started up (step S 16 ). In other words, with respect to the determined number of the unit SOFC modules to be operated, it is determined whether the number of unit SOFC modules currently operating is small. In a case where it is determined that there is a unit SOFC module to be started up (Yes in step S 16 ), the control device  62  specifies the unit SOFC modules to be started up, starts up the target unit SOFC module (step S  18 ), and finishes the process. 
         [0055]    Next, in a case where it is determined that there is no unit SOFC module to be started up (No in step S  16 ), the control device  62  determines whether there is a unit SOFC module to be stopped (step S 20 ). In other words, with respect to the determined number of the unit SOFC modules to be operated, it is determined whether the number of unit SOFC modules currently operating is large. In a case where it is determined that there is a unit SOFC module to be stopped (Yes in step S 20 ), the control device  62  specifies the unit SOFC module to be stopped, stops the target unit SOFC module (step S 22 ), and finishes the process. In a case where the control device  62  determines that there is no unit SOFC module to be stopped (No in step S 20 ), the present process is finished as is since the determined number of unit SOFC modules to be operated and the number of unit SOFC modules currently operating are the same. 
         [0056]    In the power generation system  10 , the SOFC  13  is set as a plurality of unit SOFC modules  120  arranged in parallel, it is possible to switch independently between the starting up and stopping of each of the unit SOFC modules  120 , and the number of the unit SOFC modules  120  to be operated is adjusted by the control device  62  based on the required power generation amount. Due to this, since it is possible to optionally adjust the number of unit SOFC modules  120  to be operated, it is possible to generate power efficiently with the unit SOFCs of each of the unit SOFC modules. In other words, it is possible to adjust the power generation amount of the SOFC  13  as a whole while maintaining high efficiency in each single unit SOFC. Due to this, it is possible to generate power efficiently with the SOFC  13  and it is possible to widen the range in which it is possible to generate power with high efficiency. 
         [0057]    Here, it is preferable that the control device  62  calculates the number of the unit SOFC modules to be operated to be able to output the required power generation amount while being able to be operated at a reference efficiency or higher, and sets the calculated number as the number of the unit SOFC modules to be operated. 
         [0058]    For example, it is possible to increase the efficiency of power generation by setting the flow rate of the fuel gas, the air, and the like supplied to the unit SOFC module as a reference and setting the flow rate of the unit SOFC module to 70% or more and 100% or less. In addition, the SOFC  13  is provided with ten of the unit SOFC modules. At this time, in a case where the load factor is 100%, all of the unit SOFC modules are operated and the flow rate of the unit SOFC modules is set to 100%. Next, in a case where the load factor is 90%, all of the unit SOFC modules are operated and the flow rate of the unit SOFC modules is set to 90%. Next, in a case where the load factor is 80%, all of the unit SOFC modules are operated and the flow rate of the unit SOFC modules is set to 80%. Next, in a case where the load factor is 70%, all of the unit SOFC modules are operated and the flow rate of the unit SOFC modules is set to 70%. Next, in a case where the load factor is 65%, nine of the unit SOFC modules are operated (one of the unit SOFC modules is stopped) and the flow rate of the operating unit SOFC modules is set to 72%. Next, in a case where the load factor is 60%, eight of the unit SOFC modules are operated (two of the unit SOFC modules are stopped) and the flow rate of the operating unit SOFC modules is set to 75%. Next, in a case where the load factor is 50%, seven of the unit SOFC modules are operated (three of the unit SOFC modules are stopped) and the flow rate of the operating unit SOFC modules is set to 71%. 
         [0059]    In this manner, by determining the number of unit SOFC modules to be operated in a range in which it is possible to satisfy the required power generation amount (calculated load factor) and in which the flow rate of the operating unit SOFC modules is set to 70% or more and 100% or less, it is possible for the operating unit SOFC to generate power with high efficiency and to secure the required output. 
         [0060]    In addition, in a case where the control device  62  determines the number of unit SOFC modules to be operated, it is preferable that the number of the unit SOFC modules to be operated is determined without stopping any of the unit SOFC modules. For example, in a case where the load factor (the required power generation amount) is decreased, the flow rate (driving condition) of the unit SOFC modules to be operated is adjusted such that it is possible to maintain the number of the unit SOFC modules to be operated, in other words, the flow rate (driving condition) is adjusted to approach the lower limits of the conditions under which power generation is possible with high efficiency. In the adjustment of the flow rate, it is preferable that a determination to reduce the number of unit SOFC modules to be operated is made only in a case of being at or below the lower limit at which it is possible to generate power at high efficiency. Due to this, it is possible to reduce the number of times the starting up process is performed. The starting up process is a process which is necessary to execute before operating the stopped unit SOFC modules, such as pressurizing the unit SOFC  122  or upgrading the fuel gas. 
         [0061]    In addition, in a case where the control device  62  determines the number of unit SOFC modules to be operated, it is preferable that the number of the unit SOFC modules to be operated is determined without starting up any of the unit SOFC modules. For example, in a case where the load factor (the required power generation amount) is increased, the flow rate (driving condition) of the unit SOFC modules to be operated is adjusted such that it is possible to maintain the number of the unit SOFC modules to be operated, in other words, the flow rate (driving condition) is adjusted to approach the upper limits of the conditions under which power generation is possible with high efficiency. In the adjustment of the flow rate, it is preferable that a determination to increase the number of unit SOFC modules to be operated is made only at or above an upper limit at which it is possible to generate power at high efficiency, in other words, only in a case where the output of the current number of unit SOFC modules does not satisfy the requirements. Due to this, it is possible to reduce the number of times the starting up process is performed. 
         [0062]    In addition, in a case where there are stopped unit SOFC modules, it is preferable that the control device  62  causes at least one of the stopped unit SOFC modules to execute the starting up process. By performing the starting up process in advance in the stopped unit SOFC modules in this manner, it is possible to increase the number of operating unit SOFC modules in a short time when a determination is made to increase the unit SOFC modules to be operated. 
         [0063]    Next, description will be given of another method for operating the power generation system  10  of the present embodiment described above using  FIG. 4 .  FIG. 4  is a flow chart illustrating another example of the method for operating a power generation system of the present embodiment. It is possible to realize the operating method illustrated in  FIG. 4  by the control device (controller)  62  executing a calculation process based on detection results for each component. Here, the control device  62  repeatedly executes the processes illustrated in  FIG. 4 . 
         [0064]    First, the control device  62  determines whether rotation of the unit SOFC modules is executed (step S 30 ). Here, the control device  62  determines that rotation is to be executed, for example, in a case where there is a stopped unit SOFC module and a state where the same unit SOFC module is operated has continued for a predetermined time or more. The control device  62  finishes the present process in a case where it is determined that rotation is not to be executed (No in step S 30 ). 
         [0065]    In a case where it is determined that rotation is to be executed (Yes in step S 30 ), the control device  62  specifies the rotation target unit SOFC module, starts up the target unit SOFC module from among the stopped unit SOFC modules (step S 32 ), stops the target unit SOFC modules from among the operating unit SOFC modules (step S 34 ), and finishes the present process. 
         [0066]    In this manner, by the rotation of the operating unit SOFC modules, in other words, by switching the stopped unit SOFC module in a case where there is a stopped unit SOFC module, it is possible for the control device  62  to suppress the operating unit SOFC module to be a specific unit SOFC module only. In addition, it is possible to sequentially inspect the unit SOFC modules. 
         [0067]    Here, in a case where the control device  62  executes rotation, it is preferable that the unit SOFC module for which the operation time is relatively long is stopped and the unit SOFC module for which the operation time is relatively short is started up (operated). Due to this, it is possible to suppress bias in the consumption of the unit SOFC modules and it is possible to extend the lifespan of the device as a whole. 
         [0068]    Here, for the unit SOFC modules  120 , as lines connected to the unit SOFC  122 , each unit SOFC module  120  is provided with the air branch pipe  121  supplying air, the exhaust air branch pipe  124  discharging exhaust air, the fuel branch pipe  131  supplying fuel gas, and the exhaust fuel branch pipe  134  discharging exhaust fuel gas; however, the present invention is not limited thereto. 
         [0069]      FIG. 5  is a schematic diagram illustrating another embodiment of a gas turbine, an SOFC, and a piping system. In a power generation system  10   a  illustrated in  FIG. 5 , the SOFC  13  is provided with a plurality of unit SOFC modules  120   a.  Here, the basic configuration of the unit SOFC modules  120   a  is the same as the unit SOFC modules  120 . Description will be given below of points specific to the unit SOFC modules  120   a.    
         [0070]    The unit SOFC modules  120   a  have the air branch pipe  121 , the unit SOFC (unit fuel cell)  122 , the exhaust air branch pipe  124 , the control valve  126 , the control valve  128 , the fuel branch pipe  131 , an exhaust fuel branch pipe  134 , the control valve  136 , the control valve  138 , an air discharge branch pipe  202 , a control valve  204 , a fuel gas recirculation line  212 , a recirculation blower  214 , a fuel discharge branch pipe  216 , and a control valve  218 . 
         [0071]    One end section of the air discharge branch pipe  202  is connected further to the unit SOFC  122  side than the control valve  128  of the exhaust air branch pipe  124  and the other end section is connected to the discharge line  35  discharging the compressed air A 3  to the outside. The control valve  204  is arranged in the air discharge branch pipe  202 . Similarly to the control valve described above, the control valve  204  adjusts the compressed air A 3  flowing in the air discharge branch pipe  202  by opening and closing and adjusting the degree of opening. 
         [0072]    One end section of the fuel gas recirculation line  212  is connected further to the unit SOFC  122  side than the control valve  138  of the exhaust fuel branch pipe  134  and the other end section is connected to the fuel branch pipe  131 . The recirculation blower  214  is arranged in the fuel gas recirculation line  212  and supplies exhaust fuel gas L 3  supplied from the exhaust fuel branch pipe  134  to the fuel branch pipe  131 . The fuel gas recirculation line  212  and the recirculation blower  214  are provided with the same functions as the fuel gas recirculation line  49  and the recirculation blower  50 . In other words, in the unit SOFC modules  120   a,  a mechanism for circulating the exhaust fuel gas L 3  in order to upgrade the fuel is provided for each of the unit SOFC modules  120   a.  Accordingly, only the fuel gas L 2  flows upstream of the fuel gas recirculation line  212  of the fuel branch pipe  131 . 
         [0073]    One end section of the fuel discharge branch pipe  216  is connected further to the unit SOFC  122  side than the control valve  138  of the exhaust fuel branch pipe  134  and the other end section is connected to the waste line  44  discharging the exhaust fuel gas L 3  to the outside. The control valve  218  is arranged in the fuel discharge branch pipe  216 . Similarly to the control valve described above, the control valve  218  adjusts the exhaust fuel gas L 3  flowing in the fuel discharge branch pipe  216  by opening and closing and adjusting the degree of opening. 
         [0074]    By providing the unit SOFC modules  120   a  with the air discharge branch pipe  202  and the control valve  204 , it is possible to separately discharge exhaust air to the outside. In addition, by providing the unit SOFC modules  120   a  with the fuel discharge branch pipe  216  and the control valve  218 , it is possible to discharge the exhaust fuel gas separately. Due to this, it is possible to set the unit SOFC modules  120   a  to not be easily influenced by the other unit SOFC modules  120   a  and it is possible to easily control the unit SOFC modules  120   a  independently. For example, it is possible for the unit SOFC modules  120   a  to suppress exhaust fuel gas and exhaust air gas being supplied to a shared line when starting up or when stopping, and it is possible to stabilize the quality of the exhaust fuel gas and the exhaust air gas to be supplied to the gas turbine  11 . In addition, since it is possible for the unit SOFC modules  120   a  to perform a fuel upgrading process separately, it is possible to perform the starting up process separately. In addition, it is possible to suppress changes in the quality of the fuel gas supplied to each system. 
         [0075]    Below, description will be given of an example of a method for operating the power generation system  10   a  of the present embodiment described above using  FIG. 6 .  FIG. 6  is a flow chart illustrating an example of a method for operating the power generation system  10   a.  The processing operations illustrated in  FIG. 6  are executed to stop the operating unit SOFC modules  120   a.  In a case where there is a unit SOFC module  120   a  to be stopped, the control device  62  stops the supply of the exhaust air and the exhaust fuel gas discharged from the target unit SOFC module  120   a  to the gas turbine  11  (step S 40 ). Specifically, the control valve  128  is closed and the supply of the exhaust air from the exhaust air branch pipe  124  to the exhaust air line  34  is stopped, and the control valve  138  is closed and the supply of the exhaust fuel gas from the exhaust fuel branch pipe  134  to the exhaust fuel gas supply line  45  is stopped. 
         [0076]    Next, the control device  62  starts the venting of the exhaust air and the exhaust fuel gas (step S 42 ). Specifically, the control valve  204  is opened, the exhaust air is supplied from the air discharge branch pipe  202  to the discharge line  35 , the control valve  218  is closed, and the exhaust fuel gas is supplied from the fuel discharge branch pipe  216  to the waste line  44 . Due to this, it is possible to switch the supply destination of the exhaust air and the exhaust fuel gas to a waste system from the combustor  22  of the gas turbine  11  to the outside. 
         [0077]    Next, the control device  62  changes the volume of the air and the fuel gas to be supplied to a lower limit value (minimum flow) (step S 44 ). That is, the opening degrees of the control valve  126  and the control valve  136  are reduced, and the volume of the air and the fuel gas to be supplied to the unit SOFC  122  is reduced. 
         [0078]    Next, the control device  62  determines whether cooling is completed (step S 46 ). It is possible for the control device  62  to make the determination based on the temperature of the unit SOFC  122 , the exhaust air to be vented, and the temperature of the exhaust fuel gas. In a case where the control device  62  determines that the cooling is not completed (No in step S 46 ), the process returns to step S 46 . In a case where the control device  62  determines that the cooling is completed (Yes in step S 46 ), as step S 48 , the supply and venting of the air and fuel gas are stopped and the present process is finished. In other words, by closing the control valve  126 , the control valve  136 , the control valve  204 , and the control valve  218 , the air and the fuel gas are not supplied to the unit SOFC  122 , the exhaust air and the exhaust fuel gas are not discharged, and the present process is finished. 
         [0079]    It is possible for the power generation system to separately stop each of the unit SOFC modules by stopping the unit SOFC modules using the process illustrated in  FIG. 6 , and it is possible to suppress the influence on the other unit SOFC modules. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10 ,  10   a  Power generation system 
           11  Gas turbine 
           12  Power generator 
           13  Solid oxide fuel cell (SOFC) 
           14  Steam turbine 
           15  Power generator 
           21  Compressor 
           22  Combustor 
           23  Turbine 
           25  Air intake line 
           26  First compressed air supply line 
           27  First fuel gas supply line 
           31  Second compressed air supply line 
           32  Control valve 
           33 ,  48  Blower 
           34  Exhaust air line 
           36  Compressed air circulation line 
           38  Control valve 
           41  Second fuel gas supply line 
           42  Control valve 
           43  Exhaust fuel line 
           44  Waste line 
           45  Exhaust fuel gas supply line 
           47  Control valve 
           49  Fuel gas recirculation line 
           50  Recirculation blower 
           51  Heat recovery steam generator 
           52  Turbine 
           53  Exhaust gas line 
           54  Steam supply line 
           55  Feed water line 
           56  Condenser 
           57  Feed water pump 
           62  Control device (controller) 
           120  Unit SOFC module (unit fuel cell module) 
           121  Air branch pipe 
           122  Unit SOFC (unit fuel cell) 
           124  Exhaust air branch pipe 
           126 ,  128  Control valve