Fuel cell system and control method for a turbine

Abstract

Fuel cell system comprising: a fuel cell (15); a compressor (60) that supplies a cathode gas to the fuel cell (15); a motor (62) driving the compressor (60); a cathode gas discharge passage (71) into which a cathode exhaust gas discharged from the fuel cell (15) flows; a turbine (80) arranged in the cathode gas discharge passage (71) and driven by the cathode exhaust gas, the turbine (80) having a changing mechanism (85) that changes an opening degree of a flow passage for the cathode exhaust gas passing through the turbine (80) to adjust a pressure difference between an upstream pressure and a downstream pressure of the turbine (80), the turbine (80) recovering at least part of energy of the cathode exhaust gas using the pressure difference and assisting in driving the motor (62) with the recovered energy; and a control unit (90) designed to drive the change mechanism (85) to increase or decrease the recovered energy, wherein the control unit (90) is configured to obtain a correlation temperature correlated with a first temperature of the cathode exhaust gas discharged from the turbine (80), and to perform a freezing prevention control of not setting the opening degree to be equal to or less than a predetermined opening degree when the correlation temperature is less than a predetermined threshold temperature at which the turbine (80) may freeze.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a fuel cell system with a turbine that supports driving a compressor, and a control method for the turbine. 2. Description of the state of the art

[0002] In the prior art, a fuel cell system with a compressor that supplies a cathode gas to a fuel cell is known. In a fuel cell system described in Japanese Unexamined Patent Application Laid-Open No. 2012-221657 (JP 2012-221657 A), a turbine is rotated by the energy of a cathode off-gas whose temperature has been raised due to heat emission from a fuel cell, and driving a compressor is assisted with the power generated by such rotation. SUMMARY OF THE INVENTION

[0003] However, in the fuel cell system described in JP 2012-221657 A, there is a problem that the amount of energy recovered from the cathode gas may be insufficient. Therefore, the inventor of the present invention attempted to increase the amount of recovered energy by narrowing a flow passage for the cathode exhaust gas to increase a pressure difference between an upstream pressure and a downstream pressure of the turbine. When the flow passage for the cathode exhaust gas is narrowed, the temperature decreases due to expansion of the cathode exhaust gas, and therefore, there is a problem that dew condensation and freezing of the turbine are likely to occur due to a large amount of water vapor contained in the cathode exhaust gas.

[0004] According to a first aspect of the invention, there is provided a fuel cell system comprising: a fuel cell; a compressor that supplies a cathode gas to the fuel cell; a motor that drives the compressor; a cathode gas discharge passage into which a cathode exhaust gas discharged from the fuel cell flows; a turbine disposed in the cathode gas discharge passage and driven by the cathode exhaust gas, the turbine having a changing mechanism that changes an opening degree of a flow passage for the cathode exhaust gas passing through the turbine to adjust a pressure difference between an upstream pressure and a downstream pressure of the turbine,wherein the turbine recovers at least part of the energy of the cathode exhaust gas using the pressure difference and assists driving the engine with the recovered energy; and a control unit configured to drive the change mechanism to increase or decrease the recovered energy. The control unit is configured to acquire a correlation temperature correlated with a first temperature of the cathode exhaust gas discharged from the turbine and to perform freezing prevention control of not setting the opening degree to be equal to or less than a predetermined opening degree when the correlation temperature is less than a predetermined threshold temperature at which the turbine is capable of freezing. In this fuel cell system, since the correlation temperature correlated with the first temperature is acquired,and the opening degree is not set to be equal to or less than the predetermined opening degree when the correlation temperature is less than the predetermined threshold temperature at which the turbine is capable of freezing, it is possible to restrain a decrease in an amount of energy recovered from the cathode exhaust gas, to restrain a decrease in the first temperature of the cathode exhaust gas discharged from the turbine, and to restrain freezing of the turbine.

[0005] The control unit may be configured to increase the opening degree as the freezing prevention control. According to this configuration, when the correlation temperature is lower than the predetermined threshold temperature at which the turbine is susceptible to freezing, the opening degree is increased. Accordingly, it is possible to restrain expansion of the cathode gas discharged from the turbine, further restrain a decrease in the first temperature of the cathode exhaust gas, and further restrain turbine freezing.

[0006] The control unit may be configured to acquire a second temperature of the cathode gas that has not yet been introduced into the compressor as the correlation temperature. According to this configuration, since the second temperature of the cathode gas that has not yet been introduced into the compressor is acquired as the correlation temperature, it is possible to accurately detect a low-temperature environment in which the turbine may freeze.

[0007] The control unit may be configured to perform the freezing prevention control when the correlation temperature is lower than the threshold temperature and a predetermined time has not elapsed after the fuel cell system is started. According to this configuration, the freezing prevention control is performed when the correlation temperature is lower than the threshold temperature and a predetermined time has not elapsed after the fuel cell system is started. Accordingly, it is possible to prevent a decrease in an opening degree when a predetermined time has elapsed after it is started and there is a low probability that the turbine will freeze, and to restrain a decrease in an amount of energy recovered from the cathode exhaust gas.

[0008] The fuel cell system may further include a temperature sensor that detects a third temperature of the cathode exhaust gas discharged from the fuel cell and not yet introduced into the turbine. The control unit may be configured to estimate the first temperature based on the detected third temperature of the cathode exhaust gas and acquire the estimated first temperature as the correlation temperature. According to this configuration, since the first temperature is estimated based on the detected third temperature of the cathode exhaust gas and the estimated first temperature is acquired as the correlation temperature, it is possible to control the opening degree using a temperature having a high correlation to the actual first temperature.

[0009] The control unit may be configured to calculate, as the freezing prevention controller, the opening degree at which the correlation temperature is equal to the threshold temperature and control the opening degree so that the opening degree becomes the calculated opening degree. According to this configuration, since the opening degree is calculated so that the estimated first temperature is equal to the threshold temperature and the opening degree is controlled so that the opening degree becomes the calculated opening degree, it is possible to mitigate an excessive increase in the opening degree and a decrease in the amount of energy recovered from the cathode exhaust gas.

[0010] According to a second aspect of the invention, a control method is provided for a turbine driven by cathode exhaust gas discharged from a fuel cell to recover at least a portion of energy of the cathode exhaust gas and to assist in driving a drive motor of a compressor that supplies the cathode gas to the fuel cell using the recovered energy. The control method includes: obtaining a correlation temperature correlated with a first temperature of the exhaust gas discharged from the turbine; and not setting an opening degree of a flow passage for the exhaust gas passing through the turbine to be equal to or less than a predetermined opening degree when the correlation temperature is less than a predetermined threshold temperature at which the turbine is capable of freezing.which can cause the turbine to freeze.

[0011] The invention may be embodied in various forms other than the fuel cell system and the control method for a turbine. For example, the invention may be embodied in forms such as a control method for a fuel cell and a vehicle having a fuel cell system. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Features, advantages and technical and industrial significance of example embodiments of the invention are described below with reference to the attached drawings, in which like reference numerals designate like elements, and wherein: Fig. 1 is a diagram schematically illustrating a configuration of a fuel cell system according to a first embodiment; Fig. Figure 2 is a diagram schematically illustrating a configuration of a unit; Fig. 3 is a sectional view taken along a line 3-3 in Fig. 2 is taken; Fig. 4 is a sectional view taken along a line 3-3 in Fig. 2 is taken; Fig. 5 is a diagram illustrating an example of a turbine characteristic map used in a normal control; Fig. 6 is a flowchart illustrating a procedure of freezing prevention control; Fig. 7 is a flowchart illustrating a flow of a freezing determination process; Fig. 8 is a diagram schematically illustrating a configuration of a fuel cell system according to a second embodiment; Fig. 9 is a flowchart illustrating a procedure of freezing prevention control according to the second embodiment; Fig. 10 is a flowchart illustrating a flow of a freezing determination process according to the second embodiment; Fig. 11 is a diagram illustrating an example of a turbine map indicating turbine efficiency; and Fig. 12 is a diagram illustrating an example of a turbine map indicating turbine efficiency. DETAILED DESCRIPTION OF EMBODIMENTS A. First Embodiment A-1. Configuration of a Fuel Cell System

[0013] Fig. 1 is a diagram schematically illustrating a configuration of a fuel cell system according to an embodiment of the invention. A fuel cell system 10 is a system that supplies a driving power source and is mounted in a fuel cell vehicle, which is not shown.

[0014] The fuel cell system 10 includes a fuel cell 15, a cooling system 20, an anode gas supply / discharge system 30, a turbine-mounted compressor unit 100 (hereinafter referred to simply as a “unit 100”), a cathode gas supply system 40, a cathode gas discharge system 70, and a control unit 90.

[0015] The fuel cell 15 is a so-called solid polymer fuel cell and generates electric power by supplying a reactant gas (an anode gas or a cathode gas). The fuel cell 15 has a layered or stacked structure in which a plurality of unit cells are stacked.

[0016] The cooling system 20 cools the fuel cell 15. The cooling system 20 has a coolant supply passage 21, a coolant discharge passage 22, a radiator 23 and a coolant pump 24.

[0017] The coolant supply passage 21 supplies cooling water as the coolant to the fuel cell 15. An antifreeze fluid such as ethylene glycol, air, or the like can be used as the cooling water. The coolant discharge passage 22 sends the coolant discharged from the fuel cell 15 to the radiator 23. The radiator 23 dissipates heat from the coolant. The coolant pump 24 is arranged in the coolant supply passage 21 and circulates the coolant.

[0018] The anode gas supply / discharge system 30 supplies hydrogen as an anode gas to and discharges hydrogen from the fuel cell 15. The anode gas supply / discharge system 30 includes an anode gas tank 31, an anode gas supply passage 32, a main shutoff valve 33, a pressure regulating valve 34, an anode gas circulation passage 35, a gas-liquid separator 36, a circulation pump 37, an anode exhaust / liquid valve 38, and an anode exhaust / liquid passage 39.

[0019] The anode gas tank 31 stores high-pressure hydrogen. The anode gas supply passage 32 connects the anode gas tank 31 and the fuel cell 15. The main shutoff valve 33 and the pressure regulating valve 34 are provided in the anode gas supply passage 32. The main shutoff valve 33 turns on or off supply of anode gas from the anode gas tank 31 in accordance with a control command from the control unit 90. The pressure regulating valve 34 is arranged downstream of the main shutoff valve 33 and adjusts a pressure of the anode gas supplied to the fuel cell 15 in accordance with a control command from the control unit 90.

[0020] The anode gas circulation passage 35 is connected to the fuel cell 15 and the anode gas supply passage 32, and circulates an anode off-gas discharged from the fuel cell 15 into the anode gas supply passage 32. The gas-liquid separator 36 and the circulation pump 37 are provided in the anode gas circulation passage 35. The gas-liquid separator 36 separates liquid water from the anode off-gas containing liquid water discharged from the fuel cell 15. The gas-liquid separator 36 also separates impurity gases contained in the anode off-gas, such as nitrogen gas. The anode exhaust gas containing unused hydrogen gas is circulated in the anode gas supply passage 32 by the circulation pump 37. The anode exhaust gas / liquid valve 38 is opened at a predetermined time in accordance with a control command from the control unit 90.Accordingly, the separated liquid water and the separated nitrogen gas are discharged from the system via the anode exhaust / liquid passage 39.

[0021] The unit 100 includes a compressor 60 and a turbine 80. The compressor 60 is assembled in the cathode gas supply system 40, and the turbine 80 is assembled in the cathode gas discharge system 70. The compressor 60 and the turbine 80 are connected to each other via a motor 62. The unit 100 recovers energy from a cathode exhaust gas and supplies the cathode gas to the fuel cell 15 using the recovered energy as auxiliary power.

[0022] Fig. 2 is a diagram schematically illustrating a configuration of the unit 100. In Fig. 2 shows a part of the unit 100 in a sectional view including an axis CX of the motor 62.

[0023] The compressor 60 has a housing 61, a motor 62, a shaft 63, an impeller 64, an inlet channel 65, and a compressor screw 66. The compressor 60 sucks in air as a cathode gas and compresses it and supplies the compressed air to the fuel cell 15, which is in Fig. 1 is shown.

[0024] The housing 61 has the elements of the compressor 60 housed therein. The motor 62 operates in accordance with a control command from the control unit 90 and drives the compressor 60. The shaft 63 is configured as a rotating shaft of the motor 62 and transmits torque of the motor 62 to the impeller 64. The impeller 64 is configured or formed by an impeller and compresses and conveys the cathode gas, which is sucked from the inlet channel 65, toward the compressor scroll 66 using centrifugal force as it rotates. The inlet channel 85 is connected to an upstream supply passage 41 of the cathode gas supply system 40, which is arranged in Fig. 1. The compressor screw 66, which is Fig. 2, has a spiral appearance and guides the compressed cathode gas to a downstream feed passage 51 of the cathode gas supply system 40, which in Fig. 1 is shown.

[0025] The turbine 80 includes a turbine housing 81, a turbine wheel 82, a turbine scroll 83, an exhaust duct 84, and a change mechanism 85. The turbine 80 recovers at least part of the energy of the cathode exhaust gas emitted by the fuel cell 15, which is Fig. 1, and assists in driving the motor 62 using the recovered energy.

[0026] The turbine casing 81 is formed integrally with the housing 61 and has the elements of the turbine 80 housed therein. The turbine wheel 82 is designed as an impeller and rotates using the energy of the cathode exhaust gas. The turbine wheel 82 is connected to the impeller 64 via the shaft 63. Power generated by the rotation of the turbine wheel 82 is transmitted to the motor 62 and is used as auxiliary power for the motor 62. The turbine scroll 83 has a spiral appearance and is connected to the cathode gas discharge passage 61, which is in Fig. 1, and takes the cathode exhaust gas emitted by the fuel cell 15 into the turbine housing 81. The outlet channel 84, which is shown in Fig. 2, the cathode exhaust gas passing through the turbine wheel 82 leads to the cathode gas discharge passage 71, which in Fig. 1, more specifically to a muffler 78. The temperature of the cathode exhaust gas passing through the turbine 80 decreases according to an expansion stroke of the turbine 80.

[0027] The changing mechanism 85 has a so-called variable nozzle structure, changes the opening degree of a flow passage (hereinafter referred to simply as an "opening degree") of the cathode exhaust gas passing through the turbine 80, and adjusts a pressure difference between an upstream pressure and a downstream pressure of the turbine 80. The opening degree of the flow passage for the cathode exhaust gas passing through the turbine 80 is also referred to as an opening degree of the changing mechanism 85. The changing mechanism 85 includes a changing mechanism drive motor 86, a plurality of variable vanes 87, and a plurality of shaft portions 88. The changing mechanism drive motor 86 rotates the variable vanes 87 in accordance with a control command from the control unit 90.

[0028] Fig. 3 and Fig. 4 are sectional views of the turbine 80 taken along a line 3-3 in Fig. 2 are taken. Fig. 3 schematically illustrates an example in which the opening degree is relatively large (a large opening degree), and Fig. 4 schematically shows an example in which the opening degree is relatively small (a small opening degree).

[0029] A plurality of variable vanes 87 are arranged circumferentially outside the turbine wheel 82 in a radial direction. Each variable vane 87 is configured to be rotatable by a predetermined angle around the corresponding shaft portion 88. Each shaft portion 88 is connected to the change mechanism drive motor 86 via a synchronizing ring and a link mechanism, not shown. When the variable vanes 87 pivot, the magnitude of a gap between adjacent variable vanes 87 changes, and the opening degree changes.

[0030] A passage cross-sectional area through which the cathode exhaust gas flows is large when the opening degree is large, and small when the opening degree is small. For example, the opening degree can be expressed by a ratio of an actual passage cross-sectional area when the passage cross-sectional area is set to 100% in a fully open state. As the opening degree decreases, the pressure difference of the cathode exhaust gas between the upstream pressure and the downstream pressure of the turbine 80 increases, and an expansion ratio of the cathode exhaust gas passing through the turbine wheel 82 increases.

[0031] Here, the expansion ratio of the cathode exhaust gas refers to a ratio of a pressure P4 of the cathode exhaust gas at an inlet of the turbine 80 (hereinafter referred to simply as an "inlet pressure P4") to a pressure P6 of the cathode exhaust gas at an outlet of the turbine 80 (hereinafter referred to simply as an "outlet pressure P6"). Generally, the larger the expansion ratio (P4 / P6) becomes, the lower a temperature T6 of the cathode exhaust gas at the outlet of the turbine 80 becomes (hereinafter referred to as a "discharge temperature T6").

[0032] The cathode gas supply system 40, which is Fig. 1, supplies a cathode gas to the fuel cell 15. The cathode gas supply system 40 includes an upstream supply passage 41, an air cleaner 42, an atmospheric pressure sensor 43, an air cleaner temperature sensor 44, an air flowmeter 45, a downstream supply passage 51, an intercooler 52, a supply gas temperature sensor 53, a supply gas pressure sensor 54, and an intake valve 55 in addition to the compressor 60 of the unit 100.

[0033] The upstream supply passage 41 forms a flow passage upstream of the compressor 60 in the cathode gas supply system 40. The air filter 42 removes dust as the cathode gas is taken in. The atmospheric pressure sensor 43 detects an atmospheric pressure. The air filter temperature sensor 44 detects an outside air temperature. The air flow meter 45 detects an amount of the cathode gas taken into the air filter 42. The detection results of the atmospheric pressure sensor 43, the air filter temperature sensor 44, and the air flow meter 45 are transmitted to the control unit 90.

[0034] The downstream supply passage 51 forms a flow passage downstream of the compressor 60 in the cathode gas supply system 40. The intercooler 52 cools the cathode gas that has been compressed by the compressor 60 and raised in temperature. The supply gas temperature sensor 53 measures the temperature of the cathode gas supplied to the fuel cell 15. The supply gas pressure sensor 54 measures a pressure of the cathode gas supplied to the fuel cell 15. The inlet valve 55 is arranged closer to the fuel cell 15 as a branch to a bypass flow passage 73 in the downstream supply passage 51. The inlet valve 55 adjusts a flow rate of the cathode gas in accordance with a control command from the control unit 90.

[0035] The cathode gas discharge system 70 discharges a cathode gas from the fuel cell 15. The cathode gas discharge system 70 includes a cathode gas discharge passage 71, a pressure regulating valve 72, a bypass flow passage 73, a bypass valve 74, and a muffler 78 in addition to the turbine 80 of the aforementioned unit 100.

[0036] The cathode off-gas discharged from the fuel cell 15 flows into the cathode gas discharge passage 71. The pressure regulating valve 72 is arranged closer to the fuel cell 15 as a connection point to the bypass flow passage 73 in the cathode gas discharge passage 71. The pressure regulating valve 72 adjusts a pressure of the cathode gas in the fuel cell 15 in accordance with a control command from the control unit 90. The bypass flow passage 73 connects the downstream supply passage 51 to the cathode gas discharge passage 71. The bypass valve 74 is arranged in the bypass flow passage 73. The bypass valve 74 adjusts a flow rate of the cathode gas flowing in the bypass flow passage 73 in accordance with a control command from the control unit 90.

[0037] A downstream end of the anode exhaust / liquid passage 39 of the anode gas supply / discharge system 30 is connected to a part downstream of the turbine 80 in the cathode gas discharge passage 71. The muffler 78 is arranged downstream of a connection point with the anode exhaust / liquid passage 39 in the cathode gas discharge passage 71. The muffler 78 reduces exhaust noise of the cathode exhaust.

[0038] The control unit 90 is a microcomputer having a central processing unit (CPU) and a main memory device and is configured as an electronic control unit. The control unit 90 controls the operation of the fuel cell system 10. The control unit 90 obtains output signals from various sensors, such as the atmospheric pressure sensor 43, the air filter temperature sensor 44, the air flow sensor 44, the supply gas temperature sensor 53, and the supply gas pressure sensor 54.The control unit 90 outputs drive signals to various valves, such as the main shutoff valve 33, the pressure regulating valve 34, the anode exhaust / liquid valve 38, the inlet valve 55, the pressure regulating valve 72, and the bypass valve 74, or the units associated with power generation in the fuel cell 15, such as the motor 62, the change mechanism drive motor 86, the coolant pump 24, and the circulation pump 37. The control unit 90 increases and decreases an amount of energy that can be recovered from the cathode exhaust by driving the change mechanism 85. The control unit 90 performs freezing prevention control, which will be described later.

[0039] The control unit 90 increases the amount of energy recovered from the cathode exhaust gas by determining and controlling the opening degree depending on the flow rate of the cathode exhaust gas in normal control. In this embodiment, the control unit 90 first determines a target value of an expansion ratio of the cathode exhaust gas (hereinafter referred to as a "target expansion ratio") and determines the opening degree with reference to a turbine map indicating a relationship between the target expansion ratio, the flow rate of the cathode exhaust gas, and the opening degree.

[0040] In this embodiment, an upper limit expansion ratio P4 / P6 determined depending on a pressure P3 at an outlet of the fuel cell 15 (hereinafter referred to merely as a "fuel cell outlet pressure P3") is used as the target expansion ratio. In this embodiment, the fuel cell outlet pressure P3 can be calculated based on the pressure measured by the supply gas pressure sensor 54 and the opening degrees of the inlet valve 55, the pressure regulating valve 52, and the bypass valve 74, taking into account a pressure loss in the fuel cell 15 or the like. The fuel cell outlet pressure P3 can be measured by providing a pressure sensor in the pressure regulating valve 72.In this embodiment, the outlet pressure P6 is calculated as an approximate value of the atmospheric pressure detected by the atmospheric pressure sensor 43, but may be calculated by applying a correction coefficient to the atmospheric pressure in consideration of the pressure loss in the muffler 78 or the like. It is preferable that the inlet pressure P4 be no greater than the fuel cell outlet pressure P3 in order to maintain the pressure in the fuel cell 15 at an appropriate value. Accordingly, the upper limit of the inlet pressure P4 depends on the fuel cell outlet pressure P3. Accordingly, the upper limit of the expansion ratio P4 / P6 is determined by the outlet pressure P6 and the fuel cell outlet pressure P3, and the target expansion ratio is determined.

[0041] Fig. Figure 5 is a diagram showing an example of a turbine map used for normal control. In Fig. 5, the vertical axis represents the expansion ratio P4 / P6 and the horizontal axis represents a flow rate G4 of the cathode exhaust gas flowing into the turbine 80. In Fig. In Fig. 5, for the sake of simplicity of explanation, two curves are shown in a case where the opening degree is relatively large (a large opening degree) and a case where the opening degree is relatively small (a small opening degree). Other curves with different opening degrees are not shown. The turbine map is stored in advance in the main storage device of the control unit 90.

[0042] In this embodiment, a flow rate of a cathode gas detected by the air flow meter 45 is used as a flow rate G4 of the cathode exhaust gas flowing into the turbine 80. Instead, the flow rate G4 may be calculated by applying a correction coefficient to the flow rate of the cathode gas detected by the air flow meter 45, or may be measured by providing a flow rate sensor in the cathode gas discharge passage 71.

[0043] The control unit 90 determines an opening degree from an intersection point between the flow rate G4 of the cathode exhaust gas and the determined target expansion ratio with reference to the turbine map shown in Fig. 5 is shown. In Fig. 5 shows an example where the flow rate G4 of the cathode exhaust gas is x and the target expansion ratio is y. Since the curve located at the intersection point R1 between x and y is a "large opening degree" curve, the control unit 90 determines the opening degree to be the "large opening degree." The control unit 90 outputs a drive signal to the change mechanism drive motor 86 so that the opening degree of the change mechanism 85 becomes the determined opening degree.

[0044] A large amount of water vapor or droplets, which are products generated by a chemical reaction of the fuel cell 15, is contained in the cathode exhaust gas. Accordingly, when the temperature of the cathode exhaust gas decreases at the time of passing through the turbine 80, the turbine wheel 82, the turbine housing 81, and the like are dew-condensed and frozen due to a large amount of water vapor or droplets contained in the cathode exhaust gas. Freezing of the turbine 80 is likely to occur in a cold region where an outside air temperature is low.Therefore, in the fuel cell system 10 according to this embodiment, it is possible to mitigate a reduction in the discharge temperature T6 of the cathode exhaust gas discharged from the turbine 80 to mitigate a reduction in the temperature of the turbine and to prevent freezing of the turbine 80 by performing a freezing prevention control described later.

[0045] A-2. Freeze prevention control: Fig. 6 is a flowchart illustrating a flow of freezing prevention control. The freezing prevention control is repeatedly executed after a start switch (not shown) of a vehicle mounted with the fuel cell is pressed and the fuel cell system 10 is started. The freezing prevention control may be executed at the same time as starting the fuel cell system 10 or may be executed at any other time.

[0046] The control unit 90 performs a freezing determination process (step S300). The freezing determination process refers to a process of determining whether there is a probability that the turbine 80 will freeze.

[0047] Fig. 7 is a flowchart illustrating a flow of the freezing determination process. The control unit 90 acquires an outside air temperature T0 measured by the air cleaner temperature sensor 44 (step S310). The control unit 90 determines whether the acquired outside temperature T0 is less than a predetermined threshold temperature Tmin (step S320). The threshold temperature Tmin refers to a temperature correlated with the discharge temperature T6 at which there is a probability of the turbine 80 freezing, and is stored in advance in the main storage device of the control unit 90. In this embodiment, the threshold temperature Tmin is set to 5°C, but it may be set to any other temperature correlated with the discharge temperature T6 at which there is a probability of the turbine 80 freezing.

[0048] If it is determined in step S320 that the outside air temperature T0 is not less than the threshold temperature Tmin (NO in step S320), the control unit 90 determines that there is no possibility of freezing (step S350), ends the freezing determination process, and returns to the freezing avoidance control as shown in Fig. 6 is shown.

[0049] On the other hand, if it is determined in step S320 shown in Fig. 7 that the outside air temperature T0 is lower than the threshold temperature Tmin (YES in step S320), the control unit 90 determines that the fuel cell system 10 has just been started (step S330). Specifically, the control unit 90 determines whether X seconds have not elapsed since the fuel cell system 10 was started. X seconds is determined in advance and stored in the main storage device of the control unit 90. In this embodiment, X seconds is set to 60 seconds, but may be set to any other value indicating that the fuel cell system 10 has just been started.

[0050] Therefore, when the fuel cell system 10 has just been started, the temperature of the fuel cell 15 does not become higher and accordingly the temperature of the cathode exhaust gas discharged from the fuel cell 15 is low.

[0051] Accordingly, if the fuel cell system 10 has just been started, there is a relatively high probability of the turbine 80 freezing.

[0052] If it is determined in step S330 that the fuel cell system 10 has not just been started (NO in step S330), the control unit 90 determines that there is no possibility of freezing (step S350), ends the freezing determination process, and returns to the freezing avoidance control set forth in Fig. 6 is shown.

[0053] On the other hand, if it is determined in step S330 that the Fig. 7 that the fuel cell system 10 has just been started (YES in step S330), the control unit 90 determines that there is a possibility of freezing (step S340), ends the freezing determination process, and returns to the freezing avoidance control shown in Fig. 6 is shown.

[0054] In the freezing prevention control, which is Fig. 6, it is detected whether it has been determined that there is a possibility of freezing as a result of the freezing determination process of step S300 (step S210). If it is detected in step S210 that there is no possibility of freezing (NO in step S210), the control unit 90 returns to step S300.

[0055] On the other hand, if it is detected in step S210 that there is a possibility of freezing (YES in step S210), the control unit 90 increases the opening degree of the changing mechanism 85, that is, the opening degree of a flow passage for the cathode exhaust gas passing through the turbine 80 (step S220). Specifically, the control unit 90 rotates the variable vanes 87 to increase the opening degree by outputting a control command to the changing mechanism drive motor 86. Increasing the opening degree means increasing the opening degree compared with that performed before step S220. In this embodiment, the opening degree is increased by 30%, but it may be increased by any other opening degree, such as 10% or 20%.A relatively large opening degree at which there is a low possibility of freezing of the turbine 80, at which the expansion ratio of the cathode exhaust gas passing through the turbine 80 is relatively low and the reduction in a temperature of the cathode exhaust gas is relatively small, may be set in advance, and the opening degree of the changing mechanism 85 may be increased to be larger than such an opening degree.

[0056] As the opening degree increases, a passage cross-sectional area increases, and the pressure loss of the cathode exhaust gas decreases, and accordingly, the expansion ratio (P4 / P6) decreases. Accordingly, expansion of the cathode exhaust gas discharged from the turbine 80 is mitigated, the reduction in the discharge temperature T6 is mitigated, the reduction in the temperature of the turbine 80 is mitigated, and freezing of the turbine 80 is prevented. After step S220 is performed, the control unit 90 returns to step S300.

[0057] In this embodiment, the outside air temperature T0 can be considered to be a subordinate concept of the correlation temperature in the SUMMARY OF THE INVENTION, and a subordinate concept of the temperature of the cathode gas that has not yet been introduced into the compressor.

[0058] In the fuel cell system 10 according to this embodiment, the opening degree is increased when the outside air temperature T0 is lower than the threshold temperature Tmin and the fuel cell system 10 has just been started. Accordingly, since the expansion ratio P4 / P6 of the cathode exhaust gas can be reduced, it is possible to mitigate expansion of the cathode exhaust gas discharged from the turbine 80 and mitigate a decrease in the discharge temperature T6. Accordingly, it is possible to mitigate a decrease in a temperature of the turbine 80 and prevent freezing of the turbine 80.

[0059] Freezing of the turbine 80 is likely to occur particularly in a cold region where the outside air temperature T0 is low. In the fuel cell system 10 according to this embodiment, since the freezing determination process is performed based on the outside air temperature T0, it is possible to accurately detect a low-temperature environment in which the turbine 80 is likely to freeze.

[0060] It is determined that there is a possibility of freezing of the turbine 80 when the fuel cell system 10 has just been started, and it is determined that there is no possibility of freezing of the turbine 80 when the fuel cell system 10 has not just been started. Accordingly, when a predetermined time has elapsed after the fuel cell system 10 is started, the temperature of the fuel cell 15 rises, the temperature of the cathode exhaust gas rises, and there is a low possibility of freezing of the turbine 80, it is possible to mitigate an excessive increase in the opening degree due to a determination that there is a possibility of freezing and mitigate a decrease in the amount of energy recovered from the cathode exhaust gas. B. Second embodiment:

[0061] Fig. 8 is a diagram schematically illustrating a configuration of a fuel cell system 10a according to a second embodiment. The fuel cell system 10a according to the second embodiment differs from the fuel cell system 10 according to the first embodiment in a turbine inlet temperature sensor 75a, which is additionally provided and is a specific means of the freezing determination process. The other configuration is the same as in the fuel cell system 10 according to the first embodiment, and therefore, the same elements and the same steps are denoted by the same reference numerals, and a detailed description thereof will be omitted.

[0062] The turbine inlet temperature sensor 75a is arranged between the pressure regulating valve 72 and the turbine 80 in the cathode gas discharge passage 71 of a cathode gas discharge system 70a. The turbine inlet temperature sensor 75a detects a temperature T4 of a cathode exhaust gas introduced into the turbine 80 (hereinafter also referred to as an "inlet temperature T4"). The detection result from the turbine inlet temperature sensor 75a is transmitted to the control unit 90.

[0063] Fig. 9 is a flowchart illustrating a flow of freezing prevention control according to the second embodiment. First, a freezing determination process is performed (step S500).

[0064] Fig. 10 is a flowchart illustrating a flow of the freezing determination process according to the second embodiment. The control unit 90 acquires the inlet temperature T4 detected by the turbine inlet temperature sensor 75a (step S510). The control unit 90 estimates the discharge temperature T6 of the turbine exhaust gas discharged from the turbine 80 when the above-mentioned normal control is performed based on the acquired inlet temperature T4 and a turbine map indicating a turbine efficiency ηt (step S520).

[0065] The turbine efficiency ηt is calculated as a ratio of power Lt at the outlet of the turbine 80 to a power (Lt)ad in an adiabatic change at the inlet of the turbine 80, as expressed by equation (1). ηt=Lt / (Lt(ad))

[0066] The power (Lt)ad in the adiabatic change at the inlet of the turbine 80 is calculated by equation (2) and the power Lt at the outlet of the turbine 80 is calculated by equation (3). (Lt)ad=Cpg×G4×T4{1−1(P4 / P6)κ−1κ} Lt=Cpg×G4(T4−T6)

[0067] In this case, Cpg denotes a specific heat, and k denotes a specific heat ratio (Cp / Cv). Cp denotes a specific heat at a constant pressure, and Cv denotes a specific heat at a constant volume. Equation (2) can be replaced with equation (5) by applying equation (4). A=1(P4 / P6)κ−1κ (Lt)ad=Cpg×G4×T4(1−A)

[0068] Accordingly, equation (6), which represents the discharge temperature T6, is obtained from equations (1), (3) and (5). T6=T4−T4×ηt×(1−A)

[0069] Fig. 11 and Fig. 12 are diagrams illustrating an example of a turbine map indicating a turbine efficiency ηt. Fig. 11 represents a case where the opening degree is relatively large (a large opening degree), and Fig. 12 represents a case where the opening degree is relatively small (a small opening degree). In Fig. 11 and Fig. 12, the vertical axis represents the turbine efficiency ηt and the horizontal axis represents the expansion ratio P4 / P6. In Fig. 11 and Fig. 12, for the sake of simplicity of explanation, four curves corresponding to rotational speeds of the turbine wheel 82 are shown, and other curves with other rotational speeds are not shown. A rotational speed is expressed by a ratio of an actual rotational speed when a maximum rotational speed is set to 100%. The main storage device of the control unit 90 stores a plurality of turbine maps with different opening degrees in addition to the turbine maps shown in Fig. 11 and Fig. 12 are shown.

[0070] In the above-mentioned normal control, the expansion ratio P4 / P6 is set as a target expansion ratio and the opening degree is determined from the turbine map shown in Fig. 5. The control unit 90 may calculate the turbine efficiency ηt based on the rotational speed of the turbine wheel 82 and the expansion ratio P4 / P6 with reference to the turbine map representing the turbine efficiency ηt according to the opening degree when the normal control is performed.

[0071] In step S520, which is Fig. 10, the control unit 90 estimates the discharge temperature T6 by applying the expansion ratio P4 / P6 and the turbine efficiency ηt when the normal control is performed to equation (6) representing the discharge temperature T6.

[0072] The control unit 90 obtains the estimated discharge temperature T6 and determines whether the estimated discharge temperature T6 is less than a predetermined threshold temperature Tmin (step S530). The threshold temperature Tmin is set to 5°C, for example.

[0073] If it is determined in step S530 that the estimated discharge temperature T6 is not less than the threshold temperature Tmin (NO in step S530), the control unit 90 determines that there is no possibility of freezing (step S550), ends the freezing determination process, and returns to the freezing avoidance control set forth in Fig. 9 is shown.

[0074] On the other hand, if in step S530 shown in Fig. 10, it is determined that the estimated discharge temperature T6 is lower than the threshold temperature Tmin (YES in step S530), the control unit 90 determines that there is a possibility of freezing (step S540), ends the freezing determination process, and returns to the freezing avoidance control shown in Fig. 9 is shown.

[0075] In the freezing prevention control, which is Fig. As shown in Fig. 9, as the result of the freezing determination process of step S500, it is detected whether it is determined that there is a possibility of freezing (step S410). If it is detected in step S410 that there is no possibility of freezing (NO in step S410), the control unit 90 returns to step S500.

[0076] On the other hand, if it is detected in step S410 that there is a possibility of freezing (YES in step S410), the control unit 90 calculates the opening degree at which the discharge temperature T6 is equal to the threshold temperature Tmin (step S420). More specifically, the control unit 90 applies the threshold temperature Tmin as the discharge temperature T6 in equation (6), applies the turbine efficiency ηt when the normal control is performed, and calculates the expansion ratio P4 / P6. Then, the control unit 90 calculates the opening degree based on the calculated expansion ratio P4 / P6 and the flow rate G4 of the cathode exhaust gas with reference to the turbine map shown in Fig. 5 is shown.

[0077] The control unit 90 outputs a control command to adjust the opening degree of the change mechanism 85 to the opening degree calculated in step S420 to the change mechanism drive motor 86 (step S430). Accordingly, the opening degree of the change mechanism 85 is increased (step S440), and the control unit returns to step S500. Since the discharge temperature T6 increases with an increase in the opening degree and becomes equal to the threshold temperature Tmin, a decrease in the discharge temperature T6 is mitigated, a decrease in the temperature of the turbine 80 is mitigated, and freezing of the turbine 80 is prevented.

[0078] In this embodiment, the turbine inlet temperature sensor 75a may be regarded as a subconcept of the temperature sensor that detects the temperature of the cathode exhaust gas discharged from the fuel cell and not yet introduced into the turbine in the SUMMARY OF THE INVENTION, the inlet temperature T4 may be regarded as a subconcept of the temperature of the cathode exhaust gas discharged from the fuel cell and not yet introduced into the turbine in the SUMMARY OF THE INVENTION, and the estimated discharge temperature T6 may be regarded as a subconcept of the correlation temperature in the SUMMARY OF THE INVENTION.

[0079] The aforementioned freezing prevention control according to the second embodiment achieves the same advantageous effects as the freezing prevention control according to the first embodiment. Since the discharge temperature T6 is estimated, it is possible to control the opening degree using a temperature having a high correlation with the actual discharge temperature T6. Since the discharge temperature T6 is estimated based on the inlet temperature T4 of the turbine, which is located at a position physically close to the outlet of the turbine, it is possible to mitigate a decrease in estimation accuracy of the discharge temperature T6 and a decrease in determination accuracy of the freezing determination process.

[0080] The control unit 90 calculates the opening degree at which the discharge temperature T6 is equal to the threshold temperature Tmin and increases the opening degree of the change mechanism 85 so that it becomes equal to the calculated opening degree. Accordingly, since an excessive increase in the opening degree can be mitigated, it is possible to mitigate a decrease in the amount of energy recovered from the cathode exhaust gas due to an excessive decrease in the expansion ratio P4 / P6. C. Modified example

[0081] C-1. First Modified Example: In the above-mentioned embodiments, the opening degree is increased when it is determined that there is a possibility of freezing (YES in steps S210 and S410), but the invention is not limited to this. For example, when it is determined that there is a possibility of freezing (YES in steps S210 and S410), control may be performed such that the opening degree is not equal to or less than a predetermined opening degree. The predetermined opening degree may be set in advance as a relatively large opening degree at which there is a low possibility of freezing of the turbine 80. In other words, an opening degree at which the expansion ratio of the cathode exhaust gas passing through the turbine 80 is relatively small and the reduction in temperature of the cathode exhaust gas is relatively small may be set in advance as the predetermined opening degree.With this configuration, the same advantageous effects as those in the fuel cell system 10 and 10a according to the above-described embodiment can be achieved. When a current opening degree is greater than the predetermined opening degree, the opening degree can be reduced within a range where the opening degree is not equal to or less than the predetermined opening degree, and accordingly, it is possible to prevent freezing of the turbine 80 and mitigate a decrease in the amount of energy recovered from the cathode exhaust gas. For example, if it is determined that there is a possibility of freezing (YES in steps S210 and S410), the opening degree can be maintained. Maintaining the opening degree means that the opening degree is not changed after the determination of steps S210 and S410.According to this configuration, it is possible to mitigate a decrease in the amount of energy recovered from the cathode exhaust gas due to an excessive increase in the opening degree. That is, in general, freezing prevention control of not setting the opening degree to be equal to or less than the predetermined opening degree can be performed when the correlation temperature is lower than the predetermined threshold temperature Tmin at which there is a possibility of freezing of the turbine 80. With this configuration, the same advantageous effects as those in the fuel cell systems 10 and 10a according to the aforementioned embodiments can also be achieved.

[0082] C-2. Second Modified Example: In the freezing prevention control according to the second embodiment, the opening degree at which the discharge temperature T6 is equal to the threshold temperature Tmin does not need to be detected, and the opening degree can be uniformly increased. In other words, in the second embodiment, the same freezing prevention control as in the first embodiment can be performed. In this configuration or the freezing prevention control of the first embodiment, the opening degree can be gradually increased depending on the temperature difference between the outside air temperature T0 or the estimated discharge temperature T6 and the threshold temperature Tmin.For example, the opening degree may be increased by 20% when the difference between the outside air temperature T0 or the estimated discharge temperature T6 and the threshold temperature Tmin is less than 5°C, and the opening degree may be increased by 30% when the difference is equal to or greater than 5°C. With this configuration, the same advantageous effects as those in the fuel cell systems 10 and 10a according to the above-mentioned embodiments can also be achieved.

[0083] C-3. Third Modified Example: In the freezing determination process according to the first embodiment, step S330 may be skipped. That is, the freezing determination process may be performed regardless of whether the fuel cell system 10 has just been started. A configuration in which the outside air temperature T0 can be acquired via the Internet may be adopted instead of the configuration in which the outside air temperature is detected by the air cleaner temperature sensor 44. The freezing determination process may be performed using a temperature of the cathode gas that has not yet been introduced into the compressor 60, measured by another temperature sensor arranged in the upstream supply passage 41, instead of the outside air temperature T0. With this configuration, the same advantageous effects as those in the fuel cell system 10 according to the first embodiment can be achieved.

[0084] C-4. Fourth Modified Example: In the first embodiment, the freezing determination process can be performed using the turbine inlet temperature T4 detected by the turbine inlet temperature sensor 75a arranged in the fuel cell system 10a according to the second embodiment. In this example, since the turbine inlet temperature T4 is assumed to be higher than the outside air temperature T0 due to the cathode gas passing through the compressor 60 and the fuel cell 15, the threshold temperature Tmin can be set to a higher value. For example, the threshold temperature Tmin can be set to 10°C. With this configuration, the same advantageous effects as those in the fuel cell system 10 according to the first embodiment can also be achieved.

[0085] C-5. Fifth Modified Example: In the freezing prevention control according to the above-mentioned embodiments, the freezing determination process is performed in accordance with the flowcharts shown in Fig. 7 and Fig. 10, however, the freezing prevention control may be performed using a map stored in advance in the main storage device of the control unit 90. For example, instead of the freezing determination process, the opening degree may be set to be greater than a lower limit value by storing at least the lower limit value of the opening degree and the correlation temperature as a map indicating a correlation in which freezing of the turbine 80 can be avoided in advance, applying the obtained correlation temperature to the map, and referring to the map. In the above-mentioned embodiments, the turbine map shown in Fig. 5 is used to determine the opening degree in normal control, however, the opening degree may be determined using a relative expression indicating a relationship between the flow rate G4 of the cathode exhaust gas, the expansion ratio P4 / P6, and the opening degree, instead of the turbine map. When calculating the turbine efficiency ηt in the second embodiment, a turbine map in which the inlet temperature T4 is further reflected may be used, or a correction coefficient based on the inlet temperature T4 may be applied thereto. With this configuration, the same advantageous effects as those in the fuel cell systems 10 and 10a according to the aforementioned embodiments can also be achieved.

[0086] C-6. Sixth Modified Example: The system configuration of the fuel cell system 10a according to the second embodiment is merely an example and can be modified in various forms. For example, the temperature of the cathode off-gas discharged from the fuel cell 15 and not yet introduced into the turbine 80 can be used instead of the inlet temperature T4 measured by the turbine inlet temperature sensor 75a. In this configuration, the temperature of the cathode off-gas can be measured, for example, by another temperature sensor arranged in the outlet of the fuel cell 15 or the like. For example, a pressure sensor that measures the inlet pressure P4 of the turbine 80 or a pressure sensor that measures the outlet pressure P6 of the turbine 80 can be additionally provided, and the expansion ratio P4 / P6 can be calculated based on a value measured by the pressure sensor.For example, by additionally providing the temperature sensor that measures the discharge temperature T6, steps S510 and S520 can be skipped, and the freezing determination process can be performed based on the discharge temperature T6 measured by the temperature sensor. With this configuration, the same advantageous effects as those in the fuel cell system 10a according to the second embodiment can also be achieved.

[0087] C-7. Seventh Modified Example: In the freezing determination process according to the first and second embodiments, the changing mechanism 85 has a so-called variable nozzle configuration, but the invention is not limited to this. The changing mechanism 85 may have any other configuration in which the opening degree of the flow passage for the cathode exhaust gas passing through the turbine 80 can be changed to adjust the pressure difference between the upstream pressure and the downstream pressure of the turbine 80, such as a movable flap type or a variable nozzle width type. With this configuration, the same advantageous effects as those in the fuel cell systems 10 and 10a according to the above-mentioned embodiments can also be achieved.

[0088] C-8. Eighth Modified Example: In the above-mentioned embodiments, the turbine 80 is connected to the compressor 60 via the motor 62, and driving the motor 62 is assisted by the energy recovered from the cathode exhaust gas. However, the energy of the cathode exhaust gas may be recovered as power of the turbine 80, and driving the motor 62 may be assisted by electric power generated by rotating another motor with the recovered power. That is, in general, the turbine 80 can recover at least part of the energy of the cathode exhaust gas using the pressure difference between the upstream pressure and the downstream pressure of the turbine 80, and can assist driving the motor 62 using the recovered energy.In the above-mentioned embodiment, the fuel cell systems 10 and 10a are mounted and used in a fuel cell vehicle, but they may be mounted in any other moving object or used for a stationary fuel cell.

[0089] The invention is not limited to the above-mentioned embodiments and may be embodied in various configurations without departing from its scope. For example, technical features in the embodiments that correspond to technical features in the aspects described in the SUMMARY OF THE INVENTION may be appropriately subjected to replacement or combination in order to solve some or all of the above-mentioned problems or to achieve some or all of the above-mentioned advantageous effects. The technical features may be appropriately deleted as long as they are not described as essential in this specification.

[0090] A fuel cell system comprises: a turbine (80) having a changing mechanism (85) that adjusts a pressure difference between an upstream pressure and a downstream pressure of the turbine (80), wherein the turbine (80) recovers at least a portion of an energy of the cathode exhaust gas using the pressure difference and assists driving the motor (62) with the recovered energy; and a control unit (90) configured to drive the changing mechanism (85) to increase or decrease the recovered energy.The control unit (90) acquires a correlation temperature correlated with a temperature of the cathode exhaust gas discharged from the turbine (80), and performs a freezing prevention control of not setting the opening degree to be equal to or less than a predetermined opening degree when the correlated temperature is less than a predetermined threshold temperature at which the turbine (80) may freeze.

Claims

[1] Fuel cell system comprising: a fuel cell (15); a compressor (60) that supplies a cathode gas to the fuel cell (15); a motor (62) driving the compressor (60); a cathode gas discharge passage (71) into which a cathode exhaust gas discharged from the fuel cell (15) flows; a turbine (80) arranged in the cathode gas discharge passage (71) and driven by the cathode exhaust gas, the turbine (80) having a changing mechanism (85) that changes an opening degree of a flow passage for the cathode exhaust gas passing through the turbine (80) to adjust a pressure difference between an upstream pressure and a downstream pressure of the turbine (80), the turbine (80) recovering at least part of energy of the cathode exhaust gas using the pressure difference and assisting in driving the motor (62) with the recovered energy; and a control unit (90) designed to drive the change mechanism (85) to increase or decrease the recovered energy, wherein the control unit (90) is configured to obtain a correlation temperature correlated with a first temperature of the cathode exhaust gas discharged from the turbine (80), and to perform a freezing prevention control of not setting the opening degree to be equal to or less than a predetermined opening degree when the correlation temperature is less than a predetermined threshold temperature at which the turbine (80) may freeze. [2] The fuel cell system according to claim 1, wherein the control unit (90) is configured to increase the opening degree as the freezing prevention control. [3] The fuel cell system according to claim 1 or 2, wherein the control unit (90) is configured to obtain a second temperature of the cathode gas that has not yet been introduced into the compressor (60) as the correlation temperature. [4] The fuel cell system according to claim 3, wherein the control unit (90) is configured to perform the freezing prevention control when the correlation temperature is lower than the threshold temperature and a predetermined time has not elapsed after the fuel cell system is started. [5] The fuel cell system according to claim 1 or 2, further comprising a temperature sensor (75a) that detects a third temperature of the cathode exhaust gas discharged from the fuel cell (15) and not yet introduced into the turbine (80), wherein the control unit (90) is configured to estimate the first temperature based on the detected third temperature of the cathode exhaust gas and to adopt the estimated first temperature as the correlation temperature. [6] The fuel cell system according to claim 5, wherein the control unit (90) is configured to calculate the opening degree at which the correlation temperature is equal to the threshold temperature, and to control the opening degree so that the opening degree becomes the calculated opening degree, as the freezing prevention control. [7] A control method for a turbine (80) driven by a cathode exhaust gas discharged from a fuel cell (15) to recover at least a portion of an energy of the cathode exhaust gas and to assist in driving a drive motor (62) of a compressor (60) that supplies the cathode gas to the fuel cell (15) using the recovered energy, the control method comprising: Obtaining a correlation temperature correlated with a first temperature of the cathode exhaust gas discharged from the turbine (80); and not setting an opening degree of a flow passage for the cathode exhaust gas passing through the turbine (80) to be equal to or less than a predetermined opening degree when the correlation temperature is less than a predetermined threshold temperature at which the turbine (80) is capable of freezing.

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